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Are there examples of encryption in nature?
There are examples of mathematics in nature such as Fibonacci number, fractals, etc.  Are there examples of encrypted information?
What I am looking for is a pattern that seems random on the surface, but once you put it through a "decryption" process, it is actually something else.  Not patterns that seem random because they are complex.
One example that comes close is DNA.  But I see DNA as a form of encoding, not encryption.

You may want to define what you're looking for a bit better. Isn't something that appears random, but is actually a complex formation practically the same as encryption? I don't see the difference between your first and second sentences in your second paragraph.

I see encrypted information as needing to be decrypted before it can make any sense.  Whereas given enough resources, complex patterns can be understood without modifying the data itself.
For example trying to read an encrypted text vs trying to read Chinese (if you don't know Chinese).

@KarnRatana: encryption implies the concept of "secrecy". You encrypt data so that only a certain individual or group of individuals can access it using a certain key. See also http://stackoverflow.com/questions/4657416/difference-between-encoding-and-encryption . It is difficult to imagine a biological situation where secrecy would be an issue.

@nico Thank you.  I am not looking for encoding but encryption.  Which, as you mentioned, does imply secrecy.  I'm not well versed in biology and couldn't think of any example, hence the question =)

I can think of microbiological examples of stealth technology, where viruses, such as HIV can down regulate the production of MHC I molecules in an infected cell to avoid Cytotoxic T-Cell detection or bacteria like S. pneumoniae using the virulence factor of a smooth sugar coat to avoid immune system detection.

pollen-flower interaction probably.

@KarnRatana The question is very interesting but I think it is currently unclear. Would you consider machiavelan intelligence in primates (I groom an individual but I hiding form another individuals when doing so) and camouflage or mimicry as examples of encryption when only individuals of the same species are able to see/disantangle? Can you please try to accurately define encryption for its application to biology?

One example is in the development of the immune system. V(D)J recombination, antibody production, and T-cell receptor production generates a specific "key" that, in most cases, can only be bypassed by that individual's molecular inventory.
In that sense, parasites, mutualists, and commensalists could be viewed as black, white, and grey hat hackers, respectively.
Another example is the organism-specific tRNA synthase inventory and the matching tRNAs.

This sounds similar to what @Nick mentioned (in concept not biology). Which deals more with authentication than protecting a "message".

One example is sperm-egg attraction. Many species release their egg and sperm to be joined in water. The egg must try to avoid being fertilized by the sperm of a different species. So an egg secretes a special peptide which attracts the sperm to its location. The amino acid sequence of that peptide is unique for each species and sperm can only recognize the peptide secreted by the egg of their own species. To any other sperm, the peptide is just another molecule present in the water. http://www.ncbi.nlm.nih.gov/books/NBK10010/

Isn't this just signalling?

Yes, but isn't encryption a form of signaling? If signaling is communicating biologically, it follows encryption will involve some signaling, but the message will be masked, so that not everyone will understand it.

I don't see there's anything masked here; it's just a matter of using different signals.

Perhaps protein folding is an example of decryption? From a single primary sequence (i.e. the string of amino acids), it is impossible to guess the function of a protein. The folded protein is the functional unit (of course it might be further modified).

Encryption is not about the impossibility of deciphering, that would rather be hashing. Also note that knowing folding does not immediately give you the function, just the structure.

I would think that the Bacteria - Phage restriction enzyme and methylation enzyme race could be thought of as a form of encryption. If your DNA isn't "signed" with the appropriate methylation patterns then it gets chopped into pieces and destroyed. This is the signing/verifying side of encryption rather then the encrypt/decrypt side, however.
http://en.wikipedia.org/wiki/Restriction_modification_system

This sounds similar to what @nbogard mentioned (in concept not biology).  Which deals more with authentication than protecting a "message".

R-M systems are fascinating, but it seems to me that there is no message that is encrypted.

@JackAidley modern encryption is used for signing/verifying messages as much as encrypting/decrypting them. The RM system is a form of signing/verifying.

@Nick: I disagree, my view is closer to that in Wikipedia - "[E]ncryption is the process of encoding messages (or information) in such a way that eavesdroppers or hackers cannot read it, but that authorized parties can".

@JackAidley Well if you want to be pedantic they can be examples of Cryptography rather than Encryption, but its the closest example in Biology I can think of.

I actually think I may have come up with a biological analogy to encryption.
If you define encryption as taking information, breaking it up into random pieces or just small packets of data, that are sent on their way and when they reach a source are put back together based on a key, so that unintended recipients cannot interpret the information, then I would say that how the senses process the information that we experience, process it, send it along its way and then it is recompiled to form our experience of reality, is something akin to encryption/decryption.
Take vision. Our eye receives and focuses photons of light into electrochemical signals that get relayed to the visual cortex. In the visual cortex, that data is reassembled into our experience of reality of the world around us.
If, experimentally, we were able to tap into that signal running along the optic nerve and tried to interpret it just from the data flow, then it is unlikely that we would be able to reproduce the image being collected the way we could read the electrical impulse being transmitted from a CMOS or CCD sensor chip. I say this because our visual system is predictive. There is actually more information generated by the visual cortex than is being received and transmitted from the eyes.
We develop these keys in early child development and lay down these neural networks so that we are able to build and interpret an image without all of the data. This is how we can navigate through the world, drive a car, fly a jet fighter and cope with the signal delay that is inherent in the system. This may be more an example of compression of signal, but a compressed signal without the correct decompression algorithm would be a rudimentary form of encryption.
We can also see, with synesthesia, that when there is crosstalk between the regions of the brain that are are different from the norm, messages will be perceived in possibly unintended ways. Letters and numbers have colors, sounds produce visual imagery, etc.

I think breaking information up into pieces as part of a transfer is not really encryption.
If a data is encoded, then with enough samples, we should be able to learn how to decode it.  But if a data is encrypted, then even with all the samples we want, we might not be able to decrypt it without first figuring out the key.
So if there's any data in nature that seems random and we cannot understand it even with a lot of samples, then maybe it is encrypted.  If so, then we might need to try a different approach in trying to decipher it.

Not sure we can name it encryption, but

a pattern that seems random in surface but once you put it through a
  decryption process, it is actually something else

reminds me about some forms of predatory camouflage. I'm talking from the perspective of the prey: the predator blends with the environment and what the prey perceives is just 'random' environment. When the prey 'decrypts' that there is actually something else, it is generally too late.

Predator prey situations are assumed to have a reason for encryption, as they have signaling.  Thus could be the change in appearance of a coiling snake, not only preparing to strike, but sending fake codes to prey at the same time, or coiling differently for mating.
On the recieving side, the timing of the length of gaze and head movements of deer in the midst of observing a scene might be considered cryptoanalysis, as they absorb and process data and carefully select responses and countermeasures.

Please add some references to support your answer.


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Is it a limitation of the eyes, or the brain, that we can't see a moving bullet?
Are the photoreceptors in our eyes not fast enough to register the fast moving bullet, or is the brain not powerful enough to make sense of something happening that fast?

It's less a problem of speed and more of raw photon count. Assuming a brightly lit day, the bullet will move so fast that it doesn't reflect enough photons to register against the background. High speed images of bullets usually involve a very bright flash (and other controlled settings) for the camera to pick it up. (Also, a very short flash helps the bullet to appear stationary in the image.
On the other hand, seeing where your bullets are going is pretty useful. Therefore tracer bullets were created, which are essentially a pyrotechnic flare that you substitute in every 5th bullet, typically in machine gun ammunition. Proof that if you make a bullet bright enough, you can register it just fine.
http://en.wikipedia.org/wiki/Tracer_ammunition

It's not even necessary to use tracers if one is in a shooting range where all the illumination is from behind the shooter and the background is dark.

source: Stack Exchange

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sort: votes


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Do voltage-gated calcium channels affect neuronal conduction speeds?
We have a hypothesis that a drug may block the voltage-gated calcium channels (VGCC) in annelid worms. We are wondering whether there are any correlations between VGCCs and neuron conduction velocities.  If there are fluctuations in calcium ion contents, would there be any changes in conduction velocities? Or if the channel is blocked, would it block the entire action potential along the worm's giant axon?

Most of the time, calcium is not the most important ion in terms of active propagation of an action potential along an axon. It is usually sodium and potassium which play the key roles here. In most living things the concentrations of ions for the intracellular and extracellular solutions are similar. For an action potential, you usually have an influx of sodium followed by an efflux of potassium. The sodium influx depolarizes the membrane and the potassium efflux re-polarizes it. The speed of conduction is determined by the membrane resistance and its "leakage". That is to say, how easily can a single travel along an axon? This is determined by the diameter of the axon, the greater the diameter, the faster the conduction. The leakage refers to ease with which ions leak out of the axon as the signal is propagating. Insulation via myelin sheaths help in preventing this, and, less leakage, the greater the signal at the next propagation site.
Calcium is the most important ion for the transmission of a signal from a pre to post synaptic cell. Without calcium, you have no neurotransmitter release. This is because the influx of calcium triggers a series of events involving the snare complex which eventually leads to vesicular release of neurotransmitter in quantal units. If you remove calcium from the extracellular solution, you will have no neurotransmitter release. If you block the calcium channels, you will have no neurotransmitter release.

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What determines whether a substance can diffuse across the blood-brain-barrier?
What determines whether a chemical substance is able to cross the blood-brain-barrier via passive, transmembrane diffusion?  What structurally differentiates these chemicals?

Some substances are thought to be actively transported across the BBB as well - especially in particular regions of the brain - i.e. A non-uniform fashion

Short answer
The two most important factors that favor passive diffusion across the blood-brain-barrier (BBB) are:

high lipid solubility;
low molecular weight.

And to a lesser extent:

absence of charge;
low affinity for BBB proteins;
not being a substrate for the efflux pumps present in the BBB.

Background
The blood-brain-barrier (BBB) is principally formed by the endothelial cells in the capillaries of the central nervous system (CNS), as depicted in Fig. 1. Outside the CNS, the endothelial cells lining the capillaries allow for relatively free diffusion of substances from and to the blood via transcellular diffusion. In other words, substances can diffuse relatively freely to and from the blood via the extracellular space between the endothelial cells. In contrast, endothelial cells in the capillaries in the CNS are coupled via tight junctions, which prevents most compounds from diffusing into the extracellular space in between the cells (Fig. 1). Therefore, for compounds to enter the brain, they have to diffuse into the endothelial cells and back out again into the brain. This means they have to diffuse from the aqueous milieu in the blood, through the lipid-rich cell membrane and back into an aqueous solution in the extracellular space in the brain (source: University of Texas).

Blood-brain-barrier. source: RIT
Most drugs cross the blood-brain-barrier (BBB) by transmembrane diffusion, which is mainly dependent on whether the drug is able to meld into the cell membrane. A low molecular weight and high degree of lipid solubility favor this mechanism. However, a drug taken up by the membranes in the BBB must eventually dissolve into the aqueous environment of the brain's interstitial fluid to take effect. As a result, a substance that is too lipid soluble collects in the lipid portion, but is not able to leave the BBB. I.e., it enters the BBB, but doesn't cross it.
Moreover, lipid solubility also favors uptake by the peripheral tissues; this, in turn, lowers the concentration of the drug in blood.
Thus, while lipid solubility can increase transport rate across the BBB, it can also lower the amount of the drug presented to the BBB.
Factors in addition to lipid solubility include charge, tertiary structure and degree of protein binding. Chief among these secondary factors, however, is molecular weight. The best approximation of the influence of size on BBB penetration is that it is inversely related to the square route of molecular weight. Few substances over 500 Da cross the BBB. In addition, lipid-soluble substances with molecular weights over 400 Da can be substrates for P-glycoprotein, a major brain-to-blood efflux pump located at the BBB, which prevents or greatly retards a large number of small, lipid soluble molecules from entering the CNS. However, Peptides and proteins with molecular weights in excess of 600 Da are known to cross the BBB in amounts sufficient to affect CNS function, such as the enkephalins. The largest substance found to date to cross the BBB by the mechanism of transmembrane diffusion is cytokine-induced neutrophil chemoattractant-1 at 7.8 kDa (Banks, 2009).
Note that some parts of the brain do not feature a BBB, including but not limited to, the pituitary, area postrema and pineal gland. Many of these areas participate in hormonal control. These structures hence need to be in contact with the blood stream to sense the presence of hormones and other compounds (source: University of Texas).
Reference
- Banks, BMC Neurol (2009); 9(Suppl 1): S3


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Why/how does exposure to noise cause cochlear hair-cell loss?
I am trying to understand why listening to loud music - e.g. concerts or earphones at high volume damages hearing.
According to the National Institute on Deafness the cause is physical.

Most NIHL is caused by the damage and eventual death of these hair
  cells. Unlike bird and amphibian hair cells, human hair cells don’t
  grow back. They are gone for good.

But I don't understand why/how would noise - which should basically lead to higher amplitude waves in the basilar membrane, induce damage and death of these hair cells?

There are a number of pathophysiological mechanisms that are thought to underlie noise-induced hearing loss:

Mechanical damage to the delicate cells and supporting structures of the organ of Corti;
Reduced blood flow to the inner ear;
Intense metabolic activity, which increases mitochondrial free radical formation.

Reactive oxygen species (ROS) are highly reactive. They are essential for mitochondrial function to generate energy. However, too many of them damage cellular lipids, proteins, and DNA, and upregulate apoptotic pathways. The observed impaired blood flow to the cochlea can enhance the toxic effects of ROS. Mechanical damage to the delicate hairs and membranes of the hair cells reduces their ability to converge acoustical energy into potential differences.
References
- Le Prell et al., Hear Res (2007); 226(1-2): 22–43
- Kurabvi et al., Hear Res (2017); 349: 129-37

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Allamanda blanchetti fertilization mechanism
Here is an Allamanda blanchetti flower in my garden.
My question is based on the observation that the flower seems to be devoid of any reproductive structures, i.e., Stamen or Carpel, so how does it set seed? In the picture below it can be seen that the plant isn't sterile and successfully produces seed pods.

google lens app can recognize flowers 95% of the time.

Since the species identification portion of your question is a duplicate, I encourage you to  remove that portion. That will allow you to focus on a single question, which is important because each question should be posted separately. That improves the chances that you will get answers for each question and makes the answers more accessible for future users. ——— However, we expect you to do research on your own and then, informed by what you have learned, ask any questions you still have (ideally with references to reliable sources). Start by looking up reproduction of this species. Thanks!

I have edited the question to be more specific about the fertilization process, since @Bryan Hanson has already pointed out a similar question for the species ID. But the question  as it stands now, edited, is different and not a duplicate.

Great, I've voted to reopen your question. This typically requires four other users to agree so it may take a few days. In the meantime I encourage you to dissect some of the flowers and see whether you can find out an answer yourself! A razor blade or sharp knife can be used to cut through the flowers to see what is inside — I suggest cutting along the long axis of the flower (slicing the "tube" at the base lengthwise). I think you will find this quite enlightening. However, it may lead to more questions, but that's good! (Also be careful to clean up the latex since it may be irritating.)

@tyersome sure! I will try it out as soon as possible. :)

There's now a followup question with dissection photos at What are these structures inside a cut open Allamanda blanchetti?

As far as the ID goes, this looks like Allamanda blanchetti.  See this question.

In the future I think it would be better if you just flagged the question as a duplicate of the other question (to which you gave a good answer).

I have looked up the reproduction of Allamanda genus on Wikipedia and other websites, but only taxonomy, distribution ,medicinal uses and the likes are listed. I am a high school student and don't really have much access to more scholarly articles. So please enlighten me on the matter.

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Synthetic carbon fixation pathways
Have there been carbon fixation pathways of, in particular, oxygen-producing bacteria, that have been replicated in the lab or at least the enzymes for the pathways have been successfully produced in the lab?

It is not just the enzymes. It is also the way they are arranged in the cell that dictates processes like photosynthesis.

@WYSIWYG I understand that..


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Photosynthetic efficiency and light harvesting efficiency
Is there some sort of measure of the light harvesting efficiency (possibly like transfer time) that is directly related to the photosynthetic efficiency in some sort of equation? Also, how important is light harvesting efficiency to photosynthetic efficiency?


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Senescence in space
How is senescence of individual eukaryotic cells and tissues affected by gravity and zero-gravity environments? In other words, can eukaryotic cells live forever in space (given the appropriate nutrients)?

Why would you suspect this? There’s not really any good reason to think so.

@KonradRudolph I was thinking that the zero-gravity environment might lead to some cell signalling to for example create more telomerae.. something like that...

Wouldnt all the radiation be a factor too?

@RoSiv Yes it would


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Monocot stele in roots
Why do monocots have eustele in the roots instead of protostele like dicots? I thought the roots don't need the strength of eustele because the roots are supported by the soil. So what is the evolutionary reason for the eustele in the roots of monocots?

Possible duplicate of If a trait would be advantageous to an organism, why hasn't it evolved?


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Hydra immortality
Recent research on immortality ha been on an organism named Hydra. I have heard about it being related to some FoxO gene, which has been found in several other organisms but how does telomerase affect the immortality of this amazing species?

@Dexter No, hydra is specifically investigated for its (presumed) lack of senescence, not just its regenerative abilities — unlike, say, planaria.

Yes, I realized. Just searched. You are right @KonradRudolph . I had no idea about this FoxO. I will delete that comment .

Does this answer your question: http://biology.stackexchange.com/questions/6884/why-is-the-hydra-biologically-immortal?rq=1

@AliceD no i find it incomplete..

@TanMath what do you find incomplete about it? Please [edit] your answer and clearly state exactly what you're looking for, and how the answers to existing questions, as well as other online resources, don't completely address what you're looking for.


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Is the fingerprint regenerated?
Let's say you get a a cut on your finger. Will the fingerprint be regenerated to matched your original fingerprint and if so, how?

Wow! I was just searching about this :D It looks like as far as you don't damage dermal layer, you can regenerate exact same pattern.


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height and bryophytes

Why does being taller provide advantages for Bryophyta but being
  shorter is better for Marchantiophyta? What disadvantages could being
  taller have?

I think that by being taller, you have better spore dispersal. However, the problem with being tall is that the temperature flucuates more. Am I correct? Is this the right answer? Am I on the right track?

The root of the question is going to be in the differences between Bryophyta and Marchantiophyta. What are some of those differences? Do they affect spore dispersal? Water use? Temperature?

@Resonating Bryophyta have a cuticle, stomata, and conductive tissue. But the question is guess is why does it need it? I can only think of water use.  Being tall provides advantages for spor diesperal as well.

So taller is better. Why isn't Marchantiophyta taller? Can they even be any taller than they are?


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Why do snails come out after the rain?
Many times, when there is rain after a long time, snails come out afterwards, and you can see snails and slugs everywhere! I have two questions:

Why do they come out after the rain?
Where are they usually found, not after a rain?

Note that I do emphasize when there is a rain "after a long time"

They probably need oxygen. If they are living in the dirt, then the water will force them to the surface.

A yahoo answer post suggest that they come out to flee drowning as @AMR suggested.

Snail Behavior Rain... 20,400 results; Snail Locomotion Rain 4,520 results; Snail Movement Rain... 22,500 results.

http://chat.stackexchange.com/transcript/message/25100934#25100934

In most species, slugs and snails survive droughts (which would otherwise kill them by dehydration) by creating "slime tombs" (initially-viscous bubbles of slime) around themselves. these tombs or pellets will quickly dry out, on the surface, while resisting total dehydration. When it rains, these capsules will re-hydrate or "melt," beginning on the surface, and the creatures will crawl free.
Because they've gone without food, they're ravenous, and go everywhere, "rasping" softened nutrients from the surfaces of plants and even objects where "blossoming" single-cell organisms (also recently re-hydrated) can be found.

Welcome to Biology.SE! Can you please add references? (I am not trying to be rude) I like your answer, but references would certainly complete it.


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Is Roundup toxic?
Roundup, as we all know, is a herbicide that kills weeds. It does this by preventing the production of amino acids in plants. One of these amino acids is tyrosine. Tyrosine is a nonessential amino acid so it is produced in the human body. So is Roundup toxic because it could also affect the human tyrosine synthesis pathways? Or are the pathways for tyrosine synthesis different for humans and plants?

"Glyphosate specifically inhibits an enzyme that is essential to plant growth; this enzyme is not found in humans or other animals, contributing to the low risk to human health from the use of glyphosate according to label directions." http://www.monsanto.com/products/pages/roundup-safety-background-materials.aspx

Glyphosate works by inhibiting 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase. Plants and microbes have this gene, and humans do not.
So, Roundup is toxic to plants because because it inhibits the production of critical amino acids as you surmised (EPSPS makes 5-enolpyruvyl-shikimate-3-phosphate (ESP) which is used as precursor to not only amino acids but also other plant hormones and aromatic chemicals).  It does not do this in humans.
It is a very controversial compound though, and weight of evidence for human toxicity has not clearly overwhelmingly swung one way or the other yet. There are some very impassioned believers on both sides of the fence, and I suggest reading pubmed epidemiology studies to make up your own mind.

The pathways are different. Humans synthesize tyrosine by hydroxylating phenylalanine. Plants use the shikimate pathway to produce all of the aromatic amino acids. Roundup inhibits an enzyme in the shikimate pathway but does not affect the human enzyme involved in producing phenylalanine. None of this is to say, however, that it is or isn't toxic.

The MSDS shows it to be pretty much non-toxic. LD50 is greater than 5g/kg in rats... California wants to list it as a carcinogen but that may be more political than fact based.

@AMR Thank you for the info. It seems most things are "known to the state of California" to cause cancer.


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Aptamers for DNA
I know that aptamers have a high affinity for their target and are typically made of oligonucleotide s, but can I target DNA using aptamers? If not, what possible technologies can target DNA strands?

DNA can be targeted simply based on base complementarity. That's what antisense RNAs/oligonucleotides and CRISPR do.

@AlwaysConfused aptamer. It's actually a fairly common term in molecular biology. A single Google search would have answered your question.


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Are there any completely, or 'true', Mendelian traits that do not display any polygeny at all?
Recently, I learned about polygenic traits and it got me wondering, are there any truly Mendelian traits where the trait displayed exists in a total binary?
I have looked at some questions on the forum and this one is close enough, though I suspect that the degree to which ear wax is dry or wet would vary from individual to individual.
I understand that most of our groups for anything are imperfect and so a perfect binary trait (such as yellow-green peas, which, I believe, also do have a gradient in their coloration, they can't all have the exact same pigmentation) isn't very likely, but could it exist?
Sorry if the post isn't really detailed, I'm not experienced in biology so I don't know which sources to look for.

possible duplicate https://biology.stackexchange.com/questions/76283/true-examples-of-common-variation-due-to-mendelian-inheritance

Also related: https://biology.stackexchange.com/questions/111402/what-are-the-relative-frequencies-of-dominance-and-codominance-in-genes-underlyi

George Box is credited with the aphorism "All models are wrong, but some are useful" in the context of statistics, but the same concept applies to concepts like Mendelianism which are also really a type of model applied to data.
Perhaps the simplest mechanism behind a Mendelian trait is a functional versus non-functional enzyme; let's imagine it's an enzyme that catalyzes a step in pigment formation. With the enzyme, a red-colored molecule is produced. Without it, there is no red. In many cases the enzyme concentration won't be particularly important, and you'll have a simple recessive Mendelian trait: as long as you have at least one functional copy, you get red pigment. If you have zero functional copies, no red.
If that's all the variation you have in the population you're looking at, then the Mendelian model works great. But it's also extremely unlikely that this is truly the only possible source of variation in color, even if it's the only variation you observe.
For example, there is going to be some pathway that produces the reactant for the red-making enzyme to work with, and any changes in any of the biology associated with that pathway can potentially change pigmentation, too.
There is going to be regulation of the expression of the red pigmenting enzyme, too; that regulation might start to make the copy number important if expression becomes very low. It might make the coloration more sporadic or patterned if expression becomes variable among cell populations.
Finally, there's no reason for there to be only two alleles of some gene; in a real population many different alleles will exist, and while some are going to be interchangeable with respect to phenotype (for example, it doesn't matter for phenotype what the specific mutation is that makes a gene product non-functional), others will not be.
So, the Mendelian model for phenotype is almost certainly wrong or at least not comprehensive; the question is about when it is and isn't useful, despite being wrong, and there are lots of cases when it's useful. When someone says a trait is Mendelian, you should translate that as "A Mendelian model is useful for describing the variation in this trait in the population we're talking about"; they are not necessarily (and should not be) making a claim that the trait is somehow intrinsically Mendelian, there is no such thing.

Doesn't this come down to your definition of "trait", really? In your example, if you define "trait" as the visible phenotype, the cell's color, then sure, there can be other mechanisms that would change the color. If, however, you define the trait as whether we have a functional enzyme or not, then wouldn't you say it is indeed perfectly mendelian?

@terdon In the real world it's unlikely that the only variations that exist in an enzyme are "functional" and "not". There is certainly no enzyme where alleles must sort into those two categories. I'd say at the point you've decided to sort s trait into a binary functional/non-functional you've already chosen to apply a Mendelian model to the data.

Ha! Fair point, yes.

@terdon I guess I'd also encourage people not to focus too much on Box's "wrong" and remember the "useful" part is there, too! F = ma is also a wrong model, but for many contexts you'll have only data that do not conflict with it out to many decimal places and for which it would be a waste of time to use any other model!


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Is it possible to make synthetic olfactory receptors?
Are there any electronic engineering technologies that allow people to make synthetic olfactory receptor that generate electrical impulses when exposed to certain proteins or molecules? If not, what are the primary challenges of creating such a device?

Have you performed any prior research - I'm asking, because the question is really broad. Do you mean electronic (macro)devices? or artificially bio-engineered proteins, or cells of some kind?

@AliceD At this point in time, I am interested in any detecting technology that can be customized to detect specific compounds similar to olfactory receptors. I have done some research, but I have not found anything that would be able to link into electronics systems and be reusable like our own nerves. I have done some research (my background is engineering), but it took me a good deal of time to even find anyone who could describe in detail how the olfactory receptors work to detect multiple smells.

Thanks for that, I've edited your post a little bit to clarify that point. and to hat end do you need this technology? For a sensorineural implant to replace the lost sense of olfaction (a biocompatible 'bionic nose')? Or is it more for some kind of computerized detection system that can, and should operate out of the body and 'simply' detect different odors?

My gut reaction is that this feels outside of Biology per se; there are certainly chemical
"sniffers" of a variety of types - can you explain how your question necessarily relates to biology per se? As far as how olfactory receptors work, have you tried introductory-level neuroscience textbooks? It's pretty common for them to have at least a chapter on olfaction.

Depending on just what you mean by "olfactory receptor", this might be an absolutely trivial question.  Ckips that can "smell" for instance carbon monoxide are readily available for a couple of bucks.  Others can smell CO2, alcohol, LPG, the oxygen in your car's exhaust, explosives...

@AliceD - So to clarify my purpose that isn't necessarily pertinent to the question itself: It seems like being able to create a synthetic and readily reusable smelling device that is as powerful as that of a dogs would be a world changing invention. In the medical field alone, there are numerous diseases which dogs have the ability to accurately smell and diagnose instantly. I posed this question to biologist as I thought the only way to create the wide range of detectors needed to get an accurate sensor would be to synthesize a large variety of proteins like our receptors.

@jamesqf- What is a Ckips? I can't seem to find any reference to such a thing...


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Where are examples of Interspersed repeats?
Interspersed repeats are often defined as repetitive sequences dispersed throughout the genome.
However such definition (IMO) does not exclude Tandem Repeat Sequences.
So I started to search for examples of such sequences.
After checking many Bioinformatics and Genetics books, I found many examples of Microsatellites and Minisatellites. However, I couldn't find any examples of Interspersed Repetitive DNA out there, in any of its variants (LINEs: L1, L2, L3, SINEs, etc.).
So my conclusion is Interspersed Repeats DNA Sequence looks like any other type of repeat (such as STRs or Minisatellites), the only real difference is they are flanked by Open Reading Frames (ORFs 1 and 2 for LINEs) or specific tandem duplications (like GC for SINEs)
Do anyone have one or two examples of concrete Interspersed Repeat DNA Sequences?


text:
Physiology of tricuspid and bicuspid(mitral) valves
The tricuspid and bicuspid valves are atrioventricular valves. Both are valves that prevent backflow of blood pumped from the atria to the ventricles. However, the former has 3 "flaps" while the latter has 2.
Despite searching the internet, I could not find a biological explanation for this difference.
Might this be just a relic of evolution or some sort?

An interesting question. Someone has asked it over here and you may find it useful. From fluid dynamics perspective and given heart's mechanism, there might be a really awesome reason behind it.

During embryogenesis at the level of an atrioventricular canal there is so called endocardial "cushion" tissue. This tissue bulges to the left side more than it does to the right and contribute to the fusion of the valvular ring to a one leaflet, while from the remaining part of the ring develops the second leaflet:

AVEC = atrioventricular endocardial cushions
As has been stated, the cushion tissue bulges more to the left, thus on the right side where the tricuspid valve is forming the leaflets does not "merge" and develop into three-leaflet valve.
Interestingly, this left side "suture" might sometimes fail to develop with resulting three-leaflet mitral valve.


text:
Why does gaining a hydrogen in biology considered reducing a molecule?
I've learnt in chemistry that gaining electrons means reduction, while losing electrons means oxidation. But why is it in Biology textbooks I sometimes come across the term gaining hydrogen??

An example would be nice. It's all about context. And they are confusing terms. An oxidizing agent gets reduced when oxidizing something else etc.

Additionally I think this topic is better suited at chemistry.stackexchange.

No only biology, this notion is there in organic chemistry as well. Remember reduction of alkenes? One of the most common 'reducing agents' used in organic chemistry is LiAlH₄

So in a system of two compounds participating in a redox reaction: an oxidizing agent accepts electrons, and thus the oxidizing agent becomes reduced, and the other compound which loses electrons becomes oxidized. A reducing agent donates electrons, itself becomes oxidized, and reduces the other compound. In lots of biological reactions, you can stylize the gain or loss of hydrogen as the gain or loss of an electron, because hydrogen generally donates its sole electron to any given bond.

The definition of electron acceptance and donation came later. Previously oxidation was just addition of oxygen and reduction meant removal of oxygen/addition of hydrogen.

Hydrogen can accept electrons or donate them, so it can get a bit confusing. For instance, hydrogen donates electrons from a reducing agent. For example, in biology, consider cellular respiration. A co-factor will carry electrons often by reducing an atom with a nitrogen.
Consider NADP+ to NADPH.

The nitrogen atom on the Pyridine group has an oxidation state of +1. When it picks up a hydrogen, to become NADPH, the oxidation state will go to 0 by gaining electrons brought by the hydrogen. Although the hydrogen doesn't bond to the nitrogen directly, it breaks the double bond on the para carbon (The carbon on which the new hydrogen is attached) at the top of the ring, and thus distributes the electrons to the nitrogen.
EDIT -
Hydrogen comes from some reducing agent, not a free hydride I'm sure.

Are you able to annotate the drawing of NADP so that I can follow your explanation better? Thanks :)

I agree with @Teige - a reaction instead of a single molecule would help.

There is no Hydride-Anion present in any living cell.

Oxidation state and charge are not the same thing. The pyridine in NAD+ is positively charged, but that does not mean that it is +1 oxidation state.


text:
Why is translocation of food in the phloem still a debate?
I currently take tertiary-high school biology and I am taught that translocation of food in the phloem occurs by the pressure flow hypothesis.
I would like to ask why it is still a hypothesis, and what are arguments against it. If possible are there any scientific literature or journals for me to chew on? Thanks in advance.

The wikipedia page provides some of the opposing arguments, but doesn't necessarily source them.
http://en.wikipedia.org/wiki/Pressure_Flow_Hypothesis


text:
What is the meaning of the word 'bin' in the context of RNA-Seq?
I have a question from a book about RNA-Seq. I would like to know the meaning of the word "bin" in the below cited paragraph:

RseQC has several nice features not found in the other programs: (a)
  When calculating read distribution between different genomic features,
  it reports also several bins upstream and downstream of transcripts.

It's talking about things like "TSS_up_1kb", which is a "bin" of 1kb upstream of the gene. A bin is a somewhat generic term that in this context means "a region of some given size". Another example where you might see the term is with histograms. There, values are grouped into bins, with each bin then being represented by a separate bar in the graph.


text:
How can grass-fed livestock generate fat?
How can herbivorous cattle get fat? These animals eat only grassy substances, think of cattle, buffalo, goats etc. For example, wheat grass contains 0% fat, and its nutritional value is limited to carbohydrates and fibers. Still, livestock can have substantial (10%) fat content and give milk with lots of fat on top of it.
Where does that fat come from?

Apologies for re-editing this question, but just to illustrate that grass basically contains no fat at all and to prevent discussions on nuts, seeds etc.

Perhaps I'm unduly picking nits, and I'm certainly not disagreeing with @AliceD's nice answer, but most grass does have some fat.  Here for instance are numbers for various types of hay: http://www.atascaderohayandfeed.com/hay.html (The first hit Google returned - so I'm lazy :-))

Short answer
Plants typically store energy in the form of starch. Animals, including livestock and humans for that matter, can digest starch, metabolize it into acetyl-CoA and turn it into fat.
Background
Plants contain sugars, formed by photosynthesis. Plants generally do not store energy in the form of fats, but in the form of starch, a glucose polysaccharide*. Animals digest this starch through hydrolysis catalyzed by the enzyme amylase back into glucose (Fig. 1). Cellulose is another polysaccharide. In grass-fed cattle, it is mainly cellulose that forms their source of energy. Cellulose cannot be digested by ruminants (cattle, goats, sheep) as they are unable to synthesise the enzymes needed to digest cellulose and other plant compounds. Cellulose can be hydrolyzed by cellulase enzymes in e.g. Ruminococcus species present in the ruminant tract in the guts of these animals. The cellulase enzyme endoglycosidase cleaves the disaccharide cellobiose from cellulose, and another type of enzyme, β-glucosidase hydrolyzes cellobiose into glucose monomers (sources: Microbe Wiki, University of Waikato).
Glucose is then degraded to pyruvate by aerobic glycolysis in the cytoplasm of the cell. Pyruvate is then transported into the mitochondria, where pyruvate dehydrogenase oxidatively decarboxylates pyruvate, forming acetyl-CoA and other products (Fig. 2).
Acetyl CoA can then serve as a substrate for citrate synthesis. Citrate, in turn, can be transported out of the mitochondria to the cytoplasm. There it can be split to generate cytoplasmic acetyl-CoA for fatty acid synthesis under the influence of anabolic factors like insulin in times of plenty (Fig. 3), referred to as lipogenesis.

Fig. 1. Starch is hydrolyzed by amylase. source: Biotek

Fig. 2. Acetyl-CoA formation. source: University of Utah

Fig. 3. Lipogenesis. source: Sharing in Health
*: Although plants usually store their energy reserves in the form of starch, there are important exceptions to this. Most notably certain seed pods, like peanuts and walnuts, contain substantial amounts of fat. Further, cell membranes and other components will add traces of fatty substances even in grassy plants. With credits to Marzipanherz.

Does this mean I can get fat even if i do not eat any fat?

@murmansk Getting overweight is not a problem of eating to much fat per se, but of taking up too much nutrients. Fats are more "dangerous" here, since they harbour more energy (calories) per gram than carbohydrates. But still, if you eat too much sugar and avoid fat completely, you can get overweight.

@murmansk The "Atkins diet" was a fad a few years ago based precisely on this concept: weight gain is a function of a disparity in calories in versus calories out. The source of calories is less important. More recent fad diets that take a similar approach are the "ketogenic diet" and "paleo" movement. All of them emphasize a reduction in sugars, rather than fats, for health and weight loss. (note: I'm not necessarily supporting any of these diets, just pointing out that they are somewhat popular and are based on exactly the science we are discussing here)

Same way animals can turn sugars to fats, chain reactions involving multiple enzymes. Some plants, e.g. potatoes, rice etc. will store very little fat. Most plants will only store a bit of fat in the seeds, but some - I guess nuts and olives are the main ones - will convert significant amounts of glucose (produced by photosynthesis) to fats, which we can eat, or extract oil from.
Edit: Looks like there's some good info here, but a bit dense :lipid biosynthesis


text:
What is the difference between a protein and a factor?
In terms of nomenclature/semantics, why are some proteins named proteins, and some named factors?
I've been revising on eukaryotic DNA, and I've come across some proteins that seem to serve roughly the same function, but are named differently. For example,

Replication activator protein
Replication licensing factors
Replication protein A
Replication factor C
Transcriptional factor
Eukaryotic translation initiation factor

TLDR: As far as I know, there's no specific reason some proteins are called "factors"; it's just a matter of what name was chosen.

"Protein" is a specific term meaning a long chain of amino acids. They are typically at least 50 amino acids long.
Conversely, the word "factor" is quite a loose term and is as broad as even being "an element of something". So, essentially anything in biology from a chemical to temperature could be called a factor.
In the context of biochemistry, a factor is a "substance which takes part in a biochemical reaction… …or process". This could include everything from proteins and enzymes (e.g. EGF), to non-protein chemicals such as substrates, co-factors. Even Ca$^{2+}$ (coagulation factor IV) and nitric oxide/arachidonic acid/etc. (EDHF) fall under this umbrella - a single ion is about as far from a protein as it's possible to get! So many proteins are also given the label of "factor". In fact, Protein C has been named both "Protein C" and "blood coagulation factor XIV".
One reason various proteins have been called "factors" is that the word factor used to be used to refer to genes. I'd speculate that another reason may be that certain names and acronyms were already in place. For example, protein C vs. factor C or CREB-binding protein (CBP) vs. core binding factor (CBF).
But as long as the naming of the proteins is standardized, I see no reason why we shouldn't just use acronyms to describe proteins. After all, protein names can be misleading (e.g. Sonic Hedgehog), and there is a large amount of confusion in the literature between names of proteins.

Looks like our answers are nearly identical, even down to the SHH reference - I think this one reference has to come up every time someone talks about protein nomenclature. :)

@BryanKrause I thought the exact same thing! I was slightly worried you'd think I was copying your answer with the SHH point too! I wouldn't have posted if I'd seen yours since you were first and perfectly answered the question but it's quite tricky to delete something that you've just written :)

I think it's pretty obvious with the 4 minute gap and completely different verbiage that no copying was involved. :) Welcome to Bio.SE

To be pedantic, isn't protein reserved for longer amino acid chains? With short chains referred to by the more genetic "polypeptide"?

@JackAidley Thanks for the correction; I've added a note. I was never too fond of the protein vs. peptide distinction, but it's nice to be precise

"As far as I know there is no specific reason…" See my answer for two.

Short Answer
There is no agreed upon naming convention for proteins - there are some rough standards because in language people usually try to convey their ideas in a way others can understand, but that doesn't necessarily mean fixed rules.
Longer Answer
I think it's important to recognize the process for understanding what proteins do is not always straightforward. Most often, a protein's function is first understood by seeing what happens if that protein is absent (and sometimes overexpressed).
The terminology factor implies that a protein is either modulating a process or at least is not by itself sufficient for a process. That is, it is named because when it was omitted, some other measurable process changed. It  maybe implies something that has its main function by binding, rather than catalyzing a chemical reaction, or at least that the actual mechanism of action is not yet known at the time of naming.
You would not expect, for example, an enzyme with a known target to be called a factor: it's most likely to be named according to its target and the type of reaction being catalyzed.
However, it's possible for a name to stick from when there is less complete understanding. Something might be called a factor initially because of how it influences some process, but the actual contribution is only understood later. Just for an example, take Complement factor I. Initially named because it had some role in the complement system, now it's known that it enzymatically cleaves another protein.
Importantly, there isn't any pure terminology here or consistent naming convention: in most cases, it just goes back to however the first person to describe the protein discussed it. Some proteins are named (controversially, I'll add) things like Sonic hedgehog - related to the similarly named Hedgehog signalling pathway which is all named because of one person/group's creative description of a related fruit fly phenotype.
Naming something "protein" just identifies that it's a protein, little more.

There are at least two reasons why certain compounds were historically named in the manner ‘function–factor’.

When you have some impure preparation that exerts a biological function but do not know the chemical nature of the component responsible is.

For example an unknown growth factor, could be a protein, but it could also be a steroid etc.

As a deliberate contrast to enzyme to emphasize that the activity is not catalytic. The point here is not the opposition to a different type of protein.

For example, in DNA or RNA synthesis you have the actual enzymes (DNA and RNA polymerases) that work catalytically to form phosphodiester bonds, and then various accessory proteins that are required to bind to specific regions of DNA, but do not catalyse any chemical reaction. In this case the word ‘protein’ does not distinguish the molecule from an enzyme.
In cases where the need to make such distinctions is not evident to a scientist who has established the protein nature of an activity, he may have chosen to designate it protein for greater precision.
Coda
When a field has matured sufficiently there is often an initiative to rationalize nomenclature. This is frequently resisted by the originators of the nomenclature. In this respect Fritz Lipmann is reputed to have said that “changing names is rewriting history”.

Short Answer
All factors are proteins, not all proteins are factors
Background
A factor is typically a small protein that regulates a larger target protein directly by specifically associating with it, or indirectly by affecting its substrate. For example, protein factors play key roles in protein synthesis (Berg et al., 2002) and see Fig. 1.:

An initiation factor binds to the much larger and more complex multi-subunit ribosome during the initiation of translation, a part of protein biosynthesis (fig. 1);
A release factor terminates translation by the ribosome by recognizing the termination codon or stop codon in an mRNA sequence
A transcription factor is a protein that regulates the much more complex and bigger RNA polymerase. Transcription factors control the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence.

Reference
-Berg et al., Biochemistry, 5th ed. New York: Freeman (2002)
<

Fig. 1. Inititation factors are relatively small proteins that interact with the large ribosomal multi-subunit complex. source: Concepts Of Genetics

Only problem I have with this answer is there are lots of proteins that are named "factor" that don't fit into the better-defined categories you mention like initiation/release/transcription factors. Also there are plenty of proteins in those categories that are also certainly not named factor.

@BryanKrause - they are examples, as stated, and not meant to reflect an exhaustive list. It's about the bigger picture. A factor regulates something. Nice answer btw +1. Anyway, naming conventions in Bio are history-related and their systematics murky for sure

Hi, @AliceD, nice answer! I'm not sure about the "all factors are proteins" statement though? While it's clearly true that almost all factors are proteins and that the naming paradigm is pretty nebulous, I think there are still at least a couple (coagulation factor IV, EGHF) which might not be, since they may have been named before their molecular identity was known

@Jam thanks! Factor IV is now simply designated as Calcium. Your other example I don't know what it is, sorry :(

@AliceD Whoops, that was a typo, sorry! https://en.wikipedia.org/wiki/Endothelium-derived_hyperpolarizing_factor My main point about Factor IV was that I think some things are called "factor" before we know whether they're proteins :)

I agree with @Jam, a factor need not be a protein and, AFAIAA that is why many were originally called 'factors' - because it was not known what they were. Since then, of course, the word has been more widely applied.

@JackAidley - I thought that non-protein factors were called co-factors.

@AliceD My understanding is that this is only true where the non-protein is required for the action of the protein.


text:
What are these little leaves growing on these other leaves? (plant growing near the ocean)
I saw many of these plants while hiking on a hill facing the ocean in northern Taiwan. I've included a snapshot of a nearby part of the same trail to show it's rugged, wet, green, and rocky, and another photo of some what looks to be the same kind of plant without these things, and one that has a reddish coloring.
Any idea what these little (~10 mm long) "leaf-shaped" brown/orangish things are, growing from points along two regular rows of bumps that are repeated everywhere? Is this reproduction, if so how does it work?
Also curious, any thoughts on what kind of plant to call this? I don't need a species identification necessarily, but is it a kind of fern?

below: Additional photos, click for full size.

These are really cool, and I am absolutely not certain of this, but I would hazard that they might be some kind of bulbil? Ferns (if I have started my ID correctly) can use them to asexually reproduce. For example the New Zealand endemic Asplenium bulbiferum grows little bulbils on the adaxial surface of its frond.

Thank you for that term! With it, one can search and find all kinds of crazy/weird looking things growing out of other things. Search for it along with fern and photos on pages like this and especially this and this show structures somewhat reminiscent of what I saw. I wish I could go back to the same site and see how those have grown, but that's not going to happen any time soon.

Also this and this and this and this!

@uhoh no problem, glad I could help


text:
Why won't this spray kill this bug (household insecticide); could it be so selective?
I have a can of bug spray. The label is in another language which I can not even type in order to do an online translation, but the ingredients are in romanized writing:
Metofluthrin  0.012% w/w
Phenothrin    0.100%
Cyphenothrin  0.100%

Wikipedia: Metofluthrin, Phenothrin, Cyphenothrin
I've been seeing small (3~4mm) dark flying bugs that seem to be able to take a direct hit. They may be grounded, but after a minute or two walking around and cleaning their wings, they fly away.
The spray worked very effectively on the cockroach I bought it for; this kind of thing happened about 90 seconds after a direct hit. Here they don't really infest as much as just wander in off the street and can't find their way out again.
I assume if I get rid of any residual sources of food or water the little flying things should die off. But I was really shocked to see them fly away undaunted after some serious aerosol action.
Here are some snapshots I took of the small bug trying to eat my spicy noodles. It's actually not so dark. Please please tell me it's not a tiny baby cockroach!

It's a tiny baby cockroach.  Sorry.

http://animalia-life.club/other/german-cockroach-nymph.html
I looked up your chemicals; the first (metofluthrin) is mosquito repellant and the others are for fleas and mosquitoes.  The combo makes me think that this is something intended for spraying on the body or on animals.  It is not strong enough.
You need to get a bigger boat.  Get a spray made for roaches.

aaaaahhhhhhh!!! OK thank you for the ID. It looks like I'll have a busy day today, starting with some shopping. I added a snapshot of the can. While the Wikipedia article for the last ingredient Cyphenothrin  says it's supposed to be for cockroaches, and the middle one "Phenothrin is the active agent in the branded product Raid Fly & Wasp Killer", this seems like a non-optimal mix now that I know the pest I'm after, and the repellant doesn't make sense to me. Thanks!

I've got them again, and these guys really spend a lot of time flying around like fruit flies do. They never get very big, and I think baby cockroaches don't spend their time flying around like this. So I've just asked Is this likely to be a type of fruit fly?


text:
How did sulfasalazine become a disease-modifying treatment for rheumatoid arthritis?
Sulfasalazine has been around for about fifty years, starting as an antibiotic. More recently it is used as a disease-modifying anti-rheumatic drug (DMARD, see also arthritis.org). While biologics (biopharmaceuticals) are becoming more widespread in early treatment of some types of RA, sulfasalazine has a known safety profile and is inexpensive. Sulfasalazine is on the World Health Organization's Model List of Essential Medicines. See section 17.3, Anti-inflammatory medicines.
Wikipedia says:

The mechanism of action is not clear, but it appears that sulfasalazine and its metabolites have immunosuppressive, antibacterial, and anti-inflammatory effects.[refs] It also appears to inhibit the cystine-glutamate antiporter.[ref]

Why was this "old antibiotic" tested as an anti-inflammatory medicine? Is the situation similar to Minoxidil where a "side-effect" turned out to have significant medical value? What is the evidence (if any) that sulfasalazine has a disease-modifying effect in the treatment of RA - that it actually slows down the progress of the disease rather than just temporarily reduce symptoms?

fyi I've just asked Considering its salicylate component, is Sulfasalazine contraindicated when aspirin is? Can it have similar therapeutic benefits? in Medical Sciences SE


text:
What is the purpose of the viscous gel in the Pacinian corpuscle?
I have read that 'when the stimulus (in this case pressure) is constantly applied, the gel repositions itself to prevent the formation of an action potential'. I don't really understand what this means.

Short answer
The adaptation process in Pacianian corpuscles (PCs) is mediated via the outer capsule. This capsule is an onion-like structure that quickly molds itself to pressure stimuli, thereby rapidly desensitizing the receptor.
Background
The Pacinian corpuscles are rapidly adapting (RA) skin receptors, as opposed to the slowly adapting (SA) ones, such as the Merkel cells. RA cells become swiftly unresponsive (i.e., they adapt) when a pressure stimulus is applied, but faithfully transmit rapidly changing stimuli (such as vibrations).
Pacinian corpuscles are specialized vibration receptors. Their dendritic region is shaped as an onion-like structure with layers of stacked lamellae:

Specialized dendritic capsule of the Pacinian corpuscle. Source: Biologymad
These lamellae act as high-pass filters that result in a sharp drop in sensitivity below 150 Hz (Johnson, 2001). If this capsule is dissected from the receptor and a pressure stimulus is directly applied to the sensor element underneath the capsule, the receptor response to a sustained stimulus substantially increases (Mendelson & Loewenstein, 1964). Basically, these elastic lamellae slowly mold their shape to the stimulus and pressure stimuli are only transmitted for a few milliseconds, after which the lamellae absorb the pressure. So only a one or a few spikes are generated at the start of a pressure stimulus.
You can imagine it this way, suppose you have a gel-like substance (say Play-Do) on a weight scale. Now suppose you press the Play-Do quickly and hold your finger steady at a certain point above the scale, within the Play-Do. The weight scale will shortly display a certain weight (equal to the pressure applied in Newtons), after which the Play-Do folds itself around your fingertip, and the pressure is released from the scale.
Upon release of the pressure stimulus another spike or two are generated in the PC. Hence, a continued pressure stimulus is not faithfully reproduced. However, vibratory stimuli generate spikes on the pressure onset as well as offset as well, and this happens on every phase of the vibration. Hence, vibratory stimuli are faithfully transmitted.

Response of Pacinian corpuscle to a sustained pressure stimulus (indentation of the skin) and superimposed vibratory stimuli. Note the vigorous response to vibrations, but the rapid adaptation to the static pressure stimulus. Source: Zavantag
References
- Johnson, Curr Opin Neuobiol 2001;11:455–461
- Mendelson & Loewenstein, Science 1964;3618:554-5


text:
How can Giant squid have $20,000 mm^{-2}$ photoreceptor cells, but no cones?
According to this post, Cephalopods have about $20,000 mm^{-2}$ photoreceptors in there eyes. However, Cephalopods (in my case, the Giant Squid) aren't born with cones (cells that dictate which colours you see). So are they born with about 20,000 rods (cells that help detect movement), or is there another type of cell involved?

How do you get from 20,000 $mm^2$ to 20,000 cells? Note also that in the title you said 20,000 $m^2$.

You are missing a "/". The number in that post is 20,000 PER mm2. Overall though I think you need to do some more research on this on your own... rods are not only used for detecting movement, for example. Even that post you linked has a lot of information that you seem to be lacking.

First squid lack both cones and rods, they evolved their eyes independently, the cells they use most closely resemble rods but they are by no means identical.
In vertebrates there are two types of cells rods and cones both of which can be used to see movement (movement is just detecting changes in the visual field). rods are better at it becasue they are smaller so they can be packed tighter, thus they can detect smaller changes, but that is by no means their primary function. Rods detect luminosity, that is the intensity of light, but react to light from all parts of the visual spectrum.
There is a lot of debate about how much color squid and octopi can see. behaviorally they appear to recognize color, but they don't have differentiated cells or opsins to detect them.
I appears they have stumbled on a completely different way to achieve color vision, they appear to use weird shaped pupils and chromatic aberration.
I suggest reading the linked paper as you have to dive pretty deep into the physics of optics to understand. Basically different colors of light are bend by slightly different amounts by a lens, and they are using the multiple images of their U shaped pupils to gain color information.


text:
Does an organism have only certain antibodies for life?
Before birth, in the bone marrow millions of different B-cells are formed. These cells are differentiated and express different antibodies.
Are these the only B-cell types you will ever have? Or can the body generate new types of B cells, with new antibodies to deal with newly encountered threats?

Are these the only B-cell types you will ever have? Or can the body generate new types of B cells, with new antibodies to deal with newly encountered threats?

New B cells are constantly formed throughout one's life; the B cells that are present in the newborn are constantly supplemented by newly formed B cells.

... there is no statistical difference in the rate of production of new immature B cells in the marrow of young and old animals. In total, our results confirm previous work showing that mature B cells in old mice have a slower turnover, but more importantly suggest that the defect in mature B cell turnover is not due to a decline in B lymphopoiesis, but rather an inability of the newly made cells to replenish the peripheral compartments.

--Aging and developmental transitions in the B cell lineage.
But the second part of your question seems to have a misconception.  The newly-formed B cells have no relationship to newly-encountered threats.  No matter what new pathogens you encounter, your newly-formed B cells don't reflect them; they are formed randomly, and it is only by chance that any particular B cell might be able to react with a new threat.  Fortunately, there are so many B cells that the chance of at least a few reacting is very high, even though only a tiny fraction of the whole B cell population can do so.
Although new threats (mainly) require new B cells, repeated threats don't -- once a B cell is stimulated the first time, it undergoes a pathway that functionally makes it longer lived and more effective, so that on repeat exposure these optimized B cells are called in and there's much less need for the new naive B cells.

After the body is alerted to the presence of antigens, it calls in the artillery, which in this case are the white blood cells (phagocytes). The phagocytes digest as many of the antigens as they can, and then report back to the lymphocytes (B cells). The digested antigen material inside the phagocyte is then given to the lymphocyte, so it can produce an antibody to neutralize the antigen. This is similar to using snake venom to make anti-venom.
If the antigen attacks again in the future, the lymphocytes 'remember' the antigen and release the antibody, which they already know how to make, saving time and getting rid of the antigen before the human is even aware of its presence or it's excretion from the body. This is the basic concept behind vaccines and immunity. So to answer your question, yes, we can and do form new antibodies regularly. If we didn't, every new virus strain would be fatal.
Edit: As requested this is a source I found that is way more detailed and science-y - https://www.ncbi.nlm.nih.gov/books/NBK26884/

This answer is misleading.  You are describing what happens with mature B cells following antigen exposure, which is a completely separate thing from whether newly-formed B cells continue to arise throughout an individual's life.

@iayork Damn you're actually right. Probably should have read the question better but this is all I remember so I thought I've give my two cents.

@user45808 that's fine, but unfortunately your two cents are irrelevant and somewhat misleading. Not all white blood cells are phagocytes, not by a long shot. Similarly, not all lymphocytes are B cells. Digested antigens are not "given to" the lymphocyte like a delivery person handing off a package - they are presented on the antigen-presenting cell's surface for cognate receptors to bind to, recognize, and stimulate the resting B (or T) cell. Also, not all cell-mediated immunity to repeat exposures to antigen happen without the host's knowledge - some infections still take time to fight off.


text:
What is the smallest pressure differential that humans can detect?
While cooking dinner under an extractor on its lowest setting, I noticed that none of the steam (referenced by the amount of fogging on a nearby window, compared to the absence of the fan, was effectively zero) was going anywhere else but through it.
However, placing my hand under the hood, I couldn't feel a difference in pressure. This is most likely due to two things: my hand smoothly transitioned from "normal" atmospheric pressure to the "adjusted" pressure near the hood, and the hood area is quite wide, so that extraction is done over the significant area not covered by my hand and hence less pressure applied.
But it got me thinking, and a literature check revealed nothing helpful so far, as to what is the smallest pressure differential detectable by the skin of the human body?
I would prefer to exclude anything related to hearing, as this is a well-researched area but tells us nothing about the other 99.9% of what is directly exposed to atmospheric pressure changes. Answers that cover particular parts of the body beyond skin and ears (for example, the eyes) are appreciated but only additional.

Short answer
The sensitivity to pressure differences highly depends on the study setup used. A paper resembling the conditions in your kitchen experiment concludes that the just noticeable difference ranges from 19 to 35 kPa.
Background
The smallest difference that can be perceived between two stimulus levels is called the just noticeable difference, or JND. The issue with determining pressure difference JNDs is that it may be dependent on many things. To name a few important factors are the place on the body and the type of stimulus used. The fingertips and tongue are among the most sensitive parts of the body, while the back is way less sensitive to mechanical stimulation. The type of stimulus may also dramatically affect the findings, because vibratory stimuli are sensed by other types of mechanoreceptors than static pressure stimuli.
Having said that, I found a paper describing an experiment that resembled your kitchen experiment pretty well . The authors used air puffs applied to the fingertips. They used an apparatus where the nozzle could be adjusted to accommodate different aperture sizes. The authors tested 10 normal, right handed controls and used a standard 2 alternative forced choice (2AFC) task applied as a method of constant stimuli. Personally I would have adopted an adaptive 2AFC task, but both methods are valid. The authors conclude that

Subject-averaged pressure JND values ranged from 19.4 – 24.7 kPa, with no statistical differences observed between aperture sizes.

Reference
- 33rd Bianchi et al., Annual International Conference of the IEEE EMBS Boston, Massachusetts USA, August 30 - September 3, 2011


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What is the benefit of fever during infections?
When people get sick, they often develop a fever. What is the effect of an increased body temperature on viruses and bacteria in the body? Is it beneficial to the infected body? Importantly, often fever-reducing agents like aspirin are prescribed when people are sick. Doesn't this counteract any benefits of fever?

Fever is a trait observed in warm and cold-blooded vertebrates that has been conserved for hundreds of millions of years (Evans, 2015).
Elevated body temperature stimulates the body's immune response against infectious viruses and bacteria. It also makes the body less favorable as a host for replicating viruses and bacteria, which are temperature sensitive (Source: Sci Am).
The innate system is stimulated by increasing the recruitment, activation and bacteriolytic activity of neutrophils. Likewise, natural killer cells' cytotoxic activity is enhanced and their recruitment is increased, including that to tumors. Macrophages and dendritic cells increase their activity in clearing up the mess associated with infection.
Also the adaptive immune response is enhanced by elevated temperatures. For example, the circulation of T cells to the lymph nodes is increased and their proliferation is stimulated.
In fact, taking pain killers that reduce fever have been shown to lead to poorer clearance of pathogens from the body (Evans, 2015). In adults, when body temperature reaches 104 oF (40 oC) it can become dangerous and fever reducing agents like aspirin are recommended (source: eMedicine)
Reference
- Evans, Nat Rev Immunol (2015); 15(6): 335–49

why doe not Ebola gets cured by elevating temp of body of person artificially

Now, the missing bit is "why are the viruses/bacteria not benefitted by the additional thermal energy"...

@AnoE There are a lot of examples in nature where larger creatures can ensure more extreme environments than the smaller ones.  As an example, many bacteria use simplistic tools to maintain hydrostatic equilibrium with its environment which can fail if the salinity goes up (aka gargling with salt water).  Our cells use a more complicated mechanism which can adapt to that salinity.

That's a good point about fever reducing.  In fact, most of the things we think of as "disease symptoms" are actually immune response symptoms, and many methods of treating them end up suppressing the immune response and making the overall experience worse (and longer) rather than better.

Does this also play a role in health effects due to exercise? I run every day for an hour, my body temperature is then significantly elevated one hour a day, while people who are not active or only moderately active won't have such an elevated body temperature for such a  significant time per day.

@CountIblis exercise is good always. It boosts cognitive functions and immune response alike

Interesting fact about immune system... does that mean that it's better not to treat the symptoms as long as you can tolerate them (and doesn't endanger themselves too, certainly) and let the immune system does its work? Or is it more complicated than that?

Most sources attribute elevated temperature as a mechanism to minimize pathogen replication, however, I somewhat am not satisfied with this explanation. Many mesophilic bacteria grow fine even at 45°C. Higher organisms are more susceptible to elevated body temperatures because of their complex organs. I think that elevated temperature is more of a consequence of inflammation than a potentiator.

@WYSIWYG - Yes, now many bacteria can grow fine at 45 degrees, but that they always did. Bacteria has been evolving right along with us, so responses that killed most diseases in the past may not do anything to other diseases of the present. Think about HIV. Most diseases are killed by our white blood cells, under the direction of our T-cells (high school biology version), but HIV kills T-cells and completely circumvents the process, which makes it much more dangerous than other illnesses. That doesn't mean our immune system is bunk, it just means that illnesses are getting 'smarter'.

@EvSunWoodard, illnesses have been getting "smarter" for millions of years.  The body's immune system is pretty darn good.  Imagine how good you would be with millions of years of experience at biological warfare.  Obligatory xkcd.

@EvSunWoodard Not necessarily. Pathogen certainly evolve but non-pathogenic bacteria and naive WT strains also can easily survive at elevated temperatures. Bacteria are much more resistant to heat (or other adverse conditions) than humans. Our core organs are quite sensitive to heat shock. Fever may help but not directly by affecting bacterial proteins and stalling their metabolism.

@WYSIWYG - What I'm saying is that it's easy to point to something that exists now that isn't affected by our immune system response and say, "well this response is useless", but we have no idea how many diseases existed thousands of years ago which were eliminated due to the fever response. It may be none, but more likely, the fever killed off at least some strains of illness, meaning that it makes total sense for fever to be a immune response to try and kill bacteria/viruses, even if there are some bacteria today that can survive 45 degrees C.

@AliceD I think you might be confusing correlations with causation here. sinankzlyr answer hits on the reason behind the increased efficiency of the immune response, and that is signaling through cytokines. I don't think that he was wrong in saying that it was the hypothalamus response to circulating cytokines that drive the fever, and not the other way around, i.e. Temperature increased immune activity and release of circulating cytokines. I am not sure how the endocrine system effects the immune system, though it would not surprise me that there are circulating hormones that play a role.

Fever normally under hypothalamic heat center's control which stays at limbic system of brain . Hypothalamus sets its own set point 36.4-37.2 in healthy peoples by some molecules named exogenous and endogenous pyrogens, especially PGE2 ,TNF and IL1.
The most important mechanism for fever is directing blood flow from skin to deep vascular pools and preventing heat loss from skin. Thats also cause to cold distal parts of our extremities.
When pathogen microorganisms and their toxins invade our blood, our immune response is being activated. Inflammation process starts which have four components: "Tumor ,Rubor ,Calor ,Dolor".  So we experience swelling, redness, fever and pain in other words. Our white blood cells emits prostaglandin and leukotrienes that activating inflammation and those cells to migrate there.
After interact between our defence and microbes, these pyrogens activates hypothalamus heat center. Then temperature set point rises. That activates vasomotor and cortical response. With vasomotor activation, blood flow is directed to deep blood pools and the other side smooth muscles spontaneously contracting generate extra heat. With activation cortical pathways patient feels needing to increase heat or prevent heat loss by e.g wearing more clothes, getting in bed, changing posture to decrease heat losing total area...
So what are all these mechanisms for? Our body resistance increases. Also metabolism speed rises to fight with pathogen. Microorganisms and many toxins have protein components , heat helps denaturation of those proteins and being inactivated. But also our basis contains proteins. So too high temperature is dangerous for our body. When we take analgesics which also decrease fever, thats the purpose to control fever in acceptable intervals.
   (source:Pubmed)
for more read

Welcome sinankzlyr to Biology Stack Exchange. Thanks for your contribution to the site. Please can u provide references to support your claims.

@Mesentery These are all i know, should i still reference for them? Thanks for help. I added one reference.

Most bacterial proteins are stable even at 50°C; human proteins are more susceptible to heat shock. Whatever may be the beneficial actions of fever but pathogen protein denaturation is surely not one of them.

@WYSIWYG while proteins may not denature at 40°C, that can be enough to effect protein protein interactions that pathogens rely upon to sustain infections, buying us just enough time for the adaptive immune response to take over. I have to say that I think the first four paragraphs of sinankzlyr's answer do a better job of explaining the how behind temperature elevation, at least in mammals.

@AMR I agree but I don't think there are extensive studies on this. One should compare the HSR of pathogen and host and its effect on the corresponding cells. Whereas I have seen the claim of elevated temperature directly inhibiting pathogen growth so many times without adequate reference.


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What are the effects of the common cold in an immunodeficient person?
How would the virus causing the common cold (rhinovirus) affect the human body in the absence of a normal immune response? On the linked wiki page it is said that the runny nose and fever symptoms are in fact caused by the immune response, rather than the rhinovirus itself.
Suppose the immune system is severely impaired (immunodeficiency), how would infection with the rhinovirus affect the body? Would the host not develop any symptoms then?

I've substantially edited the question to expose the question core as best as I could. The wording was unclear imo. Your sub-question on where the common cold causes its effects I have removed to keep the question focused. Feel free to ask another, separate question on that though.

In one small study, rhinovirus in the immunocompromised led to significant mortality from lower respiratory infection:

Among high-risk patients with cancer, rhinovirus infections are often fatal. In a study of 22 immunocompromised blood and marrow transplant recipients who were hospitalized with rhinovirus infections, 7 (32%) developed fatal pneumonia. The remaining patients had infections confined to the upper respiratory tract. In 6 of the 7 fatal cases, rhinovirus had been isolated in bronchoalveolar lavage fluid or an endotracheal aspirate before death.

Note that this is in hospitalized patients; it says nothing of non-hospitalized patients.
This conclusion has been disputed.
In a slightly larger study among people with hematological cancers (presumed immunosuppressed because of same):

Respiratory viral pathogens are a common cause of morbidity in patients with hematologic malignancies. ...Both a rapid viral culture with direct fluorescence antibody (DFA) staining and a PCR-based assay (MultiCode-PLx Respiratory Virus Panel) were performed on patients with hematologic malignancies, who underwent collection of a nasopharyngeal swab or bronchoalveolar lavage from October 2006 to April 2007. Eighty-two samples from 70 patients were obtained; all patients had upper respiratory tract symptoms. Respiratory viruses were detected in 10 samples (12%) by conventional virological methods and in 31 samples (38%) by the MultiCode-PLx assay. ...40% of these patients had pneumonia in addition to the upper respiratory tract symptoms. [emphasis mine; note, there is no mention of mortality.]

So long story short, they have stuffy, runny noses, sore throat, cough, etc. Clearly the virus itself causes damage to the mucosa; that is integral to viral replication After entering a mucosal cell, the virus replicates, then the progeny virus is released by lysis of the cell. This damage itself causes inflammation (not the same as an immune reaction), pain, etc. The major difference seems to be a more severe and prolonged experience.
Respiratory Consequences of Rhinovirus Infection
Detection of respiratory viruses with a multiplex polymerase chain reaction assay (MultiCode-PLx Respiratory Virus Panel) in patients with hematologic malignancies

they have stuffy, runny noses, sore throat, cough, etc. that is response of immune system not of virus right?

@murmansk - please see answer. You asked about inmmunodeficient patients. Clearly the virus itself causes damage to the mucosa; that is integral to viral replication.

Thanks for this most interesting answer +1 The question really made me wonder.


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How do snakes determine the direction of prey from their thermal infrared sensing pit organs?
This answer to How does the “heat vision” of some snakes work? explains the basic idea and there are several linked sources within that go into greater detail on the sensing mechanisms on a physical and a molecular level.
But searching these sources for terms like "spatial" or "angular" in an attempt to understand how they get direction information (specifically for striking a  pray) turned up nothing.
Some snakes have multiple pit organs that point in somewhat different directions, and I suppose it's possible that the heat-sensitive nerves across the pit membrane could have spatial resolution and the whole thing act like a pinhole camera, but so far I've found no actual explanation of how they obtain direction information from their pit organs.
Question: How do snakes determine the direction of prey from their thermal infrared sensing pit organs?

From Wikipedia:

The facial pit actually visualizes thermal radiation using the same optical principles as a pinhole camera, wherein the location of a source of thermal radiation is determined by the location of the radiation on the membrane of the heat pit. However, studies that have visualized the thermal images seen by the facial pit using computer analysis have suggested that the resolution is extremely poor

Individual pit organs act like pinhole cameras because they are arranged, well, like a pinhole camera: there is an invagination of tissue surrounded by heat-sensitive cells with a smaller hole to the outside world. From Sichert et al 2006 it looks like this:

The dashed lines here indicate heat originating from some distant point source. That point source covers some fraction of the heat-sensitive membrane. Points at different distances and directions will be picked up on different parts of the heat-sensitive membrane. For example, because this source is coming from below, only the top of the pit is illuminated; for a source coming from above, only the bottom of the pit would be illuminated, etc.
If you have access to the Sichert paper, they go into more detail about the mathematics that the pit organ geometry creates. In summary, for any cell you can map a receptive field of places in the world that can project to that spot, and through a summation of these receptive fields across the whole pit organ you can reconstruct a map of where heat is originating. At that point, the principles are no different from visual organs like the retina. The authors show that even with each individual detector having imprecise resolution, the combined activity can be decoded to form a clear image from a simple linear combination of the receptive fields.
Other studies like Chen et al 2012 show that vipers integrate retinal vision with their pit organ senses, because occluding a pit on one side and an eye on the other is worse for prey targeting success than occluding both on the same side. Neuronal evidence of this integration has been observed for quite some time, see for example Newman & Hartline, 1981.
Bothe et al 2019 is a more recent paper investigating some of the neural circuitry behind the processing of pit organ information and integration with vision.

Bothe, M. S., Luksch, H., Straka, H., & Kohl, T. (2019). Neuronal substrates for infrared contrast enhancement and motion detection in rattlesnakes. Current Biology, 29(11), 1827-1832.
Chen, Q., Deng, H., Brauth, S. E., Ding, L., & Tang, Y. (2012). Reduced performance of prey targeting in pit vipers with contralaterally occluded infrared and visual senses. PLoS One, 7(5), e34989.
Newman, E. A., & Hartline, P. H. (1981). Integration of visual and infrared information in bimodal neurons in the rattlesnake optic tectum. Science, 213(4509), 789-791.
Sichert, A. B., Friedel, P., & van Hemmen, J. L. (2006). Snake’s perspective on heat: reconstruction of input using an imperfect detection system. Physical review letters, 97(6), 068105.

Don't pit vipers have two sensors which would provide binocular sensing for a couple of feet?

@MaxW Yes, they do, though Chen et al showed that occluding one of each eye and pit (for example, left pit and right eye) impacted performance more than occluding both pits, both eyes, or both pit and eye on the same side. So, at least in their conditions, the conclusion would be that the ipsilateral integration is more critical than bilateral integration. That said, there isn't much benefit of bilateral sensing of light unless you can form an image with both individually. For sound you can use delay cues but only because sound is so slow.

@MaxW I added a citation to another paper that gets into the neurophysiology in the brain a bit more. It also has a bit of the anatomy, where there is an ipsilateral trigeminal hindbrain processing step prior to a crossing-over to the contralateral optic tectum. I don't see that they talk about any interaction with the contralateral hindbrain nuclei, though they also don't say anything to rule it out, it's just not addressed. Anyways, if that's the primary circuitry, it seems the two pits operate mostly independently within their respective 'visual' fields.

The pits may be located and angled too far laterally to have much "straight ahead" coverage.

This is fascinating! I'll download these references this afternoon and enjoy them in detail on a long train ride this weekend. There's a Phys. Rev. Letter even! :-) I got started on this yesterday talking with a theoretical physicist first about why clouds look white in thermal infrared weather satellite images and we were frustrated by the somewhat misleading graphic accompanying this Nature News item. Thank you!

@uhoh Based on your frustration with that graphic, I think you'll especially like Figure 3 of Sichert et al :)

Thanks again for your answer; this is really quite interesting! I enjoyed the ride and the read both. I think next I'll try to think about how lensed-eyes evolved and if this type of mechanism (large hole plus neurons configured to make the most of it) has any relationship to it.


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Are two colors (red + blue) necessary for LED grow lights, or would either color be sufficient?
Below are some chlorophyll absorption spectra from other answers here. There is strong absorption at both the blue/violet end, and the red end of the spectrum, and presumably both of these contribute to steps in water splitting used in photosynthesis.
Do the two broad but widely-separated peaks correspond to different steps, so that both are required? Or is it sufficient to have light present in one band alone?
If both are required, how are they used differently?
If not, why do LED grow lamps have both? Would twice-as-bright light with either wavelength be just as effective and useful to plants?

That horribly un-natural purple light looks cool in high tech settings, but it's kind-of ugly in ones living room.
note: Originally I had though that Chlorophyl A and B corresponded to the two peaks, but clearly this is not at all the case. From here:

Both Chl-a and Chl-b primarily absorb red and blue light, the most effective colors in photosynthesis. They reflect or transmit green light, which is why leaves appear green. The ratio of Chl-a to Chl-b in the chloroplast is 3:1.

above: From this answer, also seen here.

above: x2 From this answer.

It's possible that white or yellow lighting might be less efficient for growing plants. (Then again, maybe not.) Still, perhaps you'd want to splurge on running possibly-less-efficient lighting, at least during the hours when you're sitting in your living room?

The plant uses light to produce energy but also as a signal of how and when to grow (phototropism, photoperiodism). In the context of your question I'll first cover light-harvesting in photosynthesis and then phototropism.
Tl;DR
Blue and red light are important for plant growth. Red light is the main one in photosynthesis and if a plant is exposed to another low intensity, full spectrum light source there is no (theoretical) need for a blue LED.
Photosynthesis

(from WTAMU)
The light from the sun arrives in many wavelengths, no one material can use all this energy. In photosynthesis light is intercepted by an antenna, a Light Harvesting Complex, made of different pigments, each intercepting photons in a unique energy level (wavelength). These antennas are situated around a reaction center and redirect the photons to it. Only the reaction center releases an electron to the electron transport chain. The pigment in the reaction center is modified chlorophyll a. In PSII (Photo System II, beginning of the chain) it's called P680 and in PSI it's called P700, the number denotes the excitation wavelength. The goal of the antenna is to gradually redirect photons to the reaction center and to its energy level (reducing the energy).

(Pandit, Anjali & Holzwarth, Alfred & Groot, Huub. (2006). Harnessing Solar Energy for the Production of Clean Fuels.)
When a photon excites a chlorophyll molecule this energy needs to dissipate in some manner. Energy dissipation from a blue photon excitation is too fast for it to be captured by the electron chain reaction. All the of the energy difference between blue and red photons is wasted (regarding photosynthesis).

(Dr. Shilo Rosenwasser presentation)
From the chlorophyll and other pigment explanation we understand that using any light except red is wasteful regarding energy conversion efficiency, the plant photosynthetic system will "ignore" any energy extras above the red photon.
Phototropism
The stem of the plant grows towards light and the roots grow away from light, this phenomenon is easily observed by looking at plants close to a window, they tend to "look out" of the window. This means that plants have a sensor for light.
This observation was first correctly analyzed by Darwin (a historical review).
Without blue light plants will elongate in an proportional way called etiolation. Etiolation makes a deformed, fragile plant because of its mechanism. Without blue light a plant will etiolate, even with enough energy for photosynthesis.
It is important to note that the light intensity required in light-sensing responses is much lower than the intensity required for photosynthesis.
It is also important to remember that blue & red light take part in other critical steps in plant growth like germination and flowering.
In conclusion
Red light is the major contributor to photosynthesis. Blue light indicates to the plant that there is light and it should start growing normally and do photosynthesis.
In the LiCOR photosynthesis measurement instruments the default settings are 96% red light and 4% blue light, showing that empirically red light is relevant to photosynthesis and blue is for signaling.
If your plants are exposed to some natural light or white light (from a window or lamp in your living room) I don't think that you need to supply blue light as they will get the required amount from those sources. Growing in a controlled, closed environment does require the blue light.

Fascinating! Thank you for your thorough and well-organized answer.

tl;dr: Sort of?
Logically, either red or blue light should be sufficient. Chlorophyll a and chlorophyll b preferentially activate different photosystems, and both photosystems are required in green plants.
Practically, we're in luck and someone has actually done the experiment.
As the original study reports, plants need a little blue light to grow into the right shape. Red-only plants produce oxygen and grow, but they take weird shapes. It seems like plants can grow on red light, but they use blue light to work out where light is. See here for a reasonably lengthy treatment on most of the obvious combinations.

These are very interesting, thanks! It certainly seems that red-only is suggested to be all that's needed for substantial photosynthesis (though it's not clear yet if there is a slight improvement in photosynthesis or not with blue added) Has it been stated anywhere that blue-only is not sufficient? Since the absorption spectra show big peaks at both ends, I'm asking if either one is fine. Will plants not grow with blue-only?

Uh, long story short: I don't know and I also don't know how to find out. (Other than the obvious way)

OK thanks. I think the idea that multiple bands of wavelengths are important for more than just photosynthesis rate is really interesting. I have read that the newest LED modules used on the International Space Station will now include Green light as well, though I'm not sure if that's for the plants, or for the astronauts to improve their ability to do visual inspection.

I've slightly modified the wording of the question, changing "one color" to "either color" in two places. I didn't realize until now that it's did not read clearly.

Hi, I've un-accepted your answer because both of your links to hortsci.ashspublications.org have broken/rotted and you only included minimal information from them of them within your answer itself, and actually not even the titles of the works nor the authors' names. This is now an unsupported answer; is it possible to find new links to support it? Thanks!

@uhoh: Being an abuser of bare links in unpaid forum posts myself, I should illustrate that the information is recoverable:  https://pubmed.ncbi.nlm.nih.gov/11537611/ https://www.researchgate.net/publication/263124879_Plant_Productivity_in_Response_to_LED_Lighting  Just do a web search for the quoted link.

@Mike please feel free to edit the post; once it's in good shape I'll accept it. Thanks!