text:
What are the blood groups for human beings?
Human blood groups are of 4 types with negative and positive of each types (Wikipedia). But according to this forum thread there are some other types too:

A+,A-,B+,B-,O+,O-,AB+,AB-,A1+,A1-,A2+,A2-,A1B+,A1B-,A2B+,A2B-

I'm going to make software for blood donation, where I need to put a dropdown list of actual blood types. I had intended to use:

[ Select Blood Group V ]
[ O Negative           ]
[ O Positive           ]
[ A Negative           ]
[ A Positive           ]
[ B Negative           ]
[ B Positive           ]
[ AB Negative          ]
[ AB Positive          ]

But now I am uncertain whether this is appropriate. What is the most suitable list of blood types for donors?

I've edited your question, mainly because you started it with "We all know...".  This is unnecessary and is often either offensive (you have no idea what I know, and you are suggesting I am ignorant if I am not aware of your assertion) or a ploy (presenting something dubious as a fact). If you really must, use "It is well known...". I've condensed the rest, which was too verbose. Also note that in English "software" is uncountable, it never has an indefinite article — either "software" (in this case) or "the software" (referring to a specific program), but never "a software".

@David thanks for the edit and the enlightenment. You are always welcome.

As far as I am aware it is the RhD and ABO blood groups that matter with blood donation - though you may wish to contact a blood donation organisation for more specific information on how they handle & categorise the blood.
But using just RhD and ABO gives 8 combinations, positive and negative of A, B, AB, and O. You can find out more about the blood groups, blood donation, and the characteristics of blood groups here.
The subgroups you allude to are due to differences in the number of the antigens on the surface of the blood cell, for example the A1 group have ~1,000,000 A antigens per cell whereas A2 variants have ~250,000. The A group has ~20 subgroups, the vast majority of people are A1 (~80%) or A2 (~20%), and the other groups make up less than 1% of the population.

These two subgroups are interchangeable as far as transfusion is concerned, but complications can sometimes arise in rare cases when typing the blood.

If the blood donor is filling out the info, I recommend you have one dropdown that offers: A, B, O, AB, don't know....  And a second dropdown that offers Rh +, Rh -, don't know. And be aware that the average person is not likely to correctly know their blood type.
Actual laboratory blood typing checks for a large number of possible differences between blood cells beyond just ABO types. The clinician or technician will not use anything as simple as either your drop-down OR the slightly more complex drop-down in the referenced forum post.
If you still want to know more about "what the actual blood types are," recognize that there are LOTS of blood types, including the MNS system, the Kell system, and the Lewis system: http://en.wikipedia.org/wiki/Blood_typing

Thanks. And, And be aware that the average person is not likely to correctly know their blood type. - kudos for the point. :)

Gah. Yes. Corrected.

I would have assumed that someone performing the blood test is typing in the information. I doubt anyone is relying on self-reported information for blood transfusion.

Well, that's the thing. If it's the clinician, they need to enter a LOT more information about blood type than just ABO and Rh. And if if the user is the general public, they aren't likely to be correct. So I'm not seeing how this will be very helpful software.
Unless it's a dating site. Or one of those sites that gives you an ideal diet based on your blood type.


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Spider identification
This red spider has set up a web in my yard, in South Eastern, Texas, United States:

The spider itself is about an inch long (2.5 cm), and is predominantly red to brown in color, with red legs, and a very striking white line straight down the center of her back. The web is quite large—at least 4 feet (1.2 m) in diameter. It seems to be nocturnal, as I haven't seen it during the day, yet I've walked into the web (or nearly walked into its web) twice at night. Unfortunately, due to lighting conditions I wasn't able to capture the color particularly well.
I've searched but haven't found a species that looks like this yet.

After some more searching, I think stumbled across the answer. It appears to be an…
Eriophora ravilla:

Source BugGuide.net.
This species appears to have quite a diverse range of colors, and even thought I haven't found one that quite matches mine, the other similarities (the large abdomen, the stripe down the back, the four 'dimples', and the dark first leg segments) are enough to convince me. However, if anyone else can shed some light on this, I'd appreciate it.

I found the same type of spider on my backyard in Houston TX. Would love to know if it is dangerous.

@DonnaSummers They're harmless. I've found them a couple of times around my house. They tend to build their web each evening and tear it down by morning. After a few days they seem to move on to a different hunting ground.

Eriophora in general seems to be referred to as "tropical orb weavers", distinguished from araneidae (referred to as other kinds of "orb weavers"). Eriophora edax (http://bugguide.net/node/view/300786) is another possibility in your locale, and also look like they can have a stripe and vary greatly in color.

@JasonC That's another close match, but I still think E. ravilla is closer. From those images, E. edax appears to be have more dull, cryptic coloring (though not always) and have more tapered abdomen (though not always). I just noticed that my specimen also appears to have two red bumps near the rear of the abdomen, that is seen in some other individuals. Thanks for your suggestion though, I could be wrong about ravilla, but it's definitely Eriophora.

I found crab spider for mine,  they seem to vary in color and Design. Maybe this is more your spider found in Katy Texas
They have the same butt and dimples it looks like.

Could you explain why you think this specimen would be the one in the question? I'm not an expert in spiders, a tad arachnophobic even, but imho the spider in your image looks as much to the one in the question as  Margaret Thatcher looks like Napoleon Bonaparte.

Looking at the images on BugGuide.net, it seems there's quite a lot of color variation in E. ravilla. This one in particular looks a lot like yours (given relatively poor lighting). In other words, I think your image is of another E. ravilla, not a true crab spider (Thomisidae).

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Why does pH have an effect on enzymes?
I am currently studying biology and would like to know why enzymes work best in a particular narrow range of pH (the so-called pH optimum).
Unlike temperature change, I do not think this has much to do with energy. Do pH-related changes in enzymatic activity have anything to do with the active site?

What do you mean by increasing? Coming from pH 1 and increasing it until the optimum is reached?

@Chris Yes that is what i mean

"When the pH gets closer to the enzyme's pH optimum, the enzyme activity also increases." Why? Because of the way the English Language works. It would be absurd if an enzyme's activity was not highest at its optimum. I have reworded the question so it makes sense.

Enzymes have a more or less narrow optimal pH at which they work, depending on the conditions of their environment. Pepsin for example is active in the stomach which is pretty acidic and has an optimal pH of 2.0, while Trypsin, which is active in the small intestine has an optimal pH around 8.5.
Changes in the pH first affect the form of the protein, hydrogen bonds between the amino acids of the molecule and so on and also the form of the active center of the enzyme. Small changes in pH do little or nothing, then reversible changes occur and finally the enzyme gets irreversibly denaturated.
The relationship between enzyme activity and pH always looks like this (the curve can be steeper, if the optimal range is small and broader if the range is wider):

It is taken from this website, which is a nice introduction to enzymes. Another one is here.
At the optimal pH for the enzyme the conformation of the protein is as it should be (in the ideal state) while this changes when the pH is not optimal. This can lead to improper substrate binding, changes in the active center and so on.

Proteins are polymers of amino acids. Each amino acid has a side chain. Many of these side chains contain ionisable groups. The ionization state of these groups is dependent on the pH. A group that is protonated at pH=2 and neutral, for example, may become deprotonated at pH=8 and become negative. See the Henderson-Hasselbalch equation.
Protein structure is held together by many intermolecular interactions: dipole-dipole, hydrophobic, electrostatic, etc. The correct protein structure is necessary for function. At different pH levels, the protonation and deprotonation of ionisable amino acids affects the intermolecular interactions within the protein and its conformation will change. At a certain pH, the conformation of the protein will be optimal for its function and this is where it will have maximum activity. As pH deviates to either side of the optimum, the conformation changes and the structure will no longer be correct for proper function.
The ionization state of amino acids is also important in the active site but not only because of conformational changes. Catalytic residues must have certain properties to catalyze their specific reactions. As an example, two Asp residues (which have carboxylates on their side chains) in the active site of DNA polymerase are responsible for coordinating two magnesium ions that promote polymerization. If the pH were to decrease to a point where the Asp residues were protonated, they could no longer bind the Mg2+ ions and the enzyme would not function.

Don't know why this question has suddenly come up after 4 years, but this is the best answer. The ionization state of the groups involved in catalysis is the key factor, not just for interacting with magnesium ions or the substrate, but for the actual catalytic mechanism in many cases. See the chapter on mechanism of catalysis in any textbook of biochemistry.

@David Thanks. Some one posted a question as an answer which is now deleted but bumped this post up. You’re absolutely right and, if I had the time, I could write something about general acid/base catalysis.

Enzyme function is largely determined by its 3-D shape. Enzyme shape is affected by pH among a couple other things. The enzyme only retains its optimal shape at the optimal pH, as you creep outside of it the enzymes shape changes and hence so does its function.

The statement "The enzyme only retains its optimal shape at the optimal pH" convey no scientific information whatever.

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What properties (if any) do AT rich regions of DNA have?
I was looking up the fluorescent stain DAPI and seeing that it
binds strongly to A-T rich regions in DNA.

That made me wonder what is special about AT-rich regions.

In bacteria AT-rich regions are usually binding sites for initiation of replication proteins.

I edited your title as "traditional" didn't make sense. Not clear what you actually meant, but I think question works ok without it.

With regards to DAPI itself, one could tell that the molecule itself has a shape complementary to the shape of A-T rich DNA. So in this case it's "special" because DAPI happens to have the structure to form bonds to A-T rich DNA regions. (There can very well be another organic molecule that binds to C-G rich regions). A quick google search will show how it looks like

With regards to A-T in general, A-T share 2 hydrogen-bonds. However, this is not the reason why it's easier to break A-T rich regions. A-T rich regions destabilize the DNA double-helix. Refer to this http://nar.oxfordjournals.org/content/34/2/564.long. Transcription initiation occurs near T-A rich regions, e.g. TATA box in eukaryotes and Pribnow box in prokaryotes.


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What is the name of this yellow bumble bee that I photographed in New York City?
Does anybody know the name of this bumblebee? (For categorization on Wikipedia Commons: 1 and 2)

Just to help prevent other non-native English speakers make the same common mistake, you cannot say “How is something called” in English (unlike French, German and most other Germanic or Romance languages). It is “What is it called”. However, in changing the title I used what I considered a more suitable construction.

It's a Brown-belted Bumble Bee (Bombus griseocollis).

Details: http://bugguide.net/node/view/3538

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text:
Do animals exist which have good vision, but see only grayscale?
In computer vision the color information is often discarded, as most object recognition tasks seem to work just as well on the greyscale image (even better, because there is less unnecessary information).
Do animals exist which have good vision (e.g. see sharp, many details), but only see greyscale / very limited colors?

When you say sharp and many details, this is hard to convert to zoological optometry. Cats can see fantastically clearly at night (practically as though it was daytime), but if anything is more than about 8 feet away its just a blurry motion with smells and sounds. Would you consider something like that great vision? Also, animals and grayscale eyesight always attracts myths and mistruths. If I hear one more person say dogs are colour blind...

@GoodGravy With "great vision" I wanted to make sure that I am not talking about insects / mules.

@GoodGravy - what is so upsetting about dogs being called color blind?

@ChrisStronks I actually didn't mean to write colour blind (I meant grayscale). They are colour blind when it comes to red. Dogs can however see blue and yellow. http://goo.gl/iKHaiF This is probably because they never had the selection pressure to forage berries. Practically speaking a good toy for a dog would be bright blue or bright yellow, whereas red won't really stand out on a green field. People with deuteranopia see in a similar way and its surprisingly common. Thats why you shouldn't use green and red themes for presentations (particularly a problem in graphs).

@GoodGravy - thanks for clarifying. They are color blind, in a way :)

@ChrisStronks They are categorically colour blind! But its the "Dogs see in black and white" line that isn't correct. In fact I think they see into the UV range where we don't so in some ways they see more than us (Even if it is so they can see other dog's pee)! livescience.com/43461-cats-and-dogs-see-in-ultraviolet.html

@moose: Why do you think that mules don't have good vision?  Though I've little experience with them, they seem not much different than horses, which have excellent vision.

@jamesqf Sorry, I wanted to write "moles".

Preamble & Overview.
This is a rather unsatisfying answer I'm afraid. I can't seem to find any animal that has exceptional eyesight and sees in monochrome.
Baring in mind the context of humans vs machines; machines cope better with shapes rather than colours however it generally appears that those animals which rely on exceptional eyesight to survive tend to see in colour and have other neurological systems to ease "data overload".
Animals that see in grey-scale.
Some pinnipeds (seals) see in grey-scale, but note that their survival doesn't seem to depend on visibility, and it is generally regarded as poor even in a typically healthy animal. According to SeaWorld, researchers have found blind seal mothers and pups that are otherwise perfectly healthy.
Dolphins are a similar story. It is so murky that they rely on bio-sonar. Fresh water dolphins are almost completely blind.
In fact, the more species that I have read about which have monochromatic vision, the more I see that these animals rely heavily on other senses.
Some land animals with "decent" monochromatic vision:

Lorisiformes
Bats
Raccoons
Nocturnal kinkajous

Note that these often have another much more acute sense like smell or hearing.
Surprisingly, there is also not necessarily a link between monochromatic vision and nocturnal state (there are some animals with dichromatic vision that are nocturnal, and some animals with monochromatic vision are crepuscular, or diurnal and visa versa).
From wiki:

Many species, such as all marine mammals, the owl monkey, and the Australian sea lion have monochromatic vision under normal conditions...

A trend I see is that animals that have exceptional eyesight tend to have also evolved some sort of colour differentiation.
Why not ditch the colour info to make it easier on the brain?
To bring this back to your context of machines I would say that identifying the difference between a ball, a chair, a human, and a car (the sort of things machines need to know about!) is much more dependent on shape than colour. A monochromatic person wouldn't struggle with that at all.
However in a "wild" environment telling the difference between grass and tiger stripes means that an animal with trichromatic vision may get to live another day whereas the animal with monochromatic vision might not see the tiger in time. Or spotting berries and fruit in some distant trees might mean food that would have otherwise gone unnoticed.
Those are obviously very narrow examples, but generally it seems animals always have one or more exceptional senses limited to smell, hearing, or colour vision (it would be great if someone could point out an exception to this in the comments, like touch or taste!).
You are right to hint that there is a lot of information in colour vision. Like your example with loss of colour in machines, data needs to be dumped very quickly in the human brain.
I'm not a neuroscientist, but I have read pop articles about how little we can remember with photo detail. Within seconds of looking at a scene all texture details are lost, most objects are forgotten about and all we can really recall are big shapes. But importantly, we would notice the tiger in the woods before forgetting and run for it! If we had no colour input, that wouldn't be the case.

Examples of more colour being used.
Hopefully the above demonstrates that colours are important to many animals. If anything it seems that more colours even than our perception of the EM spectrum are preferable, particularly into the UV range.
For prey identification reindeer rely on ultraviolet light to spot lichens that they eat. This same sight can spot the UV-absorbent urine of predators. This article has a few more examples.
Some birds of paradise like parrots have even more brilliant patters under UV light. Even starlings look to have an identifying pattern under UV light. Both those examples are thought to be involved in mate selection.

White light does not reveal the feathers' yellow fluorescence in images (left), but short wavelength illumination does (right). Photo: Science

This all hammers home that colour is more important to animals than machines and is involved in more than just camouflage.

Excellent answer! I enjoyed it.

I second One-Face's comment... :P I was completely unaware that racoons had monochromatic vision...

Most marine mammals, some nocturnal mammals, and some fossorial mammals are monochromats -- meaning they see in what is effectively greyscale. As previous commentators have mentioned, quantifying visual acuity is not a trivial matter. Still, I'd assume that under many measures a species such as the owl monkey--a monochromat--has quite good vision, at least at night.

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Cellular Respiration/Fermentation Problem Leading to Lack of Energy
A young animal has never had much energy. He is brought to a veterinarian for help and is sent to the animal hospital for some tests. There they discover his mitochondria can use only fatty acids and amino acids for respiration, and his cells produce more lactate than normal. Of the following, which is the best explanation of his condition?
A) His mitochondria lack the transport protein that moves pyruvate across the outer mitochondrial membrane.
B) His cells cannot move NADH from glycolysis into the mitochondria.
C) His cells contain something that inhibits oxygen use in his mitochondria.
D) His cells lack the enzyme in glycolysis that forms pyruvate.
E) His cells have a defective electron transport chain, so glucose goes to lactate instead of to acetyl CoA.
Answer:  A
A) So, from this assumption, pyruvate would build up in the cytosol of cell. However, I do not see why lactate would be formed. Lactate forms when there is a lack of NAD+. In this case, there is no problem with NAD+. All NADH will be recycled into NAD+ by the Electron Transport Chain in the mitochondria.
B) I do not know what will happen in this case. Too many contradicting factors. Can anyone explain?
C) This is clearly not correct. If there were no O2,then there would only be lactic acid fermentation; the protein and fatty acids would not be used.
D) If lactate is formed, pyruvate must have been formed.
E) The electron transport chain needs to work. See C). Same explanation.
So, I am debating between A and B. Can anyone explain why the textbook says A?
EDIT :
If anyone could give me a step-by-step approach to solving this problem, I would really appreciate it. This is supposed to be a challenge problem not representative of test questions, but I still want to fully get it.

Short answer
(A) is a possible answer and is indeed cause for fatigue, as pyruvate is needed for the Krebs cycle to run. The krebs cycle is an essential step in the generation of ATP in aerobic organisms. (B) is incorrect because NADH is never transported into the mitochondria in any organism (it is a nonsense answer).
Background
NADH is not transported into mitochondria. Instead, its electrons are transported into mitochondria, and I quote from Berg et al. (2002):

NADH cannot simply pass into mitochondria for oxidation by the respiratory chain, because the inner mitochondrial membrane is impermeable to NADH and NAD+. The solution is that electrons from NADH, rather than NADH itself, are carried across the mitochondrial membrane. One of several means of introducing electrons from NADH into the electron transport chain is the glycerol 3-phosphate shuttle (Figure 18.37).

Electrons from NADH can enter the mitochondrial electron transport chain by being used to reduce dihydroxyacetone phosphate to glycerol 3-phosphate. Glycerol 3-phosphate is reoxidized by electron transfer to an FAD prosthetic group in a membrane-bound glycerol 3-phosphate dehydrogenase. Subsequent electron transfer to Q to form QH2 allows these electrons to enter the electron-transport chain.

Hence, (B) is incorrect.
EDIT: Now why would lactate be formed in (A)? Pyruvate builds up in the cell, as it cannot be transported into the mitochondria. Since lactate dehydrogenase has pyruvate as a substrate, the equilibrium of this reaction will be pushed to the right and lactate will be formed:

Source: wikipedia page on lactate dehydrogenase
The lack of glucose oxidation takes away the main power supply of aerobic organisms, causing fatigue. Lactate buildup may also induce fatigue.
Reference
Berg et al. Biochemistry 2002, Freeman, New York

why would A be a cause of fatigue? Lactate is not formed.

Let us also assume that B was referring to the electrons of NADH rather than NADH in itself just so that I can be sure I understand the biology behind this.

(B) is simply not true, as the claim is false - healthy dogs also can't move NADH across the mitochondrial membrane. It is simply a nonsense answer. I have added an explanation of why lactate is formed.

Lactate as such can cause fatigue, @ChrisStronks. Accumulation of lactate will cause acidosis which then will cause fatigue.

Very nice answer


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Why does the oxygen produced in the photosynthesis come from water and not carbon dioxide?
In the photosynthesis equation:
$$\ce{6CO2 + 6H2O ->[sunlight] C6H12O6 + 6O2}$$
The only place where we have 6 molecules of $\ce{O2}$ is in $\ce{6CO2}$. Then it reacts with $\ce{6H2O}$ to form $\ce{C6H12O6}$ and $\ce{6O2}$ that apparently comes from $\ce{CO2}$. So why do we say that the $\ce{O2}$ produced by plants comes from $\ce{H2O}$ and not $\ce{CO2}$? I don't know if I'm the one who's understanding something wrong or is it the photosynthesis formula which is wrong?

go through this http://www.howplantswork.com/2009/02/16/plants-dont-convert-co2-into-o2/

@The Last Word - great link.

and it's still like this after the edit?

Yeah, but the water might have come from mitochondria that devoured HCOH which came from CO2.

You are missing some knowledge here for sure, photosynthesis is a little complicated at A level, so I will describe it in brief.
During photosynthesis electrons and protons (A hydrogen atom without the electron) are required for a process called the electron transport chain and proton motive force. This happens during the light dependent stage of photosynthesis, (there is also a second light-independent stage called the Calvin cycle, and that is where the CO$_2$ is used), I won't go into detail about what the protons and electrons do (unless you want me to) but you need to know that these come from a water molecule, the water is split using light (photolysis, literally: cutting with light) into two hydrogens and half an oxygen molecule (or an oxygen atom). The oxygen that was released in photolysis is not required for the rest of the pathway, so it diffuses out of the cell.
For why it doesn't come from carbon dioxide, you need to consider the Calvin cycle. In the Calvin cycle, carbon dioxide is converted to glucose by enzymes, using products from the light dependent stage, so 6CO$_2$ are combined over six cycles to form one molecule of glucose. So that is where the CO$_2$ is used as well.
Hope that helped! If you want me to go into any more detail please ask!

Photosynthesis uses chlorophyll (or other pigments) for harnessing photons and water (or other compounds) as electron donor $H_2O = 1/2O_2 + 2H^+ + 2e^-$. After splitting the water it sends the electrons through the further steps of an electron transport chain and at the end it reduces $NADP^+$ into $NADPH$. Meanwhile it increases the $H^+$ concentration outside of a membrane, so it creates a $H^+$ gradient. This gradient can be used to synthesize $ATP$ from $ADP$ using a $H^+$ pump. After that $NADPH$ and $ATP$ can be used to fix $CO_2$ at any time (so that part of the photosynthesis is not light dependent).

Figure 1 - Light-dependent reactions of photosynthesis - electron transport chain - source

Splitting $CO_2$ into $CO$ and $1/2O2$ is possible as well. It can be done enzymatically or due to photolysis. You need similar energy to do that as by water splitting.
After that $CO$ can be used e.g. by shift reaction $CO + H_2O = CO_2 + H_2$ to create an electron donor ($H_2$) and play the same thing as normally. It is possible to reduce $CO$ with an electron donor ($_H2O$, $H_2$, $H_2S$, $NH_3$, etc...) further, and create for example ethanol or carbohydrates like glucose.
Afaik only artificial photosynthesis use $CO_2$ splitting and natural ones prefer $NADPH$ and $ATP$ creation and usage for example in the Calvin cycle, C4 pathway, CAM pathway, reverse Krebs cycle, Wood pathway, etc... I think this is because carbon fixation evolved before photosynthesis. So photosynthesis is just about plugging another energy source into the system.
(In programmer terms the carbon fixation is loosely coupled to the implementation of the energy producing process and the common interface is described with ATP and NADPH creation.
In mobile phone user terms it is like charging your phone's battery (ATP and NADPH storage) from line power or using a solar charger. Both chargers provide electricity (ATP and NADPH) via USB.)
The question is good, what you meant is possible, but afaik. in natural photosynthesis the $O_2$ always comes from the water.
References:

2006 - Photosynthesis in the Archean era.
2008 - When did oxygenic photosynthesis evolve?
2012 - On the photosynthetic potential in the very Early Archean oceans
2010 - Early Evolution of Photosynthesis
1988 - Photoreduction of carbon dioxide by aqueous ferrous ion: An alternative to the strongly reducing atmosphere for the chemical origin of life
wikipedia - Anoxygenic photosynthesis
2002 - Kinetic study of the reverse water gas shift reaction in high-temperature, high-pressure homogeneous systems
The Enthalpy, Entropy and Gibbs Energy of Ion Formation in Solutions
wikipedia - Standard enthalpy of formation
2012 - Splitting CO2 into CO and O2 by a single catalyst
2002 - Molecular Hydrogen as an Energy Source for Helicobacter pylori
2009 - Water-Gas Shift Reaction Catalyzed by Redox Enzymes on Conducting Graphite Platelets
2013 - Leaf-architectured 3D Hierarchical Artificial Photosynthetic System of Perovskite Titanates Towards CO2 Photoreduction Into Hydrocarbon Fuels
2014 - Comparison of CO2 Photoreduction Systems: A Review
2006 - Microorganisms pumping iron: anaerobic microbial iron oxidation and reduction
1972 - Flash photolysis of H2S
2011 - Anoxygenic Photosynthesis
2004 - Time line of discoveries: anoxygenic bacterial photosynthesis
1990 - A reverse KREBS cycle in photosynthesis: consensus at last
2009 - Molecular Approaches to the Photocatalytic Reduction of Carbon Dioxide for Solar Fuels
2014 - Electroreduction of carbon monoxide to liquid fuel on oxide-derived nanocrystalline copper
2012 - Biological conversion of carbon monoxide to ethanol: effect of pH, gas pressure, reducing agent and yeast extract.
2012 - Hydrogen, metals, bifurcating electrons, and proton gradients: The early evolution of biological energy conservation - an exciting article
2005 - Hydrogen-driven subsurface lithoautotrophic microbial ecosystems (SLiMEs): do they exist and why should we care?
2011 - Alternative Pathways of Carbon Dioxide Fixation: Insights into the Early Evolution of Life?
1998 - Hydrogen metabolism in organisms with oxygenic photosynthesis: hydrogenases as important regulatory devices for a proper redox poising?
1975 - Facultative anoxygenic photosynthesis in the cyanobacterium Oscillatoria limnetica.
2003 - On the origins of cells: a hypothesis for the evolutionary transitions from abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells
2011 - Serpentinite and the dawn of life
2010 - How did LUCA make a living? Chemiosmosis in the origin of life
2012 - The Emergence and Early Evolution of Biological Carbon-Fixation
2012 - Photosynthetic Electron Transport in an Anoxygenic Photosynthetic Bacterium Afifella (Rhodopseudomonas) marina Measured Using PAM Fluorometry
Bacterial Energetics: A Treatise on Structure and Function
2012 - Depth variation of carbon and oxygen isotopes of calcites in Archean altered upperoceanic crust: Implications for the CO2 flux from ocean to oceanic crust in the Archean
2013 - Hydrogen-Nitrogen Greenhouse Warming in Earth's Early Atmosphere
2010 - Aeronomical evidence for higher CO2 levels during Earth’s Hadean epoch
2014 - From Ionizing Radiation to Photosynthesis
2011 - Metal centers in the anaerobic microbial metabolism of CO and CO2
2014 - Evidence and arguments for methane and ammonia in Earth's earliest atmosphere and an organic compound–rich early ocean

I'd suggest you cut most of the references. Nobody is going to read them all (certainly not the poster) and they discourage people scrolling to other, perhaps more succinct answers.

@David I read them all. I think I am still somebody...

@David Btw. I think the problem is with the site. It does not provide a way to collapse a reference list or add it as an attachment.

It’s not about the oxygen!
This question indicates two misplaced concerns. One is with oxygen. I imagine that this is because of its importance to us as animals; however as far as photosynthesis is concerned oxygen is just a waste product. The other is with a chemical equation, which is as informative as the top line of a commercial balance sheet. One's focus should be on the chemical problem and what is needed to solve it.
The problem is making sugar from carbon dioxide
The purpose of photosynthesis is to make sugar from carbon dioxide. Why is this a problem?

The individual carbons in the carbon dioxide have to be joined together with C-C bonds. This requires energy (in the biological form of ATP).
In sugars most of the oxygen atoms are in OH groups. So we need to convert the C=O of the carbon dioxide to C–OH (not throw the oxygen away). This is reduction of the carbon, and requires ‘reducing power’ (in the form of the NADPH) which also requires energy input.

The sun solves the problem by supplying the energy
The energy for creating the reducing NADPH and the ATP comes from the sun in a complex series of reactions that do not involve carbon dioxide. Light of a particular wavelength is used to break a water bond so as to separate charge in a reaction that I will deliberately not balance:
(1) H2O → O2 + H
A complex series of reactions transfer the H (more strictly an electron) between different molecules, eventually reducing NADP+ to NADPH. At the same time an equally complex process builds up a H+ concentration difference across a membrane which provides the energy to convert ADP to ATP.
To reiterate, the reaction uses light energy to generate chemical energy in the form of NADPH and ATP. Oxygen is just flushed down the toilet.
Now for the Lego bricks — so easy we can do it in the dark
Having harvested the energy of the sun we have money in the bank which we can spend whenever we want — day or night — to make sugar from carbon dioxide. The overall unbalanced equation is:
(2) CO2 + NADPH + ATP → C6H12O6 + NADP+ + ADP
but this is actually a complex series of enzyme-catalysed reactions (known as the Calvin Cycle if you must know).
You still want to know the answer?
The oxygen produced in photosynthesis comes from water because this is the molecule in which a bond is broken by sunlight to separate charge allowing synthesis of molecules with reducing power and energy transfer potential. Carbon dioxide is the building block of the sugar and far from breaking a C=O bond, what is needed is for it to be converted to C-OH by reduction of the carbon.

@user1136 — I'm about to take a late vacation. I'll consider the chemical arguments you make regarding reduction of carbon dioxide and respond on my return.

@user1136 — OK, I see your point. Yes, my reference to reducing the oxygen (and even a double bond) was sloppy and incorrect. I have modified my answer to make it clear that the carbon is reduced. Trust it is ok now. If so, we can tidy up the comments.

I read on Wikipedia that basically 3 $O_2$'s come from water in the light reaction, and during the Calvin cycle $H_2O$ is produced from H+ ions and $CO_2$ to make more $H_2O$, which produces more $O_2$ in the light reaction.
Basically, 3 $O_2$'s come from the water and the other 3 $O_2$'s come from $CO_2$-made $H_2O$.
EDIT: The equation is $12H_2O$ + $6CO_2$ => $C_6H_1$$_2$$O_6$ + $6H_2O$ + $6O_2$ so therefore all the $O_2$ is from the water, and the $O_2$ in the sugar is from the $CO_2$, and the supposedly extra $O_2$ left over goes into some new $H_2O$.

It's the only answer that accounted for the discrepancy of "wait, 6 H2O only has 6 O's and so that makes 3 O2's! Also, 6 CO2 will have 3 O2's left over after making C6H12O6! Why does the missing O2 (from H2O) magically appear and the extra O2 (from CO2) magically disappear?"- well, that I could find, anyway.

Actually, I don't know if the plant reuses the CO2-generated H2O, but I know that there is H2O generated by CO2. The equation I saw later was 12H2O + 6CO2 --> C6H12O6 + 6O2 + 6H2O, but they took 6H2O from both sides to simplify the equation, but it adds confusion later when viewed without 6H2O added to each side, and the O2 is explained to come from H2O.


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Neuroscience and learning of a single neuron
Can you teach an alphabet to a neuron?
If yes, how?
How does a neuron 'learn' new information, do all individuals learns similarly ?

No, you cannot teach a single neuron an alphabet.
In general, the way the brain learns and stores information is by adjusting the strength of connections between neurons. Therefore, there is not really any information in a given neuron, but in the pattern of how it's connected to other neurons.
For the example of the alphabet, there are a lot of different pieces of information to tie together: there will be different neurons that respond to the visual shape of a letter, the word that names the letter, the sounds that the letter represents in spoken language, the words that contain the letter, etc.
A general neuroscience textbook like Purves is a good place to start learning about how brains work. You might also learn about how artificial neural networks learn. They are definitely different from biological nervous systems, and the learning mechanisms vary, but they share some characteristics such as the information being represented in the strength of connections between units, usually called weights in an artificial network.

Hmm, does that mean that if someone were to record a group of individuals recalling the same information, it is somewhat possible to predict the neuronal firing patterns that will lead to the recall of said information.      Following which, if the same electrical activity was replicated in a nascent set of neurons that has yet to learn the information, that individual is able to 'learn' that information.                                               Thanks for the book recommendations, I'll definitely check it out


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What is meant with “the chest wall and lungs are coupled together”?

Since the lung and chest are elastic, we can represent them with springs. Under normal conditions, they are coupled together: the “lung” springs are stretched and the “chest” springs are compressed. During a pneumothorax, the lungs and chest are independent and the springs representing them go to their relaxed positions, causing the lung to collapse and the chest wall to enlarge.

This analogy is honestly completely flying over my head. How are the lung and chest wall “coupled”? Heck, earlier in this book it’s mentioned that one of the two forces keeping the lungs from collapsing is “the surface tension between the lungs and the chest wall” (the other being the intrapulmonary pressure).
This and the said analogy make it sound like there is a physical-contact adhesion force between the lungs and the chest wall, which, from what I’ve looked into (and several other analogical figures the book has provided), seems to not be the case.
I believe this analogy of “coupling” is probably related to pressure relations, but I don’t understand how those are exactly set up either, and why during a pneumothorax they just cease as the two become “uncoupled”.
For those interested, the book is Medical Physics by John R. Cameron.

Then add how the diaphragm functions : the chest (ribs etc) can stay stationary and xou can breathe via the diaphragm...

@SolarMike this question isn't really about that (the dynamics of breathing). It's about the coupling of the chest wall and lungs. Of note: if the chest wall and lungs are decoupled (as in a pneumothorax or hemothorax, breathing via the diaphragm isn't effective.

How are the lung and chest wall coupled?

You have your answer in the text:

the surface tension between the lungs and the chest wall

And as you say, it is very much

like there is a physical-contact adhesion force between the lungs and the chest wall

A good, medical school level anatomy textbook will walk you through this. Moore's Clinically Oriented Anatomy gives the appropriate level of detail. Lets refer to Fig 1.30 (pg 107 of the 6th edition):
Below, you can see that there is a bag of fluid (the pleura) around the lung itself, like an under inflated balloon wrapped around the lung (see the picture of the fist, representing the lung, wrapped in a balloon, representing the pleural sac). The space inside the bag of fluid is very small, very thin, in a normal lung. The size of that space is exaggerated in the figure below in order to show the layers more clearly. In fact, in medical images of a healthy chest, it doesn't even appear as a space. The thin layer of fluid provides surface tension, keeping the two pleural walls close together. Since the chest and lungs are physically attached to the pleural walls on either side, that surface tension pulls the elastic lung outward (expanding it), and the chest wall inward (collapsing it, slightly). If the pleura is punctured, blood or air can enter, surface tension can no longer keep the two pleural walls next to each other, the chest wall returns to its preferred state (slightly more expanded than it would otherwise be), and the lung returns to its preferred state (substantially more collapsed than it would otherwise be).


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Animals as organ donors and organ's life expectancy
Recent attempts to find reliable organ donors was using genetically-engineered (GE) pigs as heart donors.
The pig's DNA is altered so that its tissues will appear identical to the patient's tissue and will not be attacked by the patient's immune system and rejected. A pig's heart is anatomically close enough to a human heart to fit to the task. All it takes is to make a GE and raise it until it is of the right age.
The issue is as follows: a human's life expectancy is much higher than that of a pig. If the patient is young enough, and the transplant succeeds, then the human will live far longer than the average pig. Will the heart hold-up throughout the lifetime of the human, or will the human need a new heart in say a decade or two?
EDIT: I have realized that actual attempts were so far limited to valves, and not entire hearts or other organs. Looking for an analogy in a human-to-human transplant, or animal-to-animal experiment, what would happen if the organ receiver is much younger than the donor itself? will the transplanted organ live the lifespan of the donor (its "original" lifespan), or will the transplant extend the life of the organ to that of the receiver?

We can't really answer such speculative questions here, please see the help. It's also encouraged to cite any supporting information when asking questions, though I don't think that will change the issue I pointed out.

http://time.com/5159889/why-pig-organs-could-be-the-future-of-transplants/  I have provided the supporting information in the link. I hoped the researchers did refer to that issue but find no reference to it. That was an attempt to understand whether all organs in the body age at the same rate or is aging related to some key factors which do not affect all organs equally.

@BryanKrause I have edited my question so that it relies on actually observed cases.

@BryanKrause there is a good deal of work on this subject. An answer can be made based on real data.

@DeNovo I think your answer is fine and will be helpful for the OP, but it doesn't directly answer the question asked which was about the life expectancy of a pig vs. human and how that impacts the lifespan of a xenotransplant, and about hearts, not valves. Allotransplants and mechanical valves also have limited lifespans.

@BryanKrause that's my point. Graft survival seems to be independent of the expected lifespan of a donor. It's about other issues.

@DeNovo For a valve. I think it's pretty unclear what the survival of a heart will be without speculation.

tl;dr
Graft survival, even with allotransplantation (human donor to a human host), is not equal to an average human lifespan. It's much shorter (11-16 years on average). Porcine grafts (which often last over 15 years) do not seem to be limited by the average pig lifespan (which, google tells me is 6-10 years). In both allo and xenotransplantation, graft failure and host survival are issues of the immune response to the graft, not of the expected lifespan of the donor.
Full answer:

Will the heart hold-up throughout the lifetime of the human, or will the human need a new heart in say a decade or two?

Lets answer this, first, for a heart transplanted from a human donor, an allograft. Even from a human donor, the heart is unlikely to hold up for a normal human lifetime. Heart transplant survival has improved greatly over the last five decades, but median survival in an adult is now 11 years. In pediatric patients, it's 16 years. The main problems are cardiac graft vasculopathy, malignancy, infection, and immune rejection. Transplant is a treatment, not a cure, and a major challenge of transplant as therapy is managing the immune response. Rejection and graft vasculopathy are (generally) related to immune injury of the transplanted organ. Malignancy and infection are related to immune suppression of the host.
Xenotransplant (transplantation from a non-human donor) adds an additional complication to the challenge of managing the immune response. Here, the range of non-self antigens is even greater, and the graft will be attacked by complement and other aspects of the innate immune system, as well as the adaptive immune system. This leads to graft destruction, often in the first hour after transplant. There have been advancements using transgenic pigs that solve some of these problems, with improvement in survival in pig to primate trials. Personally, I'm skeptical that CRISPR techniques will easily solve all of the problems of transplant immunology, but transgenic animals have moved things forward substantially over the last 15 or so years (see this review)
As far as porcine valves are concerned, they are treated to reduce their antigenicity (a protocol that hasn't worked for cardiac xenotransplants). Paradoxically, graft survival in older patients is quite good, with most valves lasting beyond 15 years. This may have something to do with age related decrease in immune function, but regardless, porcine tissue (at least from a valve), seems to hold up quite well.
There is good access to data on cardiac allotransplant survival (both adult and pediatric) directly from the international society for heart and lung transplant. There is a good review of pediatric transplants here. You can read more about cardiac graft vasculopathy here
As far as xenotransplantation is concerned, there was an excellent issue of the international journal of surgery, november 2015, that has a number of articles reviewing xenotransplant. Many are available without a paywall, and it's a good place to read up on this. This article from that issue discusses immunological challenges in xenotransplantation from pig to primate. You can read more about this here, but it's beyond a paywall. A good discussion of the overall history of xenotransplantation was reprinted in that issue, here


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Why does human facial and head hair continue to grow?
Many people can grow extremely long head hair and facial hair. Are there evolutionary theories as to why this is the case? It seems like having long hair could be a disadvantage, and extremely long hair seems to be rare in other mammals.

Closely Related Question

Another closely related question

Yet non answer the question - how is longer hair advantageous.

Just want to point out that not everything can be explained in terms of advantage or disadvantage in evolution. Most of the characters are not directly selected or selected at all.

@nbubis: why has there to be an advantage? Maybe it's just that it is not disavantageous...

@nico, I agree, but that's the OP's question.

That is my question - it seems like it would be a disadvantage - it could get caught in things, takes energy to grow, clean, etc.

How humans evolved to have head / beard hair that continues to grow longer than other animals is a topic that many anthropologists & biologists are still not sure about & there is no general consensus as to "why" yet.
The three main views that I am aware of however are :
1) Evolution of the "Aquatic Ape." (Ingram, 2000: Morgan 1997; 1982)

Infants, in order to hold onto their mothers in the water, would latch onto her hair. Limiting separation from the mother & increasing chances of survivability
Longer hair meant that infants / small children would need to swim less in order to get to their mother
Believed to be supported even further when you consider that aquatic mammals are almost always hairless, indicating that at one point, humans were highly "aquatic" mammals.

2) No real benefit, but used as a tool for "mate selection." (Darwin, 1871; Cooper 1971)

The view held by many of the Darwin school of thought (I do not mean that as a derogatory), is that at first, "hairiness" was sexually attractive, but eventually "hairlessness" became more sexually attractive in most places (i.e. the face to see facial expressions & socialize better; Wong & Simmons 2001)
A sign of "virility" & "health" as can be seen in the mate-selection behavior of lions. Which is true even today as human diagnostic material for health (Klevay, 1972).

3) Practical evolutionary benefits for the human species specifically

A lot of body heat escapes from the head, probably the most important part of your body. Hair is a good insulator that can keep in heat. This increases survivability in colder climates. (Wong & Simmons 2001; Bubenick 2003). (A disputed but considered credible reason, especially when you compare hair length and types across different regions throughout history)
Protection against damaging UV rays (while still permitting adequate Vit.D3 to come through) & some protection from free-radicals or other harmful particles. Because we became bi-pedal, the head was the main area exposed to the sun (as well as some of our back). Extending hair's usefulness to even hot environments, while other body hair became less important with the development of sweat glands (Wheeler 1985).
Heightened "Situational Awareness" through "Touch sense." A concept that may seem silly at first but has some evidence to support the theory. Though the hair is not "alive," it is connected to the follicles & your nerves. In a nutshell, it may help to increase "sensory awareness" & "data gathering" of your environment, which would favor longer hair. This would be an asset in survivability (Kardong 2002; keratin.com 2010; Sabah 1974; washington.edu)
Though not a collegiate Journal article, if reasonably credible, this small article is an interesting case for supporting hair & "Touch sense" in "recent history" & in combat-survival : http://www.sott.net/article/234783-The-Truth-About-Hair-and-Why-Indians-Would-Keep-Their-Hair-Long.

I personally lean 90% "Evolutionary benefits" & 10% "Mate selection" as to reasons why humans' hair continues to grow longer than other animals. Beard & head.
Hope this info was a little helpful.
There are copious amounts of other respectable articles on each point, but I only referenced a few.

Does anyone have a link for the "Kardong 2002; keratin.com 2010; Sabah 1974; washington.edu" study? I can't find it. I have dreadlocks and raised a discussion with a group where I claimed I can sense touch on my hair even though the roots are not stimulated in any way. I couldn't find anything to support my claim but came across this by accident. Thank you.


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Is there any research on how accurately people can follow a beat?
I'm currently helping develop a rhythm game, and as part of the scoring system, I'd like to make scores and ranks based more on accurate pattern recognition rather than hitting notes very exactly on time as is the case for many games of this variety.
In many cases, the margin of error for the best judgment in rhythm games is so small that it seems to be a matter of chance more than skill that a certain judgment is counted between "excellent" and "good" (for example, one game has the margin as small as ±0.011 s).
Of course, some people are better at hitting notes accurately than others either naturally or through practice, but I would like to find some actual research into what the base neurological limitations to rhythmic precision are and to what degree practice can actually improve it.

As a baseline benchmark for the best judgment, I was thinking of looking for some research into how accurately people can follow the tick of a metronome - for example, by measuring the average deviation of somebody tapping a button to a regular beat after about 100 beats or so when they've gotten used to it.
A Google search didn't return anything useful, probably because the search terms draw up so much other, irrelevant content. Is there any prior research on this subject? If so, where can I find it? If not, how could I conduct such an experiment myself?

You might begin your search with the characteristics of Parkinson's disease. This degenerative disorder is precipitated by the death of neurons in the substantia niagra. This brain structure is composed of multiple neurological nuclei. Its function is to connect cortical regions and nuclei together, most significantly with the basal ganglia. The difference between nuclei and cortical brain circuits is important to your question.
The symptoms of Parkinson's are directly correlated with the function of nuclei in central nervous system: the integration of information. It's the integration of conscious thought, muscle memory, sensory-motor pathways that makes voluntary movement possible. Disruption of this network in Parkinson's degrades one's ability to voluntarily execute a movement.
The cerebellum also controls voluntary movement. As a cortical structure, it is involved more with the formation and maintenance of muscle memory than information integration. Damage here results in the loss of fine motor control as opposed to gross motor control in Parkinson's.
This problem has a sensory and motor component. It might be difficult to tackle your problem as a whole. Perhaps breaking it down into smaller pieces would shed some light on what you're looking for.
Here are 2 articles I found relating motor control with the cerebellum and substantia niagra.
Relating cerebellar damage with learning sequence information
Relating the substantia niagra with interval timing

In performing a cursory Google Scholar search using the terms "keeping tempo", I found these articles that seem to cover your topic of interest:
Schulze, Cordes, and Vorberg (2005) Keeping Synchrony While Tempo Changes: Accelerando and Ritardando
Scheirer (1997) Tempo and beat analysis of acoustic musical signals
McKinnet and Moelants (2006) Ambiguity in Tempo Perception: What Draws Listeners to Different Metrical Levels?
Hopefully, these papers are a help to you. It doesn't hurt to conduct experiments that others have conducted. All that really does is add more information and data to what is currently available/known.


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Does Dracaena trifasciata really absorb carbon dioxide at night?
Dracaena trifasciata — previously known as Sansevieria trifasciata — is a common house plant (Mother-in-law’s Tongue) of West African origin. There is a claim that it generates oxygen from carbon dioxide during the night.
This seems surprising to me: that process requires energy; plants generate O₂ from CO₂ through photosynthesis which requires light. As there is no photosynthesis at night, there should be no CO₂ absorption.
Am I missing something, or is this claim false?

This is an interesting topic!
Crassulacean acid metabolism is a second CO₂ fixation pathway where CO₂ is absorbed at night.  The CO₂ is fixed into maleic acid HOOC-CH₂CH(OH)-COOH which stores some of the CO₂ in the form of carboxyl groups.  During the day carboxylases release the CO₂ for fixation during the day.
This is an adaptation where the stomata open at night to take in the CO₂ and are closed during the day - the plants sort of 'hold their breath'.  The energy for this process is derived for glycolysis which were stored during the day (nice detailed review here).
The paper says that about 6% of plants (including Mother-in-Laws Tongue) do this and you can imagine with night breathing how closing the stomata during the day would really help with dehydration.  Its interesting to note that this is found in a wide variety of plants "CAM is found in five taxonomic classes, comprising monocots and dicots, encompassing 33 families and 328 genera".  That says that CAM is a pretty old development in plant evolution and not so expensive that it is easily lost.

What does "fixation" mean in this context?

So, at night it takes in CO_2. When does it emit O_2? Also at night? ("during the day - the plants sort of hold their breath" sounds like gas exchange only happens at night)

'fixation' refers to carbon fixation - taking CO2 and putting it into a 'reduced' form or in this case a vitamin the plant uses to make food and cells - the anti GHG profile of plants :)

I think all plants emit oxygen during the day - the photons from the light are directly used to turn water into Oxygen gas at the photosynthetic reaction center - one of our most important proteins!  https://en.wikipedia.org/wiki/Photosynthetic_reaction_centre

Shigeta submitted his answer as I was writing this!
Sanseveria is one of a wide group of plants (mainly succulents) that adopt a photosynthetic strategy referred to as crassulacean acid metabolism (CAM).
Recall the basics of photosynthesis. The light-dependent reactions use energy from captured photons to generate ATP and NADPH, with the generation of O2. In the light-independent reactions (the Calvin cycle) this ATP and NADPH is used to fix CO2 to form three carbon sugars, which can then enter standard metabolic pathways to produce other metabolites.
In order to obtain CO2 for the fixation reactions plants have pores in their leaves called stomata to promote gas exchange (via diffusion). This presents a challenge for plants that grow in arid conditions: if they open their stomata during the day they lose water as water vapour. Many such plants use CAM to solve this problem. Basically what these plants do is to only open their stomata at night, capturing CO2 in the form of organic acids (oxaloacetate, malate, maleate) which are stored in vacuoles. Then, during the day, these acids can be metabolised to release the CO2 again for use in the photosynthetic reactions.
Notice that in this scheme the O2 will still be generated during the day. So when does it leave? Does it have to wait until the stomata open at night, or is it able to diffuse out through cells (recall that O2 readily diffuses through biological membranes)? I have been unable to find a definitive answer to this question so far (this is old literature), but my guess is that it is lost throughout the day.

bread and butter!

I think it's important to note that from a health point of view, baring any serious lung issues; Oxygen levels are not important. CO2 levels will reach fatal levels long before you begin to feel the effects of O2 deprivation; assuming the only process involved is mammalian respiratory in a closed environment.


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Is there a correlation between B12 deficiency and cardiovascular disease?
I've read in several health sites that state that high blood homocysteine, as well as low B12 intake, is not only correlated, but actually causes cardiovascular diseases. In order to verify those claims, I've looked for related studies and found controversial results. This one, for example, observed no positive effects from B12 supplementation:

In this randomized double-blind trial, high-dose vitamin therapy had no effect on the outcome measures of stroke, CHD events, or death.

The author speculates several possibilities for the negative results, including that high homocysteine levels could be merely correlated, but not actually causing the problem.
I wonder if there have been advancements on that matter. Is it known whether low B12 causes cardiovascular diseases, or is merely correlated? If it is just a correlation, then what is causing both low B12, high homocysteine and increased cardiovascular disease? In particular, have any study observed if a diet rich in red meat, for example, has the positive effects that pure supplementation failed to show?

Low B12 intake cannot possibly be a fundamental cause of cardiovascular disease given what we can  read here and here. At most, one might postulate that B12 intake could influence the development of cardiovascular problems due to an unhealthy lifestyle.

Can you edit your post to include a link to the health sites that state low B12 intake causes cardiovascular disease? B12 deficiency causes hematologic and neurologic problems, not cardiac or vascular. Perhaps you're thinking of B1 (thiamine)?

Is it known whether low B12 causes cardiovascular diseases?

Yes, it is known. B12 deficiency does not directly cause cardiovascular disease, and I would be suspicious of any health site that says it does. B12 deficiency causes hematologic and neurologic problems.

Why is low B12 correlated with high homocysteine:

Low B12 is correlated with high homocysteine because it is a cofactor for the conversion of homocysteine to methionine by homocysteine methyl transferase. Low folate will do the same thing (cause an elevation in homocysteine).

These cofactors and metabolites are involved in methyl donor reactions that are particularly important in the production of blood cells and myelin. Strict nutritional B12 deficiency (reduced intake) often occurs along with other nutritional deficiencies.
The primary hematologic problem is anemia, with associated changes in the morphology of certain white blood cells. The anemia, if severe and prolonged can contribute to cardiac problems of other causes. You also see paresthesias (often symmetric), a shuffling gait, and other odd neurological problems including depression and psychosis.  B12 supplementation does not always reverse the neurological problems.
You can read about B12 deficiencies in Cecil medicine Chapter 225 and 167, Harrison's Chapter 74, and in this review.


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Is blood typing still useful for analysis of ancient tissues?
Modern techniques.
In recent years, DNA sequencing has become extremely cheap. This, compounded by the ability to PCR miniscule samples to viable samples for analysis, means that aDNA can be extracted and analysed from ancient tissue samples that 20 years ago would have been impossible to sequence.
Pre-sequencing techniques.
However biochemical analysis of ancient tissues preceded the human genome project, which revolutionised sequencing technology.
Although blood typing was fiddly, grossly time consuming, and expensive, people were able to define historical lineages.
Question
How does the "old-school" approach of blood-typing compare in terms of cost, accuracy, sample required, and speed against the modern aDNA analysis? Do those sort of experiments still have a place in a modern ancient tissue lab?


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Why isn't Rh disease present in other mammals?
I have read about Rhesus D Hemolytic Disease of the Newborn, sometimes called "Rh disease". It's rare, but it can happen when an Rh+ baby is conceived by an Rh- mother.  This raises many questions. I have heard this problem only happens with humans; if so, why doesn't it affect other mammals like Rhesus monkeys?  Where along the evolutionary time-line did Rh disease come about in the first place?

Have you read this ?

I have read it now. It is a similar issue. A newborn has issues with a mother's antibodies that are in the mother's milk that they were not immunized to during pregnancy. These antibodies were brought about by sensitization (the same process that triggers RH disease). The difference being the antibodies do not crossover the placenta barrier during pregnancy. Although similar, it is not RH disease in mammals other than humans.

@biogirl essentially the difference is that one occurs during pregnancy the other during nursing.

Rh is just a proteinous(antigenic) factor in red blood cells; not a DISEASE.If it is present in blood you are Rh positive. If not Rh negative.An Rh negative person if exposed to Rh positive blood will form specific antibodies against the Rh antigens. In case of pregnancy, Rh antigens of the foetus do not get exposed to the Rh negative blood of the mother during first pregnancyas the two bloods are well separated by the placenta.
However during the delivery of the first child there is a possibility of the exposure of maternal blood to small amounts of Rh positive blood of the foetus.In such cases the mother starts preparing antibodies against Rh antigens in her blood.
In case of her subsequent pregnancies, the Rh antibodies from the Rh negative mother can leak into the blood of the Rh positive foetus and destroy the foetal RBCs. THis could be fatal to the foetus or could cause severe anaemia and jaundice to the baby.A condition called erythroblastosis foetalis arises.
This condition can be avoided by administering ant- Rh antibodies to the mother immediately after her first delivery.
This is not a disease but is just as a result of antigen- antibody reaction.As far as Rhesus monkeys are concerned, it is an innate factor in them, common to all.It is quite similar to the case of Bacillus thuringiensis in biotechnology. When inserted into plant cells these bacterial phytotoxins cause havoc as an insecticidal agent; whereas they seem to have exactly no effect inside bacteria.You may also consider the case of Penicillium notatum.These organisms are capable of causing harm to other living organisms but are not affected themselves.
Similar is the case with Rhesus monkeys.

will need references..

For just the basics on Rh factor you may check out NCERT Biology for class XI.You may check this site out: http://en.wikipedia.org/wiki/Rh_blood_group_system           You may also go through: http://www.nhlbi.nih.gov/health/health-topics/topics/rh/

This being an international forum, many would not be able to access the NCERT class XI textbook for biology. Please do include proper online links to scientific papers and articles in the body of the answer next time.

@The Last Word I provided links with the comment.Sorry for not including them with the answer. However you may check the links out, now that I have provided them.

It may not be a true disease so to speak, but essentially we are seeing children dying from naturally occurring differences in the blood types between that of the mother and of the infant. My question is why would this exist at all in the first place? What would the evolutionary benefit of such be? And why can this specific condition/occurrence not be found in other apes or other mammals?


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How Many Generation We can Run a Mycelium of Pleurotus ostreatus/Pearl Oyster?
I am able to grow mycelium successfully on the spent coffee grounds. And mycelium is successfully fruiting once/twice on the same substrate. I am cloning by taking mycelium from the existing one (which is fruiting at present) and transfer to the new jar and run it for 8-10 days and use it in new box for fruiting next generation.
What I am growing is 4th generation(I mean the transfer cycle).
As I am Newbie, I have done some sort of research and started with a master spawns (got it from govt. mushroom labs). But I am not able to understand one thing is.
Is it ok to clone using mycelium or do I need to do spore cloning or need to go for master spawn spawns now and then or its ok to continue to nth generation and keep eating the fruits.
Is there any serious issues with my cloning?


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How to categorize non-coding sequences in the genome?
I'm trying to wrap my head around the diversity of functions in a genome. Obviously, you've got coding and non-coding regions. But within "non-coding" regions, it seems to be a "wild west" to me. Transposable elements, SINEs, LINEs, transposons, retrotransposons, introns, promotors, etc. Have we annotated genomes enough to have a comprehensive list of the types of non-coding sequences?
It'd be really helpful for me to have a list of the types of non-coding sequences and sub-categories (where relevant). Wikipedia seems to break out a list but the figure on the same page for the human genome doesn't match. Can anyone provide a good reference for how to categorize non-coding sequences?

Is this the Wikipedia article, you are referring to? Which figure is the one that doesn't match?

No, I was referring to "Genome" https://en.wikipedia.org/wiki/Genome#Eukaryotic_genomes but I had just found this page which is helpful. Thanks.

thanks for adding. However, I think the figure and text do align. The text says Noncoding sequences make up 98% of the human genome and the figure says that 2% are Protein-coding genes. Everything else is non-coding :)

Check out the Sequence Ontology, for one approach to categorizing sequence features.  Alternatively, you could download a GFF3 file for the hg38 assembly of the human genome (for example from the ENSEMBL website.  One of the columns in that file contains feature types.

Here's my likely over-simplified and incomplete categorization I could come up with. Please, for those more informed, please edit!
Noncoding sequences can be broken up into the following categories:
1. RNA genes (genes that produce tRNAs, rRNAs, miRNAs, etc.)
2. Introns and untranslated regions (UTRs)
3. Cis- and trans-regulatory elements
    3.1. Cis-regulatory elements (CREs)
        3.1.1. Promoters
        3.1.2. Enhancers
        3.1.3. Silencers
    3.2. Trans-regulatory elements (genes for transcription factors)
4. Repetitive elements (repeated sequences, repeats)
    4.1. Long terminal repeats (LTRs)
    4.2. Variable number tandem repeats (VNTRs, tandem repeats)
        4.2.1. Satellites
        4.2.2. Minisatellites
        4.2.3. Microsatellites (short tandem repeats, simple sequence repeats)
    4.3 Transposable elements (TEs, transposons)
        4.3.1. Retrotransposons (Class I TEs)
            4.3.1.1. Short interspersed nuclear elements (SINEs)
            4.3.1.2. Long interspersed nuclear elements (LINEs)
        4.3.2. DNA transposons (Class II TEs)
    4.4. Telomeres
5. Pseudogenes


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How does menstruation proceed during one period?
I'm simplifying things here:

A. Does a woman bleed constantly, like a drop every 10 minutes? Two hours without pad or tampon and your underwear is dirty?

or

B. Does a woman bleed suddenly a bigger amount of menstruation fluid? Then hours might pass before it happens again?

or

C. Both.

Might be a stupid question. I'm sorry. I read all the other information on the internet but no one answers this one for some reason. I read girls should change the pads and tampons a few times a day (like every 6 hours). Does that mean A is correct?
Thanks for any helpful information!
(Well, I guess I'm male.)
Edit:
What I want to know: If I was a woman, how would I experience the days of period bleedings? From what I read in the comments and answer(s), one should expect A (constant bleeding) and should wear pads/tampons all the time (for 2 to 7 days) and sometimes one could also experience an exceptional heavier bleeding (B)? Are there (healthy) women who would experience B most of the time? I visited Wikipedia and other random sites but they all have the other important information without answering my question. (Also I will be glad to accept an answer, which satisfies my need for knowledge.)

It would help the quality of your question if you cite the websites that you read that weren't helpful.

This isn't much of an answer but here goes... Two things; it depends on which day of the period it is. There can be rather brief, heavy bleeding on the later part of the first day or sometime during the second. On most others days, the bleeding will be lighter with drop every now and again. I think most women would agree with me on the fact that when there is a rather heavy flow it can be felt. So, to boil things down, the correct answer is C :) If you want me to make this an answer I'd be happy to! :)

@L.B. Sure make an answer. See the edit in the Q first.

@Donkey, to find out, you really have to go through one yourself and find out how your own cycles are like. It's kind of like printer ink--> there is some adjustment/irregularity in the beginning, then it should be predictable/regular.

This isn't much of an answer but here goes... Two things; it depends on which day of the period it is. There can be rather brief, heavy bleeding on the later part of the first day or sometime during the second. On most others days, the bleeding will be lighter with drop every now and again. I think most women would agree with me on the fact that when there is a rather heavy flow it can be felt. So, to boil things down, the correct answer is C.
Here are a couple of links to sites that will back me up on this: Advice From AnnaMarie and Center for Young Women's Health.

I don't know, I can't vote (yet). So it's just like comments and answers suggest, you put the answer in actual words. Speaking of additional questions: How would one notice the bleeding has started? Except going to bathroom or checking, can one feel not being clean, even if it is a simple light bleeding on the first day?

I would say you have downvotes because you are providing anecdotal evidence or experience which isnt based on research. Sure you could be right but your answer is based on a small subset either you if you are a woman or women you know. It comes down to what is your proof that this is how it works? Now compare your answer with the one with the most up votes. Do you see a difference?

The answer is both. Different women experience menstruation differently depending on the levels of different hormones in their body, personal medical conditions, medications, on their age, and the activities they do in their lives. Bleeding is due to the sloughing off of the highly vascularized endometrial lining of uterus which is no longer receiving the appropriate levels of progesterone for its maintenance from the corpus luteum, in the absence of an implanted embryo in the uterus. You can imagine how different this common event would be if a woman was taking warfarin, had a bleeding or coagulation disorder, had polycystic ovarian syndrome, was a runner, or was a 13-year-old vs a 41-year-old. It varies!
Source:

Welt et al. Physiology of the Normal Menstrual Cycle. Last updated Feb 28, 2014. http://www.uptodate.com/contents/physiology-of-the-normal-menstrual-cycle.
http://www.womenshealth.gov/publications/our-publications/fact-sheet/menstruation.html

Are sudden bleeds common?

@CRags, they can be, and it could or could not be "normal." It really depends on how it affects the quality of life of the woman. Possible causes of sudden heavy menstrual bleeding may be due to leiomyomas, thick endometrial lining, hormonal imbalances, blood thinning medications, etc. For other causes, see mayoclinic. Sudden is also a subjective descriptor, because some women are more atuned to their bodies than others and can tell when they might menstruate, preparing ahead of time.

Thanks for the answer. I knew about menorrhagia. My idea was that consistent heavy bleeding would be present. The phrase “sudden bleeding” sounded like bleeding heavily but inconsistently. I wanted to know if such cases existed.

@CRags, yes I believe such cases exist, but I know this only from peers. I am in the medical field, but am not an OB-GYN, who would be the best person to give credit to my answer.

Which alternative is more common? Can a woman experience both due to changed life style, hormones, medication, etc.? (like I said: "simplifying things here", let's say a normal, healthy woman 20+, no extreme cases)

Heavy menstrual bleeding affects anywhere between 20-40% of women in countries such as the US and Britain but elsewhere is unknown. See BMJ. It is very common for women to experience changes in their menstruation in terms of quantity of blood loss and even frequency of menstruation depending on change in lifestyle, hormones, medication but it depends on what exactly she changed. A range of variation is very common even in the same woman.

Basically there is every variation under the sun possibly but I have structured my answer so that the more common variations are earlier.
On average a woman loses around 30-40 ml of fluid, this is a combination of mucous, secretions, endometrial tissue and around 50% blood. Because of this it is a slightly darker colour than just regular old blood. This of course varies with women with blood disorders or taking medication to "thin" the blood bleeding a lot more. The second day is typically the worst. On the worst day a woman may use 3-6 pads but this can be substantially more and changing every 2-3 hours is not uncommon. It is most typically more or less constant with some worse moments in the day, but there is a huge variation.
Clots are quite common but not all women get them. They're typically more common in older females and may look just like tissue. The size of these clots can very from the size of peas to grapes and even larger than 6-7cm in some women. This is a little more worrying as this may cause anaemia or iron deficiency unlike the regular bleeding above which typically doesn't. Women affected with blood clots are much more likely to have pain.
Pain itself is more common in younger women. However usually this settles down and usually it is not painful enough to affect daily activities. In some women however they get dysmennorrhea which basically means painful uterine cramps. This is because the blood flow to the uterus is restricted due to the hormone balance and the muscle is contracting really powerfully. The lack of blood flow causes the muscle to get starved of oxygen and the pain is said to be similar to angina or like a heart attack (both being caused by a similar mechanism).
Menorrhagia is the medical term for heavy periods. This is characterised by more than 80 ml of fluid each month and affects 1 in 10 women. Usually this is diagnosed by the number of pads and tampons used. Women affected by this may require to double protect, use both a tampon and pad. They may also take a number of extra underwear or trousers to work. The symptom affecting these women is termed flooding. Unexpectedly they get a "flood" which just basically goes everywhere and is uncontrollable and is usually quite a lot of volume. This is sometimes due to an underlying disease, of which, is most commonly fibroids but may also be contraceptive devices that go into the womb/uterus, blood thinners or polycystic ovary syndrome.
There is however much variation. Some women will have only 10ml or so of blood (light periods) and that's normal for them. Some women may have irregular bleeding lasting all month. Other women may have longer periods such as 10 or even 20 days per cycle. And then cycles themselves can vary from short (20 or so days) to very long with only a few cycles per year. Basically there is every variation possible under the sun.


text:
How does LDL leave the circulatory system and enter the interstitial fluid to deliver cholesterol to cells?
I have read that small molecules like oxygen and carbon dioxide diffuse through the capillaries, but larger structures can't fit through.
This answer indicates that some proteins go through via transcytosis.
The wikipedia article on capillaries shows that there are 3 types of capillaries, and it looks like LDL could fit through fenestrated or sinusoid, but sinusoid seem to only be found in certain organs, and if it did go through the holes in fenestrated capillaries, how would the LDL get to cells that are nearby only continuous capillaries? Would fenestrated capillaries be regularly dispersed so one is always nearby, or would the LDL just diffuse through the interstitial fluid originating wherever the fenestrations are?

I love these nobel prize questions! This is the same sort of question George Palade had, some 60 years ago :)

Many biologists had the same question for a number of years. In the 1950s, Nobel Laureate George Palade identified small vesicles in endothelial cells and postulated a mechanism whereby these vesicles would take in macromolecules from the luminal side of the endothelial membrane and transport them across the cell to release them to the tissues. His hypothesis has been tested, confirmed, and is now well documented.
Endothelial cells have LDL receptors that bind LDL in the capillary lumen, cause the cell to form a vesicle, and then transport it across to the membrane facing the tissues. There, the vesicle fuses with the tissue facing membrane, and LDL is released for uptake by peripheral tissue.

This process is called transcytosis. Endothelial cells use receptor mediated transcytosis to deliver many different large molecules to peripheral tissues in a regulated manner. You can read about this in some detail here (also the source of the figure), and more generally here.


text:
Humans have Cannabinoid receptors. Does that mean we're meant to consume cannabis?
I know the answer is no. But what then explains the name of these receptors being specific to Cannabinoid found in cannabis? Aren't Cannabinoid receptors exclusive to Cannabinoid? Why are they named like that, if not?

This is a case where the receptors were discovered while researching cannabinoids, but before they had discovered the endogenous chemicals that bind to the receptors. Once those were found, they were named the endocannabinoids.

You also have ammonia receptors. Are we also meant to smell it?

Receptors
Any drug or compound with specific effects has a receptor. You can read about this general concept in Goodman and Gillman's The Pharmacological Basis of Therapeutics. Chapter 1 introduces the concept, chapters 3 and 5 expand further. As far as the history of this concept is concerned, receptors mediating the specificity of action was first clearly articulated by Paul Ehrlich (in the context of dyes and toxins), and further developed by John Langley (and Ehrlich again in response). Cannabis and cannabinoids are not unique in this regard. Illicit and therapeutic drugs of all kinds have a target binding sites (receptors). In almost all cases, these receptors are bound by endogenous compounds as well.
Does having a receptor mean we should consume something?
There was a case to be made that having a great number of receptors that bind psychoactive compounds in cannabis means there is a physiologic role for some cannabinoid (especially given the pattern of expression) and suggested the existence of an endogenous cannabinoid before it was discovered. This was discussed in the nature paper that first identified the receptor and its functional expression and in the science paper that reported the discovery of endocannabinoids. Generally, it doesn't suggest that we are "meant to consume cannabis". Toxins and poisons also have receptors, and we certainly don't want to consume those.
As far as the name is concerned, the comment by @user137 is correct. Receptors are sometimes named by exogenous drugs that activate them, in particular when these compounds are discovered and studied before an endogenous ligand is found. Opioid receptors are another example.

So I think I had the misconception that cannabinoid receptors allow only cannabinoid to bind with it. Can you confirm that multiple ligands can bind to a single substrate? (So there's no exclusive lock and key mechanism)

@yathish that's correct. Cannabinoid is an entire class of compounds. Many different specific cannabinoids bind cannabinoid receptors, with varying affinities. The lock and key model is useful, but not entirely accurate. For most (possibly all) receptors, more than one ligand can bind.

In addition to De Novo's excellent answer, I would like to add that sometimes, the fact that we have receptors for something is precisely a sign that we should not consume that thing.
When it comes to taste and smell in particular, many receptors are evolutionarily conserved precisely because their physiological role is to warn us of a danger. For example, diamines such as putrescine and cadaverine smell well, terrible, as their names suggest. And we are very good at detecting these smells, because these molecules are produced during organic matter putrefaction; in other words, we have receptors (recently identified) designed specifically to tell us that certain foods are not suitable for consumption.
The same principle applies to olfactory receptors for volatile molecules harboring sulfhydryl (-SH) groups, that often smell like rotten eggs or feces - for the same reason. Mercaptan is even used to give natural gas its distinctive smell (it's initially odorless), making sure that we can detect any gas leak that could cause an accident - because we have really good receptors for it.

Cannabinoid receptors are a part of the endocannabinoid system, which encompasses native to our body ligands such as Anandamide, 2-Arachidonoylglycerol, Palmitoylethanolamide, and a few others.
The reason they are named after the cannabis plant is because of the role cannabinoids played in identifying and characterizing those receptors, a pattern that you'll see in biochemistry a lot, e.g. AMPAr and NMDAr receptors named because AMPA/NMDA are their selective ligands, even though both are glutamatergic receptors, nicotinic/muscarinic acetylcholine receptors, or CART(Cocaine- and Amphetamine-Regulated Transcript) named for the fact that cocaine and amphetamine regulate its transcription levels.
For what it's worth, not all cannabinoid system proteins are named after cannabis, such as the more recently characterized NAGly, GPR55, and GPR119 receptors.


text:
Techniques for detecting phosphorylation sites in proteins?
I would like to know how phosphorylated sites in proteins are detected in practice. I read some papers where the authors were talking about mass spectrometry techniques.
But my question is that why can't X-ray crystallography or NMR techniques capture phosphorylated sites?
(Note : My background is electronics engineering, so I am not an expert in this area.)

In principle X-ray crystallography or NMR could detect phosphorylation sites but they are much more complex and expensive techniques than mass spec. So for simply figuring out phosphorylation patterns in a protein is much easier using mass spec.
Detailed reasons:
For X-ray you need to crystallize the protein which is often very difficult/impossible and requires quite a lot of sample material. Afterwards you need a good X-ray source (ideally a synchrotron beamline) and record the scattering pattern over an extended range of angles (at least 180 degrees) before your crystal evaporates in the intense X-ray beam. Last but not least, you need additional chemical tricks, elaborate mathematical models and a large amount of computing power to compensate for the fact that in X-Ray crystallography you lose the phase information (because there are no suitable lenses for X-rays).
For NMR, you need large, very strong superconducting magnets, sophisticated radio transmitters and receivers, complicated math and computing power. However even if you have all that, you are still limited to very small proteins because otherwise the NMR spectrum is simply too complicated to be solved.
In contrast, for mass spec, you need a (highly sophisticated) spray gun, a moderate magnetic field and an electron multiplying detector. This is still a very sophisticated machine but much simpler compared to the above. In addition, data analysis is simpler than in the above cases.
Basically if you don't need to know the structural changes induced by phosphorylation (for which you would need to compare the structures of two crystals), you would stick to mass spec.

X-ray crystallography has been used to detect phosphorylated sites.
The RCSB protein database currently contains 856 structures that have both a resolution below 3 angstroms and the keyword "phosphorylated" in their listing.
It also appears to be possible to use NMR to study phosphorylated proteins.
The situations where NMR/x-ray crystallography or mass spectrometry are used differ. These differences may explain why a paper discussing the detection of phosphorylation using mass spectrometry would make no mention of NMR or x-ray crystallography.
NMR/x-ray crystallography provide structural data and almost always utilise known protein sequences.
Mass spectrometry has two major strengths for phosphorylation site detection and these highlight the reasons for using it. The first strength is the ability to gain information about many phosphorylation events, including unknown ones, from a single liquid chromatography MS/MS run. The second strength is the potential to quantify multiple phosphorylation sites from a minimum of 3 runs of a single sample.


text:
What would happen if you combined the world's deadliest diseases & viruses?
What would happen if you took the deadliest diseases/viruses in the world and combined them in a single medium (a solution of water or a test subject)? Would the strongest virus defeat the rest or would a  new deadly virus be created?

Viruses in the same cell can recombine (although they don't always do). This can happen by crossing-over of the genomic strands or reassortment of genomic segments. Therefore, you can get a new virus out of the fist two. The new one is not necessarily deadlier than the first two. If there's no recombination, the test subject will enjoy both diseases until the deadliest one kills him.

You're right : evil scientists are the best. You can start here : Viral Genetics
Have fun.

If you were to make a solution containing different viruses you would have a solution containing different viruses. Nothing would happen, they wouldn't "fight", they would just sit there. Drinking it would probably be a bad idea, but nothing would actually happen.
If you were to infect someone with different viruses then you would have given someone multiple diseases. Once again, they wouldn't fight, they would just go about their business infecting the host cells.
Now, it is possible that some sort of recombination would occur. Viruses work by inserting their genetic material (DNA or RNA depending on the virus) into the host DNA. Then, the cell's own machinery would start making copies of the virus. If a cell were infected by multiple viruses, it is possible that both would insert their genetic material into the host genome. If that were to happen, the likeliest outcome would be that the cell would simply start making copies of each of the viruses separately. It is extremely unlikely that any recombination would occur, but it is possible. In that case, assuming the viruses were similar enough, you might end up with a new virus, but there's no way of knowing if that would be more pathogenic, less pathogenic, as pathogenic as the original. Most likely it wouldn't even be viable and the combined genetic material of the two viruses would simply be inactive.
In short, viruses don't work the way you seem to think they do. They don't fight each other. They don't have anything resembling a mind. They aren't even alive by some definitions. It is not wrong to think of viruses as chemicals instead of as living things. They are incredibly simple and act following simple chemical rules. So yes, the likeliest outcome would be nothing interesting.

Kindly forgive my pedantry, but not all viruses integrate into the host genome.

@canadianer They don't? That's news to me, could you elaborate? Do some go into organelle genomes or something?

Most RNA viruses (except retroviruses) replicate in the cytoplasm. They can be translated directly by ribosomes and encode their own RNA-dependent RNA-polymerase.

@canadianer ah, of course. Yes, fair enough.

Poison + poison = poison. They aren't integers to give you a result of 2poison. Viruses are a gatewat between living and non living organisms so they would barely mate and produce a more deadly virus. You might have been a fan of Umbrella Corporation (the company working as bio-weaponary making company which tests virus and takes over world) but in real life they don't work as we think they should.
Some virus would reverse the effect of the bad virus too. Thus as other answers has told you that being evil would cost you much more money than you think.
Bacterias are living viruses are both, so some would double up the disease whereas some might lessen down the effect of other virus.

See viruses are made to destroy the host, not other viruses, so putting different types together really wouldn't have much an effect would it? as Manuella answer says, the host cell will simply suffer (or enjoy as he says) with both at once. Viruses don't mate so I sincerely doubt there would be a "combination". What some [evil] scientists are doing is that they take bacterium or viruses and they genetically modify them, and though this is a seriously tedious process, a bio-weapon could be created. Google bio-weapons and how they are made by governments.
Also, this would probably cost you a TON of money to do such a dangerous and complex project, and I doubt that anyone sensible would sponsor you.


text:
Can bacteria develop resistance to surfactants?
Bacteria can develop resistance to antibiotics. This is a problem serious enough to deserve an FAQ page on the WHO website (http://www.who.int/mediacentre/factsheets/fs194/en/).
But what about resistance to surfactants? We wash our body, our dishes, our clothes etc. with soap/detergent which are surfactants.
So can bacteria develop surfactant resistance in a way that is harmful to human health? If so, what examples are known? If not, why not?

Against detergents this is hardly possible (ok, never say never for bacteria but still), since detergents destroy the membranes of the cells. I don't think bacteria will have enough time to develop resistance mechanisms against it.

Never say never when it comes to biology!
a three part answer to the question:

Bacteria can develop resistance. These researchers did a screen for Staph and Pseudomonas which survived a detergent/soap treatment and indeed were able to select for many strains.  I'm not sure that their explanation of why they are resistant is a final answer, especially for the Pseudomonas strains which have no hypothesis at all.

Nearly all bacteria do this all the time #1  Most bacteria can go into spore or plaque forms which basically hibernate.  These plaque colonies are matrices of crosslinked bacteria and are resistant to extremes of conditions in the environment or lack of food.

Even Normal Resistance is a relative thing.  Alcohol sanitizers kill 99.9% of bacteria.  That sounds like a lot but that 0.1% can come back to full strength in a relatively short period of time.  0.1% is still millions of individuals surviving after a shower.  Resistant bacteria have a higher proportion of survivors and should improve the bounce back rate.  So resistance is a term which you need to define for a given situation.

Surfactant (soap) resistant bacteria can be nearly any sort of bacteria and a certain percentage of them are ready for a scrub down at any given time.  To get a surface completely free of bacteria takes a good deal of work.
As far as a danger to human health - resistance to being killed by soap is a real problem because the few of them which also have resistance to antibiotics and are pathogenic will be pretty much unstoppable.  Its not likely that surfactant resistance will itself make a bacteria pathogenic (never say never!) but if such a 'triple threat' emerges we'd all be in trouble.
All that being said, resistance to soap/detergents is a rare set of mutations and maintaining that resistance without constant exposure to the soap will probably not hold for long.  That is why we have massive die offs every night when we take our showers - the soap resistant bacteria have 'reverted to wild type' and mostly become susceptible over the course of the day.

I agree, I did a "death of kitchen bacteria" project and subjected pieces of sponge to different treatments. One of them was washing with lots of detergent and soap. When I grew the bacteria remaining on the sponge after the treatment, I found that it was not very effective.