text:
What is a catalytic domain of an enzyme?
Is this another name for the active site of an enzyme? What does the structure of the catalytic domain of an enzyme look like?

Quick answers: yes, and it varies tremendously. Enzymes catalyze a huge variety of reactions, from cutting DNA to adding amino acids to a growing polypeptide chain, to breaking down complex sugars, and many more besides. Catalytic domains vary from enzyme family to family, and even between enzymes in a single family. There really is no generalization you can make about structure or function, as it is so different from one to another.

AFAIK MattDMo is correct. The catalytic site is the active site where the reaction takes place that triggers the conformation change. He is also correct about there being nearly infinite potential catalytic site designs.

I would say that catalytic domains are a little different from active sites. The active site is the location where the substrates bind and the reaction occurs, the catalytic domain is the portion of the enzyme that contains the active site. Many proteins have multiple domains.

I agree with @user137. The catalytic domain is a subdomain of active site (which also contains binding domains).

Both parts overlap. Proteins are a chain of linked amino acids. This chain can be grouped into functional units which are called protein domains. Usually all parts of a domain are closely located in the protein and they form functional domains in the 3D structure of the protein. Proteins usually contain more than one domain (these are manifold but for example can be dimerization, activation or binding domains).
The catalytic domain is the part of the protein chain which contains the region where the catalyzed chemical reaction takes place. The 3D structure of the catalytic domain forms the active site, so the enzyme requires proper folding to be active. If you denature the enzyme, the catalytic domain will still be present (as this is a function of the protein sequence) but the active site will be gone.
As there are really many enzymes for all kinds of reactions it is hard to define a general form for the active site. A motif which is present in a lot of enzymes is the the form of a catalytic pocket or groove to which only the correct substrate of the enzyme has access. Additionally the binding of the substrate to the enzyme often induces a conformational change in the protein which closes this pocket. When the reaction is finished and the substrate is not bound anymore by the enzyme, the conformation changes again and releases the products of the reaction. See the image for a schematically drawing of this principle (image from here):


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What are artifacts in microbiology?
What kind of objects can be qualified as artifacts? How do we distinguish such objects?

Can you give a little context?  Where did you read or hear about it?

During a microbiology lecture, though I can't recall right now what exactly was called artifact during that lecture.

Could you perhaps mean as seen in a microscope?

I believe what you are asking about-  artifacts in biology - generally means characteristics or behavior of living things which are not advantages to reproduction.  Does that sound right?   If so lmk and I can write a response.

An artifact in biology usually refers to experimental error or other spurious results. In the context of microbiology, you will get artifacts if, for example,  you sneeze on the plate you are growing a colony on.
The term is generally used for any result that is not actually true but stems from bad analysis or experimental procedures. Take, for example, this definition from biology-online.com:

Something artificial, a distortion that does not reflect normal anatomy or pathology, not usually found in the body. For example: in radiology, the appearance on an x-ray of a surgical metal clip that obscures the clear view of an anatomical structure.

This is a very good question! There are many factors there so the list would be very long. There are many kinds of artifacts too, some related to processes and the other on technical devices. Some of the processes we do not know or we have not defined those rigorously mathematically. It depends on the methods used to get the final list. You need to specify exactly the research tool(s) and narrow the scope still to be able to define exactly some artifacts.

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text:
How can I learn about how cloning works in nature (twins)?
I would like to learn about how the process of cloning works in nature - how are twins "created" after fertilization what genetic changes can occur in each of the twins.
Where should I start? What are some interesting books on the topic, as well as recent research papers (or just the most important research topics that I could use to start looking for more information)?

A good starting point would be figuring out whether you want to study monozygotic or dizygotic twins. Monozygotic will produce genetically identical twins. Dizygotic twins are siblings who do not share a genome as there are two eggs involved.

To learn about the process of how cloning works, you need to appreciate the underlying principles. A general bio textbook should suffice (most popular one is Alberts, where you should focus on chapter 21, but maybe read other chapters for background)
Nature Scitable is also a good source.
I'd discourage reading research papers at this stage, primary research paper are often very specialized, and reviews are aimed at other researchers in the field, that is unless you find one that is focused on communicating science to the public.

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text:
What is the difference between transformation and transfection?
What is the difference between transformation and transfection? How do both of these methods work?

If you are interested in the history of molecular biology this is an interesting question.
Basically transformation came to be used to describe experiments in which the phenotype of an organism was changed by the uptake of DNA, and because of the way this developed in  bacterial systems this DNA was usually a plasmid. Then it became possible to use purified phage DNA to infect cells, whereupon the 'transformed' cells produced phage particles - for obvious reasons this was called transfection.
When efforts turned to getting the same techniques working with cultured animal cells many of the vectors were based on viral genomes (e.g. SV40) and these workers referred to this as transfection. At this point the distinction between the two terms became meaningless, and usage tends to be for historical reasons. Interestingly in molecular manipulations of yeast, where there are no viruses, the transfection word is rarely used.

@ leonardo, but I have met many scientists who work wth animal cells who refer to introducing plasmids into bacteria as transfection

@leonardo: well, in case of animal cells electroporation is also called transfection (or new words such as nucleofection). We routinely use the term transfection for animal cells but retain the use of transformation for bacteria. The usage, as Alan said, is purely due to historical reasons

As far as I know (and I haven't found no evidence against it) this is mostly a semantic difference. Both processes describe the addition of genetic material into cells using various techniques.
Transformation is here mostly used for bacterial work (transforming plasmids for example), while transfection is almost exclusively used for eukaryotic cells. The reason may be, that the term transformation is used in eukaryotic cells as well to describe the progresion of cells into cancer cells.
The techniques are not very different and there are a lot of them. Transfections often use transfection agents, which form pores in the cell membrane through which the DNA can enter the cell. This can also be done by using a short electropulse. A lot of transformations are done using "chemically treated" cells, where the cells are brought into a state in which they take up DNA which adheres to their outside.
The Wikipedia articles on Transfection and Transformation gives you more details on the methods.


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Deduce length order of 4 decapeptides based on the Ramachandran plot
I have stumbled upon the following past exam question:

4 decapeptides A, B, C, D, made only of glycine residues, have
  different conformations, and the phi and psi angles of individual
  peptides are indicated on the Ramachandran plot (unfortunately I
  cannot provide a specific plot). Order these peptides by decreasing
  length.

How should I solve this, what information useful in solving this problem can I deduce from the Ramachandran plot?

When φ = ψ = 180° the peptide is in the fully extended conformation i.e. it is the 'longest' it can be.
If it isn't possible to deduce a length order based on this information then I guess you will have to do some geometry to calculate the sums of the distances between the α C atoms using the known values for all of the bond angles - sounds laborious to me!
This site tabulates values of φ/ψ for typical structures (helix, sheet, turn) so it may be possible to decide if some of peptides are adopting these.

The Ramachandran plot describes secondary structure. If you see a lot of beta sheets in your plot you will have a more compact molecule than one which is wholly alpha helix. Gly can be almost anywhere in any quadrant. Look for residues with secondary structure to determine tertiary structure.


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What's the mechanism for being mentally tired?
I notice that after long tests, or long periods of critical thinking, my body and mind feel noticeably tired. Why does this happen? Are the neurons in my head not firing as quickly, or is there just a perceived lack of mental concentration?
What exactly is the mechanism that makes my brain feel worn out, or at least gives me the perception of my brain being worn out?

There is ongoing research on this topic, but there is a theory known as inhibition theory. It has been defined as:

Inhibition theory is based on the basic assumption that during the performance of any mental task requiring a minimum of mental effort, the subject actually goes through a series of alternating latent states of distraction (non-work 0) and attention (work 1) which cannot be observed and are completely imperceptible to the subject.

For a more in-depth explanation, see this paragraph from Wikipedia:

Directed attention fatigue occurs when a particular part of the brain’s global inhibitory system is overworked due to the suppression of increasing numbers of stimuli. This temporary condition is not a clinical illness or a personality disorder. It is rather a temporary fatigue of the inhibitory mechanisms in the brain. According to inhibition theory, it is natural for one to alternate between periods of attention and distraction. Although one’s efforts may involve very different tasks, each incoming stimulus calls upon the same directed attention mechanism.

or this paragraph from NCBI:

Recently, it has been shown that the neural mechanisms of mental fatigue related to cognitive task performance are more complex than previously thought and that mental fatigue is not caused only by impaired activity in task-related brain regions. There is accumulating evidence supporting the existence of mental facilitation and inhibition systems. These systems are involved in the neural mechanisms of mental fatigue, modulating the activity of task-related brain regions to regulate cognitive task performance.

In short, it is now believed that the brain contains mental facilitation and inhibition systems, which control mental freshness and fatigue. Basically, when you concentrate, you actually stop all distractions from taking over (and not give more focus to the main point). This mechanism is governed by the inhibitory system of the brain. After some time, this system stops working ('it gets tired') and you become unable to pay attention towards anything. That's  why it is advised that after some time of concentration, you need to distract yourself (to refresh your inhibitory system); like, say, while preparing for exams, you should have a break of 5-10 minutes after studying for about 45 minutes.
Now, why mental tiredness causes physical fatigue can be (I think) summarized by the diagram below from here:

The exact process at this part is rather very interesting. Here, the neurons are not firing as quickly as they do normally. See this Wikipedia page:

Physical fatigue can be caused by a lack of energy in the muscle, by a decrease of the efficiency of the neuro-muscular junction or by a reduction of the drive originating from the central nervous system. The central component of fatigue is triggered by an increase of the level of serotonin in the central nervous system. During motor activity, serotonin released in synapses that contact motor neurons promotes muscle contraction. During high level of motor activity, the amount of serotonin released increases and a spillover occurs. Serotonin binds to extrasynaptic receptors located on the axon initial segment of motor neurons with the result that nerve impulse initiation and thereby muscle contraction are inhibited.

The same might happen in the case of mental fatigue i.e. the excess serotonin released from frontal or parietal lobes, or even the inhibition system itself, can cause the same effect i.e. inhibition of muscle contraction.
References:

Inhibition theory
Directed Attention Fatigue
Neural mechanisms of mental tiredness
Mechanism of physical fatigue from mental tiredness
Fatigue

this is awesome. Can you please add some references for us who would like to read more? Thanks!

I've already done, I'll add 'em separately too. Sorry BTW :)

Well, there's a proximal answer and a distal answer. The distal one:
When we study zoology (and one must study zoology in order to understand the human being), we see the evolution of cells taking us back to the first animals, like Porifera, where important functions such as communication, movement and digestion - were all performed by the same type of cells.
Even in the more complex triploblastics, like Gymnotiformes and others, we still see muscle cells emiting electric discharges, like neurons do.
So, the brain is not totally dissimilar to the muscle, and just like fermentation in the muscle produces Lactic Acid, some similar system might cause us the need for rest and sleep (a need universal in the animal kingdom, by the way - the "laborious" individual ant spend more time sleeping than working, it seems).
When we do the same sort of tiresome mental work (processed in the same nerves and brain areas), the fuel - oxygen - must get scarce in that region, just like in the muscles, where the lack of oxygen activates the alternative fermentation, leading to painful skeletal muscle cramps.
So I take a guess: can it be the Lactic Acid that causes us to be "mentally tired", in a process similar to a cramp? (Though certainly different - the pain looks different - and probably more complex, enriched by layers of signals and alternative resources, increasing the versatility and effectiveness of the brain in a species where mental skills are the only weapon left.)

I could have have given an upvote if you had references. "a need universal in the animal kingdom" you know that some (though less) animals don't need sleep at all?

It was a lecture I attended many years ago, about sleep and dreams in the animal kingdom. The need for sleep is widespread in the animal kingdom. If I'm not wrong, Edward Wilson wrote about how ants spend more time sleeping than working (in Sociobiology or The Insect Societies? Not sure.) Some migratory animals "don't sleep", but actually each half of their brains sleep separately.

I think even that much of a reference would work, if you're sure about it. I think you should have a look at this.

@another'Homosapien' Nice link. That's what I meant: "Sleep is pretty much universal. Practically every animal sleeps in one way or another."


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what is meta-sympathetic nervous system?
I always knew about the sympathetic and para-sympathetic nervous systems, and today I was told about the meta-sympathetic nervous system, but I didn't understand well the man who told me about it and I didn't find enough or reliable information about this system, and if it's accepted in the scientific world or it's arguable.

The concept of a metasympathetic part of the autonomic nervous system (distinct from the sympathetic and parasympathetic parts) seems to be an idea discussed almost exclusively in the Russian scientific literature.(src)  Many, if not a majority, of the papers concerned with it seem to have one A. D. Nozdrachev as one of the authors.  He has been publishing on it at least since 1980 ("Structuro-functional organization of the vegetative (autonomic) nervous system"); most recently is from 2006 quoted in another answer ("The Metasympathetic System of the Brain").  The presentation in the latter paper seems almost incoherent to me, and apparently not due to translation.  This idea doesn't seem to have much traction, particularly outside of Russia.

As @mgkrebbs says, metasympathetic nervous system is still a matter of debate and is not fully accepted in the scientific world due to lack of research and evidence.
Metasympathetic nervous system, just like sympathetic and parasympathetic nervous system, is a part of Autonomic Nervous System.

On the basis of a number of structural and functional indices, application of pharmacological analysis as well as recording of neuronal activity, the autonomic nervous system is subdivided into the sympathetic, parasympathetic and metasympathetic parts. The metasympathetic system includes the complex of microganglia situated in the walls of visceral organs and having marked motility activity (the heart, ureters, intestine, stomach). According to the localization, separate areas of this system can be defined by its site (for example, the cardiometasympathetic area in the heart). The metasympathetic system attains the features of true autonomy. Its cells have no direct connection with the high centers: this connection is established through mono- and polysynaptic sensory and afferent units of sympathetic and parasympathetic nature.

Source

Full article here
How is it different from sympathetic & parasympathetic systems?
One major difference is in the location.
The Metasympathetic system

includes the complex of microganglia situated in the walls of visceral organs and having marked motility activity (the heart, ureters, intestine, stomach).

while Sympathetic system

consists of cells with bodies in the lateral grey column from T1 to L2/3. These cell bodies are "GVE" (general visceral efferent) neurons and are the preganglionic neurons. There are several locations upon which preganglionic neurons can synapse for their postganglionic neurons:

and Parasympathetic system

consists of cells with bodies in one of two locations: the brainstem (Cranial Nerves III, VII, IX, X) or the sacral spinal cord (S2, S3, S4). These are the preganglionic neurons, which synapse with postganglionic neurons in these locations:

For detailed location, see this image:

Did you understand what the difference between sympathetic to meta-sympathetic? I don't. In addition the heart doesn't have nerves... it has its own electrical conduction system which consist of myocardial pacemaker cells, they are not neurons.

@Assiduous this is not my theory. You wanted a reliable source for metasympathetic system and I gave you one. Also, you can never be so sure for something in biology on which research is going on. I also could never think of most of my body not being in my conscious control, but it is. I will add difference between sympathetic & metasympathetic.

don't  understand me wrong. I love your comment and I appreciate it, I just said that I didn't understand the differences:) The intonation in words is not always clear... In my case I meant to the politely intonation. Thank you and I'm very sorry if you felt inconvenience due to my first comment. :)

@Assiduous its okay, nothing about inconvenience. I was just a bit surprised as the way you asked the question in comment, it seemed to me that you think that I am expert in metasympathetic nervous system ;)

The concept of the meta-sympathetic system is founded by singles in Russian of the 80's. This theory wasn't accepted in the academic or scientific world. The most of the results in PubMed shows that they are from Russian, and a some of them are even from the same person (which published it on 80's and came back in 2006 with a new publishing of his theory, unsuccessfully.
The theory about meta-sympathetic system is not studied at the universities of the western countries such as US, Israel, UK, Australia, Canada etc. I talked with scientists from all these countries which told me these things. We can't even find any mention about this theory in the professional textbooks of biology or physiology such as Campbell biology and more.
Look at this conversion also:  https://en.wikipedia.org/wiki/Wikipedia:Reference_desk/Science#what_is_meta-sympathetic_system.3F

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Which organisms have their cell wall made of polysaccharides and amino acids?
I came across a question which somewhat goes like

In which of the following kingdom, most of the members have a cell wall made of polysaccharide and amino acids ? - Monera or Fungi.

To me it looks like both are correct answers because both are having hetero polymers of glucose and amine. Please enlighten me if I am wrong.

The correct answer is Monera.
It is so because their cell wall is made up of peptidoglycan.

Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of most bacteria, forming the cell wall. The sugar component consists of alternating residues of β-(1,4) linked N-acetylglucosamine and N-acetylmuramic acid. Attached to the N-acetylmuramic acid is a peptide chain of three to five amino acids. The peptide chain can be cross-linked to the peptide chain of another strand forming the 3D mesh-like layer.

Fungi is wrong because its cell wall is made of chitin which lacks amino acids.

Chitin $(C_8H_{13}O_5N)_n$ is a long-chain polymer of an N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world. It is a characteristic component of the cell walls of fungi, the exoskeletons of arthropods such as crustaceans (e.g., crabs, lobsters and shrimps) and insects, the radulae of molluscs, and the beaks and internal shells of cephalopods, including squid and octopuses and on the scales and other soft tissues of fish and lissamphibians.


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Why does ammonium oxalate cause swelling of RBCs?
From Pre examination procedures in laboratory diagnosis by Walter Guder :

...It consists of three parts by weight of ammonium oxalate which
  causes swelling of erythrocytes balanced by two parts of potassium
  oxalate which causes shrinkage...

This is regarding use of oxalates as anticoagulants.
To me, it makes sense that potassium oxalate causes shrinkage of erythrocytes but i don't understand why Ammonium oxalate causes swelling. Is it because it converts to Ammonia in alkaline pH of blood, diffuses into the RBC and converts back to ammonium.

After a lot of search I think the following seems a satisfactory answer.
You are correct in thinking that outside the RBC, $\ce{NH_4^+}$ converts to $\ce{NH_3}$ and once inside the RBC, it again converts to $\ce{NH_4^+}$. But this does not explain how $\ce{NH_4^+}$ swells up the RBCs. For that, let us look at the process from the beginning.
As described by Hemker et al, 2003, an important part of this process is the enzyme carbonic anhydrase (an absolutely important constituent of RBCs). Hemker et al, while studying how the Rh complex in RBCs exports $\ce{NH_4^+}$ ions out of the cell (although they used $\ce{NH_4Cl}$ in their study, it doesn't matter since blood already contains enough $\ce{Cl^-}$) they came up with the following scenario:

$\ce{NH_4^+}$ breaks into $\ce{NH_3}$ and $\ce{H^+}$ and enters inside the RBC. $\ce{H^+}$ combines with $\ce{OH^-}$ (coming from $\ce{HCO_3^}-$, we'll talk about this later) to give $\ce{H_2O}$.
Due to increased osmotic pressure (by $\ce{NH_3}$), $\ce{H_2O}$ enters inside the RBC where it is again broken into $\ce{H^+}$ and $\ce{OH^-}$. $\ce{NH_3}$ combines with $\ce{H^+}$ to give $\ce{NH_4^+}$ again (and we're done with $\ce{NH_4^+}$ here, though Hemker et al took this further).
The $\ce{OH^-}$ (from water) combines with $\ce{CO_2}$ (via carbonic anhydrase) to give $\ce{HCO_3^-}$. This $\ce{HCO_3^-}$, via Hamburger shift, is exchanged with a $\ce{Cl^-}$ while the $\ce{HCO_3^-}$ outside converts into $\ce{H^+}$, $\ce{OH^-}$ and $\ce{CO_2}$ (providing $\ce{OH^-}$ to $\ce{NH_4^+}$). See the diagram below for visual representation:

So, where does the swelling effect come in? It turns out, it is due to the increasing concentration of $\ce{NH_4^+}$ and $\ce{Cl^}-$ inside the RBC which keeps on generating higher and higher osmotic pressure. Hemker et al also found that Rhnull RBCs (lacking Rh complex) became victims to osmolysis much faster than the normal RBCs, suggesting that increased concentration of intracellular $\ce{NH_4^+}$ is the main cause of swelling and osmolysis of RBCs (in their research, they concluded that Rh factor indeed exports $\ce{NH_4^+}$ outside the RBCs).
Reference: Hemker, M. B., Cheroutre, G., Van Zwieten, R., Maaskant-van Wijk, P. A., Roos, D., Loos, J. A., Van Der Schoot, C. E. and Von Dem Borne, A. E. G. Kr. (2003), The Rh complex exports ammonium from human red blood cells. British Journal of Haematology, 122: 333–340. doi:10.1046/j.1365-2141.2003.04425.x

@gerardofurtado WYSIWYG et al coined this term (see The Biosphere chat room) for the frequency at which AliceD et al was answering questions (most probably about 10-12 answers per day, they seem to have slowed down now ;). But since my monthly/quarterly repo gain always seemed to be higher than AliceD's, I considered myself a part of this competition. Now you seem to have taken over me :P

@another'Homosapien' Since the OP is inactive, can I ask you why potassium oxalate causes shrinkage? Doesn't the addition of potassium oxalate, increase the intracellular K+ levels?

@jm97 the point is,  potassium is outside the RBC and the effect is thus reverse of what you think. You might be interested in this. Why hesitate in asking a question, its the most beautiful thing one can do...;)


text:
Effect of transpiration on plant growth
It is said that only about 1-5% of water used by plants is used in photosynthesis while the rest is transpired. Suppose that a plant grows in an environment where the temperature is not too high but humidity is very high, e.g. in the shadow of a forest. Would the plant grow faster if the humidity was lower? To what extent is the photosynthesis limited by reduced transpiration?

Simple Answer: There is no simple answer, because transpiration is just one of many factors that affect plant growth, and even in controlled experiments, the conclusions deviate according to different species.
Background: First of all, we know that increase in atmospheric humidity leads to less transpiration. A graph between transpiration and humidity, as given by Wikipedia, is as:

There have been some experiments regarding effect of transpiration on growth of plants. In an experiment, Mishra et al and team tried to get answers by reducing transpiration of Lycopersicon esculentum (tomato). They sprayed chemicals like 2-chloroethyl trimethylammonium chloride and 8-hydroxyquinoline 7 times a week and observed a reduction of as much as 80% in stomatal opening, early flowering, but no overall reduction in yield. Also, wilting was delayed for up to 8 days. Apart from this, TNAU concludes that very high and very low relative humidity severely affects grain yield. They reported reduction of 144 kg/ha yield by 1% increase in mean monthly RH.
Though it is quite common knowledge about what role transpiration plays in plant's metabolism, yet I want to include this part from University of Nebraska for the sake of convenience:

Plant growth and development relies on water for transpiration, photosynthesis, and respiration. The unique ability of water to regulate temperatures, dissolve molecules of life, and allow gas exchange, is essential for all life on earth. Transpiration is essential for evaporative cooling, CO2 acquisition, maintaining plant turgor, and mineral nutrient uptake.

Now, when we talk about transpiration along with photosynthesis, then we see formation of a positive feedback loop. When plant starts photosynthesis, it opens up stomata for exchange of CO2 and O2 with atmosphere. This also allows water to evaporate through stomata and causes transpiration, which slowly dries up the cells. To overcome this, the plant takes up more water from roots to maintain turgor pressure inside cells, which in turn leads to more transpiration. Thus, a continuous loop is formed by which photosynthesis enhances transpiration4. Also, when we talk about how photosynthesis is dependent on transpiration, then again we have no simple answer as it depends on many other factors too. For example, in a research, Graham et al concluded that the answer to this question also depends on whether the plant is grown hydroponically on in soil.
From the above examples, we can conclude that high humidity indeed causes reduction in growth. So how did I conclude that there is no simple answer? Well, there are counterexamples also present. In their research, Ford et al and colleagues found an overall increase in plant growth on increasing humidity. They found that when external humidity was high, then there was overall increase in growth in sugar beet, kale and wheat. However, water loss per plant depended on  vapor pressure deficit of air, leaf area and species. Also, water loss per unit leaf area was less for wheat than sugar beet and kale.
Thus, in short, plant growth involves so many factors, along with transpiration, that it is almost impossible to give a simple answer to this question, and it might even take a booklet to just list all the factors involved!
EDIT: As you asked in comments, I searched for more research papers on the relation between cultivation method and transpiration rate, and came up with two conclusions. First, as I am saying already, there has not been reported a straight forward relation between the two, and second, there has not been much research too on this subject. However, I succeeded in finding a couple of papers. One paper has indicated the presence of a direct relation between transpiration rate and the amount of water present in soil, be it irrigation or any other process7. On the other hand, another paper has concluded that in comparison to plants grown hydroponically, plants grown in soil require a larger root surface area to maintain the same transpiration rate and growth rate as that of plants grown hyrdropoincally8. Again, its quite difficult to draw any conclusion from these papers. So, I'll try to add some more papers as I find them.
References:

Mishra D, Pradhan GC. Effect of Transpiration-reducing Chemicals on Growth, Flowering, and Stomatal Opening of Tomato Plants . Plant Physiology. 1972;50(2):271-274
Agrometeorology - Relative Humidity and Plant Growth - TNAU Agritech Portal
Plant Growth Processes: Transpiration, Photosynthesis, and Respiration; David R. Holding, Anne M. Streich
Transpiration - Kimball's Biology Pages
The effect of increased transpiration on photosynthesis of corn part II. Comparisons between hydroponically and soil-grown plants; M.E.D Graham
Effects of Atmospheric Humidity on Plant Growth; Margaret A. Ford,  Gillian N. Thorne
Silvestre, J. and Ferreira, M.I. 2000. EFFECTS OF IRRIGATION ON TRANSPIRATION AND WATER RELATIONS OF VINEYARDS, IN THE TEJO VALLEY (CENTRAL PORTUGAL). Acta Hort. (ISHS) 537:305-312
The effect of root surface area on the water uptake and transpiration rate of maize (Zea mays) plants  (1998)
He, W.S. Nakayama, K. Yu, G.R.

The references you provided are very good. I especially like the reference about the difference between hydroponically and soil-grown plants. I don't have access to the full paper though. Do you think reduction of photosynthesis as reaction to reduced transpiration is less for hydroponically grown plants because of the increased water availability in hydroponics? If you had any further references about interactions between cultivation methods and transpiration that would be very helpful. In other words which cultivation methods are relatively unaffected by high humidity and its consequences.

Actually my former comment is wrong. Increased transpiration was associated by REDUCED photosynthesis in the paper by Graham. Still, does this have to do with higher water availability in hydroponics?

@L-Theanine I've added a couple more papers. Are those satisfactory?

You seriously made an effort to elaborate on all the facets of this question. Thank you!


text:
How fast do cancer cells divide (compared to normal cells)?
This question suggests that we have, on average, 50-70 billion cell divisions per day. I just read that cancer cells divide more often and are therefore more prone to radiation.
I am wondering, for a specific type of cancer, how fast cancer do cells divide (approximately) compared to normal cells. Does the ratio of cancer mitosis rate to the normal cell mitosis rate varies a lot?
I checked bio-numbers but there's not much data there.

I've read that healthy hepatocytes divide about once per year. However, this rate can vary quite a bit since the liver can regenerate, and regenerating liver will have a much higher rate of mitosis. Hepatocellular carcinoma will have a much faster rate of mitosis than healthy tissue but I haven't found a rate yet. There has to be a wide range in division rates from tissue to tissue, because healthy skin cells probably divide a lot more than once per year, skin cancer cells may divide faster yet.

Some literature quantifies cell growth esp. in tumors in doubling time. An article I'm having trouble recalling quoted 60 days for aggressive cancers, and 100 days for slower cancers.. sorry about having little detail. As for actual growth rate as in individual cells, that's a little harder to find, since the growth rate is dependent on the cell type, as well as the developmental characteristics of the forming tumor, which may not be symptomatic or identified early enough. Most importantly, though, cancer cell are normal cells that have lost replicative restraint, and times may not vary.

I think your question is not accurate enough. There is no such thing as "normal cell". We could use average division rate or something like that as base of comparison, but I think the exact numbers would be much better.

Tumors can be benign (they don't bother you at all eg: a mole which does not change) and malignant (also called cancer).
The difference is based on:-

Degree of differentiation - How much the tumor cells resemble the normal cells
Rate of growth - In general (over generalised) benign tumors are slow growing while malignant tumors are fast growing
Spread to nearby tissues - Benign growth don't cross tissue planes as defined by basement membranes/fascias, while malignant tumors invade across tissue planes
Metastasis - Spread to a remote location in the body through blood, lymph vessels, transcoelomic (peritoneal, pleural, pericardial spaces) routes - This is ONLY seen in malignant tumors.

Coming specifically to rate of growth, these factors must be considered:

What is stated above in 2 is the general case:

Malignant tumors a.k.a cancers grow fast
Benign tumors are slow growing
That being said some benign tumors grow faster that malignant tumors. Eg: Fibroid - grows very rapid under the influence of estrogen as in pregnancy

The rate of growth of malignant cancers depend on their degree of differentiation (look point 1 above).

Well differentiated cancers grow slow - Cancers that are very similar to normal cells are considered to be well differentiated
Poorly differentiated cancers grow faster based on the "poorness" of differentiation,i.e. the worse the degree of differentiation, the faster the growth. Totally undifferentiated (unidentifiable as any tissue type) are called anaplastic cancers and these grow the fastest

The rate of growth of cancers may vary over time

Due to excessive division probability of mutation is high and a sub-clone may emerge with faster rate of division thus a previously slow growing cancer could suddenly start growing fast
Due to the same process some cancers may suddenly decrease in their growth rate and may eventually even vanish! (become necrotic and get cleared away)

This is taken from Robbin's Text book of Pathology,Ed.8, chapter 6
Edit 1:
To know the numbers you need, you have to find out the rate of increase in volume (by measuring size at two points in time) and divide it by the approximate volume of one cell. This will give you the number of cells that has newly divided within the two measured points (growth in interval). Which can then be converted to cell divisions per second.
As pointed out, the rate is going to vary extremely, based on the cancer type.

That said I don't know if the rate of cancer growth has been measured by the rate of cell division or if the data you require exists.

I am also wondering how to measure it. Numbers like that are only estimates.

Different cancers divide at different rates. One way to qualitatively visualize this is observe hair loss in patients who are undergoing chemotherapy. Commonly, a drug like cisplatin will be administered which will cross-link DNA, inhibiting cell division by activating apoptosis. Tissues which are killed most readily by cisplatin are those which are dividing most rapidly: intestines, head hair, red and white blood cells, tumors.
Despite the loss of hair on the head, many patients do not lose slower growing hair on their arms, eyebrows, eyelashes, etc. Likewise, though cell death in the intestinal lining may be dramatic on cisplatin, the skin may not show lesions as it is a more slowly dividing population of cells.
The rate of cell division correlates with the rate of cell death while taking the chemotheraputic drug cisplatin.
You may make a rough visual survey of the cells in the body which divide so rapidly that they die, and divide so slowly that they largely survive chemotherapy.
The rate of cell division of head hair and other body hair spans the rate of cell division over which cisplatin effectively acts on tumor cells.

Hair cells are not dividing. What happens here is that you kill the cells in the hair bulb which produce the hair which leads to hair loss.

What kills the cells in the hair bulb is the process of cell division with cross-linked DNA. It's the rate of the division that determines how cytotoxic cisplatin is. What do you mean hair bulb cells aren't hair cells?

I mean mostly keratinocytes and melanocytes. These cells are usually not dividing but making proteins at a relatively high rate (otherwise the hair cannot grow and will not be pigmented) so this probably makes them vulnerable. I will check for more details here, but it might take a day.

or two...... :D

@12345678910111213 Are you saying that cisplatin damages DNA indiscriminately, and apoptosis is activated in a cell with damaged DNA only when the cell tries to divide? (Because https://en.wikipedia.org/wiki/Cisplatin#Pharmacology says "The damaged DNA elicits DNA repair mechanisms, which in turn activate apoptosis when repair proves impossible.", so I am confused.)

I think you should start with immortalized cell lines and so in vitro division rates by perfect conditions. This is easier to measure than in vivo division rates. E.g. HeLa has a division time of 23 hours. MDA-MB-231 and A549 division times are around 28 hours.

Growth of HeLa Cells
Comparative Analysis of Dynamic Cell Viability, Migration and Invasion Assessments by Novel Real-Time Technology and Classic Endpoint Assays

So I assume there is a physical barrier somewhere around one division per day or so, and it simply cannot grow faster. While bacterial division times are much lower (down to 20min), they also depend on special mechanisms (see this question/answer) and are orders of magnitude smaller than mammalian cells, so they don't count as a counter argument to this.
If you check zygote division times (10-12h, 14-16h, 22-24h, ...), you can see that they highly depend on the cell size. After some divisions the zygote depleted the reserves necessary to divide at higher rates, so after that it is limited by the ~24h barrier too.

Time-Lapse Cleavage Rating Pr
edicts Human Embryo Viability

Same 24h data here by rapidly proliferating cells.

The Cell: A Molecular Approach. 2nd edition.

So we can assume that 1/24h is the maximum rate of cell division by cancer. Let's read more about cancer in vivo, because it behaves completely different than immortalized cell lines in in vitro tests.

Originally tumours were thought to grow because they consisted of
  cells that multiplied more rapidly than cells in the surrounding
  tissue. In fact the average cell cycle of 48 hours for human tumour
  cells is slightly longer than the cycle of non-malignant cells. ...
When a normal cell divides, it does os only to replace a cell that has
  been lost and in this way a constant cell population is maintained. In
  tumour cells the control mechanism appears to have been lost: as the
  cell divides it adds to existing numbers of cells and increases the
  total population. ...
A measure of the rate of tumour growth is the time taken for a given
  population of malignant cells to double in size (doubling time). If
  the cell cycle takes between 15 and 120 hours, the doubling time can
  be between 96 hours and 500 days, depending on the histological type
  of the tumour, its age and whether it is a primary or metastatic
  growth. A shorter doubling time (less than 30 days) can be between is
  seen with teratomas, non-Hodgkin's lymphomas, and acute leukaemias;
  common solid tumours such as squamous cell carcinoma of the bronchus
  and adenocarcinoma of the breast and bowel have doubling times in
  excess of 70 days. In the patient the growth of a cacncer is only
  detectable and observable during the last 10-14 of its 35-40 doubling
  times.

Cancer Nursing: Care in Context (page 270)

So according to this book the division rate of cancer cells are similar to healthy cells.
According to another book this statement is from Dougherty & Bailey 2001, but I wasn't able to find the scientific article. :S

Tumour cells appear to have lost control mechanisms which prevent cells from growing until replacement is required. Human tumour cells are thought to have an average cycle time of 48 hours. This is not more rapid than the cycle of most normal cells. The reason tumours become larger is because their cell division creates additional cells rather than replacements (Dougherty & Bailey 2001).

Stoma Care and Rehabilitation

While ~24h is a reasonable maximum estimate for eukaryotic cells, this is very wrong for bacteria. Laboratory strains of e.coli can double the cell number in ~20 minutes (see this question)

@Nicolai Feel free to correct the answer.

The fastest carcinoma cell I have seen is the HCT-116 line, its doubling time  is about 17h. They are quite small cells.

@Roland My guess would be that it depends on the cell surface / volume ratio, but reality use to be much more complex. :-)

Please notice that cancer cells do not grow or divide faster than
  normal cells, although many people believe that, and most forms of
  chemotherapy were designed on the assumption that they grow faster.
  Actually, what makes cells cancerous is the lack of control of cell
  growth, so that they keep on growing without limit, even if slowly. On
  the other hand, some kinds of cancer cells do grow quite rapidly, even
  if not faster than certain normal cell types, such as those in the
  bone marrow that constantly replace blood cells, and those in the
  intestinal lining and skin (including hair follicles) that constantly
  renew those structures. In one form of lymphoma, the abnormality has
  nothing to do with growth or division at all, but instead is a lack of
  apoptosis (programmed cell death), so that the cells accumulate
  without limit.

Cancer in One Easy Lesson by Biology 166 Unsolved Problems in Cell Biology Albert Harris

Your answer is correct and the citation is relevant. However, it would be better if the citation is from a scientific article or a book instead of course notes.


text:
Mechanism of Myosin Head Bending in Cross Bridge Cycle Power Stroke Phase
What is the mechanism of bending of myosin head during the power stroke of the cross-bridge cycle of the muscle contraction? Does this have anything to do with the protein's 3-D structure i.e. folding of protein in space? I would prefer a physics explanation down to either the classical electrical dipole or quantum mechanical interaction amongst the proteins. I want to know what exactly produces the force that powers the bending. I am pretty sure that the force can only be electromagnetic. The question is how the force is manifested in the bending.
Any references for further reading are appreciated.

Introduction:
This is going to be quite a long answer. To have an introduction to the topic, you can have a look at articles from Wikipedia and RCSB Protein Data Bank.
The exact mechanism of physical interactions in myosin head during powerstroke cycle are not yet known. The only thing we definitely know about how release of Pi from myosin causes conformational changes (and hence, force) in it, as given at MJC website is:

This is a very basic description of the powerstroke cycle, so we'll move ahead for more detailed explanation of physical interactions during the cycle.
Details:
I have found three theories regarding physical interactions during the powerstroke cycle. Let's discuss them one by one.
THEORY 1: I could not find much explanation about physics of the cycle, the only thing I could find was at the JBC website (emphasis mine):

Mutation R759E in the myosin converter domain results in biochemical and biophysical defects as well as aberrant muscle structural and physiological properties. The central portion of the converter domain is encoded by exon 11e in indirect flight muscle (Fig., green), and the converter interfaces with the exon 9a-encoded relay domain (Fig., blue). Molecular modeling indicated that residues 508–511 in the relay loop are located near the converter residue 759 and defined weak and strong interactions of residues 509 and 511 during the rearrangements of the relay loop that are affiliated with the mechanochemical cycle. Furthermore, Ile508 can be cross-linked to Arg759 in Dictyostelium non-muscle myosin II when they are each substituted by cysteine. Therefore, we hypothesized that specific amino acids in the relay domain interact with converter residue 759 and that second site mutations in the relay residues may suppress the defects associated with converter mutation R759E.

They couldn't give much explanation of the physics involved in it. Maybe this is because I missed it or because there hasn't been much research on this because Dissecting the molecular mechanism of muscle myosin function in vivo has proved difficult. as said at JBC website.
THEORY 2: This theory, given at PNAS, provides a more detailed view of the physics, so we'll have a more thorough explanation of it:

Molecular motors produce force when they interact with their cellular tracks. For myosin motors, the primary force-generating state has MgADP tightly bound, whereas myosin is strongly bound to actin. We have generated an 8-Å cryoEM reconstruction of this state for myosin V and used molecular dynamics flexed fitting for model building. We compare this state to the subsequent state on actin (Rigor). The ADP-bound structure reveals that the actin-binding cleft is closed, even though MgADP is tightly bound. This state is accomplished by a previously unseen conformation of the $\beta$-sheet underlying the nucleotide pocket. The transition from the force-generating ADP state to Rigor requires a 9.5° rotation of the myosin lever arm, coupled to a $\beta$-sheet rearrangement. Thus, the structure reveals the detailed rearrangements underlying myosin force generation as well as the basis of strain-dependent ADP release that is essential for processive myosins, such as myosin V.

Just so that you know what it is, let me add a few points about Molecular Dynamics Flexible Fitting:

The molecular dynamics flexible fitting (MDFF) method can be used to flexibly fit atomic structures into density maps. The method consists of adding external forces proportional to the gradient of the density map into a molecular dynamics (MD) simulation of the atomic structure. For examples of MDFF applications, visit the websites on Mechanisms of Protein Synthesis by the Ribosome, Dynamics of Protein Translocation, Molecular Dynamics of Viruses, and Intrinsic Curvature Properties of Photosynthetic Proteins in Chromatophore.

Now, returning to the main point (results & discussion sections):

The equilibrium and rates of transition between the Strong-ADP and the Rigor states vary greatly among different myosin isoforms and predominantly determine how long a myosin motor can remain bound to actin in the absence of load. This kinetic tuning must be achieved by structural differences in the regions that we have seen to change in our Strong-ADP structure as well as regions involved in stabilizing the lever arm position...Our structures show that the Loop 1 conformation alters in the transition from Strong-ADP to Rigor. Thus, different sequences likely favor one conformation over the other, or promote the transition from the ADP-bound conformation to the Rigor conformation, providing a structural basis for this kinetic tuning.
For myosins, such as myosin V, that function in a cell as two-headed, processive motors, the length of processive runs and the initiation of processive runs are both enhanced by “gating” of the heads. For a two-headed molecule with both heads simultaneously attached to actin, gating refers to the fact that a lead head is essentially stalled in an ADP state strongly bound to actin, until the rear head is detached from actin by binding MgATP. This gating is attributable to the strain dependence of MgADP release. Although we do not know whether some of the subdomains of myosin may be deformed by strain, our Strong-MgADP actomyosin structure clearly reveals that strain must prevent the rearrangement of the $\beta$-sheet from the Strong-MgADP conformation to the Rigor conformation, based on the data presented in Results. Preventing this rearrangement is thus the basis of gating.

For better understanding, you should also see this video (same website) which shows animation of different conformations of myosin in different stages of the cycle:

The transitions are animated by direct morphing between the three structures Pre-Power Stroke (PPS), ADP state, and Rigor. Starting with the PPS conformation, myosin first rearranges to allow phosphate release without much change in its lever arm. Then, the first step of the powerstroke consists of a large swing of ∼58° of the lever arm toward the ADP state. The actin-binding cleft [between the U50 (blue) and L50 (white) subdomains] is closing for this transition, leading to the only state of the actomyosin cycle, which exhibits high affinity for both actin and the nucleotide. The second step of the powerstroke occurs upon ADP release and ends in the Rigor state after an additional lever arm swing of 9.5°.
THEORY 3: This theory gives even more detailed, but a bit different from the previous one, view about how the energy, released by ATP hydrolysis, is stored in myosin head. It is called Rotation-Twist-Tilt (RTT) energy storage mechanism. See this:

According to the mechanism, ATP hydrolysis in the catalytic site rotates the top of the regulatory domain, which, being connected to the coiled coils of the S-2 region, causes twist between them to increase, and leads to the twist in the myosin head. Since, at the time of hydrolysis, the myosin molecule is not bound to the actin filament, the head is free to rotate and tilt. The increase in twist is instrumental in storing the energy of ATP hydrolysis, while the rotation and tilt of the myosin head brings it sufficiently close to the actin so as to form the actomyosin complex. Untwisting of the coiled coils and the subsequent untilting and constrained reversal of rotation of the head cause the power stroke. This strain decreases the energy of interaction between actin and myosin and thus enables ATP to dissociate myosin from actin. The system is now in such a state that after the next ATP hydrolysis event, the myosin head can bind to actin, and thus, a new contractile cycle can be initiated.

The original paper of RTT mechanism (by Nath and Khurana, 2001) is available here. Since the detailed mechanism is too long, I am not posting it here. You can read the full process, just remember to have a pen and paper with you!

Conclusion:
All of the above theories provide an in-depth view of the powerstroke cycle. However, I'd still conclude that the exact physics behind the powerstroke cycle is not yet fully known, only the transitions in conformation of myosin head have been observed and interpreted to some extent.
References:

Myosin head: Wikipedia
Myosin: RCSB Protein Data Bank
Journal of Biological Chemistry: Mapping Interactions between Myosin Relay and Converter Domains That Power Muscle Function
Proceedings of the National Academy of Sciences of the United States of America: Force-producing ADP state of myosin bound to actin
Theoretical and Computational Biophysics Group: Molecular Dynamics Flexible Fitting
Cell Movements: From Molecules to Motility
Molecular mechanisms of energy transduction in cells: Biotechnology in India II
Molecular mechanism of the contractile cycle of muscle; Sunil Nath and Divya Khurana, March 2001

Thank you very much. As a matter of fact the Bonus part is most relevant to what I am most interested in, i.e. the more fundamental physical (classical electromagnetic or quantum) principle of how the force is generated of the myosin motor. Your quote in Bonus "for myosin motors, the primary force-generating state has MgADP tightly bound"only states the chemical compounds involved in the force generation but not how the force is generated. It also says "molecular dynamics flexed fitting". I suppose it refers to some physics model. But what is that physics model?

The Rotation-Twist-Tilt theory you added most recently hints at the more geometric and physical explanation of the myosin bending. I found the original paper on this theory cited by the book you linked to http://www.currentscience.ac.in/Downloads/article_id_081_01_0078_0082_0.pdf .

Yup, but since its just a theory (without any given evidences) I didn't go deeper into it. Also I was in a hurry so I couldn't look for the original paper. Thanks for finding that, I'll add it up in the answer :)

You seem to be quite knowledgeable and interested in the physics aspect of physiology. Would you mind taking a look at my other question http://biology.stackexchange.com/q/19725/8344, especially the question below the answer by James T. Fulton?


text:
Why doesn't the cell membrane just...break apart?
Forgive me if this is a silly question. I can't understand the basics. Why doesn't the cell membrane just break apart? What's keeping the layers in the phospholipid bilayer together? I know that the membrane is embedded with proteins and lipids, but I still can't wrap my head around the "why". Are the hydrophobic interactions in the middle "stronger" than the hydrophilic interactions on the outside? What's keeping the individual phosphate heads together instead of, say, one of them just drifting away due to a nearby water molecule?

Related: 1.Why should phospholipid head groups be protected in the bilayer, 2. Why do cell membranes have a bilayer

The membrane bilayer is held together by hydrophobic forces. This is an entropy driven process. When a greasy or hydrophobic molecule is suspended in water, the water molecules form an organized "cage" around the hydrophobic molecule. When two hydrophobic molecules come into contact, they force the water between them out. This increases the entropy because the freed waters don't need to be organized into the cage. Lipid bilayers have many many many hydrophobic lipids that squeeze out a lot of water and greatly increase entropy. The polar phosphates allow the water to interact with the surface of the membrane, without a polar head group the lipids would form a spherical blob instead of a membrane.
Read this section on wikipedia for more.

Perhaps worth adding that the formation of the bilayer increases the entropy of the water, but the entropy associated with the phospholipids will decrease. The net change in entropy is favourable.

Also the bilayer maximises van der Waals interactions between the closely-packed hydrocarbon chains, again contributing to the stability of the bilayer.

also surface tension.. which is basically because of the interactive forces..

Artificial vesicles can be produced using the same principle. They are quite stable, but the larger ones (giant unilamellar vesicles) are fragile to any strain: cell-sized vesicles would not resist in a tissue or blood flow, and the cell actomyosin cortex or RBC spectrin network are necessary for this.

without a polar head group the lipids would form a spherical blob instead of a membrane I am not so sure about this sentence. Even with the polar head-group, spherical blobs do form as micelles.

@James Micelles and membranes are possible. It's related to the shape of lipid. A phospholipid with two lipid tails prefers a membrane structure due to the more rectangular shape of the hydrophobic portion, while a detergent like molecule with 1 tail prefers a micelle due to the more conical shape of the hydrophobic portion.


text:
Quinone A, Quinone B, Ubiquinone, Plastoquinone: whats the difference?
I am getting very confused about this. My understanding so far is that ubiquinone is used as an electron (and proton) carrier in oxidative phosphorylation in the mitochondria whereas plastoquinone is the carrier in the light dependent reaction in photosynthesis in chloroplasts. My guess is that these carriers are similar because it is a highly conserved carrier that was present in many of the original life forms (including those that formed chloroplasts and mitochondria).
My first questions are:

is the above distinction correct?
do we find ubiquinone and plastoquinone elsewhere in cells, other than in mitochondria and chloroplasts, or in other prokaryotes?

Then I am not sure about quinone a and b. My lecturer said that quinone b is present in photosystem II and it accepts the two electrons from the lysis of H2O and then it accepts two H+ ions from the stroma to enter the membrane of the thylakoid
But I have also read that it is plastoquinone which shuttles the H+ ions in the thylakoid membrane in the form of plastoquinol. So could it be that quinone B turns into plastoquinone on accepting the electrons, and plastoquinone accepts two H+ ions to form plastoquinol?
If so, how is the quinone B from photosystem II replenished?

Difference between Ubiquinone (UQ) and Plastoquinone (PQ)
Structural Difference: Structurally, UQ and PQ are very similar. They only differ in a methyl group and 2 substituents on the benzoquinone ring (Liu et al, 2016). See the image for comparison:

Functional Difference: Functionally, PQ and UQ are very different. I'll give some differentiation points:

PQ is located on thylakoids of chloroplasts (Millner et al, 1984), whereas UQ is located primarily on the inner membrane of mitochondria, though it is found in all cells and membranes (Turunen et al, 2004).

Half-life of PQ is 15 hours while that of UQ is 30 hours in spinach cells (Wanke  et al, 2000).

UQ10 influences expression of hundreds of human genes involved in different cellular pathways, and UQ10-mediated gene-gene networks are involved in inflammation, cell differentiation, and peroxisome proliferator-activated receptor signaling (Schmelzer et al, 2011).

On the other hand, the redox state of PQ pool controls the NADPH dehydrogenase complex activity and further influences on cyclic photosystem I (Ma et al, 2008).

Although UQ has more widespread effects than PQ, most of their effects are restricted to their corresponding organelles i.e. UQ mostly influences mitochondrial activity whereas PQ mostly influences chloroplast activity. This can be understood on the basis of endosymbiotic theory since mitochondria appear to be related to proteobacteria whereas chloroplasts appear to be related to cyanobacteria (Mereschkowski et al, 1905). These two are thought to be taken in by cells about 1.5 billion years ago. We can suppose that at that time, both these organisms had similar carrier molecules (since they are very crucial for metabolism). This can justify why UQ and PQ are so similar in structure. Thus, your way of distinction can be considered as correct.
Difference between Quinone A (QA) and Quinone B (QB)
Structural Difference: In many cases, both QA and QB are structurally identical. For example, both QA and QB are ubiquinone-10 in Rhodobacter sphaeroides (Laszlo et al, 2004). Thus, the main difference between the two is functional difference.
Functional Difference: The distinct physical properties of QA and QB, including redox potential, are controlled by interactions with the binding site environment. Studies have shown that the 3-methoxy oxygen of QB interacts with the reaction center backbone (specifically amide of GlyL225). This interaction pattern serves to break the symmetry of neutral UQ between QA and QB by locking the 2-methoxy dihedral angle of QB in a position conducive to electron transfer from QA to QB (Vermaas et al, 2015). In short (not going in further details), QB interacts with the amino acid residues in reaction center in such a way that QA and QB acquire large difference in redox potentials. Apart from this, since QB is more loosely attached to the membrane, it is easily detached on being reduced to QH2 for transferring high energy electrons taken from QA to cytochrome bc1-complex8.
Thus, it is hard to differentiate QA and QB on structural basis, although QB and QA have different redox potentials due to different environmental conditions. Also, it is hard to tell whether QA and QB are structurally similar in all organisms; it is only known that they both are UQ10 in Rhodobacter sphaeroides. So, there is no simple answer (yet) on whether QB converts to plastoquinone or not. But since, in Rhodobacter, QA and QB are UQ10, thus it is not possible to convert QB to plastoquinone by this species. On the other hand, in a photosynthetic bacterium Rhodopseudomonas viridis, both QA and QB are plastoquinone molecules (Deisenhofer et al, 1994). Here, there is no need of conversion of QB to plastoquinone since it is already a plastoquinone. This finding only strengthens the fact that it is not easy to give a simple answer on whether conversion of QB to plastoquinone actually takes place or not. A much similar reaction center, from a thermophilic cyanobacterium Thermosynechococcus elongatus, looks like this (Ferreira et al, 2004; Loll et al, 2005; Umena et al, 2011):

Talking about replenishment of QB, when QH2 reaches the cytochrome bc1-complex, it is oxidized back to QB. This oxidized QB can again participate in the process of transfer of high energy electrons. Thus, QB easily gets replenished in cyclic photophosphorylation.
References:

Liu M, Lu S. Plastoquinone and Ubiquinone in Plants: Biosynthesis, Physiological Function and Metabolic Engineering. Frontiers in Plant Science. 2016;7:1898. doi:10.3389/fpls.2016.01898.

P.A. Millner, J. Barber, Plastoquinone as a mobile redox carrier in the photosynthetic membrane, FEBS Letters, Volume 169, Issue 1, 1984, Pages 1-6, ISSN 0014-5793, http://dx.doi.org/10.1016/0014-5793(84)80277-X.

Mikael Turunen, Jerker Olsson, Gustav Dallner, Metabolism and function of coenzyme Q, Biochimica et Biophysica Acta (BBA) - Biomembranes, Volume 1660, Issues 1–2, 28 January 2004, Pages 171-199, ISSN 0005-2736, http://doi.org/10.1016/j.bbamem.2003.11.012

Wanke M., Swiezewska E., Dallner G. (2000). Half-life of ubiquinone and plastoquinone in spinach cells. Plant Sci. 154 183–187. 10.1016/S0168-9452(00)00200-4

Schmelzer C., Niklowitz P., Jürgen G., Okun J. G., Haas D., Menke T., et al. (2011). Ubiquinol-induced gene expression signatures are translated into altered parameters of erythropoiesis and reduced low density lipoprotein cholesterol levels in humans. IUBMB Life 63 42–48. 10.1002/iub.413

Ma W., Deng Y., Mi H. (2008). Redox of plastoquinone pool regulates the expression and activity of NADPH dehydrogenase supercomplex in Synechocystis sp. strain PCC 6803. Curr. Microbiol. 56 189–193. 10.1007/s00284-007-9056-x

Wikipedia contributors. "Symbiogenesis." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 26 Mar. 2017. Web. 14 Apr. 2017.

Laszlo Rinyu, Erik W Martin, Eiji Takahashi, Péter Maróti & Colin A Wraight (2004). Modulation of the free energy of the primary quinone acceptor (QA) in reaction centers from Rhodobacter sphaeroides: contributions from the protein and protein–lipid(cardiolipin) interactions. Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1655, 93 - 101.

Vermaas JV, Taguchi AT, Dikanov SA, Wraight CA, Tajkhorshid E. Redox Potential Tuning Through Differential Quinone Binding in the Photosynthetic Reaction Center of Rhodobacter sphaeroides. Biochemistry. 2015;54(12):2104-2116. doi:10.1021/acs.biochem.5b00033.

Wikipedia contributors. "Photosynthetic reaction centre." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 22 Oct. 2016. Web. 14 Apr. 2017.

Yocum, Charles Photosynthetic Reaction Centers, Univ. of Mich. 5 Sept 2008. Web. 14 Aug 2014


text:
What do sphingolipids do in humans?
I want to know the significance of sphingolipids in human.
I have learnt that sphingomyelin is the most significant type of sphingolipid in human.
Also that the sphingomyelin serves as a structural constituent of myelin sheath of nerve fibres.
Is that all? I need to clarify this. Please help me. Thank you

cross-posted http://chemistry.stackexchange.com/questions/71897/what-is-the-significance-of-sphingolipids-in-human

It turns out sphingilipids have many functions in humans, many of which might still remain unknown. I will discuss here only the general, known functions  of sphingolipids, not about specific sphingolipids (as it'll make the answer too long).

Sphingolipids are believed to protect the cell surface from harmful external factors by forming a mechanically stable and chemically resistant outer leaflet of plasma membrane lipid bilayer.
Certain complex sphingolipids are known to be involved in cell signalling and cell recognition. It has been suggested that ceramide on the cell surface of one cell can be recognized by proteins on another cell's surface. Also, antibodies have been created which recognize the ceramide on a cell's surface1. All this suggests that sphingolipids play an important role in cell-cell interactions which lead to cell recognition and cell signalling.
Some simple sphingolipids, like ceramide and sphingosine-1-phosphate, have recently been shown to be mediators in signalling cascades involved in apoptosis2, proliferation3, necrosis4, inflammation5, autophagy6, senescence7, etc.
Ceramide-based lipids self-aggregate in cell membranes and form separate phases less fluid than the bulk phospholipids. These sphingolipid-based microdomains, or lipid rafts were originally proposed to sort membrane proteins along the cellular pathways of membrane transport8.
Studies indicate that proper sphingoid base synthesis is required to control protein phosphorylation events that are crucial for endocytosis and overexpression of protein kinase C1 (PKC1) and PKH1 or PKH2 (ortholgues of mammalian protein-phosphoinositide dependent protein kinase 1 or PDK1) restores the actin localization defects in the lcb1-100 mutant9,10. A suggested pathway representation would be like this11:

Decreased levels of ceramide and increased levels of sphingosine-1-phosphate are known to be mechanisms of tumor progression and drug resistance in cancer cells, and are hence being targeted in pharmacological experiments as possible cancer cures12.
Sphingolipid (mainly sphingomyelin, ceramide and gangliosides) reorganization and interaction with proteins within cellular membranes is suggested to cause many neurodegenerative disorders13.
Sphingolipids are also known to play an important role in microbial pathogenesis since in the context of bacterium- and virus-host interaction, the host is typically the source of sphingolipids, whereas in the context of the protozoan- and fungus-host interaction, both host and pathogen sphingolipids are involved14.
Sphingolipids also act as immune response modulators. Exogenously added sphingolipids (especially sphingosine) changed the receptor expression in immunological synapses, including CD3, CD4, CD8, CD45, CD54, etc. disrupting T-helper cell differentiation and prevelance of Th2 cells over Th1 cells. Sphingolipids activate macrophage function via elevation of receptor F4/80 expression and chemokine production. Fumonisin B1 regulates memory cell formation and secondary immune response, and inhibits the DNA synthesis in normal lymphocytes in response to mitogenes, more effective in T cells compared with B cells15.
Sphingosine-1-phosphate (S1P) level in lymphoid tissues, especially in thymus, is lowered as compared to blood. This creates S1P gradient that attracts lymphocytes and promotes S1PR1-dependent egress into the blood. When T cells are ready to exit the thymus and enter the blood, S1PR1 is re-expressed so that T cells can respond to the chemotactic effect of high S1P levels in circulation. When T cells re-enter the blood, S1P downregulates S1PR1. Increased lymphoid tissue S1P. blocks egress of lymphocytes by disruption of the S1P gradient or desensitization of S1PR1 on T cells. See the diagram below for representation16:

References:
 1. Gerrit van Meer, Sandra Hoetzl, Sphingolipid topology and the dynamic organization and function of membrane proteins, FEBS Letters, Volume 584, Issue 9, 3 May 2010, Pages 1800-1805, ISSN 0014-5793, https://doi.org/10.1016/j.febslet.2009.10.020.
 2. Hannun YA, Obeid LM (July 2002). "The Ceramide-centric universe of lipid-mediated cell regulation: stress encounters of the lipid kind". The Journal of Biological Chemistry. 277 (29): 25847–50. doi:10.1074/jbc.R200008200
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An interesting note about S1P: it mediates leukocyte trafficking. The major source of S1P in the blood is erythrocytes. What you can imagine is a gradient of S1P exists where it's high in the blood and perfuses to the tissues. Leukocytes express an S1PR that promotes re-entry to the blood, which is blocked by known tissue retention signals such as CD69. Fingolimod (FTY720), an S1PR inhibitor, is used to prevent T cell egress from lymphoid tissues in the treatment of multiple sclerosis. You can add sphingolipids to your list as immune response modulators!

@CMosychuk thanks, thats a very nice point! But I can't add it now since I've got some work, I had to finish it up at this much...but I'll do that as soon as I get free. Thanks again :)


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Why do we dream while sleeping?
Is there any specific reason behind why we dream? Why do we sometimes get weird and unusual dreams? Sometimes we can't even recall what we dreamt of. Why does this happen?

There're so many theories...personally, I think dreaming is the brain trying to defragment, going over the past memories and getting rid of the connections that are not deemed as important. And this process of memory consolidation is seen as a dream. This explains why we often dream about the things that has happened during the day.

This question might be a better fit on CognitiveScience.SE

@Remi.b Considering many other animal species dream, most animal species sleep, and considering there must be an evolutionary reason for both, I think it fits perfectly here. Also, sometimes the same question in two different communities will get different, thus interesting, answers.

Disclaimer: the shortest answer still remains we don't know...yet.
Talking from the evolutionary perspective, I have found as many as 8 theories about the purpose of dreaming, some of which even contradict each other(!). But I'll describe them one at a time.

One obvious theory is that we dream about something we dream of. So, dream is basically a way of wish-fulfillment; what we are dying to achieve, we get that in our dreams. Though it has been made quite scientific by Freud et al, it still doesn't look much scientific, at least from evolutionary perspective.
Another idea, made popular mainly by Hobson et al, is that dreams are purely because of accidental firing of neurons. In what they call activation-synthesis theory, they claim that during sleeping, neurons in areas of brain, including brain stem, limbic system, secondary visual cortex, etc. fire accidentally. When the brain tries to interpret those signals, the result comes out as dreams.
Another theory, proposed by Zhang et al, is that dreams are our brain's way of converting short-term moemory into long-term memory. This quite explains why some people reommend a power nap after learning something new, it helps the brain in storing the knowledge as long-term memory. Also, recent studies have concluded that it is better to remain awake for hours after a traumatic experience rather than sleeping right off, if you want to forget that experience faster.
Another theory, popularly called reverse learning theory, basically says that we dream to forget things. Dreaming is a way for our brain to prevent information overload so that memorizing new things becomes easier. Seems the opposite of Zhang et al and most likely answers why Sometimes we can't even recall what we dreamt of.
Yet another theory, seemingly more sound from evolutionary viewpoint, says that dreams are just like the playing-dead response of our body. When we dream, the neurotransmitters related to movement are shut down, resulting in what seems like a dead body. So, our body preferred to retain this mechanism because it could've helped in a way like the freeze response did.
A theory, proposed by Revonusuo et al, says that dreams are to make people ready for real-life situations. According to it, people who often dream about being chased by a tiger will learn how to overcome that situation and eventually will be able to survive such case. Seemingly answers your question Why do we sometimes get weird and unusual dreams?
Barrett et al, based on Revonusuo's theory, proposed that dreams are a way for solving problems for humans, partly because dreaming mind makes connections faster than awake one. Though this explains how Kekulé predicted benzene's structure, it doesn't look very much scientific.
A yet another one, by Coutts et al, says dreams are a way for "natural selection" of best possible method to solve a problem, including emotional situations, and is called Oneiric Darwinism. A much similar idea by Hartmann et al is called contemporary theory of dreaming.

I will not discuss the mechanism of dreaming, not only because it will make the answer too long or because others have delved into that point in their answers, but because it will answer the How? part rather than the Why? part, and also because that part would be better suited for cogsci.SE.

7 is also seen in the movie Lorenzo's Oil (1992), supposedly a true story. There's also the hypothesis of communication between the unconscious and the conscious mind, since both know things differently. But probably most of these theories are right, each in a different circumstance. Only 5 makes no sense to me: if it alone were true, wouldn't it be better not to dream at all?

@rodrigo yeah 7th one seems most backed-up with case studies, like the Kekulé one I mentioned. Of course, they all might be true until any of them is proved wrong ;) I don't know about 5th one, but they should be having some back-up studies...I'll read the paper soon ;)

Recent scientific research suggests that dopaminergic pathways are responsible for dreaming. According to this theory if something affects your dopamine system, it will also affect dreaming. My personal takeaway from this is that your dreaming can be viewed as an indicator of how your dopaminergic system works. Significant deviations from the baseline dream content may be a reaction to drugs, especially psychiatric drugs or changes in diet. These changes rarely persist for many days, as the brain compensates for the new thing.

A dopaminergic pathway runs from the ventral tegmental area, ascends
  through the lateral hypothalamus, various basal forebrain areas
  (nucleus basalis, stria terminalis, shell of nucleus accumbens) and
  terminates in the amygdala, anterior cingulate gyrus and frontal
  cortex. Damage to the dopaminergic pathway results in a loss of
  dreaming. Furthermore, chemical stimulation of the pathway (with
  L-DOPA for example) increases the frequency and vividness of dreams
  without affecting REM sleep.[10] It is interesting to note that the
  mesolimbic and mesocortical pathways are considered the seeking areas
  or the motivational command centers of the brain.

And

The evidence of the involvement of mesolimbic and mesocortical
  dopaminergic pathways suggests that dreaming occurs when a
  motivational component is activated.

Well i will like to contribute a fact which might seem a little bit out of context.
R P Feynman, the famous physicist, in his semi-autobiographical book mentioned his experiments with his consciousness and also 'dreams'. He reported that he was actually able to alter his dreams, with a little bit of practice, while he was experiencing them. Well after reading that i was quite fascinated with the possibilities and tried it myself to a fair amount of success.
Moreover although the exact mechanism or motivation of sleep hasn't been understood it is generally accepted that 'dreams' always occur in the REM stage of sleeping. In light of the 'Restorative theory of sleep',which basically says that sleep happens to make up for all we lose during daytime, i can say that dreams are actually a byproduct of the fact that maintenance of brain cells occurs by a different pathway than normal body maintenance. So all in all saying that the 'Waking up' part of our body isn't perfect and almost everytime one part of brain 'Wakes up' before the other and contributes to the bizarre yet frequent phenomena of 'Dreams'.

A couple comments: about 75% of dreaming occurs in REM, but REM is not a requirement for dreaming. The term for conscious control of dreaming is "lucid dreaming", and potentially "dream incubation" for intentionally dreaming about a certain thing

This is a difficult question to answer because we are only aware of basic mechanisms that go on in the brain during dreaming. One theory that is prevalent during sleep is that we may be replaying the day's events in order to extract and store the important information encountered. So if you block sleep after learning you can interfere with it. The function of dreaming itself is more speculative but one idea is that all that replaying back of sensory information along with normal conscious processes creates vivid imagery that is strange and new.As you know during REM the brain is very active so it can "perceive" as if it were awake. This answer is mostly directed at how we dream but hopefully you've gleamed some additional insight from it.


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How can a drone be male?
A drone bee is male and is the product of only a queen bee. How is it that the drone bee can be male. I know that a drone is produced by an unfertilized egg, but this does not explain where the two X chromosomes come from. To me, this means that the one X chromosome from the queen must some how be duplicated.

The drone bee is haploid, they only have 1 copy of each chromosome, so the chromosomes aren't duplicated. See this.

In the general case sex chromosomes in animals are different from the human XX/XY. Birds, some reptiles, some insects use ZW, where the female is ZW and the male is ZZ. Others use XO where XX is female and X is male. There are others. Fungi have hundreds of 'sexes' because they have a particularly weird system.

The short answer, which has already been pointed out by user137, is that females are diploid while males are haploid. Bees do not have sex chromosomes per se; rather sex is determined by the csd gene on chromosome III (csd stands for complementary sex determination). Diploid bees that are heterozygous at the csd locus (i.e. contain two different alleles (versions of a gene) for csd) are female (workers).
Bees that are haploid at the csd are males. Interestingly, diploid bees that are homozygous at the csd (i.e. contain two copies of the same csd allele) are also males. But in the honey bee, these bees do not survive as they are killed by workers before they mature. In some other haplodiploid species, however, both haploid and diploid males are viable (the bumble bee Bombus terrestris for example). Also note that diploid honey bee drones can be viable if they (somehow) make it to maturity without being killed by workers (e.g. see here).

Do you have a source for the diploid males?

Here is one source http://link.springer.com/article/10.1007%2FBF01240420

That's interesting, looks like 50% of all larva in those colonies are male, that would seriously handicap those colonies. Since the odds of diploid males goes up when inbreeding happens, do bees have some sort of defense against inbreeding? How many versions of the CSD gene are there? Because more versions would lead to reduced chances of diploid males. Also makes me want to know how the bee detects heterozygosity vs homozygosity.

Yes the honey bee csd is highly polymorphic, which reduces the odds of inbreeding (e.g. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1626638/). It's an interesting point within the context of bottlenecked populations (e.g. imagine an invasive bee swarm transferred by boat that has no conspecifics around) because, as you point out, if the population has low csd variability then it increases the probability that a diploid bee will be an inviable male rather than a viable worker. But as queens mate with multiple drones, a single invading queen can have multiple csd alleles in her spermatheca.

Also note that diploid honey bee drones can be viable if they (somehow) make it to maturity without being killed by workers (e.g. http://www.ncbi.nlm.nih.gov/pubmed/16475107). It looks like workers tell diploid and haploid males apart by pheromones.

@yidryi Feel free to add those links into the answer.


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Why the smell of sweat greatly varies across the body?
Having only two types of sweat glands (eccrine, apocrine) I would expect the sweat odor "spectrum" to be very "narrow". However, the smell of sweat greatly varies between scalp, arms, armpits, groin and feet. Why is that?

I think the odor is caused by bacteria, which vary in diversity across the body.

@canadianer why bacteria are so different between armpits vs. groin; hands vs arms; arms vs scalp?

Sweat is actually odorless regardless of what type of sweat gland it comes from, of which there are two types - apocrine and eccrine.
As suggested by canadianer, the odor comes from the mixture of bacteria and the resulting sweat. With regards to why your bacterial flora is so diverse is likely due to the different micro-environments found around your body.
In your armpits and feet (arguably where the strongest odor subsides), you'll find more anaerobic bacteria - bacteria that prefers low oxygen environments and sometimes, higher temperature, humid environments. Because they don't use oxygen to metabolize, their by-products in making energy are often organic acids, commonly lactic acid. Some anaerobic bacterias even have by-products containing sulfur.
Meanwhile, in less warm and damp environments on your body (i.e. face, hands), aerobic bacteria dominate. Oxygen is abundant and the temperature is just right for those strains of bacteria. Their by-products are much less putrid
EDIT: In addition, apocrine sweat is also high in the nutrients, such as lipids and proteins that bacteria can feed on. Once they metabolize the stuff in apocrine sweat, they start releasing the aforementioned by-products.

From what I understand from your answer, (1) aerobic vs. anaerobic explains why armpits and feet have the strongest odor, and (2) the variety of odors comes from the variety of bacteria. Could you please expand or add a link about how diverse are the bacteria across different micro-environments?

@Sparkler Here's a link to a good paper that explains the 3 different micro-environments found on skin and the bacteria flora that resides: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2805064/
As shown in the paper, there are 3 types of environments. Sebaceous (oily), dry and moist. Here's an excerpt from a book that gives some examples of bacteria found in skin flora: https://www.ncbi.nlm.nih.gov/books/NBK7617/#A512


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Why do sulfur compounds smell?
Many of the compounds of sulfur have a strong odor.  Hydrogen sulfide from rotten eggs, the mercaptans of a skunk, the odor compounds in onions and garlic, the bitter taste of brassicas (cabbage, Brussels sprouts, collard greens, and similar plants) are all sulfur compounds.  What is it about sulfur that makes this so common?

I think most sulfur compounds are toxic (given a certain concentration). The ability to smell or taste sulfurous compounds therefore protects us, since we can avoid them.

I was once told by a professor that rotting food releases sulfur compounds and we evolved to find their smell repugnant to avoid eating such food. Don't know if it's true, but it sounds plausible.

canadianer got it, sulfur 's stronger smell is not something intrinsic to sulfur,  we evolved to be better at detecting sulfur becasue we are very vulnerable to the bacteria in spoiled meats. And sulfur compounds are good indicators of rotting meat. Humans have not had the time to evolve the specialized immune-digestion pairing of carnivores.

Bitter taste has less to do with sulfur than the PH of the food in question, bitters is basically biological detection of alkali ph.

(I've noticed dogs are attracted to rotting organic matter.) @John immune-digestion pairing of carnivores?

Dogs (or at least the wolves we bred dogs from) like most carnivores have adaptations of both the immune system and digestive system to handle the bacteria in rotting meat. things like a much more acidic digestive system and more specialized immunological response. http://www.jwildlifedis.org/doi/abs/10.7589/2016-07-162

Nice question! It would, obviously, be a long answer if I talk about all types of sulfur compounds here. So I will take up just 2 examples to explain this.

Thiols: when we talk about sulfur compounds with foul smell, thiols (mostly) come to the top in the list. Thiols (R-SH) are a class of compounds famous for their smell. Some examples of thiols include:

compounds in onion and garlic (see this answer for more details).
a mix of compounds responsible for the smell of skunk's spray (Anderson et al, 1975)
(R)/(S)-3-methyl-3-sulfanylhexan-1-ol found in human sweat
(Methylthio)methanethiol found in male mouse urine

Some biologically important examples would include coenzyme-A, glutathione and cysteine. However, not all thiols have fowl smell. For example, furan-2-ylmethanethiol provides the aroma of roasted coffee while grapefruit mercaptan gives grapefruit its characteristic scent. You can see a complete list here.
After seeing some examples, lets come to the main point i.e. why thiols have such foul smell. Thiols are decomposition products of proteins. When amino acids, like cysteine and methionine, are decomposed, thiols are one of the last things to form. This would explain why it becomes important to smell and avoid them (you wouldn't want to eat a dead and decaying mammoth, and a blind hunter might not be able to tell if he has found a decaying mammoth if he can't smell such compounds). As a side note, this also explains why organic acids have sour taste and smell. Compounds like glucuronic acid, citric acid, oxalic acid, lactic acid, butyric acid, acetic acid, etc. are all decomposition products (if you know how vinegar is made traditionally). This is why your body tries to get rid of them (I saw that in a video, if you eat things with foul smell and sour taste (it was a Japanese dish probably), you'll suffer nausea and vomiting since your body will do all it can to prevent its harmful effects). See this Wikipedia page for more info.

Hydrogen Sulfide: H2S, or hydrogen sulfide, is another compounds famous for its smell. It smells like rotten eggs, and indeed it is because H2S is what rotten eggs release. The basic reason why H2S smells so bad is again the same. Hydrogen sulfide is one of the final products of proteolysis during decomposition (see same Wikipedia article). But there is another cause why H2S has such foul smell (and why I didn't just stop at thiols): H2S is itself toxic.

H2S has many biological signalling functions, similar to NO and CO. These include:

H2S serves as endotheium-derived relaxing factor (EDRF) and endothelium-derived hyperpolarizing factor (EDHF). In short, it acts as smooth muscle relaxant and vasodilator. It also increases the response of NMDA receptor and facilitates long term potentiation in the brain.
it is converted to sulfite by thiosulfate reductase, and further to thiosulfate and sulfate by sulfite reductase, in the mitochondria. Sulfate is excreted in the urine.
it acts on ATP-sensitive potassium channels in smooth muscles, and does the blood vessel-relaxing work in smaller blood vessels.
it blunts, reverses and promotes healing of diverse inflammatory reactions. A full list is available on this Wikipedia page.
the most pronounced effect of H2S is much similar to that of CO. It also binds to the iron in the mitochondrial cytochrome c oxidase and other mitochondrial cytochrome enzymes, thus preventing cellular respiration and causing death.

This would explain why external source of H2S would be so harmful since, being biologically such an active molecule, its amount needs to be controlled strictly. Now, if you want to know why decaying bodies would be dangerous, this Wikipedia article would be a good starting point.
EDIT: The bitter taste of some vegetables is because of a type of compounds called glucosinolates found in some plants. These compounds dissociate upon eating to form isothiocyanate and are thought to be a part of the plants' defense system (see this answer for more details). Interestingly, a similar type of compound, cucurbitacin, found in Cucurbitacae plants, does not contain sulfur, yet it is responsible for the bitter taste of these plants.

A paper was published about this in 2016: https://pubs.acs.org/doi/abs/10.1021/jacs.6b06983 Apparently the OR2T11 receptor detects thiols and requires copper to work. Also, it's interesting to note that SO2, which is not a thiol, is also quite easy to smell. You smell SO2 from burnt matches, gunpowder (e.g. fireworks), and some other sources. Evolutionary, I suppose it might be the most common chemical sign of fire that isn't also a normal part of the atmosphere, unlike CO2 and H2O, which are the main products of combustion, and carbon is not a gas, though you maybe can smell it as smoke(idk).


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DNA shared by octopuses and modern humans
What percentage of DNA do we share with octopuses? I tried looking it up but couldn't find the answer on the internet. Could someone enlighten me on the same? To make the question more specific, I am more interested in knowing how many genes humans share with octopuses, but answers regarding how different our shared genes are or even a metric of the entire genome alignment would be helpful.

Hi and welcome to Bio.SE! Where did you look it up already that wasn't satisfying? What sort of comparison are you looking for? For example are you looking for how many genes we share, or of the genes we share how different are they, or are you looking for a metric of the entire genome alignment?

@James, thank you! I did a simple Google search and did not find relevant results. Since they're three different questions and I'm obligated to keep this related to just one, for the purpose of this question, I'm looking for how many genes we share. However, I'd personally welcome answers regarding all the three degrees of analyses. Since expecting answers on all three fronts can perhaps be beyond the scope of this question, it'll be all the same to me if only the bit about how many genes we share is answered and I'm directed to different sources for further reading on the other two fronts.

I see the problem. There is a lot of sensational news stories calling them aliens, but very few quantifiable results! Good question.

Octopuses have more chromosomes and genes
Octopuses have 28 chromosomes, and 33,638 protein-coding genes (Albertin et al 2015).
Humans have 23 chromosomes, and 20,365 protein-coding genes (Uniprot).
Octopuses had an expansion of genes in only a few protein families
There is a large number of C2H2 zinc-finger proteins (C2H2 ZNFs) (~1800), interleukin-17-like genes (IL17-like), G-protein-coupled receptors (GPCRs), chitinases and sialins paralogues in the octopus genome.
However, there does not exist a comprehensive analysis of orthologous proteins in the octopus that I can find.
This means that although we can say that there are thousands of genes we only observe in octopuses, there is currently no study comparing orthologues that exist in both humans and octopuses.

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