Many hormones use , such as cAMP and cGMP, and also act on the same cells. For example calcitonin and PTH both use cAMP and have opposite effects. How is it possible that the same second messenger molecule (cAMP) can have opposite effects on the same cells? Why are there more hormones compared to 2nd messengers - doesn't that reduce the uniqueness of each hormone? Shouldn't that make the function of any hormone using e.g. cAMP be the same as that of any other hormone using the same 2nd messenger on the same cell?
122126
Glucagon stimulates glycogenolysis and gluconeogenesis, thus increasing the plasma glucose concentration — so that tissues get enough glucose in the fasting state. However glucagon also inhibits glycolysis. In this case, what is the point of having high blood glucose if it cannot be catabolized?
88967
In Feynman's Lectures on Physics , chapter 15, page 6 he writes about 2 identical, synchronized light signal clocks. These are clocks that consist of a rod (meter stick) with a mirror at each end, and the light  goes up and down between the mirrors,  making a ,click each time it goes down. He describes giving one of these clocks  to a man flying out in space, while the other remains stationary. The man in the space ship mounts the clock perpendicular to the motion of the spaceship. Feynman then writes:      "the length of the rod will not change. How do we know that   perpendicular lengths do not change? The men can agree to make marks   on each other's y-meter stick as they pass each other. By symmetry,   the two marks must come a the same y- and y' coordinates, since   otherwise, when they get together to compare results, one mark will be   above or below the other, and so we could tell who was really moving."   what exactly is the "test" with the marking of meter-sticks that Feynman is describing? why would it violate relativity, since it seems the person in the space ship would be looking outside? why would a change in a perpendicular length violate relativity, but not a change in parallel length--couldn't the men also make marks on each other's sticks, in the case of parallel lengths?  Thank you!
86821
As I understand, the different voltage between two object is caused by the total different charged between two object. Thus, the higher extra electron between two opposite charged object, the higher voltage between them, isn't it? So if i have two battery or capacitor connect in parallel, the negative connect to negative, the positive connect to the positive. Caused the charge from each plate to be double. So that's as my expected, it must be double voltage, but ironically why it's out of my expected and the reality is vice-versa?
81449
Stupid question but I need to understand:  If $ \cos = \dfrac{\text{adjacent}}{\text{hypotenuse}} $  What is arcosine? $ \text{adjacent} \cdot \text{hypotenuse}$? Is this the same for arcsine and arctangent?
7451
At school, we've been taught that human infants produce rennin/chymosin (which aids in the digestion of milk). More specifically, it is the peptic cells in the stomach which secrete prorennin, the inactive form of rennin (in addition to pepsinogen, the pepsin proenzyme).  User @another'homosapien's answer  also seems to concur with this (excellent answer by the way, I enjoyed reading it).  However  According to Mod. @AliceD's answer  (yet another excellent answer):     ...in humans there is only a chymosin pseudogene present...   Which (probably?) implies that humans (infant or otherwise) do not produce rennin.  I managed to get my hands on the Textbook of Medical Physiology (Guyton and Hall, South-Asian edition), and according to the book (Chapter Gastric secretions, page 406) peptic cells produce a large quantity of pepsinogen. There is, however, no mention of prorennin. I even flipped over to the Appendix at the back to look up "Rennin", but it turns out there is absolutely no mention of rennin in the book.    My questions,   Some sources claim that rennin is produced in human (infants). Is this true? Other sources claim that rennin is not produced in humans ( we have a pseudogene for it though). Is this correct (I mean the "rennin-is-not-produced" bit, not the "pseudogene" bit)? If rennin is produced in humans only during infancy, what stops it from being produced as we mature? (I'm asking this because every source I've seen that claims that rennin is produced in humans, explicitly states that is done so during infancy...which would suggest that rennin is not produced in adults)
123431
I get that steroid hormones need hydrophilic plasma protein carriers (eg albumin, TBG) to travel in the plasma because they're not soluble in the plasma. However, I don't see any sources talking about how these hormones can then traverse through the (presumably also hydrophilic) cytoplasm to get to their receptors either in the cytoplasm or nucleoplasm. Anyone have any clarifications on this?
2336
My bio teacher was discussing the ratios of different ions inside versus outside the cell.   $$\text{OUT:IN}$$  $$\text{K}^+ (1:20)$$ $$\text{Cl}^- (11.5:1)$$ $$\text{Ca}^{2+} (10000:1)$$ $$\text{Na}^+ (10:1)$$  Can someone provide specific details as to the relative quantities of these different ions relative to each other? In other words, given these ion ratios, provide me with how much of each type of ion there are and show why this adds to to -70 mV. I just want to make sure I have a realistic picture of the concentrations.    I think I am mixing up concentration with the ratio between inside and out. Although for every potassium ion outside there are 20 inside, the charge from the total number of potassium ions must be less than that of chloride or it would not be negative inside, right?
3502
I know that given a group $G$ and two normal subgroups $H,K \subset G$ then it is not true that:  "if $H \cong K$ then $ \frac{G}{H} \cong \frac{G}{K} $ (the counterexample is quite easy with products of cyclic groups) "  My question is: Is the converse true?   i.e.     Given that $\frac{G}{H} \cong \frac{G}{K}$ then $H \cong K$ ?   I feel that the answer is no, but I can't think of an example.
114718
I was quite fascinated by the feature  in this month's Wired magazine.  They explain that Penicillin is effective at killing bacteria because it interferes with the growth of bacterial cell walls.  How does Penicillin do that exactly? And why does it not dissolve human cells as well?
30604
I am studying the aerobic degradation of glucose and it seems that for every glucose molecule we should obtain $\ce{10H2O}$ molecules. However, it is known that we only obtain 6.  $\ce{C6H12O6 + 6O2 -&gt; 6CO2 + 6H2O}$  (I am not going to focus in all products and reactants, but just in the important ones for the formation of water molecules)   First, in the glycolysis, for each molecule of glucose we obtain two water molecules, $\ce{2NADH+}$ and 2 pyruvate molecules. By the oxidation of two pyruvate molecules we obtain $\ce{2NADH+}$ and 2 acetyl Co-A molecules. So we are going to pass twice through the Krebs cycle, obtaining $\ce{6NADH+}$ and $\ce{2FADH2}$, and requiring 4 water molecules.  So, when we arrive to the electron transport chain, we have a negative balance of 2 water molecules, and we have $\ce{10NADH+}$ and $\ce{2FADH2}$. We have been told that for every of these molecules 2 electrons go to the electron transport chain, that means that a total of 24 electrons go to the system. The problem comes here:  $\ce{4e- + 4H+ + O2 = 2H2O}$  So, bearing in mind that we have $24e^-$, 12 water molecules should be formed, so at the end, we have gained 10 water molecules, but we know that the number of water molecules formed should be 6. So, clearly there is something wrong in my explanation. I would be very pleased if you could tell me what is wrong.  Thanks in advance.
85691
I'd like to understand  and its contents as to what it implies in a bit simpler terms. I am familiar with mathematics unto Calculus 1,2,3 and some linear algebra and group theory. I am familiar with that for each symmetry there is a conservation law.    Now is there a one-one correspondence between these two always? What about laws like conservation of charge?  And conservation of energy?  And conservation of mass?  Considering our view of nature was changed as Special Relativity was introduced and thus, what earlier the theorem would have asserted as mass and energy to be originating from different symmetries then how did we reconcile these two as just one symmetry?
111143
My doctor ordered some blood tests for me.  I would like to understand what each test is for, meaning what information could be gathered from the results.   W.B.C. Platelets E.S.R. C.R.P. Alpha feto protein   C.E.A.   I have read that some of these, such as CRP, are no longer relevant or often used. Is that true?
40971
This question was posed on SE Biology some time ago, but all the answers, including the accepted one, answered a different question instead: . I therefore changed the title to match the answers, giving notice that, if the change were approved, I’d post the original separately question myself (and answer it, if necessary). Here it is:     It is believed that thymine replaced uracil (the RNA base) in DNA because of the deleterious effects of slow spontaneous deamination of cytosine to uracil: by employing thymine instead of uracil, any uracil in DNA would clearly be aberrant, allowing a specific mechanism of repair (involving uracil DNA glycosylase) to evolve with impunity. As this was clearly advantageous, why didn’t RNA also evolve to use thymine instead of uracil?   N.B. By “why?” I mean “how can one rationalize the fact that?”. Obviously, the answer to this type of question is not, and probably cannot be, known with certainty.
105071
I would be very happy if someone can help me to find the answers for the following related questions.   Can one exon have many stop codons? Can protein synthesis happen, if the stop codon is at the beginning of the exon?
5949
What is the advantage gained by the substitution of thymine for uracil in DNA?  I have read previously that it is due to thymine being "better protected" and therefore more suited to the storage role of DNA, which seems fine in theory, but why does the addition of a simple methyl group make the base more well protected?
68973
MWE:  \documentclass{article}  \usepackage{bm}  \begin{document} $$x(t), \bf{x}(t), \bf{x(t)}$$ \end{document}   Result:    Question:  Why are the second and third x(t) typeset the same? In the second  expression, it was asked to apply \bf to only x.  EDIT:  I had not payed attention to it when asking the question, but I would like to have a bold and italic x.
55997
This is something that has long bothered me, and I have asked a few physicists and chemists and never gotten a very satisfying answer. Why are particles charged? And I'm not asking (and this is the answer you often get) why some molecule might be negatively charged because the answer to that is simply is that there was one more negatively charged particle than positively charged particle. That doesn't answer my question, however, of why there are charged particles at all?  So, on a fundamental level, what makes one particle charged and another one neutral.  For instance, protons and neutrons have almost exactly the same mass, which I'm assuming that means they have very similar internal compositions, yet one is positively charged and the other neutral. What is going on that makes these particles different?  On a similar note, and I'd really like to know this, how is it that a proton and an electron have exactly equal and opposite charge, yet an electron is about 1000 times less massive than a proton?  I have particular trouble with this question because if the answer were something along the lines of "particle X which composes protons all have partial positive charges of n amount..." I would just ask why particle X has any charge and we'd be in the same predicament.  I'm sure there is a satisfactory answer to my question, I just can't find it and don't know what resources are reliable or detailed enough to be satisfactory.