At the saturation point of enzymes (from increasing enzyme conc./substrate conc.), is the rate of activity truly zero? I assumed that the rate of activity is actually still increasing, although by very very small amounts to the point where it is negligible to say it's increasing.
Do you mean that rate of increase (not the rate of activity) is zero?
Yes, it would eventually be zero. It does slow down before it gets to the point though. If you have a set number of enzymes, there's only so fast it can be working, regardless of the substrate concentration.
Think of it this way. Imagine you have a dishwasher (enzyme), the substrate in this scenario isn't any one specific thing, we're just going to refer to every time the dishwasher turns on as when the substrate enters. Now if you're loading this by hand, it's going to take a long time if you walk around the house and collect each dish individually everytime you want to turn the dishwasher on. This is like what happens when there is a low concentration of substrate, it takes longer for the substrate and enzyme to find each other. If you instead have a pile of dishes next to the dishwasher and put them in as soon as the dishwasher turns off and then hit start again, that's going to be a lot faster, like how adding more substrate decreases the amount of time it takes for a substrate to bump into an enzyme.
Here's the thing though, this dishwasher takes 20 minutes. Even if you devise a way for the dishes to instantly be transported into the dishwasher and for it to start again the second it turns off (like with very very high substrate concentration), that dishwasher is still going to take 20 minutes to wash those dishes (or to catalyse that reaction if we're talking about enzymes). There's nothing you can do to make those dishes go in faster - they're already going in instantly, so there is nothing you can change about the substrate that will make that dishwasher/enzyme do anything faster, so the rate of the reaction is not going to increase any further unless you change other factors.
I have more questions to ask. Some of these are from exams.
1.
Do B cells undergo clonal expansion and selection to differentiate into B plasma cells and B memory cells? Also where does T memory come from? Because I learnt that T helper activates T cytotoxic but does T memory undergo clonal expansion and selection to form T cytotoxic and T helper?
Yes.
cytotoxic and helper T cells are seperate things. They both already exist in your body as naive Tc and Th cells. A Tc cell will be selected when it finds a cell presenting a fragment it can bind to, a Th cell will be selected when it is presented with a fragment from an APC. When that Th cell is costimulated by a B cell that has also bound a fragment, the Th cell with release interleukins, these interleukins are what causes all the differentiation and proliferation amongst B, Th, and Tc cells. Don't worry too much about remembering the precise details, you probably won't need to talk about Th cells helping Tc cells to divide and differentiate in the exam (but you might need to talk about them helping B cells do that).
2. Question 7.c from 2015 Exam.
In the rat pituitary gland, GC stimulates the production of the growth hormone protein. However, in the rat liver, GC stimulates the production of the enzyme tryptophan oxygenase.
Given that the genetic sequence is identical in all somatic rat cells, explain how the production ofdistinct proteins in different cell types could occur.
What i wrote:
I wrote about alternative splicing and how different exons can be left in and some exons can be skipped meaing that different sequences of exons can be produced so there are different sequences of DNA. So, different proteins will be produced due to there being different amino acids having been coded for. Is this answer valid for this question?
haha I was just thinking about this question earlier today. It comes up every year and was answered incredibly badly when it was in the exam (something like 0.1/2).
The VCAA suggested answer is fairly useless too. You're in the right sort of area, but you haven't said why these would occur differently in different cells. VCAA says that you need to say that there are different transcription factors affecting it, and that's all they've said. I don't think that would get you 2 marks though.
To get the two marks you'd probably have to talk about there being different transcription factors in each cells due to them being different cells and that those transcriptions factors could alter transcription in the way you've suggested.
The question (with their answer) is a bit crap imo. You're not told in VCE that transcription factors have anything to do with post transcription modifications, so it would be a bit unreasonable to expect you to link transcriptional factors to that. You might be expected to know that factors in different cells lead to differing post transcriptional modifications, but VCAA expects you to call them transcription factors, which you aren't taught to call them.
IMO the only sufficiently correct way to answer that question would be to talk about how the receptors are different in each of the cells (both are complimentary to the same signal, but the rest of the protein is different) and how that means that the receptor/signal complex binds to a different gene in each cell, therefore producing a different protein.
From the way the VCAA answer is worded, they're implying that they want you to understand that the specific genetic sequence of both proteins is identical, when the question could easily be interpreted to mean they're saying that all the DNA is the same in all cells. If they are referring to the one specific section of DNA being identical, then that^ answer would be incorrect as its saying they attach to different sections of DNA, which doesn't explain how "identical genetic sequence" could produce different proteins.
It's a crap question, don't worry about it too much.
3. Question 11 b from 2015 Exam
For the Out-of-Africa theory, did some populations of Homo sapiens migrate out of Africa and some populations were left which explains why there is no neanderthal DNA in modern day African people?
I'm a bit confused about this theory as I also heard that evolution in Africa occurred and then they moved out?
Out of africa does say that, the evolution in africa might be a different theory you're confusing it with?
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