VCE Biology Question Thread

[Erutepa]

The Cristae has many folds which increase SA inside mitochondria. How is this increased SA useful for aerobic cell respiration?

What is the purpose of the hydrogen ion concentration gradient in ETC?

1. The cristae facilitate the ETC. Thus, the larger SA of the cristae membrane means a greater number of electron transport chains can be facilitated. If there is a greater number of ETCs, then you would have a greater ATP production (assuming the supply of energy carriers NADH and FADH2 is not a limiting factor).

2. The hydrogen ion concentration gradient produced by the 'pumping' of hydrogen ions into the intermembrane space of the mitochondria's inner and outer membranes by the ETC, creates a flow of hydrogen ions through ATP synthase protein complexes (since ions can't just diffuse across a membrane). This movement of the hydrogen ions down the concentration gradient through this ATP synthase is exploited as the mechanical energy generated from the ion movement is used to synthesize ATP from ADP.

[Erutepa]

With convergent evolution, are the two species completely unrelated or distantly related?
Also, if you were asked to list out the steps of transcription, would you include post-transcriptional modification or does that occur after transcription? Thx!

1. technically speaking, no two species are 'completely unrelated' in that they most definitely share some common ancestor whether it be a few a few MYA or as the earliest forms of life. As such I would lean towards saying distantly related, however, I don't know if you would be necessarily marked wrong if you said unrelated.

[darkz]

Also, what do you mean by different reactions between adjacent R groups

e.g. some R groups may be hydrophobic/ hydrophilic/ form specific bonds e.g. only amino acids with sulfur can form disulfide bridges

Thanks, but if a smaller prokaryote got engulfed by a eukaryote, where did that eukaryote come from? I thought it would be small prokaryote (such as photosynthetic prokaryote) engulfed by larger prokaryote, and then this endosymbiont evolved into a eukaryote.

I'm pretty sure that in the case of chloroplast, it was basically that we had photosynthesising microbes. Then they were engulfed by a eukaryote and not digested

[galaxy21]

Smaller prokaryote got engulfed by a eukaryote

Thanks, but if a smaller prokaryote got engulfed by a eukaryote, where did that eukaryote come from? I thought it would be small prokaryote (such as photosynthetic prokaryote) engulfed by larger prokaryote, and then this endosymbiont evolved into a eukaryote.

I'm pretty sure that in the case of chloroplast, it was basically that we had photosynthesising microbes. Then they were engulfed by a eukaryote and not digested

I'm pretty sure PopcornTime is right - the smaller prokaryotes (i.e. mitochondria and chloroplasts) were engulfed by a larger prokaryote.
Check out this video from Amoeba Sisters (kinda childish but really cute and explains it all clearly ) - they are suggesting that the smaller prokaryote was engulfed by a larger prokaryote.
https://www.youtube.com/watch?v=FGnS-Xk0ZqU&vl=en

EDIT: In saying that, I have just looked up a little more, and this website (https://www.nature.com/scitable/topicpage/eukaryotic-cells-14023963) is suggesting that it was a eukaryote that engulfed it. I'll keep looking later when I have a chance and let you know what I find!

[galaxy21]

I'm pretty sure PopcornTime is right - the smaller prokaryotes (i.e. mitochondria and chloroplasts) were engulfed by a larger prokaryote.
Check out this video from Amoeba Sisters (kinda childish but really cute and explains it all clearly ) - they are suggesting that the smaller prokaryote was engulfed by a larger prokaryote.
https://www.youtube.com/watch?v=FGnS-Xk0ZqU&vl=en

EDIT: In saying that, I have just looked up a little more, and this website (https://www.nature.com/scitable/topicpage/eukaryotic-cells-14023963) is suggesting that it was a eukaryote that engulfed it. I'll keep looking later when I have a chance and let you know what I find!

Okay, so I think I have got it.
I have looked up a few website and videos and looked through some of the books and resources that I have at home, and most have conflicting statements as to whether it is a prokaryote or a eukaryote that is engulfing the mitochondria and chloroplasts - turns out, there are two conflicting theories of this - one being that it was engulfed by a prokaryote and the other by a eukaryote (check it out here - https://www.nature.com/scitable/topicpage/the-origin-of-mitochondria-14232356). The theory that I think makes the most sense is in this video..its long but a really good explanation of how prokaryotes evolved into eukaryotes, and (in my opinion at least), this theory makes the most sense. (https://www.youtube.com/watch?v=Enztyji4r8E).

This theory says that the cell that engulfed the mitochondria and chloroplasts is in a prokaryotic cell that is in the process of becoming a eukaryotic cell (or an "Early pre-eukaryotic cell", as Biozone refers to them). This does make sense, as part of the general definition for a eukaryotic cell is that it contains membrane-bound organelles, and the incorporation of mitochondria and chloroplasts into the cell would make it a eukaryote.
The video above the whole process out really clearly, but in summary, the process of a cell moving from prokaryote to eukaryote is as follows:
1. A prokaryotic cell loses it's cell wall, leaving just the flexible plasma membrane that we see in many modern eukaryotes.
2. This increased flexability of the membrane allows for infolding (basically the plasma membrane folding in on itself). Because of this, internal membranes can begin to form. These start to move around the DNA, forming a nucleus region in the cell, that is bound by a nuclear envelope.
3. The membrane flexibility also allows for endocytosis to occur.
4. Cytoskeleton forms. This allows the skell to change shape and distribute daughter chromosomes during mitosis, as well as allowing for better movement in the cell as it increases in size. Flagellum also forms, allowing for cell movement.
At this stage, due to the membrane-bound nucleus, this could probably be considered to be eukaryotic, however, at this stage, there are no membrane-bound organelles.
Now, regarding the endosymbiosis bit...
We only need to know about chloroplasts and mitochondria as being engulfed, but it's also good to be aware that there were other organelles in eukaryotes that were not originally there, like peroxisomes. So...
5. What we now call mitochondria is engulfed by our larger cell.
At this stage, we can consider the basic evolution of the eukaryotic cell to be "complete". But of course...
6. What we now call chloroplasts are engulfed.

And that seems to be it! Most of this stuff about how the eukaryotic cell evolved we do not need to know about, but in my opinion, at least, it gives a better, deeper understanding of how it all works. So to answer your question, I'd say it's not necessarily a prokaryote or a eukaryote cell that is engulfing the mitochondria and chloroplasts  - it's a cell that is evolved to a stage where it is not really considered a prokaryote. Don't quote me on it, but because there is so much confusion and uncertainty on the topic, I would doubt that they would directly ask a question as to whether it is a prokaryotic or eukaryotic cell that is engulfing, and if they do ask for a definition of endosymbiosis, you probably could get away with saying a larger cell (maybe  ).
Hope that kind of helps and clears things up.

[Scribe]

Quick question. On the exam, do we write that the electron transport chain produces 32-34 ATP or do we state a specific number, either 32 or 34?
Thanks!

Edit: Also, do we need to memorise the exact quantities of each input and output of cellular respiration (eg. oxygen, FADH, NADH, etc.), or do we just need the ATP yield at each stage?

[lacitam]

I think ETC makes 32 ATP because, as I was taught, the glycolysis stage makes 4-6 ATP (you need to check with your teacher on this)

Yes, you need to know the inputs and outputs. Not sure about ATP yield but I was taught that you should know this.

Quick question. On the exam, do we write that the electron transport chain produces 32-34 ATP or do we state a specific number, either 32 or 34?
Thanks!

Edit: Also, do we need to memorise the exact quantities of each input and output of cellular respiration (eg. oxygen, FADH, NADH, etc.), or do we just need the ATP yield at each stage?

[Erutepa]

Quick question. On the exam, do we write that the electron transport chain produces 32-34 ATP or do we state a specific number, either 32 or 34?
Thanks!

Edit: Also, do we need to memorize the exact quantities of each input and output of cellular respiration (eg. oxygen, FADH, NADH, etc.), or do we just need the ATP yield at each stage?

The reason why we quote that the ETC produces 32-34 ATP is (as I have read) due to the potential conversion of the 2NADH from glycolysis into 2FADH2 when being shuttled across the membrane from the cytosol (where NADH is produced) into the mitochondria. So if 2 NADH is shuttled across such that it remains 2 NADH, then 34 ATP is made (as an NADH will allow for greater ATP production than an FADH2 molecule); and if the NADH is shuttled across to become FADH2, then 32 ATP will result from the ETC.
I have also heard some statements that the difference in ATP is due to sometimes 2 ATP being used somewhere in the reaction pathway occasionally, but I am not sure about this. Maybe someone a bit more knowledged can weigh in.
So it really is correct to say 32-34.

About memorizing the numbers of the inputs and outputs, personally, I have, although I have been told in revision lectures that we don't need to. I guess it can't hurt, but I don't think it is essential. Knowing the number of ATP output at each stage is important though.

[PhoenixxFire]

Quick question. On the exam, do we write that the electron transport chain produces 32-34 ATP or do we state a specific number, either 32 or 34?
Thanks!

Edit: Also, do we need to memorise the exact quantities of each input and output of cellular respiration (eg. oxygen, FADH, NADH, etc.), or do we just need the ATP yield at each stage?

-pick either 32 or 34. Either are accepted as correct answers but I’ve heard that VCAA doesn’t like ranges.

-Definitely need to know the ATP yield. You should also be able to write a balanced equation for each of the stages. Not sure on whether you need to know numbers of FADH or NADH. It’s not explicitly in the study design and there haven’t been any questions on it so far so I would say that it’s unlikely.

[I\'m Not A Robot]

Hey, i'm kind of confused with how glucose crosses the cell membrane.
I was pretty sure that it crossed via facilitated diffusion (passive) through a specific carrier protein... but on the 2014 exam, it is stated that it crosses via channel proteins. I asked my teacher about it and she said to just say channel proteins because it seems like that's the answer that VCAA want. Could someone clear this up...even my textbook and many other revision notes from external companies state that it crosses via a carrier protein sooo... whats up with that? What are we meant to write??

Thanks

[Bell9565]

Hey, i'm kind of confused with how glucose crosses the cell membrane.
I was pretty sure that it crossed via facilitated diffusion (passive) through a specific carrier protein... but on the 2014 exam, it is stated that it crosses via channel proteins. I asked my teacher about it and she said to just say channel proteins because it seems like that's the answer that VCAA want. Could someone clear this up...even my textbook and many other revision notes from external companies state that it crosses via a carrier protein sooo... whats up with that? What are we meant to write??

Thanks

The answers on the VCAA website are often just one of many accepted answers. If you wrote it was a carrier protein I'd assume you'd get the marks. I wonder if that was an accepted answer by VCAA (even though it may be technically wrong or correct - idk) as glucose is a hydrophillic molecule therefore could hypothetically move through the pore in a protein channel. I'd still write carrier protein BUT as glucose is no longer on the study design (like lipids) I wouldn't assume there would be a question asking whether glucose travelled through what type of facilitated diffusion. I'd just know the general rules and a couple of examples for each type of transport.

[I\'m Not A Robot]

The answers on the VCAA website are often just one of many accepted answers. If you wrote it was a carrier protein I'd assume you'd get the marks. I wonder if that was an accepted answer by VCAA (even though it may be technically wrong or correct - idk) as glucose is a hydrophillic molecule therefore could hypothetically move through the pore in a protein channel. I'd still write carrier protein BUT as glucose is no longer on the study design (like lipids) I wouldn't assume there would be a question asking whether glucose travelled through what type of facilitated diffusion. I'd just know the general rules and a couple of examples for each type of transport.

Oh okay, i didn't even know they had taken that out of the study design. Thanks heaps though!

[peachxmh]

Do restriction enzymes cut the hydrogen bonds between nitrogenous bases as well as the covalent bonds in the sugar-phosphate backbone? Just wanted to clarify bc I've read a few sources online where some say restriction enzymes only cut covalent bonds but others say they cut both.

[PhoenixxFire]

Do restriction enzymes cut the hydrogen bonds between nitrogenous bases as well as the covalent bonds in the sugar-phosphate backbone? Just wanted to clarify bc I've read a few sources online where some say restriction enzymes only cut covalent bonds but others say they cut both.

Restriction enzymes only cut the covalent binds, a few hydrogen bonds aren’t strong enough to keep the strands attached so they seperate too. This is also why sticky ends are called sticky - the hydrogen bonds temporarily bind ‘stick’ long enough for ligase to create covalents bonds whereas blunt ends up less likely to be reattached because they won’t join each other.

[Bell9565]

Oh okay, i didn't even know they had taken that out of the study design. Thanks heaps though!

Just to clarify, you need to know it in context of glucose in cellular respiration and photosynthesis but not as in the in depth knowledge that used to be taught about carbohydrates (eg the structure of more complex carbohydrates and the level of detail known for proteins)