VCE Biology Question Thread

[vox nihili]

Thank you but I wasn't really asking for the process. Sorry if my wording was a little bit off but I wanted to know how many protons are used to make a single ATP. As in when the hydrogen ions pass through ATP synthase how many need to go through to form one ATP molecule. So the ratio would either be 18:24 or 18:12. so 3 ATP : 4 H+ ions or 3 ATP : 2 H+ ions. Do all the hydrogen ions pass through the ATP synthase? Including the 12 that are left behind? If so then we would have 3 ATP molecules for every 4 hydrogen ions or 1 ATP for 0.75 of a proton which sucks really :/

My very sincere suggestion here would be to avoid trying to track the protons, not least because it's not at all required in VCE.

To put your minds at ease though, it's four protons per ATP.

[Maddie Trinh]

If the plasma membrane is amphipathic, then how can lipid-soluble molecules dissolve in the lipid bilayer? They have to cross the hydrophilic phosphate heads first in order to get to the fatty acid tails. It is also the same from the intercellular side of the membrane; they are going to get repelled by the inner hydrophilic heads as well.
Guys, how can any substance cross the plasma membrane at all (unless that same substance is also amphipathic)? And why do small molecules, like water, cross the membrane freely? Isn't water a polar substance?!!

Basically the main part to remember about the plasma membrane is that it's a fluid structure hence 'the fluid mosaic model.' The components which form the cell membrane (such as integral proteins, the phospholipid bilayer, glycolipids etc.) are not static but shift and move within the confines of the cell membrane. A good way to envision this is to think of a soap bubble and focusing on how the outer layer moves around or if you want a legit excuse to watch a youtube video, watch giant bubbles popping in slow motion by the Slow Mo Guys. Essentially, as the phospholipid layers are shifting around they create little gaps between the hydrophilic heads allowing small molecules such as (O2) and (CO2) to simply diffuse through the phospholipid bilayer of the cell membrane.

I also understand the confusion at water D:<. However because water is so small, it can sneakily diffuse through the cell membrane due to osmosis albeit not very well due to being polar. To make up for this, cells actually have specific protein channels called aquaporins which allow water to quickly move into or out of the cell but for the purpose of VCE you don't need to remember this but it's some nice little information. Well I hope that answers ur questions XD

Also for the main reason why I came here, are there any major examples of autocrine signalling outside of cytokine interleukin-1 used by monocytes?   
Thanks 

[vox nihili]

Basically the main part to remember about the plasma membrane is that it's a fluid structure hence 'the fluid mosaic model.' The components which form the cell membrane (such as integral proteins, the phospholipid bilayer, glycolipids etc.) are not static but shift and move within the confines of the cell membrane. A good way to envision this is to think of a soap bubble and focusing on how the outer layer moves around or if you want a legit excuse to watch a youtube video, watch giant bubbles popping in slow motion by the Slow Mo Guys. Essentially, as the phospholipid layers are shifting around they create little gaps between the hydrophilic heads allowing small molecules such as (O2) and (CO2) to simply diffuse through the phospholipid bilayer of the cell membrane.

I also understand the confusion at water D:<. However because water is so small, it can sneakily diffuse through the cell membrane due to osmosis albeit not very well due to being polar. To make up for this, cells actually have specific protein channels called aquaporins which allow water to quickly move into or out of the cell but for the purpose of VCE you don't need to remember this but it's some nice little information. Well I hope that answers ur questions XD

Also for the main reason why I came here, are there any major examples of autocrine signalling outside of cytokine interleukin-1 used by monocytes?   
Thanks 

Good explanation. The same holds true for hydrophobic substances. Sometimes they'll just randomly get past the heads, even though it's not really what they like to do, and then it's all happiness inside the membrane basically.


There are plenty, but you don't need to know any examples.

[haydencooper]

Hoping someone can help me with these SAC related questions!

1. Stimulation of certain regions of the mechanosensory region can receive and respond to a stimulus without any interneurons being activated (such as giving a sharp poke to the nose of the planarian which causes it to rapidly contract its body, retracting from the stimulus). What is the name of this response? What advantage would this type of response have?
-I thought this may be negative thigmotaxis

2. Explain how a single odorant molecule given off by food could be detected by a cell and therefore elicit a well co-oridnated response such as the planarian moving to the food (and provide name of process)
-I thought this may have to do with signal transduction and a chemical response to the odorant

3. A steroid hormone (hormone z) was produced by a cell (cell z). The hormone travels from its site of production to a different cell (cell x), which is located elsewhere in the body of the planaria. Explain how the characteristics of hormone z will influence the way in which it initiates a response in cell x

[HasibA]

what does 'antagonises auxins on leaf buds' mean in the context of VCE biology- specifically related to cytokines! thank you

[johntran1869]

HI just stuck on this question:
Where on the Plant (not plant cell) does respiration occur?

Thank you

[wobblywobbly]

what does 'antagonises auxins on leaf buds' mean in the context of VCE biology- specifically related to cytokines! thank you

You won't need to know much about each type of plant hormone specifically for VCE Biology, so I'm assuming this is a SAC related question. Just know that cytokinins (not cytokines - which is part of the immune system, not a plant hormone) signals for lateral bud growth, which is kind of the opposite of what auxin does, which is to promote apical dominance, inhibiting lateral bud growth. So when they say 'antagonises' they mean 'opposite in effect'.

HI just stuck on this question:
Where on the Plant (not plant cell) does respiration occur?

Thank you

Do all plant cells have mitochondria?

[QueenSmarty]

Draw a stimulus-response diagram to illustrate how an unripe tomato can be induced to ripen using ethylene gas.

Can someone please help me with this question from the Nelson Biology textbook? I know how to draw a stimulus-response model, but would the stimulus be the ethylene gas? And what would the receptor, coordinating centre and effector be?

[Jay.C]

Hoping someone can help me with these SAC related questions!

1. Stimulation of certain regions of the mechanosensory region can receive and respond to a stimulus without any interneurons being activated (such as giving a sharp poke to the nose of the planarian which causes it to rapidly contract its body, retracting from the stimulus). What is the name of this response? What advantage would this type of response have?
-I thought this may be negative thigmotaxis

2. Explain how a single odorant molecule given off by food could be detected by a cell and therefore elicit a well co-oridnated response such as the planarian moving to the food (and provide name of process)
-I thought this may have to do with signal transduction and a chemical response to the odorant

3. A steroid hormone (hormone z) was produced by a cell (cell z). The hormone travels from its site of production to a different cell (cell x), which is located elsewhere in the body of the planaria. Explain how the characteristics of hormone z will influence the way in which it initiates a response in cell x

Hey!
1. The lack of interneurons gives us a clue that this response is probably a reflex arch. As for benefits, reflex arch's allows for a very quick response to stimulus (due to the brain not being involved), thus this response can act as a protective factor against harmful stimuli e.g. if you touch something hot and thus reduces damage to the organism.   
2. I feel like there may be people in this forum better equipped to answer this in more detail then I can so ill leave it to them .
3. Hormones can be both hydrophobic and hydrophilic and thus both types will initiate a response differently in cell X. Thus because it is a steroid hormone we know that it will be hydrophobic and be able to pass through the cell membrane therefore initiating its response through a receptor located in the cell and not on the cell membrane.

[thefrog]

Just wondering if someone can answer a few queries I have

1. Why do some cells produce 36 ATP via aerobic respiration while others produce 38? (Some sources say that the extra 2 ATP comes from glycolysis while others say that it comes from the electron transport chain)
2. Why is it that if oxygen is not present, the Krebs cycle (as well as the electron transport chain) comes to a stop?
3. Can I say that hormones are released into extracellular fluid instead of the bloodstream? (Is there a difference?)
4. Does signal transduction happen with lipid-based hormones? I'm pretty sure it does but could someone just briefly explain the "cascade of events" that occurs?
5. Regarding apoptosis: where does the extrinsic signal come from?

Thank-you in advance!

[vox nihili]

Just wondering if someone can answer a few queries I have

1. Why do some cells produce 36 ATP via aerobic respiration while others produce 38? (Some sources say that the extra 2 ATP comes from glycolysis while others say that it comes from the electron transport chain)
2. Why is it that if oxygen is not present, the Krebs cycle (as well as the electron transport chain) comes to a stop?
3. Can I say that hormones are released into extracellular fluid instead of the bloodstream? (Is there a difference?)
4. Does signal transduction happen with lipid-based hormones? I'm pretty sure it does but could someone just briefly explain the "cascade of events" that occurs?
5. Regarding apoptosis: where does the extrinsic signal come from?

Thank-you in advance!

1. In some cells the ETC is more efficient than others, hence the difference. I highly doubt that it's because of different ways of doing glycolysis.

2. Because NADH/FADH₂ aren't being used by the ETC, this feeds back and halts their production by the Kreb's cycle. Not necessary to know this in VCE though, of course and it is a little more complicated than I've explained, but this should do.

3. Absolutely not. Extracellular fluid is the fluid in which all tissues are bathed, whereas blood is fluid contained within the lumina (sing. lumen, just means a hollow tube) of blood vessels. Hormones are exclusively released into blood vessels and not extracellular fluid (technically hormones can be released into both, but they're not acting as a hormone if they're in the tissue, they're acting as a paracrine in that context—basically, paracrines and hormones can be the same molecule, just in different places).

4. It does. Lipid hormone comes in, binds its receptor and the receptor moves to the nucleus where it helps to transcribe genes. Post-VCE a lot more information is added to this, but you'll just have to accept that this is signal transduction even if it doesn't have the stages you're used to.

5. Any number of places. Viruses. Damaged cells. Immune cells. It's just a signalling molecule, much like a hormone actually.

[TheAspiringDoc]

For interest: Wikipedia says that it is actually in the ETC that the amount of ATP produced varies, and this is because the 2 NADH from glycolysis can be shuttled across the mitochondrial membrane by either glycerol phosphate or malate-aspartate - of which the latter is more efficient. However, the malate-aspartate shuttle is only used in some cells (hence the variety)
And yeah this is paraphrased.

[thefrog]

1. In some cells the ETC is more efficient than others, hence the difference. I highly doubt that it's because of different ways of doing glycolysis.

2. Because NADH/FADH₂ aren't being used by the ETC, this feeds back and halts their production by the Kreb's cycle. Not necessary to know this in VCE though, of course and it is a little more complicated than I've explained, but this should do.

3. Absolutely not. Extracellular fluid is the fluid in which all tissues are bathed, whereas blood is fluid contained within the lumina (sing. lumen, just means a hollow tube) of blood vessels. Hormones are exclusively released into blood vessels and not extracellular fluid (technically hormones can be released into both, but they're not acting as a hormone if they're in the tissue, they're acting as a paracrine in that context—basically, paracrines and hormones can be the same molecule, just in different places).

4. It does. Lipid hormone comes in, binds its receptor and the receptor moves to the nucleus where it helps to transcribe genes. Post-VCE a lot more information is added to this, but you'll just have to accept that this is signal transduction even if it doesn't have the stages you're used to.

5. Any number of places. Viruses. Damaged cells. Immune cells. It's just a signalling molecule, much like a hormone actually.

Thanks so much, this really helps

For interest: Wikipedia says that it is actually in the ETC that the amount of ATP produced varies, and this is because the 2 NADH from glycolysis can be shuttled across the mitochondrial membrane by either glycerol phosphate or malate-aspartate - of which the latter is more efficient. However, the malate-aspartate shuttle is only used in some cells (hence the variety)
And yeah this is paraphrased.

Thanks for the insight!

[vox nihili]

For interest: Wikipedia says that it is actually in the ETC that the amount of ATP produced varies, and this is because the 2 NADH from glycolysis can be shuttled across the mitochondrial membrane by either glycerol phosphate or malate-aspartate - of which the latter is more efficient. However, the malate-aspartate shuttle is only used in some cells (hence the variety)
And yeah this is paraphrased.

Worst part of this is that I know I've covered this at some point....

Thanks for letting us know, that's interesting

[johnhalo]

Following on from the previous question, can someone explain clearly the signal transduction of hydrophilic (protein) hormones? My textbook (Nature of Biology) explains the role of G proteins different to what other sources say, and I'm quite confused, so a clear explanation would be wonderful.

Thanks