VCE Biology Question Thread

[Dkontro]

Can someone please explain how the body distinguishes between self and non self?

[Dkontro]

Also, does the post translation modification of insulin occur in the RER or golgi. This was a MC choice question on my exam. The modification was glycosylation.

Thanks

[vox nihili]

Can someone please explain how the body distinguishes between self and non self?

This has been explained a few times here already, so don't be afraid to scroll a bit

Self molecules are those molecules not recognised by immune cells. Non-self molecules are those molecules recognised by immune cells. This really is the fundamental difference between the two. Immune cells express receptors to non-self molecules and don't express receptors to self. That's how it tells the difference*

*obviously there's a lot more complexity here, but it would be well and truly outside of the course

Also, does the post translation modification of insulin occur in the RER or golgi. This was a MC choice question on my exam. The modification was glycosylation.

Thanks

Golgi. I think that's probably a bit outside the course, although you should know in VCE that most post-transcriptional modifications do happen in the Golgi. This isn't actually true, but that's how it's presented in VCE unfortunately.

MHC Class I markers are on every nucleated cell in the body. These are NOT responsible for presenting antigens. These MHC Class I markers are 'tagged' or 'labelled', meaning that they have these specific MHC markers so that all the other cells of the body can recognise them as self, and so they do not get destroyed.

MHC Class II markers are found on B Lymphocytes, T lymphocytes, Macrophages and Dendritic cells. These cells actually have both MHC Class I and MHC Class II. These Cells engulf foreign pathogens, digest them with their lysosomes, and present antigenic fragments on the Class II markers.

Just going to pop in here. This description of MHC class I markers is completely wrong, sorry. They do actually present antigens. I'm not sure you need this knowledge for your VCE exams, but MHC class I molecules present antigens from within the cell. So if you've got a viral infection, viral antigens are presented on MHC class I molecules. They're on all nucleated cells because all nucleated cells can be infected by viruses. CD8+ T-cells see the antigen expressed on the MHC class I and blast the shit out of the cell as a consequence.

Also really important to note that MHC molecules only present to T-cells. B-cells bind free antigen.

No one answered  me

What happens when an action potential passes along an axon in terms of the ions coming in and out? I know that Sodium ions rush in, and Potassium ions rush out. Because the sodium ions are going from the outside the neuron and into it, from a high concentration to a low one, would this be considered facilitated diffusion? My teacher says it's active transports, because energy is required, but how can energy be required if the ions are going from high to low concentration?

Thanks guys

Active transport plays no role in the propagation of an action potential along the axon. What jyodesh.com said is completely correct, though out of the course. Pumps set up concentration gradients in neurones, but it is actually diffusion that leads to the propagation of an action potential along the axon.

How do phagocytes such as macrophages know exactly where/what a foreign pathogen is? Is it because the MHC markers of the phagocyte detect the foreign antigens on the pathogen, and hence engulf it? Also, if this is true (not sure, so what im gonna say might also be wrong), is phagocytosis non-specific, although it responds to antigens, because every response is the same to any type of antigen? Like whether it's a bacterium, or a virus, it would still engulf it the same way, so it's non-specific. But specific immunity is when SPECIFIC antibodies are made for an antigen?

My main question is if you cannot be bothered reading that above : do phagocytes interact with antigens, and upon not recognising them, engulf the pathogen?

Thank you

Phagocytes have receptors for a lot of different things that tell them that they're dealing with a pathogen. Cells of the innate immune system are non-specific, but they are still able to recognise molecules associated with lots of pathogens and therefore they can generally tell whether something is or isn't an invader.
Furthermore, phagocytosis is also mediated by antibodies. Phagocytes express receptors that allow them to sense cells that have been bound by antibodies. The antibodies actually help to identify these cells and also aid the process of phagocytosis.

[cosine]

This has been explained a few times here already, so don't be afraid to scroll a bit

Self molecules are those molecules not recognised by immune cells. Non-self molecules are those molecules recognised by immune cells. This really is the fundamental difference between the two. Immune cells express receptors to non-self molecules and don't express receptors to self. That's how it tells the difference*

*obviously there's a lot more complexity here, but it would be well and truly outside of the course

Golgi. I think that's probably a bit outside the course, although you should know in VCE that most post-transcriptional modifications do happen in the Golgi. This isn't actually true, but that's how it's presented in VCE unfortunately.

Just going to pop in here. This description of MHC class I markers is completely wrong, sorry. They do actually present antigens. I'm not sure you need this knowledge for your VCE exams, but MHC class I molecules present antigens from within the cell. So if you've got a viral infection, viral antigens are presented on MHC class I molecules. They're on all nucleated cells because all nucleated cells can be infected by viruses. CD8+ T-cells see the antigen expressed on the MHC class I and blast the shit out of the cell as a consequence.

Also really important to note that MHC molecules only present to T-cells. B-cells bind free antigen.

Active transport plays no role in the propagation of an action potential along the axon. What jyodesh.com said is completely correct, though out of the course. Pumps set up concentration gradients in neurones, but it is actually diffusion that leads to the propagation of an action potential along the axon.

Phagocytes have receptors for a lot of different things that tell them that they're dealing with a pathogen. Cells of the innate immune system are non-specific, but they are still able to recognise molecules associated with lots of pathogens and therefore they can generally tell whether something is or isn't an invader.
Furthermore, phagocytosis is also mediated by antibodies. Phagocytes express receptors that allow them to sense cells that have been bound by antibodies. The antibodies actually help to identify these cells and also aid the process of phagocytosis.

So what is the difference between mhc class II and mhc class I ??

[cosine]

For the ones who said that the passage of Na+ ions and K+ ions is passive, why does the answer for this question say it's C and not D?

[grannysmith]

For the ones who said that the passage of Na+ ions and K+ ions is passive, why does the answer for this question say it's C and not D?

The initial exchange of sodium/potassium ions is passive; however, there are Na⁺/K⁺ pumps which restore the voltage potential across the cell membrane once the action potential has passed. In this case, C is more correct since D could be referring to either facilitated diffusion or active transport. But yes, this question is quite ambiguous.

[cosine]

The initial exchange of sodium/potassium ions is passive; however, there are Na⁺/K⁺ pumps which restore the voltage potential across the cell membrane once the action potential has passed. In this case, C is more correct since D could be referring to either facilitated diffusion or active transport. But yes, this question is quite ambiguous.

I agree with this, but the question says when the cell is stimulated, not depolarising?
But I do see where you're coming from

[cosine]

When it says we have 23 pairs of chromosomes, or 46 in each somatic cell, what does this even mean? Why do they always say 46 or 23 pairs, whats the difference? And you know how chromosomes can be like double stranded or single stranded, what's the difference between the two, and why are some single and some double?

Thanks

[Dkontro]

So what is the difference between mhc class II and mhc class I ??

If an MHC type 1 presents an antigen, it will be destroyed. If an MHC type 2 presents an antigen, A T helper cell will bind to to it and then activate B cells or cytotoxic T cells

[cosine]

In regards to photosynthesis, do we need to know the reactions that occur in the Calvin Benson cycle? Like conversion of CO2 into ... ?

Or just the main inputs, outputs and the location of the reactions?

Thanks

[Garden]

When it says we have 23 pairs of chromosomes, or 46 in each somatic cell, what does this even mean? Why do they always say 46 or 23 pairs, whats the difference? And you know how chromosomes can be like double stranded or single stranded, what's the difference between the two, and why are some single and some double?

Thanks

A somatic cell is any cell of the body except sperm and egg cells.

In humans, each somatic cell contains 23 pairs of chromosomes, for a total of 46. Cells containing 46 chromosomes, or 23 pairs, are called diploid cells.

Chromosomes may exist in either a duplicated, or unduplicated form. Unduplicated, single stranded chromosomes, consist of one chromatid, attached by a centromere. Duplicated, double stranded chromosomes, consist of two chromatid, attached by a centromere.

During the S phase of the cell cycle, single stranded chromosomes are replicated and become double stranded chromosomes.

[BakedDwarf]

Does the influenza virus cause inflammation?

[BakedDwarf]

In regards to photosynthesis, do we need to know the reactions that occur in the Calvin Benson cycle? Like conversion of CO2 into ... ?

Or just the main inputs, outputs and the location of the reactions?

Thanks

Well, according to the study design, it states, "the main inputs and outputs of the light
dependent and light independent stages"
So i'm guessing, yes. Just the main inputs, outputs and location.

[vox nihili]

So what is the difference between mhc class II and mhc class I ??

MHC class I molecules are expressed on all nucleated cells. They present antigens that are derived from inside the cell itself. So if a virus infects a cell, a viral antigen will be expressed on the surface of an MHC class I cell.

MHC class II molecules are only expressed on antigen presenting cells. These cells take up antigens from their environment, process them and then present them on MHC class II molecules.

Therefore, cells that express antigen on MHC class I molecules are targeted for destruction; whereas, those that present on MHC class II molecules are certainly not targeted for destruction. Indeed, they're more likely to proliferate depending on the cell type.

For the ones who said that the passage of Na+ ions and K+ ions is passive, why does the answer for this question say it's C and not D?

Because the people writing the question got it wrong. I know people have already explained the questioner's thinking behind the question, but simply, they were wrong. D is absolutely correct. On activation, sodium and potassium move through in a passive manner. There is really no ambiguity about that. The sodium and the potassium pump always works to keep the concentration gradient in check, but it does not respond to activation of the neurone.

[grannysmith]

MHC class I molecules are expressed on all nucleated cells. They present antigens that are derived from inside the cell itself. So if a virus infects a cell, a viral antigen will be expressed on the surface of an MHC class I cell.

MHC class II molecules are only expressed on antigen presenting cells. These cells take up antigens from their environment, process them and then present them on MHC class II molecules.

Therefore, cells that express antigen on MHC class I molecules are targeted for destruction; whereas, those that present on MHC class II molecules are certainly not targeted for destruction. Indeed, they're more likely to proliferate depending on the cell type.

Because the people writing the question got it wrong. I know people have already explained the questioner's thinking behind the question, but simply, they were wrong. D is absolutely correct. On activation, sodium and potassium move through in a passive manner. There is really no ambiguity about that. The sodium and the potassium pump always works to keep the concentration gradient in check, but it does not respond to activation of the neurone.

Goes to show how VCAA does indeed get it wrong sometimes

Funnily enough, it seems 68% of people chose option C, whereas only 16% chose D. Even more surprising is the fact that VCAA didn't pay the mark for D.