VCE Biology Question Thread

[Tyleralp1]

So the cells which T-helper cells stimulate to undergo colonal expansion are T-Cells, which then go onto produce Cytotoxic T-cells and memory cells? So it is not cytotoxic cells which get stimulated directly by T-helper cells?

thanks

From my understanding, yes.

However, I will stand to be corrected if that is the case.

[shivaji]

From my understanding, yes.

However, I will stand to be corrected if that is the case.

thanks! that actually makes perfect sense:D

[katiesaliba]

So in cell-mediated immunity, after the cytotoxic t-cell is activated by a Th cell, the Tc celll undergoes clonal expansion to produce more cytotoix t cells (effector T cells) and also some T-memory cells as well?

Yep! Prior to activation, cytotoxic T-cells are considered 'naive' (like unactivated B-cells).

The Cell Mediated Response.

1. Pathogen/Antigen is detected as non-self and harmful.
2. Phagocytes (Macrophages) migrate to the site of infection and engulf the pathogen, breaking it down.
3. It presents an antigen fragement on its MHC Class II markers.
4. T Helper cells recognise this antigen, and stimulate the T cells to undergo clonal expansion and divide into Cytotoxic T Cells.
5. They move to the pathogen and release toxic granules to destroy it. Memory T cells are formed in the process for future occurences, and suppresor T cells supress the immune response.

Helper-T lymphocytes need to become activated (by the means that you suggested) and then they can stimulate cytotoxic T-cells...that is, if they are bound to their complementary antigen! Interleukin-2 is the stimulatory cytokine.
'General' T-cells do not undergo clonal expansion to form cytotoxic and memory T-cells; rather it is the naive cytotoxic t-cells (CD8+) which proliferate and differentiate. This is unlike clonal expansion in B-cells, as naive B-cells act as a precursor to their plasma and memory counterparts. I've attached a screenshot from my textbook which establishes this point nicely!

So the cells which T-helper cells stimulate to undergo colonal expansion are T-Cells, which then go onto produce Cytotoxic T-cells and memory cells? So it is not cytotoxic cells which get stimulated directly by T-helper cells?

thanks

Cytotoxic T-cells are stimulated directly by helper-T Cells. It might be beneficial to know that Cytotoxic-T cells are also called 'CD8+' cells, or something. I dunno, I watch a lot of youtube videos for bio and it always seems to come up 

Also, according to NOB, suppressor T-cells are another kind of T-cell, therefore they are not created during the clonal expansion of Tc cells. So, all up there are 3 kinds of T-cells: helper, suppressor and cytotoxic!

[kk.08]

How do you count the number of chromosomes and chromatids in a cell? I know that for the number of chromosomes you count the number of centromeres, but what about chromatids...what about if you had two strands would that be considered 1 chromatid or 2?

Also, what's the difference between a centromere and kinetochore?

Thanks!

[Russ]

Just a couple of potentially confusing statements here.

I don't really understand how infected cells signal cytotoxic T cells?

Once infected with the pathogen do they break down the antigens of the pathogen and display it on the surface of their MHC class 1 markers?

thanks!

This is correct. Cytotoxic cells need a way to identify infected cells and they rely upon MHC I display of antigenic fragments.

The Cell Mediated Response.

1. Pathogen/Antigen is detected as non-self and harmful.
2. Phagocytes (Macrophages) migrate to the site of infection and engulf the pathogen, breaking it down.
3. It presents an antigen fragement on its MHC Class II markers.
4. T Helper cells recognise this antigen, and stimulate the T cells to undergo clonal expansion and divide into Cytotoxic T Cells.
5. They move to the pathogen and release toxic granules to destroy it. Memory T cells are formed in the process for future occurences, and suppresor T cells supress the immune response.

This is correct as well, but doesn't mean the previous bit about CD8/MHC I is incorrect.

So the cells which T-helper cells stimulate to undergo colonal expansion are T-Cells, which then go onto produce Cytotoxic T-cells and memory cells? So it is not cytotoxic cells which get stimulated directly by T-helper cells?

thanks

CD4 (T helper) cells do stimulate CD8 (Cytotoxic or T killer) cells to have their effects

It might be beneficial to know that T-helper cells are also called 'CD8+' cells, or something.

Th are called CD4 and Tc are called CD8 (easy to confuse, I know)

[katiesaliba]

Th are called CD4 and Tc are called CD8 (easy to confuse, I know)

Oh oppsy ahaha I meant Tc cells were CD8+ not Th! My bad 

[RazzMeTazz]

I came across a practice question which stated that the purpose of the bacterial cell wall is structure and protection.

Does the protection part apply to plant cell walls too?

[shivaji]

I came across a practice question which stated that the purpose of the bacterial cell wall is structure and protection.

Does the protection part apply to plant cell walls too?

yes.  one form of protection is that it prevents the cell from lysing as the cell wall will prevent the cell from taking up too much water by osmosis. not sure, there are probably other protective reasons too

[grannysmith]

I came across a practice question which stated that the purpose of the bacterial cell wall is structure and protection.

Does the protection part apply to plant cell walls too?

Yes, it provides protection, structure and limits the expansion of the plant cell membrane, preventing it from lysing.

[RazzMeTazz]

Would this be a sufficient explanation of the intrinsic pathway which triggers apoptosis ?

1.) Cytochrome C is released from the mitochondria of the cell
2.) As a result caspases (protein eating enzymes) are activated and lysosomes and ruptured.
3.) The structural and functional units of the cell are broken down
4.) The dying cell displays bleps on it's surface which are recognized by a padding phagocyte which consumes the dying cell

Thanks! :)

[Jason12]

what's polyploidy? polymorphism? I get the two mixed up sometimes

[katiesaliba]

what's polyploidy? polymorphism? I get the two mixed up sometimes

Polyploidy is a condition in which an individual organism has more than two full sets of chromosomes in its somatic cells (e.g. 3n, 4n, 5n...)

Polymorphism (poly=many, morph=form) is when two or more different phenotypes are present within the same population of a certain species.

[anat0my]

For Exam 1 of 2007, question 7, why can't alternative B also be correct? Thanks!

[RazzMeTazz]

In a reflex arc which neurones are involved ? I thought it was :

1.) Sensory neurone from receptor to spinal cord
2.) Interneurone transmitting message from sensory neurone to motor neurone
3.) Motor neurone stimulating effector

but some sources just say a sensory neurone and motor neurone are involved ?

[vox nihili]

In a reflex arc which neurones are involved ? I thought it was :

1.) Sensory neurone from receptor to spinal cord
2.) Interneurone transmitting message from sensory neurone to motor neurone
3.) Motor neurone stimulating effector

but some sources just say a sensory neurone and motor neurone are involved ?

Both are correct. A reflex arc can involve a monosynaptic (motor to sensory) pathway or a polysynaptic pathway (interneurone in between). Obviously that's not info you need to know.