VCE Biology Question Thread

[Evolio]

Hi pugs!
Cytokines are hydrophilic signalling molecules that are involved in the immune response. For eg, T helper cells release cytokines to B cells activating them to differentiate into plasma b cells and memory b cells.
However, cytotoxins are harmful chemicals(toxins) that kill cells. For example they may be released by cytotoxic T cells to kill infected cells in the cell-mediated response.
I hope this helps. 
Please correct me if I'm wrong!

[vox nihili]

Thank you Owlbird83, DBA-144 and vox nihili! 

Also, for the first question I meant in relation to aerobic cellular respiration, not photosynthesis i.e intermembrane space in mitochondria. I just don't understand what happens to the H+ ions after the proton gradient is established.
Sorry for not being clear !

It's actually the same thing that happens in the mitochondria and the thylakoid (but that's beyond VCE), likewise the purpose of the gradient is beyond VCE too.

For those interested (not VCE knowledge):

The purpose of the proton (H+) gradient is to drive an enzyme called ATP synthase. This is the enzyme that creates ATP. Basically, ATP synthase acts as a proton channel, allowing the protons to flow through the membrane from an area of high concentration to an area of low concentration. When they flow through, they spin a wheel inside the enzyme on the way through, which then turns a crank that smashes a phosphate onto ADP. The best analogy of this is a watermill, like this one:



When water flows down the river, it spins the wheel. The force of spinning the wheel is then used to power the mill inside, which grinds the grain. The force of the spinning wheel is transitioned to the mill inside via an axle, which is exactly the same as how the enzyme works. When hydrogen ions flow past (coming down their concentration gradient), the energy of that flow powers a wheel which then powers a mechanism that combines ADP and phosphate.

[Evolio]

Thank you vox nihili!
I really appreciate your help!

I had some more questions.
So, do Cytotoxic T cells release cytokines eg perforin that initate apoptosis in cell-infected cells? Is this correct?

Also, in apoptosis, are all the cell organelles destroyed?

What is the difference between MHC I and MHC II? I thought that MHC I were there to act as self markers and MHC II were only present on antigen presenting cells such as macrophages and dendritic cells but in the checkpoints book it says that MHC I is used to present antigens to T helper cells? I'm a bit confused about how to differentiate between these two.

Any help will be greatly appreciated!

[biologyoracle]

it is a condensation polymerisation reaction. its just that water is not released, instead a pyrophosphate is released. Still condensation polymerisation.

[DBA-144]

Thank you vox nihili!
I really appreciate your help!

I had some more questions.
So, do Cytotoxic T cells release cytokines eg perforin that initate apoptosis in cell-infected cells? Is this correct?

Also, in apoptosis, are all the cell organelles destroyed?

What is the difference between MHC I and MHC II? I thought that MHC I were there to act as self markers and MHC II were only present on antigen presenting cells such as macrophages and dendritic cells but in the checkpoints book it says that MHC I is used to present antigens to T helper cells? I'm a bit confused about how to differentiate between these two.

Any help will be greatly appreciated!

1. No. Perforin ruptures the membrane, the granzymes activate intrinsic apoptotic pathway. Don't think this is required knowledge though.
2. Mitochondria is broken down, others usually are left alone.
3. MHC 1 --> protein that binds to antigens (other proteins etc.) presenting them on the cell surface. if the presented antigens are self, T killer cells won't do anything, if not, you know the process. Should be able to see here how the MHC 1 is both used to be a self marker and for presenting antigens. There essentially the same thing, as long as the antigens are self. MHC II is found on APCs, yes.

[pugs]

Hi pugs!
Cytokines are hydrophilic signalling molecules that are involved in the immune response. For eg, T helper cells release cytokines to B cells activating them to differentiate into plasma b cells and memory b cells.
However, cytotoxins are harmful chemicals(toxins) that kill cells. For example they may be released by cytotoxic T cells to kill infected cells in the cell-mediated response.
I hope this helps. 
Please correct me if I'm wrong!

thanks Evolio!
just another quick question relating to this: does this mean that cytotoxic t cells only secrete cytotoxins and not cytokines at all?

[DBA-144]

thanks Evolio!
just another quick question relating to this: does this mean that cytotoxtic t cells only secrete cytotoxins and not cytokines at all?

Not quite. cytotoxic T cells also release cytokines and other signalling molecules which can bind to the secreting cell and cause that cytotoxic T cell to proliferate and differentiate. Please note that these details are not relevant to the course. You do not need to know what cells secrete cytotoxins, etc. but you do need to know that cytotoxic T cells secrete perforin which ruptures cell membranes and the rest of that. Also, most, if not all cells in the immune system will release cytokines, as this is the primary mode of communication between immune cells.

[Evolio]

thanks Evolio!

All good, pugs!

Hi guys.
Are dendritic cells considered to be a part of the innate (non-specific) and adaptive(specific) immune response?

Also, how are complement proteins activated?
Is this correct? Complement proteins punch holes in the cell membrane of bacteria, rupturing their cell membrane, causing their contents to spill out thus killing the bacteria?

Also, when the antigen presenting cells present the antigen to the T helper cells, do the T helper cells' T cell receptors bind to the antigen and if so is the antigen attached to the MHC class II?

Also, the release of interferon to disable virus-infected cells would be a specific response right? Because since the infected cells are being targeted it would be a part of the cell-mediated response which is a part of the specific/adaptive immune response?

Platelets adhering to a virus would be considered a non-specific response right? Because don't platelets congregate on a pathogen regardless of what type of pathogen it is, it can happen to any pathogens?
The last two questions are regarding the answers of a particular biology resource book. I am not certain about their answers.

Thank you guys for your help! 

[Healthy-Citizen]

I'll try answer what I can 

Are dendritic cells considered to be a part of the innate (non-specific) and adaptive(specific) immune response?

Yes, dendritic cells are involved in both the inflammatory response (innate) as well as antigen presentation within the adaptive immune response.

Also, the release of interferon to disable virus-infected cells would be a specific response right? Because since the infected cells are being targeted it would be a part of the cell-mediated response which is a part of the specific/adaptive immune response?

No, interferon release can be attributed to innate immunity. While it is preformed by virus infected cells, the process isn't specific to a particular antigen. I think you might be confusing the release of interferon's with the specific actions of cytotoxic T-cells.

Hope I helped!

[Chocolatemilkshake]

Hi guys.
Are dendritic cells considered to be a part of the innate (non-specific) and adaptive(specific) immune response?

Also, how are complement proteins activated?
Is this correct? Complement proteins punch holes in the cell membrane of bacteria, rupturing their cell membrane, causing their contents to spill out thus killing the bacteria?

Also, when the antigen presenting cells present the antigen to the T helper cells, do the T helper cells' T cell receptors bind to the antigen and if so is the antigen attached to the MHC class II?

Thank you guys for your help! 

Hey Evolio,

I'm going to give some of your questions a go (first time answering questions ), although it's likely that I could be wrong on some of them so anyone feel free to correct me if I am.

1. Dendritic cells are part of the innate (non-specific) immune response as they act the same way on all antigens regardless of what type of antigen it is (engulfs and destroys it). However, they aid in activating the adaptive immune system by presenting the digested antigen on MHC Class II markers to Helper T cells.

2. Complement proteins are activated when two or more antibodies bind to a pathogen (there may be other ways to activate them too though, I'm not sure??) Yes they do contribute to the lysis of pathogens by forming a membrane-attack complex (MAC) and rupturing the cell membrane .

3. The antigen would be attached to MHC Class II markers on the APC and yes I think the T cell receptor does bind to the antigen (I believe it has two receptors, one which binds to the specific antigen presented and another which binds to the MHC Class II marker, although I don't think you need to know this).

[Erutepa]

All good, pugs!

Hi guys.
Are dendritic cells considered to be a part of the innate (non-specific) and adaptive(specific) immune response?

Antigen presenting cells play roles in both the adaptive and innate immune response, hence why they are considered to be a 'branch' between the two responses. While their action in engulfing antigen is innate (non-specific), they activate the adaptive immune response and thus can be considered to be apart of both responses. I would say their action is mostly innate though.

Also, how are complement proteins activated?
Is this correct? Complement proteins punch holes in the cell membrane of bacteria, rupturing their cell membrane, causing their contents to spill out thus killing the bacteria?

The specifics regarding complement activation is not necessary at a VCE level. You just need to know that in the presence of a pathogen, compliment can be activated. In actuality there are 3 different activation pathways of the compliment system. One of them is the "classical" pathways which involves the binding to antibodies mentioned by Chocolatemilkshake. I found reading into extra depth about the compliment system quite fascinating (especially its involvement in alzheimer's).
As for the functions of compliment, I usually talk about:
 - opsonization (a certain compliment protein will bind to pathogen surface and aid in its engulfment by phagocytes)
 - chemotaxis (compliment proteins activated in presence of pathogen will attract immune cells)
 - MAC (as has been mentioned - disrupting the cell membrane and causing the lysis of the cell).

Also, when the antigen presenting cells present the antigen to the T helper cells, do the T helper cells' T cell receptors bind to the antigen and if so is the antigen attached to the MHC class II?

the antigen presenting cell (i.e. dendritic cell) presents antigen to the t helper cell via its mhc class 2. The t helper cell's t cell receptors will bind to the antigen. this clonal selection will cause the clonal expansion of this t helper cell. this is all you need to know.

Also, the release of interferon to disable virus-infected cells would be a specific response right? Because since the infected cells are being targeted it would be a part of the cell-mediated response which is a part of the specific/adaptive immune response?

This would not be considered a specific response. While it is certainly directed at virus infected cells, it is still a very general immune mechanism. Specific responses target specific antigens, and since the release of these interferon are simply in response to viruses generally, they are an innate immune response.

Platelets adhering to a virus would be considered a non-specific response right? Because don't platelets congregate on a pathogen regardless of what type of pathogen it is, it can happen to any pathogens?

correct.
But know that a specific responses refer to responses targeting a specific antigen, not a specific pathogen.

[Evolio]

Thank you Healthy-Citizen, Chocolatemilkshake and Erutepa!
I really appreciate all your help! 

Hi guys.
In apoptosis, why is the cellular response considered to be the cell blebbing/cell shrinkage and cell breaking into small apoptotic bodies? While the signal transduction part of apoptosis refers to DNA fragmentation, proteins being cleaved? Like, why isn't the DNA fragmentation and such a part of the cellular response?
Is it because the caspase enzymes which cause the signal transduction step only cause DNA fragmentation and proteins being cleaved but the actual after effect of apoptosis is the cellular response so the cell blebbing?
Would the apoptotic bodies being phagocytosed by phagocytes be considered in the cellular response?

[Erutepa]

Thank you Healthy-Citizen, Chocolatemilkshake and Erutepa!
I really appreciate all your help! 

Hi guys.
In apoptosis, why is the cellular response considered to be the cell blebbing/cell shrinkage and cell breaking into small apoptotic bodies? While the signal transduction part of apoptosis refers to DNA fragmentation, proteins being cleaved? Like, why isn't the DNA fragmentation and such a part of the cellular response?
Is it because the caspase enzymes which cause the signal transduction step only cause DNA fragmentation and proteins being cleaved but the actual after effect of apoptosis is the cellular response so the cell blebbing?
Would the apoptotic bodies being phagocytosed by phagocytes be considered in the cellular response?

I always considered dna fragmentation as a cellular response myself. Signal transduction would be the activation of caspases in that signal cascade, whereas the fragmentation of dna by caspases would be a cellular response.
I may be wrong, but this is how I've always thought of it.

As with the engulfment of the apoptic bodies, I would consider this a a process separate to apoptosis and hence not a cellular response of apoptosis.

[Evolio]

Okay, thank you!

[pugs]

Not quite. cytotoxic T cells also release cytokines and other signalling molecules which can bind to the secreting cell and cause that cytotoxic T cell to proliferate and differentiate. Please note that these details are not relevant to the course. You do not need to know what cells secrete cytotoxins, etc. but you do need to know that cytotoxic T cells secrete perforin which ruptures cell membranes and the rest of that. Also, most, if not all cells in the immune system will release cytokines, as this is the primary mode of communication between immune cells.

thanks for the clarification!
(ngl i probably should have known this oops – but thnx for clearing it up!)