VCE Biology Question Thread

[RazzMeTazz]

Does the term 'organism' only refer to cellular agents?

[grannysmith]

Why would too much glucose within a cell prevent its breakdown? If that makes sense

[That Other Guy]

I know that exogenous antigens refer to those that have been endocytosed by the body, and that these are exclusively presented by MHC Class II markers. However, I am confused as to whether 'exogenous' is synonymous with 'non-self'. Surely the cytosolic antigens presented on MHC Class I markers can be non-self and thus exogenous if a pathogenic has over ridden gene expression. If that is the case, would I be wrong to say that MHC Class II markers, in most cases, (I am aware of exceptions) display exogenous antigens? Would I be safer to simply say 'non-self'?

[sparked]

Stick to non-self. They are synonymous, examiners can be crazy though, don't give them an excuse to think you're being ambiguous. MHC II molecules present foreign antigens, non-self antigens etc.

[grannysmith]

Do we have to mention helper T cells when describing the humoral response?

[sparked]

Yes, Helper T-Cells enable the clonal selection and proliferation of B-lymphocytes which enable to humoral response, equally if it is a compare question, then obviously yes.

Learn about the kinds of cytokines that helper t-cells present to different lymphocytes. Equally make sure you understand that viruses that invade lymphocytes express on the MHC I molecule!

[simpak]

No, it wasn't a particular question
I was just wondering because my NOB textbook says: " perfect matching of tissues occurs only if identical twins are involved in organ donation"

That's not necessarily to do with MHC markers.  Even if they have the same MHC markers you can still get rejection.  There is a thing called 'minor histocompatibility antigens' rather than major histocompatibility antigens (or complex, MHC).  This refers to differences in the /peptides/ presented by MHC - so if you have /any/ different genes between individuals, you could present different peptides on the surface and these will be recognised as non-self, even if the MHC genes themselves are identical.  Rejection is slower than for when there is an MHC mismatch, but may still occur.  The classical example of this is to consider what would happen if you transplant a male organ into a female recipient - the male organ expresses genes off the Y chromosome, that isn't present in the female. The female immune system treats these 'male' antigens as if they were pathogenic or foreign because they're non-self and rejection can occur.

This is beyond the scope of your course but perhaps helps with your confusion?  What T-Rav said is correct - identical twins have identical MHC always but fraternal may or may not have this.

[RazzMeTazz]

Do plants have a second line of defense?

[sparked]

Yes. IF the pathogen manages to pass the 1st line of defense BARRIERS i.e. the keratinised leaf surface, microscopic hairs to prevent adhesion etc. Plants can either drop branches, stems etc. that are infected, or form nodes, which are areas of the plant that are essentially cornered off pathogens. They are prevented from gaining access to other parts of the organism.

[RazzMeTazz]

That's not necessarily to do with MHC markers.  Even if they have the same MHC markers you can still get rejection.  There is a thing called 'minor histocompatibility antigens' rather than major histocompatibility antigens (or complex, MHC).  This refers to differences in the /peptides/ presented by MHC - so if you have /any/ different genes between individuals, you could present different peptides on the surface and these will be recognised as non-self, even if the MHC genes themselves are identical.  Rejection is slower than for when there is an MHC mismatch, but may still occur.  The classical example of this is to consider what would happen if you transplant a male organ into a female recipient - the male organ expresses genes off the Y chromosome, that isn't present in the female. The female immune system treats these 'male' antigens as if they were pathogenic or foreign because they're non-self and rejection can occur.

This is beyond the scope of your course but perhaps helps with your confusion?  What T-Rav said is correct - identical twins have identical MHC always but fraternal may or may not have this.

Oh okay thanks so much! Yeah it definitely does help with my confusion

[Reus]

Is plant defence/immunity on the Biology Study Design?

[sparked]

Do you use bio zone? I believe there was a tiny part on plant defense. It might be worth spending like 10 minutes just getting a superficial understanding of it. Would only ever be one MC question if that.

Otherwise, think of it intuitively. Trees drop branches that are infected. Those little bumps on some leaves are often nodes.

Oh on cellular pathogens there was definitely a plant immunity focus - in terms of fungi hyphae entering leaves through the stomata. 

[The Usual Student]

The cell membrane is made out of phospholipids  that have a hyrdrophilic exterior and a hyrdrophobic interior. I was told that non charged small particles can pass through as they are lipophilic but if they are non charged then why does the hydrophilic exterior let  them through? Aren't they opposites?

[sparked]

The hydrophilic heads of the cellular membrane are essentially dissolved in the extra cellular fluid that surrounds them. Therefore the ability for molecule to enter the cell is dependent on passing the hydrophobic layer, which functionally, is exterior of the cell.

[Reus]

Do you use bio zone? I believe there was a tiny part on plant defense. It might be worth spending like 10 minutes just getting a superficial understanding of it. Would only ever be one MC question if that.

Otherwise, think of it intuitively. Trees drop branches that are infected. Those little bumps on some leaves are often nodes.

Oh on cellular pathogens there was definitely a plant immunity focus - in terms of fungi hyphae entering leaves through the stomata.

Thanks heaps! I'll get on biozone soon :p