VCE Biology Question Thread

[heids]

Okay so what about the CD4+ CD40 receptor things, do they bind to the MHC2 on B cells since they are also antigen presenting, activating them that way alongside the release of cytokines or do they only bind the the MHC2 receptors on the phagocytic cells?                                                                                                                                                               

I'll confess I didn't have a clue - this is far beyond what you need to know; in year 12, all I knew was that TH cells co-stimulated B cells.  Check out your best friend Wikipedia: http://en.wikipedia.org/wiki/B_cell#T_cell-dependent_activation, especially look at the image beside it (and ignore the stuff about T-cell independent activation, you DON'T need to know this and it's confusing... as always, the VCE Biology course simplifies things ridiculously to make them vaguely comprehensible, so don't go way beyond its scope until Uni or you'll get utterly bogged down).

Anyway, skimming Wikipedia quickly implies to me that B cell receptors bind to T cell receptors, which triggers release of cytokines from the T cell, which then activates B cells. (?)  But it DOESN'T matter, please stress thyself not.

[StupidProdigy]

In apoptosis, does a lysosome come along and lyse the cell and phagocytes consume the cell parts?

[warya]

I was out by the time you said it was beyond the scope of the course! Its hard to know how much detail you need to know things though.
Thank you!

[grannysmith]

In apoptosis, does a lysosome come along and lyse the cell and phagocytes consume the cell parts?

Enzymes, called caspases, are activated inside the cell which break down organelles including the cytoskeleton. In doing so, blebs form on the cell membrane as the cell's structure essentially collapses. The cell is then fragmented into apoptotic bodies which are then phagocytosed by phagocytes. The phagocytes contain lysosomes (organelles) which then break down the biomacromolecules into simpler molecules.

[biy]

I have a bio sac next week and it is got to do with stimuli with planaria responding to light (they usually move away from light)

what questions can I expect? and if you can fill me in the theory about responding to light, it would be greatly appreciated.

[biy]

I have a few questions:

If planaria is exposed to light, and reacts away from it, is this negative phototropism or negative phototaxis? What's the difference between the two?

I know small organisms (not microscopic, animals) have the nervous system. But such animals like planaria (earth worm) do not have a CNS, so does that mean when the stimulus is detected, where does the afferent neurons transmit the message to? I was thinking that where ever the message is transmitted to efferent neurons, the efferent neurons transmit the message to the effectors (cillia or muscles).

Thank you

[Jay.C]

Hey guys,

 I was just wondering if someone could give me a summary of how cells detect foreign materials from self materials, which cells detect this and how cells detect viruses? Another thing I am not fully understanding is the MHC markers.

Thanks!

[grannysmith]

Hey guys,

 I was just wondering if someone could give me a summary of how cells detect foreign materials from self materials, which cells detect this and how cells detect viruses? Another thing I am not fully understanding is the MHC markers.

Thanks!

From what I recall, MHC markers act as 'self' markers which the immune cells detect using specific receptors. However, foreign particles would not have these 'self' molecules as MHC protein molecules are determined by a person's genes, so they are unique to individuals. Because of this lack of 'self', immune cells would treat the foreign particle as non-self. Also, the foreign particle may possess surface antigens which can be detected by immune cells, again, by using specific receptors.

Viruses infect cells and in turn, the infected cell releases interferon - a chemical signalling molecule which causes nearby cells to become more resistant to viral infection. The infected cell also breaks down some of the viral particles and displays these antigen fragments on its MHC I marker, effectively flagging itself for destruction by immune cells. By killing the infected cell, either through apoptosis/lysing, the virus is destroyed as they are obligate intracellular parasites - that is, they can only survive inside of a host cell.

[vox nihili]

From what I recall, MHC markers act as 'self' markers which the immune cells detect using specific receptors. However, foreign particles would not have these 'self' molecules as MHC protein molecules are determined by a person's genes, so they are unique to individuals. Because of this lack of 'self', immune cells would treat the foreign particle as non-self. Also, the foreign particle may possess surface antigens which can be detected by immune cells, again, by using specific receptors.

Viruses infect cells and in turn, the infected cell releases interferon - a chemical signalling molecule which causes nearby cells to become more resistant to viral infection. The infected cell also breaks down some of the viral particles and displays these antigen fragments on its MHC I marker, effectively flagging itself for destruction by immune cells. By killing the infected cell, either through apoptosis/lysing, the virus is destroyed as they are obligate intracellular parasites - that is, they can only survive inside of a host cell.

Just gonna jump in here. Unfortunately, what you've said isn't really correct.

There are a number of ways that cells can recognise non-self. Certain cells of the immune system express receptors that can bind molecular markers that are relatively common in pathogens but never appear in humans. For instance, these receptors might bind double-stranded RNA, which occurs in some viruses but certainly not in human cells.

B-cells express receptors that are able to bind free antigens. These are molecules that are part of or produced by pathogens that float around in tissue. B-cells can bind a whole host of these molecules. They do not, however, typically express receptors to self molecules (how this happens is well beyond the course but basically any B-cells that express receptors to self antigens are deleted).

T-cells, on the other hand, express receptors that can bind antigens (i.e. molecules from pathogens) that are presented on MHC markers. MHC class I markers present antigens (they just hold them) that originate from inside the cell. Therefore, if a cell is infected by a virus, molecules from the virus are expressed on the surface of the cell. T-cells will recognise these molecules as foreign and will respond accordingly. MHC class II molecules are expressed on a limited number of cells. These molecules present antigens that have come from the environment. T-cells can bind antigens presented on these MHC molecules if they are non-self.

[grannysmith]

Just gonna jump in here. Unfortunately, what you've said isn't really correct.

There are a number of ways that cells can recognise non-self. Certain cells of the immune system express receptors that can bind molecular markers that are relatively common in pathogens but never appear in humans. For instance, these receptors might bind double-stranded RNA, which occurs in some viruses but certainly not in human cells.

B-cells express receptors that are able to bind free antigens. These are molecules that are part of or produced by pathogens that float around in tissue. B-cells can bind a whole host of these molecules. They do not, however, typically express receptors to self molecules (how this happens is well beyond the course but basically any B-cells that express receptors to self antigens are deleted).

T-cells, on the other hand, express receptors that can bind antigens (i.e. molecules from pathogens) that are presented on MHC markers. MHC class I markers present antigens (they just hold them) that originate from inside the cell. Therefore, if a cell is infected by a virus, molecules from the virus are expressed on the surface of the cell. T-cells will recognise these molecules as foreign and will respond accordingly. MHC class II molecules are expressed on a limited number of cells. These molecules present antigens that have come from the environment. T-cells can bind antigens presented on these MHC molecules if they are non-self.

Ah, my bad.

Although, I didn't intend to go into too many specifics and was more so attempting a general view of self/non self recognition.

So immune cells don't generally detect self molecules, and only respond when non self molecules are recognised?

I was always taught that, generally, immune cells have specific receptors which can distinguish between self/non self cells.

[vox nihili]

Ah, my bad.

Although, I didn't intend to go into too many specifics and was more so attempting a general view of self/non self recognition.

So immune cells don't generally detect self molecules, and only respond when non self molecules are recognised?

I was always taught that, generally, immune cells have specific receptors which can distinguish between self/non self cells.

Pretty sure the way you explained it was the way I thought about it in year 12 too, to be honest.

Immune cells have receptors that can detect non-self. What makes a particular molecular a self molecule is the absence of any immune cells with receptors to that particular molecule (technically this is still not correct, but it's closer to it)

You're right in saying that immune cells have receptors that can distinguish between self and non-self but there is actually no reason that these receptors would have to be able to interact with self antigens. To get an immune response, all you need is a system that does recognise non-self and does not recognise self.

[KingDrogba]

During both Humoral Immunity and Cell Mediated Immunity, both B and T Cells proliferate, why does this occur?
Does it amplify the response?

[Jay.C]

From what I recall, MHC markers act as 'self' markers which the immune cells detect using specific receptors. However, foreign particles would not have these 'self' molecules as MHC protein molecules are determined by a person's genes, so they are unique to individuals. Because of this lack of 'self', immune cells would treat the foreign particle as non-self. Also, the foreign particle may possess surface antigens which can be detected by immune cells, again, by using specific receptors.

Viruses infect cells and in turn, the infected cell releases interferon - a chemical signalling molecule which causes nearby cells to become more resistant to viral infection. The infected cell also breaks down some of the viral particles and displays these antigen fragments on its MHC I marker, effectively flagging itself for destruction by immune cells. By killing the infected cell, either through apoptosis/lysing, the virus is destroyed as they are obligate intracellular parasites - that is, they can only survive inside of a host cell.

Just gonna jump in here. Unfortunately, what you've said isn't really correct.

There are a number of ways that cells can recognise non-self. Certain cells of the immune system express receptors that can bind molecular markers that are relatively common in pathogens but never appear in humans. For instance, these receptors might bind double-stranded RNA, which occurs in some viruses but certainly not in human cells.

B-cells express receptors that are able to bind free antigens. These are molecules that are part of or produced by pathogens that float around in tissue. B-cells can bind a whole host of these molecules. They do not, however, typically express receptors to self molecules (how this happens is well beyond the course but basically any B-cells that express receptors to self antigens are deleted).

T-cells, on the other hand, express receptors that can bind antigens (i.e. molecules from pathogens) that are presented on MHC markers. MHC class I markers present antigens (they just hold them) that originate from inside the cell. Therefore, if a cell is infected by a virus, molecules from the virus are expressed on the surface of the cell. T-cells will recognise these molecules as foreign and will respond accordingly. MHC class II molecules are expressed on a limited number of cells. These molecules present antigens that have come from the environment. T-cells can bind antigens presented on these MHC molecules if they are non-self.

Thank you both for your help. I also have another question, what are the main differences between how the body responds to a virus and how the body responds to a bacteria?
Thanks again

[biy]

Hey guys would you lose mark if you made a little misleading wording in a bio sac that is irrelevant to the answer? The question said state why the shape and structure of neurotransmitters is important in the transmission of electrical impulses.

I wrote "Because neurotransmitters are hormones, they have specific protein receptors on the postsynaptic membrane and in order to bind with them, to initiate an action potential in the adjacent neurone, the neurotransmitters need to be complementary with the receptors"

But neurotransmitters are not hormones, I got mixed up was supposed to say ligand or signalling molecule -.-.-.-

[grannysmith]

Thank you both for your help. I also have another question, what are the main differences between how the body responds to a virus and how the body responds to a bacteria?
Thanks again

A virus would infect cells and so the cell mediated response would be involved, whereas bacteria would trigger the humoral response because they'd usually be present in bodily fluids, not 'in' cells. However, viral particles can also be detected before cell infection and hence the humoral response could also be involved. Of course, these are specific defenses and non specific defenses would also be at work e.g. natural killer cells, complement proteins etc.

^^ also @biy yeah, you probably would lose a mark, although it'd depend on how generous the marker is.