VCE Biology Question Thread

[rinaheartsx3]

Hi, my teacher has confused me with the concept of phototropism... I wanted to know whether Auxins are negatively or positively phototropic? They usually bend to the shady side of the plant which doesn't face the sun so doesn't it move away from the light? But then in checkpoints and other textbooks I read that auxins are positively phototropic... please clarify !

[grannysmith]

Hi, my teacher has confused me with the concept of phototropism... I wanted to know whether Auxins are negatively or positively phototropic? They usually bend to the shady side of the plant which doesn't face the sun so doesn't it move away from the light? But then in checkpoints and other textbooks I read that auxins are positively phototropic... please clarify !

Well, auxins are produced in the tips of plants, and accumulate on the side that is facing away from the source of light. Hence, it stimulates these cells to elongate, causing the plant to bend towards the light source.

[nhmn0301]

Hi, my teacher has confused me with the concept of phototropism... I wanted to know whether Auxins are negatively or positively phototropic? They usually bend to the shady side of the plant which doesn't face the sun so doesn't it move away from the light? But then in checkpoints and other textbooks I read that auxins are positively phototropic... please clarify !

Phototropism means the growth of the plant in response to light. Even though Auxin moves away from sunlight and accumulates in the darker side, the response of the plant is towards the source of light and hence, is positive phototropism, i.e we are concerned about the plant, not the definition about movement of auxin. The way auxin causes the plant to bend towards the source of light happens when auxin accumulates on the darker side, causing the cell wall of this region to soften, turgor pressure cause these cell to elongate and therefore, the plant bend towards the source of light.
[BEATEN]

[Chang Feng]

Can someone please explain part I. Does that mean cytotoxic T cells can just be stimulated by macrophages to replicate and not need helper T cell?? Doesn't everything need helper T cell before it can go through clonal expansion?? Thanks.

[thushan]

The cytotoxic T cells are stimulated by macrophages to proliferate and differentiate => the activated helper T cell simply amplifies this response - and the helper T cell is definitely needed, since not enough cytotoxic T cells can be produced simply from macrophage stimulation; the helper T cell help is needed.

[Chang Feng]

Thanks. Good to know, have immunity sac tomorrow. Btw- then does that mean it implies the same for activating B cells too, that macrophage can activate some B cells, but helper T cell needed to activate more.

[melons]

Could anyone help with this question: (It's from Nature of Biology Book 2 - Chapter 8 Biochallenge)

Cytokines are protein molecules that act as messengers between cells of the immune system. An immune cell may communicate with itself, an autocrine effect; with a cell nearby, a paracrine effect; or with a cell some distance away, an endocrine effect.
You will recall from chapter 5 that this is similar to communication by hormones. One difference is that hormones are more likely to act on distant cells than close ones while cytokines are more likely to act on close cells than on distant ones.
Explain why this difference is biologically logical, given the organs and cells involved in the immune and hormonal systems of the body.

[simpak]

In the immune system you wouldn't want to activate (induce effector function in) cells that are far away from the site of infection because you might cause damage to unrelated sites.  For eg if you have an infection in your leg you don't want the immune cells communicating with other immune cells that are in your brain because that would cause a whole lot of unwanted pathology.  The immune system therefore draws the necessary cells to the site before giving them activating cytokine signals.

In hormonal systems you want to communicate between organs that are far away from each other so that they induce the desired effect for that system - helps the whole body work homeostatically.  For instance the stomach might secrete a hormone (like ghrelin) that acts on the brain to tell the brain it is hungry.

[melons]

Okay. Thank you!

[Frozone]

Hey guys someone please help me with these question(see attachment).
In question 48 I  chose c because I thought non specific umminty in humans include destruction of bacteria by enzymes in saliva but the answer says a) the production of antibodies by plasma cells. I thought that only happens in specific immunity.

In question 49, I have no idea what the question is ASKING.

[Reus]

In question 48 I  chose c because I thought non specific umminty in humans include destruction of bacteria by enzymes in saliva but the answer says a) the production of antibodies by plasma cells. I thought that only happens in specific immunity.

Hmm... I would have gone for c) as well. 

[nhmn0301]

Hey guys someone please help me with these question(see attachment).
In question 48 I  chose c because I thought non specific umminty in humans include destruction of bacteria by enzymes in saliva but the answer says a) the production of antibodies by plasma cells. I thought that only happens in specific immunity.

In question 49, I have no idea what the question is ASKING.

I make a quick look through the VCAA website, you're right, the answer is C. TSFX must have stuffed up the answer .
For question 49, they are basically asking about how blood transfusion can be successful according to the characteristics of these blood type. You have to keep in mind that, IF A PERSON RECEIVES A BLOOD TRANSFUSION, THEY RECEIVE ONLY RED BLOOD CELLS ( NOT THE ANTIBODIES). Hence, for answer A e.g: if a group A person receives blood transfusion from a group B person because the group A has B antibodies in the blood, hence, the B antibodies in the patient's body will agglutinate with the transfused blood (causing death).
Whilst for answer B e.g: if a group B person (who as A antibodies) receive blood transfusion from a group O person (who has no antigens on their blood), it can be pretty safe.

[jessica666]

How do B and T cells recognise foreign material such as snake venom, dust etc. Do they have MHC markers/antigens?

[Scooby]

How do B and T cells recognise foreign material such as snake venom, dust etc. Do they have MHC markers/antigens?

If an immune response is initiated against them, then there will have been some molecule identified by the immune system as non-self (ie. an antigen). That antigen isn't necessarily going to be an MHC marker

[vox nihili]

How do B and T cells recognise foreign material such as snake venom, dust etc. Do they have MHC markers/antigens?

Some mimic antigens and bind via MHC molecules to T-cells.
Others actually mess around with self molecules and make those molecules appear as non-self.
Others can bind directly to the MHC molecule, even though it's not an antigen per se (these are called super antigens).
Things like dust cross link with IgE antibodies on mast cells.