VCE Biology Question Thread

[Jackie Chan]

Hi all,
Can somebody please explain how Tc cells are activated? Do they require Helper T cells at all?
Thanks in advance

Hi! 

Cytotoxic T cells recognise and bind to the antigens presented on the MHC-I markers on cells that are cancerous or virally infected and the binding of the Tc receptors causes a signal transduction in the T cell, resulting in it proliferating and differentiating into active Tc cells and memory T cells. And I'm pretty sure that the cytokines that the helper T cell produces helps the Tc cell to proliferate and differentiate.

The Tc cell then releases perforin to destroy the plasma membrane of the infected cell to kill it or may kill it by binding to the infected cell's death receptor.

*someone please correct me if neccessary 

[vox nihili]

Hi all,
Can somebody please explain how Tc cells are activated? Do they require Helper T cells at all?
Thanks in advance

Both answers above are pretty close.

Cytotoxic T-cells do bind to MHC class I molecules on cells that are infected with an intracellular infection. Cytokines from multiple sources help to stimulate the T-cells, as do signals directly from T-helper cells.

[M-D]

Hi all,

Can someone please explain Question 8 of the 2007 VCAA Biology Exam 2 (http://www.vcaa.vic.edu.au/Documents/exams/biology/2007biol2.pdf) page 4?

Is it relevant to the current study design:

Thanks

[PhoenixxFire]

Yes it is relevant. All they are really asking you to do is understand transcription with a lot of unnecessary information thrown in to confuse you.

They tell you that the mRNA strand has the sequence
A U G A U G C C G G A A G A C G G A C C C C C U G A U U G A

and they want to know what the strand of DNA was so you reverse transcription: the mRNA molecule would be complementary to the DNA strand so the DNA strand would be
T A C T A C G G C C T T C T G C C T G G G G G A C T A A C T

Therefore the correct answer is

Question 8
At stage I, the DNA molecule involved has the base sequence
A. TACTACGGCCTCCTGCCTGGGGGACTAACT
B. TACTACGGCCTTCTGCCTCCCGGACTAACT
C. TACTACGGCCTTCTGCCTGGGGGACTAACT
D. TACTACGGCCTTCTGTTTGGGGGACTAACT

[pikachu975]

Hi all,
Can somebody please explain how Tc cells are activated? Do they require Helper T cells at all?
Thanks in advance

Helper T cells are presented with an antigen from a macrophage or B cell on their MHC-II markers and then release interleukin-2 to stimulate production and specialisation of B and T cells. This causes cytotoxic T cells to be produced.

[Atlantis]

When talking about the inputs of the Krebs cycle, is it best to say the input is pyruvate or acetyl CoA? Does it matter which one is written in the exam?

[LifeisaConstantStruggle]

normally it wouldn't matter which one you write, but acetyl CoA is more accurate (but it still depends on the context of the question)

[vox nihili]

Helper T cells are presented with an antigen from a macrophage or B cell on their MHC-II markers and then release interleukin-2 to stimulate production and specialisation of B and T cells. This causes cytotoxic T cells to be produced.

This is close but not completely true.

They do require help from T-helper cells, up to a point. It’s really important to recognise though that cytotoxic T-cell activation is contingent upon their binding to a MHC class I molecule carrying an antigen to which they are specific.

[ezferns]

Why do hydrophilic hormones last for a shorter time than hydrophobic hormones? They both lead to molecules directly activating or suppressing genes so why is this the case?

[LifeisaConstantStruggle]

Why do hydrophilic hormones last for a shorter time than hydrophobic hormones? They both lead to molecules directly activating or suppressing genes so why is this the case?

Hydrophobic hormones are often derived from steroids, which have a much more complex structure compared to peptide-based hydrophilic hormones. This means that hydrophobic hormones generally take a longer time to degrade compared to proteins, and its subsequent reactions it initiates would last longer.
Exceptions do apply due to the relative variation of hormones, and some hydrophobic hormones may last for a shorter time compared to some hydrophilic ones, it always depends.

[littlesq]

Hey there,
How can we know which terms to memorise word for word for those "define.." questions,and the others to just know generally?
Same goes for any other specific bits of information (e.g. Carbon-14 dating can only be used for fossils up to 50,000 years.)
Sorry if that doesn't make sense! 

[LifeisaConstantStruggle]

Hey there,
How can we know which terms to memorise word for word for those "define.." questions,and the others to just know generally?
Same goes for any other specific bits of information (e.g. Carbon-14 dating can only be used for fossils up to 50,000 years.)
Sorry if that doesn't make sense! 

This question is fairly open ended, as "define" questions are different from time to time.
The best way to know what you actually need to write in the exam is to read the VCAA assessor's report as they are often very concise, but cover all bases on the essential knowledge you need to know.
You can also try to read through the study design, to see what you actually need to know for the exam.

[vox nihili]

Why do hydrophilic hormones last for a shorter time than hydrophobic hormones? They both lead to molecules directly activating or suppressing genes so why is this the case?

Hydrophobic hormones are often derived from steroids, which have a much more complex structure compared to peptide-based hydrophilic hormones. This means that hydrophobic hormones generally take a longer time to degrade compared to proteins, and its subsequent reactions it initiates would last longer.
Exceptions do apply due to the relative variation of hormones, and some hydrophobic hormones may last for a shorter time compared to some hydrophilic ones, it always depends.

You wouldn’t need to know the answer to this question for VCE biology.
I really like your reasoning Lifeisaconstantstruggle, but unfortunately it’s not quite correct.

The structural complexity of a molecule really makes little difference to how long the molecule will survive. If you take oxygen gas, as an example...it is an extremely simple structure, yet it is basically never degraded. So, unfortunately that line of thinking doesn’t hold up.

Peptide hormones have really short actions. Their influence is fleeting and they are rapidly degraded. Lipid hormones, however, remain active for a long period of time.

The reason for the difference has nothing to do with the molecules, but is actually all to do with the way cells deal with these hormones. Without going anywhere near the details, peptide hormones break down more quickly because that’s what cells do to them. Conversely, lipids stick around because that’s what cells do. These basically have to do with how long the hormone can stay bound to its receptor (which depends on the design of the receptor) and also whether there are many enzymes around to break them down.

[LifeisaConstantStruggle]

Thanks vox! I've learned something new today.
That's what my teacher explained in class a few weeks ago, that proteins are less complex compared to hydrophobic hormones. Now that I think of it, it's probably inaccurate as steroid hormones are only made up of 4 specific carbon rings but proteins can be comprised of many amino acids, but steroid hormones do not degrade as fast as protein-based hormones. Right, thank you.

[vox nihili]

Thanks vox! I've learned something new today.
That's what my teacher explained in class a few weeks ago, that proteins are less complex compared to hydrophobic hormones. Now that I think of it, it's probably inaccurate as steroid hormones are only made up of 4 specific carbon rings but proteins can be comprised of many amino acids, but steroid hormones do not degrade as fast as protein-based hormones. Right, thank you.

It depends on how you define complexity to be honest. Certainly, proteins have a very simple primary structure, but their 3D structure is horrifically complex. The converse is true of lipids.

It’s a pity that your teacher got it wrong, but I guess that’s to be expected sometimes. Most aren’t really trained beyond high school biology