VCE Biology Question Thread

[Splash-Tackle-Flail]

So osmosis is the diffusion of water in cells/semi-permeable membranes, and it seems water moves from areas of low solute concentration (or higher concentration of water) to areas of high solute concentration (or lower concentration of water), such as to equalise the solute concentrations. But is there a reason why water does this? Like did scientists just look as the behaviour of water in non-isotonic solutions and observe, or is there an explanation why water (and this can be extended to all things like high pressure->low pressure) behaves in this way on a particle/chemical/physical level? Like why do these things always strive for stable concentrations? (ok I know this is a bit of a weird q and I may be overthinking it )

[Buttercrisis]

I'm so sorry but I still don't get the protein structures; so it's not like a series of steps? Like where you go from 1-2-3-4? Does every protein have all the structures then?

1. During transcription they base pair to DNA
2. The reason for their polarity just comes down to the fact that the heads are charged and the tails are long alkane chains. Water can get through because it can, effectively, sneak past the tails.
3. A protein can't stop at a level of structure. All proteins have primary, secondary and tertiary structure. These structures are just a description of the shape of a protein at any given point (or its amino acid constituents in the case of primary structure). A protein can't be without shape
4. The information is very wrong.

So while the RNA is just floating around it attaches to the DNA and that's what they meant by complementary base pairs? :O
And thanks for helping me.

[Callum@1373]

So osmosis is the diffusion of water in cells/semi-permeable membranes, and it seems water moves from areas of low solute concentration (or higher concentration of water) to areas of high solute concentration (or lower concentration of water), such as to equalise the solute concentrations. But is there a reason why water does this? Like did scientists just look as the behaviour of water in non-isotonic solutions and observe, or is there an explanation why water (and this can be extended to all things like high pressure->low pressure) behaves in this way on a particle/chemical/physical level? Like why do these things always strive for stable concentrations? (ok I know this is a bit of a weird q and I may be overthinking it )

Haha, scientists didn't just make this up lel

So water as you know is polar, so will form ion-dipole bonds with solute molecules e.g Na+ or Cl-

Now, these solute molecules CANNOT cross the membrane. So once a water molecule is attached to that solute molecule, it isnt gonna go back onto the other side of the membrane.

So lets say we have water on the left, water and solute on the right of a membrane.

Water moves from left to right via its random movement, binds to a solute molecules and cannot diffuse across the membrane again. Now there are less free moving water molecules on the right side than the left side, so water has effectively moved from low to high solute concentration.

In other words, water binds to solute because of random movement and then can't go back across the membrane again!

[Splash-Tackle-Flail]

Haha, scientists didn't just make this up lel

So water as you know is polar, so will form ion-dipole bonds with solute molecules e.g Na+ or Cl-

Now, these solute molecules CANNOT cross the membrane. So once a water molecule is attached to that solute molecule, it isnt gonna go back onto the other side of the membrane.

So lets say we have water on the left, water and solute on the right of a membrane.

Water moves from left to right via its random movement, binds to a solute molecules and cannot diffuse across the membrane again. Now there are less free moving water molecules on the right side than the left side, so water has effectively moved from low to high solute concentration.

In other words, water binds to solute because of random movement and then can't go back across the membrane again!

That's actually really logical- thanks mate! (really wishing I got to do bio in VCE)

[Gogo14]

Can someone explain how potassium sodium pumps work and why they are used?

[TheAspiringDoc]

Could someone please explain the Heterozygote advantage?

[pi]

Could someone please explain the Heterozygote advantage?

Don't have the time to fully explain it, I'll leave that to the biology purists among us, but a good example is heterozygotes for sickle cell disease when in an area endemic with malaria.

Basically in sickle cell disease, the red blood cells 'sickle' (and hence get lysed and destroyed) in low oxygen environments (among other environments). Malaria, which is a disease caused by plasmodium which has mosquitos famously as its vector, has an effect of lowering the oxygen levels in cells. Hence, in people who are heterozygotes for the condition (which I think is autosomal recessive? google it), ie. have a sickle cell trait, those red cells that are infected with plasmodium end up sickling and get destroyed. This effectively destroys the infection those cells and hence, limits the overall infection, giving them an inherent heterozygote advantage (or in this case, 'resistance') to malaria.

Whilst people who have full sickle cell disease, ie. homozygous, will also have this "advantage"... they also still have the full sickle cell disease, which obviously sucks and without treatment is usually fatal. People who are heterozygotes don't feel the full force of the disease but just walk away with their cheeky heterozygote advantage!

[vox nihili]

Don't have the time to fully explain it, I'll leave that to the biology purists among us, but a good example is heterozygotes for sickle cell disease when in an area endemic with malaria.

Basically in sickle cell disease, the red blood cells 'sickle' (and hence get lysed and destroyed) in low oxygen environments (among other environments). Malaria, which is a disease caused by plasmodium which has mosquitos famously as its vector, has an effect of lowering the oxygen levels in cells. Hence, in people who are heterozygotes for the condition (which I think is autosomal recessive? google it), ie. have a sickle cell trait, those red cells that are infected with plasmodium end up sickling and get destroyed. This effectively destroys the infection those cells and hence, limits the overall infection, giving them an inherent heterozygote advantage (or in this case, 'resistance') to malaria.

Whilst people who have full sickle cell disease, ie. homozygous, will also have this "advantage"... they also still have the full sickle cell disease, which obviously sucks and without treatment is usually fatal. People who are heterozygotes don't feel the full force of the disease but just walk away with their cheeky heterozygote advantage!

Cannot think of a better way to more fully explain it.

Can someone explain how potassium sodium pumps work and why they are used?

Not going to explain how they work, because it's way, way beyond the VCE course, but they're used to maintain a concentration gradient for potassium and sodium across the membrane of cells. The usefulness of such a gradient is most evident in neural transmission, which relies on the diffusion of sodium and potassium to fire action potentials; however, in other cells it's also important.

I'm so sorry but I still don't get the protein structures; so it's not like a series of steps? Like where you go from 1-2-3-4? Does every protein have all the structures then?

So while the RNA is just floating around it attaches to the DNA and that's what they meant by complementary base pairs? :O
And thanks for helping me.

Pretty much. It's more or less to do with transcription though, when RNA polymerase plonks down RNA based on which nucleotides base pair to it

[Splash-Tackle-Flail]

For question 2, wouldn't the answer be a lipid intracellular receptor as oestrogen is a lipid, and for a receptor to be intracellular, the oestrogen molecule must be able to pass through the membrane, which proteins cannot (as not lipid soluble)?

For Question 7, I thought the gap junction only works for molecules that are really small, too small for cell signalling- could someone explain why I'm wrong, and why pheromones is the correct answer?

For Q8, apparently, Signals can be detected by any cell in the body if It has a receptor that is either located within the cytoplasm or on the surface of the membrane, but I thought if the receptor is in the cytoplasm then some signals won't go through? (e.g. ones that can't permeate the membrane)

Thx in advance for the help!

[geminii]

For the definition of diffusion, which of these two is better?

The net passive movement of particles from an area of high solute concentration to an area of low solute concentration.

The net passive movement of molecules from an area of high solute concentration to an area of low solute concentration.

Thanks!

[Syndicate]

For the definition of diffusion, which of these two is better?

The net passive movement of particles from an area of high solute concentration to an area of low solute concentration.

The net passive movement of molecules from an area of high solute concentration to an area of low solute concentration.

Thanks!

Diffusion is the net passive movement of molecules from an area of high solute.....

[cosine]

For question 2, wouldn't the answer be a lipid intracellular receptor as oestrogen is a lipid, and for a receptor to be intracellular, the oestrogen molecule must be able to pass through the membrane, which proteins cannot (as not lipid soluble)?

For Question 7, I thought the gap junction only works for molecules that are really small, too small for cell signalling- could someone explain why I'm wrong, and why pheromones is the correct answer?

For Q8, apparently, Signals can be detected by any cell in the body if It has a receptor that is either located within the cytoplasm or on the surface of the membrane, but I thought if the receptor is in the cytoplasm then some signals won't go through? (e.g. ones that can't permeate the membrane)

Thx in advance for the help!

Question 2: You are right, because Oestrogen is a lipid-based hormone, and there is a clue as it travels via the bloodstream with the aid of a protein carrier. But remember, when the hormone is lipid based, it cannot simply diffuse through the blood stream due to it's non-polar nature, hence it requires carrier molecule to transport it across the blood stream. Now your question is why is the carrier molecule not lipid? Well, if the lipid hormone was enclosed in a lipid carrier, then how can it diffuse through the blood stream? Hence, the hormone attaches itself to a protein molecule so that it can simply diffuse through the stream, because remember, proteins are soluble and polar.

Question 7: Your reasoning is very wrong. Remember, neurotransmitters exocytose and diffuse through the synaptic gap, to the post-synaptic membrane, and once they bind with the post synaptic receptors, they initiate the action potential in the adjacent neurone. This is by all means cell signalling. Another example occurs when the motor neurones signal the muscle/effector cells/tissue to initiate the response to the stimuli, this can only be achieved when the neurotransmitters from the motor cell have diffused down the synaptic gap and have initiated a cellular response in the effector tissue. Why is there no cell signalling with pheromones though? Well there is, but that is only when the pheromone is detected by a receptor. However, when the pheromone is released, key word released, it is released into the external environment via exocrine glands, and hence the diffusion of the pheromones into the atmosphere is not cell signalling, remember hormones/pheromones are not cells, they are molecules/proteins/lipids.

Question 8: You are sort of right here, if the signalling molecule is lipid-based, it can simply diffuse through the membrane and bind to it's protein receptor whether it is cytoplasmic or protoplasmic (in the nucleus). However, protein-based molecules/hormones cannot diffuse through the membrane, but there are transmembrane (embedded within the membrane) protein receptors that act as receptors, for these protein-based molecules. Otherwise, how else can protein-based molecules achieve their function, if they can never initiate the cellular response? Well, now you know that they cannot diffuse in, so they bind to transmembrane receptors, and from there signal transduction is initiated, until the specific cellular response is achieved.

[Splash-Tackle-Flail]

Question 2: You are right, because Oestrogen is a lipid-based hormone, and there is a clue as it travels via the bloodstream with the aid of a protein carrier. But remember, when the hormone is lipid based, it cannot simply diffuse through the blood stream due to it's non-polar nature, hence it requires carrier molecule to transport it across the blood stream. Now your question is why is the carrier molecule not lipid? Well, if the lipid hormone was enclosed in a lipid carrier, then how can it diffuse through the blood stream? Hence, the hormone attaches itself to a protein molecule so that it can simply diffuse through the stream, because remember, proteins are soluble and polar.

Question 7: Your reasoning is very wrong. Remember, neurotransmitters exocytose and diffuse through the synaptic gap, to the post-synaptic membrane, and once they bind with the post synaptic receptors, they initiate the action potential in the adjacent neurone. This is by all means cell signalling. Another example occurs when the motor neurones signal the muscle/effector cells/tissue to initiate the response to the stimuli, this can only be achieved when the neurotransmitters from the motor cell have diffused down the synaptic gap and have initiated a cellular response in the effector tissue. Why is there no cell signalling with pheromones though? Well there is, but that is only when the pheromone is detected by a receptor. However, when the pheromone is released, key word released, it is released into the external environment via exocrine glands, and hence the diffusion of the pheromones into the atmosphere is not cell signalling, remember hormones/pheromones are not cells, they are molecules/proteins/lipids.

Question 8: You are sort of right here, if the signalling molecule is lipid-based, it can simply diffuse through the membrane and bind to it's protein receptor whether it is cytoplasmic or protoplasmic (in the nucleus). However, protein-based molecules/hormones cannot diffuse through the membrane, but there are transmembrane (embedded within the membrane) protein receptors that act as receptors, for these protein-based molecules. Otherwise, how else can protein-based molecules achieve their function, if they can never initiate the cellular response? Well, now you know that they cannot diffuse in, so they bind to transmembrane receptors, and from there signal transduction is initiated, until the specific cellular response is achieved.

Thanks fam. If I was doing VCE Id ask you to be my tutor

[@#035;3]

Hi guys! I had a prac on Osmosis with de-shelled eggs.
One beaker was filled with salt solution and the other with distilled water.
Can someone please help me with the wording of this question?
thanks in advance!

Explain why the mass of the egg left in salt solution has changed. What is the role of the egg membrane in this change?
I said: The mass of the egg in the salt solution has changed due to the salt solution being hypertonic relative to the egg. Water moves out of the egg via osmosis across the partially permeable membrane until equilibrium is achieved. The role of the egg membrane is so equilibrium is achieved.

sorry that i've posted this twice now.. just really want an answer!

[cosine]

Hi guys! I had a prac on Osmosis with de-shelled eggs.
One beaker was filled with salt solution and the other with distilled water.
Can someone please help me with the wording of this question?
thanks in advance!

Explain why the mass of the egg left in salt solution has changed. What is the role of the egg membrane in this change?
I said: The mass of the egg in the salt solution has changed due to the salt solution being hypertonic relative to the egg. Water moves out of the egg via osmosis across the partially permeable membrane until equilibrium is achieved. The role of the egg membrane is so equilibrium is achieved.

sorry that i've posted this twice now.. just really want an answer!

All is good, however I would reword the second sentence to: Water diffuses out of the egg and into the hypertonic salt solution across the partially permeable membrane so that equilibrium can be established.

Also, the role of the egg membrane is not directly, to achieve equilibrium, the role of the membrane is to allow small molecules to diffuse through the small pores so that equilibrium is reached on the other side of the membrane.