VCE Biology Question Thread

[cosine]

When placed in a hypotonic solution, to balance out the number of solutes both inside and outside water needs to rush into the cell.

I know mate, but why doesn't/can't the solute in the cell balance it self with the hypotonic solution?

[grannysmith]

I know mate, but why doesn't/can't the solute in the cell balance it self with the hypotonic solution?

What makes you think the solute can pass through the membrane more easily than water?

two processes happen simultaneously, but because diffusion of water is obviously faster, the net movement occurs faster

This pretty much answers your question :p

[cosine]

What makes you think the solute can pass through the membrane more easily than water? This pretty much answers your question :p

THANK. YOU. I have been SEARCHING for the answer, i know its probably not important, but considering me i NEEDED to know why it happened, haha. Seriously, thanks

[twinkling star]

Actually had a couple of questions:

- how and why is the plasma membrane thicker than organelle membranes?

- does facilitated diffusion also involve the random movement of molecules like simple diffusion?

- does active transport involve the net movement of the substance being transported?

- can active transport occur through channel proteins, or is it restricted to carrier proteins?

- is the diffusion of water (osmosis) simple or facilitated?

Thanks!

[cosine]

Actually had a couple of questions:

- how and why is the plasma membrane thicker than organelle membranes?

- does facilitated diffusion also involve the random movement of molecules like simple diffusion?

- does active transport involve the net movement of the substance being transported?

- can active transport occur through channel proteins, or is it restricted to carrier proteins?

- is the diffusion of water (osmosis) simple or facilitated?

Thanks!

1). The plasma membrane regulates the movement of the WHOLE cell, whereas the membranes of the organelles only regulate the movement into/out the organelle, so you tell me why the cell's membrane is thicker

2). Facilitated diffusion ONLY involves the diffusion of polar molecules or ions. Non-polar substances, such as lipids cannot pass through the protein molecules in facilitated diffusion, because there is a thin lining of a hydrophillic substance on the proteins. By random movement, what exactly do you mean?

3). Active transport does not necessarily involve the overall movement of a substance, because as we know, active transport can move a molecule against a concentration gradient, which clearly isnt 'net' or 'overall' movement.

4). The diffusion of water (osmosis) is simple diffusion. Facilitated diffusion is ONLY when there is the assistance of a protein molecule. However, we know that water cannot directly diffuse through the membrane, due to its polar nature, but it is such a small molecule, that it can fit through 'pores' within the plasmamembrane, known as Aquaporins.

Hope this helped!

[grannysmith]

Actually had a couple of questions:

- how and why is the plasma membrane thicker than organelle membranes?
That's an interesting question. But what makes you think it is thicker? A bilayer is required in most (if not all) membranes, and if you imagine each phospholipid being of similar size, then it's deducible that they are roughly the same thickness.

- does facilitated diffusion also involve the random movement of molecules like simple diffusion?
What do you mean by this? Facilitated diffusion involves the movement of polar/charged molecules across the plasma membrane, via specific protein channels/carriers. That is, specific protein channels can only accomodate specific molecules. So random molecules can't just diffuse through any random transmembrane protein.

- does active transport involve the net movement of the substance being transported?
Yes. Molecules are never 'still' so we always refer to the 'net movement' or 'overall movement' of a particular substance.

- can active transport occur through channel proteins, or is it restricted to carrier proteins?
Pretty much restricted to carrier proteins.

- is the diffusion of water (osmosis) simple or facilitated?
Another interesting question. It's both, however in the context of VCE, it's mainly simple. Because water molecules can move through temporary pores in the membrane (simple), or through aquaporins which are protein channels (hence facilitated). In fact, I do not think we would survive if we relied solely on the simple diffusion of water, as most water gained is through aquaporins.

Thanks!

[cosine]

'Net movement' or 'overall' movement does not refer to the particles being still or not. Net movement refers to the movement of all the particles along the concentration gradient. Sure, when the solution reaches equilibrium, we know the particles are not just still, but they're equally moving from one side of the membrane to the other. Hence, no net movement

Should back away now, not to mess with the bio master!

[grannysmith]

'Net movement' or 'overall' movement does not refer to the particles being still or not. Net movement refers to the movement of all the particles along the concentration gradient. Sure, when the solution reaches equilibrium, we know the particles are not just still, but they're equally moving from one side of the membrane to the other. Hence, no net movement

Should back away now, not to mess with the bio master!

Haha, my bad. I should have made myself clearer :p

I'm not saying that 'because no molecule ever stays still, there is no such thing as no net movement'. That is entirely untrue.
However, in the context of active transport, which inherently involves the movement of molecules, then we refer to such movement as being a 'net movement'.

Active transport involves the 'net' movement of molecules against a concentration gradient. As aforementioned, molecules never stay still and are constantly moving. Wouldn't it be reasonable to assume that a single molecule, out of all the thousands upon thousand of molecules of the same type, may go rebel and go against its concentration gradient?
But we wouldn't consider this active transport, right? That's why personally I like to refer to active transport as the 'net movement'.

[vox nihili]

Haha, my bad. I should have made myself clearer :p

I'm not saying that 'because no molecule ever stays still, there is no such thing as no net movement'. That is entirely untrue.
However, in the context of active transport, which inherently involves the movement of molecules, then we refer to such movement as being a 'net movement'.

Active transport involves the 'net' movement of molecules against a concentration gradient. As aforementioned, molecules never stay still and are constantly moving. Wouldn't it be reasonable to assume that a single molecule, out of all the thousands upon thousand of molecules of the same type, may go rebel and go against its concentration gradient?
But we wouldn't consider this active transport, right? That's why personally I like to refer to active transport as the 'net movement'.

You could go further and say that it's not just a personal thing, but it's the correct way to describe active transport. Active transport isn't the movement of molecules against the a concentration gradient (and your example above is a fantastic way of explaining that) rather it is, unequivocally, the net movement of molecules against a concentration gradient. Without the word net, it ain't active transport.

[vox nihili]

Actually had a couple of questions:

- how and why is the plasma membrane thicker than organelle membranes?

- does facilitated diffusion also involve the random movement of molecules like simple diffusion?

- does active transport involve the net movement of the substance being transported?

- can active transport occur through channel proteins, or is it restricted to carrier proteins?

- is the diffusion of water (osmosis) simple or facilitated?

Thanks!

-refer to what grannysmith has said. I'd also add that the plasma membrane of the cell isn't actually of a uniform thickness. Way, way outside the course, but there are localised thickenings in the plasma membrane called "lipid rafts". The phospholipid's involved in these areas of membrane actually have longer tails and there is more cholesterol stacked into these areas. They're thicker so that bulky proteins can anchor in the membrane.

-yes and no. Each individual molecule moves randomly, but the sum of molecules are skewed to move in a particular direction. The nature of movement of molecules, more simply put, is the same as simple diffusion.

-as what granny smith said

-there are some weird ways it can occur via channels, but for the purposes of VCE Biol it's best to forget this. Even where channels are involved, carriers are too.

-as above as well. Water is highly polar, so it doesn't actually move through the membrane itself all that well. For most cells, this isn't such a problem, though some (particularly those with highly variable osmolarity), special channels called aquaporins are needed to get water in/out faster

[twinkling star]

Many thanks cosine, grannysmith and Mr. T-Rav! I feel a lot more confident about 'transport across cell membranes' now

[bts]

Hello,
can someone please explain what the Pi is?
Whats the difference between ATP and ADP? are they just different forms of energy one for anaerobic and the other for aerobic or something?

thank you

[Reus]

Hello,
can someone please explain what the Pi is?
Whats the difference between ATP and ADP? are they just different forms of energy one for anaerobic and the other for aerobic or something?

thank you

The Pi is a phosphate group. ATP has 3 while ADP and 2.

[aaziz17]

Hello,
can someone please explain what the Pi is?
Whats the difference between ATP and ADP? are they just different forms of energy one for anaerobic and the other for aerobic or something?

thank you

Pi is an inorganic phosphate, it is the product when you break down ATP into ADP + Pi. ATP has three phosphates where as ADP has 2. ATP is usually broken down into ADP + Pi to release energy where as ADP is attached to Pi to form ATP which is then broken to release energy. Im not really sure of the function of ADP on its own :/.

Sorry for my vague answer im not too knowledgeable on this.

[bts]

thank you

Pi is an inorganic phosphate, it is the product when you break down ATP into ADP + Pi. ATP has three phosphates where as ADP has 2. ATP is usually broken down into ADP + Pi to release energy where as ADP is attached to Pi to form ATP which is then broken to release energy. Im not really sure of the function of ADP on its own :/.

Sorry for my vague answer im not too knowledgeable on this.

The Pi is a phosphate group. ATP has 3 while ADP and 2.

so what im getting at is that ATP forms energy when broken down to ADP and Pi, but this ADP and Pi rejoins to form ATP which then breaks down again to make more energy, a never ending cycle? doesn't that mean we would have an endless supply of energy?

im a little confused :/