VCE Biology Question Thread

[vox nihili]

How do non-polar substances pass through the plasma membrane? I knot it's via diffusion, but does it occur as they are attracted to the phosphate head as they are non-polar, or are small enough to pass through the pores, and aren't repelled by the fatty acids as they too are non-polar?

Essentialy, how does non-polar substances get passed a negatively charged phosphate head?

It moves around and breaks a bit, so they just slide through like it's water. Non-polar substances don't go through the gaps though, it's incorrect to say this

[alchemy]

Essentialy, how does non-polar substances get passed a negatively charged phosphate head?

The only substances that can move across the membrane via 'simple diffusion' are lipid-soluble molecules such as steroids, or very small molecules, such as H₂O, O₂ and CO₂. Larger molecules and lipid-insoluble molecules require the aid of a protein channel to move across the membrane.

[vox nihili]

Just to make a few things clear for everyone (and if anybody does see some problems jump on them please, because it's been a little while).

As alchemy has mentioned, simple diffusion will involve small molecules or lipid soluble molecules. It's really important that you all understand this, but also understand why this is occurring. The plasma membrane is made of a phospholipid. Yes, this is an amphipathic molecule, but it probably is important to note that the major component is that lipid. The polar region is certainly much smaller. So, lipids can pass through by simple diffusion as well because it only needs to get past a pretty week polar barrier to make it into that nice, lipid environment.

Small molecules can get through because they're just that, they're small. The phospholipid bilayer can't pack in perfectly, and by virtue of the fact that the cell is at 37°C and not -273°C, the phospholipids will constantly move and jiggle around. For small molecules, they can just weave through, even if they are slightly charged. The effect of polarity isn't enough to knock them away either.

So the key points there:

-the effect of the phosphate heads' polarity will not be sufficient to deflect a lipid molecule; a phospholipid is a mainly non-polar molecule. The lipid will just barge through that tiny deflection.
-small molecules can get through for two reasons: there are tiny gaps because the packaging of phospholipids next to each other isn't perfect, and because the effect of polarity, likewise, isn't strong enough to knock them back.

Compare this last point with charged molecules. Whilst small, they bare a full charge, which would respond too much to the charge difference between itself and the phospholipid. The charge difference between an ion and a polar molecule is greater than the charge difference between two polar molecules. In this case, the difference will be great enough to keep the ions out.



The membrane is one of the most incredible things in Biology. Its design is absolutely perfect and its chemistry astounding. These systems that you're learning about, various types of transport across the membrane, reflect the complexity of its chemistry. That's why there's so much to learn, because you really are learning about an incredible structure.

[Tyleralp1]

IS the polymer of monosaccharides: Polysaccharides or Carbohydrates.
Is the polymer of glcuose: polysachharides or Carbohydrates

[alondouek]

IS the polymer of monosaccharides: Polysaccharides or Carbohydrates.
Is the polymer of glcuose: polysachharides or Carbohydrates

1. Polysaccharides
2. Complex Carbohydrates/polysaccharides

Would you like me to explain why?

[Tyleralp1]

That would be very helpful if you did

Btw,  thank you to all who helped answer my frantic pre sac questions Really apappreciate it

[alondouek]

That would be very helpful if you did

Btw,  thank you to all who helped answer my frantic pre sac questions Really apappreciate it

Monosaccharides are monomer saccharides units, such as glucose. The plural of 'mono' is 'poly', and polysaccharides are a general term for many monosaccharides joined together by glycosidic bonds. (Between the levels of 'monosaccharide' and 'polysaccharide', there is a stage called 'oligossccharides'. You probably don't need to know this for VCE, so heads up).

Using glucose as our model carbohydrate monomer unit:

- monosaccharide --> one glucose unit
- (oligosaccharide --> between 2-10 joined glucose units)
- polysaccharides --> more than 10 joined glucose units


Glucose is a specific monosaccharide; polysaccharides are the polymer units of monosaccharides, so the best definition of a polymer of glucose is a complex carbohydrate (such as starch), i.e. A polysaccharide.

[Paulrus]

ok bear with me for a second cos this is probably a stupid question, but i'm not sure why my teacher marked this as wrong.
my reasoning was that the solution the cell has been placed in has a higher concentration of the solute (5%) than the inside of the cell (2%), making it a hypertonic solution. because the outside solution has a higher concentration of solute, i thought there would be net movement of water coming out of the cell in order to reach equilibrium. my teacher corrected it and drew an arrow going inwards and i'm not sure why.
what am i missing here? i have a sac on this later today and i'm feeling kinda panicked cos of this 

[MagicGecko]

mmhm osmosis is the net movement of water molecules across a semi-permeable membrane from a region of low concentration (high water concentration) to a region of high solute concentration (low water concentration). Knowing that, the water molecules would like you said, move out of the cell. So you're actually not missing anything, I'd say your teacher is wrong. Maybe he'she did this at midnight and wasn't thinking straight? Have you tried asking your teacher why he/she marked it as wrong?

[popoy111]

I have both tsfx and neap notes from the summer school lectures which one should i use primarily and which one is better and why?

[masonruc]

I have both tsfx and neap notes from the summer school lectures which one should i use primarily and which one is better and why?

TSFX hands down!

-Notes reflect study design with tsfx, past experiences with neap is that they go way outside what is needed.

[Paulrus]

mmhm osmosis is the net movement of water molecules across a semi-permeable membrane from a region of low concentration (high water concentration) to a region of high solute concentration (low water concentration). Knowing that, the water molecules would like you said, move out of the cell. So you're actually not missing anything, I'd say your teacher is wrong. Maybe he'she did this at midnight and wasn't thinking straight? Have you tried asking your teacher why he/she marked it as wrong?

i asked her earlier today and she apologised and said she was half asleep while marking haha
i'm glad i wasn't wrong, but man that confused me 

[KanMan]

Hi everyone, I'm new to forum so I do apologise if I've accidentally responded in the wrong section. 
I was wondering if anyone could clarify to me  two questions.
1) What is the difference between proteoglycans and glycoproteins?
2) Are there amino acids which have roles other than in the formation of proteins?

Thanks in advance!

[alchemy]

Hi everyone, I'm new to forum so I do apologise if I've accidentally responded in the wrong section. 
I was wondering if anyone could clarify to me  two questions.
1) What is the difference between proteoglycans and glycoproteins?
2) Are there amino acids which have roles other than in the formation of proteins?

Thanks in advance!

1) Firstly, you don't have to know this. I don't think you ever would have in the past. Even if you did, VCAA have toned down their study design so much over the years that there's no point in hoping this will show up on your exam. But if you are inquiring from interest, that's cool. From wider reading, I recall proteoglycans to be a special class of glycoprotein. Also, glycoproteins are referred to as a type of protein but proteoglycans are more often referred to as a type of carbohydrate as they consist mostly of polysaccharide (~95%) in comparison to glycoproteins.
2) I had this question before but forgot to ask it here. Amino acids tend to stabilise pH by removing excess H⁺ or OH^- ions. They can act as neurotransmitters, and are also available as dietary supplements.

[slothpomba]

As alchemy said, both those are outside the study design and far outside VCE. They're more 2nd year biochemistry. Not sure where you got them but i'd use that less as a source in the future.