Amino acids+Plasma membrane

[paulsterio]

For your questional about ethanol (alcohol) - alcohol is actually lipophilic (lipid-loving), and thus hydrophobic.

Ethanol is not lipophilic/hydrophobic. It's a small molecule with a polar hydroxy group, it's definitely hydrophilic.

Think about it this way - alcohol dissolves in water, yes?

[Yacoubb]

Ethanol is not lipophilic/hydrophobic. It's a small molecule with a polar hydroxy group, it's definitely hydrophilic.

Think about it this way - alcohol dissolves in water, yes?

Ohh.. thank you for that, I've always thought of it as lipophilic because of how easily it crosses the membrane..

[paulsterio]

Ohh.. thank you for that, I've always thought of it as lipophilic because of how easily it crosses the membrane..

Water also crosses the membrane easily - surely water isn't lipophilic

[Graphite]

Alcohol is soluble in non polar solvents as well...but it just has a higher affinity for polar solvents...

[Yacoubb]

Water also crosses the membrane easily - surely water isn't lipophilic

Good point Thanks for that. So maybe relating to nonsense's question. Why does water actually 'readily' cross the plasma membrane? Does it have to do with concentration gradients?!

[paulsterio]

Good point Thanks for that. So maybe relating to nonsense's question. Why does water actually 'readily' cross the plasma membrane? Does it have to do with concentration gradients?!

This again, has to do with physics and not much to do with biology. It's only relevant, in biology, to know that the net movement of water is zero, that means that water continuously leaves and enters the cell at the same rate. Think about a tub, which has a hole in it, if you add in as much water as is flowing out, there will be no change in volume.

The reason why water continuously moves in and out of the cell has nothing to do with concentration gradients, because there is none. However, it has to do with a chemical/physical concept known as activity. http://en.wikipedia.org/wiki/Thermodynamic_activity

If suddenly, however, we place a cell in a hypertonic solution, then yes, a concentration gradient will form meaning water will rush out of the cell.

[Snorlax]

This again, has to do with physics and not much to do with biology. It's only relevant, in biology, to know that the net movement of water is zero, that means that water continuously leaves and enters the cell at the same rate.

We call that state equilibrium 

[pi]

Water either leaks through (being a small molecule - much like urea) or is aided by aquaporins. These of course are governed by concentration gradients and other such physical fanciness.

[Yacoubb]

Water either leaks through (being a small molecule - much like urea) or is aided by aquaporins. These of course are governed by concentration gradients and other such physical fanciness.

Okay so water can be transported across a membrane by osmosis because:
The water molecules are very small and can readily be transprted across the membrane.

Water can be transported by facilitated diffusion:
via the aquaporins protein channel.

Is that correct?!

[Snorlax]

Water either leaks through (being a small molecule - much like urea) or is aided by aquaporins. These of course are governed by concentration gradients and other such physical fanciness.

Do you know where aquaporins are introduced in the textbooks? I looked throughout Chapter 2 of my Nelson Biology and haven't seen that term...Know where it may be located?

[pi]

Okay so water can be transported across a membrane by osmosis because:
The water molecules are very small and can readily be transprted across the membrane.

I don't like the wording here, it's quite vague.

Aquaporins aren't in all cells, mainly in kidney cells. I wouldn't worry about them.

Do you know where aquaporins are introduced in the textbooks? I looked throughout Chapter 2 of my Nelson Biology and haven't seen that term...Know where it may be located?

It's not something to stress about, you'll probably never hear about them, as above.