Movement across membranes

[Smiley_]

Ok I have a really hard work sheet

can salt cross the membrane, I think so?

does protein cross a semi permeable membrane?

and why do you need to rinse a dialysis bag before completing an experiment ?

[Yacoubb]

Salt cannot pass across the membrane alone. If you were to have a solution that contained a high NaCl concentration in a water solution and it became hypertonic, water would move out of the cell, from an area of low solute concentration to high solute concentration. However, note that if salt is in the form of a solution (i.e. water + NaCl), then water can cross the plasma membrane by active transport, such as what occurs in many freshwater organisms.

Proteins cannot pass across the phopholipid bi-layer. They enter/exit via vesicular transport (endocytosis/exocytosis)

A dialysis tube must be washed prior to completing an experiment simply to maximise the credibility of your results. Often there is sodium azide within the dialysis tube that needs to be rinsed out with distilled water so that it does not hinder any transport across the dialysis membraner.

[Snorlax]

Don't think proteins can travel through the membrane even with protein channel assistance. Isn't the only way of proteins transport, through vesicular transport?

[Yacoubb]

Don't think proteins can travel through the membrane even with protein channel assistance. Isn't the only way of proteins transport, through vesicular transport?

100% it travels across the membrane by vesicular transport, but now that you mention it I'm not 100% sure of whether it can or cannot travel via the protein channel.. I haven't started looking at Chap 2 yet at my school because I do it via TAFE because my school doesn't offer it 3+4 in yr 11 and we only started last Monday..

[Snorlax]

Proteins are far too large to be facilitatedly diffused out :/

[Yacoubb]

Proteins are far too large to be facilitatedly diffused out :/

Yeah look like I said that sounds valid So proteins only travel across the membrane by endocytosis or exocytosis. I think that when I do actually start looking at the 5 means of transport across the membrane:
1) Diffusion
2) Facilitated Diffusion
3) Osmosis
4) Active Transport
5) Vesicular Transport

I'll actually put down a few common substances that travel across the membrane by these means.

[Stick]

Don't forget channel-mediated diffusion.

[Yacoubb]

Don't forget channel-mediated diffusion.

I always though channel-mediated/facilitated diffusion were the same.. what are the differences?

[Snorlax]

I always though channel-mediated/facilitated diffusion were the same.. what are the differences?

First time I'v heard of it...
Think it's just an alternative name for facilitated diffusion? Please correct me.

[Yacoubb]

First time I'v heard of it...
Think it's just an alternative name for facilitated diffusion? Please correct me.

They are the same

[Irving4Prez]

I always though channel-mediated/facilitated diffusion were the same.. what are the differences?

"Sometimes a protein channel alone is insufficient and a carrier molecule is required to move molecules down the concentration gradient through a protein channel. When a specific carrier molecule is required, this kind of movement is also called facilitated diffusion" - Nature of Biology Book 2. So they differ in that facilitated diffusion requires an additional carrier molecule whereas channel mediated doesn't.

Salt cannot pass across the membrane alone. If you were to have a solution that contained a high NaCl concentration in a water solution and it became hypertonic, water would move out of the cell, from an area of low solute concentration to high solute concentration. However, note that if salt is in the form of a solution (i.e. water + NaCl), then water can cross the plasma membrane by active transport, such as what occurs in many freshwater organisms.

Proteins cannot pass across the phopholipid bi-layer. They enter/exit via vesicular transport (endocytosis/exocytosis)

A dialysis tube must be washed prior to completing an experiment simply to maximise the credibility of your results. Often there is sodium azide within the dialysis tube that needs to be rinsed out with distilled water so that it does not hinder any transport across the dialysis membraner.

In reference to your 2nd answer, why is it that water travels via active transport and not osmosis? In order to maintain osmotic balance, the net movement of water will be down the concentration gradient thus not requiring energy.

[Bad Student]

Salt cannot pass across the membrane alone. If you were to have a solution that contained a high NaCl concentration in a water solution and it became hypertonic, water would move out of the cell, from an area of low solute concentration to high solute concentration. However, note that if salt is in the form of a solution (i.e. water + NaCl), then water can cross the plasma membrane by active transport, such as what occurs in many freshwater organisms.

In reference to your 2nd answer, why is it that water travels via active transport and not osmosis? In order to maintain osmotic balance, the net movement of water will be down the concentration gradient thus not requiring energy.

There is a higher concentration of water outside the cell of an organism in freshwater compared to the inside of the cell. When water travels out of the cell, the water is going against the concentration gradient, hence the need for active transport.

[Irving4Prez]

In reference to your 2nd answer, why is it that water travels via active transport and not osmosis? In order to maintain osmotic balance, the net movement of water will be down the concentration gradient thus not requiring energy.


There is a higher concentration of water outside the cell of an organism in freshwater compared to the inside of the cell. When water travels out of the cell, the water is going against the concentration gradient, hence the need for active transport.

Thanks for the clarification

[thushan]

Be a LITTLE careful here guys; water doesn't travel directly by active transport - as in there's no water pump per se. What happens is that if an organism is to obtain extra water, it actively transports substances that are normally non-diffusible, for instance Na+. Water then follows by osmosis.

This kind of thing happens in the intestine; our intestines do not absorb water via active transport. They absorb Na+ ions and transport them into the bloodstream by a complex process that involves at least one active transporter (the Na/K pump). Water simply follows by osmosis.

[Irving4Prez]

Aquaporins form pores in the membrane of a cell and mainly allows for the movement of water molecules across the cell membrane. Water can also travel directly through the cell membrane and be active transported out of the cell. Is there a specific moment in time were the aquaporins are used?