Moving molecules

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Moving molecules

As you wait eagerly for your food you can smell the cooking from the kitchen. But how? There are lots of the food 'smell' molecules in the kitchen, a 'high concentration' of them, and as they move about (by Brownian motion) they gradually bump each other out of the kitchen. In other words they move from a place of high concentration into the lower concentration of your room, then into your nose!

Now for a tricky question:

What would happen if the concentration of food 'smell' molecules was the same in the kitchen and your room?

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Osmosis is a special kind of diffusion,which is very important in Biology; it keeps us alive!!

There are 2 key facts:

  1. Osmosis only involves water molecules - nothing else. Osmosisis water diffusion.
  2. Osmosis occurs across a barrier such as a cell membrane which is 'selectively permeable', which means it only lets through small molecules like water.


Water diffuses like everything else, from where it is in high concentration to where it is in low concentration.

Imagine this experiment where you put a piece of potato into pure water.

Outside the potato there is a high concentration of water molecules - we say this region has a high water potential.

Inside the potato there is a low concentration of water because there are lots of solute molecules (things like sugars, salt, ions, etc.) - we say this region has a lower water potential.

Have a look at this animation to see what happens:

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The water molecules enter through the selectively permeable membrane down their 'concentration gradient', that is from the high concentration outside to their low concentration inside (or from higher water potential to lower water potential). The potato will swell up with the extra water, and so get bigger - and heavier! It's also really firm, because all its vacuoles are swollen with water. The boffins call this turgid.

Eventually there will be no difference between the concentrations outside and inside. Obviously, the opposite will happen if the potato is put into a solution with low water potential, like really syrupy sugar solution - the poor old potato cells lose their water - and their strength - so the potato feels sad and floppy. The right bit of jargon for this is flaccid.

Osmosis occurs in all plant and animal cells. It's very useful in many ways, for example, roots absorb water from the soil by osmosis; our bodies absorb water from our food by osmosis; and our kidneys retrieve water from the blood by osmosis, it's removed by filtration. This explains why, when you drink loads, you have to pay more visits to the loo!

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