Diffusion is the passive movement of particles from an area of high concentration to an area of low concentration.
Learn this phrase! Examiners like to ask for definitions, and then insist that you are word perfect, or they start taking marks off. If you learn this, you'll be fine.
Translated, it means that particles which float about randomly (e.g. liquids and gases) will naturally rearrange themselves until they are equally spread out.
If I have a room full of blindfolded people, with all the blondes in one corner and the brunettes in another and tell them to walk about randomly, after a short while they will all be thoroughly mixed up. That's pretty much how diffusion works.
NB: Diffusion can happen through a permeable/partially permeable membrane. This makes no difference, except that with a partially permeable membrane, only some things move through it. This is how substances move into and out of cells.
Rate of diffusion
The rate of diffusion depends on four main factors:
- Distance - pretty obvious. The further something has to diffuse, the longer it will take.
- Temperature - the higher the temperature, the faster the particles move, so diffusion takes less time.
- Concentration difference - going back to our room full of people, if we have two blondes and fifty brunettes, it takes less time for the blondes to be separated and mixed in with the crowd. So the greater the concentration difference, the quicker it takes.
- Surface area - the greater the surface area, the quicker the diffusion.
You won't be asked to calculate the rate of diffusion, but you may be asked a question like this: Sally is cooking a curry. If she increases the heat under the curry, will it take more or less time for the smell to reach her brother upstairs? = more, because greater temperature, so faster diffusion
Osmosis is the movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration.
That's a fancy way of saying "water diffusion" but you have to learn the posh way for the examiners. Osmosis is just diffusion of water through a membrane.
You will be asked about the dreaded potato experiment. So here are the facts:
- Someone with a racially correct name will cut up a potato into pieces of equal size.
- They will put them in beakers containing different concentrations of salt/sugar solution
- The ones with a low salt/sugar concentration will have increased in size (water gone in) and the ones with a high salt/sugar concentration will have decreased in size (water moved out)
- There will be one stubborn potato that hasn't changed.
That's it. Dreaded potato experiment complete.
Osmosis and cells
Very short and very boring card, but you may have to learn it, and it's pretty easy.
- If too much water enters a cell, it swells and becomes turgid. If it is an animal cell, it may burst and die.
- If too much water leaves a cell, it shrinks and becomes flaccid. If it is a plant cell, the membrane may rip away from the inside wall of the cell, and it may die.
That's it. Done.
Sometimes, cells need to move stuff against a concentration gradient (e.g. from somewhere where there isn't much to where there's lots). For example, a root cell must move water from the ground into the cell.
To do this, it uses active transport to physically pull the substance into the cell.
This requires energy, whereas diffusion and osmosis do not. Therefore, cells that need to use active transport have lots of mitochondria, to make the energy that they need.