What's the difference between osmosis and diffusion?

[scoobysoo]

Or are they the same? The text book I have describes both as being a way of transferring substances through a cell membrane, when the levels of concentration is greater on one side??? I am confused! Can anyone explain in layman's terms please?

[bednobs]

osmosis happens in water

[scoobysoo]

So what's diffusion? I only need to know how they work in terms of cells.

[theprof]

OK, time for a sensible answer then.

Diffusion can be defined as the gradual movement of particles from places where they are abundant to places where there are less of them. In other words, you could say that diffusion is just a natural tendency for stuff to spread out.

Now although diffusion plays an important part in biology such as the diffusion of oxygen and water vapour out of an angiosperm leaf and the diffusion of carbon dioxide into the same leaf for photosynthesis to occur, diffusion is by no means confined to biological systems. For example, if you open a container of a horrendously smelly chemical such as methyl mercaptan in a soccer stadium, it won't be long before everyone makes for the gates as the substance will diffuse into the air.

Osmosis on the other hand may be defined as the movement of water molecules across a partially ( or semi-) permeable membrane from a region of high water concentration to a region of low water concentration.

So what's a semi-permeable membrane? Well basically this is membrane with really small holes in it. These holes are so small that only water can pass through leaving behind other dissolved molecules such as glucose. The water molecules are free to pass both ways through a semi-permeable membrane, but because there are more water molecules on one side of the membrane than the other, there's a steady flow into the region of fewer water molecules. In the case of water on one side of the membrane and a strong glucose solution on the other, this would mean a net flow of water into the glucose solution. In effect, the water behaves as if it is trying to equalise the concentration either side of the membrane.

Osmosis is just a special type of diffusion.

[scoobysoo]

thank you :)

[quincy]

major difference is that osmosis requires energy, while diffusion does not

[theprof]

Hmm. OK, let's go into this in a bit more detail.

Diffusion is the passive movement of substances from regions of high concentration to regions of low concentration. It's caused by Brownian movement of molecules with each molecule following its own concentration gradient.

Another transport mechanism called Facilitated Diffusion or Carrier Mediated Passive Transportation where molecules or ions move across membranes through specific membrane channels down a concentration gradient, is again a passive mechanism and does not "require energy".

Osmosis is the diffusion of water across a semi-permeable membrane where the the process is based upon the concentration gradient concerned. Osmosis will occur spontaneously as long as a driving force is present and the direction and rate of the osmotic flow depends on the pressure gradient and concentration gradient. Osmosis is a passive process.

Passive transportation does not require additional energy. Now both diffusion and osmosis are passive processes and therefore NEITHER of them "requires energy". As I said in my first answer, osmosis is merely a special type of diffusion.

Active transportation on the other hand DOES require energy and usually this isd supplied in the form of ATP. This system often utiilises proton pumps which use ATP to pump protons across the membrane to form a proton gradient.

Incidentally, sometimes osmosis releases energy. Think of the damage done to tarmac up and down the country by expanding tree roots!