osmosis and diffusion

syllabus statements 2.12 - 2.16

what the hell is diffusion? and isn't osmosis the same thing? what about active transport? and whats all this about a selectively permeable membrane?

Diffusion is basically where particles space out really nicely and evenly, If there are a lot of particles clumped in the corner for example and not equally spread out, then they will naturally spread out and become even again, no energy needed at all. This is what diffusion is. When there are a lot of particles all together and not spread out, its called a HIGH CONCENTRATION GRADIENT because there is a HIGH CONCENTRATION of particles in that space. When there are hardly any particles there at all, its called a LOW CONCENTRATION GRADIENT, because there is a LOW CONCENTRATION of particles there. The rule with diffusion AND osmosis is that particles travel FROM a HIGH CONCENTRATION GRADIENT TO a LOW CONCENTRATION GRADIENT, they basically spread out.

Read more for explanations on Osmosis and Active transport, as well as semi permeable membranes and experiments.

So what is osmosis? Well you see osmosis is basically diffusion except with water. Basically if there is diffusion of water molecules then it ain’t called diffusion anymore it's called osmosis. The only thing to notice about osmosis is that it isn’t called concentration gradient when you’re talking about osmosis it’s called water potential (e.g. high concentration gradient becomes high water potential). Also osmosis usually occurs through a selectively permeable membrane.

Now onto selectively permeable membranes. They are SELECTIVE in what they let through them, a bit like the cell membrane of a human cell, letting out the waste and letting in the nutrients. The selectively permeable membranes let through small molecules and not bigger ones. The classic example of this that will always be on the exam paper is glucose and water, the selectively permeable membrane lets through the water (small molecule) and not the glucose (big molecule). What happens here could be a little confusing, there is this experiment where you get a selectively permeable membrane in the form of a bag (Visking Tubing) and fill it with a dilute sugar solution (glucose and water mix). Then you get that little bag and put it in a beaker of pure water. In this experiment, water moves through OSMOSIS through the selectively permeable membrane, from the beaker of water into the glucose solution. This is because the water potential in the water beaker is higher than it is in the bag so water moves into the visking tubing, get it?

There’s another osmosis experiment you need to know, its about seeing what the Surface Area to Volume ratio does to the speed of osmosis. You basically cut up a bunch of jelly into different sized cubes and put them into beakers of sugary water and then see which one soaks up the liquid the quickest. They usually do this experiment with jelly so you can see it fade as it absorbs the water. Anyway, it's time to talk about the Surface Area to Volume ratio. It’s pretty simple, it’s about a ratio that is Surface Area to volume. But it’s not about the ratio itself it’s about what it does to the speed of osmosis. So lets say I have a 4cm cube, the surface area is 96cm squared  and the volume is 64 cm cubed. This makes the SA:V ratio 96:64, or 3:2. This 3cm cube will take longer (trust) to absorb the water than a smaller cube, say 1cm. The 1cm cube has a SA:V ratio of 6:1. Basically the bigger the surface area COMPARED TO THE VOLUME the quicker the absorption of the water. This explains why cells are so tiny, they have a huge surface area compared to their volume which mean they can quickly absorb nutrients and get rid of waste quickly.

So diffusion is totally natural and requires no energy right? There is another type of transport (of molecules and things), it’s called active transport. Active transport requires energy. This is because instead of being natural and going from a high concentration gradient to a low concentration gradient, it goes the other way around, that is from a LOW CONCENTRATION GRADIENT TO A HIGH CONCENTRATION GRADIENT, which is against the concentration gradient.
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