Aims of this episode:

  • Describe diffusion and osmosis (a form of diffusion);
  • Describe active transport;
  • Explain how enzymes work.

But first…

YouTube Video Reviews


A great channel for all you need to know on GCSE topics. Handily, each episode is listed under the topic title and provides only the information you need for the exam. Don’t understand something? Start here.

In A Nutshell

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Fun Science by charlieissocoollike

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Diffusion is the movement of any substance (liquid or gas) from an area where it is in a high concentration (lots of particles) to an area of low concentration (not a lot of particles). The movement is said to happen down a concentration gradient – just as you would roll down the gradient of a hill (see Episode 005). This process will continue until the particles are evenly spread out. At this point they will still be moving about, but the over all effect will be that the concentration remains the same.


This is a type of diffusion that involves water moving through a semi-permeable membrane. The most commonly used example of this is the movement of water into the roots of a plant, through the semi-permeable membrane of the root hair cell.

Active Transport

Active Transport is the exact opposite of diffusion. This involves the movement of substances from a low concentration to a high concentration, so up the concentration gradient. Since the substances are effectively going up hill, this process requires energy. Enzymes in the cell membrane grab the molecules and pull them into the cell (a bit creepy, but it keeps us alive).

The reason this happens is that diffusion will only get you half of the molecules from – for example – the small intestine. We don’t want half of the energy from our food, we want it all! Active Transport drags the remaining molecules out of the small intestine and into the blood stream.


These little guys keep you alive and in turn you keep them working by maintaining your core body temperature around the 37 degrees Celsius mark. Get too hot or too cold and they will denature and no longer do their job.

Enzymes work by attaching to molecules in the digestive system and breaking them down into something that can be absorbed by the body. The breaking down happens on the active site of the enzyme. Different enzymes have active sites that are shaped to suit a specific molecule (or substrate).

S@S 006: Diffusion, Active Transport, Enzymes and YouTube Reviews
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