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Active transport

The process which uses energy to move substances against a concentration gradient or across a partially permeable membrane using a special transport protein.

Cell membrane

The membrane which forms the boundary between the cytoplasm of a cell and the medium surrounding it and controls the movement of substances into and out of the cell.


The spreading out of the particles of a gas or any substance in solution down a concentration gradient


A thin, flexible sheet-like structure that acts as a lining or a boundary in an organism.


The movement of water through a partially permeable membrane down a concentration gradient from a dilute solution (where there is a high concentration of water) to a concentrated solution (where there is a relatively low concentration of water).


Reusable protein molecules which act as biological catalysts, changing the rate of chemical reactions in the body without being affected themselves


The basic unit from which all living organisms are built up, consisting of a cell membrane surrounding cytoplasm and a nucleus.

What happens when you take a medicine?

There are some medicines that we take or use that go directly to that part of the body that has got the problem. Can you think of any?

If you take a medicine as a solid by mouth it ends up in the stomach which is full of water containing digestive enzymes and acid. For the drug (the biologically active ingredient) to get into the blood stream, and therefore round the body, it has to cross a number of barriers.

Medicine in stomach

First of all the solid has to dissolve in the water in the stomach, then it has to cross the wall of the stomach and then it must pass into the water of the blood stream. All of our cells are covered in a fatty membrane and drugs have to pass through this. Therefore, drugs which are absorbed well must have solubility in both water and fats otherwise they can not cross these barriers. Use the animation below to see how a medicine could cross a cell membrane through the processes of diffusion, osmosis and active transport.