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GCSE Biology Booster Tutor. Supplying the Cell.

Diffusion, Osmosis & Active Transport

· GCSE Science Notes

CELLS are the building blocks of life. They are little factories performing all the vital chemical reactions and processes that your body needs to maintain life.

Many substances have to enter and leave cells for all these processes to take place. There are a few ways they can do this.

Some substances enter and leave cells by a process called DIFFUSION.

Diffusion is the movement of particles from an area of high concentration to an area of low concentration so that they evenly spread out.

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Red blood cells carry oxygen around the body. They pick the oxygen up from the alveoli (air sacs) of the lungs. Because you have just breathed in (inhaled) the concentration of the oxygen in the alveoli is high. The red blood cells have just returned from the body having released all their oxygen to the body’s cells, so they have a low oxygen concentration. The oxygen diffuses from the alveoli into the red blood cells. Now saturated with oxygen the red blood cells commence their oxygen distribution journey around the body.

Different factors can affect the rate of diffusion. These are:

  1. · The difference in concentrations (concentration gradient)
  2. · The temperature
  3. · The surface area of the membrane


Water is essential to life. All cells need it and are required to remove excess amounts. Water enters and leaves cells by a process called OSMOSIS.

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OSMOSIS is themovement of water against the concentration gradient through a semi-permeable membrane. 

The water particles are moving from an area where there are lots of them to where there are fewer. High water content means the solute is dilute (low concentration). So, osmosis goes against the concentration gradient


Sometimes dissolved molecules are at a higher concentrationinside the cell than outside, but, because the organism needs these molecules.They still have to be absorbed. 

ACTIVE TRANSPORT is the movement of dissolved molecules into or out of a cellthrough the cell membrane, from a region of lower to a region of higher solute concentration. Because they are moving against a concentration gradient,energy from respiration is required. Carrierproteins are used to pick up the molecules and deposit them at their correct location. 

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 Examples of active transport include the uptake of glucose from the intestines in humans and the uptake of mineral ions into root hair cells of plants.