Biology AQA B2

Diffusision, Osmosis

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  • Created by: alex
  • Created on: 16-12-09 23:33


Animal Cells:

  • Cytoplasm - where a chemical reaction can take place, the enzymes control these reactions
  • Mitochondria - where respiration occurs
  • Ribosomes - where proteins are made
  • Cell Membrane -holds the structure of the cell together and controls what goes in and out of the cell
  • Nucleus - Controls what the cell does and contains genetic material

Plant cells:

Plant cells also have the same 5 as animal cells and an extra 3!

  • Cell wall - it is made of cellulose it strengthens the cell
  • Chloroplast - contains chlorophyll, it is the site for photosybthesis to occur
  • Permanent Vacuole - contains cell sap which is a weak solution of sugar and salts
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Specialised cells

Palisade leaf cells - are adapted for PHOTOSYNTHESIS, most of the cells are crammed at the top of the leaf so they are nearer the light. Also the tall thin shapes means that alot of surface area is exposed down the side for absorbing CO2.

Guard cells - are adapted for OPENING AND CLOSING THE STOMATA, when the plant has lots of water it opens the stomata to allow gases to exchange for photosynthis. So when there is a lack of water the stomata closes so that water vapour doesnt escape.

Red blood cells - are adapted for CARRYING OXYGEN, they have a big surface area for absorbing oxygen and so that they can go through the capilliraries easily. They contain loads of haemoglobin and dont contain a nucleus.

Root hair cells - are adapted for ENABLING THE PLANTS TO TAKE IN THE WATER THEY NEED, the root hairs increase the surface area for water to move into the cell. They are also normally placed near the xylem tissue that carries water to the rest of the plant.

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Diffusion is the passive moment of particles from an area of high concentration to an area of low concentration.

  • Small molecules can only diffuse through the cell membrane e.g glucose, amino acids, water and oxygen
  • Big molecules cant go through the cell membrane e.g. Proteins and Starch
  • The rate of diffusion depends on three things
    • DISTANCE - substances diffuse quicker, the closer they are to their destination
    • CONCENTRATION DIFFERENCE - if there are a lot more particles on one side , there are more to move across so it would be slower.
    • Surface Area - the more surface area there is for the molecules to move across the faster they can get from side to the other.
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Osmosis is the movement of water molecules through a partly permiable membrane (a membrane with small holes) from a region of high concentration to a region of low concentration.

  • The water molecules actually pass both ways through the membrane during this process, but this is because thee water molecules move about randomly.
  • But there is always a steady net flow of water into the region with fewer water molecules because there is always more water moledcules on one side.


  • In animal cells, if the solution on the outside of the cell is more dilute the water will move into the cell osmosis but the cell will swell and may burst!
  • However, if the cell on the outside is morfe concentrated, the cytoplasm will become to concentrated and the cell will shrivel up.
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