Biology B3a

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  • Created by: Alice
  • Created on: 08-03-13 16:55


Osmosis is the movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration. A partially permeable membrane is a membrane with very small holes in it. These holes are so amall they can only allow small molecules (like water) to pass through them, and the bigger molecules (e.g sucrose) can't. The water molecules actually move in both directions because they move randomly, however the Net movement/flow is to the region with fewer water molecules, i.e the stronger sucrose solution. This means the stronger sucrose solution gets more dilute. The water acts like it's trying to 'even up' the concentration either side of the membrane. Osmosis is a type of diffusion.

Water moves in and out of cells by Osmosis.   Tissue fluid surrounds the cells in the body - it's basically water with oxgen, glucose and stuff dissolved in it. It's squeezed out of the blood capillaries to supply the cells with everything they need. The tissue fluid will usually have a different concentration to the fluid inside the cell. This means water moves into the cell if the cell is short of water, and water can be drawn out the cell if it has too much, all via osmosis!

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Gas and Solute exchange

Substance move by Diffusion, Osmosis and Active Transport- Life processes need gases and other dissolves substances before they can happen. For example, photosynthesis needs carbon dioxide and water to get into the plant cells.Waste substances also need to move out of cells so that the organism can get rid of them. These substance move where they need to be by diffusion, osmosis or active transport.

Difussion - the net movement of particles from an area of high concentration to an area of low concentration. e.g perfume spreading through a room.

Osmosis - the net movement of water molecules from a high water concentration to a low water concentration.

Active transport - the movement of particles against the concentration gradient i.e from a low concentration to a high concentration. This uses a lot of energy!

Exchange surfaces are adapted to maximise effectiveness:

  • They are thin, so substances only have a short distance to diffuse
  • They have a large surface area so lots of a substance can diffuse at once.
  • Exchange surfaces in animals have lots of blood vessels, to get stuff in and out the blood quickly.
  • Gase exchange surfaces in animals (e.g alveoli) are often ventilated too - air moves in and out.
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