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  • Created on: 12-05-13 12:12

Biology B3 revision

B3.1.1 Dissolved substances

dissolved substances move by diffusion and by active transport.

Diffusion – from an area of high concentration to an area of low concentration. Greater the difference in concentration, the faster the rate of diffusion.


  • the diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane. Dilute solution would mean more water molecules (high concentration of water). Concentrated solution would mean less water molecules (low concentration of water) but a higher concentration of solutes.

  • Partially permeable membrane – allows water molecules through but stop solute molecules from getting through because they are too large.

  • When the concentration of solutions inside and outside of cell are different, water will either move into or out of the cell by osmosis.

  • Osmosis will gradually dilute the more concentrated solution by osmosis. E.g water moves from the soil where there is a high concentration of water, a dilute solution, to the plant root hair cells where this is a low concentration of water.

  • Along a concentration gradient.

Active Transport -

  • Substances absorbed against a concentration gradient, low to high.

  • Requires energy from respiration

  • Allows absorption of sugar and ions through a cell membrane e.g plant absorb ions from dilute solution in soil, humans sugar can be absorbed from intestine and kidney tubules by active transport.

Exchanging materials -

make exchange surfaces efficient -

  • large surface area

  • being thin, short diffusion path

  • bloody supply (capillaries)

  • ventilated for gas exchange

larger and more complex an organism, harder to exchange materials. Exchange surfaces are adapted to maximise effectiveness.

  • Villi increase surface area of small intestine.

  1. Line the walls of small intestine

  2. massive surface area for exchanging materials across

  3. extensive network of blood capillaries to absorb products of digestion by diffusion and active transport.

  4. Rich blood supply.

Sports Drinks

  • contain sugar to replace sugar lost through respiration whilst exercising.

  • Contain water and ions to replace lost during sweating.

  • If water/ions not replaced, balance of the body is disturbed. Cells do not work as efficiently.


  • lungs situated in the thorax.

  • Thorax is the upper body, divided by a sheet of muscle called diaphragm which stop organs from moving into the upper part of the body. Lungs are protected by ribcage.


  1. oxygen from air can diffuse into blood stream. Carbon dioxide can diffuse out of the bloodstream into the air.

  2. Air into lungs – ribcage moves out and up, diaphragm becomes flatter.

  3. Air out of lungs – ribcage moves in and down, diaphragm relaxes and is pushed upwards by the contents of the abdomen.

During inhaling, the diaphragm flattens because when it contracts, it lowers it's dome. This then increase volume in the thoracic cavity and creates a vacuum. This vacuum inflates the lungs by drawing in air into the body. The volume increase and the pressure decreases so that it is lower than atmospheric pressure so that the air moves from high pressure to low pressure (into the lungs)

During exhaling…


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