B3 exchange of materials

B3 1

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  • Created by: Katya
  • Created on: 22-03-11 21:46


= net movement of water from a dilute solution to a concentrated solution through a partially permeable membrane


Partially permeable membrane only allows water molecules to move across it


Movement of water molecules is random and requires no energy

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= net movement of particles from an area of high concentration to an area of low concentration

Takes place because of random movement of particles of gas or substance in solution with water so requires no energy


The bigger the concentration gradient the faster the rate of diffusion


Examples of diffusion in living organisms...

v  Oxygen diffuses from alveoli in lungs into blood and from blood into respiring cells

v  Carbon dioxide diffuses out of respiring cells into blood and from blood into alveoli in lungs

v  Food molecules diffuse from small intestine into blood and from blood into body cells

v  Carbon dioxide diffuses from air into plant leaf cells

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Active transport

= movement of substances against a concentration gradient and/or across a cell membrane

Uses up energy

Cells have high rate of respiration when high rate of active transport – cells that carry out lots of active transport usually have lots of mitochondria to provide energy (respiration happens in mitochondria)


 1.      Transport protein (=carrier protein) recognises solute molecule that needs to be transported

 2.      Solute molecule attaches itself to transport protein

3.      Transport protein rotates in membrane

4.     Transport protein releases solute inside the cell

5.      Transport protein rotates back


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Uses of active transport

  • Plants absorb mineral ions from soil by active transport (mineral ions in soil are found in dilute solutions – more dilute than inside plant)
  • Glucose is moved out of gut and kidney tubes into blood
  • Glucose is moved out of the kidney tubule by active transport
  • Reptiles and marine birds: Sodium ions move from body into salt glands near eyes and nostrils. Salt glands produce concentrated salt solution to help reptiles and marine birds get rid of excess sodium ions because they are toxic if build up
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