Diffusion

• For any organism to function properly it need to exchange substances including food molecules and waste products between itself and its environment
• Where does the exchange of substances occur in living organisms 


  Across the cell membrane
• What is diffusion


  The net movement (spreaing out) of particles from an area of higher concentration to an area of lower concentration
• Diffusion occurs because there are more particles on one side than another, so they move until both sides have equal numbers of particles.
• Particles in a solution of gas are always moving about randomly, this is because all molecules have kinetic energy so they spread out
• Unlike solutions and gases how do solids move during diffusion 


  Solids have the lowest kinetic energy meaning their particles will just vibrate
• If there happen to be more particles in one area/ region, then there tends to be an overall (net) movement of particles from this area of higher concentration to an area where there are lower particles (a region on lower concentration)
• What process is diffusion


  An entirely passive process
• What does ‘passive‘ mean in diffusion 


  That it does not require energy (small particles diffuse faster than large ones)
• The movement of particles in a fluid is dependent on how much kinetic energy they have; the higher the temp, the more kinetic energy particles have and therefore the faster they can move by diffusion
• Why can diffusion happen in solutions and gases


  Particles in those substances are free to move about randomly
• What is meant by ’concentration gradient‘


  The diference in concentration
• What happens if there is a bigger concentration gradient/ larger surface area of a membrane (in living organisms)


  The faster the diffusion rate
• In diffusion, particles always tend to move from high concentration to low concentration until equilibrium is reached
• Diffusion is commonly known for happening in living organisms
• The role of the cell membrane is to hold the cell together and control what enters and exits the cell -where diffusion takes place
• What do cell membranes let in the cell?


  Only bery small molecules like oxygen (needed for respiration), glucose, amino acids and water
• What are examples of big molecules that cannot fit through a cell membrane 


  Starch and proteins
• O2 and CO2 are examples of substances that can enter the cell membrane, what are these gases uses for in living organisms 


  -Oxygen is needed for aerobic respiration

  -Carbon dioxide is needed for photosynthesising plant cells
• What are examples of substances that leave the cell by diffusion and why they must be exiting

  -Urea is highly toxic and must diffuse out to be excreted by the kidneys

  -Carbon dioxide produced by aerobic respiration must diffuse out of cells
• What is one factor that affects the rate of diffusion?

  Difference in concentrations (concentration gradient)
• How does a greater difference in concentration affect diffusion?

  The greater the difference in concentration, the faster the overall rate of diffusion.
• What effect does higher temperature have on diffusion?

  The higher the temperature, the more kinetic energy the particles have, causing them to move faster.
• How does surface area of a membrane affect diffusion?

  A greater surface area will have a greater rate of diffusion across it.
• What is the significance of surface area to volume ratio in organisms?

  • Affects how easily substances can be exchanged with the environment
  • Larger surface area relative to volume facilitates exchange
• How to work out surface area


  (Side of a cube x side of a cube) x 6 sides
• How to work out the volume

  Length x width x height
• How to work out the surface area to volume ratio


  Divide the SA/V = answer is the SA is x times larger that the volume
• Unicellular organisms can exchange materials directly with their environment
• Unicellular organisms are composed of a single cell (means that they each live and carry out their own life process as if it is a single cell)
• As the size of an organism increases, the surface area to volume ratio decreases
• Very small organisms have a larger surface area in comparison to their volume meaning that the distance between the surface of the organism to its centre is relatively large
• Why do larger organisms have exchange surfaces and t systems

  • Exchange surfaces allow organisms to exchange molecules with their surroundings.
  • Transport systems are needed to move substances, such as oxygen, around the body to where they are needed.
  • Larger organisms have smaller surface area to volume ratios, so they need specialized systems to supply necessary materials and remove waste products.
• Many cells which are adapted for diffusion have increased surface areas in some way ( e.i root hair cells in plants and cells lining the ileum in animals)
• Adaptations for exchange in animals


  -the small intestine

  -the lungs (mammals)

  -the gills (fish)
• The highly folded surface of the small intestine increases its surface area
• What is the role of the small intestine


  Where most absorption of digested food molecules into the blood stream occurs across the wall of the small intestine
• How the small intestine is adapted to its role

  -highly folded surface and lined with specialised intestinal epithelial cells(which themselves have a highly folded membrane)=increases SA

  -one layer of epithelial cells covers surface of each villus=decreases diffusion distance

  -each villus has good blood supply=maintains a concentration gradient
• How many alveoli does each lung contain 


  250 million
• what is the role of the lungs in mammals

  gas exchange between air in the alveoli and the blood, to supply cells with oxygen foe aerobic respiration and remove CO2