Transport in cells

• Diffusion


  The movement of molecules from an area of higher concentration to an area of lower concentration
• For an organism to function, substances must move into and out of cells
• Three processes contribute to this movement - diffusion, osmosis and active transport
• Particles (molecules and ions) in a liquid and a gas

  • Move continuously
  • Spread themselves evenly throughout a liquid or a gas
  • Move from higher concentration to lower concentration
• Particles will move in both directions, but there will be a net movement from high to low concentration
• Particles will end up evenly spread throughout the liquid or gas, but will continue to move
• Examples of diffusion in biological systems

  • Substances move into and out of living cells
  • In a leaf
  • In the lungs
  • Liver cells
• Solute


  The dissolved substance in a solution
• Solvent

  The liquid in which the solute dissolves to form a solution
• Dissolve


  When a substance breaks up and mixes completely with a solvent to produce a solution
• Solution

  Mixture formed by a solute and a solvent
• Particles move from a region of high concentration


  To a low concentration in the water
• Concentration gradient

  The difference in the concentration of a chemical across a membrane
• Particles will move down a concentration gradient, from a high concentration to a low concentration
• Diffusion occurs across membranes, between the outside and inside of cells
• Factors affecting rate of diffusion

  • Concentration gradient
  • Temperature
  • Surface area of cell membrane
• Bacteria


  Single-celled microorganisms, some of which are pathogenic in humans, animals and plants
• Multicellular


  Having more than one cell
• Organism

  Living entity, eg animals, plants or microorganisms
• Osmosis

  The movement of water molecules across a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration
• As organisms increase in size

  Their surface area does not increase at the same rate as their volume
• Puppies have a larger surface area to volume ratio than adult dogs, so will lose heat more readily
• Diffusion


  One of the three processes that contribute to the movement of substances into and out of cells
• Osmosis


  One of the three processes that contribute to the movement of substances into and out of cells
• Active transport


  One of the three processes that contribute to the movement of substances into and out of cells
• Cell models

  • Cells can be modelled using cubes
  • As the volume increases, the surface area does not increase at the same rate
• If a graph is drawn of surface area to volume ratio, the ratio decreases as the size of the cube increases
• Surface area to volume ratio

  Measure of the relative amount of surface area compared to volume
• The surface area to volume ratio of a cube with length 6 is 1
• A stacked bar chart can be drawn to illustrate the proportions of surface area and volume
• Organisms

  • Bacterium
  • Blow fly
  • Whale
• Large organisms have mechanisms to increase surface area proportionately, such as additional absorbing areas or adaptations of shape
• Large organisms have transport systems and keep distances to a minimum for diffusion
• Organisms living in harsh environmental conditions may reduce their surface area, eg cacti, to reduce loss of water
• Diffusion


  The movement of molecules from an area of higher concentration to an area of lower concentration
• Osmosis

  The movement of water molecules across a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration
• Active transport

  The movement of molecules against a concentration gradient, requiring energy
• For an organism to function, substances must move into and out of cells. Three processes contribute to this movement - diffusion, osmosis and active transport
• Osmosis

  • Refers to the movement of water molecules only
• When the concentration of water is the same on both sides of the membrane


  There will be no net movement of water molecules