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
  • If there is a situation where particles of a substance are in a higher concentration, they will move from this region to where they are in a lower concentration
• Examples of diffusion in biological systems

  • Substances move into and out of living cells by diffusion
  • 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
• Concentration


  The concentration of a solution tells us how much of a substance is dissolved in water. The higher the concentration, the more particles of the substance are present
• 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 between different regions and across membranes, between the outside and inside of cells
• Factors affecting rate of diffusion

  • Concentration gradient
  • Temperature
  • Surface area of cell membrane
• The greater the difference in concentration, the quicker the rate of diffusion
• The higher the temperature, the more kinetic energy the particles will have, so they will move and mix more quickly
• The greater the surface area, the faster the rate of diffusion
• 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


  Puppies are more at risk of losing body heat than adult dogs
• 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
• The surface area to volume ratio of a cube with length 6 is 1
• Stacked bar chart


  • Can be used to illustrate the proportions of surface area and volume
• Surface area to volume ratios of various organisms

  • Bacterium: 6 000 000 : 1
  • Blow fly: 600 : 1
  • Whale: 0.06 : 1
• Large organisms


  • Have mechanisms to increase surface area proportionately, such as additional absorbing areas or adaptations of shape
  • Have transport systems and keep distances to a minimum for diffusion
• Organisms in harsh environments

  • May reduce their surface area, e.g. 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 substances across a cell membrane from a region of lower concentration to a region of higher concentration, using energy
• Osmosis refers to the movement of water molecules only
• Osmosis


  Water molecules pass through from solution one to two
• When the concentration of water is the same on both sides of the membrane, there will be no net movement of water molecules
• Solutions containing different solutes, such as salt instead of sugar, will show similar observations to osmosis