Movement of substances

• Diffusion


  The movement of molecules from an area of high concentration to an area of low concentration
• Osmosis


  The movement of water molecules from an area of high concentration to an area of low concentration through a semi-permeable membrane
• Active transport


  The movement of molecules against a concentration gradient, requiring energy
• Substances that need to go in and out of a cell

  • Oxygen
  • Glucose
  • Nutrients (e.g. amino acids)
  • Water
  • Waste products (e.g. urea, carbon dioxide)
  • Enzymes
  • Hormones
• Diffusion


  • Occurs when a substance is more concentrated in one place than another (concentration gradient)
  • Molecules move randomly due to kinetic energy, but net movement is from high to low concentration
  • Factors affecting rate of diffusion: temperature, distance, concentration gradient, surface area to volume ratio
• Increased temperature


  Particles move faster, increasing rate of diffusion
• Shorter distance for particles to travel

  Faster rate of diffusion
• Larger concentration gradient


  Faster rate of diffusion
• Larger surface area to volume ratio

  Faster rate of diffusion
• Alveoli in lungs


  • Cells are squashed flat, reducing distance for gases to diffuse
• Elephant


  • Small surface area to volume ratio, so diffusion is very slow
• Bacteria


  • Very large surface area to volume ratio, so can rely on diffusion
• Agar jelly cubes with phenol failing indicator

  • Larger cubes have smaller surface area to volume ratio, so acid diffuses in more slowly
• Where high rate of diffusion is needed, organisms have larger surface areas (e.g. alveoli, plant root hairs)
• Cell processes


  1. Substances must enter and leave the cell via the cell membrane

  2. This can happen through diffusion

  3. Osmosis

  4. Active transport
• Osmosis


  Water moves from a more dilute solution to a more concentrated one across a partially permeable membrane
• Water potential


  Higher water potential = more free water, lower water potential = less free water
• Example of osmosis


  • Placing a plant cell in pure water
  • Placing a plant cell in a concentrated solution
  • Placing an animal cell in pure water
• Plant cells in osmosis

  • Cell will swell up (become turgid) in pure water
  • Cell will become flaccid in concentrated solution
  • Cell will become plasmolysed in very concentrated solution
• Animal cells in osmosis

  • Cell will burst (lyse) in pure water
• Potato osmosis experiment

  1. Cut potato cores to same length

  2. Weigh potato cores before and after placing in salt solutions

  3. Calculate percentage change in mass

  4. Plot graph of percentage change in mass against salt concentration
• The salt concentration at which the percentage change in mass is 0 is equivalent to the concentration inside the potato cells
• How substances enter and leave the cell

  1. Diffusion

  2. Osmosis

  3. Active transport
• Active transport


  When a cell needs to take in a substance against a concentration gradient
• Diffusion


  Molecules move from high concentration to low concentration
• Active transport


  Requires energy release from respiration to move substances against concentration gradient
• Active transport in


  • Absorption of plant minerals in roots
  • Absorption of molecules from small intestine into blood
• Diffusion can happen in gases and liquids, but not solids
• Osmosis is the movement of water from high water potential to low water potential across a semi-permeable membrane
• Heating up particles gives them more kinetic energy, increasing the rate of diffusion
• Salty solutions have a low water potential
• Larger organisms have a smaller surface area to volume ratio
• Diffusion does not require a membrane, but osmosis does
• Osmosis is from high water potential to low water potential
• Active transport is against a concentration gradient and requires energy
• The shorter the distance, the faster diffusion occurs
• Salt water has a low water potential


  Water moves out of plant cells by osmosis, causing the plant to wilt