Transport in cells

• Osmosis


  The diffusion of water from a dilute solution through a partially permeable membrane
• Diffusion


  The spreading out of the particles of any substance in a solution, or particles in a gas, resulting in a net movement of particles from an area of higher concentration to an area of lower concentration, down the concentration gradient
• Diffusion


  • Passive process which means that no energy is needed
  • Molecules diffuse until they are evenly spaced apart and equilibrium is reached
  • Particles contain their own kinetic energy, resulting in random movement where the particles bump into each other
• Concentration gradient


  The difference in concentration between the two areas
• The bigger the difference in concentration


  The steeper the concentration gradient, the faster the rate of diffusion
• Partially permeable membrane


  A membrane that allows some substances to pass through but not others, for example water to pass through but not large molecules like sugar or salt
• Dilute solution


  Contains low concentration of solute but high concentration of water (the solvent)
• Concentrated solution


  Contains high concentration of solute but low concentration of water (the solvent)
• Osmosis


  The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane
• Osmosis is a passive process, no energy is used
• Cells


  • Have a partially permeable membrane
  • Water concentrations on the outside of cells can differ from the water concentrations on the inside, causing water to move into or out of cells by osmosis
• Animal cells


  • Swell and burst if water enters via osmosis
  • Shrink if too much water leaves
• Plant cells


  • Have a cell wall so water can move in and out without the cell bursting or shriveling
• Differences between active transport and osmosis

  • Active transport requires energy from mitochondria, osmosis does not
  • Active transport requires a membrane with carrier proteins, osmosis occurs over a partially permeable membrane
  • Active transport moves substances against the concentration gradient (low to high), osmosis moves water down the concentration gradient (high to low)
• Active transport examples


  • Glucose in the small intestine
  • Ions and root hair cells
• Osmosis examples


  • Plants drawing up water from the roots
  • Reabsorption of water in the kidneys
  • Carbon dioxide diffusing into a leaf through stomata for photosynthesis
  • Oxygen diffusing from the alveoli to the blood
• Isotonic solution


  Water concentrations inside and outside the cell are equal
• Hypotonic solution


  Water concentration outside the cell is higher than inside
• Hypertonic solution


  Water concentration inside the cell is higher than outside
• Animal cell in different solutions

  • Normal
  • Lysed (burst)
  • Shriveled
• Plant cell in different solutions

  • Flaccid
  • Turgid
  • Shriveled (plasmolyzed)
• Diffusion


  The spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration
• Diffusion is passive as no energy is required
• Substances that can move via diffusion

  • Oxygen, glucose, amino acids, water
  • Starch and proteins cannot
• Where diffusion takes place in the body

  • Oxygen moves into red blood cells in the lungs
  • Carbon dioxide moves from red blood cells into the lungs
  • Urea moves from liver cells into blood plasma
• Factors affecting rate of diffusion

  • Concentration gradient (greater difference = faster rate)
  • Temperature (higher = faster rate)
  • Surface area of membrane (greater = faster rate)
• Surface area to volume ratio

  The size of the surface area of the organism compared to its volume
• If surface area to volume ratio is large, the organism is less likely to require specialised exchange surfaces and a transport system because the rate of diffusion is sufficient
• Single-celled organisms can use diffusion to transport molecules into their body from the air because they have a relatively large surface area to volume ratio
• In multicellular organisms the surface area to volume ratio is small so they cannot rely on diffusion alone
• Adaptations for transport in multicellular organisms

  • Lungs: alveoli with large surface area
  • Small intestine: villi with large surface area
  • Fish gills: lamellae to increase surface area
  • Leaves: flattened shape and air spaces increase surface area
• Adaptations for efficient transport

  • Having a large surface area
  • Having a thin membrane
  • Having an efficient blood supply/being ventilated
• Osmosis


  The movement of water from a less concentrated solution to a more concentrated one through a partially permeable membrane
• Osmosis is passive (does not use energy)
• Isotonic, hypertonic, hypotonic

  Isotonic: external solution has same concentration as cell

  Hypertonic: external solution has higher concentration than cell

  Hypotonic: external solution has lower concentration than cell
• Osmosis in animals

  • Dilute external solution causes cells to burst

  Concentrated external solution causes cells to shrivel
• Osmosis in plants

  • Dilute external solution causes cells to swell with turgor pressure

  Concentrated external solution causes plasmolysis and cell death
• Experiment: Effect of sugar solutions on plant tissue

  Measure mass of potato tuber before and after placing in sugar solution

  Calculate percentage change in mass

  Plot on graph
• Active transport

  Movement of particles from lower to higher concentration, against concentration gradient, requires energy from respiration
• Active transport in root hairs and gut


  • Root hairs take up water and mineral ions from soil

  Substances like glucose and amino acids move from gut into bloodstream