Diffusion, osmosis and active transport

• Diffusion


  The movement of molecules from a region of its higher concentration to a region of its lower concentration
• Molecules move down a concentration gradient, as a result of their random movement
• Cell membrane


  A partially permeable membrane that allows some molecules to cross easily, but others with difficulty or not at all
• Cell membrane


  • The simplest sort of selection is based on the size of the molecules
• Diffusion helps living organisms to

  • Obtain many of their requirements
  • Get rid of many of their waste products
  • Carry out gas exchange for respiration
• Examples of diffusion in living organisms

  • Plants require oxygen for respiration at all times, as well as carbon dioxide for photosynthesis when conditions for photosynthesis are right (e.g. enough light and a suitable temperature)
• Brownian motion


  The random movement of molecules and ions that provides the energy for diffusion
• Factors that influence diffusion

  • Surface area to volume ratio
  • Distance
  • Temperature
  • Concentration gradient
• The higher the temperature, the faster molecules move as they have more energy
• The greater the difference in concentration on either side of the membrane, the faster movement across it will occur
• Water as a solvent

  Many substances are able to dissolve in it, making it useful for transporting dissolved substances, digesting food, and removing waste
• Osmosis


  The diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a partially permeable membrane
• Osmosis experiments with plant cells

  1. Immerse plant tissue in solutions of different concentrations

  2. Measure change in mass to determine net movement of water
• Dialysis tubing


  A non-living partially permeable membrane made from cellulose that allows smaller molecules like glucose and water to pass through but not larger molecules like sucrose
• Osmosis


  The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane
• If there is no overall change in mass

  There has been no net movement of water as the concentration in both the plant tissue and the solution surrounding it must be equal
• Investigating osmosis using dialysis tubing

  1. Filling a section of dialysis tubing with concentrated sucrose solution

  2. Suspending the tubing in a boiling tube of water for a set period of time

  3. Noting whether the water level outside the tubing decreases as water moves into the tubing via osmosis
• Water moves from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane
• Turgid

  When water moves into a plant cell, the vacuole gets bigger, pushing the cell membrane against the cell wall. This makes the cell rigid and firm.
• Importance of turgid plant cells

  • Provides support and strength for the plant, making it stand upright with leaves held out to catch sunlight
  • The pressure created by the cell wall stops too much water entering and prevents the cell from bursting
• If plants do not receive enough water


  The cells cannot remain rigid and firm (turgid) and the plant wilts
• Flaccid


  When plant cells are placed in a concentrated solution (with a lower water potential than inside the cells) water molecules will move out of the plant cells by osmosis, making them flaccid
• Plasmolysis


  The cell membrane has pulled away from the cell wall
• Effect of osmosis on animal cells

  • If an animal cell is placed into a strong sugar solution (with a lower water potential than the cell), it will lose water by osmosis and become crenated (shrivelled up)
  • If an animal cell is placed into distilled water (with a higher water potential than the cell), it will gain water by osmosis and, as it has no cell wall to create turgor pressure, will continue to do so until the cell membrane is stretched too far and it bursts
• Active transport


  The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration
• The process of active transport

  1. Substance combines with carrier protein molecule in the cell membrane

  2. Carrier transports substances across membrane using energy from respiration to give them the kinetic energy needed to change shape and move the substance through the cell membrane

  3. Substance released into cell
• Importance of active transport

  • Energy is needed because particles are being moved against a concentration gradient, in the opposite direction from which they would naturally move (by diffusion)
  • Vital process for the movement of molecules or ions across membranes, including uptake of glucose by epithelial cells in the villi of the small intestine and by kidney tubules in the nephron, and uptake of ions from soil water by root hair cells in plants
• Protein carriers


  Used in active transport to pick up specific molecules and take them through the cell membrane against their concentration gradient