movement across membranes

• Diffusion


  Constantly moving liquid and gas molecules tend to move from an area of high concentration to an area of lower concentration until evenly distributed
• Diffusion


  • It is a passive process – it does not require energy
  • Molecules move down a concentration gradient
• Visking tubing


  • Can be used to model a cell membrane as it is selectively permeable
  • Only molecules small enough can diffuse through the pores
• Osmosis


  The diffusion of water from high water concentration (dilute solute solution) to low water concentration (concentrated solute solution) across a selectively permeable membrane
• Active transport


  • Moves molecules against a concentration gradient
  • This process uses energy in the form of ATP provided by respiration
  • Glucose and oxygen are required for respiration
• Factors that affect diffusion

  • Concentration
  • Temperature
  • Pressure
• Concentration


  The greater the concentration gradient the greater the diffusion rate
• Temperature

  At higher temperatures molecules have more kinetic energy and so move and diffuse faster
• Pressure

  Molecules move quickly from an area of higher to lower pressure
• Selectively permeable membrane allows water to pass through but not large solute molecules
• High water concentration

  Low sugar concentration
• Low water concentration

  High sugar concentration
• Water moves from a high to low water concentration by osmosis across a semi-permeable membrane
• Water is the only thing that moves across the membrane during osmosis, not the solute molecules
• Water can move in both directions across the membrane, but the net movement will be from high to low water concentration
• If a cell has too much water going in, animal cells will burst but plant cells will become turgid due to the cell wall
• If a cell loses too much water, plant cells will become flaccid and animal cells will shrink