Diffusion, Osmosis, Active transport

• Diffusion


  1. Gradual movement of particles from places where there are lots of them to places where there are less of them

  2. Spreading out of particles from an area of higher concentration to an area of lower concentration
• Diffusion happens in both solutions and gases because the particles in these substances are free to move about randomly
• Diffusion of different gases

  • Smell of perfume diffusing through the air in a room
• Concentration gradient


  The difference in concentration
• Larger concentration gradient


  Faster diffusion rate
• Higher temperature


  Faster diffusion rate (because particles have more energy and move around faster)
• Cell membranes


  • Hold the cell together
  • Allow substances to move in and out
• Diffusion through cell membranes

  • Only very small molecules like oxygen, glucose, amino acids and water can diffuse through
  • Larger molecules like starch and proteins cannot fit through
• Concentration of particles on one side of the membrane

  Net (overall) movement of particles from that side
• Larger surface area of the membrane

  Faster diffusion rate (more particles pass through at once)
• Osmosis


  The diffusion of water molecules from a region of higher water concentration to a region of lower water concentration through a partially permeable membrane
• Partially permeable membrane


  • Has very small holes that only allow water molecules to pass through, not larger molecules
• Concentration of water molecules on one side of the membrane


  Net flow of water to the side with lower water concentration
• Osmosis is a type of diffusion - the passive movement of water particles from an area of higher water concentration to an area of lower water concentration
• Potato cylinder experiment


  Cut potato into cylinders, place in sugar solutions of different concentrations, measure mass change to observe osmosis
• Dependent variable


  Chip mass
• Independent variable


  Concentration of sugar solution
• Errors may arise from carrying out the method, e.g. incomplete drying of potato cylinders or evaporation of water from beakers
• Repeating the experiment and calculating mean percentage change can reduce the effect of errors
• Active transport


  The absorption of substances against a concentration gradient, from a lower to higher concentration, using energy
• Root hairs take in minerals and water

  1. Root hairs have large surface area to absorb from soil

  2. Concentration of minerals is higher in root hair cells than in soil, so diffusion can't be used

  3. Root hair cells use active transport to take up minerals, which requires energy from respiration
• Active transport in human gut

  1. Nutrients like glucose and amino acids have lower concentration in gut than in blood

  2. Active transport allows nutrients to be taken into the bloodstream against the concentration gradient, using energy
• Active transport uses energy, unlike diffusion
• Oxygen can diffuse into cells because it has a low molecular mass