Transport Across Cell Membranes

• Passive Transport

  • Movement "Down" or "with" the gradient
  • Movement "against" the gradient
• Passive Transport


  • Involves the movement of molecules and does not require energy
• Substances that cannot pass through the membrane

  • Fats
  • Large insoluble molecules
• The cell membrane is composed of a double layer of protein molecules embedded within it
• Selectively permeable


  The cell membrane allows small soluble molecules to cross but not large insoluble molecules
• Passive transport

  Molecules move from a high concentration to a lower concentration, down a concentration gradient
• Substances that can pass through the membrane

  • Sugars (glucose)
  • Amino acids
  • Proteins
  • Cytoplasm
  • Starch
• Concentration Gradient

  1. The membrane creates different conditions inside and outside of the cell

  2. This forms a concentration gradient and affects how molecules move in and out of cells
• Concentration Gradient


  The difference in concentration between an area of high concentration and an area of low concentration
• Osmosis is a passive process that does not require energy
• Cell placed in concentrated salt solution

  Water moves out of the plant cell causing it to decrease in mass and become plasmolysed
• Cell placed in pure water

  Water moves into the plant cell causing it to increase in mass and become turgid
• Effect of Osmosis on Plant Cells

  1. High water concentration: e.g. pure water - Nottural cell

  2. Low water concentration: e.g. 10% salt - cell placed in water
• Cell placed in concentrated salt solution


  Water moves out of the animal cell causing it to shrink
• Diffusion is a passive process that does not require energy
• Effect of Osmosis on Animal Cells

  1. High water concentration: e.g. pure water - red blood cell

  2. Low water concentration: e.g. 10% salt
• Diffusion is important to cells because it allows them to gain raw materials for respiration and rid themselves of harmful waste products like carbon dioxide
• Diffusion is the movement of molecules from a high concentration to a low concentration down a concentration gradient
• Osmosis is the movement of water molecules from a high water concentration to a low water concentration through a selectively permeable membrane
• Cell placed in pure water

  Water moves into the animal cell causing it to swell