Substance movement into & out of cells

1.1 Cell structure

1.3 Cell division

Substance movement into & out of cells

  1. Diffusion

    1. Diffusion is the spread of particles of a gas or substance in solution, resulting in a net movement from a region where they are of a higher concentration to a region where they are of a lower concentration
    2. The greater the difference in concentration, the faster the rate of diffusion
    3. Dissolved substances and oxygen can move through the cell membrane by diffusion

  2. Osmosis

    1. Osmosis is the diffusion of water from a less concentrated to more concentrated solution through a partially permeable membrane that allows the passage of water molecules
    2. The difference in concentration of the solutions inside & outside a cell cause water to move into or out of it by osmosis
    3. Tonicity is relative concentration of solutions: an isotonic solution has the same concentration, a hypotonic solution has a lower concentration and a hypertonic solution has a higher concentration
    4. Turgor is pressure that pushes the cell membrane of a plant cell against the cell wall
    5. Plasmolysis1 occurs when plant cells are placed in hypertonic solutions so a lot of water leaves the cell and it loses turgor pressure, causing the cell membrane to peel away from the cell wall and collapse

  3. Active transport

    1. Active transport is the absorption of substances against a concentration gradient
    2. It enables ions to be absorbed from dilute solutions in the soil by root hair cells
    3. It enables sugar to be absorbed from low concentrations in the intestine and kidney tubules
    4. It requires the use of energy from respiration

  4. Material exchange

    1. A single-celled organism has a relatively-high surface area to volume ratio and a short diffusion distance so all the necessary exchanges occur through its membrane
    2. The size & complexity of an organism increase the difficulty of exchanging materials so in multicellular organisms, many organ systems are specialised for exchanging materials
    3. Gas & solute exchange surfaces in organisms are adapted to maximise effectiveness

    4. Factors increasing the effectiveness of an exchange surface

      1. Having a large surface area that is thin to provide a short diffusion path
      2. In animals: having an efficient blood supply
      3. For gaseous exchange in animals: having good ventilation

    5. Examples

      1. The small intestine is lined with villi, which forms a large thin-walled surface area and a rich blood supply that maintains a steep concentration gradient for efficient diffusion
      2. Lungs absorb oxygen through alveoli, which form a large surface area with a rich blood supply to carry the oxygen away and maintain a concentration gradient
      3. Plant roots are long & thin to increase the surface area for water absorption and root hair cells increase the surface area further
      4. Plant leaves are flat & thin with internal air spaces and stomata to allow gases in & out of the leaves
  1. The opposite of plasmolysis is cytolysis ↩