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Cellular Transport Summary Notes

Diffusion

It is the movement of molecules from a region of higher concentration to a region of lower concentration (i.e. down a concentration gradient) until there is an equal distribution of the molecules.

  • It doesn't require energy from respiration.
  • In living organisms, mineral salts, oxygen and carbon dioxide molecules can diffuse through the cell membrane.

Factors affecting rate of diffusion

  • State of matter – diffusion is slow in solids, faster in liquids and fastest in gases.
  • Size of molecules – smaller molecules diffuses faster than larger molecules.
  • Concentration gradient – a greater concentration gradient increases the rate of diffusion.
  • Temperature – the presence of heat increases the rate of diffusion of substances.

Importance of diffusion

  • To ensure a constant supply of oxygen to living things in the water and soil.
  • To ensure that carbon dioxide diffuses into green leaves when plants carry out photosynthesis.
  • To ensure that dissolved mineral salts diffuse into the roots from the soil.
  • To enable animals to detect food by the smell.
  • To ensure that digested food diffuses into the blood through the walls of the small intestine.

Osmosis

It is movement of water molecules from a region with higher water potential to a region of lower water potential through a partially-permeable membrane.

  • The direction of osmosis is determined by the concentration of solution.
  • A dilute solution contains more water molecules. Hence it has a higher water potential than a concentrated solution. Water will then move from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution).

Types of solutions based on concentration

When a cell is placed in a:

  • Hypertonic solution – the cell shrinks as it loses water by osmosis to the solution. A hypertonic solution is more concentrated and therefore has a lower water potential than the cell. In plants, the protoplasm of a plant cell shrinks away from its cell wall as water level leaves the cell by osmosis. This process is known as plasmolysis. It causes land plants to wilt.
  • Hypotonic solution – the cells swell up as it gains water by osmosis from the solution. A hypotonic solution is more dilute and therefore has a higher water potential than the cell. If its an animal cell it swells until it bursts. In plant cells, the cell wall prevent the cells from swelling and bursting and the pressure exerted by water on the cell wall is known as turgor pressure. Turgor pressure is required for support by non-woody plants.
  • Isotonic solution – the cell remains unchanged as there is no water potential gradient, hence no net movement of water molecules in any one direction.

Active transport

It is a process by which substances are transported from a region of lower concentration to a region of higher concentration against a concentration gradient.

  • It requires energy from respiration to move against a concentration gradient.

Examples of active transport include

  1. exchange of sodium and potassium ions in nerve cells.
  2. intake of amino acids and glucose molecules in the small intestine.
  3. accumulation of iodine by marine organisms.
  4. absorption of mineral salts by root hairs.

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