Movement of substances

• Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration until the particles are evenly distributed.
• In liquids, the rate of diffusion is relatively low, compared to the rate of diffusion in gases. Hence, it takes some time for particles to move from a region of higher concentration to a region of lower concentration.
• The greater the surface area-to-volume ratio, the higher the rate of diffusion. 
• How do the microvilli increase the rate of diffusion of digested nutrients in the small intestine?

  The microvilli are finger-like projections that increase the surface area-to-volume ratio of the small intestine wall. A larger surface area-to-volume ratio increases the rate of diffusion.
• The lungs are the site of gaseous exchange in human beings. Within the lungs are many small air sacs called alveoli (singular: alveolus), which are surrounded by a network of blood capillaries. Carbon dioxide diffuses out of the blood in the blood capillaries into the alveolus. Oxygen diffuses into the blood in the blood capillaries from the alveolus..
• In plants, gaseous exchange occurs mainly through microscopic openings on the surface of leaves called stomata. When plants photosynthesise, there is a net movement of:

  • Carbon dioxide into the leaf from the atmosphere; and
  • Oxygen and water vapour out of the leaf to the atmosphere.

   When plants are not photosynthesising, there is a net movement of:

  • Oxygen into the leaf from the atmosphere; and
  • Carbon dioxide and water vapour out of the leaf to the atmosphere.
• When there is a higher concentration of mineral salts in the soil compared to the cell sap of the root hair cell, mineral salts diffuse from the soil into the root hair cells.
• Many processes involve diffusion. For example:

  • Movement of mineral salt ions into root hair cells;
  • Movement of oxygen molecules from red blood cells to body cells; and
  • Movement of digested food molecules from the blood to body cells. 

   The movement of red blood cells in the body is due to the pumping of blood by the heart and not diffusion.