top of page

Fick's Law: States that the rate of diffusion relies on the concentration gradient being large and the thickness of the membrane being very thin.

Features of efficient exchange surfaces include:

  • Large surface area.

  • Short diffusion pathway

  • Good blood supply

  • Maintained concentration gradient

Types of transport:

1. Diffusion: The movement of particles from a region of high concentration to a region of low concentration, down a concentration gradient. It is a passive process.

2. Active transport: The movement of a substance from a region of low concentration to a region of high concentration against a concentration gradient, with the use of protein pumps and ATP. It is an active process.

3. Osmosis: The movement of water molecules from a region of high water potential to a region of low water potential down the water potential gradient through a partially permeable membrane.

Factors affecting transport:

1. Temperature: The higher the temperature, the more particles move, vibrate and collide. The substance will be moving down the concentration gradient faster.

2. Concentration gradient: The higher the gradient, the faster the rate of diffusion.

3. Surface area: The higher the surface area, the more area there is for substances to move through, for example with a membrane.

4. Distance: The shorter the distance, the less time the substance will take to move through.

Revise water, osmosis and water potential with the 0.2 Water section!

Active transport in nature

Root hair cell: Root hair cells are high SA cells found in the roots of plants. They actively transport minerals from the soil into the cells. Since the cells already have minerals in them, their concentration is higher than their surroundings, so the movement is against the concentration gradient and therefore requires energy (in the form of ATP).

Gut villus: The lining of the gut contains finger-like structures called villi which absorb the products of digestion, such as sugar and amino acids. They have a high SA and good blood supply. The blood often has a higher concentration than the gut, so the movement of substances is once again against the concentration gradient.

Diffusion in nature

Diffusion of oxygen and carbon dioxide in the alveoli (air sac in the lung): Your cells require oxygen to respire, and when they respire they produce CO2, which must be removed from the blood. When you breath in, oxygen in the air moves into air sacs in your lungs, which are very thin and surrounded by blood vessels. There is little oxygen in the blood entering the air sac, so the concentration gradient means that diffusion of oxygen into the blood occurs.

Since blood meeting the air sac is oxygen poor but carbon dioxide rich, there is a concentration gradient for the CO2 to move down, and so CO2 moves into the air sac and is exhaled.


Adaptation of the air sac: Thin, good blood supply, very large surface area. Moist, so diffusion stage is liquid instead of gas.

Unicellular vs multicellular organisms:

Unicellular organisms have a high SA:vol ratio, so can rely on simple diffusion to exchange the substances they require to survive. 

Multicellular enzymes have a decreased SA:vol ratio, so need to establish exchange surfaces and systems to exchange the required substances they need to survive with their environment.

bottom of page