g) Substances are sometimes absorbed agaisnt a concentration gradient. This requires the use of energy from respiration. The process is called active transport. Active transport enables cells to absorb ions from very dilute solutions.
h) Many organ systems are specialised for exchanging materials. The effectiveness of an exchange surface is increased by: having a large surface area, being thin to provide a short diffusion path, (in animals) having an efficient blood supply and being ventilated.
i) Gas and solute exchange surfaces in humans and other organisms are adapted to maximuse effectiveness.
j) The size and complexitiy of an organism increases the difficulty of exchanging materials.
k) In humans: the surface area of the lungs is increased by the alveoli and the surface area of the small intestine is increased by villi.
l) The villi provide a large surface area with an extensive network of capillaries to absorb the products of digestion by diffusion and active transport.
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