KEGS Biology Blog


Wednesday 9th January Section 2
January 9, 2013, 10:41 pm
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Exchange surfaces

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An exchange surface is a specialised area adapted for ease of crossing (for molecules) from one side of the surface to the other. 4 important exchange surfaces:

Alveoli- Takes Oxygen into blood, and removes Carbon Dioxide through diffusion. Alveoli posses a large surface area, moist lining, thin membrane and a copious blood supply.

Small intestine- Absorbs nutrients into the blood via diffusion or active transport. The small intestine has a large surface area, and a large amount of villi. The villi have moist linings, plentiful blood supplies, lacteals and mitochondria.

Root hair cell- Absorbs nutrients and water into the plant’s roots through active transport or osmosis. They have large surface areas, a large vacuole, a thin barrier and a supply of mitochondria.

Cell membrane- Controls what enters and leaves a cell through diffusion. Cell membranes have a large surface area, are partially permeable and possess a short diffusion pathway.

A human is multicellular and has a small surface area-volume ratio, resulting in a long diffusion pathway. An amoeba, however, is single cellular and consequently has a large surface area-volume ratio, making its diffusion pathway short. This allows diffusion to take place quickly.

Due to water cohesion, if the water molecules (resulting from metabolic processes) are left inside the alveoli, they will stick together and cause the cell to collapse. To combat this, the lungs release a surfactant which prevents cohesion.

A steep concentration gradient is maintained by ventilation and the blood transport system. CO2 is brought from the tissue to the lungs, meaning the concentration of the blood is consequentially higher than that of the alveoli and thus diffusion is allowed to take place. Furthermore, oxygen is carried away from the lungs, also working to ensure a higher concentration of oxygen in the alveoli.

Comment by alexmolyneux




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