B3 Unit 1

The lungs are specialized for exchanging materials:

·        The alveoli provide a very large and moist surface area

·        The alveoli have thin walls to provide a short diffusion     path

·        The lungs are well ventilated in order to maintain a             concentration gradient (so that there is a high concentration of O₂ and a low concentration of CO₂ in the alveoli)

·        The rich supply of blood capillaries surrounding the alveoli mean that gases can readily diffuse into and out of the blood

The small intestine is specialized for absorbing digested food:

·        The villi provide a large surface area

·        There is an extensive network of capillaries to absorb the          products of digestion by diffusion and active transport.

• Plants have stomata to obtain carbon dioxide from the atmosphere.
• Plants lose water vapour from the surface of their leaves.  This loss of water vapour is called transpiration.
• Most of the transpiration is through stomata.  The size of stomata is controlled by guard cells which surround them.  If plants lose water faster than it is replaced by the roots, the stomata can close to prevent wilting.