Giving examples in each case, explain how large surface area: volume ratio, has been achieved in organisms, for exchange of gases and food material.

For exchange of gases, a large area to volume ratio has been achieved in the following ways among different organisms.

• Use of the cell surface membrane for gaseous exchange in unicellular organisms such as amoeba. Such organisms naturally have a large area to volume ratio because of their small size so that diffusion across the cell surface membrane can suffice their respiratory needs.
• In mammals, the respiratory surface is thrown into numerous spherical folds, called alveoli. These greatly increase the surface area over which exchange of gases can occur.
• In aquatic organisms such as fish, the respiratory surface is formed of highly folded lamellate structures called gills. This also increases the surface area over which gaseous exchange can take place.
• In insects, a highly branched system to tubes, called trachea ramifies through all the body tissues so as to increase the surface area over which gaseous exchange between the tissues and the respiratory surface
• In plants, leaves are numerous, flattened and bear numerous pores, called stomata, in their epidermis. This greatly increases the surface area over which gaseous exchange can take place.
• Tadpole have got external gills to increase surface area for gaseous exchange.

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