<P> Gases are constantly consumed and produced by cellular and metabolic reactions in most living things , so an efficient system for gas exchange between , ultimately , the interior of the cell ( s ) and the external environment is required . Small , particularly unicellular organisms , such as bacteria and protozoa , have a high surface - area to volume ratio . In these creatures the gas exchange membrane is typically the cell membrane . Some small multicellular organisms , such as flatworms , are also able to perform sufficient gas exchange across the skin or cuticle that surrounds their bodies . However , in most larger organisms , which have a small surface - area to volume ratios , specialised structures with convoluted surfaces such as gills , pulmonary alveoli and spongy mesophyll provide the large area needed for effective gas exchange . These convoluted surfaces may sometimes be internalised into the body of the organism . This is the case with the alveoli , which form the inner surface of the mammalian lung , the spongy mesophyll , which is found inside the leaves of some kinds of plant , or the gills of those molluscs that have them , which are found in the mantle cavity . </P>
surface area in many kinds of organs is important because the amount of surface determines
effective gas exchange