Gas exchange in single celled and simple organisms

Do single celled organisms need specialised gas exchange surfaces?

No because they have a large surface area to volume ratio so diffusion alone is sufficient to supply their needs. They can gain sufficient oxygen and glucose, and rely on concentration gradients which depend on oxygen use and carbon dioxide production

How do flatworms get their needs?

These have modest requirements so are slow moving and have a low metabolic rate. Oxygen and carbon dioxide can diffuse across their skin surface with no need for specialised gas exchange surfaces. They are long and flat so have a high surface area to volume ratio but this is smaller than unicellular organisms

How do earthworms get their needs?

These are terrestrial multicellular organisms with a long cylindrical shape providing a high surface area to volume ratio. However, the diffusion distance is too large for diffusion alone to supply sufficient oxygen to sustain metabolic requirements. They have a circulatory system to maintain diffusion gradients. The blood vessels are close to the external surface of the organism so that the diffusion path is short to reach the blood. This system is closed as blood flows inside blood vessels and contains haemoglobin which has a high affinity for oxygen to transport it to the tissues. There are no specialised gas exchange surfaces as blood enables glucose to diffuse across the surface of the animal

How do amphibia get their needs?

These are larger multicellular organisms with aquatic and terrestrial modes of life - they must return to the water to breed and their tadpoles are aquatic and use gills for gas exchange. The gas exchange surface of an adult amphibia is its thin moist skin. It has a closed circulatory system of blood containing haemoglobin to transport oxygen to the tissues. At rest, this is sufficient to supply its needs, but during periods of activity, amphibia use primitive lungs too. These increase the surface area for gas exchange but are not highly folded like those of mammals. They are also internal to reduce water loss and heat loss