Direct Diffusion The oxygen requirements of small multicellular organisms can be met by diffusion across the
outer membrane. For organisms with a diameter of less than 1 mm, direct diffusion of gases across
surface membranes is an effective method of gas exchange. Every cell in the body of simple
organisms like cnidarians and flatworms is in close proximity to the outside world. Their cells are
kept moist, and direct diffusion allows gases to spread out quickly. Flatworms are tiny, flat worms
that "breathe" by diffusing air across their outer membrane (Figure 130). Each cell in the body of
these organisms is close to the surface of the outer membrane and has access to oxygen because of
the flat shape of these organisms, which increases the surface area for diffusion. The cells in the center
of the flatworm wouldn't be able to receive oxygen if it had a cylindrical body.
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Figure 130. This flatworm’s process of respiration works by diffusion across the outer membrane
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.
Skin and Gills The skin (integument) of earthworms and amphibians serves as a respiratory system. Just
below the skin, a complex network of capillaries allows for the exchange of gases between the
circulatory system and the outside environment. The gases must be allowed to dissolve and diffuse
across cell membranes by maintaining moisture on the respiratory surface.
Water must contain oxygen for organisms that live there. Although at a lower concentration
than in the atmosphere, oxygen dissolves in water. About 21% of the oxygen in the atmosphere is
nitrogen. The oxygen content of water is significantly lower than that. Gills were developed by fish
and other aquatic organisms to absorb dissolved oxygen from water (Figure 131). Gills are highly
branched, folded, and thin tissue filaments. The dissolved oxygen in water quickly diffuses across the
gills and into the bloodstream as water flows over the gills. The oxygenated blood can then be
transported to the rest of the body by the circulatory system. Oxygen diffuses across the gill surfaces
into the coelomic fluid in animals that do not have blood. Mollusks, annelids, and crustaceans all
have gills.
Figure 131. Like many aquatic organisms, this common carp has gills that enable it to absorb
oxygen from the water
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To ensure that the fish receives enough oxygen, the folded surfaces of the gills offer a sizable
surface area. Material moves from areas of high concentration to areas of low concentration through
the process of diffusion until equilibrium is reached. In this instance, blood that has little oxygen in
it flows through the gills. The oxygen molecule concentration in water is greater than the oxygen
molecule concentration in gills. As a result, Figure 132 illustrates how oxygen molecules diffuse from
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(credit: Stephen Childs)
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(credit: "Guitardude012"/Wikimedia Commons)
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water (high concentration) to blood (low concentration). Similarly, carbon dioxide molecules in the
blood diffuse from the blood (high concentration) to water (low concentration).