Summary of Chapter Concepts

Water has many properties favorable to life. These include thermal resistance to changes in temperature, a density and viscosity that select for adaptations for movement, and the ability to dissolve many elements and compounds that are essential to life.

Aquatic environments challenge the balance of water and salt in animals. This challenge occurs because animal tissues are generally hyperosmotic or hyposmotic compared to the solute concentration of the surrounding aquatic environment.

The uptake of gases from the water is limited by diffusion. This limitation can make it difficult for organisms to exchange gases such as CO₂ and O₂. Diffusion is slowed because organisms are surrounded by a thin boundary layer of unstirred water. Although CO₂ can be abundant in water—either as a dissolved gas or in the form of bicarbonate ions—oxygen’s low solubility in water makes it less abundant. As a result, many animals have respiratory structures with large surface areas and countercurrent circulation to obtain oxygen from the water.

Temperature limits the occurrence of aquatic life. Although warmer temperatures increase the rate of chemical reactions, excessively high temperatures cause proteins and other important molecules to become unstable and denature. Cold temperatures also pose a challenge and many organisms living under near-freezing conditions have evolved adaptations that include the use of glycoproteins and supercooling to prevent the harmful effects of ice crystals growing inside their cells. Organisms that live under a wide range of temperature often use isozymes to permit proper physiological function at each temperature.