A. Metabolism--The build-up and destruction of protoplasm and other materials. Catabolism and anabolism 1. Energy to do work in most organisms is in the form of ATP 2. Most ATP is produced by aerobic process, thus the strong reason for organisms to move O2 from outside their bodies to the cells (mitochondria). B. Transfer of gases is based on their concentrations and solublities. 1. Most transfers are facilitated by movement along concentration gradients. Since the cells are constantly using up O2, there is always a very steep gradient between outside and inside. This gradient is only good if the distance is less than a mm. Thicker distances prevent effective transfers via diffusion. 2. Nitrogen is a non-biologically important gas, except in an indirect fashion and that is mostly associated with air breathing vertebrates (when nitrogen comes out of solution) 3. Oxygen and carbon dioxide are the two important gases. They both occur in atomospheric air in a relatively constant proportion any where over the globe. Because of differential solubilities, in aqueous media, the proportion of one gas to the other is very different. Solubility is dependent upon a number of factors, including temperature and pressure. 4. Organisms enhance transport of oxygen from outside of the body by having gills, lungs, etc. that facilitate the movement of oxygen from the external environment into the body fluids and circulatory systems that then move the body fluids from uptake sites to tissue use sites. 5. Enhanced uptake occurs because of flow over the respiratory surface. Enhanced transportation occurs because of blood cells or respiratory pigments or both.