TISSUE RESPIRATION | Cellular Respiration

Cellular respiration constitutes the main oxygen-consuming and adenosine triphosphate (ATP)-producing processes. Whole-animal metabolic rate is the sum of respiration from all tissues combined. ATP production by oxidative phosphorylation (OXPHOS) requires adequate delivery of both oxygen and metabolic fuels to cells. While environmental levels of oxygen (e.g., atmospheric or dissolved oxygen) and the pathway from gas-transfer organs to tissues determine oxygen delivery, fuels can be drawn from stores within the body. The pathway for oxygen from the environment to mitochondria involves the processes of diffusion and convective transport. The delivery of nonesterified fatty acids (NEFAs), glucose, and amino acids additionally relies on protein must also rely on protein transporters to cross membranes. While most, but not all, of the aspects of cellular respiration are shared across taxa, fish show some unique characteristics. Probably the biggest difference between mammals and fish are the fuels used to power muscle during exercise, and their plasticity in response to environmental and energetic stress. In addition, different fish groups can be thought of as natural knockout models for certain aspects of the fuel and oxygen delivery systems. The elasmobranchs lack NEFA plasma transport proteins and a key fat oxidation enzyme. Several species of Antarctic icefish lack the oxygen-transport proteins hemoglobin and myoglobin. Understanding how these species deal with the absence of these proteins provides windows into the regulation of fuel and oxygen-delivery pathways.