Answer: Cellular respiration is a series of chemical reactions that break down glucose to produce ATP, which may be used as energy to power many reactions throughout the body. There are three main steps of cellular respiration: glycolysis, the citric acid cycle, and oxidative phosphorylation.
Cellular respiration is the process by which animals, such as fishes, produce energy in the form of ATP to support basal and maximal rates of metabolism. This balancing of ATP supply and demand is essential to the maintenance cellular homeostasis. This article focuses on the mechanisms involved in aerobic ATP production, with a review of current knowledge and highlights differences in mechanisms observed between mammals and fishes. Interaction with the environment also impact capacities for ATP production and regulation of metabolism in fishes. These are discussed along with adaptations for alternative mechanisms that occur in novel species.
Explanation: Respiration, Metabolic Pathways
Glycolysis
As previously indicated, cellular respiration allows controlled release of free energy from carbohydrate, fat, and protein energy substrate. Cellular respiration consists of three related series of biochemical reactions:
1.
Degradative reactions resulting in the formation of acetyl coenzyme A and reducing equivalents
2.
Metabolism of acetate to carbon dioxide in the Krebs cycle with generation of additional reducing equivalents
3.
Shuttling of electrons generated from reducing equivalents along the mitochondrial electron transport chain
Acetate coupled to coenzyme A (AcCoA) is derived from carbohydrates, lipids, and proteins. Glucose is transported into cells via glucose transporter (GLUT) receptors and osmotic gradients (see Chapter 77). Ten enzymatic reactions within the cell cytoplasm define the metabolic pathway, termed glycolysis. These initial series of reactions ultimately generate two net molecules of ATP, two molecules of NADH, and two molecules of pyruvate.
The fates of pyruvate is multiple. Under anaerobic conditions, pyruvate may be reduced by NADH to lactate to regenerate NAD+. Alternatively, pyruvate is shuttled to the mitochondria, where it is further metabolized to carbon dioxide (CO2) and AcCoA. Pyruvate transamination yields alanine, whereas pyruvate carboxylase generates oxylacetate (Figure 74-6).
Catabolism of fatty acids by β-oxidation generates one molecule of AcCoA and one molecule each of FADH2 and NADH for each two-carbon fatty acid fragment cycle. These reactions occur in the mitochondria after fatty acid transport by a carnitine transport system. It should be appreciated that generation of AcCoA by fatty acid β-oxidation occurs independent of pyruvate dehydrogenase that can be rate limiting for complete glucose metabolism. Particularly as an aspect of the metabolic stress response mediated by cortisol, catecholamines, and interleukins 6 and 2, protein degradation can occur with release of amino acids. All amino acids may be catabolized to either AcCoA or some Krebs cycle intermediate. Accordingly, amino acids can be mobilized for energy production as well as de novo protein synthesis. Alternatively, amino acids can undergo gluconeogenesis, a costly process that basically requires four ATP molecules plus two GTP molecules and two NADH molecules to regenerate one molecule of glucose from two molecules of pyruvate (see Chapter 77). ATP and GTP for these series of reactions are provided by β-oxidation of fatty acids.