CHAPTER TWENTY-FOUR
METABOLISM AND ENERGETICS
24.1 An Introduction to Cellular Metabolism A. Metabolism = all the chemical reactions that occur in an organism 1. Cellular metabolism refers to collective chemical reactions that occur within cells in order to provide the energy needed to maintain homeostasis and to perform essential functions. 2. Metabolic reactions are either catabolic or anabolic.
a. Anabolism = chemical reactions where organic compounds are synthesized. b. Catabolism = chemical reactions where organic compounds are broken down.
B. In the process of metabolic turnover, cells continuously break down and replace all their organic components except DNA. 1. turnover or energy production. 2. Cells then absorb organic molecules from the surrounding interstitial fluids, supplementing those released through catabolic reactions in metabolic turnover. The components of the nutrient pool can either be used for anabolism or broken down further for ATP production. 3. During the catabolic reactions of the mitochondria, roughly 40% of the energy is used for the production of ATP and cellular work. The other 60% escapes as heat that warms the interior of the cell and the surrounding tissues. C. Cells can break down any available substrate from the nutrient pool to obtain energy. 1. Nutrients are obtained through digestion and absorption and then distributed to body cells via the blood and interstitial fluids. 2. Cells in most tissues absorb and catabolize glucose as their primary fuel source 3. Neural tissues require a continuous supply of glucose. During starvation, other tissues can shift to fatty acids or amino acids for fuel thereby conserving glucose for neural tissues. 4. Liver cells store triglycerides and glycogen reserves. If absorption by the digestive tract fails to maintain normal nutrient levels, the triglycerides and glycogen are broken down and the fatty acids and glucose are released for cells to burn. 5. Adipocytes convert excess fatty acids to triglycerides for storage. If absorption by the digestive tract and reserves in the liver fail to maintain normal nutrient levels, the triglycerides are broken down and the fatty acids are released for cells to burn. 6. Skeletal muscles at rest metabolize fatty acids and use glucose to build glycogen reserves. Amino acids are used to increase the number of myofibrils. If the digestive tract, adipocytes, and liver are unable to maintain normal nutrient levels, the contractile proteins can be broken down and amino acids release into the circulation for use by other tissues. D. RettDox Reactions 2. Oxidation -- the loss of a hydrogen ion. The two most common coenzymes of oxidation- reduction reactions are nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD).
FADH2, which are energyttcontaining molecules used to transfer energy during the creation of ATP.
24.2 Carbohydrate Digestion, Absorption, and Metabolism. A. Digestion and Absorption of Carbohydrates:
1. In the mouth, mastication helps to saturate the bolus with the secretion of the salivary glands Salivary amylase breaks down complex carbohydrates into a mixture of disaccharides and oligosaccharides. 2. Salivary amylase is denatured in the high acid conditions of the stomach