Chapter 5: Cell Respiration & Metabolism. In cellular respiration, energy is released by the stepwise breakdown of glucose and other molecules, and some of this energy is used to produce ATP. All of the reactions in the body that involve energy transformation are collectively termed metabolism. Metabolism may be divided into 2 categories: 1. Anabolism: require the input of energy and include the synthesis of large energy-storage molecules, including glycogen, fat, and protein. 2. Catabolism: release energy, usually by the breakdown of larger organic molecules into smaller molecules.

Fig. 5. 1 Overview of energy metabolism using blood glucose Fig. 5.1 Overview of energy metabolism using blood glucose. The blood glucose may be obtained from food via the digestive tract, or the liver may produce it from stored glycogen. Plasma glucose enters the cytoplasm of cells, where it can be used for energy by either anaerobic or aerobic cell respiration.

Uses of Different Energy Sources The blood serves as a common trough from which all the cells in the body are fed. If all cells used the same energy source, such as glucose, this source would quickly be depleted and cellular starvation would occur. The energy sources in blood are: Glucose from the liver Ketone bodies from the liver Fatty acids from the adipose tissue Amino acids from the muscles Lactic acid from the muscles Some organs preferentially use one energy source more than the others, so that each energy source is “spared” for organs with strict energy needs. During severe starvation, the brain also gains some ability to metabolize ketone bodies for energy.

Table 5.5 Relative Importance of Different Molecules in the Blood with Respect to the Energy Requirements of Different Organs Organ Glucose Fatty Acids Ketone Bodies Lactic Acid Brain +++ _ + Skeletal (muscles resting) Liver ++ Heart

Figure 5. 10 Glycogenesis and glycogenolysis Figure 5.10 Glycogenesis and glycogenolysis. Blood glucose entering tissue cells is phosphorylated to glucose 6-phosphate. This intermediate can be metabolized for energy in glycolysis, or it can be converted to glycogen (1) in a process called glycogenesis. Glycogen represents a storage from of carbohydrates that can be used a source for new glucose 6-phosphate (2) in a process called glycogenolysis. The liver contains an enzyme that can remove the phosphate from glucose 6-phosphate; liver glycogen thus serves as a source for new blood glucose.

Figure 5.18 The interconversion of glycogen, fat, and protein. Most reactions are reversible, the reaction from pyruvic acid to acetyl CoA is not. (Only plants can use CO2 to produce glucose.)