Chapter 11: Glycogen Metabolism

Overview A constant source of blood glucose is an absolute requirement for human life. Glucose is the greatly preferred energy source for the brain, and the required energy source for cells with few or no mitochondria, such as mature erythrocytes. Glucose is also essential as an energy source for exercising muscle, where it is the […]

Chapter 12: Metabolism of Monosaccharides and Disaccharides

Overview Glucose is the most common monosaccharide consumed by humans, and its metabolism has been discussed extensively. However, two other monosaccharides—fructose and galactose—occur in significant amounts in the diet (primarily in disaccharides), and make important contributions to energy metabolism. In addition, galactose is an important component of cell structural carbohydrates. Figure 12.1 shows the metabolism […]

Chapter 13: Pentose Phosphate Pathway and NADPH

Overview The pentose phosphate pathway (also called the hexose monophosphate pathway, or 6-phosphogluconate pathway) occurs in the cytosol of the cell. It includes two, irreversible oxidative reactions, followed by a series of reversible sugar-phosphate interconversions (Figure 13.1). No ATP is directly consumed or produced in the cycle. Carbon 1 of glucose 6-phosphate is released as […]

Chapter 14: Glycosaminoglycans, Proteoglycans, and Glycoproteins

Overview of Glycosaminoglycans Glycosaminoglycans are large complexes of negatively charged hetero-polysaccharide chains. They are generally associated with a small amount of protein, forming proteoglycans, which typically consist of over 95% carbohydrate. [Note: This is in comparison to the glycoproteins, which consist primarily of protein with a small amount of carbohydrate (see Overview of Glycoproteins).] Glycosaminoglycans […]

Chapter 15: Metabolism of Dietary Lipids

Overview Lipids are a heterogeneous group of water-insoluble (hydrophobic) organic molecules that can be extracted from tissues by nonpolar solvents (Figure 15.1). Because of their insolubility in aqueous solutions, body lipids are generally found compartmentalized, as in the case of membrane-associated lipids or droplets of triacylglycerol in white adipocytes, or transported in plasma in association […]

Chapter 16: Fatty Acid and Triacylglycerol Metabolism

Overview Fatty acids exist “free” in the body (that is, they are unesterified), and are also found as fatty acyl esters in more complex molecules, such as triacylglycerols. Low levels of free fatty acids occur in all tissues, but substantial amounts can sometimes be found in the plasma, particularly during fasting. Plasma free fatty acids […]

Chapter 17: Complex Lipid Metabolism

Overview of Phospholipids Phospholipids are polar, ionic compounds composed of an alcohol that is attached by a phosphodiester bridge to either diacylglycerol or sphingosine. Like fatty acids, phospholipids are amphipathic in nature, that is, each has a hydrophilic head (the phosphate group plus whatever alcohol is attached to it, for example, serine, ethanolamine, and choline, […]

Chapter 18: Cholesterol and Steroid Metabolism

Overview Cholesterol, the characteristic steroid alcohol of animal tissues, performs a number of essential functions in the body. For example, cholesterol is a structural component of all cell membranes, modulating their fluidity, and, in specialized tissues, cholesterol is a precursor of bile acids, steroid hormones, and vitamin D. It is therefore of critical importance that […]

Chapter 19: Amino Acids: Disposal of Nitrogen

Overview Unlike fats and carbohydrates, amino acids are not stored by the body, that is, no protein exists whose sole function is to maintain a supply of amino acids for future use. Therefore, amino acids must be obtained from the diet, synthesized de novo, or produced from normal protein degradation. Any amino acids in excess […]

Chapter 20: Amino Acid Degradation and Synthesis

Overview The catabolism of the amino acids involves the removal of ?-amino groups, followed by the breakdown of the resulting carbon skeletons. These pathways converge to form seven intermediate products: oxaloacetate, pyruvate, ?-ketoglutarate, fumarate, succinyl coenzyme A (CoA), acetyl CoA, and acetoacetate. These products directly enter the pathways of intermediary metabolism, resulting either in the […]