CHAPTER 22: Unit 3. Carbohydrate Metabolism
Carbohydrate metabolism denotes the various biochemical processes responsible for the formation, breakdown and interconversion of carbohydrates in living organisms. Organisms capable of aerobic respiration metabolize glucose and oxygen to release energy with carbon dioxide and water as byproducts.
Carbohydrates play several crucial roles in the metabolic processes of living organisms. They serve as energy sources and as structural elements in living cells. Because the monosaccharide glucose is a prominent energy source in almost all living cells, major emphasis is placed on its synthesis, degradation, and storage.
Carbohydrates are an important source of the energy that drives these reactions. This chapter discusses the energy-generating pathways of carbohydrate metabolism are discussed. During glycolysis, an ancient pathway found in almost all organisms, a small amount of energy is captured as a glucose molecule is converted to two molecules of pyruvate. Glycogen, a storage form of glucose in vertebrates, is synthesized by glycogenesis when glucose levels are high and degraded by glycogenolysis when glucose is in short supply. Glucose can also be synthesized from noncarbohydrate precursors by reactions referred to as gluconeogenesis. The pentose phosphate pathway enables cells to convert glucose-6-phosphate, a derivative of glucose, to ribose5-phosphate (the sugar used to synthesize nucleotides and nucleic acids) and other types of monosaccharides. NADPH, an important cellular reducing agent, is also produced by this pathway. In Chapter 9, the glyoxylate cycle, used by some organisms (primarily plants) to manufacture carbohydrate from fatty acids, is considered. Photosynthesis, a process in which light energy is captured to drive carbohydrate synthesis, is described in Chapter 13. Any discussion of carbohydrate metabolism focuses on the synthesis and usage of glucose, a major fuel for most organisms. In vertebrates, glucose is transported throughout the body in the blood. If cellular energy reserves are low, glucose is degraded by the glycolytic pathway. Glucose molecules not required for immediate energy production are stored as glycogen in liver and muscle. The energy requirements of many tissues (e.g., brain, red blood cells, and exercising skeletal muscle cells) depend on an uninterrupted flow of glucose. Depending on a cell’s metabolic requirements, glucose can also be used to synthesize, for example, other monosaccharides, fatty acids, and certain amino acids. Figure 8.2 summarizes the major pathways of carbohydrate metabolism in animals.
The most important carbohydrate is glucose, a simple sugar that is metabolized by nearly all known organisms. Glucose and other carbohydrates are part of a wide variety of metabolic pathways across species: plants synthesize carbohydrates from carbon dioxide and water by photosynthesis storing the absorbed energy internally, often in the form of starch or lipids. Plant components are consumed by animals and fungi, and used as fuel for cellular respiration. Oxidation of one gram of carbohydrate yields approximately 4 kcal of energy and from lipids about 9 kcal. Energy obtained from metabolism is usually stored temporarily within cells in the form of ATP. Organisms capable of aerobic respiration metabolize glucose and oxygen to release energy with carbon dioxide and water as byproducts. Carbohydrates can be chemically divided into complex and simple. Simple carbohydrates consist of single or double sugar units. Sucrose or table sugar is a common example of a simple carbohydrate. Complex carbohydrates contain three or more sugar units linked in a chain. They are digested by enzymes to release the simple sugars. Starch, for example, is a polymer of glucose units and is typically broken down to glucose. Simple and complex carbohydrates are digested at similar rates, so the distinction is not very useful for distinguishing nutritional quality. Cellulose is also a polymer of glucose but it cannot be digested by most organisms. Some bacteria that produce enzymes for cellulose live inside the gut of some mammals such as cows, and when cows eat plants, the cellulose is broken down by the bacteria and some of it is released into the gut.
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Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen atoms. The family of carbohydrates includes both simple and complex sugars. Glucose and fructose are examples of simple sugars, and starch, glycogen, and cellulose are all examples of complex sugars. The complex sugars are also called polysaccharides and are made of multiple monosaccharide molecules. Polysaccharides serve as energy storage (e.g., starch and glycogen) and as structural components (e.g., chitin in insects and cellulose in plants).
During digestion, carbohydrates are broken down into simple, soluble sugars that can be transported across the intestinal wall into the circulatory system to be transported throughout the body. Carbohydrate digestion begins in the mouth with the action of salivary amylase on starches and ends with monosaccharides being absorbed across the epithelium of the small intestine. Once the absorbed monosaccharides are transported to the tissues, the process of cellular respiration begins (Figure 1). This section will focus first on glycolysis, a process where the monosaccharide glucose is oxidized, releasing the energy stored in its bonds to produce ATP.
Further important pathways in carbohydrate metabolism include the pentose phosphate pathway (conversion of hexose sugars into pentoses), glycogenesis (conversion of excess glucose into glycogen, stimulated by insulin), glycogenolysis (conversion of glycogen polymers into glucose, stimulated by glucagon) and gluconeogenesis (de novo glucose synthesis).
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https://www.youtube.com/watch?v=-Ow8dbOfU8Q
There are multiple diseases that arise from improper carbohydrate metabolism. Diabetes mellitus is caused by a lack of, or a resistance to, insulin leading to hypo- or hyperglycemia. Lactose intolerance is a common allergy in adults and results from a lack of the enzyme lactase, which converts lactose disaccharides (found in dairy products) into glucose monosaccharides. Much rarer diseases such as galactosemia and von Gierke’s diseases are caused by congenital mutations in enzymes involved in glucose metabolic pathways.
Any discussion of carbohydrate metabolism focuses on the synthesis and usage of glucose, a major fuel for most organisms. In vertebrates, glucose is transported throughout the body in the blood. If cellular energy reserves are low, glucose is degraded by the glycolytic pathway. Glucose molecules not required for immediate energy production are stored as glycogen in liver and muscle. The energy requirements of many tissues (e.g., brain, red blood cells, and exercising skeletal muscle cells) depend on an uninterrupted flow of glucose. Depending on a cell’s metabolic requirements, glucose can also be used to synthesize, for example, other monosaccharides, fatty acids, and certain amino acids. The following Figure summarizes the major pathways of carbohydrate metabolism in animals.
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| Reference: https://global.oup.com/us/companion.websites/fdscontent/uscompanion/us/static/companion.websites/9780199730841/McKee_Chapter8_Sample.pdfMajor Pathways in Carbohydrate Metabolism In animals, excess glucose is converted to its storage form, glycogen, by glycogenesis. When glucose is needed as a source of energy or as a precursor molecule in biosynthetic processes, glycogen is degraded by glycogenolysis. Glucose can be converted to ribose-5-phosphate (a component of nucleotides) and NADPH (a powerful reducing agent) by means of the pentose phosphate pathway. Glucose is oxidized by glycolysis, an energy-generating pathway that converts it to pyruvate. In the absence of oxygen, pyruvate is converted to lactate. When oxygen is present, pyruvate is further degraded to form acetyl-CoA. Significant amounts of energy in the form of ATP can be extracted from acetyl-CoA by the citric acid cycle and the electron transport system. Note that carbohydrate metabolism is inextricably linked to the metabolism of other nutrients. For example, acetyl-CoA is also generated from the breakdown of fatty acids and certain amino acids. When acetyl-CoA is present in excess, a different pathway converts it into fatty acids.
Carbohydrate metabolism is a fundamental biochemical process that ensures a constant supply of energy to living cells. The most important carbohydrate is glucose, which can be broken down via glycolysis, enter into the Kreb’s cycle and oxidative phosphorylation to generate ATP. Oxidative phosphorylation is a combination of two simultaneous processes; the electron transport chain and chemiosmotic coupling. The electron |