An Introduction to Carbohydrates

Carbohydrates, commonly referred to as sugars and starches, are polyhydroxy aldehydes and ketones, or compounds that can be hydrolyzed to them. Along with proteins, fatty acids, and nucleotides, they form one of the four main groups of biomolecules responsible for the structure and function of all living cells. 

Many carbohydrates contain cyclic acetals or hemiacetals. Examples include glucose, the most common simple sugar, and lactose, the principal carbohydrate in milk. 

Glucose is the carbohydrate that is transported in the blood to individual cells. The hormone insulin regulates the level of glucose in the blood. Diabetes is a common disease that results from a deficiency of insulin, resulting in increased glucose levels in the blood and other metabolic abnormalities. Insulin injections control glucose levels. 

Hemiacetals in sugars are formed in the same way that other hemiacetals are formed—that is. by cyclization of hydroxy aldehydes. Thus, the hemiacetal of glucose is formed by cyclization of an acyclic po/yhydroxy aldehyde (A), as shown in the accompanying equation. This process illustrates two important features. 

Many more details on this process and other aspects of carbohydrate chemistry are pre.sented in Chapter 27. 

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Chapter 11, with which this part begins, is an introduction to carbohydrates and other carbonaceous materials (e.g., fatty acids, terpenes, steroids Figure III-A) derived, initially, from the absorption of carbon dioxide (CO2) by plants and passing through acetate (CH3C02 ) or its equivalent. Where appropriate, some references to enzymes involved in these conversions will be indicated. 

AN INTRODUCTION TO CARBOHYDRATES, ACETOGENINS, AND STEROIDS Bicvck>r4.4.01 ffused 6.6>... 

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