Carbohydrates Sugars and Sugar Polymers

The second class of biological molecules, the carbohydrates, is a diverse group of compounds. Carbohydrates contain primarily carbon atoms flanked by hydrogen atoms and hy-droxyl groups (H—C—OH). They have two major biochemical roles:

► They act as a source of energy that can be released in a form usable by body tissues.

The protein binds to the chaperonin "cage" and enters it.

Protein

"Cage"

The protein folds into its appropriate shape and is released.

Protein

The protein folds into its appropriate shape and is released.

The protein binds to the chaperonin "cage" and enters it.

Cage Protein

"Cage"

3.12 Chaperonins Protect Proteins from Inappropriate Binding

Chaperonins surround new or denatured proteins and prevent them from binding to the wrong ligand.

► They serve as carbon skeletons that can be rearranged to form other molecules that are essential for biological structures and functions.

Some carbohydrates are relatively small, with molecular weights of less than 100. Others are true macromolecules, with molecular weights in the hundreds of thousands.

There are four categories of biologically important carbohydrates, which we will discuss in turn:

► Monosaccharides (mono-, "one"; saccharide, "sugar"), such as glucose, ribose, and fructose, are simple sugars. They are the monomers out of which the larger carbohydrates are constructed.

► Disaccharides (di-, "two") consist of two monosaccharides linked together by covalent bonds.

► Oligosaccharides (oligo-, "several") are made up of several (3 to 20) monosaccharides.

► Polysaccharides (poly-, "many"), such as starch, glycogen, and cellulose, are large polymers composed of hundreds or thousands of monosaccharides.

The general formula for carbohydrates, CH2O, gives the relative proportions of carbon, hydrogen, and oxygen in a monosaccharide (i.e., the proportions of these atoms are 1:2:1). In disaccharides, oligosaccharides, and polysaccha-rides, these proportions differ slightly from the general formula because two hydrogens and an oxygen are lost during each of the condensation reactions that form them.

Monosaccharides are simple sugars

Green plants produce monosaccharides through photosynthesis, and animals acquire them directly or indirectly from plants. All living cells contain the monosaccharide glucose. Cells use glucose as an energy source, breaking it down through a series of reactions that release stored energy and produce water and carbon dioxide.

Glucose exists in two forms, the straight chain and the ring. The ring form predominates in more than 99 percent of circumstances because it is more stable under cellular conditions. There are two forms of the ring structure (a-glucose and P-glucose), which differ only in the placement of the —H and —OH attached to carbon 1 (Figure 3.13). The a and P forms interconvert and exist in equilibrium when dissolved in water.

Different monosaccharides contain different numbers of carbons. (The standard convention for numbering carbons in carbohydrates shown in Figure 3.13 is used throughout this book.) Most of the monosaccharides found in living systems belong to the D series of optical isomers (see Chapter 2). But some monosaccharides are structural isomers, which have the same kinds and numbers of atoms, but arranged differently. For example, the hexoses (hex-, "six"), a group of structural isomers, all have the formula C6H12O6. Included among the hexoses are glucose, fructose (so named because it was first found in fruits), mannose, and galactose (Figure 3.14).

Pentoses (pent-, "five") are five-carbon sugars. Some pen-toses are found primarily in the cell walls of plants. Two pentoses are of particular biological importance: Ribose and deoxyribose form part of the backbones of the nucleic acids RNA and DNA, respectively. These two pentoses are not isomers; rather, one oxygen atom is missing from carbon 2 in deoxyribose (de-, "absent") (see Figure 3.14). As we will see in Chapter 12, the absence of this oxygen atom has important consequences for the functional distinction of RNA and DNA.

Glycosidic linkages bond monosaccharides together

The disaccharides and polysaccharides described above are all constructed from monosaccharides that are covalently bonded together by condensation reactions that form glyco-sidic linkages. One such linkage between two monosaccha-rides forms a disaccharide. For example, a molecule of su-

The numbers in red indicate the standard convention for numbering the carbons.

H 1 .O Aldehyde group

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  • Cotman
    What areboilogical molecules that are sugar polymers?
    6 years ago

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