Carbohydrates

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Chapter 9 Carbohydrates

Chapter contents

Plant metabolism 63
Formation of carbohydrates 64
Function of carbohydrates 64
Monosaccharides 64
Nomenclature of sugars 66
Disaccharides, e.g. sucrose 67
Polysaccharides 67
The formation of carbohydrate polymers 68
Why is cellulose difficult to break down? 68
Fat-free foods 68
Polysaccharides with special uses 68

Plant Metabolism

Two systems are at work in a plant:

1. Primary metabolism: products of photosynthesis, which provide the food necessary if the plant is to live, grow and reproduce.
2. Secondary metabolism: very diverse and thought to provide secondary functions for the plant, such as protection against animals that might eat it or preservation again microbial attack, thus preventing decomposition. In some cases, secondary metabolites interact with symbiotic organisms. For example, cyanogenic glycosides made by sugars linking to highly toxic cyanide are released when the plant is attacked by an insect or disease-causing organism.

Photosynthesis is the means by which plants use sunlight to produce sugar. Through various chemical reactions in the plant, this ultimately produces energy that the plant can use. Structures called chloroplasts, which are present in the leaves, enable plants to absorb light. The chloroplasts contain photosynthetic pigments called chlorophylls; chlorophyll A is the main photosynthetic pigment in all organisms except bacteria. Combinations of pigments in the plant increase the spectrum of light that plants can absorb for photosynthesis. Carotenoids (Figure 9.1; see Chapter 22 ‘Terpenes’, p. 173) help to transfer this energy. Carbon dioxide and water combine to create chemical energy that can be stored.

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Figure 9.1 The structures of compounds (carotenoids) able to absorb light energy.

Formation of Carbohydrates

The energy that plants gain from the light is stored as carbohydrates. The classic equation for this is shown below:

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Function of Carbohydrates

• Energy storage (starch in plants and glycogen in animals): humans obtain some of their energy from carbohydrates that come from plants. Effectively, plants store carbohydrates as energy. When the carbohydrates are metabolized energy is released.
• Structural (cellulose cell walls or chitin exoskeletons; Figure 9.9, p. 70).
• Recognition (carbohydrates of glycoproteins and glycolipids; pp. 71–72).
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Figure 9.9 The structures of chitin and cellulose.

Monosaccharides

Monosaccharides are the simplest form of sugar. They have more than one hydroxyl group and are based on a backbone of 3, 4, 5; or 6 carbons (triose, tetrose, pentose, or hexose, respectively). Figure 9.2 demonstrates the varieties of sugars of different numbers of carbons:

• Trioses: found in the initial stages after photosynthesis.
• Ribose, deoxyribose, fructose and glucose: can convert between a ring and straight structure.
• Glucose: usually referred to as blood sugar and used as a measure of how effectively the body deals with sugars.
• Fructose: very often used as a sweetener in drinks; in many cases it is derived from corn syrup.
• Ribose and deoxyribose: the sugar components of RNA and DNA.
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Figure 9.2 The variety of sugars.

General Characteristics of Simple Sugars

• These simple sugars are polar (as a result of all the —OH groups) and therefore are readily water soluble. Because they are small, their passage through cell membranes is relatively easy and they are probably pulled through by simple osmosis (see Figure 16.1, p. 124) as the water is pulled though.
• Most (but not all) have a sweet taste; a sugar moiety is responsible for the sweet taste of a glycoside.
• Each carbon that supports a hydroxyl group, except for the first and last carbon (atoms) is a chiral centre (see Figures 6.3, 6.5, 6.6 and 6.7, pp. 36–39) and can therefore give rise to a number of isomers all with the same chemical formula (see Chapter 6 ‘Isomers’, p. 35), for example, glucose and galactose have the same chemical formula but different chemical and physical properties (Figure 9.3).
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Figure 9.3 Comparison of glucose and galactose structures. These tend not to be in a position to rotate as mostly they are thought of a being in a ring structure.

Furanose and Pyranose

Sugars can form ring structures. There are two main types (Figure 9.4):

• Furanose: a five-membered-ring sugar molecule (fructose).
• Pyranose: a six-membered-ring sugar molecule (glucose).

image

Figure 9.4

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