Two systems are at work in a plant:

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.

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

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

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:

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).

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).

Figure 9.4 The differences between furanose (fructose) and pyranose (glucose) rings (note the thickened line indicating the ring coming out of the page towards you).

When reading the literature on sugars, particularly those as part of the structure of glycosides (see Chapter 24 ‘Glycosides’, p. 181) you might come across complex shorthand denoting the type of sugar or sugars to which the chemically active compound is attached. It is only really necessary to understand this if you want to pursue pharmacology to a very high level. Usually, it is enough to know you are looking at an active chemical attached to a glycoside.

Numbering Sugars

The carbon atoms in a sugar ring are numbered. This enables a chemist to see where functional groups are attached (Figure 9.5).

Figure 9.5 Numbering of a sugar molecule.

Abbreviations are used for some of the more common sugars, for example:

• Glc: glucose

• Man: mannose

• Gal: galactose

• Xyl: xylose.

Type of Ring Structure

p and f indicate pyranose and furanose rings.

• Alpha or Beta Sugars

Sugars in a ring are also known as either alpha or beta sugars:

• An —OH in the beta position is on the same side as the C6 carbon.

• An —OH in the alpha position is on the opposite side of the ring to the C6 carbon.

• ‘Boats and Chairs’

Sugars that form a ring structure, such as glucose, have several options as far as shape is concerned. Figure 9.6 demonstrates these variations.

Figure 9.6 The different forms of cyclic sugar structures.

The chair structure is most commonly seen attached to various plant chemicals to form a glycoside. Its shape is more conducive to forming a compact shape than a boat structure, and it is considered the most stable form. The lower bold edge of the ring represents this part of the molecule projecting out of the page towards the reader.

Monosaccharides can join together by condensation (see Figure 4.7B, p. 26). When sugars are linked together the links can be one of two sorts: either at the fourth carbon (1–4 bond) or the sixth carbon (1–6 bond).

The bonds created between individual sugars can vary between the 1, 4 or 6 carbon atoms on each monosaccharide. This will lead to combinations that result in different chemical and physical properties (as shown in Figure 9.7).

When beta-glucose (the —OH is on the same side as the C6 carbon) forms the polymer cellulose, the linkage occurs through beta 1–4 glycosidic bonds. This arrangement allows the hydroxyl groups sticking off the sugar backbone to form hydrogen bonds (see Figure 3.13, p. 19, Figure 4.7B, p. 26 and Figure 9.7) with other strings of cellulose. This effectively ‘glues’ the strings together, so that as a whole the structure has a high tensile strength. This is important in plant cells walls because it makes them rigid. It also explains why cellulose provides fibre in the diet; cellulose is difficult to break down.

Carbohydrates such as starch or glycogen contain alpha 1–4 glycosidic bonds (the —OH is on the opposite side to the C6 glycosidic bond). This type of bonding creates coiling, which is not conducive to forming chains linked by hydrogen bonds.

Foods that are called ‘fat free’ are not really fat free. Somewhere, the label will list some type of sugar in the ingredients. Sugar is transformed to fat in the body and stored if the sugar is not used up.