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Each hydrogen shares a pair of electrons with the oxygen. The oxygen has a greater affinity for electrons than the hydrogens, so it ‘pulls’ the electrons closer.

Water (H2O) consists of two hydrogen atoms covalently bonded to one oxygen atom.

This makes the oxygen slightly negative (indicated by δ–) and the hydrogens slightly positive (indicated by δ+).

This creates different charged regions, making water a polar molecule. Because it has two charged regions it is dipolar.

104.5 °

δ– δ+

δ+

Hydrogen bonds

The slight negative charge on the oxygen atom makes it attract the slightly positive hydrogen atom of another water molecule.

Many of the properties of water are due to its ability to form hydrogen bonds.

The numerous hydrogen bonds in water make it a very stable structure.

hydrogen bond

Water as a solvent

Properties and biological roles of water

Carbohydrates are a group of substances used as both energy sources and structural materials in organisms.

All carbohydrates contain carbon, hydrogen and oxygen, with the general formula: Cx(H2O)y.

There are three main groups of carbohydrates:

monosaccharides – these are simple sugars, with the general formula (CH20)n, where n can be 3–7

disaccharides – these are ‘double sugars’, formed from two monosaccharides

polysaccharides – these are large molecules formed from many monosaccharides.

The structure of glucose can be represented in different ways:

Glucose is an abundant and very important monosaccharide. It contains six carbon atoms so it is a hexose sugar. Its general formula is C6H12O6.

Glucose is the major energy source for most cells. It is highly soluble and is the main form in which carbohydrates are transported around the body of animals.

straight chain ring ring (simplified)

Glucose exists in different forms called structural isomers. Two common isomers are alpha glucose and beta glucose.

alphaglucose

betaglucose

The only difference between these two isomers is the position of the –OH group attached to carbon 1. In alpha glucose it is below the carbon and in beta glucose it is above the carbon.

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This minor structural difference has a major effect on the biological roles of alpha and beta glucose.

Galactose is not as soluble as glucose and has an important role in the production of glycolipids and glycoproteins.

Two other important hexose monosaccharides are fructose and galactose.

fructose galactose

Fructose is very soluble and is the main sugar in fruits and nectar. It is sweeter than glucose.

Pentose monosaccharides contain five carbon atoms. Like hexoses, pentoses are long enough to form a ring.

Two important pentose molecules are the structural isomers ribose and deoxyribose. These are important constituents of RNA and DNA.

ribose deoxyriboseThe only difference between them is that ribose has one H atom and one –OH group attached to carbon 2, whereas deoxyribose has 2 H atoms and no –OH group.

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The formation of disaccharides

Maltose (malt sugar) is formed from two glucose molecules joined by an alpha 1–4 glycosidic bond.

Sucrose (table sugar) is formed from glucose and fructose joined by an alpha 1–4 glycosidic bond.

Lactose (milk sugar) is formed from galactose and glucose joined by a beta 1–4 glycosidic bond.

Benedict’s test for reducing sugars

Mono- and disaccharides

Polysaccharides are polymers containing many monosaccharides linked by glycosidic bonds. Like disaccharides, polysaccharides are formed by condensation reactions.

The major polysaccharides are starch and cellulose in plants, and glycogen in animals.

Polysaccharides are mainly used as an energy store and as structural components of cells.

The structure of starch

Properties and uses of starchStarch is the major carbohydrate storage molecule in plants.

Starch is produced from glucose made during photosynthesis. It is broken down during respiration to provide energy and is also a source of carbon for producing other molecules.

It is usually stored as intracellular starch grains in organelles called plastids.

Plastids include green chloroplasts (e.g. in leaves) and colourless amyloplasts (e.g. in potatoes).

Iodine test for starch

Unlike starch, cellulose is very strong, and prevents cells from bursting when they take in excess water.

Cellulose is another polysaccharide and is the main part of plant cell walls. It is the most abundant organic polymer.

Cellulose consists of long chains of beta glucose molecules joined by beta1–4 glycosidic bonds.

The glucose chains form rope-like microfibrils, which are layered to form a network.

The structure of cellulose

Animals do not store carbohydrate as starch but as glycogen.

Glycogen has a similarstructure to amylopectin, containing many alpha 1–6 glycosidic bonds that produce an even more branched structure.

Glycogen is less dense and more soluble than starch, and is broken down more rapidly. This indicates the higher metabolic requirements of animals compared with plants.

Glycogen is stored as small granules, particularly in muscles and liver.

Polysaccharides: true or false?