The straight chain structure is in equilibrium with a 6-membered ring structure in solution:
or
The ring structure occurs because the straight chain can turn back on itself and the reactive (aldehyde) group on carbon 1 can react with a hydroxyl (-OH) group on carbon 5. The reason why glucose is soluble is because the –OH group can hydrogen bond with water molecules.
The ring structure can be of two types at carbon 1, α glucose is the standard glucose; β glucose is not common in cell metabolism but is found in cellulose. Stereoisomerism occurs when the same atoms or groups are joined together but differ in their arrangement in space. α and β glucose are isomers – can you spot the one difference?
α-glucose β-glucose
Disaccharides:
Glucose can be linked to one other monomer to make a disaccharide.
Examples of disaccharides:
Glucose + glucose = Maltose + water
Glucose + galactose = Lactose (milk sugar) + water
Glucose + fructose = Sucrose (cane sugar) + water
When the monosaccharide units join together they form a new covalent bond, with the elimination of water. This is called a condensation reaction. The addition of water is necessary if a disaccharide is to split back into monosaccharides. This is the opposite of condensation and is called hydrolysis. The bond formed is called a glycosidic bond and is usually formed between the C1 of one monosaccharide and the C4 of the other. This is known as an α 1-4 glycosidic bond.
Maltose is a common disaccharide in living things. It is a breakdown product of carbohydrate metabolism in animals. Sucrose is common in plants. It is a condensation product of a glucose and fructose molecule. This time both reducing groups are involved in the glycosidic bond and so sucrose has no free reducing group and is therefore not a reducing sugar. Maltose does have a free reducing group on the right hand glucose so is a reducing sugar. Lactose is found in mammalian milk and is a condensation product of glucose and galactose and is also a reducing sugar. Disaccharides are sweet, soluble and crystalline.
Polysaccharides:
There are three main types: Starch
Glycogen
Cellulose
The first two are storage molecules and cellulose is a structural molecule.
Polysaccharides are formed when many hundreds of monosaccharides condense (join) to form chains. The chains formed may be:
- Variable in length
- Branched or unbranched
- Folded – ideal for energy storage
- Straight or coiled
The most important polysaccharides are made from many hexose units. Polysaccharides are large, insoluble in water and not sweet – making them ideal for energy storage because they exert no osmotic influence and don’t easily diffuse out of cells. Upon hydrolysis, polysaccharides are converted into their constituent monosaccharides to be used as respiratory substrates.
Starch is made of two carbohydrate polymers: Amylose – a linear molecule that is coiled and Amylopectin – a branching molecule. Amylose makes up 20% and amylopectin 80% of the total starch molecule.
Amylose has repeating glucose units in the commonest linkage – α1-4 glycosidic linkages, just as in maltose. Hydrogen bonds help to make the linear molecules into a helical structure (a coil).
Amylopectin has linear stretches and some branches (with α1-6 links). The overall structure of starch is the fan shaped amylopectin with the coiled amylose tangled in the branches.
Glycogen has a very similar structure to that of amylopectin, except that the straight parts are shorter i.e. it is more tightly branched. It is the only carbohydrate energy store found in animals and fungi. Found in liver and muscle tissue.
Starch and glycogen are metabolic stores of glucose molecules. The branched nature of amylopectin and glycogen means that several enzymes, working at the ends of the branches can quickly release glucose. Because they are joined by the simplest linkage, the units of glucose are easily released by the appropriate enzyme.
Cellulose is a linear polysaccharide made of glucose units, but linked in the unusual linkage of β1-4. β glucose units are rotated by 180º, allowing glycosidic bonds to form between the hydroxyl (-OH) groups of adjacent glucose molecules. This effectively means that one glucose unit is ‘up’ and the next is ‘down’. This arrangement alters the 3D shape and enzymes that can break 1-4 linkages are useless with cellulose, which makes cellulose resistant to being broken down. Only certain bacteria have a β1-4 cellulase enzyme. This makes it a valuable structural material.
The cellulose structure is a linear spiral (i.e. no branches) with strong hydrogen bonds that link the parallel strands making the cell wall a very stable structure. Molecules of cellulose can wrap around each other, rather like the filaments in a rope, to form even stronger, thick strands. Cellulose makes up 50% of plant cell walls and 90% of cotton.
Glucose is linked by α linkages in:
Maltose (disaccharide – glucose + glucose)
Starch (polysaccharide in plants)
Glycogen (polysaccharide in animals)
And by β linkages in: Sucrose (disaccharide – glucose + fructose)
Cellulose (polysaccharide in plant cell walls)
Hydrolysis of disaccharides and polysaccharides:
All of the condensation reactions (reactions where a glycosidic bond is formed) can be reversed by adding water. This is known as hydrolysis, and the monomers, such as glucose, are produced.
Summary:
Carbohydrates are split into three main groups:
-
Monosaccharides – single sugars. A general formula for monosaccharides is (CH2O)n. E.g. the hexose sugar, glucose. C6H12O6
-
Disaccharides – double sugars – Cx(H20)y
- Polysaccharides – many sugars
Uses of carbohydrates:
- Sources of energy in organisms.
- Storage of energy in organisms.
- Structural component of cell membranes, cell walls and some skeletons.
Lipids
All lipids are composed of carbon, hydrogen and oxygen atoms. They are insoluble in water but readily dissolve in organic solvents e.g. alcohol.
There are two basic types of lipids:
- Fats – solid at room temp.
- Oils – liquids at room temp.
The simplest forms are neutral lipids. They are made of two kinds of molecule:
Glycerol (C3H8O3)
Fatty acid
Glycerol is a 3 carbon, simple carbohydrate – a ‘polyol’
Fatty acids are carboxylic acids and usually have more than 12 carbons in their chains. Their structural formula is R ── COOH. All fatty acids (there are 70 different ones) are lipids, and have a long hydrocarbon chain forming a pleated backbone of carbon atoms with hydrogen atoms attached, and a carboxyl group (- COOH) at one end.
If the fatty acid contains the maximum number of hydrogen atoms possible, it is said to be saturated. (CnH2nO2) If however, some adjacent carbon atoms form a double covalent bond (-C=C-), the fatty acid is said to be unsaturated. The simplest molecule is ethanoic acid:
A common fatty acid found in lipids is stearic acid: C17H35COOH:
NB. This is a saturated fatty acid as all the bonds between the carbons are single bonds.
Each fatty acid can join via its carboxylic acid group to one of the –OH groups on the glycerol. If the fatty acid is stearic acid the resulting neutral lipid (or triglyceride) is glyceryl tristearate. The reaction is a special type of condensation reaction known as an esterification reaction:
1 glycerol + 3 fatty acids produce:
A triglyceride + 3 waters
If only one fatty acid combines, it is called a monoglyceride. Two is a diglyceride, three is a triglyceride.
Neutral lipids are often used as energy stores e.g. in adipose cells (animals) or seeds (plants). However, the plant fatty acids are often more unsaturated than those in animals. Unsaturation (double bonds in the carbon chain) leads to a more liquid nature i.e. oils are made of predominantly unsaturated fatty acids. Unsaturated fatty acids are important in the cell membrane structure and help to account for its fluid nature.
