AQA A-Level Biology

Monomers and polymers

Living things are built from a small number of carbon-based molecules, and the same few building blocks turn up in every organism. A monomer is a small unit that can be joined to many others, and a polymer is the long molecule made when a large number of monomers are joined in a chain. Monosaccharides, amino acids and nucleotides are the three monomers you meet across the course.

Two reactions make and break these chains.

• A condensation reaction joins two molecules by forming a new chemical bond, and a molecule of water is released as it happens.
• A hydrolysis reaction breaks a bond by using up a molecule of water, which is added across the bond.

Here two glucose molecules form maltose.

The reaction runs to the right by condensation and to the left by hydrolysis. Each type of bond in the course has its own name (glycosidic, ester, peptide, phosphodiester), but the chemistry of making and breaking it is the same.

Monosaccharides

Monosaccharides are single sugar units. Glucose, galactose and fructose are the three to know, and each has the formula $\ce{C6H12O6}$, so they are isomers of one another: the same atoms arranged differently.

Glucose exists in two forms, α-glucose and β-glucose. Both are six-carbon rings, and they differ only in the position of the hydroxyl group on carbon 1. In α-glucose the $\ce{-OH}$ on carbon 1 sits below the plane of the ring, and in β-glucose it sits above it. This small difference decides which polysaccharides each form can build, because it changes how neighbouring units can be linked.

Monosaccharides such as glucose are the main respiratory substrate in most cells and are soluble, so they are carried in blood and cell fluid easily.

Disaccharides and the glycosidic bond

A condensation reaction between two monosaccharides forms a glycosidic bond and produces a disaccharide.

Hydrolysis of the glycosidic bond, with water, splits a disaccharide back into its monosaccharides. This is what digestive enzymes do to the sugars in food.

Polysaccharides

A polysaccharide is a polymer of many monosaccharide units joined by glycosidic bonds. Three matter here, and all are polymers of glucose. Starch and glycogen are made from α-glucose, and cellulose is made from β-glucose.

Starch

Starch is the energy store of plants. It is a mixture of two polymers:

• amylose, an unbranched chain joined by 1,4 glycosidic bonds, which coils into a helix and so packs a lot of glucose into a small space;
• amylopectin, a chain with 1,4 bonds and occasional 1,6 bonds that produce side branches.

Starch suits a storage role for several reasons. It is insoluble, so it does not change the water potential of a cell and does not leave it by osmosis. It is large, so it cannot diffuse out through the membrane. It is compact, so a lot of energy can be held in a small volume, and its branched ends can be hydrolysed at many points at once by enzymes to release glucose.

Glycogen

Glycogen is the energy store of animals and fungi, held mainly in the liver and muscles. It has the same basic structure as amylopectin (α-glucose, 1,4 and 1,6 bonds) but is more highly branched, with shorter chains. Because there are more free ends, enzymes can release glucose from it very quickly, which suits the faster metabolic rate of animals. It is also very compact and insoluble, for the same reasons as starch.

Cellulose

Cellulose is the main structural component of plant cell walls. In β-glucose the hydroxyl group on carbon 1 points the opposite way from α-glucose, so in the chain every second glucose unit has to be rotated through 180 degrees to make a glycosidic bond. The result is a long, straight, unbranched chain and not a helix.

Many hydrogen bonds form between neighbouring chains. Each bond is weak, but together they hold chains side by side in bundles called microfibrils, which are grouped into fibres. This gives cellulose great tensile strength, so the wall can resist the pressure of water pushing outwards on it and prevent the cell bursting when it takes in water by osmosis. Most animals cannot digest cellulose because their enzymes do not fit the β-glycosidic bond.

Biochemical tests for carbohydrates

Benedict's test for reducing sugars

All monosaccharides and some disaccharides (maltose and lactose) are reducing sugars, because they can donate electrons to another substance.

1. Add Benedict's solution (blue) to the sample.
2. Heat in a boiling water bath.
3. Read the colour: blue means no reducing sugar, and with more reducing sugar it moves through green, yellow and orange to a brick-red precipitate.

The colours run in order of concentration, so the test is semi-quantitative. A colorimeter can turn it into a more accurate measure by finding how much light a filtered sample lets through.

Test for non-reducing sugars

Sucrose is a non-reducing sugar, so it gives no colour change in the ordinary Benedict's test. To test for it:

1. Do the Benedict's test first. It stays blue.
2. Take a fresh sample and boil it with dilute hydrochloric acid, which hydrolyses the glycosidic bond and releases the reducing sugars glucose and fructose.
3. Cool the sample and neutralise it with sodium hydrogencarbonate, because Benedict's reagent only works in alkaline conditions.
4. Repeat the Benedict's test. A brick-red precipitate now means a non-reducing sugar was present.

Iodine test for starch

Add iodine dissolved in potassium iodide solution. The yellow-brown solution turns blue-black when starch is present, because iodine fits inside the amylose helix.

Lipids

Lipids are a mixed group that share one property: they are insoluble in water. They contain the elements carbon, hydrogen and oxygen, but with far less oxygen than carbohydrates. Two groups are needed, triglycerides and phospholipids.

Triglycerides

A triglyceride forms by the condensation of one molecule of glycerol and three fatty acids. Each fatty acid joins to glycerol by an ester bond, and one molecule of water is released for each bond, so three are released in all. A fatty acid has a carboxyl group ($\ce{-COOH}$) at one end, written RCOOH, and a hydrocarbon tail (the R group).

The R group can be saturated or unsaturated.

• In a saturated fatty acid every carbon in the tail is bonded to as many hydrogen atoms as possible, so there are no carbon-carbon double bonds and the tail is straight.
• In an unsaturated fatty acid there is at least one carbon-carbon double bond. Each double bond bends the tail, so the molecules pack together less closely. Fats made mostly from saturated fatty acids are therefore solid at room temperature and oils made from unsaturated ones are liquid.

Triglycerides are used as:

• an energy store, because the many carbon-hydrogen bonds in the tails release more energy per gram when oxidised than carbohydrate does;
• a source of metabolic water, released when the fat is respired;
• thermal insulation and protection around organs, since fat conducts heat poorly;
• a waterproofing layer, because they are hydrophobic;
• an insoluble store that has no effect on the water potential of a cell.

Phospholipids

A phospholipid is like a triglyceride in which one fatty acid has been replaced by a phosphate-containing group. The phosphate group is charged and polar, so it is hydrophilic (attracted to water). The two remaining fatty acid tails are non-polar, so they are hydrophobic (repelled by water). One molecule therefore has a hydrophilic head and hydrophobic tails.

In water, phospholipids arrange themselves so that the heads face the water and the tails face each other, away from it. This produces a double layer, the phospholipid bilayer, which is the basis of every cell membrane. Because the tails form a hydrophobic centre, ions and polar molecules cannot easily pass through it.

Emulsion test for lipids

1. Shake the sample with ethanol so that any lipid dissolves.
2. Pour the ethanol into water.
3. If lipid is present, it comes out of solution and forms a cloudy white emulsion, because tiny droplets of lipid scatter light.

A clear liquid means there is no lipid.