Molecules

A condensation reaction joins two molecules together to form a larger molecule, with the removal of a water molecule. In carbohydrates, condensation between two monosaccharides forms a glycosidic bond; in lipids, condensation between glycerol and a fatty acid forms an ester bond. Each new bond formed releases one molecule of water.

Water is a polar molecule: the oxygen is slightly negative (δ⁻) and the hydrogens slightly positive (δ⁺). Because of this, water is an excellent solvent for polar and ionic substances such as glucose, mineral ions and amino acids — its charged regions surround and separate the solute particles, holding them in solution.

Dissolved substances can therefore be transported in aqueous fluids such as blood plasma in animals and in the xylem and phloem of plants.

In addition, hydrogen bonding between water molecules gives water cohesion, so a continuous column of water can be drawn up the xylem in the transpiration stream without breaking.

Sucrose is broken down by hydrolysis: a water molecule is added to break the glycosidic bond between the glucose and fructose units, releasing the two monosaccharides.

Sucrose is a non-reducing sugar because the reactive groups of both monosaccharides are involved in the glycosidic bond, so it cannot reduce the copper(II) ions in Benedict's solution and gives a negative (blue) result.

Once hydrolysed, the products glucose and fructose are reducing sugars — their reactive groups are now free. They reduce the copper(II) ions in Benedict's solution to copper(I) oxide, producing a brick-red precipitate, a positive result.

Glycogen is a polymer of α-glucose joined by α-1,4-glycosidic bonds with α-1,6-glycosidic bonds at the branch points. Several features make it an excellent store in animals.

It is highly branched, so it has many free ends. Enzymes such as glycogen phosphorylase can act on all these ends at once, allowing glucose to be released rapidly when respiration rate suddenly increases — important for animals with high and fluctuating energy demands, for example during exercise.

It is compact (the branched, coiled structure packs a large amount of glucose into a small volume), so a lot of energy can be stored in the limited space of liver and muscle cells.

Glycogen is insoluble in water, so it has no effect on the water potential of the cell. Storing free glucose instead would lower the water potential of the cytoplasm and cause water to enter by osmosis, which could damage the cell.

Finally, because it is built from α-glucose, glycogen is readily hydrolysed back to glucose, which can be respired immediately to release energy.

A triglyceride is formed from one glycerol and three fatty acids joined by ester bonds. The physical state of the triglyceride depends on the fatty acids it contains.

Saturated fatty acids contain only carbon–carbon single bonds, so their hydrocarbon tails are straight. Straight tails pack closely together, allowing many van der Waals forces to act between neighbouring molecules. A relatively large amount of energy is needed to overcome these forces, so the melting point is high and the triglyceride is solid at room temperature, such as the fats stored by many animals.

Unsaturated fatty acids contain one or more carbon–carbon double bonds (C=C). Each double bond introduces a kink into the hydrocarbon tail. The kinked tails cannot pack closely, so there are fewer van der Waals forces between molecules, a lower melting point, and the triglyceride is liquid (an oil) at room temperature, as in plant and fish oils.

This difference is biologically important. Plants and cold-water organisms tend to have a higher proportion of unsaturated fatty acids, which keeps their membranes and stored lipids fluid at low temperatures, maintaining membrane function and the ability to mobilise energy stores. Conversely, a solid fat store is space-efficient and stable for animals in warmer conditions. In both cases the triglyceride remains a highly reduced, energy-dense store releasing more than twice the energy per gram of carbohydrate, and being insoluble it does not affect the cell's water potential.

(a) Between two α-glucose molecules a glycosidic bond forms (specifically an α-1,4-glycosidic bond in maltose).

(b) Between glycerol and a fatty acid an ester bond forms.

(c) Between two amino acids a peptide bond forms.

In every case the bond is made by condensation, removing one water molecule per bond.

Both amylose and amylopectin are polymers of α-glucose joined by glycosidic bonds.

In amylose the α-glucose monomers are joined only by α-1,4-glycosidic bonds in the same orientation, so the chain coils into a tight helix. This makes amylose compact, storing a large quantity of glucose in a small volume.

Amylopectin has the same α-1,4 backbone but also has α-1,6-glycosidic bonds at branch points, giving a branched molecule with many free ends. Enzymes can act on these ends, so glucose can be released by hydrolysis when the plant needs it for respiration.

Starch is also insoluble, so it has no effect on the water potential of the cell and does not cause water to enter by osmosis. Because it is made of α-glucose, it is readily hydrolysed back to glucose for respiration.

(a) Ratio = energy of lipid ÷ energy of carbohydrate = 38 ÷ 17 = 2.24 (to 3 significant figures). The lipid therefore releases about 2.2 times more energy per gram than the carbohydrate.

(b) A triglyceride's fatty-acid tails are long hydrocarbon chains that are highly reduced — they contain many C–H bonds and proportionally much less oxygen than a carbohydrate. When the molecule is oxidised, these C–H bonds release a large amount of energy, so per gram a lipid yields more than twice the energy of a carbohydrate, in which the carbon is already partly oxidised (carbohydrates have the general formula $C_n(H_2O)_n$).

(c) Triglycerides are insoluble in water, so they have no effect on the water potential of the cell and do not cause water to enter by osmosis. (Alternatively: oxidation yields metabolic water, useful to desert animals, or the store provides insulation/buoyancy.)

All three are polysaccharides made by condensation, but differences in the type of glucose and the bonding give each a structure matched to its role.

Glycogen is a polymer of α-glucose with α-1,4 bonds and frequent α-1,6 branches, making it highly branched with many free ends. This allows enzymes such as glycogen phosphorylase to hydrolyse many ends simultaneously, so glucose is released rapidly — ideal for animals, which have high and fluctuating respiratory demands. It is compact and insoluble, so it does not affect the cell's water potential.

Starch (amylose + amylopectin) is also a polymer of α-glucose (α-1,4, with α-1,6 branches in amylopectin). Amylose coils into a compact helix, packing a large amount of glucose into a small space, and being insoluble it has no osmotic effect. It is the energy store of plants, which have lower, less sudden energy demands than animals, so it is somewhat less branched than glycogen.

Cellulose is a polymer of β-glucose with β-1,4 bonds; every alternate monomer is rotated 180°, giving straight, unbranched chains. Many hydrogen bonds form between the —OH groups of adjacent parallel chains, bundling them into microfibrils of high tensile strength. This makes cellulose ideal as the structural component of plant cell walls, resisting turgor pressure so the cell does not burst.

The contrast shows structure dictating function: the same monomer, in α or β form with different bonding, becomes either a rapidly mobilised store (glycogen), a compact plant store (starch), or a strong structural fibre (cellulose).

(a) As the concentration of reducing sugar increases from 0.2% to 0.8%, the mass of copper(I) oxide precipitate increases from 8 mg to 34 mg. The relationship is positive and approximately linear — each 0.2% rise in concentration adds roughly 8–9 mg of precipitate (8 → 17 → 25 → 34 mg).

(b) Benedict's test relies on the reducing sugar reducing copper(II) ions to copper(I) oxide (the brick-red precipitate). A higher concentration of reducing sugar means more reactive (aldehyde/ketone) groups are present to donate electrons, so more copper(II) is reduced to copper(I) oxide and a greater mass of precipitate forms.

(c) The student must control variables such as the volume of Benedict's solution added, the temperature/heating time in the water bath, and the volume of sugar solution used, so that the only variable affecting the result is the sugar concentration.