Carbohydrates: from one sugar to a store of thousands

One sugar, one ring, and two versions of it

A monosaccharide is a single sugar unit, and most of the ones an A-level course asks for fit the general formula (CH₂O)_n. Glucose is a hexose, six carbons, C₆H₁₂O₆. Ribose fits too: it is a pentose, five carbons, C₅H₁₀O₅. Deoxyribose is the exception on the list. As a deoxy sugar it has lost one oxygen, so its formula C₅H₁₀O₄ breaks the pattern, which is worth noticing rather than glossing over. Both pentoses are worth learning here rather than in the nucleic acid unit, because the sugar in RNA and the sugar in DNA are ordinary monosaccharides doing an unusual job.

In solution glucose closes into a ring, and it can close in two ways. The hydroxyl group on carbon 1 ends up either below the ring or above it. Below gives α-glucose, above gives β-glucose, and that is the entire difference between them: same atoms, same formula, one group moved.

Deoxyribose is ribose with one oxygen fewer, on carbon 2, which is what the name says. Both are sugars in their own right and both are reducing sugars, so a Benedict's test shows only that a reducing sugar is present, never which one it is.

Monosaccharide

A single sugar unit, the monomer from which disaccharides and polysaccharides are built.

Monomer

A small molecule from which a larger one is built by joining many of them together.

Polymer

A large molecule made of many similar monomers joined together.

Condensation

A reaction that joins two molecules with a covalent bond and releases a molecule of water.

Hydrolysis

A reaction that breaks a covalent bond between two molecules using a molecule of water.

Joining sugars: one bond, one water

Two monosaccharides join when a hydroxyl group on one reacts with a hydroxyl group on the other. A hydrogen comes off one and a hydroxyl off the other, the two leave together as water, and the sugars are left joined through an oxygen atom. That link is a glycosidic bond, and the reaction is condensation, exactly as it is when amino acids join into a polypeptide.

The three disaccharides worth knowing by name are maltose (glucose and glucose), sucrose (glucose and fructose) and lactose (glucose and galactose). Sucrose is the sugar plants move in phloem and the one in a bag on a kitchen shelf; lactose is the sugar in milk.

Adding water across the bond runs the reaction backwards. That is hydrolysis, and it is what maltase, sucrase and lactase do in the small intestine, and what amylase does further up the chain. A person who makes little or no lactase does not hydrolyse lactose, so it reaches the large intestine intact and the bacteria there ferment it.

Starch and glycogen: two stores of the same sugar

Starch is the plant store and is not one molecule but two. Amylose is an unbranched chain of α-glucose joined by 1,4 glycosidic bonds, and because of the angle those bonds make, the chain coils into a helix. Amylopectin has the same 1,4 backbone with extra 1,6 bonds along it, and each of those is a branch point.

Glycogen is the animal store, held in liver and muscle cells. It is built the same way as amylopectin and branched more often, with shorter chains between the branches.

Every property of a store follows from that structure, and mark schemes want the link made rather than the property recited. The molecules are large and insoluble, so they do not move out of the cell and do not affect the water potential of the cytoplasm. They coil or branch into a compact shape, so a great deal of glucose is held in a small space. They are hydrolysed back to α-glucose, which is the respiratory substrate the cell actually uses.

Cellulose: the same glucose, every other one turned over

Cellulose is a chain of β-glucose joined by 1,4 glycosidic bonds, and the geometry of β-glucose will not allow that unless every second molecule is rotated through 180 degrees. It is the hydroxyl group on carbon 1 sitting above the ring that forces the flip: only then are the two groups that must react close enough to each other.

A chain built that way does not coil. It runs straight, and straight chains lie alongside each other, so hydroxyl groups on neighbouring chains sit close enough to form hydrogen bonds between them. Each of those bonds is weak. There are so many of them along a chain thousands of units long that the bundle they hold together, a microfibril, has a tensile strength worth quoting in an answer.

That is what a plant cell wall is for. The wall resists the outward push of a cell taking in water, so the cell becomes turgid rather than bursting, and turgid cells are what hold a non-woody plant up.

Humans produce no enzyme that hydrolyses a β-1,4 bond, so cellulose passes through us as fibre. A cow does not produce one either. What it has is a rumen full of bacteria that do, which is why the digestion of cellulose in a ruminant is a partnership rather than an adaptation of the animal's own gut.

Glycosidic bond

The covalent bond formed by condensation between two monosaccharides, joining them through an oxygen atom.

Disaccharide

Two monosaccharides joined by a glycosidic bond.

Polysaccharide

Many monosaccharides joined by glycosidic bonds into one large molecule.

Testing for sugars and for starch

Benedict's solution contains copper(II) ions in alkaline solution, and it is blue because of them. A reducing sugar reduces those copper(II) ions to copper(I), which comes out of solution as a brick-red precipitate of copper(I) oxide. Add Benedict's solution to the sample and heat it in a water bath at 80 °C or above for five minutes: blue means no reducing sugar, and green, yellow, orange and brick red mean progressively more.

A negative result therefore does not mean no sugar. To test for a non-reducing sugar such as sucrose, take a fresh sample, boil it with dilute hydrochloric acid to hydrolyse the glycosidic bond, neutralise it with sodium hydrogencarbonate, and only then add Benedict's solution and heat it again. Miss the neutralisation and the test fails, because Benedict's solution has to be alkaline to work.

For starch, use iodine dissolved in potassium iodide solution. It is orange-brown on its own and turns blue-black in the presence of starch, at room temperature and with no heating. Iodine on its own does not dissolve in water, which is why the reagent is always named in full in a mark scheme.

Putting a number on a colour

The colour series is a rough scale, and questions increasingly ask for a measurement rather than a description. There are two accepted ways to get one, and both begin the same way: make up a set of glucose solutions of known concentration, treat every tube identically, and use them to calibrate the method.

The colorimeter method uses what is left behind. After heating, filter off the copper(I) oxide precipitate and measure the absorbance of the filtrate with a red filter. The more reducing sugar there was, the more copper(II) was reduced, so the less blue remains and the lower the reading. Plot reading against known concentration to get a calibration curve, treat the unknown the same way, and read its concentration off the curve.

The alternative is to filter the precipitate, dry it and find its mass, which measures the same thing without a colorimeter and takes rather longer.

Either way, the same variables have to be controlled or the calibration means nothing: the same volume and concentration of Benedict's solution, the same volume of sample, the same temperature, and the same time in the water bath. Benedict's solution must be in excess as well, or the tubes with the most sugar all run out of copper(II) ions and give the same reading as each other.