Monomers and polymers

1. A monomer is a small molecule that can be joined to many others of the same kind, and a polymer is the long chain that results.
2. Cells build almost every large molecule this way, because it needs only a small set of subunits and one joining reaction to make an enormous variety of products.
3. A carbohydrate is a molecule made of carbon, hydrogen and oxygen only, and its monomers are simple sugars.
4. Glucose, the commonest of those sugars, has the formula $C_6{H}_{12}{O}_6$, so its ratio of carbon to hydrogen to oxygen is $1:2:1$.
5. Every carbohydrate keeps roughly that ratio, which is why carbohydrates hold so much oxygen compared with the other groups of biological molecule.
6. The same joining reaction builds a carbohydrate from sugars, and the reverse reaction takes it apart again when the cell needs the sugars back.

Sugars as the monomers of carbohydrates

1. A monosaccharide is a single sugar unit, and it is the monomer from which every carbohydrate is assembled.
2. Glucose, fructose and galactose are the monosaccharides you need to recognise, and all three share the formula $C_6{H}_{12}{O}_6$.
3. They share a formula but not a shape, because their atoms are arranged differently, so they taste different and enzymes treat them differently.
4. Glucose and galactose form six-sided rings while fructose forms a five-sided ring.
5. A plant cell makes its own glucose by photosynthesis, while an animal cell receives glucose from digested food carried in the blood.
6. A disaccharide is made when two monosaccharides are joined together.
7. Maltose is glucose joined to glucose, sucrose is glucose joined to fructose, and lactose is glucose joined to galactose.
8. Sucrose is the sugar a plant moves around in its phloem, and lactose is the sugar in milk.
9. Both monosaccharides and disaccharides are soluble in water and taste sweet, which is what everyday use of the word sugar refers to.

Building and breaking carbohydrates

1. A condensation reaction joins two sugars together and releases one molecule of water as the new bond forms.
2. The water comes from the two sugars themselves, because a hydrogen atom is taken from one and a hydroxyl group from the other.
3. Building a long chain means repeating that reaction, so joining $n$ monomers into one polymer releases $n-1$ molecules of water.
4. The reactions are controlled by enzymes and driven by energy from ATP, which is why a growing cell needs a fast supply of it.
5. Hydrolysis is the reverse reaction, in which a molecule of water is added at each bond to break the chain back into its monomers.
6. Hydrolysis is what happens when a carbohydrase enzyme digests a carbohydrate, so amylase hydrolyses starch and maltase hydrolyses maltose.
7. Hydrolysis is essential because a polymer is far too large to cross a cell membrane, while a single sugar is small enough to be absorbed.
8. Taken together, the two reactions let a cell store glucose safely when it is plentiful and release it again the moment it is needed.

The three polysaccharides

1. A polysaccharide is a carbohydrate polymer of many monosaccharides joined in a long chain.
2. Starch is the polysaccharide a plant uses to store glucose, and it is built from glucose monomers alone.
3. Its chains are coiled, and some of them branch, which packs a very large number of glucose units into a small space inside a cell.
4. Starch is insoluble, so it does not dissolve in the cytoplasm and does not draw water into the cell by osmosis the way stored glucose would.
5. A plant lays down starch in its leaves during the day and in storage organs such as potato tubers and seeds for the longer term.
6. Glycogen is the equivalent store in animals and fungi, and it is also built from glucose monomers alone.
7. Its chains are much more highly branched than those of starch, which means many chain ends are exposed at once.
8. Every exposed end is a place an enzyme can start hydrolysing, so glycogen can be broken back down to glucose very quickly when demand rises.
9. That matters because an animal can move suddenly and needs glucose released within seconds, which is why glycogen is stored in the liver and muscles.
10. Cellulose is not a store at all; it is a structural polysaccharide, and it too is built from glucose.
11. Its glucose units are joined so that the chains stay straight rather than coiling, and neighbouring chains are then cross-linked to one another.
12. Bundles of cross-linked chains form strong fibres that are laid down in layers to make the cell wall of a plant cell.
13. Those fibres are what let a plant cell resist the outward pressure of the water inside it without bursting, and what makes celery stringy and wood strong.
14. All three are made from the same monomer, so it is the arrangement of the chains, not the subunit, that decides what each polysaccharide can do.