1The idea in one paragraph

Life is built on carbon because a carbon atom makes four strong covalent bonds, so it can sit at the centre of chains, branches and rings of almost any size. Small units called monomers are joined into large polymers by condensation, which releases water, and split again by hydrolysis, which uses it. Carbohydrates are built from sugar monomers: glucose is soluble and easy to move, starch and glycogen pack glucose into compact stores, and cellulose turns the same glucose into strong fibres. Lipids are the molecules that do not dissolve in water. Triglycerides store twice as much energy per gram as carbohydrate and insulate the body; phospholipids have one water-loving end and one water-hating end, so they build membranes on their own; steroids are non-polar enough to slip straight through those membranes. Every one of those functions is explained by a detail of shape.

2Why life is built on carbon

A covalent bond is a pair of electrons shared between two atoms. It is strong, so a molecule held together by covalent bonds does not fall apart at the temperatures cells live at.

Carbon has four electrons available for bonding, so a carbon atom forms up to four covalent bonds. They can be four single bonds, or a mixture of single and double bonds (a double bond counts as two). Carbon bonds readily to other carbon atoms and to the other non-metals that living things use: hydrogen, oxygen, nitrogen, phosphorus and sulfur.

That combination is what makes carbon special. Because each carbon can bond to further carbons, you can build a backbone of any length, and because each one still has spare bonds, the backbone can branch, close into a ring, or carry side groups. Figure 1 shows the four basic skeleton shapes you should be able to name.

• Unbranched chains: the hydrocarbon tail of a fatty acid.
• Branched chains: many amino acid side chains, and, at a larger scale, glycogen.
• Single rings: glucose and ribose.
• Several fused rings: the steroids, such as testosterone.

A note on units (nature of science). Biology moves between scales constantly, and the prefixes that do it are fixed by international agreement, so that "a micrometre" means exactly the same thing in every laboratory in the world. That agreement is what lets one scientist check another's measurements.

3Building and breaking polymers: condensation and hydrolysis

A monomer is a small molecule that can join to others like it; a polymer is a long chain of monomers. A macromolecule is simply a very large molecule, and the biological ones are mostly polymers.

Condensation joins two monomers with a covalent bond and releases a molecule of water. One monomer gives up an –OH group, the other gives up an –H, and those two become H₂O. Figure 2 shows the exchange.

The rule for counting follows directly: one water molecule for every bond made. Join 2 glucose molecules and one water is released. Join 100 glucose molecules into a chain and there are 99 bonds, so 99 water molecules are released.

Hydrolysis is the reverse. A water molecule is split: its –H goes to one monomer and its –OH to the other, and the bond between them breaks. The name says it: hydro for water, lysis for splitting. Digestion in your gut is hydrolysis, catalysed by enzymes, of polysaccharides to monosaccharides, proteins to amino acids, and nucleic acids to nucleotides.

The same pair of reactions builds all three kinds of biological polymer, which is why the guide asks you to know them together:

• polysaccharides, from monosaccharides such as glucose (starch, glycogen, cellulose);
• polypeptides, from amino acids (B1.2);
• nucleic acids, from nucleotides (DNA and RNA, A1.2).

4Monosaccharides: pentoses, hexoses and glucose

A monosaccharide is a single sugar unit, the monomer of carbohydrates. Two sizes matter here. A pentose has five carbon atoms: ribose, in RNA, and deoxyribose, in DNA. A hexose has six: glucose, and also fructose and galactose.

In water most of these sugars close into a ring, and in the exam you will see them drawn that way. Figure 3 shows ribose and the two forms of glucose.

How to tell a pentose from a hexose on a diagram. Count the carbon atoms, not the corners of the ring. The ring always contains one oxygen atom, and one carbon sits outside the ring on a CH₂OH group. So:

• a five-sided ring (four carbons and an oxygen) plus one carbon outside = 5 carbons = pentose;
• a six-sided ring (five carbons and an oxygen) plus one carbon outside = 6 carbons = hexose.

Counting ring corners gives the right answer for these two only by luck, and fails the day a diagram leaves the ring oxygen unlabelled. Count carbons.

α-glucose and β-glucose differ in one place only: the –OH group on carbon 1. In α-glucose it points down, below the ring; in β-glucose it points up. That small difference decides whether a chain of glucose becomes starch or cellulose, sections 5 and 6.

Glucose, and why its properties suit its job. Glucose is the sugar that animals carry in their blood and that every cell can respire. The guide names four properties, and each one connects to a use.

The weakness of glucose is the other side of its solubility. Dissolved glucose raises the solute concentration of a cell, which draws water in by osmosis (B2.1). A cell that stored its fuel as free glucose would swell. The solution is to lock it up in a polymer.

5Starch and glycogen: storing glucose compactly

Plants store glucose as starch; animals and fungi store it as glycogen, mainly in the liver and muscles. Both are polymers of α-glucose, and Figure 4 shows their three forms.

Starch is a mixture of two molecules. Amylose is an unbranched chain of α-glucose joined by 1,4 links (carbon 1 of one glucose to carbon 4 of the next), and the angle of that link makes the chain coil into a helix. Amylopectin is also 1,4-linked, but every so often a 1,6 link starts a branch. Glycogen is built like amylopectin with far more branches, so it forms dense, bush-like granules.

The guide wants three properties, each explained by structure.

Compact. Coiling and branching fold a very long chain into a small space. A lot of glucose fits into a starch grain in a potato cell or a glycogen granule in a liver cell.

Relatively insoluble, so osmotically inactive. A single glucose dissolves; a molecule of thousands of glucose units does not, because it is too large to be surrounded and carried by water molecules. This is the reason, and it is not that starch is non-polar (it is covered in –OH groups). Because the store is insoluble, it does not raise the solute concentration inside the cell, and the cell does not take in water by osmosis.

Easy to build up and break down. Glucose is added to the ends of chains by condensation and removed from the ends by hydrolysis. A branched molecule has many ends, so many enzymes can work at once. That is why glycogen, the most branched, is also the fastest to mobilise: an animal that suddenly has to run needs glucose in seconds, and a plant never does.

6Cellulose: the same monomer as a building material

Cellulose is the main component of plant cell walls, and it is a polymer of β-glucose joined by 1,4 links. Because the –OH on carbon 1 points up in β-glucose, the only way to link carbon 1 of one unit to carbon 4 of the next is to turn every other glucose upside down. Figure 5 shows the result.

Follow the chain of consequences, because this is exactly what a four-mark "explain" answer contains.

1. Monomers alternate in orientation, each turned 180° relative to its neighbour.
2. So the chain does not coil. It stays straight.
3. Straight chains can lie side by side in parallel, close together.
4. The many –OH groups of neighbouring chains form hydrogen bonds between the chains, cross-linking them.
5. Bundles of cross-linked chains form microfibrils, which have very high tensile strength: they resist being pulled apart.
6. So the cell wall can resist the pressure of water pushing outwards, and the plant cell becomes turgid instead of bursting.

One hydrogen bond is weak. Millions of them along parallel chains are not, which is the whole trick of cellulose.

Put sections 5 and 6 side by side and you have a textbook case of form and function. Starch and cellulose are both made of glucose, both joined by condensation, and both insoluble. One small change in the monomer, α against β, decides whether the polymer coils into a store or straightens into a fibre.

7Glycoproteins and cell–cell recognition

A glycoprotein is a protein with a carbohydrate chain attached. In the plasma membrane the carbohydrate sits on the outside of the cell, where other cells can meet it. The chains differ between cell types, between individuals and between species, so they work as identity tags: cells use them to recognise one another, and the immune system uses them to tell self from non-self.

The guide's example is the ABO blood group system, shown in Figure 6.

Every person's red blood cells carry the same core carbohydrate chain on their surface glycoproteins. What differs is the final sugar.

• Group O: the core chain only.
• Group A: an enzyme adds one extra sugar to the tip of the chain, making the A antigen.
• Group B: a slightly different enzyme adds a different sugar, making the B antigen.
• Group AB: both enzymes are present, so both antigens are made.

An antigen is a molecule the immune system can recognise. A person with group A blood makes antibodies against the B antigen, because B is not part of "self" for them. If they are given group B blood, their antibodies bind to the B antigens on the donated cells and the cells clump together, which can be fatal. That is why blood is matched before a transfusion, and it shows the point of the whole section: one sugar, at the tip of one chain, is enough for the body to recognise a cell as its own or as foreign.

8Lipids: defined by what they will not dissolve in

Lipids are substances in living organisms that dissolve in non-polar solvents such as hexane but are only sparingly soluble in water. They are defined by that behaviour, not by one shared structure, so the group is varied: fats, oils, waxes (the waxy cuticle on a leaf) and steroids are all lipids.

The reason for the behaviour is in their structure. Lipids are built mostly of carbon and hydrogen in long chains or rings, and C–H bonds are almost non-polar. Water molecules are more attracted to each other than to a non-polar surface, so they hydrogen-bond among themselves and push the lipid away. That property, being hydrophobic ("water-fearing"), is the source of everything lipids do: they separate from water, they store energy without drawing water into cells, and they form the barrier of every membrane.

9Triglycerides and phospholipids, built by condensation

Both are built on glycerol, a small molecule with three carbon atoms, each carrying an –OH group. A fatty acid is a long hydrocarbon chain with a carboxyl group (–COOH) at one end. Figure 7 shows the two lipids made from them.

A triglyceride forms when one glycerol links to three fatty acids. Each link is a condensation reaction between an –OH on glycerol and the –COOH of a fatty acid, forming an ester bond and releasing one water molecule. So:

glycerol + 3 fatty acids → triglyceride + 3 water

A phospholipid forms when one glycerol links to two fatty acids and one phosphate group. The two fatty acids join by ester bonds; the phosphate takes the third –OH position. It is still three condensation reactions, so three water molecules are released. The phosphate often carries a further polar group, but you are not asked to name one.

The difference in that third position is the difference in function. A triglyceride has three non-polar tails and nothing polar worth mentioning, so it is entirely hydrophobic and gathers in droplets. A phospholipid has a polar, charged head, which changes everything (section 12).

10Saturated, monounsaturated and polyunsaturated fatty acids

The chain of a fatty acid can hold different numbers of C=C double bonds, and Figure 8 shows what they do to its shape.

• Saturated: no C=C double bonds. Every carbon in the chain holds as many hydrogen atoms as it can, so the chain is "saturated" with hydrogen, and it is straight.
• Monounsaturated: one C=C double bond.
• Polyunsaturated: two or more C=C double bonds.

In natural fatty acids the double bonds are almost always of the kind that bends the chain, so each one puts a kink in it.

Why that changes the melting point. Straight saturated chains can pack tightly against each other, and chains in close contact attract one another more strongly. More heat is needed to separate them, so the melting point is high. Kinked chains cannot pack closely, the attractions between them are weaker, and the melting point is lower. Each extra double bond adds a kink and lowers the melting point further. The rule: more double bonds, lower melting point.

Where you find each kind. Lipids rich in saturated fatty acids are usually solid at room temperature: we call them fats. Lipids rich in unsaturated fatty acids are usually liquid: we call them oils.

• Plants store energy mainly as oils, rich in unsaturated fatty acids, in seeds such as sunflower and olive. A plant does not keep itself warm, so its stores must stay liquid and usable at whatever temperature the seed or tissue happens to be at, often cool.
• Endotherms, the birds and mammals that keep their bodies at a steady high temperature, store more saturated fat. Inside a warm body this fat is soft enough to be used; out of the body at room temperature it is solid, which is why butter and lard stand up on a kitchen table while olive oil pours.

11Triglycerides in adipose tissue: energy storage and insulation

Adipose tissue is tissue whose cells are filled with droplets of triglyceride. Mammals keep much of it under the skin and around organs.

Why triglycerides suit long-term energy storage. Oxidising a gram of lipid releases about 37 kJ, against about 17 kJ for a gram of carbohydrate, because lipids are richer in C–H bonds and poorer in oxygen. They are stored without water: triglyceride droplets exclude it, whereas glycogen is stored bound to water that adds mass but no energy. They are insoluble, so they have no osmotic effect, and chemically stable, so they keep for months. In numbers, an adult carrying 10 kg of stored triglyceride holds:

For an animal that carries its fuel, above all one that flies or migrates, that difference in mass decides the long-term store. Glycogen is kept anyway because it is quicker to mobilise, and its glucose can be respired with or without oxygen, whereas fat is broken down more slowly and only aerobically. So animals use both, as the table sets out.

Thermal insulation. Fat conducts heat poorly, so a layer of adipose tissue under the skin slows the loss of heat from a warm body to cold surroundings. How much insulation an animal needs depends on two things the guide asks you to relate.

• Body temperature: an endotherm holds its body at a steady high temperature, so there is a gap between inside and outside, and heat flows across it continuously. The bigger the gap, the more insulation pays.
• Habitat: water conducts heat away more than twenty times faster than air, so marine mammals in polar seas, such as seals and whales, carry a thick layer of blubber. Animals of hot, dry places tend to do the opposite: a camel stores fat concentrated in its hump rather than spread under the skin, so the rest of its body can lose heat freely.

12Phospholipids form bilayers because they are amphipathic

A phospholipid has two regions with opposite properties. The phosphate head is polar and charged, so it is hydrophilic ("water-loving"). The two fatty acid tails are non-polar, so they are hydrophobic. A molecule with a hydrophilic part and a hydrophobic part is amphipathic. Use that word; the guide requires it.

In water, the heads are attracted to water molecules and the tails are pushed away from them. The arrangement that satisfies both at once is a bilayer, shown in Figure 9: two sheets of phospholipids, tails pointing inwards to form a hydrophobic core, heads pointing out into the water on both sides. No energy is spent building it and no enzyme is needed. The bilayer forms spontaneously, because it is the arrangement in which no tail is in contact with water. That self-assembly is the basis of every cell membrane, and B2.1 builds the whole membrane on it.

13Steroids pass through the bilayer

Steroids are lipids built on a skeleton of four fused rings: three six-sided rings and one five-sided ring. Different steroids carry different small groups on that skeleton, but the skeleton is how you recognise one. The guide names two, both hormones, drawn in Figure 10.

• Oestradiol, the main oestrogen, a female sex hormone. Its first ring has alternating double bonds (drawn as a ring with a circle), and it carries an –OH at each end.
• Testosterone, the main male sex hormone. It has an =O group at one end, an –OH at the other, and two methyl (–CH₃) groups.

Both molecules are almost entirely carbon and hydrogen, with one or two small polar groups. So they are non-polar, and a non-polar molecule can dissolve in the hydrophobic core of the phospholipid bilayer and pass straight through it by simple diffusion. That is why steroid hormones need no transport protein to enter a cell: they diffuse in and bind to receptors in the cytoplasm or nucleus. Glucose, by contrast, is polar and cannot cross the hydrophobic core, which is why it needs protein channels (B2.1).

How to identify a steroid in a Paper 1A diagram. Look for the four fused rings, three of six and one of five. If a molecule has one ring, it is not a steroid; if it has two separate rings joined by a bridge, it is not a steroid. The small groups tell you which steroid; the rings tell you it is one.