Water & carbohydrates & lipids

Just four families of large molecule account for almost everything your body is made of and everything it does: carbohydrates, lipids, proteins and nucleic acids. Chemists call them the biological macromolecules, and although their jobs range from storing energy to carrying genetic information, they are all assembled from a small set of simple building blocks using the same handful of chemical tricks. Before we can understand those macromolecules, though, we have to start with the substance in which all of them are dissolved and around which all of them are shaped: the remarkable molecule that makes up roughly two-thirds of your body weight and without which none of life's chemistry could take place at all. That molecule is water.

Almost every special property of water traces back to a single fact about its shape. A water molecule is one oxygen atom bonded to two hydrogen atoms, but the oxygen nucleus pulls the shared bonding electrons far more strongly than the small hydrogen nuclei do, a property called electronegativity. Because the two hydrogens sit on the same side of the bent, V-shaped molecule, the oxygen end carries a small negative charge while the hydrogen end carries a small positive charge. A molecule with separated regions of positive and negative charge like this is called polar. Water is not charged overall, yet this uneven distribution of charge turns each tiny molecule into a miniature magnet with a positive pole and a negative pole, and that single feature is the key that unlocks nearly everything water can do inside a living cell.

Because water is polar, the positive hydrogen end of one molecule is attracted to the negative oxygen end of a neighbour, forming a weak link called a hydrogen bond. Any one hydrogen bond is feeble and constantly breaking and re-forming, but in a glass of water each molecule is bonded to several neighbours at once, and the sheer number of these bonds gives liquid water a surprising internal strength. This mutual clinging of water molecules to one another is called cohesion, and it is why water forms rounded droplets and why the surface behaves like a stretched skin, the surface tension that lets small insects walk across a pond. Water molecules also cling to other polar surfaces, an attraction called adhesion; together, cohesion and adhesion let water climb upward against gravity through the narrow tubes of a plant, a process called capillary action that helps carry water from the roots of a tall tree to its highest leaves.

The hydrogen bonds that hold water together also give it a set of properties living things depend on. First, water has a high specific heat: it takes a great deal of energy to raise its temperature, because much of the heat goes into breaking hydrogen bonds rather than speeding molecules up. This lets oceans, lakes and even the watery inside of your body resist sudden temperature swings, keeping conditions stable for delicate chemistry. Second, evaporating water carries away a lot of heat, so sweating cools the body efficiently. Third, water is often called the universal solvent: its polarity lets it surround and dissolve any charged or polar substance, from salts to sugars, so that the cell's reactions can take place in solution. Finally, water shows a rare and vital anomaly: solid ice is less dense than liquid water and therefore floats, because hydrogen bonds lock the molecules into an open, spacious lattice as they freeze.

These physical quirks translate directly into survival. Because ice floats, ponds and lakes freeze from the top down, leaving a liquid layer beneath the ice where fish and other organisms can survive the winter; if ice sank, bodies of water would freeze solid from the bottom and life in them would perish. Water's high specific heat means large organisms and whole ecosystems stay thermally stable, and because water is such a good solvent it serves as the transport medium of life: blood plasma, plant sap and the cytoplasm inside every cell are all mostly water carrying dissolved substances from place to place. Water even takes part directly in chemistry, acting as a reactant in hydrolysis and a product in dehydration synthesis. It is no exaggeration to say that where there is life there is liquid water, and that the search for life elsewhere in the universe is largely a search for it.

The other pillar of biological chemistry is the element carbon. Every large molecule that builds and runs a living body, from carbohydrates and lipids to proteins and nucleic acids, is built on a skeleton of carbon atoms, and the reason lies in carbon's bonding. A carbon atom has four electrons in its outer shell and needs four more to be stable, so it forms four covalent bonds with other atoms. This gives carbon an unmatched versatility: it can bond to hydrogen, oxygen, nitrogen and, crucially, to other carbon atoms, linking them into long straight chains, branched chains, or closed rings. Because each carbon in a chain still has spare bonds for other atoms and groups, the number of possible carbon-based molecules is essentially limitless. This is why chemists call carbon compounds organic molecules, and why the astonishing diversity of life can be built from just a handful of chemical elements arranged around carbon frameworks.

The giant molecules of life are called macromolecules, and most are polymers, long chains built from many small repeating units called monomers, rather as a train is built from identical carriages. Three of the four major groups are polymers: carbohydrates are chains of sugars, proteins are chains of amino acids, and nucleic acids are chains of nucleotides. Cells build and break these polymers using two opposite reactions. To join two monomers, the cell removes the equivalent of a water molecule, taking a hydroxyl group from one monomer and a hydrogen atom from the other, and uses the freed bonds to link them. This building reaction is called dehydration synthesis (or a condensation reaction) because water is taken out. The reverse reaction, hydrolysis, splits a polymer by adding water back across the bond, from 'hydro' for water and 'lysis' for splitting, which is exactly what happens when your digestive system breaks food polymers back down into monomers you can absorb.

Carbohydrates are the cell's most immediate source of energy, and in their simplest form they are the single sugars called monosaccharides. The most important is glucose, whose formula is C_6H_{12}O_6: six carbons, twelve hydrogens and six oxygens, roughly the one-carbon-to-one-water ratio that gives the whole group its name (carbo-hydrate). Glucose is the fuel that cellular respiration burns to release energy, and it is the sugar your blood carries to every cell. Other monosaccharides such as fructose (fruit sugar) and galactose share the identical formula C_6H_{12}O_6 but arrange their atoms differently; molecules like these, with the same formula but different structures, are called isomers, and the small change in shape is enough to give them different tastes and different chemical behaviour. Monosaccharides are also the units from which every larger carbohydrate is built.

When two monosaccharides join by dehydration synthesis they form a disaccharide: glucose plus fructose makes sucrose (table sugar), glucose plus galactose makes lactose (milk sugar), and two glucose units make maltose. Link hundreds or thousands of sugars together and you get a polysaccharide, and here structure decides function. Starch is the storage polysaccharide of plants, a coiled chain of glucose that seeds and tubers stockpile as an energy reserve; glycogen is the animal equivalent, a highly branched glucose polymer stored in your liver and muscles and broken down whenever blood sugar falls. Cellulose, by contrast, is a structural polysaccharide: its glucose units are joined by a slightly different type of bond that makes the chains straight and rigid, and bundles of them form the tough fibres of plant cell walls. That single difference in bonding is why we can digest starch for energy but pass cellulose through as indigestible fibre.

The reason cells prize glucose so highly is that it is the fuel of cellular respiration, the controlled series of reactions that releases the chemical energy locked in its bonds. When glucose is broken down in the presence of oxygen it is oxidised step by step to carbon dioxide and water, and the energy set free is captured to power the cell. That is why glucose is delivered continuously to every tissue and why an active muscle or a busy brain must have a steady supply. Storing energy as glucose polymers is therefore a way of banking fuel: starch and glycogen are, in effect, glucose kept in reserve, ready to be released by hydrolysis and burned the moment the body needs energy, which links the carbohydrates directly to the energy chemistry you will meet again with ATP.

Because carbohydrates are the body's first-choice fuel, the level of glucose in the blood is controlled with great care. After a meal, excess glucose is stored by joining the units into glycogen; between meals, glycogen is broken back down by hydrolysis to keep blood glucose steady and the brain, which runs almost entirely on glucose, supplied. Carbohydrates also do more than fuel: short sugar chains attached to the outside of cells act as identity tags that let cells recognise one another, and in plants the cellulose of cell walls provides the rigid support that lets a stem stand upright and a tree grow tall. From the sweetness of fruit to the wood of a forest, an enormous range of biological structures and functions comes down to different ways of arranging the same simple sugar building blocks.

Lipids are the great exception among biological molecules, because they are not polymers and, above all, they do not mix with water: they are hydrophobic ('water-fearing'), a property that comes from their long chains of nonpolar carbon and hydrogen. The most familiar lipids are the triglycerides, the fats and oils, each built from one molecule of glycerol joined to three long fatty acid chains. Triglycerides are superb energy stores, packing more than twice the energy per gram of carbohydrates, which is why animals lay down body fat and seeds store oils. Fatty acids come in two forms. In a saturated fat every carbon in the tail is 'saturated' with hydrogen and holds only single bonds, so the straight chains pack tightly and the fat is solid at room temperature, like butter or lard. In an unsaturated fat one or more double bonds put kinks in the chains, keeping them apart, so the fat stays liquid as an oil, like olive or sunflower oil.

Two other classes of lipid are essential even though they store little energy. A phospholipid resembles a triglyceride but with one fatty acid replaced by a phosphate group, and this gives it a split personality: the phosphate 'head' is charged and hydrophilic (water-loving) while the two fatty-acid 'tails' are hydrophobic. In water, phospholipids therefore line up automatically into a double layer, a phospholipid bilayer, with their water-loving heads facing outward and their water-fearing tails tucked inside, and this bilayer is the basic fabric of every cell membrane. The other class, the steroids, share a distinctive skeleton of four fused carbon rings. Cholesterol is the best known: it stiffens animal cell membranes and is the raw material from which the body makes steroid hormones such as testosterone and oestrogen, the chemical messengers that control growth and reproduction.

Beyond energy storage and membranes, lipids serve the body in several further ways. A layer of fat beneath the skin provides insulation against the cold, which is why seals and whales in polar seas carry thick blubber, and pads of fat cushion and protect delicate organs such as the kidneys. Some vitamins, A, D, E and K, are fat-soluble, meaning they dissolve in and are carried by lipids rather than water, and waxy lipids waterproof surfaces such as the leaves of plants, the feathers of birds and the outer layer of your own skin, cutting down water loss. Because lipids are hydrophobic, however, the blood cannot carry them dissolved the way it carries glucose; instead it packages them with proteins for transport, which is why doctors measure different kinds of blood cholesterol when they assess the risk of heart disease.

Seen together, water, carbohydrates and lipids show how life is built from simple ingredients combined in clever ways. Water provides the polar, hydrogen-bonded medium in which all of biology's chemistry happens, and carbon provides the versatile four-bond skeleton on which the large molecules are assembled. Carbohydrates, from single glucose molecules to vast starch and cellulose chains, supply quick energy and structural support, while lipids store energy densely, waterproof surfaces, build the membranes that enclose every cell, and act as hormones. In the next topic we turn to the proteins, the most varied and versatile macromolecules of all, but the same two reactions you met here, dehydration synthesis to build and hydrolysis to break, will govern how they too are assembled from their monomers.