Carbohydrates & lipids

Sugars: the monomers

Carbohydrates are sugars and chains of sugars. Their building blocks are monosaccharides ("single sugars"), which usually have carbon, hydrogen and oxygen in a 1 : 2 : 1 ratio. Glucose, C₆H₁₂O₆, is the most important one. Fructose (fruit sugar) and galactose have the same formula but their atoms are arranged differently.

In water, glucose mostly exists as a ring. When the ring closes, the –OH group on carbon 1 can end up pointing one of two ways. If it points below the ring, it's α-glucose; if it points above, it's β-glucose. That tiny difference turns out to decide whether you can digest a polymer or not.

Linking sugars: glycosidic bonds

Two monosaccharides join by dehydration synthesis to form a disaccharide, linked by a glycosidic bond. Bonds are named by the carbons they connect: an α(1→4) bond links carbon 1 of an α-glucose to carbon 4 of the next sugar.

• Maltose = glucose + glucose.
• Sucrose (table sugar) = glucose + fructose.
• Lactose (milk sugar) = glucose + galactose.

Long chains of sugars are polysaccharides. They come in two broad jobs: storing energy and building structures.

Energy stores: starch and glycogen

Plants store glucose as starch. It has two forms. Amylose is an unbranched chain of glucose joined by α(1→4) bonds. Amylopectin has the same chains but with branches every so often, joined by α(1→6) bonds. The geometry of the α(1→4) bond makes the chain twist into a helix, a compact coil.

Animals store glucose as glycogen, in the liver and muscles. It's built like amylopectin but branched much more heavily. Lots of branches means lots of chain ends, and enzymes remove glucose units from the ends, so glycogen can be broken down quickly when you need energy.

Structural polysaccharides: cellulose and chitin

Cellulose, the main material of plant cell walls (and of cotton), is also a chain of glucose, but joined by β(1→4) bonds. Because of the β geometry, every other glucose is flipped relative to its neighbours. The chain comes out straight and flat instead of coiled.

Straight chains can line up side by side. The –OH groups on neighbouring chains hydrogen-bond to each other, bundling the chains into strong fibres called microfibrils. That's a big part of what makes wood and plant stems so tough.

Chitin, which forms the exoskeletons of insects and crabs and the cell walls of fungi, is built like cellulose, but each sugar carries a nitrogen-containing group.

Why you can digest starch but not cellulose

Your digestive enzymes, such as amylase, have active sites that fit the shape of α-linked glucose. They can't grip a β(1→4) bond, so cellulose passes through your small intestine undigested. It isn't useless though: this undigested fibre helps food move through your intestines.

Cows, sheep and termites can live on plants because microbes in their guts make cellulase, an enzyme that does hydrolyse β(1→4) bonds. The animals then absorb the products.

Lipids: defined by avoiding water

Lipids aren't grouped by a shared monomer. What they have in common is that they're mostly hydrophobic: built largely from carbon and hydrogen, so they don't dissolve in water. They aren't polymers either. The three types you need to know are triglycerides, phospholipids and steroids.

Triglycerides: fats and oils

A triglyceride (a fat) is made from one glycerol, a small three-carbon molecule with three –OH groups, and three fatty acids, long hydrocarbon chains with a –COOH group at one end. Each fatty acid is joined to glycerol by an ester bond in a dehydration reaction, so three water molecules are released per fat molecule.

Fats are the body's long-term energy store. They pack about 9 kcal per gram, compared with about 4 kcal per gram for carbohydrate or protein, because their chains are rich in C–H bonds and contain very little oxygen.

A fatty acid with only single C–C bonds holds as many hydrogen atoms as possible; it's saturated with hydrogen. Its chain is straight, so saturated fats pack tightly together and are solid at room temperature, like butter. An unsaturated fatty acid has one or more C=C double bonds, and so fewer hydrogen atoms. In natural fats these double bonds are usually cis, meaning the two hydrogen atoms on the double bond sit on the same side. That forces the chain to carry on at an angle, putting a bend in it.

Bent chains can't pack closely, so the molecules are held together more weakly and melt at a lower temperature. That's why olive oil, rich in unsaturated fatty acids, is liquid at room temperature.

Phospholipids and steroids

A phospholipid is like a triglyceride with one fatty acid swapped for a phosphate-containing group. The result has a polar, hydrophilic head and two hydrophobic fatty-acid tails. A molecule with both a water-loving and a water-avoiding part is amphipathic.

In water, phospholipids arrange themselves into a double layer, the bilayer, with heads facing the water on both sides and tails hidden in the middle. That's the basic structure of cell membranes, driven by the hydrophobic effect. A triglyceride can't do this, because it has no hydrophilic head; it just gathers into oily droplets.

Tail saturation matters here too. Membranes with more unsaturated, kinked tails are more fluid, because the tails can't pack tightly. Organisms living in the cold tend to build their membranes with more unsaturated tails, which keeps the membranes from stiffening.

Steroids look quite different: four carbon rings fused together. Cholesterol is the most common. It sits in animal cell membranes, where it steadies fluidity, making membranes less runny when warm and preventing tight packing when cold. It's also the starting material for steroid hormones such as oestrogen and testosterone.

Key terms

Monosaccharide

A single sugar unit, such as glucose or fructose; the monomer of carbohydrates.

Glycosidic bond

The covalent link between two sugars, formed by dehydration synthesis.

α- and β-glucose

Two ring forms of glucose that differ in whether the –OH on carbon 1 points below or above the ring.

Polysaccharide

A long chain of monosaccharides, such as starch, glycogen, cellulose or chitin.

Triglyceride

A fat or oil: one glycerol joined to three fatty acids by ester bonds.

Saturated / unsaturated

A fatty acid with only single C–C bonds / one with at least one C=C double bond.

Cis / trans

Double bonds with their two hydrogens on the same side, which kinks the chain / on opposite sides, which leaves it nearly straight.

Phospholipid

A lipid with a hydrophilic phosphate head and two hydrophobic tails; the main component of membranes.

Amphipathic

Having both a hydrophilic part and a hydrophobic part.

Steroid

A lipid built from four fused carbon rings, such as cholesterol or testosterone.

Check yourself

Try answering in your head before you open each answer.

• 1.A person with no cellulase-producing gut microbes eats a meal of rice and celery. Predict what happens to the glucose in each.Show answerHide

  The rice's starch has α-linked glucose, which amylase hydrolyses, so its glucose is absorbed. The celery's cellulose has β(1→4) links that human enzymes can't hydrolyse, so it passes through as fibre and its glucose isn't absorbed.

• 2.Bacteria are moved from 37 °C to 10 °C. Predict how they would change the fatty acids in their membranes over the next few generations, and why.Show answerHide

  They would use more unsaturated (cis) fatty acids and/or shorter ones. The cold makes membranes stiffen; kinked tails can't pack tightly, so they keep the membrane fluid enough for proteins to move and work.

• 3.Drop some phospholipids and some triglycerides into water. Which forms a bilayer, and which forms droplets? Explain from their structure.Show answerHide

  Phospholipids form a bilayer, because each has a hydrophilic head that can face the water and hydrophobic tails that hide inside. Triglycerides have no hydrophilic head, so the whole molecule is hydrophobic, and they simply cluster into oil droplets.