Sugars and starch

Section 1 of 4

Living things are built from four big families of molecules: sugars, fats, proteins and DNA. Sugars, fats and proteins are put together from the functional groups you met in lesson 5.

Glucose, C₆H₁₂O₆, is the sugar plants make from carbon dioxide and water using the energy in sunlight, and the sugar your blood carries to every cell. In water, almost every glucose molecule closes up into a ring of five carbon atoms and one oxygen atom, which folds into a chair like the cyclohexane ring. Five -⁠OH groups stick out around it, so glucose is an alcohol five times over, and like ethanol it mixes easily with water.

Glucose, C₆H₁₂O₆A sugar. Five carbon atoms and one oxygen atom make a six-membered ring, and a sixth carbon hangs off the ring. Four ring carbons and the outside carbon each carry an -OH group.OOHOHOHOHOH
Glucose as a skeletal formula: a ring of five carbons and one oxygen, with five -⁠OH groups around it.

Glucose joins into starch

Two glucose molecules can join. An -⁠OH group on one reacts with an -⁠OH group on the other, a water molecule leaves, and the two rings stay linked through one oxygen atom. This is a condensation, the reaction you have met before: two molecules join and give off water, just as an acid and an alcohol do when they make an ester.

Glucose, C₆H₁₂O₆A sugar. Five carbon atoms and one oxygen atom make a six-membered ring, and a sixth carbon hangs off the ring. Four ring carbons and the outside carbon each carry an -OH group.OOHOHOHOHOH plus Glucose, C₆H₁₂O₆A sugar. Five carbon atoms and one oxygen atom make a six-membered ring, and a sixth carbon hangs off the ring. Four ring carbons and the outside carbon each carry an -OH group.OOHOHOHOHOH
make
Maltose, C₁₂H₂₂O₁₁Two glucose rings joined through one oxygen atom, the same link that joins the glucose units in starch.OOOOHOHOHOHOHOHOHOH plus Water, H₂OOne oxygen atom joined by single bonds to two hydrogen atoms, in a bent shape.OHH
Two glucose molecules join into maltose, malt sugar, and give off a water molecule. The new link is the oxygen atom between the two rings.

Keep adding glucose units, one link at a time, and you get starch: chains of hundreds or thousands of glucose units. Starch is a polymer, like poly(ethene) and nylon, with glucose as its monomer. Plants store energy as starch. Potatoes, rice and the flour in bread are full of it.

Cellulose: the same units, flipped

Glucose rings come in two forms, which differ only in the way the -⁠OH group on one carbon points. Starch is built from one form, and every link turns the same way, so the chain curls round into a coil. Cellulose is built from the other form. Each glucose unit sits flipped over against the one before it, so the chain stays straight. Straight chains lie side by side, and the -⁠OH groups on neighbouring chains hold on to each other, making tough fibres.

Glucose units joined into starch and into cellulose Two drawings of glucose rings, each ring a hexagon, with every oxygen atom shown as a red dot: one in each ring and one at each link. Top, starch: seven rings, each turned the same way as the one before, so each link turns the same way and the chain curves round. Bottom, cellulose: three straight chains, each with every other ring flipped upside down. Dashed lines between the chains show the -OH groups of neighbouring chains holding on to each other. starch every link turns the same way, so the chain coils cellulose every other glucose is flipped, so straight chains stack
Starch and cellulose are both chains of glucose, drawn here as hexagons, with every oxygen atom in red: one in each ring and one at each link. In starch the chain coils; in cellulose every other glucose is flipped, so the chains stay straight and stack into fibres.

Cellulose makes the walls of plant cells. Cotton is almost pure cellulose, and wood and paper are largely made of it. Your enzymes cut the links in starch, and cellulose passes through you as the fibre in your food. Cows and termites live on cellulose with the help of microbes in their guts, which break it down for them.

Quick check

Each time two glucose units join in a starch chain, which small molecule is given off?