A cotton shirt, a wool jumper and a polyester fleece are all made of long chain molecules, lined up along thin fibres. Cotton is the simplest of them: it is almost pure cellulose.
Each cotton fibre is a single plant cell, a hair that grows from a seed inside the cotton boll, the plant's seed pod. It grows about 2 to 3 cm long and far thinner than a human hair. While it grows, it is a round tube full of water. When the boll opens, the fibre dries, collapses flat and twists over and over, and the twists help the fibres grip each other when they are spun into yarn.
The fibre's wall is cellulose: chains of glucose, with each glucose unit flipped over against the one before, so the chain stays straight. One chain in cotton can hold 10,000 glucose units or more. The straight chains lie side by side along the fibre, and the -OH groups of neighbouring chains hold on to each other with hydrogen bonds: the δ+ hydrogen of an -OH on one chain reaches a lone pair on an oxygen of the next.
Starch and cellulose
Starch is made of glucose too. In starch every link turns the same way, so the chains curl into coils that pack loosely. Hot water works its way in between them, which is how starch thickens a sauce, and your enzymes cut starch back into sugar. Cellulose's straight chains hold each other with hydrogen bonds all along their length, and in most of a cotton fibre they pack in a regular order, like the molecules of a crystal, so a cotton shirt comes through a hot wash whole.
| Starch | Cellulose | |
|---|---|---|
| glucose units | all turned the same way | every other one flipped |
| chains | coiled | straight, side by side |
| in hot water | swells and thickens it | stays as it is |
| found in | potatoes, rice, flour | cotton, wood, paper |
Creases and ironing
Hydrogen bonds explain why cotton creases. Crumple a cotton shirt and its chains bend; some hydrogen bonds break, and new ones form with the chains in their bent positions, which holds the crease. A steam iron puts hot water in among the chains, and the water molecules make hydrogen bonds of their own with the -OH groups, loosening the chains' hold on each other. The hot plate presses the cloth flat, and as it dries, new hydrogen bonds form with the chains lying flat.