Water and carbohydrates: transport, fuel and storage

What you’ll learn

• Relate water polarity to its role as a transport solvent.
• Compare the structures and functions of sugars, starch and glycogen.
• Explain condensation and hydrolysis of glycosidic bonds.

Understand the idea

A water molecule has an uneven distribution of charge: oxygen is partially negative and the hydrogens are partially positive. Water molecules surround ions and interact with other polar molecules, allowing many substances to dissolve. Blood plasma can therefore carry dissolved glucose, ions and other solutes. A substance being transported in an aqueous fluid does not imply that every component dissolves freely; hydrophobic substances may need transport proteins.

Glucose, fructose and galactose are monosaccharides. Joining two monosaccharides by condensation forms a glycosidic bond and releases water: glucose with glucose forms maltose, glucose with fructose forms sucrose, and glucose with galactose forms lactose. Hydrolysis uses water to split the bond. Small soluble sugars can be transported and used in respiration, whereas a storage polymer contains many sugar units in one large molecule.

Starch contains amylose, an unbranched, coiled polymer of $\alpha$-glucose with $\alpha$-1,4 links, and amylopectin, which also has $\alpha$-1,6 branch points. Glycogen has the same types of links but is more extensively branched. Branches provide many ends at which enzymes can remove glucose units. These compact, largely insoluble stores exert much less osmotic effect than an equivalent number of free glucose molecules.

Key formulae

$$\text{monosaccharide}+\text{monosaccharide}\rightleftharpoons\text{disaccharide}+\mathrm{H_2O}$$

$$\text{linear chain of }n\text{ monomers: }n-1\text{ glycosidic bonds}$$

Worked example

A linear chain is made by joining 120 glucose molecules using condensation. Find the number of bonds formed and explain one advantage of storing glucose in this form.

1. The first glucose needs no joining bond; each subsequent glucose adds one bond.
2. The chain therefore contains 120 − 1 = 119 glycosidic bonds, with 119 water molecules released.
3. A large storage polymer has a smaller osmotic effect than 120 separate dissolved glucose molecules; an insoluble store also does not diffuse out readily.

Answer: 119 bonds; polymer storage reduces the osmotic effect of the stored glucose.