Water and carbohydrates

MDCAT Biology · Biology · Biological molecules. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A water molecule is polar: the oxygen is slightly negative and the hydrogens slightly positive. Neighbouring molecules attract each other with hydrogen bonds. Almost every special property of water comes from this: it dissolves ions and polar molecules, it heats up and cools down slowly, it takes much heat to evaporate, and its molecules cling together (cohesion).

Carbohydrates are made of carbon, hydrogen and oxygen. Monosaccharides are single sugars. Two join by a glycosidic bond, with the loss of one water molecule (condensation), to give a disaccharide. Many join to give a polysaccharide for storage (starch, glycogen) or structure (cellulose, chitin). Hydrolysis reverses this and uses water.

Rules to remember

High specific heat capacity: keeps body and lake temperatures steady. High heat of vaporisation: sweating and transpiration cool. Ice is less dense than water, so it floats
Monosaccharides by carbon number: triose 3C (glyceraldehyde), pentose 5C (ribose, deoxyribose, ribulose), hexose 6C (glucose, fructose, galactose). Aldose = aldehyde group (glucose); ketose = ketone group (fructose)
Maltose = glucose + glucose; sucrose = glucose + fructose; lactose = glucose + galactose
Starch = amylose (unbranched, α 1→4) + amylopectin (branched, α 1→4 and α 1→6): plant store
Glycogen: α-glucose, more branched than amylopectin: animal store in liver and muscle
Cellulose: β-glucose, β 1→4, straight unbranched chains: plant cell walls
Iodine test: starch blue-black, glycogen red-brown, cellulose no colour. n monosaccharides in a chain have (n − 1) glycosidic bonds

Common mistake

Mixing up condensation and hydrolysis. Condensation joins units and releases water; hydrolysis splits a bond and uses up water.

Worked example

An unbranched amylose chain of 50 glucose units is completely hydrolysed. How many water molecules are used and how many glucose molecules are formed?

50 units in one chain are joined by 50 − 1 = 49 glycosidic bonds.
Hydrolysis of each bond uses one water molecule: 49 × 1 = 49.
Every unit is set free as glucose: 50 molecules.

Answer: 49 water molecules are used and 50 glucose molecules are formed.

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