Carbohydrates and lipids

Biology AS · 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

Carbohydrates are built from sugar units. A monosaccharide (glucose, fructose) is a single unit and is the monomer. Two units make a disaccharide and many units make a polysaccharide, which is a polymer and a macromolecule. Units are joined by covalent glycosidic bonds in condensation reactions, each releasing one water molecule. Hydrolysis uses water to break the bond again.

The shape of the glucose decides the polysaccharide. α-glucose gives coiled, compact storage molecules (starch, glycogen). β-glucose gives straight chains of cellulose for strength.

Triglycerides and phospholipids are lipids. They are not polymers. Triglycerides are non-polar and hydrophobic, so they do not dissolve in water.

Rules to remember

α-glucose has the –OH on carbon 1 below the ring; β-glucose has it above the ring
Maltose = α-glucose + α-glucose; sucrose = α-glucose + fructose. Glucose, fructose and maltose are reducing sugars; sucrose is non-reducing
Amylose: α-glucose, 1,4 glycosidic bonds, unbranched helix. Amylopectin: 1,4 bonds with 1,6 branches. Glycogen: like amylopectin but more branched
Starch and glycogen are compact and insoluble, so they store glucose without affecting water potential; many branch ends allow fast release of glucose
Cellulose: β-glucose, 1,4 bonds, every second unit rotated 180°; straight chains held by hydrogen bonds into microfibrils with high tensile strength
Triglyceride = glycerol + three fatty acids joined by three ester bonds (three water molecules released). Saturated fatty acids have no C=C bonds; unsaturated have one or more
Phospholipid = glycerol + two hydrophobic (non-polar) fatty acid tails + one hydrophilic (polar) phosphate head

Common mistake

Counting one water molecule per monomer. A chain of n monomers has n − 1 glycosidic bonds, so n − 1 water molecules are released when it forms.

Worked example

50 triglyceride molecules are completely hydrolysed. How many water molecules are used and what are the products?

Each triglyceride has three ester bonds.
Breaking each ester bond uses one water molecule: 50 × 3 = 150 water molecules.
Each triglyceride gives one glycerol and three fatty acids: 50 glycerol and 50 × 3 = 150 fatty acids.

Answer: 150 water molecules are used, giving 50 glycerol molecules and 150 fatty acid molecules.

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