B1.1 Carbohydrates and Lipids

Monosaccharide

A single sugar unit and the simplest carbohydrate (e.g. glucose, fructose, galactose) that can combine to form disaccharides and polysaccharides.

Disaccharide

A carbohydrate made of two monosaccharides joined by a glycosidic bond (e.g. maltose, sucrose, lactose).

Polysaccharide

A large carbohydrate polymer made of many monosaccharides joined by glycosidic bonds (e.g. starch, glycogen, cellulose).

Glycosidic bond

The covalent bond formed between two monosaccharides during a condensation reaction, with the removal of a water molecule.

Condensation reaction

A reaction that joins two molecules (e.g. monosaccharides) to form a larger one, releasing a molecule of water.

Hydrolysis reaction

A reaction that breaks a larger molecule into smaller units by adding a molecule of water (the reverse of condensation).

Lipid

A diverse group of hydrophobic (non-polar) biological molecules that are insoluble in water; includes triglycerides, phospholipids and steroids.

Triglyceride

A lipid formed from one glycerol molecule and three fatty acids, joined by ester bonds. Used mainly for long-term energy storage.

Fatty acid

A hydrocarbon chain with a carboxyl (–COOH) group at one end; can be saturated or unsaturated.

Saturated fatty acid

A fatty acid with no carbon–carbon double bonds; the chain is "saturated" with hydrogen. Tends to be solid at room temperature.

Unsaturated fatty acid

A fatty acid with one or more carbon–carbon double bonds (monounsaturated = one; polyunsaturated = more than one). Double bonds create kinks, so these tend to be liquid (oils) at room temperature.

Ester bond

The covalent bond formed between a fatty acid and glycerol during a condensation reaction.

Phospholipid

A lipid with a hydrophilic ("head" containing a phosphate group) and two hydrophobic ("tails", fatty acids) regions. This amphipathic nature allows it to form bilayers in water — the basis of cell membranes.

Amphipathic

Describes a molecule that has both hydrophilic and hydrophobic regions (e.g. phospholipids).

Glycogen

The main storage polysaccharide in animals; highly branched for rapid hydrolysis, stored in liver and muscle.

Starch

The main storage polysaccharide in plants; made of amylose (unbranched) and amylopectin (branched).

Cellulose

A structural polysaccharide in plant cell walls; straight, unbranched chains held by hydrogen bonds, giving high tensile strength.

Carbon atom

Carbon forms four covalent bonds, enabling a vast diversity of stable molecules (chains, branches, rings). The basis of all organic molecules.

Monomer

A single small unit that can be joined to others to build a polymer (e.g. a monosaccharide).

Polymer

A large molecule made of many repeating monomers joined together (e.g. a polysaccharide).

Glycoprotein

A protein with attached carbohydrate chains on the cell surface, functioning in cell–cell recognition (e.g. antigens such as the ABO blood group markers).

Hydrophobic property of lipids

Lipids are largely non-polar, so they cannot form hydrogen bonds with water and are therefore insoluble in it.

Hydrophilic

Polar or charged; attracted to and soluble in water.

Monounsaturated fatty acid

One carbon–carbon double bond, creating a single kink in the chain.

Polyunsaturated fatty acid

More than one carbon–carbon double bond, creating multiple kinks; tends to be liquid (oil) at room temperature.

Cis-fatty acid

An unsaturated fatty acid with the two hydrogens at the double bond on the same side, producing a kink; the form found in most natural fats.

Trans-fatty acid

An unsaturated fatty acid with the hydrogens at the double bond on opposite sides, keeping the chain relatively straight; mostly produced industrially and associated with cardiovascular disease.

Adipose tissue

Tissue that stores triglycerides beneath the skin and around organs; provides long-term energy storage and thermal insulation.

Why lipids store more energy than carbohydrates

Triglycerides are more reduced (more C–H bonds, less oxygen) than carbohydrates, so their oxidation releases roughly twice the energy per gram (~38 kJ/g vs ~17 kJ/g). Being hydrophobic, they also store without associated water mass.

Phospholipid bilayer

Forms spontaneously in water: the hydrophilic heads face outward toward the water and the hydrophobic tails face inward away from it. The structural basis of all cell membranes.

Steroids and the bilayer

Steroids (e.g. cholesterol, steroid hormones) are non-polar, so they can pass directly through the hydrophobic core of the phospholipid bilayer.