Carbohydrates are organic molecules made of carbon, hydrogen and oxygen.
They are used as a source of energy and for structural purposes.
Monosaccharides are single sugar units.
Examples include glucose, fructose and galactose.
Glucose is important in respiration.
Monosaccharides can join together by condensation reactions to form disaccharides or polysaccharides.
Disaccharides are made of two monosaccharides joined together.
Examples include maltose (glucose + glucose), sucrose (glucose + fructose) and lactose (glucose + galactose).
Polysaccharides are long chains of monosaccharides.
Starch is a storage polysaccharide in plants, made of amylose and amylopectin. It is compact and insoluble.
Glycogen is a storage polysaccharide in animals. It is highly branched, allowing rapid release of glucose.
Cellulose is a structural polysaccharide in plant cell walls. Its beta-glucose chains form strong microfibrils.
Lipids are organic molecules that are insoluble in water but soluble in organic solvents.
They are used for long-term energy storage, insulation and as components of cell membranes.
Triglycerides are made of one glycerol and three fatty acids joined by ester bonds.
They are hydrophobic and store energy efficiently.
Phospholipids are similar to triglycerides, but one fatty acid is replaced by a phosphate group.
They are amphipathic, meaning they have a hydrophilic head and hydrophobic tails.
Phospholipids form the bilayer of cell membranes.
Steroids, such as cholesterol, are lipids with a four-ring structure.
They are used in membranes and as precursors to hormones.
Proteins are polymers of amino acids.
They are used as structural components, enzymes, hormones and antibodies.
Amino acids are joined by peptide bonds in condensation reactions to form polypeptides.
Proteins have four levels of structure:
Primary structure: the sequence of amino acids.
Secondary structure: alpha helices or beta pleated sheets formed by hydrogen bonds.
Tertiary structure: the 3D folding due to hydrogen, ionic, disulphide bonds and hydrophobic interactions.
Quaternary structure: more than one polypeptide chain forming a functional protein.
Proteins can be globular or fibrous.
Globular proteins are soluble and functional (eg: enzymes, haemoglobin).
Fibrous proteins are insoluble and structural (eg: collagen, keratin).
Water is a polar molecule, meaning it has an uneven distribution of charge.
It is a solvent, allowing substances to dissolve and react in solution.
Water is important in transport, for example in blood and in xylem.
It has a high specific heat capacity, which stabilises temperature in organisms and environments.
Water has cohesion and adhesion, which allows capillary action and surface tension.
Inorganic ions are ions that do not contain carbon.
They have important roles in biological processes: