Biological molecules

All large biological molecules, known as polymers, are constructed from smaller, repeating subunits called monomers. Think of it like a train (the polymer) made of many identical carriages (the monomers). Organisms build the specific polymers they need by joining monomers together in a process called a condensation reaction. This reaction forms a chemical bond and releases a single molecule of water. Conversely, to break down polymers into monomers (for example, during digestion), a process called hydrolysis occurs. Hydrolysis means 'splitting with water', and it uses a water molecule to break each bond.

Carbohydrates are molecules made of carbon, hydrogen, and oxygen atoms. Their primary role is to provide and store energy. The simplest carbohydrates are single sugar units called monosaccharides, like glucose. Glucose is the main fuel for respiration. Two monosaccharides can join to form a disaccharide, like sucrose (table sugar). When many monosaccharides join together, they form large polymers called polysaccharides. Key examples are starch (energy store in plants), glycogen (energy store in animals), and cellulose (forms the structural component of plant cell walls).

Lipids are a diverse group of molecules that are insoluble in water, commonly known as fats and oils. They are made of carbon, hydrogen, and oxygen, but have a much lower proportion of oxygen than carbohydrates. A typical lipid, a triglyceride, is formed from one molecule of glycerol joined to three molecules of fatty acids via condensation reactions. Lipids have several vital functions: they are an excellent long-term energy store, they provide thermal insulation, they protect vital organs, and they are a key component of cell membranes.

Proteins are polymers that carry out a vast range of functions in living organisms. They are made from carbon, hydrogen, oxygen, and nitrogen (and sometimes sulfur). The monomers of proteins are amino acids. There are about 20 different types of amino acids. They are joined together by peptide bonds in condensation reactions to form long chains called polypeptides. The specific sequence of amino acids in a polypeptide chain determines how it folds into a unique three-dimensional (3D) shape. This 3D shape is critical to the protein's function. Examples of proteins include enzymes (biological catalysts), antibodies (part of the immune system), and haemoglobin (transports oxygen).

Although it is an inorganic molecule (it does not contain carbon), water is essential for all living organisms. Its most important role in the context of biological molecules is as a solvent. Most of the chemical reactions that happen inside cells, collectively known as metabolism, take place in aqueous solution. This means the reacting substances (solutes) are dissolved in water (the solvent). Water's ability to dissolve a wide range of substances allows molecules like glucose and amino acids to be transported around the body in the blood plasma. Furthermore, as we have seen, water is a key reactant in all hydrolysis reactions, which are vital for breaking down large food molecules during digestion.