Phospholipids are fundamental components of cell membranes, playing a crucial role in determining their structure and function. A phospholipid molecule consists of a glycerol backbone, two fatty acid tails (hydrophobic), and a phosphate group attached to a head (hydrophilic). This amphipathic nature (having both hydrophobic and hydrophilic parts) is crucial for forming the lipid bilayer of cell membranes. In water, phospholipids spontaneously arrange themselves into a bilayer, with the hydrophobic tails pointing inwards, away from the water, and the hydrophilic heads facing outwards, interacting with the aqueous environment. This arrangement creates a stable barrier that separates the interior of the cell from its external environment. The fluid mosaic model of the cell membrane describes how these phospholipids, along with proteins and cholesterol, move laterally within the layer, contributing to membrane fluidity and flexibility. This fluid nature allows for the controlled entry and exit of substances, signalling, and cell recognition, making phospholipids indispensable in cell membrane function.

Saturated and unsaturated fats differ in the structure of their fatty acid chains. Saturated fats have no double bonds between carbon atoms, resulting in straight chains that pack tightly together, typically making them solid at room temperature. Common sources include butter and red meat. In contrast, unsaturated fats contain one or more double bonds, causing kinks in the chain that prevent tight packing, often making them liquid at room temperature, like olive oil. Regarding health, saturated fats are associated with increased levels of LDL cholesterol (low-density lipoprotein), which can contribute to cardiovascular diseases. Unsaturated fats, particularly monounsaturated and polyunsaturated fats, are considered healthier as they can lower LDL cholesterol levels and provide essential fatty acids like omega-3 and omega-6, which are crucial for brain function and cell growth. It is important to balance the intake of different types of fats for optimal health.

Monosaccharides are the simplest form of carbohydrates, consisting of single sugar molecules like glucose, fructose, and galactose. They typically have a basic formula of (CH2O)n and are the building blocks for more complex carbohydrates. Monosaccharides are readily absorbed and used for immediate energy. Polysaccharides, on the other hand, are long chains of monosaccharide units linked together. They include starch, glycogen, and cellulose. Starch, found in plants, and glycogen, found in animals, serve as energy storage forms. Cellulose, a major component of plant cell walls, provides structural support. The key difference lies in their structure: monosaccharides are single units, quick to digest and provide fast energy, whereas polysaccharides are complex, providing sustained energy release and structural functions.

The R-group, or side chain, in amino acids is critical in determining the properties and function of proteins. Each of the 20 amino acids has a unique R-group, which can range from a simple hydrogen atom to more complex structures with rings or charged groups. These side chains influence the amino acid's behaviour and interactions. For example, amino acids with hydrophobic (water-repelling) side chains tend to be found in the interior of proteins, stabilising the structure, while hydrophilic (water-attracting) side chains are often on the surface, interacting with the aqueous environment. The R-groups also facilitate specific interactions between amino acids, like hydrogen bonding, ionic interactions, and disulphide bridges, which are crucial in forming and stabilising the protein's tertiary and quaternary structures. The diversity of R-groups contributes to the immense variety of protein structures and functions, from enzymes and antibodies to transport proteins and hormones.

Enzymes are made of proteins primarily because of the versatile and complex nature of proteins, which allows them to form highly specific and intricate structures necessary for enzyme function. The sequence of amino acids in a protein determines its three-dimensional shape, which is essential for enzyme activity. Enzymes have active sites, specific regions where substrate molecules bind and undergo chemical reactions. The specificity of these active sites arises from the unique arrangement of amino acids, forming a shape that perfectly complements the substrate. Additionally, the flexibility of proteins allows enzymes to undergo slight conformational changes to facilitate binding and catalysis (induced fit model). This specificity and ability to change shape are not as readily achievable with other biomolecules like lipids or carbohydrates, making proteins the ideal candidates for enzymatic roles.