A DNA molecule is a complex structure composed of two long, strands that are coiled together in a double helix. Each strand is a polymer made up of repeating units called nucleotides.

Each nucleotide consists of three parts:

• A deoxyribose sugar
• A phosphate group
• A nitrogenous base

There are four different nitrogenous bases found in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). The two strands of DNA are held together by hydrogen bonds between the bases.

The base pairing rules are specific: adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G). This is due to the specific chemical properties of these bases. The A-T pairing involves two hydrogen bonds, while the C-G pairing involves three hydrogen bonds.

The double helix structure is important because it provides stability to the DNA molecule and allows for easy replication. The complementary base pairing ensures that the two strands can be used as templates for making new strands during DNA replication. The double helix also protects the genetic information within the DNA molecule.

It is important that large biological molecules are formed from many small molecules for several reasons, which directly relate to the properties of the resulting macromolecules:

• Energy Storage: The formation of large molecules like starch and glycogen allows for the storage of energy. Glucose, a small molecule, contains a limited amount of energy. By linking many glucose molecules together, a much larger amount of energy can be stored in a compact form. This stored energy can be released later through hydrolysis.
• Structural Support: Large molecules like cellulose provide structural support to plants. The long, linear chains of cellulose molecules create strong fibres that give plants rigidity. This is crucial for maintaining their shape and resisting external forces.
• Biochemical Processes: Proteins are essential for a vast array of biochemical processes, including enzyme catalysis, transport, and structural roles. The complex 3D structures of proteins, formed from the specific sequence of amino acids, are crucial for their function. These structures are only possible through the polymerization of amino acids.
• Efficiency: Polymerisation is a more efficient way to store and utilize energy and build complex structures than dealing with numerous individual small molecules. It allows for a more stable and manageable system.

Fats, also known as lipids, are primarily composed of carbon (C), hydrogen (H), and oxygen (O). However, the proportion of oxygen is significantly lower than in carbohydrates. The arrangement of atoms in fats is crucial. Fats are composed of glycerol and fatty acids. Fatty acids consist of a long hydrocarbon chain (primarily C and H) with a carboxyl group (-COOH). The hydrocarbon chains can be saturated (only C-H bonds) or unsaturated (containing C=C double bonds). The presence of C-H bonds contributes to the hydrophobic (water-repelling) nature of fats, while the arrangement of the hydrocarbon chains influences their physical state (solid or liquid) at room temperature. Saturated fats tend to be solid due to their straight chains packing tightly, while unsaturated fats are often liquid due to their kinks preventing tight packing.