Cell level systems Study Guide

DNA, or deoxyribonucleic acid, is a biological macromolecule essential for all known forms of life. Its primary role is to carry genetic instructions for the development, functioning, growth, and reproduction of all organisms. DNA is classified as a polymer, which means it is a large molecule made up of many smaller, repeating units called monomers. In the case of DNA, these monomers are called nucleotides. Each nucleotide has three key components: a deoxyribose sugar, a phosphate group, and a nitrogen-containing base. There are four types of nitrogenous bases in DNA: adenine (A), guanine (G), cytosine (C), and thymine (T). These nucleotides link together to form a long strand, with the phosphate group of one nucleotide attaching to the sugar of the next. DNA typically consists of two such strands wound around each other in a characteristic 'double helix' structure, resembling a twisted ladder. The two strands are held together by weak hydrogen bonds that form between specific pairs of bases: adenine always pairs with thymine (A-T), and guanine always pairs with cytosine (G-C). This specific pairing is crucial for DNA replication and protein synthesis. The sequence of these bases along the DNA strand forms the genetic code, determining the characteristics of an organism.

Lipids are a diverse group of organic compounds that include fats, oils, and waxes. In biological systems, the most common type of lipid, particularly fats and oils, is the triglyceride. Triglycerides are fundamental to life as they serve as a major energy store, provide insulation, and protect organs. The structural components that make up a triglyceride molecule are fatty acids and glycerol.

Glycerol is a small, three-carbon alcohol molecule. It forms the 'backbone' of the triglyceride. Fatty acids are long hydrocarbon chains with a carboxyl group at one end. They can be saturated (containing only single bonds between carbon atoms) or unsaturated (containing one or more double bonds). Typically, a single glycerol molecule combines with three fatty acid molecules to form one triglyceride molecule. This joining process occurs through a condensation reaction, where a molecule of water is removed for each fatty acid attached to the glycerol. This forms ester bonds between the glycerol and each fatty acid.

Conversely, when the body needs to access the energy stored within lipids, or if lipids need to be broken down for other metabolic processes, the triglyceride molecule can be hydrolysed. This is essentially the reverse of synthesis. During hydrolysis, water molecules are added, breaking the ester bonds and releasing the glycerol molecule and the three fatty acid molecules. These released fatty acids and glycerol can then be used in various metabolic pathways. For instance, fatty acids can be broken down through beta-oxidation to produce acetyl-CoA, which enters the Krebs cycle for energy production. Glycerol can be converted into an intermediate of glycolysis, also leading to energy production. Therefore, fatty acids and glycerol are not just building blocks but also the very units that are processed and utilised when lipids are broken down, making them central to lipid metabolism.