Biological Molecules: MDCAT Biology notes
Biological molecules MDCAT notes: why carbon is central, main classes of biomolecules, condensation and hydrolysis, anabolism, catabolism, cell makeup.
What biochemistry studies
Biochemistry (molecular biology) studies the chemical compounds of living things and the reactions between them. All life is built from a few elements. About 99% of the mass of a cell is made of carbon, hydrogen, oxygen and nitrogen, with smaller amounts of phosphorus, sulphur, calcium, potassium, sodium and others.
Why carbon is the backbone of life
- Carbon is tetravalent. It forms four covalent bonds.
- It bonds with other carbon atoms to make long chains, branched chains and rings. It also forms single, double and triple bonds.
- It combines with H, O, N, S and P, which gives a huge variety of molecules.
- Every organic biomolecule contains carbon and hydrogen.
- The C–H bond is the main store of chemical energy. Food is oxidized by breaking C–H bonds, and the energy released drives cell activities.
Major classes of biomolecules
| Class | Monomer | Main role |
|---|---|---|
| Carbohydrates | Monosaccharides (for example glucose) | Immediate energy source, structural (cellulose) |
| Proteins | Amino acids | Enzymes, structure, transport, hormones, defence |
| Lipids | Fatty acids and glycerol (not true polymers) | Energy storage, membranes, insulation |
| Nucleic acids | Nucleotides | Storage and transmission of genetic information |
Composition of a cell
Water is the most abundant compound in cells, making up about 70–90% of cell mass. The human body is roughly two-thirds to three-quarters water by weight. Among organic compounds, proteins are the most abundant in mammalian cells, ahead of lipids, carbohydrates and nucleic acids. Water is inorganic, so in a question about the most abundant organic molecule, water is the wrong choice.
Building up and breaking down
Condensation and hydrolysis
- Condensation (dehydration synthesis) joins two monomers and removes a molecule of water. It builds polymers such as starch, proteins and nucleic acids.
- Hydrolysis splits a polymer into monomers by adding water. It is the exact reverse of condensation. Digestion is hydrolysis.
Metabolism
| Anabolism | Catabolism |
|---|---|
| Builds complex molecules from simple ones | Breaks complex molecules into simple ones |
| Energy is consumed (endergonic) | Energy is released (exergonic) |
| Examples: photosynthesis, protein synthesis | Examples: respiration, digestion |
Metabolism is the sum of anabolism and catabolism. The energy released by catabolism is captured as ATP and used to drive anabolism.
Common MDCAT traps
- Hydrolysis is the reverse of condensation, not photolysis or oxidation.
- Hydrolysis adds a whole water molecule, not just H or OH.
- The most abundant compound overall is water, but the most abundant organic compound is protein.
- The energy-rich bond is C–H, not C=O. C=O is found in low-energy products like $\mathrm{CO_2}$.
- Nucleic acids, not proteins, store and transmit information.
Quick revision
- Carbon is tetravalent.
- All organic biomolecules contain carbon and hydrogen.
- Catabolic reactions release energy; anabolic reactions use it.
- Condensation removes water; hydrolysis adds water.
- C, H, O and N make up about 99% of cell mass.