AQA A-Level Biology: Biological Molecules overview
An overview of AQA A-Level Biology Topic 1: monomers and polymers, carbohydrates, lipids, proteins, enzymes, nucleic acids, ATP, water and inorganic ions, with a guide to each.
Biological molecules are the building materials of every living thing. Topic 1 asks you to understand what these molecules are made of, how they are built and broken down, and how the structure of each one explains what it does. This page sets out how the topic fits together and points to the guide for each part of it.
The one idea the topic is built on
Structure determines function. It sounds like a slogan, but on this topic it is a working method. Ask what a molecule is shaped like and what it is attracted to, and its job follows: a straight chain makes a fibre, a branched one makes a fast-release store, a molecule with a water-loving end and a water-hating end makes a membrane.
Almost as useful is that the whole topic runs on two reactions. Condensation joins molecules and releases water; hydrolysis splits them and uses water. Learn them once and they apply to every group here.
The guides
Monomers and polymers What monomers and polymers are, condensation and hydrolysis, and the bond each group of molecules forms. Short, and worth reading first.
Carbohydrates Alpha and beta glucose, glycosidic bonds, starch, glycogen and cellulose, and the Benedict's and iodine tests.
Lipids Triglycerides and ester bonds, saturated and unsaturated fatty acids, phospholipids and the bilayer, and the emulsion test.
Proteins Amino acids, the peptide bond, the four levels of protein structure and the bonds holding each, and the Biuret test.
Enzymes How enzymes work, the induced fit model, and what temperature, pH and concentration do to the rate of reaction.
Nucleic acids Nucleotide structure, the DNA double helix and complementary base pairing, how RNA differs, and semi-conservative replication.
ATP, water and inorganic ions The structure and hydrolysis of ATP and why cells use it, the properties of water, and the roles of four key ions.
Where the marks tend to go
Three things are worth knowing before you start, because they come up across the whole topic.
- Name the bond. Glycosidic, peptide, phosphodiester and ester are not interchangeable, and a question that gives you the bond and asks for the molecule is as common as the reverse.
- Give the reason, not just the fact. "Glycogen is branched" earns nothing on its own. "Glycogen is branched, so it has many ends, so glucose is released quickly" earns the marks.
- Get the test wording exact. A brick-red precipitate for Benedict's, a milky-white emulsion for lipids, purple for Biuret, blue-black for starch. The emulsion test in particular is marked on the word emulsion.
How this fits together
These molecules are the materials. How they are assembled into a working cell, and how things get in and out of one, is Topic 2.