AS and A'Level Topics Covered

Biological molecules

All life on Earth shares a common chemistry. This provides indirect evidence for evolution. Despite their great variety, the cells of all living organisms contain only a few groups of carbon-based compounds that interact in similar ways. Topics covered include:
Carbohydrates
Lipids
Proteins
Nucleic acids  
Water

Cells

All life on Earth exists as cells. These have basic features in common. Differences between cells are due to the addition of extra features.
Topics covered include:
Prokaryotic cells
Eukaryotic cells
Cell membranes
Antigens and disease

Organisms exchange substances with their environment

The internal environment of a cell or organism is different from its external environment. The exchange of substances between the internal and external environments takes place at exchange surfaces. Topics include:
Diffusion
Osmosis
Active transport
Surface area to volume ratio
Mass transport

Genetic information, variation and relationships between organisms

Biological diversity – biodiversity – is reflected in the vast number of species of organisms, in the variation of individual characteristics within a single species and in the variation of cell types within a single multicellular organism. The topic includes:
Differences as a result of genetic factors, of environmental factors, or a combination of both.
Genes, loci and amino acids during protein synthesis.
Mutations
Meiosis and fertilisation.
Natural selection 
Index of diversity.

Energy transfers in and between organisms (A-level only)

Life depends on continuous transfers of energy. In photosynthesis, light is absorbed by chlorophyll and this is linked to the production of ATP. In respiration, various substances are used as respiratory substrates. The hydrolysis of these respiratory substrates is linked to the production of ATP. 

Organisms respond to changes in their internal and external environment (A-level only)

A stimulus is a change in the internal or external environment. This topic includes:
The nervous system
Mammalian hormones
Plants  growth substances.

Genetics, populations, evolution and ecosystems (A-level only)

The theory of evolution underpins modern Biology. This topic includes:
Phylogenetic classification
DNA as the genetic material and a ‘universal’ Genetic code
Genetic drift 
Natural selection
Evolution.
Competition