AQA A-Level Biology: Biological Molecules and Cells -- Complete Revision Guide

Topics 1 and 2 of AQA A-Level Biology -- Biological Molecules and Cells -- form the biochemical and cellular foundation of the entire course. Almost every topic you study later, from respiration and photosynthesis to genetics and gene expression, builds on the concepts introduced here. Getting these two topics right is not just about doing well on Paper 1; it is about setting yourself up for success across all three papers.

This guide works through every key area of Topics 1 and 2, highlighting the detail that examiners expect and the connections that will strengthen your understanding.

How These Topics Fit Into the AQA A-Level Biology Exams

AQA A-Level Biology (specification 7402) is assessed entirely through written examinations -- there is no coursework. The qualification is made up of three papers:

• Paper 1 covers Topics 1--4 (Biological Molecules, Cells, Organisms Exchange Substances, Genetic Information). It is 2 hours long, worth 91 marks, and accounts for 35% of your A-Level grade.
• Paper 2 covers Topics 5--8 (Energy Transfers, Organisms Respond to Changes, Genetics/Populations/Evolution/Ecosystems, Control of Gene Expression). It is also 2 hours, 91 marks, and 35%.
• Paper 3 can draw on any content from Topics 1--8 and includes questions on practical skills. It is 2 hours, 78 marks, and 30%.

Required Practicals are assessed across all three papers through written questions that test your understanding of practical methods, results analysis, and experimental design.

Biological Molecules and Cells are examined directly on Paper 1, but they are also fair game on Paper 3, and the principles underpin many questions across the entire qualification. A strong grasp of enzyme kinetics, membrane transport, and protein structure will serve you in nearly every topic you encounter.

Topic 1: Biological Molecules

Monomers, Polymers, and the Reactions That Link Them

Many biological molecules are polymers -- long chains built from smaller repeating units called monomers. Monosaccharides, amino acids, and nucleotides are all monomers. They are joined together by condensation reactions, which release a molecule of water for each bond formed. The reverse process -- breaking polymers back into their monomers -- is hydrolysis, which requires a molecule of water to break each bond.

This principle applies consistently across carbohydrates, proteins, and nucleic acids. Examiners frequently test whether you understand that condensation and hydrolysis are not specific to one type of molecule but are a general feature of polymer biochemistry.

Carbohydrates

Carbohydrates are built from monosaccharide monomers. Glucose is the most important monosaccharide at A-Level, and you must know the difference between its two isomers:

• Alpha-glucose has the hydroxyl group on carbon 1 pointing downward (same side as the CH2OH group on carbon 6 in a ring diagram).
• Beta-glucose has the hydroxyl group on carbon 1 pointing upward (opposite side).

This structural difference has enormous consequences for the polysaccharides that each isomer forms.

Disaccharides are formed when two monosaccharides join by a glycosidic bond through a condensation reaction. The three you need to know are:

• Maltose -- glucose + glucose (alpha-1,4 glycosidic bond)
• Sucrose -- glucose + fructose
• Lactose -- glucose + galactose

Polysaccharides are where carbohydrate chemistry becomes especially interesting for biologists:

• Starch is the main energy storage molecule in plants. It is made of two components: amylose (a long, unbranched chain of alpha-glucose with alpha-1,4 glycosidic bonds, coiled into a helix) and amylopectin (a branched chain with alpha-1,4 and alpha-1,6 glycosidic bonds). The branching in amylopectin means enzymes can hydrolyse it from many points simultaneously, allowing rapid glucose release.
• Glycogen is the animal equivalent of starch. It has a similar structure to amylopectin but is more highly branched, making it even more compact and more rapidly broken down -- suited to the high metabolic demands of animal cells.
• Cellulose is a structural polysaccharide found in plant cell walls. It is made from beta-glucose monomers linked by beta-1,4 glycosidic bonds. Every other glucose molecule is flipped 180 degrees, which allows hydrogen bonds to form between adjacent chains. These hydrogen bonds create strong microfibrils that give the cell wall its rigidity.

Lipids

Lipids are not polymers, but they are still large biological molecules assembled from smaller components.

Triglycerides are formed when one molecule of glycerol bonds with three fatty acid chains through ester bonds (formed by condensation reactions). Triglycerides are the body's main long-term energy store. They are hydrophobic, meaning they do not dissolve in water and therefore do not affect the water potential of cells. They yield more energy per gram than carbohydrates because of their high proportion of hydrogen atoms.

Fatty acids can be saturated (no carbon-carbon double bonds, meaning the hydrocarbon chain is straight and the molecules pack closely together -- solid at room temperature) or unsaturated (one or more carbon-carbon double bonds, creating kinks in the chain that prevent close packing -- liquid at room temperature).

Phospholipids have a similar structure to triglycerides, but one fatty acid is replaced by a phosphate group. This gives them a hydrophilic head and two hydrophobic tails, making them amphipathic. This property is fundamental to the structure of cell membranes.

Proteins

Proteins are polymers of amino acids, joined by peptide bonds formed through condensation reactions.

The four levels of protein structure are central to understanding protein function:

• Primary structure -- the specific sequence of amino acids in the polypeptide chain, determined by the base sequence of the gene that codes for it.
• Secondary structure -- the polypeptide chain folds into regular patterns held by hydrogen bonds between the C=O and N-H groups of the peptide backbone. The two main forms are the alpha helix (a coiled structure) and the beta pleated sheet (a flat, folded structure).
• Tertiary structure -- the overall three-dimensional shape of the polypeptide, determined by interactions between the R groups of amino acids: hydrogen bonds, ionic bonds, disulfide bridges, and hydrophobic interactions. Tertiary structure is what gives a protein its specific shape and therefore its function.
• Quaternary structure -- some proteins consist of more than one polypeptide chain (or subunit) working together. Haemoglobin, for example, is made of four polypeptide subunits and four haem groups.

You should also understand the distinction between globular proteins (compact, roughly spherical, soluble, and functional -- such as enzymes, antibodies, and haemoglobin) and fibrous proteins (long, insoluble, and structural -- such as collagen and keratin).

Enzymes

Enzymes are biological catalysts -- globular proteins that lower the activation energy of metabolic reactions.

The lock and key model proposes that the substrate fits precisely into the enzyme's active site, like a key fitting a lock. However, the induced fit model is now accepted as more accurate: the active site changes shape slightly as the substrate binds, placing strain on bonds within the substrate and lowering the activation energy further.

Factors affecting enzyme activity:

• Temperature -- increasing temperature increases kinetic energy and the rate of enzyme-substrate collisions, up to the optimum temperature. Beyond this point, the enzyme denatures as hydrogen bonds and other weak interactions that maintain its tertiary structure break, altering the shape of the active site.
• pH -- each enzyme has an optimum pH. Extreme pH values alter the ionisation of R groups, disrupting ionic and hydrogen bonds and changing the shape of the active site.
• Substrate concentration -- at low concentrations, increasing substrate increases the rate of reaction. At high concentrations, the rate plateaus because all active sites are occupied (enzyme saturation, or Vmax).

Enzyme inhibition:

• Competitive inhibitors have a similar shape to the substrate and compete for the active site. Their effect can be overcome by increasing substrate concentration.
• Non-competitive inhibitors bind to a site other than the active site (an allosteric site), causing a conformational change that alters the active site shape. Increasing substrate concentration does not overcome their effect.

Nucleic Acids and ATP

DNA is a double-stranded polymer of nucleotides. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases (adenine, thymine, cytosine, guanine). The two strands are held together by hydrogen bonds between complementary base pairs: A pairs with T (two hydrogen bonds), and C pairs with G (three hydrogen bonds). The strands run antiparallel and form a double helix.

RNA is typically single-stranded, with ribose sugar instead of deoxyribose and uracil replacing thymine.

Semi-conservative replication was confirmed by the Meselson and Stahl experiment. Bacteria grown in heavy nitrogen (15N) were transferred to light nitrogen (14N) medium. After one generation, all DNA was of intermediate density (one old strand, one new). After two generations, half was intermediate and half was light. This ruled out conservative and dispersive models.

ATP (adenosine triphosphate) is the cell's immediate energy currency. It consists of adenine, ribose, and three phosphate groups. Hydrolysis of the terminal phosphate bond by ATPase releases energy for cellular work. ATP is rapidly regenerated from ADP and inorganic phosphate, making it ideal for providing energy in small, manageable amounts.

Water

Water's properties arise from hydrogen bonding between its polar molecules:

• Excellent solvent -- its polarity allows it to dissolve ionic and polar substances, making it the medium for most biochemical reactions.
• High specific heat capacity -- large amounts of energy are needed to raise water's temperature, which helps organisms maintain a stable internal environment.
• Cohesion -- hydrogen bonds between water molecules create strong cohesive forces, important for water transport in the xylem of plants.

Biochemical Tests

You must know the standard biochemical tests and what they detect:

• Benedict's test -- for reducing sugars. Heat the sample with Benedict's reagent; a colour change from blue to green, yellow, orange, or brick-red indicates reducing sugars. For non-reducing sugars, first hydrolyse with hydrochloric acid, neutralise with sodium hydrogen carbonate, then repeat the Benedict's test.
• Iodine test -- for starch. Add iodine solution; a colour change from brown-orange to blue-black indicates starch.
• Biuret test -- for proteins. Add sodium hydroxide followed by copper sulfate solution; a colour change from blue to purple/lilac indicates protein (peptide bonds).
• Emulsion test -- for lipids. Dissolve the sample in ethanol, then add water; a cloudy white emulsion indicates lipids.

Topic 2: Cells

Cell Structure and Ultrastructure

You need detailed knowledge of eukaryotic cells (animal and plant) and prokaryotic cells, and you must be able to compare them.

Key eukaryotic organelles and their functions:

• Nucleus -- contains the cell's genetic material (DNA) within chromatin. Surrounded by a nuclear envelope (double membrane with nuclear pores). The nucleolus within it is the site of ribosomal RNA synthesis.
• Rough endoplasmic reticulum (rough ER) -- studded with ribosomes; involved in the synthesis and transport of proteins, particularly those destined for secretion.
• Smooth endoplasmic reticulum (smooth ER) -- no ribosomes; involved in lipid and steroid synthesis and the detoxification of drugs and alcohol.
• Golgi apparatus -- modifies, packages, and distributes proteins and lipids received from the ER. Produces lysosomes and secretory vesicles.
• Mitochondria -- the site of aerobic respiration (the Krebs cycle occurs in the matrix; oxidative phosphorylation occurs on the inner membrane/cristae). Double membrane; the inner membrane is folded into cristae to increase surface area.
• Chloroplasts (plant cells only) -- the site of photosynthesis. Double membrane; contain thylakoid membranes arranged in grana (where the light-dependent reactions occur) and stroma (where the Calvin cycle occurs).
• Lysosomes -- membrane-bound vesicles containing hydrolytic enzymes. Involved in intracellular digestion, recycling of worn-out organelles, and apoptosis.
• Ribosomes -- the site of protein synthesis (translation). Found free in the cytoplasm (80S in eukaryotes) or attached to rough ER.
• Centrioles -- composed of microtubules arranged in a 9+0 pattern. Involved in organising the spindle fibres during cell division.
• Flagella (and cilia) -- long, whip-like projections used for cell movement. Composed of microtubules in a 9+2 arrangement.

Prokaryotic cells (such as bacteria) lack membrane-bound organelles, have smaller ribosomes (70S), have a single circular DNA molecule (not associated with histone proteins), and may have plasmids, a cell wall made of peptidoglycan, a capsule, and flagella.

Cell Fractionation and Ultracentrifugation

To study individual organelles, cells must first be broken open (homogenisation) in a cold, isotonic, buffered solution. The homogenate is then filtered to remove cell debris, and subjected to differential centrifugation -- spinning at increasing speeds. The heaviest organelles (nuclei) sediment first at the lowest speed; lighter organelles (ribosomes) require the highest speeds. Each pellet is resuspended and centrifuged at higher speeds to separate the next organelle.

The Cell Membrane: Fluid Mosaic Model

The cell membrane is described by the fluid mosaic model. It consists of a phospholipid bilayer in which proteins, cholesterol, glycolipids, and glycoproteins are embedded or attached.

• Phospholipids form the basic bilayer structure, with hydrophilic heads facing outward and hydrophobic tails facing inward. The bilayer is fluid because phospholipids can move laterally within their layer.
• Channel proteins and carrier proteins allow the passage of specific ions and molecules across the membrane (facilitated diffusion and active transport).
• Cholesterol sits between the phospholipid tails, regulating fluidity -- it prevents the membrane from becoming too rigid at low temperatures or too fluid at high temperatures.
• Glycolipids and glycoproteins (carbohydrate chains attached to lipids or proteins on the outer surface) play roles in cell recognition, signalling, and acting as receptor sites.

Transport Across Membranes

This is one of the most commonly examined areas in Topic 2:

• Diffusion -- the net movement of molecules or ions from a region of higher concentration to a region of lower concentration. It is passive (no energy required) and continues until equilibrium is reached.
• Facilitated diffusion -- passive transport of molecules or ions across the membrane through channel proteins or carrier proteins. Still down the concentration gradient; no ATP required.
• Osmosis -- the net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower (more negative) water potential. Understanding water potential (measured in kPa, always zero or negative in solution) is essential.
• Active transport -- the movement of molecules or ions across the membrane against the concentration gradient, requiring ATP and carrier proteins. The carrier protein changes shape when ATP is hydrolysed, moving the substance across the membrane.
• Co-transport -- a specific form of active transport in which the movement of one substance down its concentration gradient is used to drive the transport of another substance against its gradient. A key example is the sodium-glucose co-transporter in the epithelial cells of the small intestine: sodium ions are actively pumped out by the sodium-potassium pump, creating a gradient; sodium then moves back into the cell by facilitated diffusion through the co-transporter protein, bringing glucose with it.
• Endocytosis -- the cell membrane invaginates to engulf substances into a vesicle (bringing material into the cell).
• Exocytosis -- vesicles fuse with the cell membrane, releasing their contents outside the cell.

Cell Recognition and the Immune Response

Cells carry antigens on their surface -- molecules (usually proteins or glycoproteins) that can trigger an immune response. The immune system distinguishes self from non-self using these antigens.

The immune response involves several stages:

1. Phagocytes (such as macrophages and neutrophils) are non-specific. They engulf pathogens by endocytosis, fuse lysosomes with the vesicle to digest the pathogen, and then present the pathogen's antigens on their own cell surface (antigen presentation).

2. T lymphocytes are activated when their specific receptors bind to the antigen presented by a phagocyte. Different types of T lymphocytes carry out different roles: helper T cells release cytokines that stimulate B lymphocytes and other immune cells; cytotoxic T cells destroy infected body cells.

3. B lymphocytes are activated (with stimulation from helper T cells) when their surface antibodies bind to a complementary antigen. They undergo clonal selection and then divide rapidly by mitosis (clonal expansion). They differentiate into plasma cells, which secrete large quantities of specific antibodies, and memory cells, which remain in the body long-term and allow a faster, stronger secondary immune response if the same antigen is encountered again.

4. Antibodies are Y-shaped glycoproteins. Each antibody has two identical antigen-binding sites that are specific to one antigen. Antibodies work by agglutinating pathogens (clumping them together), neutralising toxins, or marking pathogens for phagocytosis (opsonisation).

Vaccines contain antigens (from dead or attenuated pathogens, or isolated antigen molecules) that stimulate the primary immune response and the production of memory cells, without causing disease. If the individual is later exposed to the live pathogen, the memory cells mount a rapid secondary response that prevents the disease from developing.

Monoclonal antibodies are produced from a single clone of B lymphocytes and are all identical, binding to the same specific antigen. They are used in medical diagnosis (e.g., pregnancy tests, ELISA tests) and in targeted cancer treatment.

HIV and the immune system: HIV is a retrovirus that infects and destroys helper T cells. As the number of helper T cells declines, the immune system becomes progressively less able to mount effective responses, eventually leading to AIDS -- a condition in which opportunistic infections and cancers can develop because the immune system is severely compromised.

The Cell Cycle and Mitosis

The cell cycle is the sequence of events between one cell division and the next. It consists of:

• Interphase -- the longest phase, during which the cell grows, carries out its normal functions, and replicates its DNA. Interphase is divided into G1 (cell growth and protein synthesis), S phase (DNA replication), and G2 (further growth and preparation for division, including organelle replication).
• Mitosis -- the division of the nucleus, producing two genetically identical daughter nuclei. It is divided into four stages:
  • Prophase -- chromosomes condense and become visible, the nuclear envelope breaks down, the centrioles move to opposite poles and spindle fibres begin to form.
  • Metaphase -- chromosomes line up along the cell equator (metaphase plate), attached to spindle fibres at their centromeres.
  • Anaphase -- the centromeres divide and sister chromatids are pulled to opposite poles of the cell by the shortening of spindle fibres.
  • Telophase -- chromatids reach the poles, the nuclear envelope reforms around each set, chromosomes decondense, and the spindle fibres break down.
• Cytokinesis -- the division of the cytoplasm, producing two separate daughter cells. In animal cells, the membrane pinches inward (cleavage furrow). In plant cells, a cell plate forms along the middle of the cell.

Required Practical: Investigating Osmosis in Plant Tissue

This practical involves placing pieces of plant tissue (typically potato cylinders) into a range of sucrose solutions of different concentrations, then measuring the change in mass (or length) after a set time.

Key points for exam questions on this practical:

• The independent variable is the concentration of sucrose solution; the dependent variable is the change in mass (expressed as percentage change to account for variation in initial mass).
• Control variables include the volume of solution, temperature, duration of immersion, and the dimensions of the plant tissue.
• In hypotonic solutions (higher water potential than the cell), water enters by osmosis and the tissue gains mass. In hypertonic solutions (lower water potential), water leaves and the tissue loses mass.
• The concentration at which there is no change in mass indicates the point at which the external water potential equals the water potential of the cell contents.
• A graph of percentage change in mass against sucrose concentration typically shows a curve that crosses the x-axis at this equilibrium point.

Exam Strategy for Topics 1 and 2

These topics contain a large amount of factual content, but the exams do not just test recall. AQA questions frequently require you to apply your knowledge to unfamiliar contexts -- for example, interpreting data from an enzyme experiment you have never seen, or explaining why a mutation in a gene encoding a membrane transport protein might cause a specific disease.

Prioritise understanding over memorisation. If you understand why cellulose is strong (hydrogen bonds between beta-glucose chains), you can answer a question about it even if the wording is unfamiliar. If you only memorised a definition, you may struggle.

Practise biochemical tests until they are automatic. These are easy marks on Paper 1 if you know the reagents, methods, and expected results -- but they are marks easily lost if you confuse the details.

Draw and annotate diagrams from memory. The fluid mosaic model, enzyme-substrate interactions, DNA replication, and mitosis are all commonly examined through diagrams. Practise sketching these without your notes, then check for accuracy.

Link Topics 1 and 2 to later content. The biochemistry of nucleotides connects to DNA replication and protein synthesis in Topic 4. Membrane transport connects to gas exchange in Topic 3 and to nerve impulse transmission in Topic 6. Making these connections will prepare you for synoptic questions on Paper 3.

Prepare with LearningBro

If you are looking for structured practice on the content covered in this guide, try our AQA A-Level Biology courses:

• AQA A-Level Biology: Molecules and Cells -- covers the full breadth of Topics 1 and 2 with exam-style questions.
• AQA A-Level Biology: Biological Molecules in Depth -- focused practice on carbohydrates, lipids, proteins, enzymes, nucleic acids, and biochemical tests.
• AQA A-Level Biology: Cells and Transport in Depth -- detailed coverage of cell ultrastructure, membrane transport, the immune response, and the cell cycle.

Each course is designed to test your understanding with questions that mirror the style and demand of AQA exam papers, helping you identify gaps in your knowledge and build confidence before the real thing.