241 terms
All cells arise from other cells
In a multicellular organism, most cells lose the ability to divide once they specialise. Only certain cell types keep that ability throughout life.
Cells
All cells arise from other cells
Some eukaryotic cells — cells that contain a nucleus — repeatedly follow a fixed sequence of stages called the cell cycle. Each cycle ends with the cell dividing to produce new cells.
Cells
All cells arise from other cells
Before a cell divides, it must copy all of its DNA. This copying happens during interphase — the preparation stage of the cell cycle.
Cells
All cells arise from other cells
Mitosis is a type of cell division. It splits one cell into two new cells, and each new cell carries an exact copy of the original cell's DNA.
Cells
All cells arise from other cells
Mitosis splits one cell into two identical cells. Chromosomes go through five named stages as this happens. Each stage has a distinct, testable behaviour.
Cells
All cells arise from other cells
During cell division, protein fibres called spindle fibres attach to chromosomes and pull the two identical copies apart. Each copy moves to opposite ends of the cell.
Cells
All cells arise from other cells
After a cell copies and separates its DNA, cytokinesis splits the cytoplasm to produce two new daughter cells. The word 'usually' matters — the process can occasionally fail.
Cells
All cells arise from other cells
Normally, cells divide in a tightly controlled way. When that control breaks down, cells divide without stopping — forming a tumour, which can become cancer.
Cells
All cells arise from other cells
Cancer cells divide too fast and without control. Many treatments work by targeting specific stages of cell division to slow or stop this process.
Cells
All cells arise from other cells
Bacteria divide by splitting in two — a process called binary fission. The cell copies its DNA first, then splits its cytoplasm to make two new daughter cells.
Cells
All cells arise from other cells
Viruses are not alive, so they cannot divide like cells do. Instead, a virus injects its genetic material into a host cell and forces that cell to make new virus copies.
Cells
All cells arise from other cells
Meiosis is a different type of cell division from mitosis. AQA covers it separately in section 3.4.3, not here.
Cells
All cells arise from other cells
New cells can only come from pre-existing cells, and in eukaryotes (organisms whose cells have a nucleus) this happens through a tightly regulated sequence of events called the cell cycle, which culminates in mitosis — a type of cell division that produces two genetically identical daughter cells. Prokaryotes (cells without a nucleus, such as bacteria) divide by a simpler process called binary fission, while viruses bypass cell division entirely by hijacking a host cell's machinery to copy themselves. Understanding how cell division is controlled matters beyond the basics: when that control breaks down, cells can divide uncontrollably, leading to tumours and cancer — which is why many cancer treatments specifically target the cell cycle.
Cells
Alteration of the sequence of bases in DNA can alter the structure of proteins
A gene mutation is a change to the base sequence of a DNA molecule. Several types exist, and they most often arise when DNA copies itself before cell division.
The control of gene expression
Alteration of the sequence of bases in DNA can alter the structure of proteins
Gene mutations can happen by chance at any time. Certain environmental factors, called mutagenic agents, make mutations happen more often.
The control of gene expression
Alteration of the sequence of bases in DNA can alter the structure of proteins
A mutation changes the base sequence of a gene. Because bases code for amino acids, a changed sequence can produce a different chain of amino acids — a different polypeptide.
The control of gene expression
Alteration of the sequence of bases in DNA can alter the structure of proteins
Each amino acid in a protein is coded for by a three-base sequence called a codon (or triplet). A substitution mutation swaps one base for another, producing a new codon. The genetic code is described as degenerate, meaning most amino acids are coded for by more than one codon. Because of this redundancy, a substitution that changes the third base of a codon very often still codes for the same ami
The control of gene expression
Alteration of the sequence of bases in DNA can alter the structure of proteins
Adding or deleting a base shifts every codon after that point. This scrambles the amino acid sequence from that position onwards.
The control of gene expression
Alteration of the sequence of bases in DNA can alter the structure of proteins
A gene mutation is a change to the sequence of bases in DNA, and these changes can arise spontaneously during DNA replication or be triggered by mutagenic agents — environmental factors such as UV radiation or certain chemicals that increase the rate at which mutations occur. Because the base sequence of a gene codes for the order of amino acids in a protein, some mutations alter the resulting polypeptide's structure and potentially its function. However, the impact of a mutation depends on its type: a substitution (one base swapped for another) may have no effect due to the degenerate genetic code, whereas an insertion or deletion can cause a frame shift that disrupts every codon downstream, with far-reaching consequences for the protein produced.
The control of gene expression
ATP
ATP is the molecule cells use to carry and transfer energy. It is built from three parts: a sugar called ribose, a base called adenine, and three phosphate groups.
Biological molecules
ATP
An enzyme called ATP hydrolase breaks ATP apart using water. It splits ATP into ADP and a free phosphate group, releasing energy the cell can use.
Biological molecules
ATP
When ATP breaks down, it releases energy. Cells link this release directly to processes that need energy, such as muscle contraction or building proteins.
Biological molecules
ATP
When ATP breaks down, it releases a phosphate group. That phosphate can attach to other molecules, making them more chemically reactive and easier for cells to use.
Biological molecules
ATP
Cells rebuild ATP by joining ADP and inorganic phosphate (Pi) together. The enzyme ATP synthase catalyses this reaction during photosynthesis and respiration.
Biological molecules
ATP
Adenosine triphosphate (ATP) is the universal energy currency of cells — a small nucleotide derivative built from ribose, adenine and three phosphate groups, whose structure allows it to store and release energy on demand. When a cell needs energy, the enzyme ATP hydrolase breaks one phosphate bond in a hydrolysis reaction (splitting using water), releasing inorganic phosphate (Pi) and converting ATP into ADP; this released energy can directly power energy-requiring processes such as muscle contraction, active transport and biosynthesis. Understanding ATP is essential because it bridges the energy-releasing reactions of respiration and photosynthesis — where ATP synthase rebuilds ATP from ADP and Pi — with virtually every energy-consuming process in a living organism.
Biological molecules
Biodiversity within a community
Biodiversity means the variety of living organisms in an area. Scientists measure it at any scale — from a single pond to the whole planet.
Genetic information, variation and relationships between organisms
Biodiversity within a community
Species richness tells you how many different species live in a community. A woodland with 40 different plant species has a higher species richness than one with only 10.
Genetic information, variation and relationships between organisms
Biodiversity within a community
An index of diversity is a single number that measures biodiversity. It accounts for both how many species exist and how many individuals belong to each species.
Genetic information, variation and relationships between organisms
Biodiversity within a community
The index of diversity (d) uses the formula d = N(N–1) / Σn(n–1), where N is the total number of all organisms counted and n is the count for each individual species. The symbol Σ means 'sum of', so you calculate n(n–1) separately for every species and then add all those values together. Follow these steps for any dataset: 1. Count all organisms across every species to get N. Calculate N(N–1). 2.
Genetic information, variation and relationships between organisms
Biodiversity within a community
Modern farming methods increase food production, but they also destroy habitats and remove species. This drives down the variety of living organisms in an area.
Genetic information, variation and relationships between organisms
Biodiversity within a community
Farming feeds people but damages biodiversity. Conservation protects biodiversity but can limit food production. Society must find ways to do both at the same time.
Genetic information, variation and relationships between organisms
Biodiversity within a community
Measuring biodiversity — the variety of living organisms in an area — requires more than simply counting how many different species are present; it also matters how individuals are distributed across those species. An index of diversity captures both of these factors in a single calculated value, giving a more meaningful picture of how healthy and stable a community is than species richness alone. Understanding how human activities such as intensive farming reduce biodiversity then sets up the real-world tension between food production and conservation — the protection of natural communities — that you will need to evaluate.
Genetic information, variation and relationships between organisms
Carbohydrates
Monosaccharides are the smallest sugar units in biology. Glucose, galactose, and fructose are three common examples — they act as building blocks for larger carbohydrates.
Biological molecules
Carbohydrates
When two sugar units join together, they release a water molecule. This reaction creates a covalent link called a glycosidic bond.
Biological molecules
Carbohydrates
Two simple sugars (monosaccharides) can join together to form a double sugar called a disaccharide. Maltose, sucrose, and lactose are the three disaccharides you need to know.
Biological molecules
Carbohydrates
Glucose exists in two slightly different forms, called α-glucose and β-glucose. They have the same chemical formula but differ in the position of one hydroxyl group.
Biological molecules
Carbohydrates
A polysaccharide is a large carbohydrate molecule. Many glucose units join together through condensation reactions, each releasing a water molecule, to build it.
Biological molecules
Carbohydrates
Both starch and glycogen are large energy-storage molecules. Cells build them by linking many α-glucose sugar units together, releasing water each time two units join.
Biological molecules
Carbohydrates
Cellulose is a large carbohydrate made by joining many β-glucose molecules together. Each join releases a water molecule in a condensation reaction.
Biological molecules
Carbohydrates
Glycogen, starch, and cellulose are all large carbohydrates made from glucose. Their different structures make them suited to very different jobs — energy storage or physical support.
Biological molecules
Carbohydrates
The shape and bonding of a polysaccharide directly determines what job it does in a cell. Glycogen and starch store energy; cellulose builds rigid plant cell walls.
Biological molecules
Carbohydrates
Two chemical tests identify carbohydrates in a sample. Benedict's solution detects sugars, and iodine solution detects starch — each test produces a visible colour change as its positive result.
Biological molecules
Carbohydrates
Carbohydrates are built from monosaccharides — simple sugar units such as glucose — which join together through condensation reactions (reactions that release water) to form increasingly complex molecules, from two-unit disaccharides through to large polysaccharides. The precise structure of a carbohydrate determines its function: small differences, such as whether glucose exists in its α or β form, dictate whether the resulting polysaccharide acts as a compact energy store like starch and glycogen, or a rigid structural material like cellulose. Understanding this structure-to-function relationship is central to explaining how both animal and plant cells are organised and fuelled.
Biological molecules
Cell recognition and the immune system
The surface of every cell is studded with molecules — mostly glycoproteins (proteins with sugar chains attached) and glycolipids — that act as chemical identity markers. Your immune system continuously scans these markers and compares them against a reference set of "self" molecules established early in development. This scanning system allows the immune system to recognise four distinct categori
Cells
Cell recognition and the immune system
An antigen is a molecule on a cell's surface that the immune system can recognise as foreign. When antigens change shape, the immune system can no longer recognise them, making diseases harder to prevent.
Cells
Cell recognition and the immune system
Phagocytes are white blood cells that engulf pathogens whole. They then use digestive enzymes called lysozymes to break the pathogen apart and destroy it.
Cells
Cell recognition and the immune system
When a pathogen invades, specialised white blood cells called helper T cells act as coordinators. They receive a signal from antigen-presenting cells and then activate the immune system's other defenders.
Cells
Cell recognition and the immune system
The humoral response is the antibody-based branch of the adaptive immune system. It targets antigens (foreign molecules, usually proteins) that are free in body fluids or on pathogen surfaces. The sequence of events runs as follows: 1. A B lymphocyte with a complementary receptor binds to a specific foreign antigen. Helper T cells (TH) release cytokines that activate this B cell. 2. Clonal selec
Cells
Cell recognition and the immune system
A vaccine trains your immune system to fight a specific pathogen without you getting ill. When enough people in a population are vaccinated, even unvaccinated individuals gain protection — this is herd immunity.
Cells
Cell recognition and the immune system
Active immunity means your body makes its own antibodies after meeting an antigen. Passive immunity means your body receives ready-made antibodies from an outside source.
Cells
Cell recognition and the immune system
HIV is a virus that invades helper T cells — the white blood cells that coordinate your immune response. It hijacks those cells to copy itself, gradually destroying your immune system.
Cells
Cell recognition and the immune system
HIV destroys the immune cells that coordinate your body's defences. Over time, this leaves the body unable to fight off infections that a healthy person would easily survive.
Cells
Cell recognition and the immune system
Scientists engineer identical antibodies that bind to one specific target on a cell. Doctors use these monoclonal antibodies to deliver drugs directly to diseased cells or to detect specific substances in the body.
Cells
Cell recognition and the immune system
AQA does not ask you how factories or laboratories make monoclonal antibodies. You only need to know what they do and how they are used.
Cells
Cell recognition and the immune system
Both vaccines and monoclonal antibodies raise ethical debates about safety, consent, animal use, and fair access. Scientists and society must weigh these benefits against the risks and concerns.
Cells
Cell recognition and the immune system
The ELISA test uses antibodies to detect whether a specific protein or pathogen is present in a sample. A colour change signals a positive result.
Cells
Cell recognition and the immune system
Your body constantly distinguishes between its own healthy cells and anything foreign or dangerous, using surface molecules called antigens — proteins or glycoproteins on a cell's surface that the immune system can recognise. When a pathogen (a disease-causing organism) enters the body, a coordinated immune response is triggered, involving specialised white blood cells such as T lymphocytes and B lymphocytes, which work together to destroy the threat and build immunological memory. Understanding this system explains how vaccines protect populations, why HIV is so damaging, and how monoclonal antibodies — laboratory-produced proteins designed to bind to a single specific target — are used in medicine and diagnosis.
Cells
Cell structure
All living organisms are built from cells, and understanding how those cells are organised internally is the foundation of the entire A-level course. Eukaryotic cells — cells that contain a membrane-bound nucleus, found in animals, plants and fungi — contain a range of specialised organelles (small structures within the cell, each with a specific function), whereas prokaryotic cells, such as bacteria, are structurally simpler and lack a true nucleus. Knowing what each organelle does, and how eukaryotic and prokaryotic cells differ, directly underpins everything that follows: how cells divide, how substances move in and out, and how the immune system recognises threats.
Cells
Digestion and absorption
Your body breaks large food molecules into small pieces using digestion. Only these small pieces are tiny enough to pass through cell membranes and enter the bloodstream.
Organisms exchange substances with their environment
Digestion and absorption
Enzymes are biological catalysts that speed up hydrolysis — the breaking of chemical bonds using water. Each food type requires a specific set of enzymes. **Carbohydrates:** 1. Amylase (produced in the salivary glands and pancreas) breaks starch into maltose, a disaccharide. 2. Membrane-bound disaccharidases — enzymes fixed to the surface of intestinal epithelial cells — then split disaccharides
Organisms exchange substances with their environment
Digestion and absorption
The ileum (the final section of the small intestine) absorbs digested nutrients through two distinct mechanisms depending on the molecule type. **Co-transport — for glucose and amino acids:** 1. Sodium-potassium ATPase pumps on the basolateral (blood-side) membrane actively pump sodium ions out of the epithelial cell, keeping the sodium concentration inside the cell low. 2. This low intracellular
Organisms exchange substances with their environment
Digestion and absorption
Food molecules such as starch, proteins and lipids are too large to cross cell membranes, so the body uses hydrolysis — enzyme-driven reactions that break chemical bonds using water — to split them into smaller, absorbable units. Specific enzymes including amylases, lipases, endopeptidases and exopeptidases each target different molecule types, breaking them down in a precise sequence along the digestive tract. Once digestion is complete, the small intestine absorbs the products through specialised mechanisms, including co-transport (where glucose or amino acids hitch a ride alongside ions moving into cells) and micelles (tiny fat-soluble droplets that ferry lipid products to the intestinal lining).
Organisms exchange substances with their environment
DNA and protein synthesis
The genome is the complete set of genes found in a cell. In humans, this means roughly 20,000–25,000 genes, all encoded in the DNA across 46 chromosomes. Every body cell carries the same genome — a liver cell and a neuron contain identical DNA. The proteome, however, is the full range of proteins a particular cell is able to produce. Unlike the genome, the proteome is not fixed. Consider a liver
Genetic information, variation and relationships between organisms
DNA and protein synthesis
mRNA is a single-stranded molecule that carries a copy of a gene's instructions. tRNA is a small molecule that reads those instructions and delivers the correct amino acid to build a protein.
Genetic information, variation and relationships between organisms
DNA and protein synthesis
Transcription is the process where a cell copies a gene from DNA into a single-stranded molecule called mRNA. The enzyme RNA polymerase carries out this copying.
Genetic information, variation and relationships between organisms
DNA and protein synthesis
In prokaryotic cells, transcription produces a finished mRNA molecule straight away. No editing steps happen before the mRNA is used to make a protein.
Genetic information, variation and relationships between organisms
DNA and protein synthesis
In eukaryotic cells, copying a gene first produces a rough draft called pre-mRNA. The cell then cuts out non-coding sections and joins the useful parts together to make the final mRNA.
Genetic information, variation and relationships between organisms
DNA and protein synthesis
Translation is the process where a ribosome reads the mRNA sequence and builds a protein. Transfer RNA molecules bring the correct amino acids, joining them into a chain called a polypeptide.
Genetic information, variation and relationships between organisms
DNA and protein synthesis
You do not need to memorise which codon codes for which amino acid. AQA will always give you that information in the exam if you need it.
Genetic information, variation and relationships between organisms
DNA and protein synthesis
Every cell contains a genome — the complete set of genes — but it only ever produces a specific proteome, meaning the particular range of proteins that cell actually makes, and understanding how that happens is what this subtopic is about. The base sequence of DNA is first copied into mRNA (a single-stranded messenger molecule) during transcription, and in eukaryotes — organisms with a nucleus — this involves an editing step called splicing before the mRNA is ready to use. That mRNA is then read by ribosomes during translation, where tRNA molecules (which carry amino acids) match their anticodons to the codons on the mRNA, assembling a polypeptide chain. Mastering this flow of information from DNA to protein underpins almost everything else in genetics, from how mutations cause disease to how cells become specialised.
Genetic information, variation and relationships between organisms
DNA, genes and chromosomes
Prokaryotes are simple cells with no nucleus, such as bacteria. Their DNA forms a small, closed loop and floats freely in the cell, unattached to any proteins.
Genetic information, variation and relationships between organisms
DNA, genes and chromosomes
Eukaryotic cells (cells that contain a membrane-bound nucleus, such as human liver cells or plant leaf cells) store their genetic information as chromosomes inside that nucleus. Each chromosome consists of a single, extremely long DNA molecule wound tightly around spool-like proteins called histones. This winding is essential: a single human DNA molecule, if stretched out, would be roughly 5 cm lo
Genetic information, variation and relationships between organisms
DNA, genes and chromosomes
Mitochondria and chloroplasts carry their own DNA. That DNA is short, circular, and not wrapped around any proteins.
Genetic information, variation and relationships between organisms
DNA, genes and chromosomes
A gene is a specific sequence of DNA bases. It either carries instructions for building a protein, or it produces an RNA molecule that the cell uses directly.
Genetic information, variation and relationships between organisms
DNA, genes and chromosomes
Every gene sits at a precise, unchanging location on a chromosome. Biologists call this fixed address the gene's locus.
Genetic information, variation and relationships between organisms
DNA, genes and chromosomes
Every group of three bases in a DNA strand acts as a code word for one specific amino acid. Amino acids are the building blocks that join together to make proteins.
Genetic information, variation and relationships between organisms
DNA, genes and chromosomes
Every living organism uses the same DNA code to build proteins. Each set of three bases is read separately, and multiple triplets can code for the same amino acid.
Genetic information, variation and relationships between organisms
DNA, genes and chromosomes
Most DNA in a eukaryotic cell's nucleus does not carry instructions for making proteins. Large stretches of DNA sit between genes and do not code for any polypeptide.
Genetic information, variation and relationships between organisms
DNA, genes and chromosomes
Inside a gene, coding sections called exons carry the instructions for building a protein. Non-coding sections called introns sit between the exons and do not code for amino acids.
Genetic information, variation and relationships between organisms
DNA, genes and chromosomes
Genetic information is stored as DNA, but its organisation differs between cell types — in eukaryotic cells (cells with a nucleus), DNA is long, linear and wound around proteins called histones to form chromosomes, whereas in prokaryotic cells and in organelles like mitochondria, DNA is short and circular. A gene is a specific sequence of DNA bases that either codes for a polypeptide (a chain of amino acids that folds into a protein) or produces a functional RNA molecule, with every three bases — a triplet — specifying a particular amino acid. Understanding this structure is the foundation for everything that follows in this section, from how proteins are built to how mutations arise and drive genetic diversity.
Genetic information, variation and relationships between organisms
Energy and ecosystems
Plants use carbon dioxide from the air or water to build organic molecules, such as sugars. This process makes plants the entry point for carbon and energy in every ecosystem.
Energy transfers in and between organisms
Energy and ecosystems
Plants produce sugars — primarily glucose — during photosynthesis. These sugars have two possible fates once made. The majority enter respiration, where cells break them down to release ATP (the cell's usable energy currency). This ATP powers every active process in the plant: active transport, cell division, protein synthesis, and more. The energy released during respiration is ultimately lost t
Energy transfers in and between organisms
Energy and ecosystems
Biomass (the total mass of organic material in organisms) can be measured in two main ways: 1. **Dry mass of tissue per unit area** — fresh tissue contains variable amounts of water, which stores no chemical energy and skews comparisons. Drying the sample (usually in an oven at around 80 °C until mass is constant) removes all water, leaving only organic and inorganic solids. Results are expressed
Energy transfers in and between organisms
Energy and ecosystems
Gross primary production (GPP) measures all the chemical energy that plants lock into their biomass through photosynthesis. It covers a specific area or volume of an ecosystem.
Energy transfers in and between organisms
Energy and ecosystems
Plants use some of their captured energy for their own respiration. Net primary production (NPP) is the energy that remains after subtracting those respiratory losses from the total energy fixed.
Energy transfers in and between organisms
Energy and ecosystems
After plants use some energy for respiration, the remaining energy — called net primary production — goes into growing new plant tissue. Herbivores and other organisms can then access that stored energy by eating the plant.
Energy transfers in and between organisms
Energy and ecosystems
Animals cannot use all the energy in their food. Net production is the energy left over after losses in faeces, urine, and respiration — energy available to build new biomass.
Energy transfers in and between organisms
Energy and ecosystems
Productivity measures how fast an organism builds up biomass. Primary productivity applies to plants; secondary productivity applies to animals. Both use units of energy per area per year.
Energy transfers in and between organisms
Energy and ecosystems
Energy efficiency in farming means maximising the proportion of energy from crops or livestock that reaches humans. Two broad strategies achieve this. **Strategy 1 — Simplifying food webs.** In a natural ecosystem, energy leaks into many non-human food chains: insects eat crops, foxes eat chickens, weeds compete with wheat. Farmers reduce these losses by: 1. Using pesticides to kill insects and o
Energy transfers in and between organisms
Energy and ecosystems
Plants capture energy from sunlight and lock it into organic molecules, but not all of that energy is available to the rest of the ecosystem — some is lost through respiration (the process cells use to release energy for life processes). By distinguishing between gross primary production (the total chemical energy fixed by plants) and net primary production (what remains after respiratory losses), you can quantify exactly how much energy is available to pass up through trophic levels — the feeding levels of a food chain. Understanding these energy budgets explains why food chains are short, why farming practices aim to reduce energy losses, and how productivity — the rate at which biomass is built up — can be measured and improved.
Energy transfers in and between organisms
Evolution may lead to speciation
Individuals in a population look and function differently from one another — this is phenotypic variation. Genes and the environment both cause it, but mutation is the original source of all genetic differences.
Genetics, populations, evolution and ecosystems
Evolution may lead to speciation
Sexual reproduction shuffles genetic information in two ways. Meiosis creates genetically unique sex cells, and random fertilisation combines them unpredictably, producing new gene combinations every time.
Genetics, populations, evolution and ecosystems
Evolution may lead to speciation
Not every organism survives long enough to reproduce. Predators, disease, and competition for resources mean that individuals with helpful traits survive and breed more than others.
Genetics, populations, evolution and ecosystems
Evolution may lead to speciation
Organisms whose traits help them survive in their environment tend to reproduce more. They pass the alleles — gene variants — behind those helpful traits to their offspring.
Genetics, populations, evolution and ecosystems
Evolution may lead to speciation
Some individuals reproduce more than others because their traits suit the environment better. Over generations, the alleles — gene variants — behind those traits become more common in the population.
Genetics, populations, evolution and ecosystems
Evolution may lead to speciation
Natural selection can change a population's traits in three ways. It can keep traits the same, shift them toward one extreme, or split the population into two distinct groups.
Genetics, populations, evolution and ecosystems
Evolution may lead to speciation
Evolution means the allele frequencies in a population shift over generations. An allele is one version of a gene, and its frequency is how common it is in the population.
Genetics, populations, evolution and ecosystems
Evolution may lead to speciation
When two populations stop interbreeding, their alleles change independently over time. Mutations and natural selection build up separately in each group, making the two gene pools increasingly different.
Genetics, populations, evolution and ecosystems
Evolution may lead to speciation
A species is defined by the biological species concept: members of the same species can interbreed and produce fertile offspring. Speciation — the formation of a new species — occurs when this ability breaks down permanently. The process typically follows these steps: 1. A single population splits into two groups that are reproductively isolated (prevented from interbreeding), for example by a p
Genetics, populations, evolution and ecosystems
Evolution may lead to speciation
Speciation is the formation of new species. It can happen when a physical barrier splits a population apart (allopatric), or when reproductive isolation develops within the same location (sympatric).
Genetics, populations, evolution and ecosystems
Evolution may lead to speciation
Genetic drift is random chance changing how common an allele is in a population. It has the biggest effect when a population is small.
Genetics, populations, evolution and ecosystems
Evolution may lead to speciation
Natural selection acts on the genetic variation present in a population, meaning individuals with phenotypes — observable characteristics — that suit their environment are more likely to survive, reproduce, and pass on their alleles (the variants of a gene responsible for those traits). Over generations, this shifts allele frequencies within the gene pool (the total collection of alleles in a population), which is what evolution actually is at a genetic level. When populations become reproductively isolated — separated so that they can no longer interbreed — their gene pools diverge independently until members can no longer produce fertile offspring together, giving rise to new species through a process called speciation.
Genetics, populations, evolution and ecosystems
Gas exchange
All gas exchange surfaces share the same core features: they are thin, moist, and large in surface area relative to volume. How each organism achieves this depends on its body plan and habitat. **Single-celled organisms** (e.g. Amoeba) are so small that their surface area to volume ratio (SA:V ratio — the proportion of outer surface relative to body size) is naturally high. Gases diffuse directly
Organisms exchange substances with their environment
Gas exchange
Insects and drought-resistant plants both need to let gases in and out for respiration and photosynthesis. Every adaptation that helps gas exchange also risks letting water escape, so each organism uses structures that balance both needs.
Organisms exchange substances with their environment
Gas exchange
The human gas exchange system moves air from your mouth and nose down into your lungs. It branches from one large tube into millions of tiny air sacs where oxygen enters your blood.
Organisms exchange substances with their environment
Gas exchange
Alveoli are tiny air sacs in the lungs. Their walls have special features that allow oxygen and carbon dioxide to diffuse rapidly between air and blood.
Organisms exchange substances with their environment
Gas exchange
Ventilation moves fresh air into the lungs and stale air out. This keeps oxygen levels high and carbon dioxide levels low at the alveolar surface, so diffusion into the blood continues rapidly.
Organisms exchange substances with their environment
Gas exchange
Breathing moves air by changing pressure inside the thoracic cavity (the airtight space in your chest that contains the lungs). Boyle's Law explains the link: when volume increases, pressure falls; when volume decreases, pressure rises. During inspiration (breathing in): 1. The diaphragm — a dome-shaped sheet of muscle below the lungs — contracts and flattens downward. 2. The external intercostal
Organisms exchange substances with their environment
Gas exchange
Every living organism needs to move oxygen in and carbon dioxide out, and the structures that make this possible are shaped by the same core problem: maximising the rate of diffusion across an exchange surface while minimising harmful water loss. This subtopic compares how organisms as different as insects, fish, flowering plants, and humans have each evolved specialised gas exchange systems — from the tracheal system (a network of air-filled tubes) in insects to the alveoli (tiny air sacs) deep in the human lung — revealing the structural trade-offs each solution involves. Understanding these adaptations also underpins how ventilation (the active movement of air or water over an exchange surface) maintains the concentration gradients that drive diffusion, which connects directly to mass transport and whole-body physiology.
Organisms exchange substances with their environment
Gene expression is controlled by a number of features
Not every gene in a cell is active at the same time — which genes are switched on or off is tightly controlled, and mutations (spontaneous, heritable changes to DNA base sequences) can disrupt this control in ways that range from harmless to life-threatening. Much of the genome consists of non-coding DNA — sequences that do not code for proteins but play a crucial role in regulating when and how much a gene is expressed. Understanding these control mechanisms explains how genetically identical cells can become specialised, and why some mutations cause disease while others have no effect at all.
The control of gene expression
Gene technologies allow the study and alteration of gene function
Modern gene technologies give scientists the tools to read, manipulate, and alter the genetic information inside cells — opening up both medical and agricultural applications that were impossible just decades ago. Techniques such as automated sequencing (using machines to rapidly decode the order of bases in DNA) and genome projects have revealed how genes are organised and regulated across entire organisms. Understanding these technologies matters because they allow scientists to investigate what individual genes actually do, correct faults caused by mutations (changes to the base sequence of DNA), and deliberately engineer new traits — building directly on the gene expression principles covered in earlier subtopics.
The control of gene expression
Genetic diversity and adaptation
Genetic diversity measures how many different versions of genes exist across all individuals in a population. More different versions means higher genetic diversity.
Genetic information, variation and relationships between organisms
Genetic diversity and adaptation
Natural selection needs variation to work on. Genetic diversity — the range of different alleles in a population — supplies that variation, giving selection something to act on.
Genetic information, variation and relationships between organisms
Genetic diversity and adaptation
Natural selection follows a clear causal chain: 1. A random mutation — a change in the DNA base sequence — produces a new allele (a new version of a gene). 2. By chance, that allele gives some individuals an advantage in their current environment. For example, a mutation in a bacterium might alter the active site of an enzyme that an antibiotic normally targets, making the antibiotic ineffective.
Genetic information, variation and relationships between organisms
Genetic diversity and adaptation
Directional selection happens when one extreme version of a trait gives organisms a survival advantage. Over generations, that extreme trait becomes more common in the population.
Genetic information, variation and relationships between organisms
Genetic diversity and adaptation
Stabilising selection is a type of natural selection that favours average individuals. It removes extreme traits from a population, keeping most individuals close to the middle of the range.
Genetic information, variation and relationships between organisms
Genetic diversity and adaptation
Natural selection acts as a filter over many generations. Individuals with traits that suit their environment survive and reproduce more, so those traits become more common in the species.
Genetic information, variation and relationships between organisms
Genetic diversity and adaptation
Adaptations are features that help an organism survive and reproduce in its environment. They fall into three types: structural (body shape or parts), chemical/functional (internal processes), or behavioural (what the organism does).
Genetic information, variation and relationships between organisms
Genetic diversity and adaptation
Genetic diversity — the total number of different alleles, meaning different versions of genes, present in a population — is the raw material that makes natural selection possible. When an allele gives an organism a survival or reproductive advantage in its environment, that allele is passed on more frequently until it becomes more common across generations. This process drives two distinct patterns: directional selection, where one extreme trait is favoured (such as antibiotic resistance in bacteria), and stabilising selection, where intermediate traits are favoured (such as average birth weight in humans). Understanding these mechanisms explains how populations gradually accumulate adaptations — anatomical, physiological or behavioural features that suit them to their environment.
Genetic information, variation and relationships between organisms
Genetic diversity can arise as a result of mutation or during meiosis
A gene mutation is a permanent change to the sequence of DNA bases in a chromosome. This change alters the genetic instructions carried by that gene.
Genetic information, variation and relationships between organisms
Genetic diversity can arise as a result of mutation or during meiosis
During DNA replication, copying errors can occur by accident. These errors — called mutations — change the sequence of bases in the DNA, and they happen without any external cause.
Genetic information, variation and relationships between organisms
Genetic diversity can arise as a result of mutation or during meiosis
The genetic code is degenerate — multiple codons can code for the same amino acid. So swapping one DNA base for another sometimes makes no difference to the protein produced.
Genetic information, variation and relationships between organisms
Genetic diversity can arise as a result of mutation or during meiosis
Some chemicals and types of radiation damage DNA. They are called mutagenic agents, and they make gene mutations happen more often than normal.
Genetic information, variation and relationships between organisms
Genetic diversity can arise as a result of mutation or during meiosis
Sometimes chromosomes fail to separate properly during meiosis. This error, called non-disjunction, produces sex cells with the wrong number of chromosomes.
Genetic information, variation and relationships between organisms
Genetic diversity can arise as a result of mutation or during meiosis
Meiosis is a type of cell division that makes sex cells. Each sex cell it produces carries a unique combination of genetic information.
Genetic information, variation and relationships between organisms
Genetic diversity can arise as a result of mutation or during meiosis
Meiosis starts with one diploid parent cell — a cell containing two full sets of chromosomes (46 in humans). It produces four haploid daughter cells, each with only one set (23 in humans). These daughter cells become gametes: sperm or eggs. The process involves two sequential nuclear divisions: 1. Before division begins, DNA replicates so each chromosome consists of two identical sister chromati
Genetic information, variation and relationships between organisms
Genetic diversity can arise as a result of mutation or during meiosis
Genetic diversity — the range of different alleles (versions of genes) present within a population — can arise through two main routes: gene mutations, which are changes to the DNA base sequence that may alter the protein an organism produces, and meiosis, the type of cell division that produces sex cells. During meiosis, processes called independent segregation (the random sorting of chromosome pairs) and crossing over (the exchange of DNA segments between paired chromosomes) shuffle alleles into new combinations, ensuring offspring are genetically unique. Understanding these mechanisms explains where heritable variation comes from — the raw material that natural selection acts on.
Genetic information, variation and relationships between organisms
Inheritance
The genotype is the complete set of alleles an organism carries in its DNA. Think of it as the genetic instruction manual — written in the cells, not always visible from the outside.
Genetics, populations, evolution and ecosystems
Inheritance
Your phenotype is every observable characteristic you have — like eye colour or height. It comes from your genes working together with your environment.
Genetics, populations, evolution and ecosystems
Inheritance
A single gene can exist in more than two versions, called alleles. Human blood group is a classic example — one gene has three different alleles.
Genetics, populations, evolution and ecosystems
Inheritance
Alleles are different versions of a gene. A dominant allele controls the phenotype even when only one copy is present. A recessive allele only shows its effect when both copies match. Codominant alleles both contribute to the phenotype together.
Genetics, populations, evolution and ecosystems
Inheritance
Diploid organisms carry two copies of every gene. When both copies are the same allele, the organism is homozygous. When the two copies differ, it is heterozygous.
Genetics, populations, evolution and ecosystems
Inheritance
A genetic diagram uses a Punnett square — a grid showing every possible combination of parental gametes — to predict offspring outcomes. Follow these steps for any cross: 1. Write the parental phenotypes and genotypes (e.g. Tall Tt × short tt). 2. Identify the gametes each parent can produce and place them along the grid axes. 3. Fill the grid to find all possible offspring genotypes. 4. Convert
Genetics, populations, evolution and ecosystems
Inheritance
Some genes break the simple rules of inheritance. They can sit on sex chromosomes, travel together on the same chromosome, exist in more than two versions, or have one gene switch another gene off entirely.
Genetics, populations, evolution and ecosystems
Inheritance
The chi-squared test is a statistical calculation. It tells you whether the difference between your actual breeding results and your predicted ratio is due to chance or something real.
Genetics, populations, evolution and ecosystems
Inheritance
Inheritance is the study of how alleles — the different versions of a gene — are passed from parents to offspring and how they determine an organism's characteristics. By constructing genetic diagrams, you can predict the phenotype (the observable traits an organism displays) that results from any combination of alleles, whether those alleles are dominant, recessive, codominant, sex-linked, or influenced by other genes through epistasis. Mastering these patterns is essential groundwork for understanding how variation arises within populations, which underpins everything covered in evolution and speciation later in this section.
Genetics, populations, evolution and ecosystems
Inorganic ions
Organisms contain charged mineral particles called inorganic ions, dissolved in their fluids. Some ions are present in large amounts, while others exist in tiny traces.
Biological molecules
Inorganic ions
Every inorganic ion does a particular job in the body. Its chemical properties — such as its charge or size — determine exactly what that job is.
Biological molecules
Inorganic ions
Each of the four ions below has a distinct structural or functional role: **Hydrogen ions (H⁺) and pH** Hydrogen ions are simply protons released when acids dissolve in water. The more H⁺ present, the lower the pH. pH controls enzyme activity by altering the shape of the active site — a small change in H⁺ concentration can denature an enzyme entirely. **Iron ions (Fe²⁺) and haemoglobin** Haemogl
Biological molecules
Inorganic ions
Inorganic ions — charged mineral particles dissolved in the cytoplasm and body fluids of organisms — are present in varying concentrations and each plays a precise role determined by its chemical properties. For example, iron ions are essential components of haemoglobin (the oxygen-carrying protein in red blood cells), phosphate ions form the backbone of DNA and the energy-carrying molecule ATP, and sodium ions drive the co-transport of glucose and amino acids across cell membranes. Understanding these roles ties together several biological molecules you have already studied and shows how small, simple ions underpin large-scale physiological processes.
Biological molecules
Investigating diversity
Genetic diversity refers to the range of different alleles — alternative versions of a gene — present within or between species. Scientists use four methods to measure it: 1. **Observable or measurable characteristics** — traits such as shell banding patterns in snails, or height in humans, can be recorded across a population. A wider spread of trait frequencies indicates greater diversity. 2. **
Genetic information, variation and relationships between organisms
Investigating diversity
When biologists investigate variation within a species — for example, measuring the shell length of garden snails — they follow a structured approach to make their conclusions reliable. 1. Collect data from random samples. Random sampling means every individual has an equal chance of selection. This removes bias and makes the sample representative of the whole population. Techniques include rando
Genetic information, variation and relationships between organisms
Investigating diversity
You will never need to calculate a standard deviation in an AQA Biology exam. You do need to read, interpret, and compare standard deviation values that are given to you.
Genetic information, variation and relationships between organisms
Investigating diversity
Genetic diversity — the range of alleles, or different versions of genes, present within or between species — can be measured by comparing DNA base sequences, the mRNA transcribed from them, or the amino acid sequences of the proteins they encode. The more these sequences differ, the greater the genetic diversity. When investigating variation within a species, scientists collect data from random samples and use the mean and standard deviation — a measure of how spread out the values are around the average — to draw reliable conclusions. These methods give you the tools to turn biological observations into evidence you can actually interpret and evaluate.
Genetic information, variation and relationships between organisms
Lipids
Lipids are a group of biological molecules that includes fats and oils. Triglycerides and phospholipids are the two main types you need to know.
Biological molecules
Lipids
A triglyceride forms when one glycerol molecule joins to three fatty acid molecules. Each join releases a water molecule — this type of reaction is called condensation.
Biological molecules
Lipids
When glycerol and a fatty acid join together, they release a water molecule. This reaction creates a new chemical link called an ester bond.
Biological molecules
Lipids
Every fatty acid has a long hydrocarbon tail called the R-group. This tail is either saturated (contains only single bonds) or unsaturated (contains one or more double bonds between carbon atoms).
Biological molecules
Lipids
A phospholipid looks like a triglyceride, but with one fatty acid swapped out for a phosphate-containing group. This swap gives the molecule a water-attracting head and two water-repelling tails.
Biological molecules
Lipids
Triglycerides and phospholipids have different structures, so they behave differently and do different jobs. Triglycerides store energy; phospholipids form the membranes that surround every cell.
Biological molecules
Lipids
The emulsion test detects lipids in a sample. You dissolve the sample in ethanol, then add water — a cloudy white emulsion forms if lipids are present.
Biological molecules
Lipids
Lipids are a group of biological molecules that includes triglycerides — energy-storage molecules built from one glycerol and three fatty acids joined by ester bonds (formed during condensation reactions) — and phospholipids, which have a similar structure but with one fatty acid replaced by a phosphate-containing group. This structural difference is what makes phospholipids ideal for forming cell membranes, while triglycerides excel at storing energy and insulating the body. Understanding how structure determines function here is a key principle that runs throughout A-level Biology.
Biological molecules
Mass transport
Once substances have been absorbed across an exchange surface, larger organisms cannot rely on diffusion alone to move them around — diffusion is too slow over long distances. Mass transport solves this by using a bulk flow system, such as the circulatory system in animals or the vascular tissue in plants, to carry substances rapidly from one part of the organism to another. Understanding mass transport explains how the specialised exchange surfaces covered in earlier subtopics are actually connected to the cells that need the substances, completing the picture of how organisms supply every tissue with what it needs to survive.
Organisms exchange substances with their environment
Monomers and polymers
A monomer is a small molecule that acts as a building block. Many monomers join together to build much larger molecules inside living organisms.
Biological molecules
Monomers and polymers
A polymer is a large molecule built by joining many smaller units called monomers together. Think of it like a long chain made from many identical links.
Biological molecules
Monomers and polymers
Some molecules act as building blocks that join together to form larger biological molecules. Monosaccharides, amino acids and nucleotides are the three key building blocks you need to know.
Biological molecules
Monomers and polymers
When two monomers (the small repeating units that build larger molecules) join together, they undergo a condensation reaction. The reaction works in three steps: 1. Two monomers come close together, each carrying a hydroxyl group (–OH) or an amino group (–NH₂) at the point where they will bond. 2. A new covalent bond forms between the two monomers, linking them into a single, longer molecule. 3.
Biological molecules
Monomers and polymers
The word hydrolysis comes from the Greek for water (hydro) and splitting (lysis) — so the name itself tells you what happens. When a polymer needs to be broken down into its monomers, the body uses hydrolysis to split the bonds holding them together. Here is how the reaction works: 1. A water molecule (H₂O) targets the chemical bond between two monomers. 2. The water molecule splits: one hydroge
Biological molecules
Monomers and polymers
Many of the large molecules found in living organisms are polymers — long chains built by joining many smaller repeating units called monomers together. This linking happens through condensation reactions, where a chemical bond forms between two monomers and a molecule of water is released as a by-product; the reverse process, hydrolysis, breaks those bonds by consuming water. Understanding these two reactions is the foundation for everything that follows in this section, because carbohydrates, proteins and nucleic acids are all built and broken down in exactly this way.
Biological molecules
Nervous coordination
Skeletal muscle contraction depends on a precise molecular mechanism in which the proteins actin and myosin slide past one another — shortening the muscle fibre — driven by calcium ions and the breakdown of ATP. Understanding this sliding filament model explains not just how muscles contract, but how the nervous system translates an electrical signal into physical movement. This underpins everything from reflex arcs to the sustained activity of slow muscle fibres and the explosive power of fast muscle fibres.
Organisms respond to changes in their internal and external environments
Nucleic acids are important information-carrying molecules
DNA and RNA are nucleic acids — polymers (long chains) built from smaller units called nucleotides — and their primary role is to store and transfer the genetic information that instructs cells how to build proteins. Understanding their structure, including how nucleotides bond together and how the two strands of DNA pair up, explains how that information is copied accurately and passed on during cell division. This subtopic also introduces how RNA differs from DNA in structure and function, which is essential groundwork for understanding gene expression later in the course.
Biological molecules
Nutrient cycles
Unlike energy, nutrients such as nitrogen and phosphorus are not lost from ecosystems. Living organisms, dead matter, and the environment continuously pass these elements between each other in repeating cycles.
Energy transfers in and between organisms
Nutrient cycles
Microorganisms break down dead organic matter and convert chemical elements into forms that living organisms can reuse. Without them, essential elements like nitrogen and phosphorus would stay locked in dead material forever.
Energy transfers in and between organisms
Nutrient cycles
Saprobionts are organisms that break down dead organic matter. They release nutrients locked inside dead material back into the environment for other organisms to reuse.
Energy transfers in and between organisms
Nutrient cycles
Mycorrhizae are fungi that grow into and around plant roots. They massively increase the surface area available for absorbing water and mineral ions from the soil.
Energy transfers in and between organisms
Nutrient cycles
Bacteria drive the nitrogen cycle by converting nitrogen compounds between different forms. Each type of bacterium performs a specific job — from breaking down dead matter to fixing atmospheric nitrogen gas into usable compounds.
Energy transfers in and between organisms
Nutrient cycles
You do not need to memorise the scientific names of specific bacteria. Focus on what each type of bacteria does in the nitrogen cycle, not what it is called.
Energy transfers in and between organisms
Nutrient cycles
When farmers harvest crops or remove animals, they take nutrients out of the soil permanently. Fertilisers — either natural or artificial — replace the nitrates and phosphates that would otherwise be gone.
Energy transfers in and between organisms
Nutrient cycles
Fertilisers added to farmland can wash into rivers and lakes. This triggers a chain of events that removes oxygen from the water and kills aquatic life.
Energy transfers in and between organisms
Nutrient cycles
Unlike energy, which flows in one direction through an ecosystem and is lost as heat, chemical elements such as nitrogen and phosphorus are continuously recycled — broken down, transformed, and made available again for living organisms to use. Microorganisms are central to this process: saprobionts (decomposers that feed on dead organic matter) and specialised bacteria convert nutrients between different chemical forms, while mycorrhizae (fungi that form close associations with plant roots) help plants absorb the inorganic ions that result. Understanding these cycles also explains why human activities like applying fertilisers can disrupt ecosystems, leading to problems such as eutrophication — where excess nutrients trigger algal overgrowth that ultimately depletes oxygen in water.
Energy transfers in and between organisms
Photosynthesis
The light-dependent reaction takes place on the thylakoid membranes inside the chloroplast — think of these as stacked, flattened sacs packed with green chlorophyll pigment. 1. Chlorophyll absorbs light energy. This causes photoionisation: electrons in chlorophyll gain enough energy to escape the molecule entirely. 2. These high-energy electrons pass along a series of proteins called the electron
Energy transfers in and between organisms
Photosynthesis
The light-independent reaction takes molecules made during the light-dependent reaction and uses their energy to build a simple sugar. It needs both reduced NADP and ATP to do this.
Energy transfers in and between organisms
Photosynthesis
The Calvin cycle runs continuously in the stroma (the fluid-filled space) of the chloroplast. Think of it as a molecular assembly line that fixes atmospheric carbon dioxide into usable organic molecules. 1. Carbon dioxide (CO₂) from the air combines with a five-carbon acceptor molecule called ribulose bisphosphate (RuBP). The enzyme rubisco (ribulose bisphosphate carboxylase/oxygenase) catalyses
Energy transfers in and between organisms
Photosynthesis
Photosynthesis converts light energy into chemical energy stored in organic molecules, and at A-level you study exactly how this happens across two linked stages inside the chloroplast. The light-dependent reaction uses light to split water and generate ATP (the cell's energy currency) and reduced NADP (an electron carrier), while the light-independent reaction — the Calvin cycle — uses those products to fix carbon dioxide into triose phosphate, a simple sugar that can be built into glucose and other organic compounds. Understanding these mechanisms underpins the whole of energy transfer in ecosystems, and connects directly to how respiration, productivity, and nutrient cycles are driven by the organic molecules plants produce.
Energy transfers in and between organisms
Populations
A species can live as a single group in one place, or as several separate groups spread across different locations. Each of these groups is called a population.
Genetics, populations, evolution and ecosystems
Populations
A population has three defining features that you must be able to state precisely. 1. Same species — all individuals share enough genetic similarity to produce fertile offspring with one another. 2. Same space — they occupy a defined area, for example all the red deer living on the Isle of Rum in Scotland. 3. Same time — they exist together at the same moment, so you are not mixing individuals fr
Genetics, populations, evolution and ecosystems
Populations
A gene pool is every allele (version of a gene) that exists across all individuals in a population. Allele frequency measures how common each allele is within that pool.
Genetics, populations, evolution and ecosystems
Populations
The Hardy–Weinberg principle is a mathematical model. It predicts that, in a stable population, the proportion of each allele (version of a gene) stays the same across generations.
Genetics, populations, evolution and ecosystems
Populations
The Hardy–Weinberg principle only holds true when a population meets specific conditions. These conditions include no mutation, no natural selection, random mating, no migration, and a large population size.
Genetics, populations, evolution and ecosystems
Populations
Every gene in a population exists in different versions called alleles. The Hardy–Weinberg equation, p² + 2pq + q² = 1, describes the expected frequencies of all three possible genotypes when a gene has two alleles. Here p is the frequency of one allele (usually the dominant one) and q is the frequency of the other (usually the recessive one). Because these are the only two alleles, p + q = 1. T
Genetics, populations, evolution and ecosystems
Populations
Within a species, individuals live in populations — groups occupying the same area at the same time that can interbreed — and together these individuals share a gene pool, meaning the complete set of alleles (versions of genes) present across the group. The Hardy–Weinberg principle is a mathematical model that predicts allele frequencies will remain stable between generations, provided certain conditions are met, such as no mutation, no natural selection, and random mating. Using the Hardy–Weinberg equation, p² + 2pq + q² = 1, you can calculate the expected frequencies of alleles, genotypes, and phenotypes in a population — a skill that underpins your understanding of how and why populations change over time through evolution.
Genetics, populations, evolution and ecosystems
Populations in ecosystems
A community is all the different species living together in one place. An ecosystem includes that community plus all the non-living surroundings, such as soil, water, and temperature.
Genetics, populations, evolution and ecosystems
Populations in ecosystems
Every species fills a unique role in its habitat, called a niche. Adaptations to non-living conditions (like temperature) and living ones (like predators) define that role.
Genetics, populations, evolution and ecosystems
Populations in ecosystems
Carrying capacity is the maximum population size an ecosystem can sustain over time, given its available resources. A population typically grows rapidly when numbers are low, then levels off as it approaches carrying capacity — producing an S-shaped (logistic) growth curve. Three categories of factor cause carrying capacity to vary: 1. Abiotic factors — non-living conditions such as temperature,
Genetics, populations, evolution and ecosystems
Populations in ecosystems
A quadrat is a square frame placed on the ground to define a sample area. Researchers count every individual of the target species inside it, then repeat this across many randomly placed quadrats and calculate a mean density. Multiplying that mean by the total habitat area gives a population estimate. For species distributed unevenly across a habitat — for example, where bluebells grow more densel
Genetics, populations, evolution and ecosystems
Populations in ecosystems
The mark-release-recapture method only gives an accurate population estimate if certain conditions hold true. These conditions are the assumptions scientists must accept when using this technique.
Genetics, populations, evolution and ecosystems
Populations in ecosystems
Primary succession is the process where living things gradually colonise bare, lifeless ground. Over time, each wave of species changes the environment until a stable final community establishes itself.
Genetics, populations, evolution and ecosystems
Populations in ecosystems
As succession progresses, each group of species alters the environment around it. Those changes make conditions better for new species and worse for the ones already there.
Genetics, populations, evolution and ecosystems
Populations in ecosystems
As organisms grow and die, they physically alter their surroundings. These changes make the environment less harsh, allowing new species to survive there and increasing biodiversity.
Genetics, populations, evolution and ecosystems
Populations in ecosystems
Left alone, most habitats change over time until a dense climax community takes over. Conservationists actively intervene to hold a habitat at an earlier stage, protecting species that would otherwise disappear.
Genetics, populations, evolution and ecosystems
Populations in ecosystems
Every ecosystem — a community of interacting species together with the non-living environment they inhabit — has a carrying capacity, meaning a maximum population size it can sustainably support, shaped by factors such as competition, predation, and abiotic conditions like temperature and water availability. Each species occupies a niche, the unique role and set of conditions it is adapted to within that habitat, and understanding niches helps explain why populations fluctuate over time. This subtopic also covers how ecosystems change through primary succession, the gradual process by which pioneer species colonise bare ground and progressively alter conditions until a stable climax community is established, and how scientists use field techniques such as quadrats and mark-release-recapture to estimate population sizes reliably.
Genetics, populations, evolution and ecosystems
Proteins
Proteins are polymers — long chains built from smaller monomer units called amino acids, joined together by peptide bonds through condensation reactions. Each amino acid has a variable region called an R-group, and it is the sequence and variety of these R-groups that determines how a protein folds into its precise three-dimensional shape. Because a protein's shape dictates its function, understanding protein structure explains how molecules like haemoglobin carry oxygen and how enzymes catalyse reactions — making this subtopic essential for almost every biological process you will study at A-level.
Biological molecules
Respiration
Cells need energy to do work. Respiration is the process that releases energy from glucose and uses it to build ATP — the molecule that directly powers everything a cell does.
Energy transfers in and between organisms
Respiration
Glycolysis — from the Greek for 'sugar splitting' — is the universal opening stage of respiration. Every living cell that respires begins here, whether or not oxygen is available. The process takes place in the cytoplasm, the fluid that fills the cell, rather than inside any organelle. Think of glycolysis as the entry gate to respiration: glucose must pass through it before anything else can happ
Energy transfers in and between organisms
Respiration
Glycolysis splits glucose into two smaller molecules called pyruvate. The cell spends a little ATP to start the process, then gains more ATP back — along with an energy-carrying molecule called reduced NAD.
Energy transfers in and between organisms
Respiration
When cells run out of oxygen, they convert pyruvate into either lactate or ethanol. This regenerates NAD so glycolysis can keep making ATP.
Energy transfers in and between organisms
Respiration
After glycolysis, pyruvate moves into the mitochondria to continue aerobic respiration. The cell uses energy to actively pump pyruvate across the mitochondrial membranes.
Energy transfers in and between organisms
Respiration
Aerobic respiration after glycolysis runs through three connected stages inside the mitochondria. 1. **Link reaction (mitochondrial matrix):** Pyruvate loses a carbon dioxide molecule and gets oxidised, forming a two-carbon acetate group. This oxidation reduces NAD to reduced NAD — think of NAD as an empty energy-carrier that picks up hydrogen here. Acetate immediately joins coenzyme A (a carrier
Energy transfers in and between organisms
Respiration
Every energy-demanding process in a living cell depends on ATP — a small molecule that acts as the cell's universal energy currency — and respiration is the metabolic pathway that produces it. Starting with glycolysis in the cytoplasm, glucose is broken down through a sequence of reactions that, in aerobic conditions, continues inside the mitochondria through the link reaction, Krebs cycle, and oxidative phosphorylation — a process in which electrons pass down a chain of proteins, driving the synthesis of large amounts of ATP. Understanding respiration explains not only how organisms release energy from glucose, but also how fats and amino acids feed into the same pathway, linking directly to how energy flows through entire ecosystems.
Energy transfers in and between organisms
Skeletal muscles are stimulated to contract by nerves and act as effectors
Muscles can only pull — they cannot push. So two muscles work as a pair: one contracts to create movement, and the other contracts to reverse it.
Organisms respond to changes in their internal and external environments
Skeletal muscles are stimulated to contract by nerves and act as effectors
Skeletal muscle has a layered structure, from the whole muscle you can see down to tiny fibres visible only under a microscope. Each level of organisation helps the muscle generate and transmit force.
Organisms respond to changes in their internal and external environments
Skeletal muscles are stimulated to contract by nerves and act as effectors
A myofibril is a long, cylindrical strand inside a muscle fibre. It contains repeating units called sarcomeres, which are the sections that actually shorten when a muscle contracts.
Organisms respond to changes in their internal and external environments
Skeletal muscles are stimulated to contract by nerves and act as effectors
Muscle fibres contract when two proteins, actin and myosin, repeatedly grab and pull past each other. Calcium ions start the process, and ATP powers each pull.
Organisms respond to changes in their internal and external environments
Skeletal muscles are stimulated to contract by nerves and act as effectors
Calcium ions remove a blocking protein called tropomyosin from actin filaments. This exposes binding sites and allows myosin heads to attach and pull the filament, causing contraction.
Organisms respond to changes in their internal and external environments
Skeletal muscles are stimulated to contract by nerves and act as effectors
AQA does not ask you to explain what troponin does. You only need to know the role of tropomyosin and calcium ions in muscle contraction.
Organisms respond to changes in their internal and external environments
Skeletal muscles are stimulated to contract by nerves and act as effectors
Muscles need ATP to contract. They store a backup molecule called phosphocreatine, which rapidly rebuilds ATP when supplies run low during exercise.
Organisms respond to changes in their internal and external environments
Skeletal muscles are stimulated to contract by nerves and act as effectors
Skeletal muscles contain two fibre types. Slow fibres sustain low-level effort for a long time. Fast fibres produce powerful, rapid contractions but tire quickly.
Organisms respond to changes in their internal and external environments
Skeletal muscles are stimulated to contract by nerves and act as effectors
Skeletal muscle is the effector — the structure that carries out a response — when the nervous system detects a stimulus, and understanding how it works means tracing that response all the way down to the molecular level. Inside each muscle fibre, contractile units called sarcomeres contain two proteins, actin and myosin, which repeatedly bind and pull past one another in a process driven by ATP and triggered by calcium ions. Mastering this sliding filament mechanism unlocks your ability to explain not just how muscles contract, but why energy supply, fibre type, and antagonistic muscle pairs all determine how the body moves and responds.
Organisms respond to changes in their internal and external environments
Species and taxonomy
Two organisms belong to the same species if they can mate and produce offspring that are themselves able to reproduce. Producing offspring is not enough — that offspring must also be fertile.
Genetic information, variation and relationships between organisms
Species and taxonomy
Before animals mate, they perform courtship behaviours. These rituals allow individuals to identify others of the same species, so that mating produces fertile offspring.
Genetic information, variation and relationships between organisms
Species and taxonomy
Phylogenetic classification groups species by their evolutionary history — how closely related they are. Species that share a more recent common ancestor sit in the same group.
Genetic information, variation and relationships between organisms
Species and taxonomy
Classification arranges living things into a series of nested groups. Every small group sits entirely inside a larger one, and no group ever belongs to two different larger groups at the same time.
Genetic information, variation and relationships between organisms
Species and taxonomy
Biologists sort living things into named groups. Each of those groups — at any level — is called a taxon. The plural of taxon is taxa.
Genetic information, variation and relationships between organisms
Species and taxonomy
Biologists sort all living things into eight ranked groups. From broadest to most specific, these are: domain, kingdom, phylum, class, order, family, genus and species.
Genetic information, variation and relationships between organisms
Species and taxonomy
Every species gets a unique two-part Latin name called a binomial. The first part names the genus; the second names the species — for example, Homo sapiens.
Genetic information, variation and relationships between organisms
Species and taxonomy
AQA does not ask you to memorise different classification systems. You only need to know the single hierarchy taught in this subtopic.
Genetic information, variation and relationships between organisms
Species and taxonomy
Organisms are grouped into the same species if they can interbreed to produce fertile offspring, with courtship behaviour playing a key role in ensuring individuals recognise suitable mates. To make sense of the enormous diversity of life, biologists use phylogenetic classification — a system that organises species into nested groups called taxa (singular: taxon) based on their shared evolutionary history. This hierarchy runs from the broadest level (domain) down to the most specific (species), and every species is given a unique two-part Latin name — its binomial — made up of its genus and species, such as Homo sapiens.
Genetic information, variation and relationships between organisms
Stimuli, both internal and external, are detected and lead to a response
Survival depends on an organism's ability to detect changes — called stimuli — in both its external surroundings and its internal environment, and then produce a coordinated response. Receptors are specialised cells or proteins that convert a specific stimulus into an electrical or chemical signal, which is then communicated to effectors — the muscles or glands that carry out the response. Understanding this stimulus–response framework is the foundation for everything that follows in this section, from how nerve impulses are transmitted to how skeletal muscle contracts and how the body maintains a stable internal state.
Organisms respond to changes in their internal and external environments
Surface area to volume ratio
As an organism grows larger, its volume increases faster than its surface area. This means larger organisms have less outer surface available, relative to the amount of living tissue inside.
Organisms exchange substances with their environment
Surface area to volume ratio
As organisms get larger, their surface area to volume ratio falls. Larger organisms compensate by changing body shape or evolving specialised exchange systems, such as lungs or gills.
Organisms exchange substances with their environment
Surface area to volume ratio
Smaller organisms have a higher surface area to volume ratio. This means they lose heat faster and must run their chemical reactions — their metabolic rate — more quickly to compensate.
Organisms exchange substances with their environment
Surface area to volume ratio
As an organism gets larger, its volume — the amount of living tissue that needs supplying — grows much faster than its surface area, meaning there is proportionally less outer surface available to exchange substances like oxygen and carbon dioxide with the environment. This reduction in surface area to volume ratio creates a problem for larger organisms, because simple diffusion across a body surface is no longer fast enough to meet the demands of a high metabolic rate (the speed at which chemical reactions occur in cells). Understanding this constraint explains why larger organisms have evolved specialised exchange surfaces and transport systems — such as lungs and circulatory systems — which are explored in the subtopics that follow.
Organisms exchange substances with their environment
Transport across cell membranes
Every cell membrane — whether surrounding a cell or enclosing an organelle — shares the same basic structure. A double layer of fat-based molecules forms the core, with proteins embedded throughout.
Cells
Transport across cell membranes
The fluid-mosaic model describes the cell membrane as a flexible, constantly moving double layer of fat-based molecules. Proteins, glycoproteins, and glycolipids sit within or on this layer, each doing a different job.
Cells
Transport across cell membranes
Cholesterol molecules sit between the fatty acid tails of phospholipids in the membrane. They hold the tails together and reduce how freely the membrane can move.
Cells
Transport across cell membranes
The five transport mechanisms each solve a different problem. **Simple diffusion** — small, non-polar (uncharged, fat-soluble) molecules such as oxygen and CO₂ dissolve directly through the phospholipid bilayer, moving down their concentration gradient. Larger or charged molecules cannot cross this way; the hydrophobic (water-repelling) core of the bilayer blocks them. **Facilitated diffusion**
Cells
Transport across cell membranes
Some cells need to move substances very quickly. They do this by having a larger membrane surface area or more transport proteins embedded in the membrane.
Cells
Transport across cell membranes
Cell membranes are selectively permeable barriers — meaning they control which substances can enter and leave the cell — and understanding how molecules cross them is central to almost every process in biology. The fluid-mosaic model describes the membrane as a flexible phospholipid bilayer (a double layer of fat-based molecules) embedded with proteins, glycoproteins, and cholesterol, each playing a specific role in regulating transport. Substances move across membranes by several distinct mechanisms — including simple diffusion, facilitated diffusion, osmosis, active transport, and co-transport — each suited to different molecules and conditions, and cells can be structurally adapted to make these processes faster or more efficient.
Cells
Using genome projects
Scientists have used large-scale projects to read the complete DNA sequence of many organisms. These projects include the Human Genome Project, which finished mapping all human DNA in 2003.
The control of gene expression
Using genome projects
Sequencing the complete DNA of a simple organism reveals every protein it can make. Scientists can use this protein list to find antigens — molecules that trigger an immune response — and design vaccines against them.
The control of gene expression
Using genome projects
In complex organisms like humans, knowing the full DNA sequence does not tell you all the proteins the organism makes. Non-coding DNA and genes that control other genes make the relationship far more complicated.
The control of gene expression
Using genome projects
Scientists keep improving the technology used to read DNA sequences. Modern sequencing machines now do this work automatically, making the process much faster and cheaper than before.
The control of gene expression
Using genome projects
Large-scale genome sequencing projects have read the complete DNA instructions of many organisms, including humans, using increasingly automated technology. In simpler organisms, knowing the genome makes it possible to predict the proteome — the full set of proteins an organism can produce — which opens up practical applications such as identifying antigens (molecules that trigger an immune response) for vaccine development. In more complex organisms like humans, this translation from genome to proteome is far less straightforward, because much of the DNA does not code for proteins and some genes act as regulators that control whether other genes are switched on or off.
The control of gene expression
Water
Water makes up the majority of every living cell. Most animal cells are roughly 70–80% water by mass.
Biological molecules
Water
Water takes part directly in chemical reactions inside cells. Cells consume water in some reactions and produce it in others.
Biological molecules
Water
Water dissolves substances inside cells. This allows the chemical reactions that keep organisms alive to take place.
Biological molecules
Water
Water resists changes in temperature better than most liquids. This keeps cells and aquatic environments stable when heat is gained or lost.
Biological molecules
Water
Latent heat of vaporisation is the energy required to convert a liquid into a vapour without changing its temperature. Water has an unusually large value for this property. This is because water molecules form hydrogen bonds with each other — weak attractions between the slightly positive hydrogen atoms of one molecule and the slightly negative oxygen atom of another. Before a water molecule can e
Biological molecules
Water
Cohesion is the attraction between molecules of the same substance. In water, each molecule carries a slight positive charge on its hydrogen atoms and a slight negative charge on its oxygen atom. These opposite charges attract neighbouring water molecules, forming hydrogen bonds (weak electrostatic attractions between a δ+ hydrogen on one molecule and the δ− oxygen on another). This produces two
Biological molecules
Water
Despite being a simple molecule, water is essential to life — it makes up the majority of every cell and is directly involved in metabolic reactions (the chemical reactions that keep organisms alive), acting as both a metabolite (a molecule consumed or produced in those reactions) and a solvent (the liquid in which other substances dissolve so reactions can take place). Its unique physical properties — including a high heat capacity (resistance to temperature change), a large latent heat of vaporisation (energy needed to convert liquid water to vapour), and strong cohesion (attraction between water molecules) — make it ideally suited to maintaining the stable internal conditions that living organisms depend on.
Biological molecules