This is a complete revision guide for AQA Combined Science Trilogy Biology Paper 2 (specification code 8464), covering Topics 4.5 to 4.7: Homeostasis and Response, Inheritance and Evolution, and Ecology. This guide is written specifically for Combined Science students. If you are sitting Triple Science Biology, a separate guide is available for that specification.
Combined Science Biology Paper 2 has two required practicals: the Reaction Time practical and the Field Investigations practical using quadrats and transects. Both are covered in full below. Content labelled Higher Tier only is for Higher students. Foundation students can skip those sections.
Work through each section, then cover the page and recall the key points from memory. Focus extra time on the areas where your recall is weakest. Active recall is what moves content into long-term memory before an exam.
Topic 4.5: Homeostasis and Response
What is homeostasis?
Homeostasis is the regulation of the internal conditions of the body so they remain constant, allowing cells to survive within narrow limits and enzymes to work properly. The body controls blood glucose concentration, body temperature and water levels. Without homeostasis, enzyme activity would be disrupted and cells could not function.
All control systems share the same structure:
- Receptors: detect changes (stimuli)
- Coordination centres: process information (brain, spinal cord, pancreas)
- Effectors: carry out the response (muscles or glands)
This forms a feedback system that returns conditions to normal whenever they deviate from the set point.
The nervous system
The nervous system allows the body to respond quickly to changes in the environment. Receptors detect stimuli and send electrical impulses through neurones to the central nervous system (CNS), which consists of the brain and spinal cord. The CNS coordinates responses carried out by effectors such as muscles and glands.
A reflex action is automatic, rapid and does not involve conscious thought. The reflex arc pathway is:
Reflexes are important because they protect the body and allow fast responses without waiting for conscious thought.
Sensory neurones carry signals from receptors to the CNS. Motor neurones carry signals from the CNS to effectors. These are frequently confused in exam answers. Remember: sensory goes in, motor goes out.
Required Practical: Reaction time
Aim: investigate how a factor affects reaction time, for example caffeine, exercise or distractions.
Method: one person holds a ruler vertically while another places their thumb and finger at the zero mark without touching it. The ruler is dropped without warning and the person catches it. Record the distance fallen before catching. Repeat several times and calculate a mean. Change the independent variable and repeat.
- Independent variable: the factor being tested (for example, before and after caffeine)
- Dependent variable: reaction time, measured from the distance fallen
- Control variables: same ruler, same person, same hand, same conditions
A larger distance fallen means a slower reaction time. A smaller distance means a faster reaction time. Repeat readings are needed to identify anomalies and calculate a reliable mean. Control variables must be kept constant to make the test fair.
The endocrine system
The endocrine system is made up of glands that release hormones directly into the blood. Hormones act on target organs. The effects are slower to start but longer-lasting than nervous system responses. Key glands include the pituitary gland (the master gland that controls other glands), pancreas, thyroid, adrenal glands, ovaries and testes.
Blood glucose control
Blood glucose is controlled by the pancreas through a negative feedback system. When blood glucose is too high, the pancreas releases insulin. Insulin causes cells to take up glucose and the liver to store it as glycogen, lowering blood glucose back to normal. When blood glucose is too low (Higher Tier only), the pancreas releases glucagon, which causes the liver to convert glycogen back into glucose and release it into the blood.
The pancreas does not produce insulin. Blood glucose cannot be regulated without external help. Treated with insulin injections.
The body no longer responds to insulin properly. Linked to obesity and poor diet. Treated with dietary changes and exercise.
Hormones in reproduction
Four hormones control the menstrual cycle. FSH causes egg maturation in the ovaries. LH triggers egg release (ovulation). Oestrogen maintains the uterus lining. Progesterone maintains the uterus lining after ovulation. Testosterone stimulates sperm production in males.
Contraception
Methods of contraception include hormonal methods (pill, implant, injection), barrier methods (condoms, diaphragm), IUDs, spermicides, sterilisation and abstinence. Hormonal contraception typically works by preventing FSH release so eggs do not mature, or by preventing ovulation.
Fertility treatment (Higher Tier only)
FSH and LH can be given as fertility drugs to stimulate egg production, increasing the chance of natural conception. IVF involves collecting eggs, fertilising them with sperm in the laboratory, and placing the resulting embryos into the uterus. Issues include emotional stress, low success rates and the risk of multiple births.
Feedback systems (Higher Tier only)
Adrenaline prepares the body for fight or flight by increasing heart rate, releasing glucose into the blood and increasing oxygen delivery to muscles. Thyroxine controls the metabolic rate and influences growth and development. Both hormones are regulated by negative feedback systems, where rising levels of the hormone reduce further secretion.
Topic 4.6: Inheritance, Variation and Evolution
Reproduction
Involves the fusion of gametes (sperm and egg in animals, pollen and egg cell in plants). Produces genetic variation in offspring. Requires two parents.
One parent only. No gametes involved. Produces genetically identical offspring (clones). Happens by mitosis. Fast and requires no mate.
Meiosis
Meiosis is the type of cell division that produces gametes. It halves the number of chromosomes and produces four genetically different cells. This is essential for sexual reproduction and is the source of genetic variation between offspring.
Meiosis produces four genetically different gametes with half the chromosome number. Mitosis produces two genetically identical cells with the full chromosome number, used for growth and repair. Confusing these is one of the most common errors in this topic.
DNA and the genome
DNA is the genetic material found in the nucleus of cells. It is a double helix polymer. A gene is a section of DNA that codes for a specific protein. A chromosome contains many genes. The genome is the complete set of genetic material in an organism. Understanding the human genome is important for identifying genes linked to inherited diseases and tracing patterns of human migration.
Genetic inheritance
- Allele: a version of a gene
- Genotype: the combination of alleles an organism has
- Phenotype: the physical characteristic that results
- Dominant allele: always expressed when present
- Recessive allele: only expressed when two copies are present
- Homozygous: both alleles are the same (AA or aa)
- Heterozygous: alleles are different (Aa)
Most characteristics are controlled by multiple genes rather than just one. Punnett squares show all possible offspring genotypes from a cross. Example: Aa crossed with Aa.
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
This gives a 3:1 ratio of dominant phenotype to recessive phenotype. Always express genetic results as probabilities or ratios, not certainties.
Inherited disorders
Polydactyly is caused by a dominant allele, meaning only one copy is needed for it to be expressed. It causes extra fingers or toes. Cystic fibrosis is caused by a recessive allele, so both alleles must be recessive for the condition to appear. Embryo screening allows embryos to be tested for genetic conditions before implantation, but raises ethical, social and economic issues that must be considered.
Sex determination
Humans have 23 pairs of chromosomes. The 23rd pair determines sex. Females have XX chromosomes and males have XY. Eggs always carry an X chromosome. Sperm carry either X or Y. There is a 50% probability of a male and a 50% probability of a female offspring from any pregnancy.
Variation and evolution
Variation comes from genes, the environment, or a combination of both. Mutations are random changes in DNA that create new alleles. Most mutations have no effect on the phenotype, some do affect it, and very few are beneficial.
Evolution is the change in inherited characteristics in a population over time. The process: variation exists in a population; natural selection acts on that variation; individuals best adapted to their environment survive and reproduce; their alleles become more common over time; this can eventually produce a new species.
Better-adapted individuals survive and reproduce, passing on their advantageous alleles. Evolution happens to populations over generations, not to individual organisms. This distinction is regularly tested.
Selective breeding
Selective breeding is when humans choose parents with desired traits and breed them over many generations. The process: select parents with the desired trait, breed them, select offspring with the best version of the trait, and repeat over many generations. Risks include inbreeding, which reduces genetic diversity and increases susceptibility to disease.
Genetic engineering
Genetic engineering transfers genes between organisms to give desired traits. Uses include disease-resistant and higher-yield crops, and the production of human insulin in bacteria. Concerns include unknown environmental effects, uncertainty about long-term health impacts, and ethical objections to modifying living organisms.
Evidence for evolution
Evidence comes from fossils, antibiotic resistance in bacteria, and genetic comparisons between species. Fossils are formed when hard parts are preserved in rock, when minerals replace the original material, or when organisms leave traces such as footprints. The fossil record is incomplete because soft-bodied organisms rarely fossilise and geological activity can destroy evidence.
Extinction occurs when no individuals of a species remain. Causes include environmental change, competition from other species, and new predators or pathogens. Antibiotic resistance is a direct, observable example of evolution: random mutations in bacteria produce resistant individuals; antibiotics kill non-resistant bacteria; resistant bacteria survive and reproduce, spreading the resistance gene. To slow the development of resistance, antibiotics should only be used when necessary, the full course must be completed, and agricultural use should be restricted.
Classification
The Linnaean system organises life from broadest to most specific: Kingdom, Phylum, Class, Order, Family, Genus, Species. Species are named using the binomial system using Genus and species together (for example, Homo sapiens). Modern classification has been improved by microscopy and biochemical analysis. The three-domain system divides all life into Archaea, Bacteria and Eukaryota (which includes plants, animals, fungi and protists).
Topic 4.7: Ecology
Ecosystems and communities
Ecology is the study of interactions between organisms and their environment. An ecosystem is the interaction between living (biotic) and non-living (abiotic) factors in an area.
Abiotic factors are non-living factors that affect ecosystems: light intensity, temperature, moisture, soil pH and mineral content, wind speed, carbon dioxide levels and oxygen levels. Biotic factors are living factors: predators, food availability, pathogens and competition between species.
Organisms compete for food, water, light, space, mates and territory. Species are interdependent: they rely on each other for food, pollination, seed dispersal and shelter. Removing one species can affect the entire ecosystem through its food web connections.
Adaptations
Organisms are adapted to survive in their specific environment. Structural adaptations are physical features. Behavioural adaptations are actions or responses. Functional adaptations are internal physiological processes. Extremophiles are organisms adapted to survive in extreme conditions such as very high temperatures, high salinity or high pressure environments.
Food chains and trophic levels
Food chains start with producers, which are plants or algae that make food by photosynthesis. Energy flows from producers to primary consumers, then secondary consumers, then tertiary consumers. Only around 10% of biomass is transferred from one trophic level to the next. The rest is lost through respiration (heat and movement), waste products and parts of organisms that are not eaten.
Predator-prey cycles
Predator and prey populations rise and fall in linked cycles. When prey numbers increase, more food is available for predators, so predator numbers rise. As predator numbers increase, more prey is eaten, so prey numbers fall. With less prey available, predator numbers fall too, allowing prey populations to recover, and the cycle continues.
Required Practical: Field Investigations (quadrats and transects)
Aim: estimate population size and investigate how environmental factors affect the distribution of organisms.
Quadrat method: place a quadrat randomly in the study area, count the organisms inside, repeat many times across the area, calculate the mean count per quadrat, then estimate the total population using the formula:
Transect method: lay a tape measure across a habitat, place quadrats at regular intervals along it, and record species abundance at each point. Transects are used to show how the distribution of organisms changes across an environmental gradient, for example moving from a path into a meadow or from the top to the bottom of a beach.
- Independent variable: the environmental factor being investigated (light intensity, soil moisture, pH)
- Dependent variable: the abundance or distribution of organisms
- Control variables: quadrat size, counting method, sampling intervals
Random sampling avoids bias in quadrat placement. Using many quadrats produces a more representative sample. A transect is used rather than random quadrats when investigating change across a gradient.
Material cycles
The carbon cycle: plants remove carbon dioxide from the atmosphere through photosynthesis, incorporating carbon into organic molecules. Carbon is returned to the atmosphere as carbon dioxide through respiration by all organisms, through decomposition of dead matter by microorganisms, and through combustion of fossil fuels. The water cycle: water evaporates from land and transpires from plants, condenses to form clouds and returns to Earth as precipitation, providing fresh water for organisms and ecosystems.
Biodiversity and human impact
Biodiversity is the variety of all living species in an ecosystem. High biodiversity leads to stable ecosystems with resilient food webs. Human activities reduce biodiversity through waste pollution (affecting water, air and land), deforestation (causing habitat loss, reduced biodiversity and increased atmospheric carbon dioxide), global warming (caused by increased carbon dioxide and methane, leading to ecosystem disruption and species extinction), and changes in land use for farming and urban development.
Methods to maintain biodiversity include captive breeding programmes to protect endangered species, habitat protection through nature reserves and national parks, reforestation to restore lost habitats, recycling to reduce demand for raw materials and pollution, and sustainable farming practices to reduce environmental damage.
For 6-mark questions on maintaining biodiversity, always include benefits to the environment, the economic or social costs of the method, and a comment on its long-term effectiveness or limitations. A one-sided answer will not reach full marks.
Key equations for Biology Paper 2
The population estimate formula is the main calculation you need to know for Combined Science Biology Paper 2:
Most common exam mistakes across all three topics
- Mixing up insulin and glucagon: insulin lowers blood glucose, glucagon raises it
- Forgetting negative feedback: the response reduces the original change, returning conditions to normal
- Confusing sensory and motor neurones
- Confusing meiosis and mitosis: meiosis makes gametes with half the chromosome number, mitosis makes identical cells for growth
- Mixing up genotype and phenotype
- Forgetting to express genetic results as probabilities or ratios, not certainties
- Saying evolution happens to individual organisms: it happens to populations over many generations
- Saying energy is recycled in ecosystems: energy flows and is lost as heat. Matter such as carbon and water is what is recycled.
- Not linking human activity to biodiversity loss in ecology answers
- Forgetting to state why random sampling is used in quadrat questions: to avoid bias
