Growth and Development
What you must understand
Growth is a permanent increase in size or mass; development is the increase in complexity, such as the formation of organs. Know the stages of human development: zygote, embryo, fetus, birth, infancy, childhood, adolescence, adulthood.
Learn the main features of growth: it is measured by height and mass, plotted on growth charts, and occurs at different rates in different parts of the body. The brain grows fastest early; the reproductive organs grow fastest at puberty. Puberty is triggered by hormones: testosterone in boys and oestrogen in girls, causing secondary sexual characteristics.
In plants, growth occurs at meristems, and growth can be measured by height, mass or leaf area. Learn that growth requires the raw materials of nutrition and that a deficiency in childhood can cause stunting.
Traps examiners set
- Confusing growth (increase in size) with development (increase in complexity).
- Saying growth stops at a fixed age for everyone; growth rates vary.
- Forgetting that growth charts compare an individual with a population average, not a target.
- Mixing up which hormone causes which secondary sexual characteristic.
Worked example
A student says a plant grew taller overnight because it absorbed water. Challenge: temporary water uptake is not growth. Growth is a permanent increase in dry mass, from cell division and cell enlargement.
Rule of thumb: if the change would reverse when water is lost, it is not growth.
Practical link
When measuring growth, take repeat readings, use the same method each time, and control variables such as time of day, to make comparisons fair and reliable.
Inheritance
What you must understand
Genetic information is carried in the nucleus on chromosomes, which are made of DNA. Genes are sections of DNA that code for characteristics. Humans have 46 chromosomes in 23 pairs; gametes have 23 single chromosomes. Know the terms: allele, dominant, recessive, homozygous, heterozygous, genotype, phenotype.
Be able to use a genetic diagram (Punnett square). For a monohybrid cross between two heterozygotes (Tt × Tt), the offspring ratio is 1 TT : 2 Tt : 1 tt, giving a 3:1 phenotype ratio.
Understand sex determination: females are XX, males are XY, so the father's sperm determines the sex of the child. Learn that some characteristics are controlled by multiple genes or by the environment, and that mutations are changes in DNA that can produce new alleles.
Traps examiners set
- Writing the phenotype ratio when the question asks for the genotype ratio, or vice versa.
- Forgetting that a recessive characteristic only appears when both alleles are recessive.
- Saying the mother determines the sex of the baby.
- Confusing a gene with an allele: a gene is the section of DNA; alleles are different versions of it.
Worked example
Two heterozygous parents (Tt × Tt) have a child. What is the chance the child shows the recessive characteristic? The square gives TT, Tt, Tt, tt. Only tt shows the recessive trait, so the probability is 1 in 4, or 25%.
Rule of thumb: always write the parental genotypes, set out the gametes, complete the square, then state the ratio with the phenotype named.
Variation and Evolution
What you must understand
Variation is the differences between individuals. It can be genetic (caused by genes, including mutation and sexual reproduction) or environmental (diet, exercise, lifestyle). Most characteristics are affected by both. Variation can be continuous, such as height, shown on a line graph, or discontinuous, such as blood group, shown on a bar chart.
Evolution is the change in inherited characteristics of a population over generations, driven by natural selection. The steps: there is variation; some individuals are better adapted; they survive and reproduce; their alleles are passed on; the frequency of advantageous alleles increases. This is often summarised as "survival of the fittest", where fitness means reproductive success, not strength.
Learn how antibiotic resistance develops in bacteria as a modern example of natural selection, and understand that resistance arises from random mutation, not from the antibiotic causing the change.
Traps examiners set
- Saying an organism "adapts" during its lifetime and passes that on. Acquired changes are not inherited.
- Writing that the environment causes the mutation. Mutations occur randomly; selection acts afterwards.
- Confusing continuous and discontinuous variation graphs.
- Describing evolution as an individual changing, rather than a population changing over generations.
Worked example
Explain why a population of insects becomes resistant to a pesticide. Some insects already have a random mutation giving resistance. The pesticide kills non-resistant insects, so resistant ones survive and reproduce, passing on the resistance allele. Over generations, most of the population is resistant.
Rule of thumb: variation first, selection second, inheritance third, change in frequency last.
Health, Disease and Immunity
What you must understand
A pathogen is a microorganism that causes disease. Know examples: bacteria (cholera, tuberculosis), viruses (influenza, HIV), fungi (athlete's foot) and protoctists (malaria). Learn how diseases spread (air, water, food, vectors, contact) and how to reduce spread: sanitation, clean water, vaccination, vector control and hygiene.
The body has defences. The first line includes skin, mucus, cilia and stomach acid. The immune system responds with white blood cells: some engulf pathogens (phagocytes), others produce antibodies that are specific to an antigen. Memory cells give long-lasting immunity after infection or vaccination.
Vaccination introduces a harmless form of the pathogen so the body makes antibodies and memory cells without becoming ill. Antibiotics kill bacteria but do not affect viruses, and overuse leads to resistance.
Traps examiners set
- Saying antibiotics cure colds or flu, which are viral.
- Forgetting that antibodies are specific to one antigen.
- Confusing the role of phagocytes (engulf) with lymphocytes (produce antibodies).
- Describing vaccination as "injecting antibodies". It stimulates the body to make its own.
Worked example
Explain how vaccination protects a person. A weakened or inactive pathogen is introduced. White blood cells produce antibodies against its antigens, and memory cells remain. If the real pathogen enters later, antibodies are produced quickly, so the person does not become ill.
Rule of thumb: for any immunity question, name the antigen, the antibody, the white blood cell and the memory response.
Ecology
What you must understand
An ecosystem is a community of organisms interacting with each other and their physical environment. Know the levels: organism, population, community, ecosystem. Learn the terms habitat, niche, producer, consumer, decomposer, herbivore, carnivore and omnivore.
Food chains show energy flow. Arrows point in the direction of energy transfer. Energy is lost at each level through respiration, movement, heat and undigested material, so pyramids of energy and biomass are usually narrower at the top. Food webs show interconnected chains; removing one organism affects others.
Learn the carbon and nitrogen cycles in outline: photosynthesis removes carbon dioxide, respiration and combustion return it; decomposers release minerals. Understand competition (for food, light, space) and interdependence.
Traps examiners set
- Drawing arrows from predator to prey. Arrows show energy flow, so they point to the consumer.
- Saying energy is lost as "food". Energy is lost as heat, movement and waste.
- Confusing a food chain with a food web, or a population with a community.
- Forgetting that decomposers recycle nutrients, which is why they matter.
Worked example
A food chain is grass → rabbit → fox. If the foxes are removed, rabbits increase at first because there is less predation, then food becomes scarce and the rabbit population falls again as competition for grass increases.
Rule of thumb: in every food chain question, follow the energy and name the loss at each step.
Fieldwork
Use quadrats to sample plants and capture-recapture to estimate animal populations. Always sample randomly to avoid bias.
Environment and Human Impact
What you must understand
Human activity affects the environment in several ways. Learn these with causes, effects and solutions:
- Pollution: sulfur dioxide from burning fossil fuels causes acid rain; carbon dioxide and methane are greenhouse gases causing global warming; sewage and fertiliser cause eutrophication of rivers and lakes.
- Deforestation: reduces biodiversity, increases soil erosion and carbon dioxide levels.
- Overfishing and hunting: can drive species to extinction.
- Waste and plastic: harms wildlife and persists in the environment.
Know the process of eutrophication in order: fertiliser runs into water → nutrients increase → algae grow rapidly → algae die → decomposers use oxygen → oxygen levels fall → fish die. Learn the greenhouse effect: gases trap heat in the atmosphere; more gases mean more heat trapped.
Conservation measures include recycling, renewable energy, protected areas, sustainable fishing quotas and reforestation.
Traps examiners set
- Saying the greenhouse effect is entirely harmful. Without it, Earth would be too cold; the problem is the enhanced effect.
- Confusing global warming with ozone depletion.
- Getting the order of eutrophication wrong, especially placing fish death before oxygen depletion.
- Saying deforestation only affects animals, ignoring soil erosion and carbon dioxide.
Worked example
Explain how fertiliser can kill fish in a lake. Fertiliser adds nutrients, so algae multiply. When algae die, decomposers break them down and use up oxygen in the water. Oxygen levels fall, so fish cannot respire and die: this is eutrophication.
Rule of thumb: for any impact question, give the cause, the effect on organisms and a possible solution.