All living things are made of cells. Animal cells have a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes. Plant cells have all of these plus a cell wall (made of cellulose), a permanent vacuole (filled with cell sap) and chloroplasts (contain chlorophyll, site of photosynthesis).
Bacterial cells are prokaryotic: no nucleus, instead a single circular loop of DNA plus small rings called plasmids, and much smaller than animal or plant cells (typically 0.5-5 micrometres vs 10-100 micrometres for eukaryotic cells).
Magnification = image size divided by actual size. Always keep units the same before dividing, then convert. Light microscopes can resolve down to about 200 nanometres; electron microscopes resolve down to about 0.1-1 nanometre, giving much higher resolution to see organelles like mitochondria and ribosomes in detail. Remember 1 mm = 1000 micrometres (um), and 1 um = 1000 nanometres (nm).
Mitosis produces two genetically identical diploid cells for growth and repair. Stem cells (embryonic and adult) can differentiate into different cell types; embryonic stem cells are more versatile. Common exam mistake: confusing mitosis (growth/repair, identical cells) with meiosis (sex cells, four genetically different cells) — meiosis is on the biology specification too but is a separate topic.
Three key processes move substances across cell membranes:
Rate of diffusion is increased by a bigger surface area, a shorter diffusion distance, a bigger concentration gradient, and higher temperature. This is why the small intestine has villi (surface area) and alveoli have thin, moist walls with a huge surface area and rich blood supply.
Cells are the building blocks of life. They organise into tissues, tissues into organs, and organs into organ systems, which work together to form the whole organism.
A common exam mistake is mixing up tissue and organ definitions, so learn the order: cell, tissue, organ, system, organism.
The digestive system breaks down large insoluble food molecules into small soluble molecules that can be absorbed into the blood. This uses both physical breakdown (chewing, churning) and chemical breakdown (enzymes).
Key organs and their jobs:
Enzymes are biological catalysts made of protein that speed up reactions without being used up. Each enzyme has an active site with a specific shape that fits its substrate, known as the lock and key model.
Enzymes are denatured by high temperatures (above about 37 to 40 degrees C for human enzymes) and by extreme pH, which changes the active site shape so the substrate no longer fits. Do not say the enzyme is killed, enzymes are proteins, not living things.
Bile is made in the liver, stored in the gall bladder, and released into the small intestine. It is alkaline, so it neutralises stomach acid, and it emulsifies fats, breaking them into small droplets to increase surface area for lipase to work on. Bile is not an enzyme, a very common mix-up.
Pathogens are microorganisms that cause disease: bacteria, viruses, fungi and protists. They cause disease by damaging cells directly or by producing toxins.
Diseases spread by direct contact, water, or air (droplet infection). Common mistake: students confuse a vector (carrier, e.g. mosquito) with the pathogen itself (e.g. Plasmodium).
Plants defend themselves with a physical barrier (cellulose cell wall, waxy cuticle, bark), chemical defences (antibacterial chemicals, poisons) and mechanical defences (thorns, hairs, leaves that droop or curl).
The body has non-specific defences: skin acts as a barrier and produces antimicrobial secretions, the nose has hairs and mucus to trap pathogens, the trachea and bronchi have mucus and cilia to trap and move pathogens away, and the stomach produces hydrochloric acid to kill pathogens.
White blood cells defend the body in three main ways: phagocytosis (engulfing and digesting pathogens), antibody production (specific to each antigen, causes pathogens to clump for destruction) and antitoxin production (neutralises toxins).
Vaccination introduces small quantities of dead or inactive pathogen to stimulate white blood cells to produce antibodies. If the same pathogen infects again, the response is much faster, giving immunity. Common mistake: vaccines do not give you the disease, and herd immunity protects those who cannot be vaccinated.
Antibiotics kill bacteria (not viruses) but overuse increases antibiotic resistance, e.g. MRSA. Painkillers only treat symptoms, they do not kill pathogens. New drugs must be tested for toxicity, efficacy and dosage, first in the lab and on cells/tissue, then on animals, then in clinical trials on healthy and ill volunteers using a placebo and double-blind trials to avoid bias.
Bioenergetics is how living things get and use energy. In GCSE Biology this covers photosynthesis (making food/energy) and respiration (releasing energy from food).
Photosynthesis is an endothermic reaction (it takes in energy) that happens in chloroplasts, using light energy to convert carbon dioxide and water into glucose and oxygen.
The rate of photosynthesis can be limited by three main factors: light intensity, carbon dioxide concentration, and temperature. Whichever factor is in shortest supply limits the rate - this is the 'limiting factor'.
A common required practical uses pondweed (like Elodea or Cabomba) to measure oxygen bubbles produced at different distances from a lamp, testing the effect of light intensity.
Respiration is an exothermic reaction (releases energy) that happens continuously in all living cells, in the mitochondria.
During exercise, heart rate, breathing rate and breath volume all increase to supply muscles with more oxygen and glucose and remove carbon dioxide faster.
Homeostasis is keeping the body's internal environment stable despite changes outside or inside the body. It controls things like body temperature, blood glucose, water levels and carbon dioxide levels. All homeostatic systems work by negative feedback - a change is detected and the body acts to reverse it, bringing things back to normal.
The nervous system lets the body respond quickly to changes (stimuli) using electrical impulses. The pathway is: receptor -> sensory neurone -> coordinator (brain or spinal cord, part of the CNS) -> motor neurone -> effector (a muscle or gland).
Neurones do not touch. There is a tiny gap called a synapse between them. Electrical impulses cannot cross this gap, so the signal is passed by chemicals (neurotransmitters). These diffuse across the synapse and trigger a new electrical impulse in the next neurone. A common mistake is saying the impulse 'jumps' the gap - it does not, it is chemical transmission.
A reflex is an automatic, rapid response that does not involve conscious thought, protecting the body from harm. The reflex arc pathway is: stimulus -> receptor -> sensory neurone -> relay neurone (in the spinal cord) -> motor neurone -> effector -> response. Because the signal usually bypasses the brain (going through the spinal cord instead), reflexes are much faster than a considered response. Example: touching something hot causes an instant hand withdrawal before you 'feel' the pain.
The brain has different regions with different jobs:
Neuroscientists map the brain's regions using MRI scanning, studying patients with brain damage, and electrically stimulating regions and observing the effect. Investigating the brain is difficult because the skull protects it, it is complex, and damage can be permanent, so treatments must be very carefully tested.
Most body cells are diploid (2 sets of chromosomes, 46 in humans, 23 pairs). Gametes (sperm and egg) are haploid, containing 23 chromosomes each. Fertilisation restores the diploid number.
A gene is a section of DNA that codes for a protein and controls a characteristic. Different versions of a gene are called alleles. An allele can be dominant (shown even with only one copy, written as a capital letter) or recessive (only shown if both alleles are recessive, lower case letter).
Genotype is the alleles an organism has (e.g. Bb). Phenotype is the physical characteristic that results (e.g. brown eyes). Homozygous means both alleles are the same (BB or bb). Heterozygous means the alleles are different (Bb).
Use a Punnett square to predict offspring ratios from a cross. A classic monohybrid cross between two heterozygotes (Bb x Bb) gives a 3:1 ratio of dominant to recessive phenotypes in the offspring, but a 1:2:1 genotype ratio (BB:Bb:bb).
Common mistake: students forget that a 3:1 ratio is a probability, not a guarantee for every family — small numbers of offspring can easily deviate from the expected ratio by chance.
Humans have 23 pairs of chromosomes; pair 23 determines sex. Females are XX, males are XY. All eggs carry an X chromosome; sperm carry either X or Y, so the father's sperm determines the sex of the child, giving a 50:50 ratio.
Variation between individuals can be genetic (inherited), environmental (caused by surroundings), or a combination of both. Mutations are random changes to the DNA base sequence. Most mutations have no effect on the protein produced or the phenotype; a small number are harmful, and very rarely one is beneficial.
Darwin's theory of natural selection: individuals in a population show variation; those with characteristics best suited to the environment are more likely to survive and reproduce; they pass the advantageous alleles to their offspring; over many generations the useful characteristic becomes more common. This is how species evolve over time.
A community is all the populations of different species living in a habitat. An ecosystem is the community plus the non-living (abiotic) parts of the environment. Abiotic factors include temperature, light intensity, moisture and pH; biotic factors include predation, competition and disease.
Energy is lost between trophic levels (as heat from respiration, in waste, and in uneaten/undigested parts), so only around 10% of energy transfers to the next trophic level — this limits food chains to usually no more than 4-5 trophic levels.
Common mistake: confusing a food chain (one path of energy transfer) with a food web (multiple interconnected chains) — exam questions often ask what happens to a web if one species is removed.