GCSE AQA Biology Revision Guide

Cell Structure & Organisation

Ever wondered how scientists can see things thousands of times smaller than what your eyes can detect? Light microscopes use rays of light to magnify specimens up to 2000 times – they're affordable and work anywhere, making them perfect for school labs. Electron microscopes are the heavy-hitters, using electron beams to achieve magnifications up to 2 million times, but they're massive, expensive, and need special rooms.

Prokaryotic cells like bacteria are the simplest living things – just single cells measuring 0.2-2.0 micrometres. They pack everything into one compartment: cytoplasm for chemical reactions, a cell membrane controlling what goes in and out, and a tough cell wall for protection. Many bacteria have flagella (whip-like tails) for swimming around and plasmids – little DNA loops that often carry antibiotic resistance genes.

Eukaryotic cells are much more sophisticated, with the nucleus acting as the control centre containing all your genetic information. The cytoplasm is where most chemical reactions happen, whilst mitochondria power the cell through respiration. Plant cells get extra features: chloroplasts for photosynthesis, a cellulose cell wall for strength, and a large vacuole to keep the plant rigid.

Quick Tip: Remember that all cells share basic features – they all need a membrane, genetic material, and cytoplasm to function!

Cell Division & Differentiation

Your body is constantly creating new cells – about 25 million every second! The cell cycle is how this happens, involving three key stages that ensure each new cell gets exactly the same genetic information as the original.

Stage 1 sees cells growing bigger and copying their DNA to create duplicate chromosomes. Stage 2 is mitosis, where the nucleus divides and chromosomes separate to opposite ends of the cell. Stage 3 completes the process as the cytoplasm splits, creating two identical daughter cells. This whole process is essential for growth, repair, and replacing worn-out cells.

Stem cells are the body's repair kit – they're unspecialised cells that can develop into any type of cell needed. Embryonic stem cells can become absolutely anything, whilst adult stem cells from bone marrow are more limited but still incredibly useful for treating diseases like paralysis and blindness.

Plant cells are more flexible than animal cells – they can differentiate throughout their entire lives at growing points called meristems. This ability makes plant cloning possible, which could help save endangered species from extinction.

Fascinating Fact: A single plant cell can potentially grow into an entire new plant – imagine growing a whole oak tree from just one cell!

Organisation & Enzymes

Your body is brilliantly organised, like a company with different departments working together. Tissues are groups of similar cells doing the same job, organs are collections of tissues (like your stomach), and organ systems are groups of organs working as a team.

The digestive system is your body's food processor, breaking down large molecules into smaller ones you can absorb. Enzymes are the molecular scissors making this possible – these protein catalysts speed up reactions by millions of times. Each enzyme has a uniquely shaped active site that fits perfectly with its target molecule, following the lock and key model.

Temperature and pH dramatically affect enzyme activity. Most human enzymes work best at 37°C – your body temperature. Go much higher and the enzymes denature, permanently losing their shape and becoming useless. Different enzymes prefer different pH levels, which is why stomach acid doesn't destroy the enzymes in your mouth.

Your digestive system produces specific enzymes for different foods: amylase breaks down starch into sugars, protease splits proteins into amino acids, and lipase converts fats into fatty acids and glycerol. The small intestine's villi provide a massive surface area for absorbing all these nutrients into your bloodstream.

Remember: Enzymes are like molecular machines – they need the right conditions to work properly, just like you need the right environment to study effectively!

Circulation & Plant Systems

Your blood is like a busy motorway system carrying everything your cells need. Plasma is the yellow liquid transport medium, whilst red blood cells are oxygen taxis packed with haemoglobin. White blood cells are your immune system's soldiers, and platelets act as emergency repair crews for cuts.

Your double circulatory system is incredibly efficient – one loop takes blood to your lungs for gas exchange, whilst the other pumps oxygenated blood around your entire body at high pressure. Arteries carry blood away from your heart with thick, muscular walls to handle the pressure. Veins bring blood back with valves preventing backflow, whilst capillaries form networks for exchanging substances with tissues.

Your heart is a four-chambered pump with atria receiving blood and ventricles pumping it out. Coronary heart disease occurs when fatty deposits block coronary arteries, but stents can hold them open and statins reduce cholesterol buildup.

Plants have their own transport systems: xylem carries water and minerals from roots to leaves, whilst phloem transports sugars made during photosynthesis around the plant through translocation. Transpiration creates the pressure difference that pulls water up through the plant.

Cool Connection: Your circulatory system and a plant's transport system both rely on pressure differences – it's physics powering biology!

Disease & Defence

Communicable diseases spread from person to person through pathogens – bacteria, viruses, fungi, or protists. Bacteria multiply rapidly and may produce toxins, whilst viruses hijack your cells' machinery to reproduce, eventually destroying them.

Diseases spread through droplet infection (coughing and sneezing), direct contact, or contaminated water and food. Salmonella food poisoning comes from undercooked food, gonorrhoea spreads through sexual contact, and malaria is carried by mosquito vectors containing protist parasites.

Your body has multiple defence lines: skin acts as a physical barrier with antimicrobial secretions, your respiratory system produces mucus to trap pathogens, and stomach acid destroys many microorganisms. Your immune system uses white blood cells to engulf pathogens, produce specific antibodies, and create antitoxins against bacterial poisons.

Plants defend themselves too, using cellulose cell walls, waxy cuticles, and bark as physical barriers. Many plants produce antibacterial chemicals or develop thorns and poisons to deter attackers. Some even mimic diseased plants to avoid being eaten!

Top Tip: Prevention is always better than cure – simple hygiene like handwashing prevents most infections from taking hold!

Immunity & Treatment

Your immune system is like having a personal army with an incredible memory. Every cell has unique antigens on its surface – when foreign antigens enter your body, white blood cells produce specific antibodies to neutralise them. Memory cells remember the right antibody, making you immune to future infections by the same pathogen.

Vaccination trains your immune system without causing disease. Dead or inactive pathogens in vaccines stimulate antibody production, so your body responds rapidly if you encounter the real pathogen later. Herd immunity protects entire populations when enough people are vaccinated.

Antibiotics kill bacteria without harming your cells, but they're useless against viruses and antibiotic resistance is becoming a serious problem. New medicines take up to 12 years and cost around £1700 million to develop, requiring extensive testing for effectiveness, safety, and stability.

Monoclonal antibodies are laboratory-produced proteins targeting specific cells. They're made by fusing mouse B lymphocytes with fast-growing tumour cells, creating hybridoma cells that mass-produce identical antibodies. These are used in pregnancy tests, disease diagnosis, and targeted cancer treatments.

Science Insight: Your immune system is constantly learning and adapting – it's like having a personal tutor that never forgets a lesson!

Non-communicable Diseases

Non-communicable diseases don't spread between people but can seriously impact your health. Smoking is particularly dangerous – carbon monoxide reduces oxygen-carrying capacity, tar accumulates in lungs causing COPD, and various chemicals increase cardiovascular disease risk.

Malignant tumours (cancers) spread around the body, unlike benign tumours that stay in one place. Most cancers result from mutations caused by carcinogens like tobacco smoke, ionising radiation, or UV light. Treatment involves radiotherapy (targeted radiation) or chemotherapy (cell-destroying chemicals).

Alcohol is processed by your liver, but excessive consumption leads to cirrhosis, liver cancer, and brain damage. Obesity increases type 2 diabetes risk, where your body either doesn't produce enough insulin or cells stop responding to it properly.

Regular exercise reduces cardiovascular disease and diabetes risk by increasing muscle tissue, improving heart fitness, and lowering blood cholesterol. A balanced diet prevents excess energy storage as fat whilst providing essential nutrients for cellular processes.

Life Hack: Small lifestyle changes like taking stairs instead of lifts or walking to school can significantly reduce your risk of developing serious diseases later!

Photosynthesis

Photosynthesis is arguably the most important reaction on Earth – it captures light energy and converts carbon dioxide and water into glucose and oxygen. This endothermic reaction requires energy input and happens mainly in chloroplasts containing the green pigment chlorophyll.

Three main factors limit photosynthesis rates: light intensity, temperature, and carbon dioxide concentration. Brighter light increases the rate until another factor becomes limiting. Higher temperatures speed up enzyme-controlled reactions until around 40-50°C when enzymes denature. CO₂ concentration is often the limiting factor on sunny days.

Greenhouses exploit these limiting factors commercially, controlling temperature, light, and CO₂ levels whilst growing plants in nutrient-rich water (hydroponics). This produces faster growth, cleaner crops, and higher profits, despite expensive monitoring equipment.

Plants use glucose in multiple ways: immediate respiration for energy, conversion to cellulose for structural strength, storage as starch, and combination with nitrate ions to make amino acids for proteins. Starch is insoluble, preventing osmotic problems that soluble glucose would cause.

Amazing Fact: Every breath of oxygen you take was produced by photosynthesis – you're literally breathing the waste product of plants!

Respiration

Aerobic respiration is your cellular power station, combining glucose with oxygen to release energy for everything your body does. This exothermic reaction happens mainly in mitochondria, which have folded membranes providing massive surface areas for the enzyme-controlled reactions.

When oxygen runs short during intense exercise, your muscles switch to anaerobic respiration, breaking down glucose incompletely to produce lactic acid. This creates an oxygen debt that must be repaid after exercise, explaining why you keep breathing heavily even after stopping.

Yeast uses anaerobic respiration (fermentation) to produce ethanol and carbon dioxide – the basis of bread-making and alcohol production. Plant cells can also respire anaerobically, though they produce ethanol rather than lactic acid.

Metabolism encompasses all the chemical reactions in your body. Your liver is metabolically hyperactive, detoxifying poisons like alcohol, recycling old blood cells, storing iron, and converting lactic acid back to glucose. The liver also breaks down excess proteins to form urea for kidney excretion.

Energy Connection: Every movement you make, every thought you think, and every heartbeat relies on energy from respiration – you're essentially a walking power station!

Nervous System & Control

Homeostasis keeps your internal environment stable – regulating temperature, blood glucose, and water content for optimal cellular function. Your nervous system provides rapid responses through electrical impulses, whilst the hormone system gives slower, longer-lasting chemical control.

Your brain has specialised regions: the cerebral cortex handles consciousness and intelligence, the cerebellum coordinates balance, the medulla controls unconscious activities like breathing, and the hypothalamus regulates body temperature. Scientists study brain function through brain damage cases, electrical stimulation, and MRI scans.

Reflex actions bypass conscious thought for rapid responses to danger. Sensory neurones carry impulses from receptors to the CNS, relay neurones process information in the spinal cord, and motor neurones trigger responses in effector organs. Synapses between neurones use chemicals to transmit electrical impulses across gaps.

Your eyes automatically adjust to different conditions through accommodation – ciliary muscles and suspensory ligaments change lens shape for focusing. Vision problems like myopia (short-sightedness) occur when eyeballs are too long, but can be corrected with concave lenses.

Mind-Blowing: Your nervous system processes millions of signals every second – it's more powerful than any computer ever built!