Biology

90 guides

Cells

• Animal and plant cells: GCSE structures and differencesAnimal and plant cells for GCSE Biology: labelled diagrams, what each part does, the differences between them, and how bacterial cells compare.
• Binary fission: how bacteria divide, stages and mathsBinary fission is how bacteria divide. The stages with a diagram, binary fission vs mitosis, and how to work out bacterial numbers for GCSE and A-level.
• Cell simulator: organelles, diffusion and osmosisFree cell simulator: explore labelled animal, plant and bacterial cells, then run diffusion and osmosis across a membrane with live water potential.
• Chloroplast structure and function: labelled guideChloroplast structure explained: envelope, thylakoids, grana, stroma, DNA and starch, what each part does in photosynthesis, and how it is adapted.
• Co-transport: how glucose is absorbed in the ileumWhat co-transport is, how sodium ions and glucose are absorbed together in the ileum, why it counts as active transport, and exam answers.
• Cytoskeleton: structure and functions explainedWhat the cytoskeleton is, its three protein fibres (microfilaments, intermediate filaments and microtubules), and how it supports and moves cells.
• Eukaryotic and prokaryotic cells: differences and examplesThe differences between eukaryotic and prokaryotic cells for GCSE and A-level Biology: nucleus, DNA, plasmids, size and cell walls, in a comparison table.
• Facilitated diffusion: channel and carrier proteinsWhat facilitated diffusion is, how channel and carrier proteins work, examples, the rate graph, and how it compares with diffusion and active transport.
• Fluid mosaic model: membrane structure explainedThe fluid mosaic model of membranes for A-level Biology: phospholipids, proteins, glycoproteins and cholesterol, why it is fluid, and a key practical.
• Interphase: G1, S and G2 phases of the cell cycleInterphase is when a cell grows and copies its DNA before dividing. What happens in G1, S and G2, how long it lasts, checkpoints, and worked examples.
• Magnification calculator: image size ÷ actual sizeFind magnification, image size or actual size in cm, mm, µm or nm with the working, or calibrate an eyepiece graticule against a stage micrometer.
• Microscope diagram: labelled parts and what they doThe labelled parts of a light microscope and what each one does, how to use it step by step, and how to calculate magnification, for GCSE Biology.
• Mitochondria diagram: structure and function, labelledA labelled mitochondria diagram, the structure of a mitochondrion, what each part does in aerobic respiration, and how to draw and label one in the exam.
• Osmosis required practical: potato method and resultsThe osmosis required practical with potato cylinders: equipment, method, variables, results, percentage change in mass, the graph, errors and exam tips.
• Products of cellular respiration: equation, stages, ATPThe products of cellular respiration are carbon dioxide, water and ATP. See the equation, the reactants, each stage, and aerobic vs anaerobic respiration.
• Specialised cells: GCSE examples and adaptationsSpecialised cells for GCSE Biology: how sperm, nerve, muscle, root hair, xylem and phloem cells are adapted to their jobs, with a table and exam tips.
• The cell cycle GCSE: the three stages explainedThe cell cycle for GCSE Biology: the three stages, what happens to DNA and organelles, why mitosis matters, exam answers, and what A-level adds.
• Totipotent cells: meaning, examples and other stem cellsWhat totipotent means, where totipotent cells are found, how they differ from pluripotent, multipotent and unipotent stem cells, and totipotency in plants.
• Virtual microscope: focus, magnify and measure cellsFree virtual microscope: focus on onion cells, blood, a leaf and live pond water, switch objectives from ×4 to ×100 and measure cells with a scale bar.
• What does the cell membrane do? Structure and functionsWhat the cell membrane does: control what enters and leaves the cell, transport, signalling and recognition, with its fluid mosaic structure explained.
• What is mitosis? Stages, purpose and mitosis vs meiosisMitosis is the cell division that makes two identical cells for growth and repair. Learn every stage, the purpose of mitosis, cytokinesis and examples.
• What is osmosis? Definition, examples and diagramsWhat osmosis is, in exam ready wording: why water moves, whether it is active or passive, what it does to plant and animal cells, and the potato practical.
• Where does photosynthesis take place? Equation and stagesWhere photosynthesis takes place, from leaf to chloroplast, the equation, reactants and products, both stages, the rate, and how it links to respiration.

Biological molecules

• Alpha glucose and beta glucose: structure and differencesThe structure of alpha glucose and beta glucose, the one difference between them, how glycosidic bonds form, and why it matters for starch and cellulose.
• Biological catalysts are enzymes: how enzymes workBiological catalysts are called enzymes. How enzymes work, the active site and lock and key model, temperature, pH and denaturation, and digestive enzymes.
• Biological molecules: A-level Biology topic overviewBiological molecules for A-level Biology: carbohydrates, lipids, proteins, nucleic acids, ATP, water and ions, with their monomers, bonds and food tests.
• Disaccharides: maltose, sucrose and lactose explainedWhat a disaccharide is, how maltose, sucrose and lactose form and are hydrolysed, glycosidic bonds, the non-reducing sugar test and lactose intolerance.
• Ester bond: what it is and how it forms in lipidsWhat an ester bond is, how condensation forms three in a triglyceride, how hydrolysis breaks them, and how they compare with glycosidic and peptide bonds.
• Food tests: Benedict's, iodine, biuret and lipid testsThe food tests required practical: Benedict's test for sugars, iodine test for starch, biuret test for protein and the lipid test, with method and results.
• Induced fit model of enzyme action explainedWhat the induced fit model of enzyme action says, how it differs from lock and key, how it lowers activation energy, and model answers for A-level.
• Monosaccharides: definition, examples and structureMonosaccharides for A-level Biology: trioses, pentoses and hexoses, glucose, fructose and galactose, reducing sugars, and how disaccharides form.
• Phospholipid structure: head, tails and the bilayerPhospholipid structure for A-level Biology: the phosphate head and fatty acid tails, how they differ from triglycerides, and why they form a bilayer.
• Primary structure of protein: amino acids and peptide bondsThe primary structure of a protein is its sequence of amino acids. How peptide bonds form, how the sequence decides the 3D shape, and worked examples.
• Starch, glycogen and cellulose: structure and functionThe structure of starch, glycogen and cellulose: alpha and beta glucose, glycosidic bonds, branching, microfibrils, and how each fits its function.

Organisation and transport

• Alveoli adaptations: how the lungs exchange gasesAlveoli adaptations for GCSE Biology: huge surface area, thin walls, a rich blood supply and ventilation, and how each one speeds up diffusion.
• Cardiac output calculator: CO = HR × SV, with stepsFind cardiac output from heart rate and stroke volume, rearrange for either, and work out stroke volume and ejection fraction from ventricle volumes.
• Digestive system GCSE: organs, enzymes and bileThe GCSE digestive system: what each organ does, where amylase, protease and lipase are made, what bile does, and how villi absorb food.
• Heart diagram GCSE: labelled parts and blood flowA labelled heart diagram for GCSE Biology: the four chambers, the blood vessels, the valves, the path of blood through the heart and cardiac output.
• Leaf structure: labelled diagram and what each part doesThe structure of a leaf, with a labelled cross-section: epidermis, palisade and spongy mesophyll, veins, stomata and guard cells, and what each does.
• Organisation biology: GCSE topic summary and key factsThe GCSE Biology organisation topic in one place: cells to organ systems, digestion, the heart and blood, heart disease, cancer and plant transport.
• Oxygen dissociation curve: shape, Bohr effect, examplesWhat the oxygen dissociation curve shows, why it is S-shaped, how the Bohr effect shifts it to the right, fetal haemoglobin, and how to read the curve.
• Should an ideal exchange surface be thick or thin?An ideal exchange surface is thin, for a short diffusion path. Why it matters, the other features, Fick's law, and examples from lungs to leaves.
• Tissue fluid: how it forms and returns to the bloodWhat tissue fluid is, how hydrostatic pressure forms it at the arteriole end, how water returns at the venule end, the lymph system and oedema.
• What does the P wave represent? ECG and earthquake P wavesWhat the P wave represents on an ECG (atrial depolarisation), how to read it with the PR interval, QRS and T wave, and what a P-wave is in an earthquake.
• Xylem and phloem: structure, function and differencesXylem and phloem compared: what each carries, how their structure suits the job, transpiration and translocation, and what affects transpiration.

Nerves and homeostasis

• Eye diagram GCSE: labelled parts and what they doA labelled GCSE eye diagram with the job of every part, how the eye focuses on near and distant objects, short and long sight, and exam tips.
• Homeostasis and response: GCSE Biology revision guideGCSE homeostasis and response explained: control systems, the nervous system, hormones, blood glucose, body temperature, water balance and plant hormones.
• Muscle fibres: structure, types and how they get energyWhat a skeletal muscle fibre is, its parts and their jobs, slow and fast twitch fibres compared, and how fibres make ATP, with A-level worked examples.
• Neuromuscular junction: structure and how it worksNeuromuscular junction for A-level Biology: its structure, each step from impulse to contraction, acetylcholinesterase, and how it differs from a synapse.
• Osmoregulation: ADH, the kidneys and negative feedbackOsmoregulation for GCSE and A-level Biology: how the kidneys and ADH control the water potential of the blood by negative feedback, plus the nephron.
• Pacinian corpuscle: structure and generator potentialHow the Pacinian corpuscle works: its lamellae, stretch-mediated sodium channels, the generator potential and action potentials, for A-level Biology.
• Reflex arc diagram: GCSE labels and how it worksA labelled reflex arc diagram for GCSE Biology: receptor, sensory, relay and motor neurones, synapses and effector, with a worked example and exam tips.
• Saltatory conduction: how myelin speeds up nerve impulsesSaltatory conduction explained: how nerve impulses jump between nodes of Ranvier on myelinated axons, why it is faster, and other factors affecting speed.
• Sliding filament theory: muscle contraction step by stepThe sliding filament theory for A-level Biology: sarcomere bands, the roles of calcium, tropomyosin and ATP, the cross-bridge cycle and muscle fibre types.
• The ornithine cycle: how the liver makes ureaWhat the ornithine cycle is, why the liver turns toxic ammonia into urea, deamination, the steps through citrulline and arginine, and the equations.

Respiration and photosynthesis

• Calvin cycle A-level Biology: the 3 stages explainedThe Calvin cycle for A-level Biology: fixation by rubisco, reduction of GP to triose phosphate, regeneration of RuBP, the ATP and reduced NADP it uses.
• Glycolysis for A-level Biology: stages, products, ATPGlycolysis for A-level Biology: where it happens, the three stages from glucose to pyruvate, the net gain of ATP and reduced NAD, and what happens next.
• Krebs cycle for A-level Biology: steps, products, diagramThe Krebs cycle for A-level Biology: where it happens, each step with a diagram, the products per turn and per glucose, and its place in respiration.
• Link reaction: what happens, products and equationWhat happens in the link reaction of aerobic respiration, where it takes place, its equation, and its products per pyruvate and per glucose.
• Oxidative phosphorylation: electron transfer chain and ATPOxidative phosphorylation for A-level Biology: the electron transfer chain, proton gradient, ATP synthase and chemiosmosis, and why oxygen is needed.
• Photosynthesis required practical: pondweed and lightThe photosynthesis required practical: oxygen from pondweed as a lamp is moved, with equipment, method, variables, the inverse square law, errors and tips.

Disease and immunity

• Antibody structure: what antibodies are and how they workAntibody structure explained: the chains, variable regions and binding sites, how antibodies fight infection, and how monoclonal antibodies are made.
• Infection and response: GCSE Biology topic summaryGCSE Biology infection and response in one place: pathogens, the diseases you need, how the body defends itself, vaccines, antibiotics and new drugs.
• Is malaria a protist? What causes malaria, explainedYes: malaria is caused by Plasmodium, a single-celled protist spread by mosquitoes. Learn its life cycle, symptoms, control and the GCSE key facts.
• White blood cell diagram: types, features and jobsA white blood cell diagram drawn to scale, the five types and how to tell them apart, what phagocytes and lymphocytes do, and GCSE exam tips.

Genetics and inheritance

• Autosomal linkage: linked genes, crosses and ratiosWhat autosomal linkage is, why linked genes change dihybrid ratios, how crossing over makes recombinants, and how to spot linkage with chi-squared.
• Codon chart: the full mRNA codon table and how to read itThe full codon chart for all 64 mRNA codons, how to read it, start and stop codons, and a free DNA to mRNA converter that translates any sequence.
• Dihybrid cross: Punnett square, 9:3:3:1 ratio, examplesHow to do a dihybrid cross: working out the gametes, the 16-box Punnett square, why it gives 9:3:3:1, other ratios, and testing results with chi-squared.
• Gel electrophoresis: how it works, steps and resultsHow gel electrophoresis separates DNA fragments by size, the method step by step, how to read a gel against a DNA ladder, and its use in DNA profiling.
• Hardy-Weinberg calculator: p, q, 2pq and chi-squaredFree Hardy-Weinberg calculator: find p, q, p², 2pq and q² from a phenotype share, an allele frequency or genotype counts, with a chi-squared test.
• Lac operon: how it works, with and without lactoseThe lac operon for A-level Biology: its parts, what happens with and without lactose, the role of glucose and cAMP, and why bacteria control genes so.
• Protein synthesis GCSE: transcription and translationProtein synthesis for GCSE Biology in simple steps: DNA as a template, mRNA, ribosomes and carrier molecules, folding, and how mutations change proteins.
• Punnett square calculator: monohybrid and dihybridMake a Punnett square for any monohybrid or dihybrid cross, with genotype and phenotype ratios, the chance of each outcome and worked examples.
• Selective breeding: process, examples, benefits and risksSelective breeding for GCSE Biology: what it is, the steps breeders follow, examples, benefits and risks including inbreeding, and how it compares with GM.
• What does DNA stand for? DNA meaning, structure and functionDNA stands for deoxyribonucleic acid. Learn what DNA is made of, the sugar in it, what it does, who discovered it and why it matters, with examples.
• What is meiosis? Purpose, stages and meiosis vs mitosisMeiosis makes four genetically different sex cells with half the chromosomes. Learn its purpose, every stage, and how it differs from mitosis.
• When does DNA replication occur? Steps, enzymes, whereDNA replication happens in S phase, before a cell divides. Learn where it occurs, what each enzyme does, the steps and semi-conservative replication.

Ecology and evolution

• Belt transect: method, results and how to analyse themWhat a belt transect is, how to carry one out step by step, recording percentage cover and frequency, and analysing the results, with worked examples.
• Capture recapture formula: N = MC ÷ R, with examplesThe capture recapture formula N = MC ÷ R (the Lincoln index) explained: how mark-release-recapture works, examples, assumptions and sources of error.
• Index of diversity: Simpson's diversity index with examplesHow to calculate an index of diversity: the AQA formula and Simpson's diversity index, what the values mean, and worked examples with a free calculator.
• Natural selection simulation: watch a population evolveFree natural selection simulation: set predators, ground colour, climate and food, and watch a population's colour, speed and size evolve over generations.
• Nutrient cycles: the nitrogen and phosphorus cyclesHow nutrient cycles work: the nitrogen and phosphorus cycles, saprobionts, mycorrhizae, fertilisers, leaching and eutrophication, with exam answers.
• Population growth calculator: exponential and logisticWork out exponential and logistic population growth: N after a time, doubling time, growth rate and carrying capacity, with a simulation and examples.
• Simpson's diversity index calculator and mark-recaptureFree ecology calculator: find Simpson's diversity index from species counts, or estimate a population by mark-release-recapture with a 95% interval.
• Spearman's rank: formula, steps and critical valuesHow to do a Spearman's rank test: the formula, ranking with ties, a worked biology example, critical values for p = 0.05 and how to write the conclusion.
• Sympatric speciation: definition, examples and polyploidySympatric speciation for A-level Biology: how a new species forms without a barrier, polyploidy step by step, examples, and allopatric vs sympatric.
• The 5 kingdoms of life: features and examples of eachThe 5 kingdoms explained: the features of prokaryotes, protoctists, fungi, plants and animals, with examples, the taxonomic hierarchy and three domains.

Courses

• Cell biology course: organelles, transport and divisionAn interactive cell biology course in 25 short sections: organelles, the membrane, diffusion, osmosis, active transport, respiration and mitosis.
• Genetics course: DNA, inheritance and CRISPR, interactiveAn interactive genetics course in 24 short sections: DNA structure, replication, protein synthesis, mutations, Punnett squares, meiosis and CRISPR.
• Human physiology course: how the body's systems workAn interactive human physiology course in 25 short sections: the heart, blood, breathing, gas exchange, nerves, digestion, immunity and homeostasis.