Cambridge O level Biology

Biology exams reward students who can describe an experiment and explain what it proves, not just recite definitions. This book delivers exactly that pairing, so it suits O level and IGCSE candidates, homeschooling parents, and teachers hunting for clear practicals. If you want definitions, demonstrations and exam-ready reasoning in one place, this is your workbook.

1. 1Classification

   Five kingdoms and one name for you

   Humans sit in the binomial system as Homo sapiens, a two-part name that works the same way for every organism on Earth. Around that naming scheme, the book sorts all living things into five kingdoms: animals, plants, fungi, protoctists and prokaryotes.

   The idea

   Naming is not bookkeeping; it is the foundation of every claim biology makes. Once every organism has a two-part name and a kingdom, you can say something precise about a new species and have it understood anywhere.

   Try this: Pick five organisms you saw this week and assign each a kingdom, then write its binomial-style name from memory.

2. 2Movement in and out of cells

   Glucose gets through, starch stays put

   The reason glucose passes out of the tubing while starch never leaves is size — starch molecules are simply too large to diffuse through the tubing. The definitions land: diffusion is the net movement of molecules from higher to lower concentration down a concentration gradient, driven by random movement, and osmosis is the same story for water, moving from a dilute to a concentrated solution through a partially permeable membrane.

   The idea

   'Partially permeable' is not a vague phrase but a physical fact: molecule size decides who travels and who stays. Diffusion and osmosis are defined by this behaviour, and the practical appears in exams year after year.

   Try this: Soak a strip of Visking tubing, fill it with glucose and starch solution, and test the surrounding water for glucose after an hour.

3. 3Enzymes

   A potato that fizzes

   The enzyme catalase in the potato tissue — liver works too — tears hydrogen peroxide apart. Enzymes are protein catalysts that speed reactions without being used up. The lock and key hypothesis explains why catalase only does this job — the substrate fits the active site like a key fits a lock. Temperature has the final word: activity climbs to an optimum of about 37°C for human enzymes, then falls as the enzyme denatures.

   The idea

   The lock-and-key picture explains the fierce specificity that lets thousands of reactions run side by side. More heat is not always better — past the optimum, the catalyst destroys itself.

   Try this: Add hydrogen peroxide to a small piece of raw potato and relight a splint to test the bubbles for oxygen.

4. 4Plant nutrition

   Foil on a leaf settles the argument

   The word equation is carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll. The destarched plant with a partly covered leaf, tested with iodine, shows starch present only where light reached — proof that light is necessary for photosynthesis.

   The idea

   The plant becomes its own control — same leaf, same CO2, same chlorophyll, only light differs — and the iodine test reads out the answer as a colour map.

   Try this: Clip foil over part of a leaf on a sunny windowsill, then iodine-test the leaf tomorrow and compare the two halves.

5. 5Animal nutrition

   Nine metres of you

   The alimentary canal runs from mouth to anus: mouth, oesophagus, stomach, small intestine split into duodenum and ileum, large intestine with colon and rectum, then the anus. The balanced diet consists of carbohydrates, fats, proteins, vitamins, minerals, water and dietary fibre, in correct proportions to maintain good health.

   The idea

   Each region of the canal is specialised for a stage of the job. The diet framework lists seven nutrient classes in correct proportions, with one goal of good health.

   Try this: Sketch the alimentary canal from memory and label each region, then check it against the book's list.

6. 6Transport in plants

   Counting bubbles up a glass tube

   The potometer measures the rate of water uptake — a stand-in for the transpiration rate. Two transport systems run behind it: xylem vessels carry water and dissolved mineral salts from roots to leaves, their lignin-strengthened walls also propping the plant up, while phloem moves sucrose and amino acids from leaves to the rest of the plant — translocation, in one word.

   The idea

   The potometer turns a slow, invisible process into a visible moving mark you can time. Xylem's lignin serves transport and structural support in the same tissue.

   Try this: Set up a simple potometer with a geranium shoot and time how far the air bubble travels in ten minutes.

7. 7Transport in animals

   Four chambers, one thick-walled pump

   The heart has four chambers: right atrium, right ventricle, left atrium, left ventricle. The left ventricle has the thickest muscular wall, built to push blood around the whole body. Red blood cells contain haemoglobin, which combines with oxygen in the lungs to form oxyhaemoglobin.

   The idea

   The left ventricle's thick wall is the muscle needed for the longest, highest-pressure route in the circulation. Haemoglobin gives the blood its high-capacity oxygen ferry.

   Try this: Draw the heart's four chambers and mark which has the thickest wall, then trace one red blood cell's full circuit.

8. 8Respiration

   Seeds that warm a flask

   Germinating seeds in a vacuum flask show a temperature rise: respiration is releasing heat energy, and the flask's insulation traps it. The chemistry is aerobic respiration, glucose + oxygen → carbon dioxide + water. In vigorous exercise, human muscles run anaerobic respiration when oxygen supply falls short, producing lactic acid and an oxygen debt you repay breathing hard afterwards. Exhaled air turns limewater milky faster than inhaled air does — respiration's carbon dioxide, caught on camera.

   The idea

   Respiration is an energy release you can weigh as heat. The limewater test catches the second product, CO2, and the exercise case shows what happens when oxygen cannot keep up.

   Try this: Fill a vacuum flask with soaked, germinating seeds and log the thermometer reading every few hours for a day.

9. 9Excretion

   The kidney's million tiny filters

   The nephron is the functional unit of the kidney. At the glomerulus and Bowman's capsule, ultrafiltration strains the blood; the filtrate then runs through the proximal convoluted tubule, the loop of Henle and the distal convoluted tubule, where glucose, some water and salts are selectively reabsorbed. What remains — urea, excess water and salts — drains as urine down the ureters to the bladder. The kidneys are the main excretory organs, and excretion is the removal from the body of toxic materials, metabolic waste products and substances in excess of requirements.

   The idea

   The kidney filters almost everything, then takes back exactly what the body needs — selective reabsorption matters more than the filtering itself. Excretion is waste from metabolism, not just anything the body ejects.

   Try this: Write the nephron's parts in order from Bowman's capsule down, and note beside each what is reabsorbed there.

10. 10Coordination and response

    A hand snatched from a hot pan

    In the reflex arc, pain and touch receptors in the skin fire impulses along a sensory neurone to a relay neurone in the spinal cord, which hands the signal to a motor neurone running to a muscle. Between the neurones sit synapses, tiny gaps crossed by chemical transmitter substances released to pass the impulse on. In the eye, to focus near, ciliary muscles contract, suspensory ligaments slacken and the lens turns more convex; for distance, the muscles relax, ligaments tighten, lens flattens. When the situation is frightening, adrenal glands release adrenaline — heart rate up, blood pressure up, blood diverted to muscles, the body braced for fight or flight.

    The idea

    Reflex arcs bypass the brain for speed, and hormones broadcast slower, longer-lasting orders through the blood. The lens does not focus itself; it is squeezed and released by muscles around it.

    Try this: Close your eyes, have someone hold a pen at different distances, and notice how your focus shifts — then describe what your ciliary muscles are doing.

11. 11Reproduction

    Clones, gametes and a 28-day clock

    Asexual reproduction makes clones, whether by binary fission in bacteria, budding in yeast or vegetative propagation in plants. Sexual reproduction fuses male and female gamete nuclei into a zygote, and the offspring carry genetic variation. In humans, the menstrual cycle averages about 28 days with ovulation around day 14; the uterine lining thickens ready for implantation, and progesterone maintains it if fertilisation happens. Fertilisation occurs in the oviduct, where sperm nucleus meets egg nucleus; the zygote implants, develops a placenta and umbilical cord, and the amniotic sac fills with fluid to cushion the fetus. In plants, a typical flower — a buttercup or lily — carries sepals, petals, stamens with anther and filament, and carpels with stigma, style and ovary, and pollination is pollen transferred from anther to stigma.

    The idea

    Cloning copies a proven success; sexual reproduction shuffles the deck so some offspring survive whatever comes next. Flowers are structures built around one transfer, pollen from anther to stigma.

    Try this: Dissect a large flower, tape each part — sepal, petal, stamen, carpel — to paper and label them all.

12. 12Inheritance

    Mendel's peas and the 3:1 fingerprint

    Gregor Mendel crossed tall and short pea plants: the first generation was all tall, and the second generation split roughly 3 tall to 1 short — the ratio that became the fingerprint of inheritance. His work was not recognised until many years after his death. A gene is a length of DNA coding for a particular protein, alleles are different forms of the same gene, and chromosomes are thread-like DNA structures in the nucleus. Punnett squares make it mechanical — cross pure-breeding tall (TT) with dwarf (tt), or two heterozygotes Tt × Tt, and the 3:1 dominant-to-recessive ratio falls out; the same logic gives Bb × Bb for brown and blue eyes, brown dominant. Sex chromosomes settle the last question: females XX, males XY, a 1:1 ratio of sons to daughters.

    The idea

    The Punnett square turns Mendel's insight into arithmetic anyone can check. Genes, alleles and chromosomes are the vocabulary; the ratio is the proof they were real all along.

    Try this: Draw a Punnett square for Tt × Tt, count the genotypes, and confirm the 3:1 phenotype ratio yourself.

13. 13Variation and selection

    A moth that turned black to survive

    Before the Industrial Revolution, peppered moths — Biston betularia — rested on lichen-covered tree trunks, and light-coloured wings made them nearly invisible. Then pollution blackened the bark. On sooty trunks the pale moths stood out, and bird predators found them; dark-coloured moths, once conspicuous, now had the advantage and became the more common form. Variation comes in two kinds: continuous variation shows a range of values like height or weight, while discontinuous variation falls into distinct categories like blood group.

    The idea

    Selection needs variation, an environmental change, and a difference in survival — and the peppered moth supplies all three within a single human lifetime. Continuous and discontinuous traits are inherited and measured differently.

    Try this: Cut pale and dark paper moth shapes, scatter them on light and dark backgrounds, and time how long each takes a friend to find.

14. 14Organisms and their environment / Human influences / Biotechnology

    Grass, rabbit, fox — and a warming sky

    The food chain grass → rabbit → fox traces energy from a producer — always a plant — through the consumers that eat it. The carbon cycle recycles the same atoms: plants take up carbon dioxide in photosynthesis, animals get carbon by feeding, and respiration, decomposition and the combustion of fossil fuels send it back to the atmosphere. Deforestation reduces biodiversity, increases soil erosion, and raises atmospheric carbon dioxide because the trees that would absorb CO2 are removed or burned. In a bakery, yeast fermenting sugars releases carbon dioxide that makes dough rise; the same microbe produces ethanol and CO2 in brewing.

    The idea

    Every arrow in a food chain is energy moving once, never backwards. The carbon cycle applies the same discipline to atoms, with fossil fuels adding a new, one-way withdrawal. Deforestation breaks the cycle at its intake end — fewer trees, less CO2 absorbed, more released when they burn.

    Try this: Write a four-organism food chain from your garden or local park and label the producer and each consumer's level.

O level and IGCSE biology students who want definitions tied to experiments they can picture, plus teachers and homeschooling parents building practical-led lessons. Also ideal for anyone returning to biology after years away and wanting the fundamentals re-anchored in memorable demonstrations.

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