Edexcel IGCSE Science (Double Award) · Biology · 4SD0/1B

Double Award Biology · Topics 1–5 — Revision Pack

Pearson Edexcel International GCSE Science (Double Award) 4SD0 — Biology Paper 1 (4SD0/1B), same core Biology as 4BI1/1B. English revision sheets for Year 10 / IGCSE.

Double Science Biology (Paper 1 core)

This pack covers Double Award Biology only. It excludes bold Biology-only content that appears in the separate Biology specification for Paper 2 (4BI1/2B). Cross-check with your latest Issue of the 4SD0 specification.

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Topic 1 · Nature & variety of living organisms

MRS GREN characteristics; plants, animals, fungi, protoctists, bacteria, viruses & pathogens.

1. Outline

  • All living organisms share the same life processes (MRS GREN).
  • Organisms are grouped by common features: plants, animals, fungi, protoctists, bacteria.
  • Viruses are not cells and are not considered living; they are pathogens.
  • Pathogens include some fungi, bacteria, protoctists and all viruses.

2. Characteristics of life — MRS GREN

Living organisms share these characteristics:

LetterProcessMeaning
MMovementChange position or orientation (animals move; plants may grow toward light).
RRespirationRelease energy from food (aerobic or anaerobic).
SSensitivityDetect and respond to stimuli (light, chemicals, temperature…).
GGrowthPermanent increase in size and dry mass; development of cells/tissues.
RReproductionProduce offspring (asexual or sexual).
EExcretionRemove waste products of metabolism (e.g. CO2, urea).
NNutritionObtain food / nutrients for energy and materials.
Exam tip

Homeostasis (controlling internal conditions) is also listed in the specification with these processes — link it to excretion, temperature control and water balance later in Topic 2.

3. Variety of living organisms

Plants (eukaryotic)

  • Multicellular; cells have chloroplasts → photosynthesis.
  • Cellulose cell walls; store carbohydrate as starch or sucrose.
  • Examples: flowering plants (maize, peas/beans).

Animals (eukaryotic)

  • Multicellular; no chloroplasts / no photosynthesis; no cell walls.
  • Usually nervous coordination; can move from place to place.
  • Store carbohydrate as glycogen. Examples: mammals (humans), insects (housefly, mosquito).

Fungi (eukaryotic)

  • Cannot photosynthesise. Body often a mycelium of hyphae (many nuclei); some are single-celled (yeast).
  • Cell walls of chitin. Saprotrophic nutrition: secrete enzymes onto food → absorb products.
  • May store carbohydrate as glycogen. Examples: Mucor, yeast.

Protoctists (eukaryotic)

  • Microscopic, usually single-celled.
  • Some animal-like (Amoeba); some plant-like with chloroplasts (Chlorella).
  • Pathogenic example: Plasmodium (malaria).

Bacteria (prokaryotic)

  • Microscopic single cells; cell wall, membrane, cytoplasm, plasmids; no true nucleus — circular DNA chromosome.
  • Most feed on living/dead organisms; a few photosynthesise.
  • Examples: Lactobacillus bulgaricus (yoghurt); Pneumococcus (pneumonia).

Viruses (not living cells)

  • Much smaller than bacteria; protein coat + DNA or RNA; no cellular structure.
  • Parasitic — reproduce only inside living host cells.
  • Examples: tobacco mosaic virus (TMV); influenza; HIV.

4. Pathogens

A pathogen is a microorganism that causes disease. Pathogens may be:

  • Fungi (e.g. athlete’s foot)
  • Bacteria (e.g. Pneumococcus → pneumonia)
  • Protoctists (e.g. Plasmodium → malaria)
  • Viruses (e.g. influenza, HIV, TMV)
Remember

Viruses are pathogens but are not living organisms — they lack cell structure and cannot respire or grow independently.

5. Self-check questions

Try these yourself first. Answers are in the next section.

  1. List the seven MRS GREN life processes.
  2. Give two differences between plant and animal cells / organisms from this topic.
  3. How do fungi obtain nutrients?
  4. Why are viruses not classified as living organisms?
  5. Name one pathogenic bacterium, one pathogenic protoctist and one virus.

6. Self-check answers

Tap / click to show answers
  1. Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition.
  2. Plants: chloroplasts / photosynthesis, cellulose walls, store starch/sucrose. Animals: no chloroplasts/walls, store glycogen, usually nervous coordination & locomotion.
  3. Saprotrophic nutrition — extracellular enzyme secretion then absorption of products.
  4. No cellular structure; only reproduce inside host cells; cannot carry out life processes independently.
  5. e.g. Pneumococcus; Plasmodium; influenza / HIV / TMV.
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Topic 2 · Structure & functions in living organisms

Cells, molecules, enzymes, transport, nutrition, respiration, gas exchange, circulation, excretion, coordination.

1. Levels of organisation

Organisationorganelles → cells → tissues → organs → organ systems → organism

  • Tissue — group of similar cells with a shared function (e.g. muscle tissue).
  • Organ — several tissues working together (e.g. leaf, heart).
  • Organ system — organs cooperating (e.g. digestive system, circulatory system).

2. Cell structure

Animal cell nucleus mitochondrion cytoplasm cell membrane Plant cell cell wall nucleus chloroplast vacuole
Animal cell (left) vs plant cell (right): plant cells also have a cell wall, chloroplasts and a large permanent vacuole.
StructureAnimalPlantFunction
Nucleus✓✓Contains DNA; controls cell activities
Cytoplasm✓✓Site of chemical reactions
Cell membrane✓✓Controls entry/exit of substances
Mitochondria✓✓Aerobic respiration → ATP
Ribosomes✓✓Protein synthesis
Cell wall (cellulose)—✓Support / shape
Chloroplasts—✓Photosynthesis
Permanent vacuole—✓Cell sap; support (turgor)

Specialised cells (examples): root hair (large surface area for water/minerals); palisade mesophyll (many chloroplasts); red blood cell (no nucleus, haemoglobin, biconcave); sperm (flagellum, mitochondria); egg (nutrient stores); ciliated epithelial (cilia move mucus).

3. Biological molecules & food tests

MoleculeElementsRoleFood test
Carbohydrates (sugars, starch)C, H, OEnergy store / sourceStarch: iodine → blue-black. Reducing sugar: Benedict’s + heat → brick red
ProteinsC, H, O, N (S)Enzymes, structure, hormonesBiuret → purple / lilac
Lipids (fats/oils)C, H, OEnergy store, membranes, insulationEthanol emulsion → cloudy white
Practical note

Benedict’s test needs heat (water bath). Iodine and Biuret do not. Emulsion test: crush food + ethanol, then add water.

4. Enzymes

Enzymes are biological catalysts (usually proteins). They speed up reactions without being used up. Specific: each enzyme has an active site that fits a substrate (lock and key).

Lock-and-key model enzyme (active site) substrate → enzyme–substrate complex → products + enzyme unchanged
Substrate fits the active site; reaction happens; products leave; enzyme is reused.
  • Temperature: rate rises to an optimum, then falls as enzyme denatures (active site shape changes).
  • pH: each enzyme has an optimum pH; extremes denature.
  • Substrate concentration: rate rises until all active sites are occupied (plateau).

Digestive enzymes (examples)Amylase: starch → maltose  |  Protease: protein → amino acids  |  Lipase: lipids → fatty acids + glycerol

5. Movement of substances

ProcessDefinitionEnergy?
DiffusionNet movement of particles from high → low concentration (down gradient)No (passive)
OsmosisNet movement of water from dilute → concentrated solution across a partially permeable membraneNo (passive)
Active transportMovement against a concentration gradient using energy from respirationYes (ATP)
Examples

Diffusion: O2/CO2 in lungs & leaves. Osmosis: water into root hairs. Active transport: mineral ions into roots; glucose into blood from ileum.

6. Plant nutrition — photosynthesis

Word / symbol equationcarbon dioxide + water → glucose + oxygen
6CO2 + 6H2O → C6H12O6 + 6O2  (light + chlorophyll)

  • Limiting factors: light intensity, CO2 concentration, temperature (enzymes).
  • Leaf adaptations: large surface area; thin; stomata for gas exchange; palisade cells packed with chloroplasts; xylem/phloem in veins.
  • Minerals: nitrate → amino acids/proteins; magnesium → chlorophyll.

7. Human nutrition & digestion

Balanced diet: carbohydrates, proteins, lipids, vitamins, minerals, fibre, water.

RegionKey points
MouthChewing; salivary amylase starts starch digestion
StomachAcid (HCl) kills pathogens; protease (pepsin); churning
Small intestineBile emulsifies fats; pancreatic enzymes; absorption in ileum (villi ↑ surface area)
Large intestineWater absorbed; faeces formed

Peristalsis — wave of muscle contraction that moves food along the gut.

8. Respiration

Respiration releases energy from glucose for life processes (not the same as breathing).

Aerobicglucose + oxygen → carbon dioxide + water (+ lots of energy / ATP)
C6H12O6 + 6O2 → 6CO2 + 6H2O

Anaerobic (humans / animals)glucose → lactic acid (+ little energy)

Anaerobic (yeast — fermentation)glucose → ethanol + carbon dioxide (+ little energy)

  • Aerobic gives much more energy per glucose than anaerobic.
  • After anaerobic exercise, oxygen debt: lactic acid must be oxidised / removed.

9. Gas exchange

Plants

  • Gases diffuse through stomata (mainly underside of leaf).
  • Day: net CO2 in / O2 out (photosynthesis > respiration). Night: O2 in / CO2 out (respiration only).
  • Guard cells open/close stomata; balance gas exchange vs water loss.

Humans

  • Air pathway: nose/mouth → trachea → bronchi → bronchioles → alveoli.
  • Alveoli: large surface area, thin walls, moist, rich capillary network → efficient diffusion of O2/CO2.
  • Inhalation: diaphragm contracts & flattens; ribcage up/out → volume ↑, pressure ↓ → air in. Exhalation: reverse.

10. Transport

Plants — xylem & phloem

  • Xylem: water & mineral ions from roots → leaves (transpiration stream). Dead lignified tubes.
  • Phloem: sugars (sucrose) / dissolved food from leaves → rest of plant (translocation). Living cells.
  • Transpiration: water evaporates from mesophyll → out stomata; pulls water up xylem. Factors: temperature, humidity, wind, light.

Humans — blood, heart & circulation

Double circulation (schematic) Heart Lungs Body deox. blood O2 blood to body from body
Pulmonary circuit (heart ↔ lungs) and systemic circuit (heart ↔ body). Arteries carry blood away from the heart; veins return it.
  • Blood components: plasma (transport); red cells (O2); white cells (defence); platelets (clotting).
  • Arteries: thick muscular/elastic walls, high pressure. Veins: thinner, valves. Capillaries: one-cell thick for exchange.
  • Double circulation: blood passes through heart twice per full body circuit — keeps pressure high for systemic flow.

11. Excretion

  • Removal of metabolic waste: CO2 (lungs); urea, excess water & salts (kidneys → urine).
  • Kidneys filter blood; useful substances reabsorbed; urine stored in bladder.
  • Urea made in liver from excess amino acids (deamination).

12. Coordination & homeostasis (Paper 1 / DA level)

Nervous system & eye

  • CNS = brain + spinal cord. Peripheral nerves carry impulses.
  • Reflex arc: stimulus → receptor → sensory neurone → relay neurone (CNS) → motor neurone → effector (muscle/gland). Fast & automatic.
  • Eye: cornea/lens focus light on retina; iris controls pupil size; rods/cones detect light; optic nerve to brain.

Hormones & homeostasis basics

  • Hormones: chemical messengers from endocrine glands in blood.
  • Adrenaline: fight-or-flight — ↑ heart rate, breathing, blood glucose.
  • Insulin (pancreas): lowers blood glucose (glucose → glycogen in liver).
  • Homeostasis: keeping internal conditions steady (temperature, water, glucose).
  • Skin & temperature: sweating, vasodilation cool; shivering, vasoconstriction warm.

13. Self-check questions

  1. Name three structures found in plant cells but not animal cells.
  2. Describe the Benedict’s and iodine food tests and positive results.
  3. What happens to an enzyme above its optimum temperature?
  4. Distinguish diffusion, osmosis and active transport.
  5. Write the word equation for photosynthesis and name two limiting factors.
  6. Where are amylase, protease and lipase produced / acting?
  7. Compare aerobic and anaerobic respiration in humans.
  8. State the roles of xylem and phloem.
  9. Why is the human circulation described as double?
  10. Outline a simple reflex arc.

14. Self-check answers

Tap / click to show answers
  1. Cell wall, chloroplasts, large permanent vacuole.
  2. Benedict’s + heat: reducing sugar → brick red. Iodine: starch → blue-black.
  3. Denatures — active site shape changes; substrate no longer fits; rate falls.
  4. Diffusion: any particles high→low, passive. Osmosis: water across PPM dilute→concentrated, passive. Active transport: against gradient, needs energy.
  5. CO2 + water → glucose + oxygen (light, chlorophyll). Limiting: light, CO2, temperature.
  6. Amylase: saliva / pancreas → mouth & small intestine. Protease: stomach & pancreas → stomach & SI. Lipase: pancreas → SI.
  7. Aerobic: needs O2, products CO2+H2O, much ATP. Anaerobic: no O2, lactic acid, little ATP; causes oxygen debt.
  8. Xylem: water & minerals up. Phloem: sugars / dissolved food around plant.
  9. Blood goes through heart twice per circuit — pulmonary then systemic circuits.
  10. Stimulus → receptor → sensory neurone → CNS (relay) → motor neurone → effector.
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Topic 3 · Reproduction & inheritance

Asexual vs sexual; plant & human reproduction; DNA, genes, inheritance; mitosis/meiosis; variation & selection.

1. Outline

  • Reproduction can be asexual (one parent, clones) or sexual (gametes, genetic variation).
  • Genes on DNA control inherited characteristics; alleles can be dominant or recessive.
  • Mitosis → growth/repair/asexual; meiosis → gametes.

2. Asexual vs sexual reproduction

AsexualSexual
ParentsOneUsually two
Gametes / fertilisationNoYes
Offspring geneticsIdentical (clones)Variation (mix of alleles)
AdvantageFast; no mate neededVariation → better survival if environment changes

3. Reproduction in flowering plants

  • Flower parts: sepals, petals, stamens (anther + filament = male), carpels (stigma, style, ovary = female).
  • Pollination: pollen from anther → stigma (insect or wind adapted).
  • Fertilisation: pollen tube grows to ovule; male gamete fuses with egg → zygote → seed; ovary → fruit.
  • Seed germination needs water, oxygen and a suitable temperature.

4. Human reproduction (overview)

  • Male: testes produce sperm & testosterone; sperm ducts, prostate, penis.
  • Female: ovaries produce eggs & oestrogen/progesterone; oviducts, uterus, vagina.
  • Fertilisation usually in oviduct; zygote implants in uterus lining.
  • Placenta exchanges nutrients/O2/waste between mother and fetus; umbilical cord connects; amniotic fluid protects.
  • Secondary sexual characteristics at puberty driven by hormones.

5. DNA, genes & inheritance

  • DNA — genetic material in nucleus; double helix; bases A–T, C–G.
  • Gene — length of DNA coding for a protein / characteristic.
  • Chromosome — DNA structure carrying many genes. Humans: 23 pairs (46).
  • Allele — different version of a gene. Dominant expressed if present; recessive only if two copies.
  • Genotype — alleles present (e.g. Bb). Phenotype — observed characteristic.
  • Homozygous (BB or bb) vs heterozygous (Bb).

Monohybrid cross (example)Parents Bb × Bb → offspring genotypes 1 BB : 2 Bb : 1 bb → phenotypes 3 dominant : 1 recessive

Use Punnett squares in exams. Sex chromosomes: XX female, XY male — roughly 50% each sex.

6. Mitosis & meiosis (basics)

MitosisMeiosis
PurposeGrowth, repair, asexual reproductionProduce gametes
Daughter cells2, genetically identical, diploid4, genetically different, haploid
Chromosome numberSame as parentHalf of parent

7. Variation & natural selection (DA level)

  • Continuous variation (e.g. height) vs discontinuous (e.g. blood group, tongue rolling).
  • Sources of variation: genetic (alleles, mutation, sexual reproduction) and environmental.
  • Natural selection: variation → competition → better-adapted individuals survive & reproduce → alleles become more common (evolution over generations).
  • Mutation: random change in DNA; may be harmful, neutral or (rarely) useful.

8. Self-check questions

  1. Give two advantages of sexual reproduction over asexual.
  2. What is the difference between pollination and fertilisation in plants?
  3. Define gene, allele, genotype and phenotype.
  4. In a cross Aa × Aa (A dominant), what fraction of offspring show the recessive phenotype?
  5. How do mitosis and meiosis differ in chromosome number of daughter cells?
  6. Outline natural selection in three steps.

9. Self-check answers

Tap / click to show answers
  1. Genetic variation; better chance of survival if environment changes; may combine useful traits from two parents.
  2. Pollination = pollen transfer to stigma. Fertilisation = fusion of male gamete with egg in ovule.
  3. Gene: DNA section coding for a protein/characteristic. Allele: version of a gene. Genotype: alleles present. Phenotype: observed trait.
  4. 1/4 (aa).
  5. Mitosis: same (diploid). Meiosis: half (haploid).
  6. Variation exists → struggle for survival → best adapted reproduce → advantageous alleles increase in frequency.
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Topic 4 · Ecology & the environment

Ecosystems, feeding relationships, pyramids, carbon & nitrogen cycles, human influences.

1. Ecosystems

  • Population — all organisms of one species in an area.
  • Community — all populations in an area.
  • Habitat — place where an organism lives.
  • Ecosystem — community + non-living environment interacting.
  • Factors affecting distribution/abundance: temperature, light, water, nutrients, competition, predation.

2. Feeding relationships & pyramids

Food chain → pyramid of numbers grass → rabbit → fox → fleas producer → primary → secondary → (many parasites) producers herbivores carnivores Energy lost at each level as heat / waste / movement
Arrows in a food chain show energy flow. Pyramids of numbers or biomass show trophic levels; energy decreases up the chain.
  • Producer — makes own food (photosynthesis). Consumer — eats other organisms. Decomposer — breaks down dead material.
  • Food web: many interlinked food chains.
  • Only ~10% of energy passes to the next trophic level (rest lost as heat, waste, movement).

3. Carbon & nitrogen cycles

Carbon cycle (key processes)

  • Photosynthesis removes CO2 from air → organic compounds.
  • Respiration & combustion return CO2.
  • Decomposition of dead organisms releases CO2 (and returns nutrients to soil).
  • Fossilisation locks carbon in fossil fuels over long timescales.

Nitrogen cycle (DA level)

  • Nitrogen needed for amino acids / proteins.
  • Nitrifying bacteria: ammonia → nitrites → nitrates (plants absorb nitrates).
  • Nitrogen-fixing bacteria (root nodules of legumes / soil): N2 → ammonium compounds.
  • Decomposers: proteins → ammonia. Denitrifying bacteria: nitrates → N2 gas.

4. Human influences on the environment

  • Greenhouse effect: CO2, methane trap heat → enhanced greenhouse effect → global warming / climate change. Sources: fossil fuels, deforestation, livestock.
  • Deforestation: less photosynthesis / less CO2 uptake; habitat loss; soil erosion; biodiversity ↓.
  • Pollution: water (sewage, fertiliser → eutrophication); air (SO2/NOx → acid rain); plastics / pesticides.
  • Eutrophication outline: excess nitrate/phosphate → algal bloom → light blocked → plants die → decomposers use O2 → aquatic animals die.

5. Self-check questions

  1. Define population, community and ecosystem.
  2. Why is a pyramid of energy always pyramid-shaped?
  3. Name two processes that add CO2 to the atmosphere and one that removes it.
  4. What role do nitrogen-fixing bacteria play?
  5. Explain eutrophication in four steps.

6. Self-check answers

Tap / click to show answers
  1. Population: one species in an area. Community: all species. Ecosystem: community + abiotic environment.
  2. Energy is lost at each trophic level (heat, waste, movement), so less energy is available higher up.
  3. Add: respiration, combustion (decomposition). Remove: photosynthesis.
  4. Convert atmospheric N2 into ammonia / compounds plants can eventually use as nitrates.
  5. Nutrient runoff → algal bloom → death of plants (light blocked) → decomposition uses O2 → fish/animals die.
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Topic 5 · Use of biological resources

Crop yields, microorganisms, fish farming, selective breeding, genetic modification (DA).

1. Outline

Humans manage plants, animals and microbes to increase food production and useful products. Methods include fertilisers, pest control, controlled environments, fermentation, selective breeding and genetic modification.

2. Increasing crop yields

  • Fertilisers: supply nitrates, phosphates, potassium → better growth. Risk: leaching → eutrophication.
  • Pest control: pesticides / insecticides / herbicides / fungicides kill pests/weeds/fungi. Risks: toxicity, bioaccumulation, kill useful insects; pests may become resistant.
  • Biological control: use natural predators/parasites (fewer chemicals).
  • Glasshouses / polytunnels: control temperature, light, CO2, water → higher yields year-round; pest management easier.

3. Microorganisms in food production

  • Yeast: anaerobic respiration (fermentation) → ethanol + CO2 for bread (CO2 rises dough) and brewing/wine.
  • Yoghurt / Lactobacillus: bacteria ferment lactose → lactic acid; milk proteins thicken; flavour changes.
  • Fermenters: large vessels for growing microbes under controlled temperature, pH, oxygen, nutrients, aseptic conditions. Used for antibiotics (e.g. penicillin from Penicillium), enzymes, mycoprotein, etc.
Why aseptic?

Unwanted microbes compete for nutrients, spoil the product or produce toxins — sterilise equipment and use clean air filters.

4. Fish farming

  • Fish reared in tanks/cages: controlled water quality, feeding, breeding.
  • Advantages: reliable supply; can reduce pressure on wild stocks if managed well.
  • Issues: disease spreads easily (high density) → antibiotics; waste pollution; escapees may affect wild populations; feed may use wild fish.

5. Selective breeding & genetic modification

Selective breeding

  1. Choose parents with desired traits.
  2. Breed them; select best offspring.
  3. Repeat over many generations.

Examples: high-yield wheat; cattle for milk/meat; dogs for behaviour. Risk: reduced genetic variation / inbreeding problems.

Genetic modification (GM) — DA basics

  • Transfer a gene from one organism to another so the recipient makes a useful protein / trait.
  • Steps (overview): isolate gene → insert into vector (e.g. plasmid) → introduce into host cells → select modified organisms.
  • Examples: insulin-producing bacteria; crops with herbicide resistance or better nutrition.
  • Debate: benefits (yield, medicine) vs concerns (ecology, ethics, labelling).

6. Self-check questions

  1. How do fertilisers increase crop yield, and what environmental risk do they pose?
  2. Why is CO2 sometimes added to glasshouses?
  3. Explain the role of yeast in bread making.
  4. Give two problems associated with intensive fish farming.
  5. Outline the steps of selective breeding.
  6. What is a plasmid used for in genetic modification?

7. Self-check answers

Tap / click to show answers
  1. Provide minerals (e.g. nitrates) for growth/proteins. Risk: leaching → eutrophication.
  2. CO2 can be a limiting factor for photosynthesis; raising it increases rate/yield.
  3. Anaerobic respiration produces CO2, which makes dough rise; ethanol evaporates on baking.
  4. Disease spread; pollution from waste; antibiotic use; escapees; feed sustainability (any two).
  5. Select parents with desired traits → breed → select best offspring → repeat generations.
  6. As a vector to carry the desired gene into a host cell (e.g. bacterium).