IGCSE Biology High-Yield Theory Questions

Mark Scheme Answer:

• • Speeds up / increases rate of, (chemical) reaction
• • Without being used up / changed / consumed

Mark Scheme Answer:

• • Enzymes / active site have denatured
• • Enzyme / active site has changed shape
• • Active site does not fit / is not complementary to substrate
• • Enzyme-substrate complex cannot form

Mark Scheme Answer:

• • Active site has specific shape
• • Shape is complementary to / fits, substrate
• • Enzyme-substrate complex forms
• • Only one type of substrate can bind (specificity)

Mark Scheme Answer:

Carbon, Hydrogen, Oxygen, Nitrogen (CHON)

Mark Scheme Answer:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Or: carbon dioxide + water → glucose + oxygen (word equation)

Mark Scheme Answer:

Magnesium (ions)

Mark Scheme Answer:

• • In light: photosynthesis occurs, CO₂ is used/absorbed
• • pH increases / becomes more alkaline (indicator turns purple)
• • In dark: only respiration occurs, CO₂ is released
• • pH decreases / becomes more acidic (indicator turns yellow)
• • In light: rate of photosynthesis > rate of respiration

Mark Scheme Answer:

• • Temperature
• • Carbon dioxide concentration
• • Amount of chlorophyll / number of chloroplasts
• • Water availability

Mark Scheme Answer:

Aerobic respiration (site of aerobic respiration)

Mark Scheme Answer:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

Mark Scheme Answer:

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

glucose → ethanol + carbon dioxide

Mark Scheme Answer:

Lactic acid

Mark Scheme Answer:

Bread-making / making dough rise / leavening agent

Mark Scheme Answer:

• • Tissue: Phloem
• • Other molecule: Amino acids

Mark Scheme Answer:

• • Hollow / no cytoplasm / no end walls → allows continuous water flow
• • Lignified walls → provides strength/support, prevents collapse
• • Long tubes / continuous → efficient transport
• • Narrow diameter → capillary action

Mark Scheme Answer:

• • Translocation / transport of sugars/sucrose
• • Transport of amino acids
• • From leaves (source) to other parts (sink)
• • Bidirectional (up and down)

Mark Scheme Answer:

1. Physical/mechanical digestion by teeth (in mouth) / churning (in stomach)
2. Increases surface area for enzyme activity
3. Chemical digestion: breaks insoluble molecules into soluble ones
4. Salivary glands / pancreas secrete amylase
5. Amylase breaks starch → maltose
6. Maltase found on epithelium of small intestine
7. Maltase breaks maltose → glucose
8. Optimum pH for amylase/maltase is neutral (pH 7-8)

Mark Scheme Answer:

Absorption / transport of fatty acids / glycerol / fat / lipids

Mark Scheme Answer:

• • Taking food into mouth: Ingestion
• • Teeth that cut: Incisors
• • Teeth that grind: Molars and premolars
• • Process of grinding: Chewing / mastication

Mark Scheme Answer:

• • Group of receptor cells/tissues
• • That detect/sense/respond to specific stimuli
• • Transmits impulse to sensory neurone

Mark Scheme Answer:

1. Neurotransmitter molecules involved
2. Released from vesicles (from presynaptic neurone into synaptic gap)
3. Diffuse across gap/cleft
4. Bind/fit/attach to receptor proteins on next neurone

Mark Scheme Answer:

1. Ciliary muscle contracts
2. Suspensory ligaments slacken/loosen
3. Lens becomes more convex / thicker / rounder
4. More refraction of light
5. Light/image focused onto retina/fovea

Mark Scheme Answer:

• • Both found in retina, both are light receptors
• • High concentration of cones / no rods at fovea
• • Rods: sensitive to low light intensity / dim light / night vision; give black & white image
• • Cones: sensitive to high light intensity / bright light; detect colour / absorb different wavelengths

Mark Scheme Answer:

• • Cervix: dilates during birth
• • Ovary: produces eggs/ova and oestrogen
• • Oviduct (Fallopian tube): site of fertilisation
• • Uterus lining: site of implantation

Mark Scheme Answer:

• • Gas exchange: O₂ to fetus, CO₂ to mother
• • Transfer of nutrients from mother to fetus
• • Transfer of excretory products from fetus to mother
• • Transfer by diffusion
• • Produces hormones
• • Transfer of antibodies (passive immunity)
• • Separates fetal and maternal blood

Mark Scheme Answer:

• • Provides support to fetus
• • Protects fetus from mechanical shock
• • Maintains temperature
• • Allows movement / muscle development

Mark Scheme Answer:

Allele

Mark Scheme Answer:

• • Allows genetic variation / diversity
• • Different combinations of alleles/DNA/mutations from parents
• • Allows adaptation/evolution/natural selection
• • In response to changing environment
• • Reduces chances of extinction

Key Points:

• • Meiosis: produces gametes, halves chromosome number, creates variation
• • Mitosis: growth, repair, produces identical cells, maintains chromosome number
• • Gamete formation = meiosis
• • Embryo growth = mitosis

Mark Scheme Answer:

1. Start as unspecialised/undifferentiated cells
2. Divide by mitosis
3. Become specialised/differentiated
4. Specific genes are expressed
5. Produce specific proteins

Mark Scheme Answer:

• • Carbohydrates: source of energy
• • Proteins: growth and repair of tissues
• • Fats/Lipids: energy store, insulation, cell membranes
• • Vitamins: various functions (e.g., Vitamin D for calcium absorption)
• • Minerals: e.g., calcium for bones/teeth, iron for haemoglobin
• • Fibre: prevents constipation, aids peristalsis
• • Water: solvent for reactions, transport medium

Mark Scheme Answer:

• • Vitamin D needed for absorption of calcium
• • Calcium needed for strong bones and teeth
• • Children are growing / developing bones
• • Prevents rickets (soft/weak bones)

Mark Scheme Answer:

• • Starch: Iodine solution → blue-black (positive)
• • Reducing sugars: Benedict's + heat → orange/red (positive)
• • Protein: Biuret → purple (positive)
• • Fats: Ethanol emulsion test → cloudy white (positive)
• • Vitamin C: DCPIP → decolourises (blue to colourless)

Mark Scheme Answer:

• • Genus (capital letter)
• • Species (lowercase)
• • Written in italics or underlined

Mark Scheme Answer:

Kingdom → Phylum → Class → Order → Family → Genus → Species

Memory aid: King Philip Came Over For Good Soup

Mark Scheme Answer:

• • Animals: multicellular, heterotrophic, no cell wall
• • Plants: multicellular, autotrophic, cellulose cell wall
• • Fungi: chitin cell wall, saprophytic/parasitic
• • Protoctists: mostly unicellular, have nucleus
• • Prokaryotes: no nucleus, no membrane-bound organelles

Mark Scheme Answer:

• • Large surface area (many alveoli) - more diffusion
• • Thin walls (one cell thick) - short diffusion distance
• • Good blood supply (capillaries) - maintains concentration gradient
• • Moist lining - gases dissolve

Mark Scheme Answer:

• • Oxygen: 21% inspired → ~16% expired
• • Carbon dioxide: 0.04% inspired → ~4% expired
• • Water vapour: variable → saturated in expired air
• • Nitrogen: ~79% (unchanged)

Mark Scheme Answer:

1. Identify the gene for the desired characteristic
2. Cut the gene using restriction enzymes
3. Cut the plasmid/vector using same restriction enzyme
4. Insert gene into plasmid using ligase
5. Insert plasmid into host cell (e.g., bacterium)
6. Host cell replicates, producing the protein

Mark Scheme Answer:

Advantages:

• • Higher yield / more food
• • Pest/disease resistance → less pesticide
• • Herbicide resistance
• • Improved nutritional content

Disadvantages:

• • Genes may spread to wild plants
• • May affect food chains
• • Unknown long-term effects
• • Ethical concerns

Mark Scheme Answer:

• • Red blood cells: transport oxygen (contain haemoglobin)
• • White blood cells: destroy pathogens (phagocytosis, antibodies)
• • Platelets: blood clotting
• • Plasma: transports dissolved substances (glucose, CO₂, urea, hormones)

Mark Scheme Answer:

• • Biconcave shape → large surface area for gas exchange
• • No nucleus → more space for haemoglobin
• • Contains haemoglobin → carries oxygen
• • Flexible → can squeeze through capillaries

Mark Scheme Answer:

1. Platelets release clotting factors at wound
2. Fibrinogen (soluble) converted to fibrin (insoluble)
3. Fibrin forms mesh of fibres
4. Red blood cells trapped → forms clot/scab

Mark Scheme Answer:

• • Phagocytosis: phagocytes engulf and digest pathogens
• • Antibody production: lymphocytes produce specific antibodies
• • Antibodies cause pathogens to clump together (agglutination)
• • Antibodies neutralise toxins
• • Memory cells remain for faster secondary response

Mark Scheme Answer:

1. Vaccine contains dead/weakened pathogen or antigens
2. Antigens stimulate immune response
3. Lymphocytes produce antibodies
4. Memory cells are formed
5. If infected later, memory cells produce antibodies quickly
6. Pathogen destroyed before symptoms develop

Mark Scheme Answer:

• • Antibiotics target bacterial cell structures (cell wall, ribosomes)
• • Viruses don't have these structures
• • Viruses replicate inside host cells

Mark Scheme Answer:

The removal of toxic waste products of metabolism and substances in excess of requirements from the body.

Mark Scheme Answer:

• • Kidneys: urea, excess water, excess salts (as urine)
• • Lungs: carbon dioxide, water vapour
• • Skin: water, salts, urea (in sweat)

Mark Scheme Answer:

1. Excess amino acids cannot be stored
2. Deamination occurs in the liver
3. Amino group removed from amino acid
4. Amino group converted to ammonia
5. Ammonia is toxic so converted to urea
6. Urea transported in blood to kidneys

Mark Scheme Answer:

• • Food chain: shows transfer of energy from one organism to next
• • Food web: interconnected food chains
• • Producer: makes own food (photosynthesis) - plants
• • Consumer: eats other organisms for energy
• • Trophic level: position in a food chain

Mark Scheme Answer:

• • Energy lost in respiration (as heat)
• • Energy lost in excretion (urine, CO₂)
• • Not all parts eaten (bones, fur)
• • Not all food digested (egested as faeces)
• • Only ~10% transferred to next level

Mark Scheme Answer:

• • Broad/flat → large surface area for light absorption
• • Thin → short diffusion distance for gases
• • Transparent epidermis → lets light through
• • Palisade cells packed with chloroplasts, near top surface
• • Air spaces in spongy mesophyll → gas exchange
• • Stomata → allow CO₂ in, O₂ out
• • Veins (xylem/phloem) → transport water and sugars

Mark Scheme Answer:

• • Stomata: pores that allow gas exchange
• • Guard cells: control opening/closing of stomata
• • When turgid (full of water) → stomata open
• • When flaccid (lost water) → stomata close
• • Close at night or in dry conditions to reduce water loss

Mark Scheme Answer:

The loss of water vapour from the leaves through the stomata.

Mark Scheme Answer:

• • Temperature: higher temp → faster transpiration (more evaporation)
• • Humidity: lower humidity → faster transpiration (steeper gradient)
• • Wind speed: faster wind → faster transpiration (removes water vapour)
• • Light intensity: more light → stomata open → faster transpiration

Mark Scheme Answer:

Mark Scheme Answer:

• • Insulin: lowers blood glucose (converts glucose to glycogen)
• • Glucagon: raises blood glucose (converts glycogen to glucose)
• • Both secreted by pancreas
• • Act on liver

Mark Scheme Answer:

• • Oestrogen: repairs uterus lining, stimulates LH release
• • Progesterone: maintains uterus lining for pregnancy
• • FSH: stimulates follicle development in ovary
• • LH: triggers ovulation

Mark Scheme Answer:

• • Sepal: protects flower in bud
• • Petal: attracts insects (coloured, scented)
• • Stamen (anther + filament): male part, produces pollen
• • Carpel (stigma + style + ovary): female part, contains ovules
• • Nectary: produces nectar to attract pollinators

Mark Scheme Answer:

Mark Scheme Answer:

• • Water pollution (sewage/fertilizers): eutrophication, algal bloom, oxygen depletion, fish death
• • Air pollution (SO₂, NOₓ): acid rain, damages plants, corrodes buildings
• • CO₂: enhanced greenhouse effect, global warming, climate change
• • CFCs: ozone layer depletion, more UV radiation

Mark Scheme Answer:

• • Protected areas / nature reserves / national parks
• • Captive breeding programmes
• • Seed banks / gene banks
• • Education and awareness
• • Laws to protect endangered species
• • Sustainable harvesting / fishing quotas

Mark Scheme Answer:

The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane.

Mark Scheme Answer:

• • In pure water (hypotonic): water enters by osmosis → cell becomes turgid → cell wall prevents bursting
• • In concentrated solution (hypertonic): water leaves by osmosis → cell becomes flaccid → plasmolysis occurs (membrane pulls away from wall)

Mark Scheme Answer:

• • In pure water: water enters → cell swells → bursts (lysis) - no cell wall!
• • In concentrated solution: water leaves → cell shrinks (crenation)

Mark Scheme Answer:

Brain and spinal cord

Mark Scheme Answer:

Stimulus → Receptor → Sensory neurone → Relay neurone (in CNS) → Motor neurone → Effector → Response

• • Reflexes are fast, automatic, protective
• • Do not involve conscious thought
• • Example: withdrawing hand from hot object

Mark Scheme Answer:

• • Sensory neurone: carries impulses from receptor to CNS
• • Relay neurone: connects neurones in CNS
• • Motor neurone: carries impulses from CNS to effector

Mark Scheme Answer:

• • Food supply - more food = larger population
• • Predation - more predators = smaller prey population
• • Disease - disease reduces population
• • Competition - limits resources available

Mark Scheme Answer:

1. Place quadrats randomly in the area
2. Count organisms in each quadrat
3. Calculate mean number per quadrat
4. Calculate area of quadrat
5. Estimate: (mean count / quadrat area) × total area

Mark Scheme Answer:

• • Eye: light
• • Ear: sound, balance
• • Nose: chemicals (smell)
• • Tongue: chemicals (taste)
• • Skin: touch, temperature, pressure, pain

Mark Scheme Answer:

• • Cornea: refracts light
• • Iris: controls amount of light entering (pupil size)
• • Lens: focuses light onto retina
• • Retina: contains light receptors (rods and cones)
• • Optic nerve: carries impulses to brain
• • Ciliary muscle: changes shape of lens
• • Suspensory ligaments: hold lens in place

Mark Scheme Answer:

• • Phototropism: growth response to light direction
• • Gravitropism (geotropism): growth response to gravity
• • Positive = towards stimulus, Negative = away from stimulus
• • Shoots: positive phototropism, negative gravitropism
• • Roots: negative phototropism, positive gravitropism

Mark Scheme Answer:

1. Light causes auxin to move to shaded side
2. More auxin on shaded side
3. Auxin promotes cell elongation in shoots
4. Cells on shaded side elongate more
5. Shoot bends towards light

Mark Scheme Answer:

The net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient.

Mark Scheme Answer:

• • Concentration gradient: steeper = faster
• • Temperature: higher = faster (more kinetic energy)
• • Surface area: larger = faster
• • Distance: shorter = faster

Mark Scheme Answer:

Movement of particles from a region of lower concentration to higher concentration (against concentration gradient), requiring energy from respiration.

Mark Scheme Answer:

• • Root hair cells absorb mineral ions from soil
• • Mineral ion concentration higher inside cell than in soil
• • Requires energy (ATP) from respiration
• • Uses carrier proteins in membrane

Mark Scheme Answer:

• • Continuous: range of values, no distinct categories (e.g., height, mass)
• • Discontinuous: distinct categories, no intermediates (e.g., blood group, tongue rolling)

Mark Scheme Answer:

• • Genetic factors: different alleles inherited
• • Environmental factors: diet, climate, exercise
• • Mutation: random changes in DNA
• • Most features affected by both genes AND environment

Mark Scheme Answer:

1. Variation exists within a population
2. Some individuals have advantageous characteristics
3. These individuals are more likely to survive (survival of the fittest)
4. They reproduce and pass on their alleles
5. Over generations, advantageous alleles become more common

Mark Scheme Answer:

1. Humans select organisms with desired characteristics
2. These are bred together
3. Offspring with best characteristics selected
4. Process repeated over many generations
5. Examples: high-yield crops, fast-growing livestock

Mark Scheme Answer:

• • Growth of organisms
• • Repair of damaged tissues
• • Asexual reproduction
• • Produces genetically identical cells (clones)
• • Maintains diploid chromosome number

Mark Scheme Answer:

Mark Scheme Answer:

• • Slows reactions (depressant)
• • Reduces self-control
• • Damages liver (cirrhosis)
• • Can cause addiction
• • Impairs judgement

Mark Scheme Answer:

• • Nicotine: addictive, raises blood pressure
• • Tar: carcinogen (causes lung cancer), damages cilia
• • Carbon monoxide: combines with haemoglobin, reduces oxygen transport
• • Causes chronic bronchitis, emphysema
• • Increases risk of heart disease

Mark Scheme Answer:

• • Right atrium: receives deoxygenated blood from vena cava
• • Right ventricle: pumps blood to lungs via pulmonary artery
• • Left atrium: receives oxygenated blood from pulmonary vein
• • Left ventricle: pumps blood to body via aorta (thicker wall - higher pressure)
• • Valves: prevent backflow of blood
• • Septum: separates left and right sides

Mark Scheme Answer:

• • Supply blood to heart muscle itself
• • Provide oxygen and glucose for heart muscle respiration
• • Blockage causes heart attack (coronary heart disease)

Mark Scheme Answer:

Adds CO₂:

• • Respiration (by all organisms)
• • Combustion (burning fossil fuels)
• • Decomposition

Removes CO₂:

• • Photosynthesis

Mark Scheme Answer:

• • Decomposers: break down proteins → ammonia
• • Nitrifying bacteria: convert ammonia → nitrates
• • Denitrifying bacteria: convert nitrates → nitrogen gas
• • Nitrogen-fixing bacteria: convert nitrogen gas → ammonia (in root nodules)

Mark Scheme Answer (MRS GREN):

• • Movement
• • Respiration
• • Sensitivity (response to stimuli)
• • Growth
• • Reproduction
• • Excretion
• • Nutrition

Mark Scheme Answer:

• • Carbohydrates: Carbon, Hydrogen, Oxygen (CHO)
• • Proteins: Carbon, Hydrogen, Oxygen, Nitrogen (CHON) - some have sulfur
• • Fats/Lipids: Carbon, Hydrogen, Oxygen (CHO) - less oxygen than carbs

Mark Scheme Answer:

• • Starch/Glycogen: made of glucose molecules
• • Proteins: made of amino acids
• • Fats: made of fatty acids + glycerol
• • DNA: made of nucleotides