Animal cell components
- Nucleus — contains DNA; controls cell activity
- Cytoplasm — site of most chemical reactions
- Cell membrane — controls what enters/leaves
- Mitochondria — site of aerobic respiration; releases energy
- Ribosomes — where proteins are made
Plant cell extras
- Cell wall — made of cellulose; gives structural support
- Chloroplasts — contain chlorophyll; site of photosynthesis
- Large permanent vacuole — stores cell sap; maintains turgor pressure
- (Also has nucleus, cytoplasm, cell membrane, mitochondria, ribosomes)
Common mistake: Students often forget that plant cells still have mitochondria. Photosynthesis happens in chloroplasts; respiration happens in mitochondria — both occur in plants.
Specialised cells
| Cell | Special features | How features help |
|---|
| Root hair cell | Long extension; thin membrane; no chloroplasts | Large SA for absorbing water & minerals; no light underground so no chloroplasts needed |
| Red blood cell | Biconcave shape; no nucleus; haemoglobin | Large SA for O₂ absorption; no nucleus = more room for haemoglobin |
| Sperm cell | Long tail; many mitochondria; streamlined head | Tail for swimming; mitochondria provide energy |
| Nerve cell | Very long; many dendrites | Transmits electrical signals over long distances quickly |
Diffusion & Osmosis
Diffusion — movement of particles from high to low concentration (down a concentration gradient). Passive — no energy needed.
Rate of diffusion increases with: larger surface area · steeper concentration gradient · higher temperature · shorter distance
Osmosis — movement of water through a partially permeable membrane from higher to lower water concentration. This is how water enters root hair cells from soil.
Levels of organisation
Cell→
Tissue→
Organ→
Organ system→
Organism
If asked to explain why a specialised cell is adapted, always match each feature to a specific function — never just list features without explaining what they do.
This topic appears in almost every past paper. Know the pathway, vessel names, and transpiration factors perfectly.
Pathway of water
Soil→
Root hair (osmosis)→
Xylem vessels→
Leaf cells→
Stomata→
Atmosphere (transpiration)
Xylem vessels
- Dead cells, hollow tubes
- Lignified (strengthened) walls
- Carry water and mineral salts upward only
Phloem vessels
- Living cells
- Carry dissolved sugars (sucrose)
- Transport in both directions (translocation)
Mineral salts
| Mineral | Used for | Deficiency symptom |
|---|
| Nitrates | Making proteins and DNA; growth | Stunted growth, yellowing of older leaves |
| Magnesium | Making chlorophyll | Yellow leaves (chlorosis) |
| Phosphates | DNA and cell membranes | Poor root growth, purple leaves |
Transpiration factors
| Factor increases | Effect | Reason |
|---|
| Temperature ↑ | Rate increases | Water molecules have more kinetic energy; evaporate faster |
| Wind speed ↑ | Rate increases | Removes water vapour; maintains steep concentration gradient |
| Light intensity ↑ | Rate increases | Stomata open wider for more CO₂ for photosynthesis |
| Humidity ↑ | Rate decreases | Less gradient between leaf and air; slower diffusion out |
Always explain WHY a factor changes transpiration rate — say what happens to water molecules or the concentration gradient, not just "it increases".
carbon dioxide + water → glucose + oxygen (requires light energy, in chloroplasts)
Key facts
- Takes place in chloroplasts, which contain the green pigment chlorophyll
- CO₂ enters through stomata; water absorbed through roots via xylem
- Glucose used for: respiration · starch storage · cellulose (cell walls) · proteins (with nitrates)
- Oxygen released as by-product through stomata
Limiting factors
| Factor | Effect when increased | Why |
|---|
| Light intensity | Rate increases (up to a point) | Light provides energy to drive the reaction |
| CO₂ concentration | Rate increases (up to a point) | CO₂ is a raw material |
| Temperature | Rate increases then falls sharply | Too hot denatures enzymes |
Common mistake: Plants respire continuously day and night. During the day, photosynthesis rate exceeds respiration rate, so there is net uptake of CO₂ — but respiration never stops.
Starch test: boil leaf in ethanol to remove chlorophyll → wash in water → add iodine. Blue-black = starch present. Know every step of this method.
Aerobic respiration
glucose + oxygen → carbon dioxide + water + energy
- Occurs in mitochondria
- Happens continuously in all living cells
- Energy used for: movement · growth · temperature · active transport
Anaerobic respiration
| In animals / humans | In yeast |
|---|
glucose → lactic acid + energy Causes muscle fatigue and cramp during intense exercise. |
glucose → ethanol + CO₂ + energy Used in fermentation — CO₂ makes bread rise; ethanol gives alcohol. |
Anaerobic releases far less energy than aerobic. It is a temporary solution when oxygen supply is insufficient.
When comparing aerobic and anaerobic: aerobic needs O₂, anaerobic does not; aerobic produces CO₂ and water, anaerobic (animals) produces lactic acid; aerobic releases more energy.
Key vocabulary
| Term | Definition |
|---|
| Gene | A section of DNA that codes for a characteristic |
| Allele | A version of a gene (e.g. blue or brown eye colour allele) |
| Dominant | Allele expressed if present (capital letter, e.g. B) |
| Recessive | Only expressed if two copies present (e.g. b) |
| Genotype | The alleles an organism has (e.g. Bb) |
| Phenotype | The physical characteristic shown (e.g. brown eyes) |
| Homozygous | Both alleles identical (BB or bb) |
| Heterozygous | Two different alleles (Bb) |
Sex determination & Punnett squares
XX = female · XY = male. The father's sperm determines sex (carries X or Y; eggs always carry X).
Example: Tall (T) dominant over short (t). Cross Tt × Tt:
| T | t |
|---|
| T | TT (tall) | Tt (tall) |
| t | Tt (tall) | tt (short) |
Ratio: 3 tall : 1 short
Always show parent genotypes above and beside the grid, fill in ALL four boxes, then state the ratio of phenotypes — not just genotypes.
Digestive system — enzymes
| Enzyme | Substrate | Product | Made in |
|---|
| Amylase | Starch | Sugars | Salivary glands, pancreas |
| Protease | Proteins | Amino acids | Stomach, pancreas |
| Lipase | Fats | Fatty acids + glycerol | Pancreas |
Circulatory system
- Arteries — away from heart; thick muscular walls; high pressure
- Veins — toward heart; thinner walls; valves prevent backflow
- Capillaries — one cell thick; exchange gases, nutrients and wastes
- Double circulation: pulmonary (heart–lungs) and systemic (heart–body)
Excretory system
Liver makes urea→
Kidneys filter blood→
Urine → ureter → bladder → urethra
Know the difference: ureter (kidney to bladder) vs urethra (bladder to outside). Frequently confused and examiners penalise the error.
Food chains & energy transfer
Producer (plant)→
Primary consumer→
Secondary consumer→
Tertiary consumer
- Arrows show direction of energy flow
- Energy is lost as heat and movement at each level — only ~10% is transferred
- This is why food chains rarely exceed 4–5 links
Decomposers & nutrient cycling
- Bacteria and fungi break down dead organisms
- Release minerals (including nitrates) back into soil for plants
- Without decomposers, nutrients would be locked in dead matter
Human impacts
- Deforestation — destroys habitats; reduces biodiversity; releases CO₂
- Pollution / eutrophication — fertilisers cause algal blooms; deplete oxygen in water
- Overfishing — reduces populations faster than they can reproduce
- Climate change — shifts habitats; causes extinction
When asked about removing a species, trace the effect in both directions: what eats it (population falls) and what it eats (population rises).