AQA GCSE: Biology

Cell Structure

  • Animal cells: nucleus, cytoplasm, cell membrane, mitochondria, ribosomes

  • Plant cells: all animal cell parts + cell wall, chloroplasts, permanent vacuole

  • Bacteria: cell wall, cell membrane, cytoplasm, plasmids, circular DNA (no nucleus)

  • Specialised cells:

    • Sperm → tail, lots of mitochondria

    • Nerve → long, myelin sheath

    • Muscle → lots of mitochondria, contractile fibres

    • Root hair → large surface area, thin walls

Cell Transport

  • Diffusion: movement of particles high → low concentration

  • Osmosis: movement of water across partially permeable membrane

  • Active transport: movement against gradient using energy

  • Factors affecting diffusion/osmosis: surface area, concentration gradient, temperature

Cell Division

  • Mitosis:

    • Growth & repair

    • Produces 2 identical daughter cells

    • Steps:

      1. DNA replicates

      2. Chromosomes line up

      3. Chromatids pulled apart

      4. Two identical cells form

  • Meiosis:

    • Produces gametes

    • 4 genetically different cells

    • Halves chromosome number

Animal Organisation

  • Tissues: groups of similar cells (e.g., muscular tissue)

  • Organs: groups of tissues (e.g., stomach)

  • Organ systems: groups of organs (e.g., digestive system)

  • Circulatory system: heart → arteries → capillaries → veins

  • Blood components:

    • RBC → O₂ transport

    • WBC → fight infection

    • Platelets → clotting

    • Plasma → transport

  • Lungs: alveoli for gas exchange, diffusion of O₂ → blood, CO₂ → alveoli

Infectious Disease

  • Pathogens: bacteria, viruses, fungi, protists

  • Transmission: direct contact, air, water, vectors

  • Defence mechanisms: skin, mucus, cilia, stomach acid, blood clotting

  • Immune response:

    • Phagocytosis (WBC engulfs pathogen)

    • Antibodies (specific to pathogen)

    • Vaccination → stimulates immunity

  • Antibiotics vs antivirals: antibiotics kill bacteria, not viruses

Enzymes

  • Biological catalysts: speed up reactions

  • Active site: fits substrate

  • Lock & key model: substrate fits enzyme exactly

  • Factors affecting enzyme activity: temperature, pH, substrate concentration, enzyme concentration

  • Denaturation: extreme conditions change enzyme shape → stops working

Photosynthesis

  • Equation:
    6CO2 + 6H2O → C6H12O6 + 6O2

  • Factors affecting rate: light intensity, CO₂ concentration, temperature

  • Adaptations of leaves: large surface area, thin, stomata, chlorophyll

Respiration

  • Aerobic:
    C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

  • Anaerobic:

    • Humans: glucose → lactic acid

    • Plants/yeast: glucose → ethanol + CO₂

  • Occurs in mitochondria (aerobic)

Inheritance

  • DNA → genes → chromosomes → proteins

  • Alleles: dominant vs recessive

  • Homozygous: same alleles, heterozygous: different alleles

  • Phenotype: physical appearance

  • Genotype: allele combination

  • Punnett squares: predict offspring ratios

  • Variation: genetic vs environmental

  • Evolution: natural selection → adaptation → evolution

Required Practicals

1. Microscopy (Cells)

Aim: Observe and identify cells using a light microscope.

Method:

  1. Place a drop of water on a clean microscope slide.

  2. Put the specimen (e.g. onion epidermis) onto the drop.

  3. Add a drop of iodine solution (stain).

  4. Carefully lower a coverslip using a needle to avoid air bubbles.

  5. Place the slide on the microscope stage.

  6. Start with the lowest magnification lens.

  7. Focus using the coarse, then fine focus.

  8. Increase magnification if needed.

  9. Draw and label what you see (no shading, clear lines).

Key exam words: stain, magnification, resolution

2. Osmosis (Potato Cylinders)

Aim: Investigate the effect of sugar solution concentration on mass of potato.

Method:

  1. Use a cork borer to cut equal-sized potato cylinders.

  2. Measure and record initial mass of each cylinder.

  3. Place cylinders into different concentrations of sugar solution.

  4. Leave for 24 hours.

  5. Remove, blot dry, and measure final mass.

  6. Calculate percentage change in mass.

  7. Plot a graph of concentration vs % change.

Variables:

  • IV: Sugar concentration

  • DV: % change in mass

  • Controls: Time, temperature, size of potato

🍞 3. Food Tests

Aim: Identify biological molecules in food.

Method:

  • Starch: Add iodine → blue-black

  • Sugars: Add Benedict’s + heat → green/yellow/orange

  • Protein: Add Biuret → lilac

  • Lipids: Ethanol + water → milky emulsion

Control: Distilled water (should not change colour)

4. Enzymes (Amylase)

Aim: Investigate the effect of pH (or temperature) on enzyme activity.

Method (pH example):

  1. Set up water baths at constant temperature.

  2. Mix amylase + buffer solution (different pH values).

  3. Add starch solution.

  4. Every 30 seconds, place a drop onto iodine on a spotting tile.

  5. Record time when iodine stays brown.

  6. Repeat for each pH.

  7. Calculate rate = 1 ÷ time.

Variables:

  • IV: pH

  • DV: Time for starch to be broken down

  • Controls: Temperature, volume, concentration

5. Photosynthesis (Pondweed)

Aim: Investigate effect of light intensity on rate of photosynthesis.

Method:

  1. Place pondweed in water with sodium hydrogen carbonate.

  2. Position a lamp at a measured distance.

  3. Leave for 2 minutes to acclimatise.

  4. Count oxygen bubbles for 1 minute.

  5. Repeat at different distances.

  6. Calculate light intensity = 1 ÷ distance².

Variables:

  • IV: Light intensity

  • DV: Bubbles per minute

  • Controls: Temperature, CO₂ concentration