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Year 8 Science · KS3 notes

Digestion, respiration and electricity

Clear notes with worked examples, then 4 practice questions with answers. Written by Beyond Tutors tutors for the KS3 National Curriculum.

Digestion

mouth → oesophagus → stomach → small intestine (nutrients absorbed) → large intestine (water absorbed)

Enzymes break big food molecules into small ones the body can absorb.

RESPIRATION (in every cell)

Aerobic: glucose + oxygen → carbon dioxide + water (+ energy)

Anaerobic (muscles, short of oxygen): glucose → lactic acid (+ less energy)

PHOTOSYNTHESIS (in plants)

carbon dioxide + water → glucose + oxygen (needs light, happens in chloroplasts)

Breathing

Gas exchange happens in the alveoli: oxygen into the blood, carbon dioxide out.

Electricity

Current (I) in amps, potential difference (V) in volts, resistance (R) in ohms.

V=I×RV = I \times R, so R=V÷IR = V \div I. 6 V and 0.5 A → 12 Ω

Magnets

Like poles repel, opposite poles attract.

Practice questions

Try each one, then tap to see the answer and why.

  1. Question 1 · Digestion

    Where is most water absorbed back into the body?

    • AStomach
    • BLarge intestine
    • CSmall intestine
    • DOesophagus
    Show the answer

    B: Large intestine

    The large intestine absorbs water from undigested food.

  2. Question 2 · Respiration

    What is produced by anaerobic respiration in human muscles?

    • ACarbon dioxide and water
    • BAlcohol
    • CLactic acid
    • DOxygen
    Show the answer

    C: Lactic acid

    Without enough oxygen, muscles make lactic acid.

  3. Question 3 · Digestion

    What do enzymes do in digestion?

    • AMake food bigger
    • BTurn food into water
    • CBreak large molecules into small ones the body can absorb
    • DKill all bacteria
    Show the answer

    C: Break large molecules into small ones the body can absorb

    Enzymes are biological catalysts that break down large food molecules.

  4. Question 4 · Respiration

    What is the word equation for aerobic respiration?

    • Aglucose → lactic acid
    • Bcarbon dioxide + water → glucose + oxygen
    • Cglucose + oxygen → carbon dioxide + water
    • Doxygen + water → glucose
    Show the answer

    C: glucose + oxygen → carbon dioxide + water

    Aerobic respiration uses oxygen to release energy from glucose.

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