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Unit 1 · Nutrition, Respiration & Gas Exchange

Unit 1 — Nutrition, Respiration and Gas Exchange — Edexcel IGCSE Biology Modular - UNIT 1 Revision Notes & Topical Questions

5 min read 7 parts Lesson 1 of 4 Unit quiz

📚 Edexcel IGCSE Biology Modular - UNIT 1 Hub / Topical Revision
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Lesson summary

Nutrition, Respiration and Gas Exchange: Overview — EDEXCEL MODULAR - OL notes: Summarise photosynthesis, human diet and digestion, respiration and gas exchange, with key investigations.Photosynthesis is the process by which plants make glucose from carbon dioxide and water using…

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Absorption
  • Aerobic respiration

Section 1 of 7Plant nutrition: photosynthesis

Photosynthesis is the process by which plants make glucose from carbon dioxide and water using light energy absorbed by chlorophyll: carbon dioxide + water → glucose + oxygen.

Plant nutrition: photosynthesis
The glucose is used for respiration, converted to starch for storage, to cellulose for cell walls, and with nitrate to make amino acids and proteins.
Magnesium ions are needed for chlorophyll and nitrate ions for amino acids; deficiency causes yellow leaves and stunted growth.
The palisade layer absorbs light, and the spongy layer has air spaces for gas exchange.

Testing a leaf for starch (to show photosynthesis took place):

1
De-starch the plant in the dark for 24 hours.
2
Expose the plant to the test condition (for example light with part of the leaf covered by foil, or carbon dioxide removed with potassium hydroxide).
3
Boil the leaf in water to kill it and soften it.
4
Heat it in ethanol (in a water bath, never a flame) to remove chlorophyll.
5
Rinse in water and add iodine solution. Blue-black means starch is present; yellow-brown means none.

Section 2 of 7Human nutrition

A balanced diet contains all food groups in the correct proportions: carbohydrates (energy), proteins (growth and repair), lipids (energy store, insulation), vitamins and minerals (health), fibre (bulk to push food through the gut) and water.

Human nutrition
Digestion breaks large insoluble molecules into small soluble ones that can be absorbed.
Mechanical digestion (chewing, churning, bile emulsification) increases surface area; chemical digestion uses enzymes (amylase, proteases, lipase) in the mouth, stomach (with hydrochloric acid, pH about 2) and small intestine (alkaline, pH 8-9).
Absorption happens in the villi of the small intestine.

Energy content of foodburn the food under water and use energy per gram = mass of water x temperature rise x 4.2 / mass of food.

Section 3 of 7Respiration and gas exchange

Aerobic respirationC6H12O6 + 6O2 → 6CO2 + 6H2O + energy.

Respiration and gas exchange
Anaerobic respiration in muscles produces lactic acid; in yeast it produces ethanol and carbon dioxide.
Germinating seeds in a flask with limewater turn it cloudy; boiled seeds act as a control.
A vacuum flask reduces heat loss so the small temperature rise from respiring seeds can be detected,
  • and sterilising the seeds removes microorganisms that would respire too.

In the lungs, air passes via the trachea, bronchi and bronchioles to the alveoli. Alveoli are adapted for diffusion: a large surface area, walls one cell thick, a good blood supply and a moist lining. Oxygen diffuses into the blood and carbon dioxide diffuses out.

Section 4 of 7Putting the topics together in extended answers

Six-mark questions in this area need a linked line of reasoning: link nutrition to energy, to respiration, to gas exchange.

1
For example: food is digested to glucose, absorbed into the blood, carried to cells; oxygen from alveoli is carried by red blood cells; in the mitochondria aerobic respiration releases energy; carbon dioxide diffuses back into the blood and is breathed out.
2
Avoid vague words such as 'it', 'stuff' or 'energy is made'; name the substance and process.
3
Do not extrapolate a graph beyond the data.
4
The rate of photosynthesis can be limited by light intensity, carbon dioxide concentration or temperature, whichever is in shortest supply.

Section 5 of 7Worked examples

  1. Write the balanced equation for aerobic respiration and check the atoms. [3]3 marks

  2. Tube A has germinating seeds and tube B boiled seeds, both with limewater. Predict and explain. [3]3 marks

  3. Why is ethanol heated in a water bath when testing a leaf? [1]1 mark

  4. Which feature of the alveolus mainly increases the rate of diffusion? [1]1 mark

Section 6 of 7Key terms

TermMeaning
PhotosynthesisMaking glucose from carbon dioxide and water using light energy.
DigestionBreaking large insoluble molecules into small soluble ones.
AbsorptionMovement of digested molecules into the blood or lymph.
Aerobic respirationRelease of energy from glucose using oxygen.
AlveolusAir sac in the lung where gases are exchanged.
EmulsificationBreaking large fat droplets into smaller ones by bile.

Section 7 of 7Exam technique

1
Heating ethanol over a naked flame.
2
Saying plants only respire at night.
3
Saying bile contains enzymes - it emulsifies fats.
4
Saying emulsification is chemical digestion.

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