OxfordAQA International GCSE Biology: Breathing and respiration (9201) – Revision Notes

These notes condense sections 3.2.5 (Breathing) and 3.2.6 (Respiration) of the OxfordAQA International GCSE Biology (9201) specification, Version 5.2, for exams in May/June 2018 onwards. For full explanations and worked examples, read the study guide. Test yourself with the practice set, track progress on the checklist, find other topics in the course hub, and see where you stand with the free diagnostics. Related existing pages: bioenergetics revision notes.

3.2.5 Breathing at a glance

The system (3.2.5 a)

• Function: moves air in and out, letting oxygen pass into the blood and carbon dioxide pass out by diffusion.
• Lungs are in the thorax, protected by the ribcage, separated from the abdomen by the diaphragm.
• Air path: trachea, bronchi, bronchioles, alveoli.
• Recognise on a diagram: ribs, intercostal muscles, diaphragm, lungs, trachea, bronchi, bronchioles, alveoli.

Inhaling and exhaling (3.2.5 b)

	Inhale	Exhale
Intercostals	contract, ribcage up	relax, ribcage down
Diaphragm	contracts, flattens	relaxes, domed
Thorax volume	up	down
Pressure	below outside air	above outside air
Air	enters	leaves

Method in steps for a 4-mark explanation:

1. Name the muscles and what they do.
2. State the volume change.
3. State the pressure change compared with the surrounding air.
4. State which way the air moves.

Alveoli (3.2.5 c)

• Very large surface area.
• Richly supplied with blood capillaries.
• Result: gases diffuse readily into and out of the blood.
• Helpful extras: thin walls (short diffusion distance), breathing keeps renewing the air.

Ventilators (3.2.5 d)

	Negative pressure	Positive pressure
Effect	air is drawn into the lungs	air is forced into the lungs
Outline	pressure lowered around the chest in a sealed chamber	air pushed into the airway above outside pressure
Used when	spontaneous breathing has stopped through disease or injury	same

3.2.6 Respiration at a glance

Key facts (3.2.6 a-f)

• Respiration transfers energy in cells; aerobic uses oxygen, anaerobic does not.
• Aerobic: glucose + oxygen → carbon dioxide + water.
• C6H12O6 + 6O2 → 6CO2 + 6H2O
• Happens continuously in plants and animals; most reactions occur inside mitochondria.
• Energy is used to: build larger molecules from smaller; contract muscles (animals); keep a steady body temperature in cold surroundings (mammals and birds); build sugars, nitrates and other nutrients into amino acids and then proteins (plants).

Exercise (3.2.6 g-h)

• Heart rate up, so more blood flow to the muscles.
• Breathing rate and depth up.
• Muscle glycogen converted back to glucose.
• Effect: more glucose and oxygen supplied; carbon dioxide removed faster.

Anaerobic respiration (3.2.6 i-m)

	Muscle (animals)	Plants and some microorganisms
Equation in words	glucose → lactic acid	glucose → ethanol + carbon dioxide
Symbols	C6H12O6 → 2C3H6O3	C6H12O6 → 2C2H5OH + 2CO2
Energy transferred	much less than aerobic	much less than aerobic
Extra point	oxygen debt, fatigue	used by yeast

• Anaerobic respiration in muscle: incomplete breakdown of glucose, so lactic acid builds up.
• Oxygen debt: oxygen needed afterwards to oxidise lactic acid to carbon dioxide and water.
• Fatigue: muscles stop contracting efficiently; one cause is lactic acid build-up; blood flowing through the muscles eventually removes it.

Must-know distinctions

• Breathing (moving air) versus respiration (chemical reactions in cells).
• Aerobic (needs oxygen, complete breakdown, more energy) versus anaerobic (no oxygen, incomplete in muscle, less energy).
• Negative pressure (draws air in by lowering pressure around the chest) versus positive pressure (forces air in).
• Lactic acid (animal muscle) versus ethanol and carbon dioxide (plants and yeast).
• Oxygen debt is repaid after exercise; it is not a store of oxygen.

Required practical: effects of exercise

1. Record resting pulse and breathing rate; repeat and take a mean.
2. Exercise for a set time at a set intensity (for example stepping on and off a step).
3. Measure pulse and breathing straight afterwards.
4. Keep measuring at regular intervals until values return to resting.
5. Control: same person, same step, same timing; change only one thing at a time.

Shortcut: count beats or breaths for 15 seconds and multiply by 4.

Quick calculations

Percentage change = (new - original) / original x 100.

Worked reminder: pulse 72 bpm at rest, 126 bpm after exercise. Increase = 54. 54 / 72 x 100 = 75%.

Worked reminders

Balancing the equations

Aerobic: carbon 6 on each side; hydrogen 12 = 12; oxygen 6 + 12 = 18 on the left and 12 + 6 = 18 on the right.

Lactic acid: 2C3H6O3 has 6 carbon, 12 hydrogen and 6 oxygen, matching glucose.

Ethanol: 2C2H5OH + 2CO2 has 6 carbon, 12 hydrogen and 6 oxygen, matching glucose.

Reading a recovery table

Time after exercise (min)	0	10	20	30
Lactic acid (mmol/dm3)	12.5	8.0	4.5	2.0

• Fall from 0 to 10 min: 4.5 mmol/dm3 (36%).
• Rate from 10 to 20 min: 3.5 / 10 = 0.35 mmol/dm3 per minute.
• Fastest fall is at the start, when the concentration is highest.
• Link to theory: the blood removes lactic acid and oxygen oxidises it, repaying the oxygen debt.

Rate from a count

38 bubbles of gas from yeast in 5 minutes is 38 / 5 = 7.6 bubbles per minute. The gas is carbon dioxide.

Model answers to learn

Why do breathing rate and heart rate rise in exercise?

Muscles need more energy transferred, so more glucose and oxygen must reach them and more carbon dioxide must be removed. A faster, deeper breathing rate takes in more oxygen. A faster heart rate increases blood flow to the muscles. Glycogen in the muscles is converted back to glucose, adding to the glucose available.

Why is less energy transferred in anaerobic respiration?

Glucose is only partly broken down, to lactic acid. The lactic acid still holds energy, which is only released when it is later oxidised using oxygen.

Why do you keep breathing hard after you stop?

Oxygen is needed to oxidise the lactic acid that built up, to carbon dioxide and water. This is repaying the oxygen debt.

Why do muscles become fatigued?

During long vigorous activity lactic acid builds up in the muscles. Muscles stop contracting efficiently. Blood flowing through the muscles eventually removes the lactic acid.

Why can a ventilator save a life?

When spontaneous breathing stops, oxygen cannot reach the blood. A mechanical ventilator moves air into the lungs so oxygen can still diffuse into the blood and carbon dioxide can diffuse out.

Plants and respiration

• Plants respire continuously, including at night.
• Energy from plant respiration builds sugars, nitrates and other nutrients into amino acids, then proteins.
• When oxygen is short, plant cells can respire anaerobically and produce ethanol and carbon dioxide.
• Do not confuse respiration (transfers energy from glucose in all living cells) with photosynthesis (builds glucose in leaves in the light).

Quick self-test

1. Name the muscles that move the ribcage during inhalation.
2. What happens to the shape of the diaphragm when it contracts?
3. State the change in thorax pressure that makes air enter the lungs.
4. Give two adaptations of alveoli for gas exchange.
5. What is the difference between a negative pressure and a positive pressure ventilator?
6. Write the word equation for aerobic respiration.
7. A student counts 21 heartbeats in 15 seconds. What is the pulse per minute?
8. Breathing rate rises from 12 to 30 breaths per minute during exercise. Calculate the percentage increase.
9. Give two ways the energy from respiration is used in a plant.
10. What gases are produced when yeast respires anaerobically?
11. What is the oxygen debt used for?
12. Blood lactic acid falls from 9.0 to 3.0 mmol/dm3 in 30 minutes. What percentage is that?

Answers

1. Intercostal muscles (and the diaphragm, which is also involved).
2. It flattens.
3. Pressure falls below that of the air outside the body.
4. Very large surface area; richly supplied with blood capillaries.
5. Negative pressure draws air in; positive pressure forces air in.
6. glucose + oxygen → carbon dioxide + water.
7. 21 x 4 = 84 bpm.
8. (30 - 12) / 12 x 100 = 150%.
9. Any two: build larger molecules from smaller ones; build sugars, nitrates and other nutrients into amino acids; build amino acids into proteins.
10. Ethanol and carbon dioxide.
11. To oxidise lactic acid to carbon dioxide and water.
12. (9.0 - 3.0) / 9.0 x 100 = 66.7%, a fall of about 67%.

Where marks are usually lost

• Saying the diaphragm moves up and relaxes on inhaling.
• Missing the pressure step: volume change alone does not explain air entering.
• Describing a ventilator without saying whether pressure is lowered around the chest or raised in the airway.
• Writing “respiration is breathing”.
• Writing “energy is made” instead of transferred.
• Naming lactic acid for yeast or ethanol for muscle.
• Percentage change divided by the new value rather than the original.
• Forgetting to give units (bpm, breaths per minute, mmol/dm3).
• Leaving out the “repaid by oxidising lactic acid” part of oxygen debt.

Official syllabus

OxfordAQA International GCSE Biology (9201), Version 5.2, for exams in May/June 2018 onwards, published by OxfordAQA. Sections 3.2.5 and 3.2.6.