OxfordAQA International GCSE Biology: Breathing and respiration (9201)

This guide covers 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. The qualification is untiered, so everything here is for every candidate. Use the course hub for the other topics and the printable checklist to tick off each statement. When you want short recall pages, use the revision notes; for exam-style questions with worked answers, use the practice set. A quick check of where you stand is on the diagnostics page. For wider context, see the existing bioenergetics study guide.

What this guide covers

Section	What you must be able to do
3.2.5 a	Describe what the breathing system does and recognise its parts on a diagram
3.2.5 b	Explain inhalation and exhalation in terms of muscles, volume and pressure
3.2.5 c	Explain how alveoli suit gas exchange
3.2.5 d	Describe negative and positive pressure mechanical ventilators
3.2.6 a-e	Aerobic and anaerobic respiration, the equation, where it happens
3.2.6 f	Uses of the energy transferred
3.2.6 g-h	Body changes during exercise and why they help
3.2.6 i-l	Anaerobic respiration in muscle, oxygen debt, fatigue
3.2.6 required practical	Investigate the effects of exercise on the human body
3.2.6 m	Anaerobic respiration in plants and microorganisms

3.2.5 a: the breathing system

The breathing system takes air in and out so that oxygen can diffuse from the air into the blood and carbon dioxide can diffuse from the blood into the air. The lungs sit in the thorax. The ribcage protects them. The diaphragm, a sheet of muscle, separates the thorax from the abdomen below.

Air travels in this order: mouth or nose, trachea, two bronchi (one to each lung), many branching bronchioles, then the alveoli at the ends. You must recognise on a diagram the ribs, intercostal muscles (between the ribs), diaphragm, lungs, trachea, bronchi, bronchioles and alveoli.

Diagram tip: the trachea is the single tube down the middle; it splits into two bronchi; the diaphragm is the dome under the lungs; the intercostal muscles lie between neighbouring ribs, not inside the lungs. Lungs contain no muscle of their own, so they cannot pull air in themselves.

3.2.5 b: inhaling and exhaling

Breathing depends on a change in thorax volume, and so a change in the pressure within it. Air always moves from higher to lower pressure.

Stage	Inhalation	Exhalation
Intercostal muscles	Contract, pulling the ribcage upwards	Relax, ribcage moves downwards
Diaphragm	Contracts and flattens	Relaxes and returns to its domed shape
Volume of thorax	Increases	Decreases
Pressure in thorax	Falls below the pressure of the surrounding air	Rises above the pressure of the surrounding air
Air movement	Atmospheric air enters the lungs	Air leaves the lungs

A full answer links the chain: muscle action, then volume change, then pressure change, then air movement. Answers lose credit when they say the lungs “suck in air” or that the diaphragm “pushes up” on inhalation. On inhalation the diaphragm flattens, moving down.

Worked example 1: reading a pressure trace

A sensor records the pressure inside the thorax compared with the air outside. A negative value means lower than the outside air.

Time (s)	0	0.75	1.5	2.25	3.0
Pressure difference (kPa)	0.0	-0.3	0.0	+0.3	0.0

1. From 0 to 1.5 s the pressure is below the outside air, so air is entering: this is inhalation, with the diaphragm flattening and the intercostal muscles contracting.
2. From 1.5 to 3.0 s the pressure is above the outside air, so air is leaving: exhalation, with both muscle groups relaxed.
3. One full breath takes 3.0 s, so the breathing rate is 60 / 3.0 = 20 breaths per minute.

3.2.5 c: alveoli

Alveoli are tiny air sacs at the ends of the bronchioles. Two adaptations are required: they give a very large surface area, and they are richly supplied with blood capillaries. A large surface area gives many places for gas to cross at once. A rich blood supply carries oxygen away and brings carbon dioxide in, which helps keep the concentration difference high so diffusion continues.

You can strengthen an explanation by adding that alveolus walls and capillary walls are very thin, so the diffusion distance is short, and that breathing keeps replacing the air in the alveoli. Both are consistent with the specification wording, but the two required adaptations must be stated first. Oxygen moves from the air in the alveoli into the blood; carbon dioxide moves from the blood into the alveolar air, to be breathed out.

3.2.5 d: mechanical ventilators

A healthy person breathes automatically. Spontaneous breathing may stop because of disease or injury, and then the patient can be helped by a mechanical ventilator. There are two main types.

Type	What it does	How it works in outline
Negative pressure	Air is drawn into the lungs	The chest sits in a sealed chamber; air pressure around the chest is lowered, the chest wall expands, and air is drawn in through the mouth
Positive pressure	Air is forced into the lungs	A machine pushes air into the airway at a pressure above that of the surrounding air; the lungs inflate

The key contrast is the direction of the pressure difference. Negative pressure lowers the pressure outside the lungs so air is drawn in. Positive pressure raises the pressure of the air entering so air is forced in. In both cases air flows from high pressure to low pressure.

3.2.6 a-e: respiration

Respiration in cells transfers energy. It can be aerobic (using oxygen) or anaerobic (without oxygen). Respiration is not the same as breathing: breathing is the movement of air; respiration is the chemical process in cells.

Aerobic respiration uses glucose and oxygen:

glucose + oxygen → carbon dioxide + water

C6H12O6 + 6O2 → 6CO2 + 6H2O

Worked example 2: checking the equation balances

Atom	Left side	Right side
Carbon	6 (in glucose)	6 (six CO2)
Hydrogen	12 (in glucose)	12 (six H2O)
Oxygen	6 + 12 = 18	12 + 6 = 18

Both sides match, so the equation is balanced. If a question asks you to complete it, check that the number of oxygen atoms is the same on both sides.

Plants and animals both respire aerobically without a break, and a plant does so at night as well as by day. Most of the reactions happen inside mitochondria. Active cells, including muscle cells, contain large numbers of mitochondria.

3.2.6 f: what the energy is used for

• to build larger molecules from smaller ones
• in animals, to make muscles contract
• in mammals and birds, to maintain a steady body temperature in colder surroundings
• in plants, to build sugars, nitrates and other nutrients into amino acids, which are then built into proteins

Notice that the energy transferred is used by the cell; it is not “made”. Energy is transferred, not created.

3.2.6 g-h: the body during exercise

Exercise increases the demand for energy. Three changes follow:

1. Heart rate increases, so blood flow to the muscles increases.
2. The rate and depth of breathing increase.
3. Glycogen stored in the muscles is converted back to glucose.

These changes increase the supply of glucose and oxygen to the muscles and increase the rate of removal of carbon dioxide from them. Write all three effects when asked for the benefit.

Worked example 3: calculating changes from exercise data

At rest, a volunteer’s pulse is 68 beats per minute (bpm) and breathing rate is 15 breaths per minute. After 3 minutes of exercise, pulse is 112 bpm and breathing rate is 31 breaths per minute.

Percentage increase = (new value - original value) / original value x 100

• Pulse: (112 - 68) / 68 x 100 = 64.7%, so about 65%
• Breathing rate: (31 - 15) / 15 x 100 = 106.7%, so about 107%

Breathing rate rose by a larger percentage than pulse. Counting for 15 seconds and multiplying by 4 gives a rate per minute, for example 17 beats in 15 s is 68 bpm.

3.2.6 i-l: anaerobic respiration in muscle

If insufficient oxygen reaches the muscles, energy is transferred by anaerobic respiration:

glucose → lactic acid

C6H12O6 → 2C3H6O3

Count the atoms: glucose has 6 carbon, 12 hydrogen and 6 oxygen, and 2C3H6O3 has the same totals. This is an incomplete breakdown of glucose, so lactic acid builds up and much less energy is transferred than in aerobic respiration.

Afterwards, an oxygen debt needs to be repaid to oxidise the lactic acid to carbon dioxide and water. That is why you keep breathing hard after stopping.

During long periods of vigorous activity muscles become fatigued and stop contracting efficiently. One cause of muscle fatigue is the build-up of lactic acid. Blood flowing through the muscles eventually removes the lactic acid.

Worked example 4: reading lactic acid data

Blood lactic acid after a sprint:

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

• Fall in the first 10 minutes: 12.5 - 8.0 = 4.5 mmol/dm3, which is 36% of the starting value (4.5 / 12.5 x 100).
• Average rate of fall from 10 to 20 minutes: (8.0 - 4.5) / 10 = 0.35 mmol/dm3 per minute.
• The fall is faster at the start: the first interval falls by 4.5, the third by only 2.5.
• Explanation: lactic acid is removed by the blood and oxidised, which needs oxygen; this is the oxygen debt being repaid.

Required practical: effects of exercise on the human body

A typical plan: measure pulse and breathing rate at rest, then after a set exercise such as stepping on and off a step for 2 minutes, then at intervals during recovery.

• Independent variable: the exercise (type, duration or intensity).
• Dependent variables: pulse rate and breathing rate.
• Control: same person, same step height and rate, same recording time, same room.
• Repeat and take a mean. For resting pulse readings of 66, 70 and 68 the mean is 68.
• Record recovery time as how long the pulse takes to return to the resting value.
• Safety: stop if anyone feels unwell; check that nobody has a medical condition that makes the exercise unsuitable.

3.2.6 m: anaerobic respiration in plants and microorganisms

In plant cells and some microorganisms, such as yeast, anaerobic respiration produces ethanol and carbon dioxide, not lactic acid:

glucose → ethanol + carbon dioxide

C6H12O6 → 2C2H5OH + 2CO2 (carbon 6 = 4 + 2; hydrogen 12 = 12; oxygen 6 = 2 + 4)

Worked example 5: yeast and bubbles

A yeast suspension in glucose solution is sealed under oil. 38 bubbles of gas are counted in 5 minutes.

Rate = 38 / 5 = 7.6 bubbles per minute.

The gas is carbon dioxide, which turns limewater cloudy. The liquid left behind contains ethanol.

Common errors

• Writing that the diaphragm moves up on inhaling.
• Saying that lungs “inflate by themselves”.
• Mixing up breathing and respiration.
• Saying anaerobic respiration “makes energy” rather than transfers less energy.
• Giving lactic acid as the product in yeast.
• Using the wrong base in a percentage change: divide by the original value.
• Forgetting to explain that lactic acid is oxidised using oxygen when asked about 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.