Respiration & Gas Exchange

Why organisms respire

Every living cell needs a constant supply of energy. That energy powers muscle contraction, active transport across membranes, building large molecules like proteins from smaller ones, cell division, nerve impulses, and — in mammals and birds — keeping the body warm.

Respiration is the chemical process that releases this energy from food molecules, usually glucose. It happens in every living cell, all the time. Do not confuse it with breathing: breathing is the physical movement of air, while respiration is a reaction inside cells.

Key terms Respiration — the release of energy from glucose inside cells.

Aerobic respiration — respiration using oxygen, releasing a large amount of energy.

Anaerobic respiration — respiration without oxygen, releasing a smaller amount of energy.

Aerobic respiration

Aerobic respiration uses oxygen to break glucose down completely into carbon dioxide and water, releasing a large amount of energy.

The word equation is:

$$\text{glucose} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$$

The balanced symbol equation is:

$$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$$

The energy released is used for the activities listed above — movement, active transport, making molecules, growth, cell division and (in mammals/birds) generating heat.

Exam tip Learn the symbol equation exactly — the numbers must balance. A common slip is writing $O_2$ without the 6 in front, or forgetting that water is a product. Count the atoms: 6 carbon, 12 hydrogen and 18 oxygen on each side.

Anaerobic respiration in humans

When you exercise hard, your muscles cannot get enough oxygen quickly enough. Cells then respire anaerobically — without oxygen. Glucose is only partly broken down into lactic acid:

$$\text{glucose} \rightarrow \text{lactic acid}$$

This releases far less energy than aerobic respiration, because glucose is not fully broken down. It is a useful short-term boost during sprinting or heavy lifting.

Lactic acid builds up in the muscles and blood, causing the burning feeling and fatigue. After exercise you keep breathing hard and fast to take in extra oxygen. This oxygen is needed to break the lactic acid down — the amount required is called the oxygen debt.

Watch out Human anaerobic respiration produces lactic acid only — no carbon dioxide and no water. Carbon dioxide is not a product here. That CO₂ trap catches many students.

Anaerobic respiration in yeast and plants

In yeast and plant cells, anaerobic respiration produces ethanol (alcohol) and carbon dioxide instead of lactic acid:

$$\text{glucose} \rightarrow \text{ethanol} + \text{carbon dioxide}$$

In yeast this process is called fermentation. It is used industrially: the carbon dioxide makes bread dough rise, and the ethanol is the alcohol in beer and wine.

Comparing aerobic and anaerobic respiration

The key idea is energy yield: aerobic respiration releases much more energy per glucose molecule because the glucose is broken down fully. Anaerobic respiration is faster to start and useful in emergencies, but it is inefficient and its products (lactic acid or ethanol) can be harmful if they build up.

Investigating respiration

Heat released by germinating peas. Germinating peas respire rapidly. Place living, germinating peas in one vacuum flask and dead (boiled, then cooled) peas that have been disinfected in a second flask as a control. Insert a thermometer into each, seal with cotton wool, and invert. Over a day or two the flask with living peas warms up, showing respiration releases heat (energy). The control with dead peas does not, proving the warmth comes from living respiration and not from microbes.

Carbon dioxide produced. You can show organisms release CO₂ using an indicator that detects it:

Limewater turns from clear to milky/cloudy when CO₂ bubbles through.
Hydrogencarbonate indicator is red in normal air; rising CO₂ turns it yellow.

Place small organisms (such as germinating peas or woodlice) in a sealed tube connected to the indicator. As they respire, CO₂ collects and the indicator changes colour. A control tube with no organisms shows no change.

Exam tip Always describe the control in respiration practicals — dead/boiled peas for the heat experiment, or an empty tube for the CO₂ experiment. Marks are awarded for explaining that the control rules out other causes.

Features of a good gas exchange surface

Whether in lungs, gills or leaves, efficient gas exchange surfaces share the same adaptations:

Large surface area — more space for gases to diffuse across.
Thin (often one cell thick) — short diffusion distance.
Moist — gases dissolve before diffusing.
Good supply of blood / good ventilation — keeps a steep concentration gradient by removing arriving gas and bringing more.

The human gas exchange system

Air travels in through the nose and mouth, then down this pathway:

trachea → bronchi → bronchioles → alveoli

The trachea (windpipe) is held open by C-shaped rings of cartilage.
It branches into two bronchi, one to each lung.
Bronchi divide into smaller bronchioles.
These end in tiny air sacs, the alveoli, where gas exchange happens.

The alveoli are superbly adapted: there are millions of them giving a huge surface area; their walls are one cell thick; they are moist; and each is wrapped in capillaries carrying blood, maintaining a steep gradient. Oxygen diffuses from the alveolar air into the blood, and carbon dioxide diffuses from the blood into the alveolus to be breathed out.

Breathing (ventilation)

Ventilation moves fresh air in and out, keeping the oxygen high and carbon dioxide low in the alveoli. It relies on the ribs, intercostal muscles and the diaphragm (a sheet of muscle below the lungs).

Inhalation (breathing in):

1. Intercostal muscles contract, pulling the ribs up and out.
2. The diaphragm contracts and flattens, moving down.
3. Chest volume increases, so pressure inside decreases.
4. Air is pushed in from outside (high to low pressure).

Exhalation (breathing out):

1. Intercostal muscles and diaphragm relax.
2. Ribs move down and in; the diaphragm domes upward.
3. Chest volume decreases, so pressure increases.
4. Air is forced out.

Exam tip Muscles only pull — they never push. So during exhalation in normal breathing the muscles relax; the chest gets smaller passively. Write "muscles relax", not "muscles push the air out".

Gas exchange in plants and the leaf

Plant gas exchange happens mainly through small pores on the underside of the leaf called stomata, which open and close using guard cells. Inside, the spongy mesophyll has air spaces and a large internal surface area for gases to diffuse across the thin, moist cell walls.

Plants both respire (all the time) and photosynthesise (in light), so the net gas exchange depends on light:

Real world A sealed jar of pondweed in sunlight releases bubbles of oxygen, but at night the same plant takes in oxygen like an animal. Plants respire 24 hours a day — they do not "breathe out only oxygen". Remembering this stops the classic exam mistake of saying plants only release O₂.