Cellular Respiration

Lessons

Senior 3 · Biology

Cellular Respiration

4 lessons

Lesson 2 of 4

Aerobic respiration and how to measure it

25 min

A straw and a tube of limewater

Breathe out gently through a straw into a test tube of clear limewater and, within seconds, it turns milky. Limewater turns milky with carbon dioxide, and the air you breathe out has about a hundred times as much carbon dioxide as the air you breathe in. That carbon dioxide was not in the air: it was made a minute earlier, in the cells of your body, by aerobic respiration.

Definition

Aerobic respiration: the release of energy from glucose using oxygen, producing carbon dioxide and water. It takes place mostly in the mitochondria.

Respirometer: a piece of apparatus that measures the rate of respiration of small organisms, usually by measuring how fast they take in oxygen.

Hydrogencarbonate indicator: a solution that changes colour with the amount of carbon dioxide: red at normal levels, yellow when carbon dioxide rises, purple when it falls.

The equation

In words:

\text{glucose} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water} \quad (+\ \text{energy released})

As a balanced chemical equation:

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

Complete breakdown of one mole (180 g) of glucose releases about 2870 kJ. Part of this energy is used to make ATP; the rest is released as heat.

How many ATP? Many textbooks and examination papers give 38 ATP for each glucose molecule in aerobic respiration. This is the traditional, theoretical maximum. Modern measurements show that real cells make about 30 to 32 ATP, because some energy is lost on the way. In this unit we use 38 in calculations, and quote 30 to 32 as the real value. Either way, it is far more than the 2 ATP made without oxygen (Lesson 3).

Aerobic respiration happens in stages: the first stage, splitting glucose in two, takes place in the cytoplasm; the later stages, which need oxygen and make most of the ATP, take place in the mitochondria. That is why very active cells, such as muscle and sperm cells, are packed with mitochondria.

Showing that living things respire

Germinating seeds are ideal for experiments: they respire fast, because they are growing, and they cannot photosynthesise yet. Every experiment needs a control of seeds that have been killed by boiling, then cooled and rinsed in a weak disinfectant so that bacteria and fungi on them cannot respire either.

Carbon dioxide is produced. Seal germinating seeds in a tube above hydrogencarbonate indicator (on a gauze, not touching it). Within an hour or so the indicator turns from red to yellow; with the boiled seeds it stays red.
Heat is released. Put germinating seeds in one vacuum flask and boiled, disinfected seeds in another, each with a thermometer held in cotton wool, and read the temperatures daily for a few days.
Oxygen is used. Use a respirometer.

A simple respirometer

Figure 1: A simple respirometer. The control tube is set up in the same way, with its own capillary tube, but with glass beads instead of seeds.

How it works. The seeds take in oxygen and give out carbon dioxide. The soda lime absorbs the carbon dioxide, so the only change in the gas in the tube is the loss of oxygen. The volume of gas shrinks and the pressure falls, so the drop of coloured liquid is pushed along the capillary towards the seeds by the air outside. The distance the drop moves in a set time measures the volume of oxygen used.

The water bath keeps the temperature steady, because gas expands when it is warmed and would move the drop even without respiration. The control tube shows any movement caused by changes in temperature or air pressure, so it can be subtracted.

Warning

Safety. Soda lime is corrosive: handle it with a spatula, keep it below the gauze so the organisms cannot touch it, and wear eye protection. If you use small animals such as woodlice or maggots instead of seeds, handle them gently, keep the experiment short and return them to their habitat afterwards.

Worked example 1: a respiration rate

In a respirometer with germinating beans, the drop moved 30 mm in 10 minutes; in the control it moved 2 mm in the same direction. The capillary has an internal radius of 0.5 mm. Calculate the rate of oxygen uptake in mm³ per minute. (Volume of a cylinder $= \pi r^2 \times \text{length}$ .)

Correct for the control: $30 - 2 = 28\ \text{mm}$ is due to respiration.
Volume of oxygen: $\pi \times 0.5^2 \times 28 = \pi \times 0.25 \times 28 \approx 22.0\ \text{mm}^3$ .
Rate: $\frac{22.0\ \text{mm}^3}{10\ \text{min}} = 2.2\ \text{mm}^3$ per minute.

Worked example 2: the vacuum flasks

Results of the heat experiment: (illustrative data)

Day	Flask A: germinating peas (°C)	Flask B: boiled, disinfected peas (°C)
0	21	21
1	24	21
2	27	21
3	29	21

Result: flask A warmed by $29 - 21 = 8\ ^\circ\text{C}$ in 3 days; flask B did not warm at all.
Explanation: the living peas respired and released part of the energy from their food as heat; the vacuum flask stopped the heat escaping.
Why disinfect B? Bacteria and fungi on dead peas would respire too, and warm the flask; then B would not be a fair control.
Why are the flasks turned upside down in some versions? Carbon dioxide is denser than air, so in an upright flask it would collect around the seeds; inverting the flask lets it sink out through the cotton wool plug.

Warning

Common mistakes. (1) Saying the drop moves because the seeds "suck in" the liquid: the pressure in the tube falls as oxygen is used and carbon dioxide is absorbed, so outside air pushes the drop in. (2) Forgetting the soda lime: without it, the carbon dioxide given out would roughly replace the oxygen used, and the drop would hardly move. (3) Writing 38 ATP as if it were measured: it is the traditional theoretical figure; real cells make about 30 to 32.

Practical: Investigation of the rate of respiration in germinating seeds, Oxford Mastering Biology. Watch how the respirometer is set up and how the movement of the liquid is read; compare it with Figure 1.

Tip

Memory hook: "6, 6, 6". One glucose needs 6 oxygen and makes 6 carbon dioxide and 6 water: $\mathrm{C_6H_{12}O_6} + 6\mathrm{O_2} \rightarrow 6\mathrm{CO_2} + 6\mathrm{H_2O}$ .

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Practice: aerobic respiration

Write the word equation and the balanced chemical equation for aerobic respiration.
In which part of the cell does most aerobic respiration take place?
Using the traditional figure, how many ATP molecules are made from 5 glucose molecules in aerobic respiration? What does the modern figure give?
Why are germinating seeds, not leaves, used to show that plants produce carbon dioxide in respiration?
What is the job of the soda lime in a respirometer?
In a respirometer the drop moved 45 mm in 15 minutes; the control drop did not move. The capillary radius is 0.5 mm. Calculate the rate of oxygen uptake in mm³ per minute.
Predict what would happen to the rate in question 6 if the water bath were 10 °C cooler, and explain.
In the vacuum flask experiment, why is a thermometer in boiled peas included, and why are those peas disinfected?

Show answersHide answers

Glucose + oxygen → carbon dioxide + water (+ energy released); $\mathrm{C_6H_{12}O_6} + 6\mathrm{O_2} \rightarrow 6\mathrm{CO_2} + 6\mathrm{H_2O}$ .
In the mitochondria.
$5 \times 38 = 190$ ATP; with about 30 to 32 per glucose, about 150 to 160.
Leaves photosynthesise in light and take up carbon dioxide, hiding the carbon dioxide made by respiration; germinating seeds have no chlorophyll yet.
It absorbs the carbon dioxide given out, so the movement of the drop measures only the oxygen used.
$\pi \times 0.5^2 \times 45 \approx 35.3\ \text{mm}^3$ in 15 minutes, so about $2.4\ \text{mm}^3$ per minute.
The rate would fall: respiration is controlled by enzymes, which work more slowly at lower temperatures.
It is the control: it shows that the rise in temperature is caused by living, respiring seeds. Disinfecting kills bacteria and fungi whose respiration would also release heat.

Key points

$\mathrm{C_6H_{12}O_6} + 6\mathrm{O_2} \rightarrow 6\mathrm{CO_2} + 6\mathrm{H_2O}$ ; about 2870 kJ per mole of glucose; 38 ATP per glucose by the traditional count, about 30 to 32 in real cells.
Experiments with a dead control show carbon dioxide production (indicator), heat (vacuum flasks) and oxygen uptake (respirometer).
In a respirometer, soda lime absorbs carbon dioxide; the drop's movement × the capillary's cross-section gives the oxygen used.

Sources and credits

Video: Practical 21.7 Investigation of the rate of respiration in germinating seeds, Oxford Mastering Biology, embedded from YouTube.

Written by Ganzaa: the text, Figure 1, the worked examples (illustrative data) and the practice questions.

1A straw and a tube of limewater

2The equation

3Showing that living things respire

4A simple respirometer

A straw and a tube of limewater

The equation

Showing that living things respire

A simple respirometer