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Topic 12 · 12.1–12.3

Respiration

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • State the uses of energy in living organisms, and investigate the effect of temperature on respiration in yeast.
  • Describe aerobic respiration and state its word equation; state the balanced equation EXTENDED.
  • Describe anaerobic respiration and state the word equations for yeast and muscles; the balanced equation for yeast EXTENDED.
  • Describe the oxygen debt and how it is removed EXTENDED.

📚The biology

Why organisms respire

Respiration releases energy from nutrient molecules (mainly glucose). It happens in every living cell, all the time. It is not the same as breathing.

Energy released by respiration is used for muscle contraction, protein synthesis, cell division, active transport, growth, the passage of nerve impulses, and keeping body temperature constant.
The seven uses on the syllabus.

Aerobic respiration

Aerobic respiration is the chemical reactions in cells that use oxygen to break down nutrient molecules to release energy. It takes place mainly in the mitochondria.

glucose + oxygen → carbon dioxide + water

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

EXTENDED The balanced equation is Supplement. It is photosynthesis in reverse, but the two are not the same process run backwards: they use different enzymes in different organelles.

Anaerobic respiration

Anaerobic respiration is the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen. It releases much less energy per glucose molecule than aerobic respiration.

Three rows. Aerobic respiration in all cells with oxygen: glucose plus oxygen gives carbon dioxide plus water, releasing much more energy. Anaerobic respiration in yeast: glucose gives alcohol plus carbon dioxide. Anaerobic respiration in muscles during vigorous exercise: glucose gives lactic acid. Both anaerobic types release much less energy.
Anaerobic respiration breaks glucose down only partly, so most of its energy stays in the products (alcohol or lactic acid).

In yeast: glucose → alcohol + carbon dioxide. In muscles during vigorous exercise: glucose → lactic acid.

\[ \mathrm{C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2} \]

EXTENDED The balanced equation for yeast is Supplement. Yeast’s anaerobic respiration is used in bread-making and biofuels (topic 21).

The oxygen debt EXTENDED

A graph of oxygen against time. Before exercise, oxygen taken in matches oxygen needed. During exercise from 5 to 15 minutes, the oxygen needed jumps up, but the oxygen taken in rises more slowly and stays below it. After exercise the oxygen needed returns to the resting value, but oxygen taken in stays high and falls slowly: the extra oxygen repays the oxygen debt.
EXTENDED During exercise the muscles need more oxygen than the blood can supply, so they also respire anaerobically. (Schematic.)

EXTENDED During vigorous exercise, lactic acid builds up in muscles and blood, causing an oxygen debt. After exercise it is removed by:

  • EXTENDED a continued fast heart rate, transporting lactic acid in the blood from the muscles to the liver;
  • continued deeper and faster breathing, supplying oxygen for aerobic respiration of lactic acid;
  • aerobic respiration of lactic acid in the liver.

Investigating respiration in yeast

A test-tube of yeast in glucose solution stands in a water bath. A delivery tube from the top leads into a tube of water, where bubbles of gas are counted per minute. Limewater could be used instead to show the gas is carbon dioxide.
Change the water-bath temperature; keep the volume and concentration of yeast and glucose the same.

✏️Worked example

The yeast apparatus was used at six temperatures. The results are plotted below. (a) State the temperature with the fastest rate of respiration. [1] (b) Explain the increase in rate from 10 °C to 40 °C. [2] (c) Explain the result at 60 °C. [2] (d) Suggest why the glucose solution was boiled and cooled before the yeast was added. [1]
A line graph of bubbles per minute against temperature: 4 at 10 degrees Celsius, 10 at 20, 22 at 30, 34 at 40, 18 at 50 and 2 at 60.

(a) 40 °C (34 bubbles per minute).

(b) Respiration is controlled by enzymes. As temperature rises, molecules have more kinetic energy, so enzyme and substrate collide more often and the reactions of respiration are faster, releasing more carbon dioxide.

(c) The yeast’s enzymes are denatured at high temperature, so respiration almost stops and very little carbon dioxide is produced.

(d) Boiling removes dissolved oxygen, so the yeast respires anaerobically (and kills any other microorganisms).

Check it. The rate at 40 °C is 34 ÷ 4 = 8.5 times the rate at 10 °C: the sharp rise then sharp fall is the enzyme pattern from topic 5.
Counting bubbles. Bubbles vary in size, so volume collected in a gas syringe is more accurate. Say so if asked for an improvement.

📝Practise

In the style of the multiple-choice, theory and practical papers. EXTENDED marks Supplement content.

1. (Multiple choice.) What are the products of anaerobic respiration in human muscles? A: carbon dioxide and water. B: alcohol and carbon dioxide. C: lactic acid only. D: lactic acid and carbon dioxide.
C.
2. (Theory.) State three uses of energy from respiration in the human body. [3]
Any three: muscle contraction; protein synthesis; cell division; active transport; growth; passage of nerve impulses; maintaining a constant body temperature.
3. (Theory.) State the word equation for aerobic respiration. [2]
glucose + oxygen → carbon dioxide + water.
4. (Theory.) State one difference between aerobic and anaerobic respiration other than the use of oxygen. [1]
Aerobic releases much more energy per glucose molecule (or: different products — carbon dioxide and water, against lactic acid or alcohol and carbon dioxide).
5. (Theory.) Explain why bread dough rises when yeast is added. [2]
Yeast respires the sugar in the dough, producing carbon dioxide gas, which is trapped as bubbles and makes the dough expand.
6. (Theory.) EXTENDED Write the balanced equation for anaerobic respiration in yeast. [2]
\( \mathrm{C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2} \).
7. (Theory.) EXTENDED After a sprint, an athlete keeps breathing deeply and quickly for several minutes. Explain why. [3]
Lactic acid built up during anaerobic respiration (an oxygen debt). Extra oxygen is needed for the aerobic respiration of the lactic acid, mainly in the liver; the fast heart rate carries the lactic acid there.
8. (Practical.) In the yeast experiment, suggest a control and explain its purpose. [2]
An identical tube with boiled (killed) yeast, or with no yeast. It shows that the gas is produced by the living yeast and not by anything else.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • Royal Society of Biology — investigating respiration in yeast
  • BBC Bitesize — aerobic and anaerobic respiration