Unit 8 Cellular respiration
Key unit competence: Explain aerobic and anaerobic respiration and their applications in daily life.
Key ideas
- Cellular respiration is the chemical breakdown of food (mainly glucose) in all living cells to release energy, which is stored in ATP (adenosine triphosphate). It is controlled by enzymes and happens all the time in every living cell.
- ATP is the energy currency of the cell: when ATP is broken into ADP + phosphate, energy is released for muscle contraction, active transport, making proteins (growth), cell division, nerve impulses and keeping body temperature.
- Aerobic respiration uses oxygen: glucose + oxygen → carbon dioxide + water + much energy (about 38 ATP per glucose). C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. It starts in the cytoplasm (glycolysis) and is completed in the mitochondria.
- Anaerobic respiration releases energy without oxygen and produces only 2 ATP per glucose, because glucose is broken down only partly.
- In yeast and plants (alcoholic fermentation): glucose → ethanol + carbon dioxide + a little energy.
- In human muscles during hard exercise: glucose → lactic acid + a little energy. Lactic acid causes fatigue and cramp. The extra oxygen needed afterwards to break down lactic acid is the oxygen debt; we keep breathing fast after exercise to repay it.
- Comparison: aerobic needs O₂, gives CO₂ and water, much energy, complete breakdown; anaerobic needs no O₂, gives lactic acid or ethanol + CO₂, little energy, incomplete breakdown.
- Experiments: limewater turns milky with CO₂ (or hydrogencarbonate indicator turns yellow); germinating seeds in a vacuum flask produce heat; a respirometer with soda lime (absorbs CO₂) measures oxygen uptake by small organisms.
- Applications of anaerobic respiration: bread making (yeast CO₂ makes dough rise; alcohol evaporates in baking), brewing (banana and sorghum beer, wine: ethanol), yoghurt and cheese (lactic acid bacteria), biogas production, silage for cattle.
Common misconceptions
- "Respiration is breathing." No: respiration is a chemical process in cells; breathing only moves air.
- "Plants respire only at night." No: plants respire all the time; in light photosynthesis hides the CO₂ they produce.
- "Anaerobic respiration produces no carbon dioxide." No: yeast produces CO₂ and ethanol; only lactic acid fermentation in muscles produces no CO₂.
- "Respiration makes energy." No: energy cannot be made; respiration releases energy stored in food.
Worked examples
Example 1. Explain why a sprinter keeps panting for several minutes after a 100 m race.
Solution
During the race the muscles needed energy faster than oxygen could be supplied, so they respired anaerobically, producing lactic acid. This created an oxygen debt. After the race, deep fast breathing supplies the extra oxygen needed to break down the lactic acid (in the liver) to CO₂ and water or back to glucose.
Example 2. Explain why bread dough containing yeast rises in a warm place.
Solution
Yeast respires the sugar in the dough anaerobically (fermentation), producing carbon dioxide gas and ethanol. The CO₂ forms bubbles that make the dough rise. Warmth speeds up yeast enzymes. During baking the heat kills the yeast and the ethanol evaporates.
Practice questions
Define respiration and write the word equation for aerobic respiration.
Show answer
Respiration is the breakdown of food in living cells to release energy. Glucose + oxygen → carbon dioxide + water + energy (ATP).
Common mistake: Writing oxygen as a product.
State four uses of energy released in respiration.
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Muscle contraction; active transport; building proteins and growth; cell division; nerve impulses; maintaining body temperature.
Common mistake: Saying energy is used to make oxygen.
Give three differences between aerobic and anaerobic respiration.
Show answer
Aerobic uses oxygen, anaerobic does not; aerobic gives CO₂ and water, anaerobic gives lactic acid or ethanol (+CO₂); aerobic releases much energy (38 ATP), anaerobic little (2 ATP); aerobic completes in mitochondria, anaerobic in cytoplasm.
Common mistake: Saying anaerobic gives more energy.
Write the word equations for anaerobic respiration in (a) yeast; (b) muscles.
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(a) Glucose → ethanol + carbon dioxide + little energy. (b) Glucose → lactic acid + little energy.
Common mistake: Including oxygen as a reactant.
Explain what is meant by oxygen debt.
Show answer
The extra oxygen needed after vigorous exercise to break down lactic acid produced during anaerobic respiration; it is repaid by continued fast, deep breathing.
Common mistake: Saying oxygen debt means the body has too much oxygen.
Describe an experiment to show that germinating seeds produce carbon dioxide.
Show answer
Put germinating seeds in a flask connected to a tube of limewater (with a control flask of boiled seeds); after some time the limewater with living seeds turns milky, the control stays clear.
Common mistake: Leaving out the control.
State three uses of anaerobic respiration in industry or at home.
Show answer
Bread making (CO₂ raises dough); brewing beer and wine (ethanol); yoghurt and cheese (lactic acid); biogas production; silage.
Common mistake: Giving uses of photosynthesis.
Explain why flooding a field kills many crops.
Show answer
Water fills soil air spaces so roots lack oxygen; they respire anaerobically, releasing little energy and toxic ethanol; active transport stops, and roots die.
Common mistake: Saying crops drown because they breathe water.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Banana beer (urwagwa) is made by: a. fermentation by yeast 2. Aerobic respiration is completed in the: b. anaerobic microbes digesting waste 3. Biogas is produced by: c. lactic acid 4. Anaerobic respiration in human muscles produces: d. mitochondria Show answer
1 → a (fermentation by yeast) · 2 → d (mitochondria) · 3 → b (anaerobic microbes digesting waste) · 4 → c (lactic acid)
- Yeast produces ethanol.
- Glycolysis starts in the cytoplasm.
- It contains methane.
- It causes fatigue and cramp.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
nucleus · cytoplasm · carbon dioxide and water · oxygen is being used up- In a respirometer with soda lime, the coloured liquid moves towards the insects. This shows .
- The breakdown of glucose to pyruvate occurs in the .
- The products of aerobic respiration are .
Show answer
1: oxygen is being used up · 2: cytoplasm · 3: carbon dioxide and water
- CO₂ is absorbed, so the volume drop is O₂ used.
- This is glycolysis.
- Much energy is also released.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Anaerobic respiration releases less energy because: a. lactic acid 2. Making yoghurt uses bacteria that produce: b. glucose is only partly broken down 3. The energy currency of the cell is: c. rise 4. Germinating seeds in a vacuum flask make the temperature: d. ATP Show answer
1 → b (glucose is only partly broken down) · 2 → a (lactic acid) · 3 → d (ATP) · 4 → c (rise)
- Energy remains in lactic acid or ethanol.
- The acid thickens and sours milk.
- ATP releases energy when it becomes ADP.
- Respiration releases heat.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
ethanol and carbon dioxide · lactic acid only · food such as glucose · muscle contraction and active transport- Anaerobic respiration in yeast produces .
- Energy from respiration is used for .
- Respiration releases energy from .
Show answer
1: ethanol and carbon dioxide · 2: muscle contraction and active transport · 3: food such as glucose
- This is alcoholic fermentation.
- Also growth and nerve impulses.
- The energy is stored in ATP.