Respiration

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Respiration in Cells

Respiration is a chemical process that breaks down nutrient molecules (especially glucose) to release energy stored in their bonds.
Respiration is enzyme-controlled and occurs in all living cells; most aerobic reactions take place in mitochondria.
Respiration can be aerobic (with oxygen) or anaerobic (without oxygen). Aerobic releases much more energy per glucose molecule.
Energy released is used for: muscle contraction, protein synthesis, cell division, growth, active transport, nerve impulses, and maintaining body temperature.

Uses of energy released in respiration

Aerobic Respiration

Aerobic respiration uses oxygen to completely break down glucose, releasing a relatively large amount of energy.
Word equation: glucose + oxygen → carbon dioxide + water
Balanced symbol equation (Extended): C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Products are carbon dioxide and water; energy is released for cellular processes.

Word equation for aerobic respiration

Anaerobic Respiration in Animals

Anaerobic respiration does not use oxygen; it is the incomplete breakdown of glucose, releasing less energy.
In animals (e.g., during vigorous exercise), glucose is converted to lactic acid.
Word equation: glucose → lactic acid
Lactic acid build-up lowers pH in muscle cells, which can denature enzymes; it must be removed.

Word equation for anaerobic respiration in animals

Anaerobic Respiration in Yeast

In yeast, anaerobic respiration produces ethanol and carbon dioxide.
Word equation: glucose → ethanol + carbon dioxide
Balanced symbol equation (Extended): C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Used in bread-making (CO₂ makes dough rise) and brewing (ethanol gives alcohol, CO₂ gives fizz).

Word equation for anaerobic respiration in yeast

Oxygen Debt (Extended)

After exercise, lactic acid is transported to the liver and oxidised (using oxygen) to CO₂ and H₂O.
This requires extra oxygen, known as repaying the oxygen debt.
Breathing rate and heart rate remain high after exercise to deliver oxygen to the liver for lactic acid oxidation.

Comparing Aerobic and Anaerobic Respiration

Aerobic: uses oxygen, complete breakdown, large energy release, products CO₂ + H₂O.
Anaerobic (animals): no oxygen, incomplete, small energy release, product lactic acid only.
Anaerobic (yeast): no oxygen, incomplete, small energy release, products ethanol + CO₂.
Anaerobic respiration in animals does not produce CO₂ – that is a common exam mistake.

Comparing aerobic and anaerobic respiration

Effect of Temperature on Respiration (Yeast Experiment)

Methylene blue dye is added to a yeast-glucose suspension; it turns from blue to colourless as respiration occurs.
The time for colour change measures the rate of respiration (faster change = faster rate).
Independent variable: temperature; dependent variable: rate of respiration (time for colour change).
Control variables: volume/concentration of dye, yeast, glucose; pH (use buffer).
Rate increases up to ~40 °C (more kinetic energy, more enzyme-substrate collisions), then decreases as enzymes denature above optimum temperature.

Methylene blue apparatus

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1.Which of these shows the correct products of aerobic respiration?
Easy
- Aenergy, water, carbon dioxide
- Benergy, lactic acid, carbon dioxide
- Cenergy, water, lactic acid
- Dwater, carbon dioxide, lactic acid
2.What is the correct word equation for aerobic respiration in plants?
Easy
- Aglucose + oxygen → carbon dioxide + water
- Bcarbon dioxide + water → glucose + oxygen
- Cglucose + water → carbon dioxide + oxygen
- Dglucose + carbon dioxide → water + oxygen
3.Which of the following processes require, and are dependent on, the activity of enzymes? 1. aerobic respiration 2. glucose → lactic acid 3. osmosis 4. anaerobic respiration in yeast
Medium
- A1 and 2
- B1 and 3
- C1, 2 and 4
- DAll of them
4.What is produced by anaerobic respiration in humans?
Easy
- Aalcohol and carbon dioxide
- Balcohol and lactic acid
- Clactic acid only
- Dcarbon dioxide only
5.Which substances are used during aerobic respiration? 1 carbon dioxide 2 glucose 3 oxygen 4 water
Easy
- A1 and 3
- B1 and 4
- C2 and 3
- D2 and 4
6.What is the balanced chemical equation for anaerobic respiration in yeast?
Medium
- AC6H12O6 → 2CO2 + 6H2O
- BC6H12O6 → 6CO2 + 6H2O
- CC6H12O6 → 2C2H5OH + 2CO2
- DC6H12O6 → 2C2H5OH + 2H2O
7.Which of these correctly shows the products produced by anaerobic respiration in yeast and in animals?
Medium
- AAnimals: lactic acid yes, carbon dioxide no; Yeast: lactic acid no, carbon dioxide no
- BAnimals: lactic acid yes, carbon dioxide no; Yeast: lactic acid no, carbon dioxide yes
- CAnimals: lactic acid no, carbon dioxide yes; Yeast: lactic acid yes, carbon dioxide no
- DAnimals: lactic acid yes, carbon dioxide yes; Yeast: lactic acid no, carbon dioxide yes
8.During vigorous exercise, lactic acid builds up in muscles. How is this lactic acid removed during recovery?
Medium
- Aexcretion of lactic acid by the kidneys
- Banaerobic respiration of lactic acid in the muscles
- Caerobic respiration of lactic acid in the liver
- Dexcretion of lactic acid by the lungs

Plus de questions

9.Why does anaerobic respiration of glucose release less energy in muscle cells than aerobic respiration?
Medium
- AEnergy is lost in oxygen.
- BEnergy remains trapped in the chemical bonds in the molecule lactic acid.
- CEnergy remains trapped in the chemical bonds in the molecule ethanol.
- DEnergy is lost in carbon dioxide.
10.Which of the following processes would release the most energy in a cell from one molecule of glucose?
Medium
- AAerobic respiration in yeast
- BAerobic respiration in animals
- CAerobic respiration in plants
- DAll of the above release the same amount
11.In which conditions would the cells of a dicotyledonous green plant respire?
Medium
- Amesophyll cells in darkness only, root hair cells in darkness only
- Bmesophyll cells in bright light only, root hair cells in darkness only
- Cneither mesophyll nor root hair cells respire
- Dboth mesophyll and root hair cells respire in both darkness and bright light
12.Which part of a mitochondrion is the site of the Krebs cycle: the outer membrane, inner membrane, cristae, or matrix?
Easy
- Aouter membrane
- Binner membrane
- Ccristae
- Dmatrix
13.A student set up an experiment to measure the rate of aerobic respiration in yeast using methylene blue. They added 5 cm³ of yeast suspension and 5 cm³ of glucose solution to a test tube, then added a few drops of methylene blue. The tube was placed in a water bath at 30°C. The time taken for the dye to turn colourless was recorded. Which of the following is a controlled variable in this experiment?
Medium
- AVolume of yeast suspension
- BTemperature of the water bath
- CTime taken for colour change
- DRate of respiration
14.The graph shows the rate of respiration of yeast at different temperatures. At 50°C, the rate is very low. Which statement best explains this observation?
Medium
- AEnzymes are denatured at high temperatures, so they cannot catalyse respiration.
- BThe yeast cells die at 50°C because the temperature is too high.
- COxygen becomes less soluble in water at higher temperatures, reducing respiration.
- DGlucose is used up faster at high temperatures, so the rate decreases.
15.In an experiment using methylene blue to measure the rate of aerobic respiration in yeast, a student recorded the time taken for the dye to turn colourless at 20°C, 30°C, 40°C and 50°C. The results are shown in the table. | Temperature / °C | Time for colour change / s | |---|---| | 20 | 120 | | 30 | 60 | | 40 | 30 | | 50 | 180 | At which temperature was the rate of respiration fastest?
Easy
- A20°C
- B30°C
- C40°C
- D50°C
16.A student investigates the effect of temperature on the rate of anaerobic respiration in yeast. They set up five flasks containing yeast and glucose solution, each at a different temperature, and measure the volume of gas produced in 10 minutes. Which gas is being produced?
Easy
- AOxygen
- BCarbon dioxide
- CHydrogen
- DNitrogen
17.A student used apparatus consisting of a sealed container with germinating peas, a tube connected to a U-tube manometer containing coloured liquid, to investigate the rate of aerobic respiration in germinating peas. As the peas respire, the coloured liquid moves. Which change would cause the coloured liquid to move more quickly?
Medium
- AIncreasing the temperature from 20°C to 30°C
- BReplacing the germinating peas with boiled peas
- CAdding a chemical that absorbs carbon dioxide
- DDecreasing the number of peas
18.During a 100 m sprint, a runner's leg muscles respire anaerobically. Which substance accumulates in the muscles as a result?
Easy
- AEthanol
- BLactic acid
- CCarbon dioxide
- DOxygen
19.The table shows the energy released from one glucose molecule in different organisms. | Organism | Energy released (kJ) | |---|---| | Yeast (aerobic) | 2870 | | Yeast (anaerobic) | 210 | | Human muscle (anaerobic) | 150 | Which statement is correct?
Medium
- AAerobic respiration releases more energy than anaerobic respiration.
- BAnaerobic respiration in yeast releases more energy than in humans.
- CBoth A and B are correct.
- DNeither A nor B is correct.
20.A student investigates the effect of temperature on the rate of anaerobic respiration in yeast by measuring the rate of carbon dioxide production. The results are shown in the graph. At 45°C, the rate is lower than at 35°C. What is the most likely reason?
Medium
- AThe yeast cells have used up all the glucose.
- BThe enzymes controlling anaerobic respiration have denatured.
- CCarbon dioxide is more soluble at higher temperatures.
- DThe yeast switches to aerobic respiration at high temperatures.
21.A person runs a marathon. After the race, they continue to breathe heavily for some time. This is mainly to provide oxygen for the breakdown of which substance?
Medium
- AGlucose
- BLactic acid
- CCarbon dioxide
- DEthanol
22.Which of the following is a use of the energy released by respiration in humans?
Easy
- AMuscle contraction
- BPhotosynthesis
- CDiffusion of oxygen into the blood
- DExcretion of urea
23.A student set up the following experiment to investigate anaerobic respiration in yeast. They mixed yeast and glucose solution in a flask, added a layer of oil on top, and connected a delivery tube to a limewater container. After 24 hours, the limewater turned milky. What was the purpose of the oil layer?
Medium
- ATo prevent oxygen from entering the flask
- BTo keep the yeast warm
- CTo stop carbon dioxide from escaping
- DTo provide nutrients for the yeast
24.The balanced chemical equation for anaerobic respiration in yeast is: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ Which pair correctly identifies Product 1 and Product 2?
Easy
- Aethanol and carbon dioxide
- Blactic acid and carbon dioxide
- Cethanol and water
- Dlactic acid and water
25.In an experiment to investigate the effect of temperature on aerobic respiration in yeast, methylene blue is added to a yeast-glucose suspension. What does the time taken for the dye to turn colourless indicate?
Medium
- AThe rate of respiration – a shorter time means a faster rate
- BThe rate of respiration – a shorter time means a slower rate
- CThe amount of glucose used – a shorter time means less glucose used
- DThe amount of oxygen produced – a shorter time means more oxygen produced
26.A student investigates the effect of temperature on yeast respiration using the methylene blue method. They set up five test tubes with equal volumes of yeast suspension and glucose solution, add the same amount of methylene blue, and place each in a water bath at a different temperature. Which variable is the independent variable?
Easy
- ATemperature
- BTime for colour change
- CVolume of yeast suspension
- DVolume of glucose solution
27.In the methylene blue experiment, which of the following should be kept constant to ensure a fair test?
Easy
- ATemperature
- BTime for colour change
- CVolume of yeast suspension added to each tube
- DNumber of bubbles produced
28.The graph shows the effect of temperature on the rate of respiration in yeast. At 50 °C, the rate is very low. Which statement best explains this observation?
Medium
- AThe enzymes involved in respiration have denatured at this temperature
- BThe yeast cells have used up all the glucose
- CThe methylene blue has been used up
- DThe oxygen supply is limited
29.A student uses a gas syringe to measure the volume of gas produced by yeast in a sealed flask. Which gas is being produced during anaerobic respiration in yeast?
Easy
- ACarbon dioxide
- BOxygen
- CHydrogen
- DNitrogen
30.In an experiment on anaerobic respiration in yeast, a layer of oil is added on top of the yeast-glucose mixture. What is the purpose of the oil layer?
Easy
- ATo prevent oxygen from entering the mixture
- BTo provide nutrients for the yeast
- CTo keep the mixture warm
- DTo absorb carbon dioxide
31.The balanced equation for anaerobic respiration in yeast is C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. How many carbon atoms are there in the products?
Medium
- A2
- B4
- C6
- D8
32.After a 100 m sprint, a runner breathes heavily for several minutes. This is mainly to provide oxygen for the breakdown of which substance?
Medium
- ALactic acid
- BGlucose
- CEthanol
- DCarbon dioxide
33.A student set up an experiment to measure the rate of anaerobic respiration in yeast. They placed 10 cm³ of yeast suspension and 10 cm³ of glucose solution in a flask, added a layer of oil, and attached a gas syringe. They recorded the volume of gas produced every minute for 20 minutes. Which of the following would be a suitable control variable?
Medium
- AThe volume of yeast suspension
- BThe volume of gas produced
- CThe temperature of the water bath
- DThe type of yeast
34.Which of the following is the correct word equation for aerobic respiration?
Easy
- Aglucose + oxygen → carbon dioxide + water
- Bglucose + carbon dioxide → oxygen + water
- Ccarbon dioxide + water → glucose + oxygen
- Dglucose + water → carbon dioxide + oxygen
35.Which of the following statements correctly defines respiration?
Easy
- AA chemical process that breaks down nutrient molecules to release energy
- BThe process of breathing air into and out of the lungs
- CA process that only occurs in the presence of oxygen
- DA process that produces glucose from carbon dioxide and water
36.What is the word equation for aerobic respiration?
Easy
- Aglucose + oxygen → carbon dioxide + water
- Bcarbon dioxide + water → glucose + oxygen
- Cglucose → lactic acid
- Dglucose → ethanol + carbon dioxide
37.Which of the following is a product of anaerobic respiration in humans?
Easy
- Alactic acid
- Bethanol
- Ccarbon dioxide
- Doxygen
38.Respiration is controlled by which type of biological molecule?
Easy
- Aenzymes
- Bhormones
- Cantibodies
- DDNA
39.In an experiment measuring the rate of aerobic respiration in yeast using methylene blue, what does the time taken for the dye to become colourless indicate?
Medium
- Athe rate of respiration (faster time = higher rate)
- Bthe amount of glucose used
- Cthe volume of carbon dioxide produced
- Dthe temperature of the mixture
40.What is the definition of respiration?
Easy
- AA chemical process that breaks down nutrient molecules to release energy
- BThe process of breathing in and out
- CThe production of carbon dioxide and water
- DThe breakdown of glucose to produce lactic acid
41.Respiration is controlled by enzymes.
Easy
True or false?
42.What is the only product of anaerobic respiration in animals?
Easy
- ALactic acid
- BEthanol
- CCarbon dioxide
- DOxygen
43.The word equation for anaerobic respiration in yeast is: glucose → ethanol + carbon dioxide. Which of these is a use of this process in industry?
Medium
- ABread making (the carbon dioxide causes dough to rise)
- BMaking yoghurt (lactic acid production)
- CCheese production (curdling milk)
- DBrewing (ethanol gives beer its alcoholic nature)
44.Match each type of respiration with its correct description.
Medium
- Aerobic respiration
- Anaerobic respiration
- Requires oxygen, complete breakdown of glucose, releases a large amount of energy
- Does not require oxygen, incomplete breakdown of glucose, releases a small amount of energy
45.Carbon dioxide is a product of anaerobic respiration in animals.
Easy
True or false?
46.In the methylene blue experiment to investigate the rate of aerobic respiration in yeast, what does the time taken for the dye to become colourless indicate?
Medium
- AThe rate of respiration (faster colour change = faster respiration)
- BThe amount of glucose used
- CThe amount of oxygen produced
- DThe temperature of the yeast suspension
47.Place the following events in the correct order to describe what happens during inspiration (breathing in).
Medium
- External intercostal muscles contract
- Volume of thorax increases
- Air rushes into the lungs
- Pressure in thorax decreases
- Diaphragm moves down, ribs move upwards and outwards
48.Which of the following are products of anaerobic respiration in yeast? (Select all that apply.)
Medium
- AEthanol
- BCarbon dioxide
- CLactic acid
- DWater
- EOxygen
49.A student investigates the effect of temperature on the rate of aerobic respiration in yeast using methylene blue. The results are shown in the table. | Temperature (°C) | Time for methylene blue to become colourless (s) | |---|---| | 10 | 300 | | 20 | 150 | | 30 | 75 | | 40 | 40 | | 50 | 120 | Which statement explains the change in time between 40 °C and 50 °C?
Hard
- AThe enzymes involved in respiration have denatured at 50 °C.
- BThe yeast cells have died at 50 °C.
- CThe glucose has been used up by 50 °C.
- DThe methylene blue is less sensitive at higher temperatures.
50.Complete the word equation for aerobic respiration.
Easy
glucose + ____ → carbon dioxide + water (+ energy)
51.In the balanced chemical equation for aerobic respiration, how many molecules of carbon dioxide are produced from one molecule of glucose?
Easy
52.Complete the sentence about oxygen debt.
Easy
After vigorous exercise, lactic acid is removed by reacting with ____ in the liver.
53.In an experiment, a student measures the volume of carbon dioxide produced by yeast over 10 minutes at 30 °C. The total volume collected is 24 cm³. Calculate the average rate of carbon dioxide production in cm³ per minute.
Medium
54.State the word equation for anaerobic respiration in yeast.
Easy
55.Explain why the rate of respiration in yeast decreases when the temperature is raised above 40 °C.
Medium
56.In the balanced chemical equation for aerobic respiration, how many molecules of carbon dioxide are produced from one molecule of glucose?
Easy
- A5
- B8
- C6
- D7
57.In an experiment, a student measures the volume of carbon dioxide produced by yeast over 10 minutes at 30 °C. The total volume collected is 24 cm³. Calculate the average rate of carbon dioxide production in cm³ per minute.
Medium
- A1.4 cm³/min
- B3.4 cm³/min
- C2.4 cm³/min
- D4.4 cm³/min

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