Respiration

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Notes

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**.

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.

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.

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).

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.

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.

Aerobic respiration in a cell showing inputs and outputs

Anaerobic respiration in muscle and yeast

Practice questions

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1.Which row of the table below 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
Easy
- 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?
Easy
- AC6H12O6 → 2CO2 + 6H2O
- BC6H12O6 → 6CO2 + 6H2O
- CC6H12O6 → 2C2H5OH + 2CO2
- DC6H12O6 → 2C2H5OH + 2H2O
7.Which row in the table below shows the correct 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

More 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?
Hard
- 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?
Hard
- 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.The diagram below shows a mitochondrion. Which part is the site of the Krebs cycle?
Medium
- 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?
Hard
- 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?
Medium
- 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?
Medium
- AOxygen
- BCarbon dioxide
- CHydrogen
- DNitrogen
17.A student used the apparatus shown to investigate the rate of aerobic respiration in germinating peas. The apparatus consists of a sealed container with germinating peas, a tube connected to a U-tube manometer containing coloured liquid. As the peas respire, the coloured liquid moves. Which change would cause the coloured liquid to move more quickly?
Hard
- 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?
Hard
- 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?
Hard
- 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 row correctly identifies the products? | | Product 1 | Product 2 | |---|--------|--------| | A | ethanol | carbon dioxide | | B | lactic acid | carbon dioxide | | C | ethanol | water | | D | lactic acid | water |
Medium
- AA
- BB
- CC
- DD
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?
Hard
- 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?
Medium
- 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?
Hard
- 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?
Medium
- 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?
Hard
- 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?
Hard
- 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.The diagram shows a mitochondrion. Which part is the site of the Krebs cycle?
Medium
- AThe matrix
- BThe inner membrane
- CThe outer membrane
- DThe cristae
36.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
37.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
38.Which of the following is a product of anaerobic respiration in humans?
Easy
- Alactic acid
- Bethanol
- Ccarbon dioxide
- Doxygen
39.Respiration is controlled by which type of biological molecule?
Easy
- Aenzymes
- Bhormones
- Cantibodies
- DDNA
40.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

Slides

What is Respiration?

Respiration is a chemical process that breaks down glucose to release energy.
It is enzyme-controlled and occurs in all living cells.
Most aerobic respiration reactions take place in the mitochondria.
Respiration can be aerobic (with oxygen) or anaerobic (without oxygen).

Which of the following is the correct definition of respiration?

Breathing air in and out of the lungs
A chemical reaction that releases energy from glucose
The production of glucose from carbon dioxide and water
The breakdown of proteins to release energy

Uses of Energy from Respiration

Energy released by respiration is used for:
Muscle contraction, protein synthesis, cell division, growth.
Active transport, generation of nerve impulses, maintaining body temperature.

Which of these processes use energy released by respiration?

Muscle contraction
Diffusion of oxygen into blood
Protein synthesis
Cell division

Aerobic Respiration

Aerobic respiration uses oxygen to break down glucose completely.
It releases a relatively large amount of energy.
Word equation: glucose + oxygen → carbon dioxide + water
Balanced equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

The word equation for aerobic respiration is: ____ + oxygen → carbon dioxide + ____

Anaerobic Respiration

Anaerobic respiration occurs without oxygen.
It is the incomplete breakdown of glucose, releasing less energy.
In animals (muscle cells): glucose → lactic acid
In yeast: glucose → ethanol + carbon dioxide

What are the products of anaerobic respiration in yeast?

Lactic acid only
Ethanol and carbon dioxide
Carbon dioxide and water
Lactic acid and carbon dioxide

Anaerobic Respiration in Animals

During vigorous exercise, muscle cells may not get enough oxygen.
They switch to anaerobic respiration, producing lactic acid.
Lactic acid build-up causes muscle fatigue and lowers pH.
Oxygen debt: extra oxygen needed to break down lactic acid after exercise.

Which statements about anaerobic respiration in animals are true?

It produces lactic acid.
It produces carbon dioxide.
It releases more energy than aerobic respiration.
It occurs when oxygen supply is insufficient.

Comparing Aerobic and Anaerobic Respiration

Aerobic: uses oxygen, complete breakdown, large energy release, products CO₂ + H₂O.
Anaerobic: no oxygen, incomplete breakdown, small energy release.
Animal anaerobic product: lactic acid; yeast: ethanol + CO₂.

Which row correctly compares aerobic and anaerobic respiration?

Aerobic uses oxygen; anaerobic does not.
Aerobic releases less energy than anaerobic.
Anaerobic produces carbon dioxide and water.
Aerobic occurs only in plants.

Investigating Respiration in Yeast

Methylene blue dye is used as an indicator of respiration rate.
It turns from blue to colourless as oxygen is used up.
Faster colour change = faster respiration rate.
Temperature affects respiration rate: increases up to ~40°C, then decreases as enzymes denature.

In an experiment on yeast respiration, what is the independent variable when investigating the effect of temperature?

Rate of respiration
Temperature
Time for colour change
Concentration of glucose

Oxygen Debt (Extended)

Lactic acid from anaerobic respiration is transported to the liver.
In the liver, lactic acid is oxidised (reacts with oxygen) to CO₂ and H₂O.
This requires extra oxygen, known as repaying the oxygen debt.
That's why we breathe heavily after exercise.

Lactic acid is broken down in the ____ using oxygen, producing ____ and water.

Summary

Respiration releases energy from glucose, controlled by enzymes.
Aerobic: glucose + oxygen → CO₂ + H₂O + lots of energy.
Anaerobic (animals): glucose → lactic acid + little energy.
Anaerobic (yeast): glucose → ethanol + CO₂ + little energy.
Energy is used for contraction, synthesis, division, transport, impulses, and heat.

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