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GCSE Biology · GCSEChapter 10 · Aerobic and anaerobic respiration
GCSE · Chapter 10

Aerobic and anaerobic respiration

By the end of this chapter you will be able to

  • Write the word and symbol equations for aerobic respiration
  • Name the uses of the energy released by respiration
  • Compare aerobic and anaerobic respiration, and explain oxygen debt
  • Carry out and interpret the yeast practical, and calculate a rate of gas production
  • [H] Compare the energy values of fats, carbohydrates and proteins, and link exercise to metabolic rate

You should already know

  • Glucose and its role in photosynthesis (Chapter 9)
  • Enzymes and temperature (Chapter 5)
  • Mitochondria (Chapter 2)

Where this leads

  • Muscle contraction and the nervous system (Chapter 11)
  • Respiration and the heart (Chapter 6)
  • Metabolic rate and hormones (Chapter 12)

GCSE Biology. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe respiration as an exothermic reaction in every cell, and write the equations for aerobic respiration, Describe the uses of the energy released by respiration, Describe anaerobic respiration in muscles and in yeast, and oxygen debt, Investigate the effect of temperature on the rate of anaerobic respiration in yeast, using gas production, [H] Compare the energy values of food, and describe metabolic rate and exercise.

Exercise P  Do you remember?

1
FWhich organelle releases energy by aerobic respiration? Write the word.
2
FWhich gas do we breathe in for respiration? Write the word.
3
FWhat is 3 x 6 x 10? Write the number.
4
FWhat is the name of the sugar that is made by photosynthesis? Write the word.
5
FWhich way does water move in osmosis: from dilute to concentrated, or the other way? Write the words.
10.1

Respiration in every cell

GCSE
Describe respiration as an exothermic reaction in every cell, Write the word and symbol equations for aerobic respiration
Video lesson: Respiration in every cellComing soon
Context. Ali is tired after a long walk, and he feels warm. “Where does the energy for my walk come from?” he asks. The energy is released by respiration, which happens in every living cell of his body, all the time.

Respiration is the process that releases energy from glucose. It is an exothermic reaction, meaning that it releases energy, some of which is given off as heat. It happens in every living cell, all the time. In aerobic respiration, glucose and oxygen react in the mitochondria to give carbon dioxide and water, and a large amount of energy is released.

Word equation: glucose + oxygen -> carbon dioxide + water (energy released).

Symbol equation: C6H12O6 + 6O2 -> 6CO2 + 6H2O (energy released).

glucose+ oxygenmitochondriaaerobic respirationcarbondioxide + waterenergy is released (exothermic), and some is wasted as heat
Aerobic respiration in the mitochondria: glucose and oxygen give carbon dioxide, water and energy

Key ideas

  • Respiration releases energy from glucose in every living cell.
  • It is exothermic: energy is released, and some is given off as heat.
  • Aerobic respiration uses oxygen and takes place in the mitochondria.
  • Products of aerobic respiration: carbon dioxide and water.
Worked example 10.1.1
Write the word equation for aerobic respiration. Write the words.

Method. Name the reactants and the products.

  1. Reactants. The reactants are glucose and oxygen.
    glucose and oxygen
  2. Products. The products are carbon dioxide and water, with energy released.
    carbon dioxide and water
  3. Write it. Glucose + oxygen -> carbon dioxide + water.
    glucose + oxygen
  4. Write the answer.
    glucose + oxygen -> carbon dioxide + water

Check. Energy is released, not used up. ✓

Worked example 10.1.2
Name the two products of aerobic respiration. Write the words.

Method. Recall the products.

  1. Recall. Aerobic respiration produces carbon dioxide and water.
    carbon dioxide and water
  2. Name them. The two products are carbon dioxide and water.
    carbon dioxide and water
  3. Write the answer.
    carbon dioxide and water

Check. Energy is also released. ✓

Worked example 10.1.3
Say what it means that respiration is exothermic. Write one sentence.

Method. Think about the energy change.

  1. Think. In an exothermic reaction, energy is transferred to the surroundings.
    transferred to the surroundings
  2. Conclude. So respiration releases energy, and some is given off as heat.
    given off as heat
  3. Write the answer.
    Respiration releases energy, and some of it is given off as heat to the surroundings.

Check. Exothermic means energy is released. ✓

Watch out

Watch out: respiration is not the same as breathing. Breathing moves air in and out; respiration is the chemical reaction in cells that releases energy.

Exam tip

Exam tip. Write the equations with the energy word included, so the marker can see that energy is released.

Try it (with your teacher). With your teacher, use a warm yeast and sugar mixture and a thermometer, and feel the warmth of the tube. Explain in one sentence why the tube gets warmer.

Exercise 10A  Respiration in every cell

1
FName the reactants of aerobic respiration. Write the words.
2
FName the products of aerobic respiration. Write the words.
3
FIn which organelle does aerobic respiration take place? Write the word.
4
FIs respiration an exothermic reaction? Write yes or no.
5
FIn which cells does respiration take place? Write the words.
6
FWrite the symbol equation for aerobic respiration, using C6H12O6, O2, CO2 and H2O. Write the equation.
7
FHow many molecules of oxygen does one glucose molecule need in aerobic respiration? Write the number.
8
FName the gas that is breathed out as a product of aerobic respiration. Write the word.
9
PSWhy does the body feel warm during exercise? Write one sentence. Accept any sentence that says respiration releases energy as heat.
10
FHow many carbon atoms are there in one glucose molecule, C6H12O6? Write the number.
11
PSWrite one difference between the word equation and the symbol equation for aerobic respiration. Accept any sentence that says the symbol equation shows the numbers of molecules and atoms.
12
RChallenge: explain why respiration is needed in every cell, in two sentences. Accept any answer that says every cell needs energy for its processes, and respiration releases that energy from glucose.
10.2

Using the energy from respiration

GCSE
Describe the uses of the energy released by respiration
Video lesson: Using the energy from respirationComing soon
Context. Priya does a press-up. Her muscles contract, her body moves, and her heart rate rises. Each of these actions needs energy. “Where does the energy for each of these things come from?” she asks her teacher.

The energy released by respiration is used in several ways. It is used to build up molecules, such as proteins from amino acids. It is used for muscle contraction, so that the muscles can move. It is used for active transport, moving substances against a concentration gradient. In mammals, some energy is used to keep the body warm.

Use of energyExample
building up moleculesmaking proteins from amino acids
muscle contractionmoving the arm when doing a press-up
active transportabsorbing mineral ions into root hair cells
keeping warmmaintaining the body temperature
Four uses of the energy released by respiration

Key ideas

  • Respiration releases energy for building molecules, muscle contraction, active transport and keeping warm.
  • Muscles need a lot of energy during exercise.
  • Active transport uses energy from respiration to move particles against the gradient.
  • Building proteins from amino acids needs energy.
Worked example 10.2.1
Name one use of the energy released by respiration in a muscle. Write the words.

Method. Think about what a muscle does.

  1. Think. A muscle uses energy to contract and move the body.
    contract
  2. Name it. So the use is muscle contraction.
    muscle contraction
  3. Write the answer.
    muscle contraction

Check. The energy is used, not stored. ✓

Worked example 10.2.2
Name the process that uses energy from respiration to move mineral ions into root hair cells. Write the words.

Method. Recall the process against a gradient.

  1. Recall. Moving particles against a concentration gradient needs energy.
    needs energy
  2. Name it. This process is active transport.
    active transport
  3. Write the answer.
    active transport

Check. Diffusion needs no energy. ✓

Worked example 10.2.3
Name one use of energy in building molecules. Write the words.

Method. Think about what the cell builds.

  1. Think. Cells build large molecules, such as proteins, from smaller units.
    build proteins
  2. Name it. Building proteins from amino acids uses energy.
    building proteins
  3. Write the answer.
    building proteins from amino acids

Check. Energy is needed to join the units together. ✓

Watch out

Watch out: the energy from respiration is not stored in the molecules that are built. It is used to build them, and it is used up in the building.

Exam tip

Exam tip. When you name a use of energy, give the process and an example: “muscle contraction, for example moving the arm”.

Try it (with your teacher). With your teacher, do ten gentle squats and then feel your heart rate. Explain in one sentence which use of respiration energy the muscles are using.

Exercise 10B  Using the energy from respiration

1
FName the use of respiration energy that moves the muscles. Write the words.
2
FName the use of respiration energy that builds proteins. Write the words.
3
FWhich process moves particles against a concentration gradient using energy? Write the words.
4
FName one use of respiration energy that keeps a mammal warm. Write the words.
5
FIs respiration used for muscle contraction during exercise? Write yes or no.
6
FName one example of active transport in a plant. Write the words.
7
PSWhy does an athlete need more energy from respiration than a person sitting still? Write one sentence. Accept any sentence that says the muscles use more energy to contract during exercise.
8
PSWhich use of respiration energy would be most reduced if a person stopped moving their muscles? Write the words. Accept muscle contraction.
9
PSExplain why a plant needs energy from respiration to absorb mineral ions. Write one sentence. Accept any sentence that says active transport moves the ions against the gradient, which needs energy.
10
RChallenge: a pupil says that the energy from respiration is stored in the proteins built from amino acids. Explain why this is not correct. Write two sentences. Accept any answer that says the energy is used up in building the protein, not stored in it.
10.3

Anaerobic respiration and oxygen debt

GCSE
Describe anaerobic respiration in muscles (lactic acid) and in yeast (ethanol and carbon dioxide), Explain oxygen debt
Video lesson: Anaerobic respiration and oxygen debtComing soon
Context. Hana sprints up a hill. Her breathing cannot keep up with her muscles, and for a while the muscles work without enough oxygen. After she stops, she keeps breathing hard for several minutes. “Why am I still out of breath?” she asks.

When there is not enough oxygen for aerobic respiration, muscles use anaerobic respiration. Glucose is converted to lactic acid, and only a small amount of energy is released. Lactic acid builds up and causes the muscles to tire.

Yeast also respires anaerobically, in a process called fermentation. Glucose is converted to ethanol and carbon dioxide. This is used in bread-making and brewing.

After exercise, the body needs extra oxygen to break down the lactic acid and to restore the body to its resting state. This extra oxygen is the oxygen debt. Breathing stays fast for a while after exercise.

glucosemuscles: lactic acid(little energy released)yeast: ethanol + carbon dioxide(little energy released)without oxygen, both reactions release far less energy than aerobic respiration
Anaerobic respiration in muscles (lactic acid) and in yeast (ethanol and carbon dioxide)
FeatureAerobicAnaerobic (muscles)Anaerobic (yeast)
oxygen neededyesnono
productscarbon dioxide and waterlactic acidethanol and carbon dioxide
energy released per glucosea lota littlea little
where it happensmitochondriacytoplasmcytoplasm
Aerobic and anaerobic respiration compared

Key ideas

  • Anaerobic respiration in muscles: glucose to lactic acid.
  • Anaerobic respiration in yeast: glucose to ethanol and carbon dioxide.
  • Anaerobic respiration releases much less energy than aerobic respiration.
  • Oxygen debt is the extra oxygen needed after exercise to clear lactic acid.
Worked example 10.3.1
Name the product of anaerobic respiration in muscles. Write the words.

Method. Recall the product in muscles.

  1. Recall. In muscles, glucose is converted to lactic acid without oxygen.
    lactic acid
  2. Name it. So the product is lactic acid.
    lactic acid
  3. Write the answer.
    lactic acid

Check. Lactic acid causes muscle fatigue. ✓

Worked example 10.3.2
Name the two products of anaerobic respiration in yeast. Write the words.

Method. Recall the products of yeast.

  1. Recall. Yeast converts glucose to ethanol and carbon dioxide.
    ethanol and carbon dioxide
  2. Name them. The products are ethanol and carbon dioxide.
    ethanol and carbon dioxide
  3. Write the answer.
    ethanol and carbon dioxide

Check. Yeast does this in the absence of oxygen. ✓

Worked example 10.3.3
Explain what oxygen debt is. Write one sentence.

Method. Think about the oxygen needed after exercise.

  1. Think. After exercise the body needs extra oxygen.
    extra oxygen
  2. Conclude. This extra oxygen clears lactic acid and restores the resting state.
    clears lactic acid
  3. Write the answer.
    Oxygen debt is the extra oxygen the body needs after exercise to clear lactic acid and restore the resting state.

Check. The debt is paid back by breathing harder. ✓

Watch out

Watch out: anaerobic respiration does not produce much energy. It is a short-term supply, and it is not a replacement for aerobic respiration.

Exam tip

Exam tip. Name the organism or tissue with its product: “muscles: lactic acid”, “yeast: ethanol and carbon dioxide”.

Try it (with your teacher). With your teacher, compare the breathing rate after a minute of gentle stepping and after a minute of brisk stepping. Describe the difference, and link it to oxygen debt.

Exercise 10C  Anaerobic respiration and oxygen debt

1
FName the product of anaerobic respiration in muscles. Write the words.
2
FName the alcohol that yeast produces during anaerobic respiration. Write the word.
3
FWhat gas does yeast produce during anaerobic respiration? Write the word.
4
FIs anaerobic respiration in muscles a process that needs oxygen? Write yes or no.
5
FName the extra oxygen needed by the body after exercise. Write the words.
6
PSWhy does breathing stay fast after exercise? Write one sentence. Accept any sentence that says the body must repay the oxygen debt, so breathing stays fast.
7
PSCompare aerobic and anaerobic respiration: name one product that is only in anaerobic respiration. Write the word. Accept lactic acid, or ethanol.
8
PSName the part of the cell where anaerobic respiration takes place. Accept cytoplasm.
9
PSA runner feels a burning pain in the legs. Explain why, in one sentence. Accept any sentence that says lactic acid builds up in the muscles when there is not enough oxygen.
10
RChallenge: explain why anaerobic respiration can only supply energy for a short time, in two sentences. Accept any answer that says it releases little energy per glucose, and the lactic acid build-up stops the muscles from working.
10.4

Required practical: yeast and temperature

GCSE
Investigate the effect of temperature on the rate of anaerobic respiration in yeast, Calculate a rate of gas production
Video lesson: Required practical: yeast and temperatureComing soon
Context. Ruth mixes yeast with sugar solution and seals it with a gas-collecting tube. Over five minutes, bubbles of carbon dioxide fill the tube. “I will repeat it at four temperatures,” she says. “Which temperature gives the most gas?”

In this practical the independent variable is the temperature of the yeast and sugar mixture, and the dependent variable is the volume of gas collected in 5 minutes. The gas is carbon dioxide, a product of anaerobic respiration (fermentation). The rate is found from the volume: rate = gas volume / time. The control variables are the mass of yeast, the concentration and volume of sugar solution, and the time.

Yeast is a safe, school-approved organism. The water bath is set by the teacher, and the apparatus is checked before use.

Temperature (C)Gas in 5 min (cm3)Rate = gas / 5 (cm3 per min)
2051
30153
40255
50102
Gas collected in 5 minutes at four temperatures, and the rate (invented data)
Gas from yeast at each temperature (invented data)05101520253001020304050temperature (degrees C)gas in 5 minutes (cm3)
Line graph of gas from yeast against temperature (invented data)

Key ideas

  • Independent variable: temperature. Dependent variable: volume of gas in 5 minutes.
  • Rate = gas volume / time, so the rate is cm3 per minute.
  • Keep the mass of yeast, the sugar solution and the time the same.
  • The gas is carbon dioxide, produced by fermentation.
Worked example 10.4.1
At 30 degrees C, the gas volume in 5 minutes is 15 cm3. Calculate the rate, in cm3 per minute. Write the number.

Method. Divide the gas volume by the time.

  1. Divide. Rate = 15 / 5.
    15 / 5
  2. Calculate. The rate is 3 cm3 per minute.
    15 / 5 = 3
  3. Write the answer.
    3

Check. 3 x 5 = 15. ✓

Worked example 10.4.2
Read the table. At which temperature is the rate highest? Write the number, in degrees C.

Method. Find the highest rate in the table.

  1. Compare. The rates are 1, 3, 5 and 2 cm3 per minute.
    1, 3, 5, 2
  2. Read. The highest rate, 5 cm3 per minute, is at 40 degrees C.
    40
  3. Write the answer.
    40

Check. The rate at 50 degrees C falls to 2. ✓

Worked example 10.4.3
Explain why the gas volume falls at 50 degrees C. Write one sentence.

Method. Think about the enzymes in the yeast.

  1. Think. The enzymes in the yeast are denatured at 50 degrees C.
    denatured
  2. Conclude. So the rate of anaerobic respiration falls.
    rate falls
  3. Write the answer.
    At 50 degrees C the yeast enzymes denature, so the rate of anaerobic respiration falls.

Check. Denaturing is permanent. ✓

Watch out

Watch out: the gas is measured over a set time. Use the same time for each temperature, or the results cannot be compared.

Exam tip

Safety. Yeast is a safe organism, but do not taste the mixture. The water bath is controlled by the teacher, and the apparatus is kept stable.

Try it (with your teacher). With your teacher, set up yeast and sugar solution at two temperatures in gas-collecting tubes. Record the gas volume after 5 minutes, and calculate the rate for each.

Exercise 10D  Required practical: yeast and temperature

1
FIn the yeast practical, what is the independent variable? Write the words.
2
FWhat is the dependent variable in the yeast practical? Write the words.
3
FThe gas volume at 20 degrees C is 5 cm3 in 5 minutes. Calculate the rate. Write the number, in cm3 per minute.
4
FThe gas volume at 40 degrees C is 25 cm3 in 5 minutes. Calculate the rate. Write the number, in cm3 per minute.
5
FAt which temperature is the gas volume lowest in the table? Write the number, in degrees C.
6
FWhich temperature gives the highest rate in the table? Write the number, in degrees C.
7
PSWhy must the mass of yeast be kept the same for each test? Write one sentence. Accept any sentence that says the mass of yeast is a control variable, so only the temperature changes the result.
8
PSDescribe the pattern in the rate as the temperature rises from 20 to 40 degrees C, in one sentence. Accept any sentence that says the rate rises as the temperature rises.
9
PSSuggest one improvement to the yeast practical. Accept any sensible improvement, such as repeating each temperature or testing a temperature closer to the best value.
10
RChallenge: explain why the gas volume is a measure of the rate of anaerobic respiration, in two sentences. Accept any answer that says the carbon dioxide is produced by anaerobic respiration, so more gas means a faster rate.
10.5

Energy in food and metabolic rate [H]

GCSE
[H] Compare the energy values of carbohydrates, fats and proteins, [H] Describe metabolic rate and its link to exercise
Video lesson: Energy in food and metabolic rate [H]Coming soon
Context. Dan wants to know which food gives the most energy per gram. His teacher gives him a table of approximate values, and a short question to answer with them.

[H] Higher tier only. This section is higher-tier content. It uses approximate energy values per gram, for practice.

[H] Fats give the most energy per gram, about twice as much as carbohydrates or proteins. Metabolic rate is the rate at which the body uses energy. It rises during exercise, because the muscles use more energy, and it is also higher in people with more muscle mass.

FoodEnergy (kJ per gram, approximate)
carbohydrate16
fat37
protein17
Approximate energy values per gram of three food types (for practice)

Key ideas

  • [H] Fat releases the most energy per gram.
  • [H] Carbohydrate and protein release a similar energy per gram.
  • [H] Energy released = mass of food x energy per gram.
  • [H] Metabolic rate rises during exercise.
Worked example 10.5.1
[H] Find the energy from 10 g of fat, using the value from the table. Write the number, in kJ.

Method. Multiply the mass by the energy per gram.

  1. Multiply. Energy = 10 x 37.
    10 x 37
  2. Calculate. 10 x 37 = 370 kJ.
    370
  3. Write the answer.
    370

Check. 370 divided by 10 is 37. ✓

Worked example 10.5.2
[H] How many times more energy per gram does fat give than carbohydrate? Write the number, to one decimal place.

Method. Divide the fat value by the carbohydrate value.

  1. Read. Fat gives 37 kJ per gram and carbohydrate gives 16 kJ per gram.
    37 and 16
  2. Divide. Divide 37 by 16 to compare them.
    37 / 16 = 2.3
  3. Write the answer.
    2.3

Check. 16 x 2.3 is about 37. ✓

Worked example 10.5.3
[H] Explain why metabolic rate rises during exercise. Write one sentence.

Method. Think about the muscles.

  1. Think. Muscles use more energy during exercise.
    more energy
  2. Conclude. So the rate at which the body uses energy rises.
    rate rises
  3. Write the answer.
    The muscles use more energy during exercise, so the metabolic rate rises.

Check. More respiration supplies the extra energy. ✓

Watch out

Watch out: the energy values are approximate and are for practice. Do not treat them as exact values for a particular food.

Exam tip

Exam tip. [H] Show the calculation: mass x energy per gram. Give the unit with the answer.

Try it (with your teacher). With your teacher, use a food label with energy and fat values. Work out the energy from 20 g of fat using the value on the label, and check it against the table.

Exercise 10E  [H] Energy in food and metabolic rate

1
F[H] Which food gives the most energy per gram: carbohydrate, fat or protein? Write the word.
2
F[H] What is the energy per gram of carbohydrate? Write the number, in kJ.
3
PS[H] Find the energy from 25 g of carbohydrate, using 16 kJ per gram. Write the number, in kJ.
4
PS[H] Find the energy from 5 g of protein, using 17 kJ per gram. Write the number, in kJ.
5
PS[H] Which gives more energy: 10 g of fat or 25 g of carbohydrate? Write the words.
6
PS[H] Explain why metabolic rate is higher during exercise than at rest. Write one sentence. Accept any sentence that says the muscles use more energy and so respiration rate rises.
7
R[H] Challenge: a student says that fats are bad for energy because they give less energy per gram. Explain why this is wrong. Write two sentences. Accept any answer that says fat gives the most energy per gram, so it is an efficient energy store.

Chapter summary

SkillMethod
Aerobic respirationGlucose + oxygen -> carbon dioxide + water, with a lot of energy released in the mitochondria.
Uses of energyBuilding molecules, muscle contraction, active transport and keeping warm.
Anaerobic in musclesGlucose -> lactic acid, in the cytoplasm, with little energy released.
Anaerobic in yeastGlucose -> ethanol and carbon dioxide (fermentation).
Oxygen debtThe extra oxygen needed after exercise to clear lactic acid.
Yeast practicalRate = gas volume / time; temperature is the independent variable.
[H] Energy in foodEnergy = mass x energy per gram; fat gives the most energy per gram.

Can you…?

  • I can write the word and symbol equations for aerobic respiration
  • I can name the uses of the energy released by respiration
  • I can compare aerobic and anaerobic respiration, and explain oxygen debt
  • I can carry out the yeast practical and calculate a rate of gas production
  • [H] I can compare the energy values of food and link exercise to metabolic rate

Key terms

respiration
the process that releases energy from glucose in every living cell
exothermic
a reaction that releases energy, some of it as heat
aerobic respiration
respiration using oxygen, which releases a lot of energy
anaerobic respiration
respiration without oxygen, which releases little energy
lactic acid
the product of anaerobic respiration in muscles
fermentation
anaerobic respiration in yeast, producing ethanol and carbon dioxide
oxygen debt
the extra oxygen needed after exercise to clear lactic acid
metabolic rate
the rate at which the body uses energy
[H] energy per gram
the energy released by one gram of a food

Exercise 10Z  Check-up

1
Write the word equation for aerobic respiration. Write the words.
2
Name the organelle where aerobic respiration takes place. Write the word.
3
Name the product of anaerobic respiration in muscles. Write the words.
4
Name the two products of anaerobic respiration in yeast. Write the words.
5
Name the extra oxygen the body needs after exercise. Write the words.
6
Name one use of energy from respiration. Accept any one: muscle contraction, building molecules, active transport, or keeping warm.
7
In the yeast practical, the gas volume at 30 degrees C is 15 cm3 in 5 minutes. Find the rate, in cm3 per minute. Write the number.
8
Name the gas produced by yeast during anaerobic respiration, and measured in the practical. Write the words.
9
Explain in one sentence why anaerobic respiration releases less energy than aerobic respiration. Accept any sentence that says glucose is not fully broken down, so less energy is released.
10
Name the energy-releasing process that happens in the cytoplasm when there is no oxygen. Write the words.

Total 22 marks. Allow about 22 minutes.

Exercise 10X  Exam-style questions

1
FName the reactants of aerobic respiration. Write the words.[1 mark]
2
FName the product of anaerobic respiration in muscles. Write the words.[1 mark]
3
FName the process by which yeast respires without oxygen. Write the word.[1 mark]
4
FWrite the symbol equation for aerobic respiration. Write the equation.[2 marks]
5
FExplain why a person breathes faster after a run. Write two sentences. Accept any answer that says the oxygen debt must be repaid, and lactic acid is cleared.[2 marks]
6
FName three uses of the energy released by respiration. Accept any three of: muscle contraction, building molecules, active transport, keeping warm.[3 marks]
7
PSIn the yeast practical, the gas volumes are 5, 15, 25 and 10 cm3 at 20, 30, 40 and 50 degrees C, all in 5 minutes. Calculate the rate at 40 degrees C, and say why the rate at 50 degrees C is lower. Accept 5 cm3 per minute, with the denaturing of the enzymes explained.[3 marks]
8
FDescribe the difference between aerobic and anaerobic respiration in muscles, using one product and one energy statement. Write two sentences. Accept any answer that says aerobic respiration gives carbon dioxide and water and releases a lot of energy, while anaerobic gives lactic acid and releases little energy.[3 marks]
9
FExplain why respiration is described as exothermic, and say what happens to the energy it releases. Write two sentences. Accept any answer that says the reaction releases energy, and some is given off as heat to the surroundings.[2 marks]
10
RExplain why oxygen debt occurs after exercise, and why the rate of breathing stays high for a while. Write three sentences. Accept any answer that says lactic acid builds up during anaerobic respiration, the body needs extra oxygen to clear it, and breathing stays fast until the debt is paid.[4 marks]
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