Respiration

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The key idea

Respiration transfers energy from glucose. Aerobic respiration uses oxygen; anaerobic respiration releases less energy.

Aerobic and anaerobic respirationOxygen changes the products and energy transferred.

Revision notes

The bit that matters

Learn the process in clean chunks. If a sentence explains a cause, make sure you can say the effect too.

Aerobic respiration

Aerobic respiration uses oxygen to release energy from glucose and is an exothermic reaction occurring continuously in the mitochondria of living cells.The word equation is glucose + oxygen produces carbon dioxide + water.It releases far more energy per glucose molecule than anaerobic respiration because the glucose is completely broken down.

Anaerobic respiration

Anaerobic respiration takes place without oxygen and releases much less energy because glucose is not fully broken down.In muscle cells glucose is converted to lactic acid.In yeast and plant cells anaerobic respiration is called fermentation and produces ethanol and carbon dioxide, which is used in brewing and baking.

Uses of energy from respiration

The energy released by respiration is used for muscle contraction, for building larger molecules from smaller ones such as proteins from amino acids, and to keep warm in mammals and birds.Metabolism is the sum of all the reactions in a cell or body, and respiration supplies the energy that drives these reactions.

Oxygen debt and recovery

During hard exercise muscles respire anaerobically, producing lactic acid that causes muscle fatigue.The oxygen debt is the amount of extra oxygen the body needs after exercise to react with the accumulated lactic acid and remove it.Blood transports the lactic acid to the liver, where it is converted back into glucose.

Key terms

Definitions to learn

The exothermic process that releases energy from glucose in every living cell.

Aerobic respiration

Respiration using oxygen that fully breaks down glucose to carbon dioxide and water.

Anaerobic respiration

Respiration without oxygen that releases less energy and produces lactic acid in muscles.

Fermentation

Anaerobic respiration in yeast that produces ethanol and carbon dioxide.

Metabolism

The sum of all the chemical reactions taking place in a cell or the body.

Oxygen debt

The extra oxygen needed after exercise to remove the lactic acid that built up.

Worked example

Compare aerobic respiration with anaerobic respiration in human muscle.

Both release energy from glucose.

Aerobic respiration uses oxygen and produces carbon dioxide and water.

Anaerobic respiration does not use oxygen and produces lactic acid.

Final answer

Anaerobic respiration releases less energy and produces lactic acid.

Exam habit

Distinguish aerobic from anaerobic in every answer. Anaerobic in animals: lactic acid.In yeast and plants: ethanol and CO₂. Never confuse the two products.Always write 'respiration releases energy' — it does not produce it.

Watch out

Respiration releases energy. Energy is not created.

Examiner tips

How to score full marks

1State clearly that respiration is exothermic and happens in all living cells, not only during exercise.
2For anaerobic respiration give the correct product for the organism: lactic acid in muscle, ethanol and carbon dioxide in yeast.
3Do not confuse respiration with breathing; respiration is a chemical reaction in cells.

Practice questions

Try these yourself

Open each answer only after you have explained the full biological process.

1Write the word equation for aerobic respiration.

Mark scheme

1.Include oxygen as a reactant.

glucose + oxygen → carbon dioxide + water

2Why can vigorous exercise cause an oxygen debt?

Mark scheme

1.Link anaerobic respiration to lactic acid.

Extra oxygen is needed after exercise to process accumulated lactic acid.

3Where in a cell does aerobic respiration occur?

Mark scheme

1.Recall the organelle.

Mitochondria.

4Write the word equation for aerobic respiration.[1 mark]

Mark scheme

1.Reactants on the left, products on the right.

glucose + oxygen produces carbon dioxide + water (1)

5Name the products of anaerobic respiration in muscle cells.[1 mark]

Mark scheme

1.Recall the muscle product, not the yeast product.

Lactic acid (1)

6State two uses of the energy released during respiration.[2 marks]

Mark scheme

1.Think about movement, building molecules and temperature.

Any two from: muscle contraction or movement (1); building larger molecules from smaller ones such as proteins (1); maintaining a constant body temperature in mammals and birds (1)

7Explain why anaerobic respiration releases less energy than aerobic respiration.[3 marks]

Mark scheme

1.Compare how fully glucose is broken down.

Anaerobic respiration does not use oxygen (1); the glucose is only partly broken down or oxidised incompletely (1); so less energy is released per glucose molecule than in aerobic respiration (1)

8A sprinter feels tired and their muscles ache after a 200 metre race. Explain in terms of respiration why this happens and how the body recovers.[4 marks]

Mark scheme

1.Explain the switch to anaerobic respiration.
2.Explain oxygen debt and lactic acid removal.

During intense exercise muscles cannot get enough oxygen so they respire anaerobically (1); this produces lactic acid which causes muscle fatigue and aching (1); after exercise the body must repay the oxygen debt by taking in extra oxygen (1); blood carries lactic acid to the liver where it is converted back to glucose (1)

9Name the organelle in which aerobic respiration takes place.[1 mark]

Mark scheme

1.Recall the site of aerobic respiration.

Mitochondria (1)

10State the word equation for anaerobic respiration in yeast.[1 mark]

Mark scheme

1.Recall the two products of fermentation.

glucose produces ethanol + carbon dioxide (1)

11Explain why bread dough rises when yeast is added and then collapses when baked in an oven.[4 marks]

Mark scheme

1.Link yeast fermentation to gas production.
2.Explain what heat does.

Yeast respires anaerobically (fermentation) and produces carbon dioxide (1); the gas gets trapped in the dough, causing it to rise (1); during baking the high temperature denatures the yeast enzymes so fermentation stops (1); the heat also causes the air bubbles to expand before the gluten structure sets, but the yeast protein is denatured, so no more gas is made (1).

12A student measures the volume of carbon dioxide produced by yeast in glucose solution at 20°C and 35°C. Predict and explain the difference in results.[4 marks]

Mark scheme

1.Apply enzyme rate theory to respiration.
2.Link to increased collision frequency.

At 35°C the volume of carbon dioxide produced per minute will be greater (1); higher temperature increases the kinetic energy of molecules (1); enzyme-substrate collisions occur more frequently (1); so the rate of anaerobic respiration and fermentation in the yeast is faster, producing more carbon dioxide (1).

13Explain why cells in actively contracting muscles have more mitochondria than resting cells.[4 marks]

Mark scheme

1.Link muscle contraction to energy demand.
2.Connect energy to aerobic respiration in mitochondria.

Contracting muscles need a large amount of energy (1); this energy is released by aerobic respiration (1); aerobic respiration takes place in mitochondria (1); more mitochondria means more respiration can occur simultaneously to meet the high energy demand (1).

14Compare aerobic and anaerobic respiration in human muscle cells in terms of reactants, products and the amount of energy transferred.[6 marks]

Mark scheme

1.Produce a direct comparison using a structured approach.

Aerobic: reactants are glucose and oxygen (1); products are carbon dioxide and water (1); large amount of energy transferred (1). Anaerobic: reactant is glucose only, no oxygen required (1); product is lactic acid (1); much smaller amount of energy transferred because glucose is only partially broken down (1).

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