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AQA BiologyBioenergetics

Exercise and metabolism

Explain responses to exercise and how metabolism supports the body.

Start here

The key idea

During exercise, muscles need more energy.Heart rate, breathing rate and breathing volume increase to supply more oxygen and remove carbon dioxide.

The exercise responseWorking muscles increase demand for oxygen and glucose.
The exercise responseWorking muscles increase demand for oxygen and glucose.exerciseheart rate risesbreathing risesmoreaerobicrespiration
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.

1

Response to exercise

During exercise muscles need more energy, so the body increases the rate and depth of breathing and increases heart rate.These changes deliver more oxygen and glucose to the muscles and remove carbon dioxide faster.The increased blood flow also removes lactic acid produced during anaerobic respiration.

2

Metabolism

Metabolism is the sum of all the chemical reactions in a cell or the body, controlled by enzymes.It includes building reactions, such as making proteins from amino acids and glycogen from glucose, and breaking down reactions, such as breaking excess proteins to form urea.The energy for metabolism is transferred by respiration.

3

Storage molecules

Glucose can be stored as glycogen in the liver and muscles, which provides a quick source of glucose for respiration during exercise.Excess glucose may also be converted to lipids for longer term storage.In plants glucose is stored as starch and converted to cellulose and lipids.

4

The liver and metabolism

The liver plays a central role in metabolism.It breaks down excess amino acids in a process called deamination, forming ammonia which is converted to urea for excretion by the kidneys.The liver also converts lactic acid produced in muscles back into glucose after exercise.

Key terms

Definitions to learn

Metabolism

The sum of all chemical reactions in a cell or the body, controlled by enzymes.

Glycogen

A storage carbohydrate made from glucose, stored in the liver and muscles.

Deamination

The breakdown of excess amino acids in the liver, producing ammonia then urea.

Lactic acid

The product of anaerobic respiration in muscle that causes fatigue.

Oxygen debt

The extra oxygen needed after exercise to remove lactic acid.

Worked example

Explain why heart rate increases during exercise.

1

Muscles respire faster to release more energy.

2

They need more oxygen and glucose.

3

A faster heart rate delivers these and removes carbon dioxide.

Final answer

Heart rate rises to support increased respiration in muscles.

Exam habit

Explain the chain: exercise → muscles need more energy → faster respiration → more oxygen and glucose needed → heart and breathing rate rise.Do not list responses without linking them to increased respiration.

Watch out

Do not describe only breathing rate. Explain how the response supports respiration.

Examiner tips

How to score full marks

  • 1When explaining responses to exercise, link each change to delivering more oxygen and glucose and removing carbon dioxide.
  • 2Define metabolism precisely as the sum of all reactions in the body controlled by enzymes.
  • 3Remember the liver, not the muscle, converts lactic acid back to glucose.
Practice questions

Try these yourself

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

1Define metabolism.
Mark scheme
  1. 1.Give the broad biological definition.
The sum of all chemical reactions in a cell or body.
2Give one metabolic use of energy released by respiration.
Mark scheme
  1. 1.Think building, temperature or movement.
Any one from: movement, keeping warm, building larger molecules or active transport.
3Why does breathing rate stay high briefly after exercise?
Mark scheme
  1. 1.Link to recovery from anaerobic respiration.
Extra oxygen is needed to remove or process lactic acid.
4Define metabolism.[1 mark]
Mark scheme
  1. 1.State it is all the reactions in the body.
The sum of all the chemical reactions in a cell or the body (1)
5Name the storage carbohydrate made from glucose in the liver and muscles.[1 mark]
Mark scheme
  1. 1.Recall the animal storage molecule.
Glycogen (1)
6State two changes that happen to the body during exercise to supply muscles with more energy.[2 marks]
Mark scheme
  1. 1.Think about breathing and heart rate.
Any two from: heart rate increases (1); breathing rate increases (1); breath depth or volume increases (1)
7Explain why heart rate and breathing rate increase during exercise.[3 marks]
Mark scheme
  1. 1.Link to oxygen, glucose and carbon dioxide.
Muscles respire faster and need more energy (1); increased heart and breathing rate deliver more oxygen and glucose to the muscles (1); and remove carbon dioxide produced by respiration more quickly (1)
8After a long run an athlete continues to breathe heavily for several minutes. Explain why, referring to metabolism in the muscles and liver.[4 marks]
Mark scheme
  1. 1.Explain anaerobic respiration during the run.
  2. 2.Explain oxygen debt and the role of the liver.
During the run muscles respired anaerobically producing lactic acid (1); after exercise extra oxygen is needed to repay the oxygen debt (1); the heavy breathing supplies this oxygen (1); lactic acid is carried in the blood to the liver and converted back into glucose (1)
9State what is meant by deamination and name the organ where it occurs.[3 marks]
Mark scheme
  1. 1.Recall the process and the organ.
Deamination is the breakdown of excess amino acids (1) in the liver (1); forming ammonia which is converted to urea (1).
10Explain why athletes who train regularly over many weeks develop larger hearts and more mitochondria in their muscle cells.[4 marks]
Mark scheme
  1. 1.Link training to increased demand.
  2. 2.Link physiological adaptation to meeting that demand.
Regular training means muscles respire faster and for longer periods (1); a larger heart can pump more blood per beat, delivering more oxygen and glucose to muscles (1); more mitochondria in muscle cells means more aerobic respiration can occur to meet the increased energy demand (1); these are physiological adaptations that improve athletic performance (1).
11A person eats a large meal containing carbohydrates. Describe what happens to the glucose in the blood after absorption, referring to storage and metabolic use.[4 marks]
Mark scheme
  1. 1.Link to blood glucose regulation, glycogen storage and respiration.
Blood glucose rises after the meal is absorbed (1); insulin is released causing glucose to move into cells (1); excess glucose is converted to glycogen for storage in the liver and muscles (1); glucose is also used in respiration to release energy for metabolic processes (1).
12Explain why excess protein in the diet cannot be stored in the body, unlike excess carbohydrate or fat.[5 marks]
Mark scheme
  1. 1.State that protein has no storage form.
  2. 2.Describe what happens to excess amino acids.
There is no storage molecule equivalent to glycogen or fat for proteins (1); excess amino acids are broken down by deamination in the liver (1); the amino group is removed and converted to ammonia, then to urea (1); the remaining carbon compound is used in respiration or converted to fat (1); urea is excreted by the kidneys in urine (1).
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