Response to Exercise and Metabolism

During exercise, your muscles contract far more often than when you are at rest. Contraction needs energy, and that energy is transferred by respiration. So the harder you work, the more glucose and oxygen your muscle cells need, and the more carbon dioxide they produce.

The body reacts to this increased demand for energy by changing three things together:

• Heart rate increases — the heart beats faster.
• Breathing rate increases — you take more breaths per minute.
• Breath volume increases — each breath becomes deeper, moving more air.

All three changes have one purpose: to supply the muscles with more oxygenated blood.

Key idea: During exercise the body reacts to the increased demand for energy. Heart rate, breathing rate and breath volume all increase to deliver more oxygen and glucose to the muscles and to remove carbon dioxide faster.

Walk-through — why heart rate and breathing rate rise:

1. Muscles contract more during exercise, so they respire faster and need more oxygen and glucose.
2. Breathing faster and deeper gets more oxygen into the blood at the lungs and removes carbon dioxide more quickly.
3. A faster heart rate pumps that oxygenated blood around the body more often, so it reaches the muscles sooner.
4. More oxygen and glucose delivered to the muscles means aerobic respiration can keep transferring the energy needed for continued contraction.

While enough oxygen reaches the muscles, they respire aerobically. Aerobic respiration is the complete breakdown (oxidation) of glucose using oxygen, releasing the most energy per glucose molecule:

$\text{glucose}+\text{oxygen}\to \text{carbon dioxide}+\text{water}$

This is why heart rate, breathing rate and breath volume rise during exercise — the body is trying to keep supplying enough oxygen for aerobic respiration to continue.

But there is a ceiling. During vigorous exercise, muscle cells can demand oxygen faster than the heart and lungs can deliver it. When the oxygen supply is insufficient, the muscles switch to anaerobic respiration to keep some energy being transferred.

Feature	Aerobic respiration	Anaerobic respiration (in muscles)
Oxygen used?	Yes	No
Energy released per glucose	Large amount	Much smaller amount
Glucose breakdown	Complete oxidation	Incomplete oxidation
Products	Carbon dioxide + water	Lactic acid

Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration, because the glucose is only partially broken down. That is the trade-off for working without enough oxygen.

When oxygen supply to the muscles is insufficient, glucose is broken down without oxygen. This incomplete oxidation of glucose produces lactic acid:

$\text{glucose}\to \text{lactic acid}$

There is no oxygen, carbon dioxide or water in this reaction — the only product you need to name at GCSE is lactic acid.

Because the glucose is not fully broken down, anaerobic respiration releases far less energy from each glucose molecule than aerobic respiration. To keep the muscles supplied with energy, the cells must respire a much larger amount of glucose, and the lactic acid begins to build up.

Worked walk-through — sprinting up a flight of stairs:

1. Your leg muscles contract rapidly and demand energy very quickly.
2. Your heart and lungs cannot deliver oxygen fast enough to match that demand.
3. With insufficient oxygen, the muscle cells respire anaerobically.
4. Each glucose molecule is only partially broken down, so lactic acid accumulates in the muscles.
5. The incomplete oxidation creates an oxygen debt — extra oxygen will be needed afterwards to deal with that lactic acid.

Be precise: anaerobic respiration in muscles produces lactic acid (not carbon dioxide and water). The incomplete oxidation of glucose is what causes lactic acid to build up and creates the oxygen debt.

During long periods of vigorous activity, lactic acid keeps building up in the working muscles. As it accumulates, the muscles become fatigued — they stop contracting efficiently.

Muscle fatigue is the reason you cannot keep sprinting at full effort indefinitely: the muscles feel weak or sore and lose power, forcing you to slow down or stop.

• Cause: anaerobic respiration during prolonged vigorous exercise.
• What builds up: lactic acid in the muscle cells.
• Effect: muscles become fatigued and stop contracting efficiently.

Key definition — muscle fatigue: during long periods of vigorous activity, muscles become fatigued and stop contracting efficiently. This follows the build-up of lactic acid from anaerobic respiration.

Linking it back: the body raised heart rate, breathing rate and breath volume to try to keep up the oxygen supply. When even that is not enough for vigorous exercise, anaerobic respiration takes over, lactic acid accumulates, and fatigue sets in.

After hard exercise stops, you keep breathing heavily and your heart keeps pounding for a while. This is the body repaying its oxygen debt.

(Higher Tier only — oxygen debt is the amount of extra oxygen the body needs after exercise to react with the accumulated lactic acid and remove it from the cells.)

So that the lactic acid is dealt with, blood must carry it away from the muscles and the body must take in extra oxygen.

(Higher Tier only — blood flowing through the muscles transports the lactic acid to the liver, where it is converted back into glucose.)

Worked walk-through — why you pant after sprinting:

1. During the sprint, anaerobic respiration produced lactic acid that built up in your muscles.
2. When you stop, that lactic acid still has to be removed — it cannot simply disappear.
3. You keep breathing fast and deep to take in extra oxygen beyond your normal resting need. This extra oxygen is the oxygen debt.
4. Blood flowing through the muscles transports the lactic acid to the liver.
5. In the liver the lactic acid is converted back into glucose, and the extra oxygen lets the body react with and remove the accumulated lactic acid.
6. Once the oxygen debt is repaid and the lactic acid is cleared, your breathing and heart rate return to resting levels.

Higher Tier exam tip: oxygen debt is repaid after exercise, not during it. Quote it precisely — the extra oxygen needed after exercise to react with and remove the accumulated lactic acid.

The energy transferred by respiration does not just power muscle contraction. Cells use it to build and break down molecules constantly, and the sum of all these reactions is called metabolism.

Key definition — metabolism: metabolism is the sum of all the reactions in a cell or in the body.

The energy transferred by respiration in cells is used by the organism for the continual enzyme-controlled processes of metabolism that synthesise new molecules. Every reaction in the list below is controlled by enzymes and depends on energy from respiration.

These reactions fall into two broad groups:

• Synthesis — building large molecules from smaller ones (for example, joining glucose molecules into starch, glycogen or cellulose).
• Breakdown — splitting large molecules into smaller ones (for example, breaking down excess protein in the liver).

(Extra context — not required by AQA 8461.) Reactions that build molecules and reactions that break them down are sometimes called anabolic and catabolic. You do not need these terms for 8461 — "synthesis" and "breakdown" are enough.

Metabolism relies on a small set of building blocks: sugars, amino acids, fatty acids and glycerol are used in the synthesis and breakdown of carbohydrates, proteins and lipids.

The specification lists specific reactions you should be able to recognise as part of metabolism. Learn them as reactant(s) → product(s):

Reactant(s)	Product	Type of process
Glucose	Starch, glycogen and cellulose	Synthesis of carbohydrates
One glycerol + three fatty acids	A lipid molecule	Synthesis of a lipid
Glucose + nitrate ions	Amino acids → proteins	Synthesis of proteins
Glucose + oxygen (or glucose alone, anaerobically)	Energy transferred	Respiration
Excess proteins	Urea (for excretion)	Breakdown of protein

Two details examiners look for:

• A lipid molecule is formed from one molecule of glycerol and three molecules of fatty acids. Get the numbers right.
• Plants and animals make amino acids by combining glucose with nitrate ions; the amino acids are then joined to synthesise proteins.

Key fact: excess protein cannot be stored, so it is broken down. This breakdown forms urea, which is then excreted. Respiration is itself counted as one of the reactions of metabolism.

1. Saying anaerobic respiration in muscles produces carbon dioxide and water

In muscles, anaerobic respiration produces only lactic acid. Carbon dioxide and water are products of aerobic respiration. Do not mix the two equations up.

2. Saying oxygen debt is repaid during exercise

Oxygen debt is the extra oxygen needed after exercise to react with and remove the accumulated lactic acid. It is repaid once you stop, which is why you keep breathing heavily afterwards.

3. Confusing where lactic acid goes (HT)

On the Higher Tier, lactic acid is transported by the blood to the liver, where it is converted back into glucose — not "broken down in the muscles" and not "turned into carbon dioxide in the muscles".

4. Treating metabolism as only "breaking things down"

Metabolism is the sum of all reactions in a cell or the body — both building molecules up (synthesis) and breaking them down. Listing only digestion or respiration misses half the definition.

5. Getting the lipid numbers wrong

A lipid is made from one glycerol and three fatty acids. Writing "three glycerol and one fatty acid" or leaving out the numbers loses easy marks.

6. Forgetting that all three exercise responses change together

Heart rate, breathing rate and breath volume all increase during exercise. Answers that mention only heart rate are incomplete — name all three and link them to supplying more oxygenated blood to the muscles.