Aerobic and Anaerobic Respiration

GCSE Biology · Bioenergetics

What Is Respiration?

Respiration is an exothermic chemical reaction that occurs in every living cell to release energy from glucose. It is not breathing — breathing is ventilation. Respiration is a metabolic process controlled by enzymes.

The energy released by respiration is used for:

Muscle contraction (movement)
Active transport (moving substances against a concentration gradient)
Maintaining body temperature (in mammals and birds)
Building larger molecules from smaller ones (e.g. proteins from amino acids, glycogen from glucose)
Cell division (mitosis)
Transmitting nerve impulses

Aerobic Respiration

Aerobic respiration uses oxygen and occurs in the mitochondria of cells. It is the most efficient way to release energy from glucose.

Word equation:

glucose + oxygen → carbon dioxide + water (+ energy)

Balanced symbol equation:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Aerobic respiration releases a large amount of energy per molecule of glucose.

Anaerobic Respiration

Anaerobic respiration occurs without oxygen. It happens when oxygen cannot be supplied fast enough, such as during intense exercise.

In animals (and humans):

glucose → lactic acid (+ a small amount of energy)

Occurs in the cytoplasm (not mitochondria)
Releases much less energy than aerobic respiration (because glucose is only partially broken down)
Produces lactic acid, which builds up in muscles and causes fatigue, cramps, and a burning sensation

In plants and yeast (fermentation):

glucose → ethanol + carbon dioxide (+ a small amount of energy)

Used in brewing (making beer and wine — yeast ferments sugars to produce alcohol)
Used in baking (yeast produces CO₂, which makes bread rise)
Anaerobic respiration in yeast is called fermentation

Comparing Aerobic and Anaerobic Respiration

Feature	Aerobic	Anaerobic (animals)	Anaerobic (yeast/plants)
Oxygen needed?	Yes	No	No
Where in cell	Mitochondria	Cytoplasm	Cytoplasm
Products	CO₂ + water	Lactic acid	Ethanol + CO₂
Energy released	A lot	A little	A little
Glucose broken down	Completely	Incompletely	Incompletely

Exercise and Respiration

During exercise:

Muscles need more energy, so the rate of respiration increases
The body responds by: increasing heart rate, increasing breathing rate and depth, and dilating blood vessels supplying the muscles
These changes deliver more oxygen and glucose to the muscles, and remove carbon dioxide faster

During vigorous exercise, the body cannot supply oxygen fast enough. Muscles begin anaerobic respiration in addition to aerobic respiration.

Oxygen Debt

After vigorous exercise, lactic acid must be broken down. This requires oxygen.

The oxygen debt is the extra oxygen the body needs after exercise to:

1. Break down lactic acid — lactic acid is transported in the blood to the liver, where it is converted back to glucose

2. This is why you continue to breathe heavily after exercise — to repay the oxygen debt

EPOC (Excess Post-exercise Oxygen Consumption) is the scientific term for this process.

Required Practical: Effect of Exercise on Breathing Rate

Aim: Investigate the effect of exercise on breathing rate (or heart rate).

Method:

1. Measure resting breathing rate (breaths per minute) while sitting

2. Perform exercise (e.g. step-ups or star jumps) for a set time (e.g. 2 minutes)

3. Immediately measure breathing rate again

4. Continue measuring at intervals (e.g. every minute) until it returns to resting rate

5. Repeat for different intensities of exercise or durations

Expected results: Breathing rate increases during and immediately after exercise, then gradually returns to the resting rate. More intense exercise = higher peak breathing rate and longer recovery time.

Exam Tips

Respiration happens in all living cells all the time — not just when exercising
Learn both equations (word and symbol) for aerobic respiration
Anaerobic respiration in humans produces lactic acid (not ethanol) — do not confuse with yeast
Oxygen debt is repaid in the liver where lactic acid is converted back to glucose