2f Respiration¶

Part of 2 Structure and Functions in Living Organisms.

Respiration is how every living cell releases usable energy from food, and it never stops. It can happen with oxygen (aerobic) or without it (anaerobic), and the two differ in how much energy they release and in what they produce.

What You Need to Learn¶

Further detail: Pearson Edexcel International GCSE Biology specification.

How aerobic and anaerobic respiration release energy, the equations for each, and the practical investigations that show respiration happening.

ATP and Aerobic Respiration¶

Respiration takes place continuously in living cells, releasing energy from nutrient molecules. This energy is stored in a molecule called ATP, which is then used to drive processes such as active transport, muscle contraction and protein synthesis.

Aerobic respiration uses oxygen and oxidises glucose fully, releasing the most energy:

Word equation: glucose + oxygen → carbon dioxide + water

Symbol equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Most of the reactions of aerobic respiration occur in the mitochondria, which is why cells with high energy demand — such as muscle cells and sperm cells — contain many mitochondria.

Exam technique

Give an equation in the form the question asks for: words for a word equation, and a balanced set of symbols for a symbol equation. Aerobic respiration is the reverse of photosynthesis, so check which one the question is about before writing it down.

Anaerobic Respiration¶

Anaerobic respiration occurs when oxygen supply is insufficient, such as during a sprint. In humans, most respiration is aerobic, but muscles can respire anaerobically for short periods when oxygen cannot be supplied quickly enough. Because glucose is not fully oxidised, less energy is released than in aerobic respiration. The lactic acid still holds energy that has not been released, which is why it can later be broken down using oxygen.

In animals (including humans), the equation is:

glucose → lactic acid

Lactic acid accumulates in muscles during intense exercise, contributing to fatigue, muscle cramps and sometimes the sensation of a stitch. An oxygen debt builds up because lactic acid must be broken down and removed after exercise. This is why breathing and heart rate can stay elevated for a time after exercise: extra oxygen is needed to remove the lactic acid.

In plant cells and yeast (called fermentation), the equation is:

glucose → ethanol + carbon dioxide

The carbon dioxide released by yeast causes bread to rise. The ethanol produced is the basis of alcoholic drinks.

Comparing Aerobic and Anaerobic Respiration¶

Feature	Aerobic	Anaerobic
Oxygen required?	Yes	No
Energy released	More	Less
End products (animals)	CO₂ + water	Lactic acid
End products (yeast/plants)	CO₂ + water	Ethanol + CO₂
Location in cell	Mainly mitochondria	Cytoplasm

Respiration in Practical Work¶

Investigating carbon dioxide production from respiring organisms: Germinating seeds or small organisms are placed in a sealed flask connected by a capillary tube to hydrogen-carbonate indicator solution. A separate flask containing sodium hydroxide (to absorb CO₂ from the air) and a control flask (with dead or boiled seeds) ensure that colour changes are due to respiration. Hydrogen-carbonate indicator is red at normal atmospheric CO₂ concentration, purple when CO₂ falls and yellow when CO₂ rises. The indicator near respiring seeds turns yellow as CO₂ from respiration dissolves in it.

Worked example: reading the indicator

The indicator beside germinating seeds turns from red to yellow. The indicator beside boiled seeds, the control, stays red.

Yellow means carbon dioxide has risen, and the only difference between the two flasks is that the seeds in the first are alive. The control shows that the colour change is caused by respiration in living seeds, and not by anything else in the apparatus.

Investigating heat production from respiring organisms: Living germinating seeds and dead (boiled) seeds as a control are placed in separate vacuum flasks with thermometers insulated in cotton wool. After a few days, the flask with living seeds shows a greater temperature rise because respiration releases energy, some of which appears as heat.

Explore a respirometer model¶

Use the interactive below to load organisms into a sealed chamber, absorb carbon dioxide with potassium hydroxide, and watch capillary fluid rise as oxygen is used. It helps visualise one way aerobic respiration can be measured in a closed apparatus, but it complements the carbon-dioxide and heat practicals described above rather than replacing them. Open full interactive.

In this model, the organisms use oxygen and release carbon dioxide. The potassium hydroxide absorbs the carbon dioxide, so the total gas volume in the sealed tube falls. That pressure drop allows atmospheric pressure on the open side to push the capillary fluid upwards, so the fluid movement is being used here as a model of oxygen uptake during aerobic respiration.

What this simulation does not show¶

The fluid movement is generated from a simplified rate model rather than from measured gas-volume changes from real organisms.
Potassium hydroxide is treated as absorbing carbon dioxide completely, so the remaining volume change reflects oxygen uptake only.
The named course practical focuses on carbon dioxide release and heat production. This model is mainly useful for visualising oxygen uptake during aerobic respiration in a sealed chamber.

Common Confusions¶

Respiration vs breathing: Respiration is the chemical process in cells that releases energy. Breathing is ventilation, which moves air in and out of the lungs.
ATP is not energy: ATP is the molecule that carries the energy released in respiration to the parts of the cell that use it.
Anaerobic in humans: Muscles respire anaerobically when oxygen cannot be supplied as fast as it is used, such as in a sprint. It is not the normal way humans respire.
Lactic acid and oxygen debt: Lactic acid builds up when muscles respire anaerobically. Oxygen debt is the extra oxygen needed afterwards to break it down, which is why breathing stays fast after exercise.

Check Yourself¶

Write the word equation for aerobic respiration.
State where most of the reactions of aerobic respiration take place, and explain why muscle cells have many of these structures.
Explain why anaerobic respiration releases less energy than aerobic respiration.
Describe what happens to breathing after a sprint, and explain why.
A flask of boiled seeds is used as a control in a respiration investigation. Explain why.
Compare the products of anaerobic respiration in a muscle cell and in a yeast cell.

Answers

Glucose + oxygen → carbon dioxide + water.
In the mitochondria. Muscle cells use a lot of energy for contraction, so they need many mitochondria to release it by aerobic respiration.
Glucose is not fully oxidised, so some of its energy stays in the product, which is lactic acid in animals and ethanol in yeast.
Breathing and heart rate stay raised for a time after the sprint. The muscles respired anaerobically, and extra oxygen is needed to break down the lactic acid, which is the oxygen debt.
Boiled seeds are dead, so they cannot respire. If the control shows no change, any change in the flask with living seeds must be caused by respiration.
Muscle cells produce lactic acid. Yeast cells produce ethanol and carbon dioxide.

Key Terms¶

Respiration: the chemical process in cells that releases energy from food molecules.
ATP: an energy-carrying molecule used to power cell activities.
Aerobic respiration: respiration that uses oxygen and releases a large amount of energy.
Anaerobic respiration: respiration that occurs without oxygen and releases less energy.
Lactic acid: the product of anaerobic respiration in animal cells.
Fermentation: anaerobic respiration in yeast or plant cells, producing ethanol and carbon dioxide.
Oxygen debt: the extra oxygen needed after anaerobic exercise to remove lactic acid from the body.
Hydrogen-carbonate indicator: a solution that changes colour with CO₂ concentration, used to detect respiration.