K327 / K328 / 2027

Lesson 4 of 5 / Respiration in humans

Aerobic respiration releases energy

What changes during aerobic respiration, and where does the energy come from?

I can define aerobic respiration in human cells and state its word equation.

  • I can define aerobic respiration and state its word equation.Syllabus

    Syllabus K327 / K328, Topic 6(d). Define aerobic respiration and state its word equation.

About 5 min

Make a guess. You are not marked.

Where in your body does aerobic respiration happen?

  1. In almost every living cell.
  2. In the lungs, where you breathe air in.
  3. In muscle cells, when you exercise hard.
Show the answer

In almost every living cell.

Respiration releases energy inside cells, mostly in mitochondria. The lungs only exchange the gases.

The key idea

Aerobic respiration breaks down glucose using oxygen, releasing energy and producing carbon dioxide and water.

Explore the idea

What goes in, and what comes out?

Respiringhuman cellGlucoseOxygen

Glucose + oxygen
-> carbon dioxide + water

Glucose is broken down in the presence of oxygen. Oxygen supports the aerobic process; it is not a replacement for glucose.

Word-equation model. Arrows indicate inputs and outputs; neither particle counts nor reaction rates are represented.

Explanation

Aerobic respiration is the release of energy by the breakdown of glucose in the presence of oxygen. It is a chemical process in living cells. Mitochondria are major sites of aerobic respiration, connecting this process to the cell structures studied earlier.

The word equation is glucose + oxygen -> carbon dioxide + water. Glucose and oxygen are the reactants used in the process; carbon dioxide and water are products. Energy is released and transferred for cell activities.

Cells use this energy for activities such as muscle contraction, making new cell material and maintaining cell functions. These activities do not all stop when exercise ends, so cells continue to respire while a person rests.

Oxygen is needed for the aerobic pathway, but it does not replace glucose as the food molecule being broken down. Carbon dioxide is transported in blood to the lungs, where it diffuses into alveoli and is exhaled. This links cellular respiration to gas exchange without treating them as the same process.

In a supervised respiration investigation, carbon dioxide can be detected because it turns clear limewater milky. Compare gas from a living, respiring sample with an appropriate control under the same conditions. Record the change before drawing the conclusion; cloudiness detects carbon dioxide but does not directly measure the amount of energy released.

A respirometer measures how fast living organisms take in oxygen. Germinating seeds or woodlice sit in a sealed tube above sodium hydroxide solution, which absorbs carbon dioxide. A capillary tube with a coloured drop of liquid is attached. As the organisms take in oxygen and the carbon dioxide they release is absorbed, the volume of gas in the tube falls, so the drop moves towards the organisms. The distance moved per minute measures the rate of oxygen uptake. A control tube holds glass beads of the same volume instead of living organisms, showing that the drop moves only because of respiration. Sodium hydroxide is corrosive, so it is kept below a gauze where the organisms cannot touch it.

Step by step
  1. 1

    Write the reactants

    Glucose and oxygen go before the arrow.

  2. 2

    Write the products

    Carbon dioxide and water go after the arrow.

  3. 3

    State the purpose

    Energy is released for cell activities; it is not a new material made by breathing.

Worked example

A resting muscle

A student says a resting muscle has stopped respiring because it is not contracting strongly. Explain the error and give the aerobic word equation.

One way to explain it

Resting muscle cells still need energy for ongoing cell activities, so aerobic respiration continues. The word equation is glucose + oxygen -> carbon dioxide + water, with energy released.

Why this answer works
  • Rest is not absence of cell activity.
  • Use both reactants and both products.
  • Distinguish cellular energy release from breathing movements.

Watch out for this

A student says: "Aerobic respiration happens only in the lungs." What is wrong with this?

Show the answer

The lungs exchange gases. Many body cells use oxygen to respire; for example, muscle cells release energy for contraction.

Check your understanding

Which statement correctly describes aerobic respiration in a human cell?

  1. Glucose is broken down to lactic acid, and oxygen is released as a product.
  2. Breathing takes in oxygen, which the lungs turn into glucose for the cells.
  3. Glucose and oxygen are used; carbon dioxide and water form; energy is released.
Show the answer

Glucose and oxygen are used; carbon dioxide and water form; energy is released.

This is the aerobic word equation and its energy consequence.

Exam-style questions

Check your understanding

Which is a correct word equation for anaerobic respiration in human muscle?

  1. glucose + oxygen -> lactic acid + energy released
  2. glucose -> lactic acid + energy released
  3. glucose -> ethanol + carbon dioxide + energy released
  4. lactic acid + oxygen -> glucose + energy released
Show the answer

glucose -> lactic acid + energy released

Human muscle without enough oxygen breaks glucose down to lactic acid, releasing a little energy.

Quick recall

Card 1 of 3. Answer in your head, then check.

What changes during aerobic respiration, and where does the energy come from?

You can return to this lesson any time.