K224 / K225 / 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 K224 / K225, 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 releases energy by breaking down glucose using oxygen. It is a chemical process that happens inside living cells. Mitochondria are where most aerobic respiration takes place.

The word equation is: glucose + oxygen -> carbon dioxide + water. Glucose and oxygen are used up. Carbon dioxide and water are made. Energy is released for the cell to use.

Cells use this energy to contract muscles, build new cell parts and keep working. Cells keep needing energy even when you are resting, so they keep respiring.

Glucose is the food that is broken down; oxygen is needed to break it down. The blood carries carbon dioxide to the lungs, where it diffuses into the alveoli and is breathed out.

You can show that living things make carbon dioxide, because it turns clear limewater milky. Compare gas from living things, such as germinating seeds, with a control that has no living things. Milky limewater shows carbon dioxide, but it does not measure how much energy was released.

A respirometer measures how fast living things take in oxygen. Germinating seeds or woodlice sit in a sealed tube above sodium hydroxide, which absorbs carbon dioxide. A narrow tube with a coloured drop is attached. As the organisms use oxygen, the gas volume falls and the drop moves towards them. The distance moved per minute shows the rate of oxygen uptake. A control tube has glass beads instead of living things. Sodium hydroxide is corrosive, so it is kept under a gauze.

Step by step
  1. 1

    Write what goes in

    Glucose and oxygen go before the arrow.

  2. 2

    Write what comes out

    Carbon dioxide and water go after the arrow.

  3. 3

    Say what it is for

    Energy is released for the cell to use. Breathing does not make this energy.

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