Lesson 4 of 5 / Respiration in humans
Aerobic respiration releases energy
What changes during aerobic respiration, and where does the energy come from?
In this lesson: Define aerobic respiration in human cells and state its word equation.
About 5 min
The key ideaAerobic respiration breaks down glucose using oxygen, releasing energy and producing carbon dioxide and water.
Explore the idea
What goes in, and what comes out?
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.
Step by step
- 1
Write the reactants
Glucose and oxygen go before the arrow.
- 2
Write the products
Carbon dioxide and water go after the arrow.
- 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.
Is this true? "Aerobic respiration happens only in the lungs."
The lungs exchange gases. Many body cells use oxygen to respire; for example, muscle cells release energy for contraction.