Lesson 5 of 6 / 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 and use its word and balanced symbol equations.
About 5 min
The key ideaAerobic respiration breaks down glucose using oxygen, releasing energy and producing carbon dioxide and water.
C6H12O6 + 6O2
-> 6CO2 + 6H2O
Molecule symbols are simplified. Red circles represent oxygen atoms, dark circles carbon and pale circles hydrogen. The glucose icon stands for one complete molecule.
Explanation
Aerobic respiration is the release of energy by the breakdown of glucose in the presence of oxygen. It is a series of enzyme-controlled reactions in cells. Much of the process takes place in mitochondria, although the initial breakdown steps occur in the cytoplasm.
The word equation is glucose + oxygen -> carbon dioxide + water. The balanced symbol equation is C6H12O6 + 6O2 -> 6CO2 + 6H2O. Energy is released and transferred for cell activities; it is not a new kind of matter created by the reaction.
The equation balances atoms. One glucose molecule supplies six carbon atoms, so the products include six carbon dioxide molecules. Twelve hydrogen atoms form six water molecules. Eighteen oxygen atoms occur on each side when all reactants and products are counted.
Cells use released energy for activities such as muscle contraction, active transport and building large molecules. These activities continue when a person is resting, so respiration does not stop between breaths or when exercise ends.
Oxygen is a reactant, not an energy source that replaces glucose. Carbon dioxide is a waste product that blood carries to the lungs, where it can diffuse into alveoli and be exhaled.
Step by step
- 1
Write the reactants
Place glucose and oxygen before the arrow.
- 2
Write the products
Place carbon dioxide and water after the arrow, and state that energy is released.
- 3
Check the atoms
Use coefficients 1, 6, 6, 6; do not change the chemical formula of a substance.
Worked example
Balance the oxygen
Why is the coefficient of O2 six in C6H12O6 + 6O2 -> 6CO2 + 6H2O?
One way to explain it
The products contain 12 oxygen atoms in carbon dioxide and 6 in water, making 18. Glucose already supplies 6 oxygen atoms, so 12 more are needed from 6 molecules of O2.
Why this answer works
- Count atoms, not just the number of molecules.
- Include oxygen already inside glucose.
- Change coefficients rather than changing O2 into a different formula.
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.