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Aerobic and Anaerobic Respiration

Compare the equations for respiration with and without oxygen, and their uses.

  • Define and explain Aerobic and Anaerobic Respiration in your own words
  • Use key terms such as aerobic respiration accurately
  • Apply what you have learned to new examples and questions
  • Avoid the common mistakes learners make with this topic

This lesson focuses on Aerobic and Anaerobic Respiration: compare the equations for respiration with and without oxygen, and their uses.

Definition: Aerobic and Anaerobic Respiration

Compare the equations for respiration with and without oxygen, and their uses.

Key ideas

Respiration

Aerobic respiration releases energy from glucose using oxygen: glucose + oxygen → carbon dioxide + water. It powers muscle contraction, active transport and keeping warm. When oxygen runs short, anaerobic respiration takes over: in human muscle it produces lactic acid (glucose → lactic acid), while yeast produces ethanol and carbon dioxide (glucose → ethanol + carbon dioxide), which is used in brewing and bread-making.

Photosynthesis

Photosynthesis takes place in the chloroplasts of green cells: carbon dioxide + water → glucose + oxygen, using light energy trapped by chlorophyll. Leaves are adapted with a large surface area, thin structure for a short diffusion distance, and stomata that let gases in and out. Glucose is used for respiration, converted to starch for storage, or built into cellulose for cell walls.

Key term — aerobic respiration: Respiration using oxygen, which releases far more energy from glucose than anaerobic respiration.

Worked example: Aerobic and Anaerobic Respiration

Compare the products of anaerobic respiration in human muscle and in yeast.

In human muscle, anaerobic respiration converts glucose to lactic acid. In yeast, it converts glucose to ethanol and carbon dioxide.

Answer: In human muscle, anaerobic respiration converts glucose to lactic acid. In yeast, it converts glucose to ethanol and carbon dioxide.

Common mistakes
  • Writing the photosynthesis equation backwards Photosynthesis builds glucose from carbon dioxide and water; respiration breaks glucose down. Mixing them up reverses the direction of energy flow.
  • Saying plants only respire at night Plants respire all the time to release energy. At night photosynthesis stops but respiration continues.

Practice

Write the word equation for aerobic respiration.
It is the reverse of photosynthesis.

Glucose + oxygen → carbon dioxide + water (energy is released).

Write the word equation for photosynthesis.
What goes in, and what comes out?

Carbon dioxide + water → glucose + oxygen (using light energy).

Grass at one trophic level contains 20 000 kJ of energy, and the rabbits feeding on it contain 2 400 kJ. Calculate the percentage efficiency of energy transfer.
efficiency = (energy transferred ÷ energy available) × 100

2 400 ÷ 20 000 = 0.12; 0.12 × 100 = 12% efficiency. The rest is lost through respiration, movement, excretion and uneaten parts.

Give two ways a leaf is adapted for efficient photosynthesis.
Think about catching light and gases.

Any two of: large surface area to catch light; thin for a short diffusion distance; chlorophyll in chloroplasts to trap light energy; stomata for gas exchange; veins to transport water and glucose.

Quick check

Aerobic and Anaerobic Respiration — quick check

Which of these best defines "aerobic respiration"?

Respiration using oxygen, which releases far more energy from glucose than anaerobic respiration.

On a graph of photosynthesis rate against light intensity, the curve levels off at high light. Explain why.

Light is no longer the limiting factor; something else, such as carbon dioxide concentration or temperature, is now in shortest supply, so extra light cannot raise the rate further.
Key takeaways
  • Aerobic and Anaerobic Respiration: compare the equations for respiration with and without oxygen, and their uses.
  • Respiration: Aerobic respiration releases energy from glucose using oxygen: glucose + oxygen → carbon dioxide + water.
  • chlorophyll: The green pigment in chloroplasts that absorbs light energy for photosynthesis.
  • Watch out for: writing the photosynthesis equation backwards
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