Lesson overview

Respiration: How do cells release energy from food?

Collect the gas that yeast gives off in a gas syringe, test it with limewater and time fresh yeast at five temperatures to find where it respires fastest. Learn what living things use energy for, how aerobic and anaerobic respiration release it, and why a sprint leaves your muscles with an oxygen debt.

Suggested sequence

Lesson 1 · Energy from food. What living things use energy for; aerobic respiration as chemical reactions in cells that use oxygen to release energy from nutrient molecules; its word equation and (Supplement) balanced equation; germinating peas in vacuum flasks. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
Lesson 2 · Yeast, bread and temperature. Investigating the effect of temperature on respiration in yeast; anaerobic respiration and its word equation in yeast, with (Supplement) the balanced equation; much less energy per glucose molecule than aerobic respiration; bread and brewing. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
Lesson 3 · Muscles and the oxygen debt. Anaerobic respiration in muscles during vigorous exercise; (Supplement) lactic acid and the oxygen debt, and how the debt is removed after exercise. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
End of module · final check. 9 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
One to two weeks later · return questions. 11 questions, 15 min, lesson closed.

Learning goals

Part 1: Energy from food

State the uses of energy in living organisms: muscle contraction, protein synthesis, cell division, active transport, growth, the passage of nerve impulses and the maintenance of a constant body temperature (12.1.1)
Describe aerobic respiration as the chemical reactions in cells that use oxygen to break down nutrient molecules to release energy; show the heat it releases with germinating peas (12.2.1)
State the word equation for aerobic respiration: glucose + oxygen → carbon dioxide + water (12.2.2)
Supplement: state the balanced chemical equation for aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (12.2.3)

Part 2: Yeast, bread and temperature

Investigate and describe the effect of temperature on respiration in yeast: collect the carbon dioxide in a gas syringe for a fixed time and work out the rate in cm³ per minute (12.1.2)
Describe anaerobic respiration as the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen, and state that it releases much less energy per glucose molecule than aerobic respiration (12.3.1, 12.3.2)
State the word equation for anaerobic respiration in yeast: glucose → alcohol + carbon dioxide (12.3.3)
Supplement: state the balanced chemical equation for anaerobic respiration in yeast: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (12.3.5)

Part 3: Muscles and the oxygen debt

State the word equation for anaerobic respiration in muscles during vigorous exercise: glucose → lactic acid; it releases much less energy per glucose molecule than aerobic respiration (12.3.4, 12.3.2)
Supplement: state that lactic acid builds up in muscles and blood during vigorous exercise, causing an oxygen debt (12.3.6)
Supplement: outline how the oxygen debt is removed after exercise: a fast heart rate continues to transport lactic acid in the blood from the muscles to the liver; deeper and faster breathing continues to supply oxygen for the aerobic respiration of lactic acid; lactic acid is respired aerobically in the liver (12.3.7)

Misconceptions to listen for

“Diffusion needs energy from respiration.” — Diffusion is passive: particles spread out down a concentration gradient by their own random movement.
“Osmosis needs energy from respiration.” — Osmosis is the diffusion of water through a partially permeable membrane. It is passive too.
“Only muscles need energy.” — Every living cell needs energy: to make proteins, for active transport, to divide and more.
“Taking in nitrate ions is diffusion, so it needs no energy.” — The ions move into the cell from soil that has fewer of them: against the gradient. That is active transport, and it uses energy.
“Making amylase is cell division.” — Amylase is an enzyme, and enzymes are proteins. Making it is protein synthesis.
“Row A.” — The lungs are where gases enter and leave the blood. Respiration is the reactions inside every living cell.
“Row C.” — Aerobic respiration uses oxygen. Carbon dioxide is one of the substances it gives off.
“Row D.” — Energy cannot be made. It is released from the nutrient molecules as they are broken down.
“glucose + carbon dioxide → oxygen + water” — That is the wrong way round: respiration uses oxygen and gives off carbon dioxide.
“glucose + oxygen → energy + water” — Energy is released, but it is not a substance. The products are carbon dioxide and water.

Scope

The uses of energy in living organisms (12.1.1); investigating the effect of temperature on respiration in yeast (12.1.2); aerobic respiration and its word equation (12.2.1, 12.2.2) and balanced equation (Supplement 12.2.3); anaerobic respiration, much less energy per glucose molecule, the word equations in yeast and in muscles (12.3.1–12.3.4) and the balanced equation in yeast (Supplement 12.3.5); lactic acid and the oxygen debt, and how the debt is removed (Supplement 12.3.6, 12.3.7). The gas volumes in the experiment come from a teaching model.

Open the first part as pupils see it