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2.6Structures and Functions in Living Organisms

Respiration — Edexcel Linear - OL Revision Notes & Topical Questions

10 min read 13 parts Lesson 21 of 25 1 quiz

📚 Edexcel Linear - OL Hub / Topical Revision
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Curriculum Equation Key Balanced Cellular Respiration EquationFast Exam Solution
C₆H₁₂O₆ + 6O₂ ➔ 6CO₂ + 6H₂O + 36-38 ATP
Word Equation & Conditions: Glucose + Oxygen -> Carbon Dioxide + Water + Energy (36-38 ATP)
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Lesson summary

Respiration: Aerobic and Anaerobic — EDEXCEL LINEAR - OL notes: Define respiration, state the uses of the energy released, write the equations for aerobic and anaerobic respiration and compare the products.Respiration is a chemical reaction carried out in all living organisms, in…

  • mitochondria
  • oxygen debt
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4

Part 1 of 6What is respiration?

Respiration is a chemical reaction carried out in all living organisms, in which energy is released from glucose, either in the presence of oxygen (aerobic) or in its absence (anaerobic).

What is respiration?
It is not the same as breathing or gas exchange.
The energy is transferred to ATP,
  • which cells use for: muscle contraction and movement; protein synthesis and building larger molecules from smaller ones; cell division and growth; active transport; transmitting nerve impulses;
    • and keeping body temperature constant in mammals and birds.

Part 2 of 6Aerobic respiration

Aerobic respiration requires oxygen and takes place in the mitochondria. It is defined as the chemical process in cells that uses oxygen to break down nutrient molecules such as glucose to release energy.

Word equation

  • glucose + oxygen → carbon dioxide + water (+ energy).
  • Symbol equation: C6H12O6 + 6O2 → 6CO2 + 6H2O.
  • It releases a large amount of energy per glucose molecule, and is the reverse of photosynthesis.

Part 3 of 6Anaerobic respiration

Anaerobic respiration does not require oxygen. It is the chemical process in cells that breaks down nutrient molecules to release energy without using oxygen. It releases much less energy per glucose molecule because the glucose is not fully broken down, but it releases the energy quickly, which helps in short bursts of intense exercise.

AerobicAnaerobic in animals (muscles)Anaerobic in plants and yeast
Oxygen needed?YesNoNo
WhereMitochondriaCytoplasmCytoplasm
Equationglucose + oxygen → carbon dioxide + waterglucose → lactic acidglucose → ethanol + carbon dioxide
Energy releasedLargeSmallSmall

In animal cells, lactic acid builds up during vigorous exercise and causes muscle fatigue and pain. After exercise the person keeps breathing hard to take in extra oxygen to break down the lactic acid, repaying the oxygen debt. Yeast anaerobic respiration is called fermentation and is used to make bread (carbon dioxide makes dough rise) and alcoholic drinks (ethanol).

Part 4 of 6Practical start: carbon dioxide from respiring seeds

Respiration can be investigated by measuring the carbon dioxide and heat released by germinating seeds. Set up tubes with germinating seeds, dead (boiled) seeds and glass beads, each connected to limewater or hydrogencarbonate indicator. Limewater turns cloudy (milky) in the tube with germinating seeds, because respiring seeds release carbon dioxide, but stays clear with boiled seeds or beads.

Part 5 of 6Comparing respiration and gas exchange, and exam hints

Respiration happens in every living cell and releases energy; gas exchange is the supply of oxygen to cells and removal of carbon dioxide, and happens at exchange surfaces such as alveoli and stomata.

1
Breathing (ventilation) is the movement of air in and out of the lungs.
2
Do not use these terms interchangeably.
3
When asked why anaerobic respiration releases less energy, say that glucose is only partly broken down,
  • so the lactic acid or ethanol still contains energy.
4
In calculations of respiratory quotient or exam data tasks, compare the volumes of oxygen used and carbon dioxide produced.
5
The ATP made in respiration is the immediate source of energy for cells; it is rebuilt continuously.

Quick check

  • aerobic respiration needs oxygen, takes place in the mitochondria and releases a large amount of energy, giving carbon dioxide and water;
    • anaerobic respiration needs no oxygen, takes place in the cytoplasm and releases little energy, giving lactic acid in muscles or ethanol and carbon dioxide in yeast and plants.

Part 6 of 6Worked examples

  1. Write the balanced equation for aerobic respiration. [2]2 marks

  2. Name the products of anaerobic respiration in yeast. [2]2 marks

  3. Explain why muscles hurt after sprinting. [3]3 marks

  4. Why does a person pant after exercise? [2]2 marks

  5. State two uses of the energy released in respiration. [2]2 marks

Key terms

TermMeaning
RespirationChemical reaction in cells that releases energy from glucose.
Aerobic respirationRespiration using oxygen.
Anaerobic respirationRespiration without oxygen.
Lactic acidProduct of anaerobic respiration in animal muscles.
Oxygen debtExtra oxygen needed after exercise to break down lactic acid.
FermentationAnaerobic respiration in yeast producing ethanol and carbon dioxide.

Avoid these mistakes

1
Saying respiration is breathing.
2
Saying anaerobic respiration releases more energy.
3
Saying plants make lactic acid anaerobically — they make ethanol and carbon dioxide.
4
Forgetting that respiration occurs in plants too.

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