Respiration
What respiration is and its distinction from breathing; the seven uses of energy released; aerobic respiration word equation and Extended balanced equation; anaerobic respiration in animals (lactic acid) and yeast (ethanol + CO₂); Extended balanced fermentation equation; and Extended oxygen debt mechanism.
Respiration
COREWhat is Respiration?
Respiration is the chemical process in which organic molecules are broken down to release energy for metabolism.
Key points:
- Respiration happens in every living cell, all the time — it is not the same as breathing
- The primary substrate is glucose (though fats and proteins can also be used)
- Energy released is used to make ATP — the cell's energy currency
Breathing (ventilation) is the mechanical movement of air in and out of the lungs — a physical process. Respiration is a chemical process occurring in every cell. Many students confuse these two. The question "where does respiration occur?" is answered "in every living cell", not "in the lungs".
Uses of Energy in Living Organisms
The energy released by respiration (as ATP) is used for all energy-requiring processes in living organisms. The syllabus requires you to know these specific uses:
| Use of energy | Example |
|---|---|
| Muscle contraction | All movement — walking, heartbeat, peristalsis, breathing muscles |
| Protein synthesis | Ribosomes assembling amino acids into proteins; making enzymes, structural proteins, antibodies |
| Cell division | Mitosis during growth, repair, and reproduction |
| Active transport | Moving ions and molecules against a concentration gradient across cell membranes |
| Growth | Cell division and protein synthesis during growth of new tissues require energy (ATP) |
| Passage of nerve impulses | Maintaining ion gradients across neurone membranes and restoring them after each impulse requires active transport (ATP) |
| Maintenance of body temperature | In mammals and birds — heat generated as a by-product of metabolic reactions keeps body warm |
Investigating Respiration — Yeast and Temperature
Setup: Mix yeast with glucose solution in a boiling tube. Seal with a bung. Connect via tubing to a test tube of limewater (or hydrogen carbonate indicator). Place the boiling tube in a water bath set at a given temperature. Measure CO₂ production over time (limewater turns cloudy, or count bubbles per minute).
Variable: Temperature of water bath (independent variable). Rate of CO₂ production (dependent variable — proxy for respiration rate).
Control variables: Volume and concentration of glucose solution, volume and concentration of yeast suspension, time allowed for each temperature, same apparatus.
Expected result: Respiration rate increases with temperature (up to an optimum, typically ~35–40°C for yeast enzymes). Above the optimum, rate falls as enzymes denature. At 0°C, little or no CO₂ is produced.
Link to enzyme theory: Respiration is controlled by enzymes. Higher temperature = more kinetic energy = more enzyme-substrate collisions per second = faster rate. Above optimum = denaturation of enzyme active sites.
Which of the following correctly describes respiration?
- A. The movement of air in and out of the lungs to obtain oxygen
- B. The process by which plants absorb CO₂ and release O₂ during photosynthesis
- C. A chemical process in cells in which organic molecules are broken down to release energy
- D. The diffusion of oxygen from the alveoli into the bloodstream
Aerobic Respiration
CORE EXTENDEDDefinition and Word Equation
Aerobic respiration is the chemical process that uses oxygen to break down glucose, releasing a relatively large amount of energy.
glucose + oxygen → carbon dioxide + water (+ energy released)
Balanced Chemical Equation — Extended
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy released as ATP)
Read it as: one molecule of glucose reacts with six molecules of oxygen to produce six molecules of carbon dioxide and six molecules of water. The large amount of energy released is used to synthesise ATP from ADP and inorganic phosphate.
Glucose is C₆H₁₂O₆ — 6 carbons, 12 hydrogens, 6 oxygens. Each carbon becomes one CO₂ (need 6 CO₂). Each pair of hydrogens becomes one H₂O (12 H → 6 H₂O). Count oxygens on right: 6×2 + 6×1 = 18 O. Oxygens already in glucose: 6. So O₂ needed: (18−6)÷2 = 6 O₂. Balanced.
Mitochondria — Site of Aerobic Respiration — Extended
Most of the ATP produced by aerobic respiration is generated in the mitochondria. Cells that have high energy demands have many more mitochondria — for example:
Require large amounts of ATP for repeated contraction. Skeletal muscle cells are densely packed with mitochondria, particularly in slow-twitch muscle fibres adapted for endurance.
The midpiece of a sperm cell is packed with mitochondria to provide energy (ATP) for the flagellum to beat, driving the sperm toward the egg.
Active transport of mineral ions into root hair cells against concentration gradients requires large amounts of ATP — hence many mitochondria.
Which row correctly shows the reactants and products of aerobic respiration?
| Reactants | Products | |
|---|---|---|
| A | Glucose + carbon dioxide | Oxygen + water |
| B | Glucose + oxygen | Carbon dioxide + water |
| C | Carbon dioxide + water | Glucose + oxygen |
| D | Glucose + water | Carbon dioxide + oxygen |
A student claims that sperm cells and red blood cells both need large amounts of energy and should therefore both have many mitochondria. Evaluate this claim. [3 marks]
- The claim is partly correct: sperm cells do have many mitochondria in their midpiece to provide ATP for flagellum movement [1 mark]
- However, red blood cells have no mitochondria at all — they lost their nucleus and organelles (including mitochondria) during maturation to maximise space for haemoglobin [1 mark]
- Red blood cells obtain energy via anaerobic respiration (glycolysis) in the cytoplasm; this produces far less ATP than aerobic respiration but is sufficient for their limited metabolic needs [1 mark]
Anaerobic Respiration
CORE EXTENDEDDefinition and Comparison with Aerobic
Anaerobic respiration is the chemical process that breaks down glucose to release energy without using oxygen, releasing a relatively small amount of energy.
| Feature | Aerobic respiration | Anaerobic respiration |
|---|---|---|
| Oxygen required? | ✓ Yes | ✗ No |
| Glucose broken down completely? | ✓ Yes — to CO₂ and H₂O | ✗ No — partially broken down |
| Energy released (ATP yield) | Relatively large amount | Relatively small amount |
| Products in animals/humans | CO₂ + H₂O | Lactic acid only |
| Products in yeast | CO₂ + H₂O | Ethanol + CO₂ |
| Location | Cytoplasm + mitochondria | Cytoplasm only |
Word Equations
glucose → lactic acid (+ energy released)
Occurs in muscle cells during intense exercise when oxygen delivery cannot keep up with demand. Lactic acid accumulates in muscles and blood, causing an oxygen debt and contributing to fatigue.
glucose → ethanol + carbon dioxide (+ energy released)
Used in brewing (alcohol production) and bread-making (CO₂ makes dough rise). Called fermentation. Yeast is killed by ethanol accumulation above ~12–15% — hence the upper limit of fermentation-produced alcohol.
Balanced Chemical Equation for Yeast — Extended
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (+ energy released as ATP)
One molecule of glucose yields two molecules of ethanol and two molecules of carbon dioxide. Only a small amount of ATP is produced compared to aerobic respiration.
Oxygen Debt — Extended
During intense exercise, muscles respire anaerobically and accumulate lactic acid. After exercise stops, extra oxygen is still consumed at an elevated rate — this extra oxygen consumed above the resting level is the oxygen debt (also called excess post-exercise oxygen consumption, EPOC).
| Stage | What happens |
|---|---|
| During intense exercise | Oxygen supply to muscles is insufficient for aerobic respiration; anaerobic respiration begins; lactic acid accumulates in muscle cells and blood; fatigue develops |
| After exercise — repaying the debt | Breathing rate and heart rate remain elevated; heart continues to transport lactic acid from muscles via blood to the liver; extra O₂ is used for aerobic respiration of lactic acid in the liver; debt is repaid and lactic acid concentration returns to normal |
The elevated breathing rate continues to supply extra oxygen to the liver, which uses it to break down lactic acid by aerobic respiration. Heart rate remains elevated to transport lactic acid from muscle cells through the blood to the liver. Both return to resting levels only once all lactic acid has been cleared and the oxygen debt is fully repaid.
A runner sprints 400 m and continues to breathe heavily for several minutes after finishing. Explain, in terms of oxygen debt and lactic acid, why breathing rate remains elevated after exercise. [4 marks]
- During the sprint, muscles could not receive enough oxygen for aerobic respiration; anaerobic respiration occurred [1 mark]
- Lactic acid accumulated in muscle cells and blood [1 mark]
- After exercise, extra oxygen is needed for aerobic respiration of lactic acid in the liver — this is the oxygen debt [1 mark]
- Elevated breathing rate supplies this extra oxygen until lactic acid is fully cleared and blood lactic acid returns to resting levels [1 mark]
During a marathon, a runner's muscles begin to produce lactic acid. What does this indicate?
- A. Aerobic respiration has stopped completely
- B. The rate of respiration exceeds the rate of oxygen delivery to the muscles
- C. Glucose in the muscles has been completely used up
- D. Carbon dioxide has accumulated to a level that is toxic to muscle cells
Comprehensive Practice Questions
Mixed questions across all of Topic 12.
Which statement correctly compares aerobic and anaerobic respiration?
- A. Aerobic respiration produces lactic acid; anaerobic produces carbon dioxide and water
- B. Both types of respiration require oxygen
- C. Aerobic respiration releases more energy per molecule of glucose than anaerobic respiration
- D. Anaerobic respiration only occurs in yeast; aerobic only in animals
(a) State the word equation for aerobic respiration. [1 mark]
(b) State the word equation for anaerobic respiration in yeast. [1 mark]
(c) Give two uses of the energy released by respiration in a mammal. [2 marks]
(d) Explain why a yeast culture in a sealed container will eventually stop producing ethanol, even if glucose is still available. [2 marks]
- (a) glucose + oxygen → carbon dioxide + water [1 mark]
- (b) glucose → ethanol + carbon dioxide [1 mark]
- (c) Any two of: muscle contraction / protein synthesis / cell division / active transport / maintaining body temperature / growth / passage of nerve impulses [2 marks]
- (d) Ethanol accumulates in the sealed container [1 mark]; ethanol reaches a concentration that is toxic / kills the yeast cells [1 mark]
(a) Write the balanced chemical equation for aerobic respiration. [1 mark]
(b) Explain why muscle cells contain many more mitochondria than skin cells. [2 marks]
(c) A student measures blood lactic acid levels before, during, and after a 10-minute period of intense exercise. Describe and explain the expected changes in blood lactic acid concentration during and after exercise. [4 marks]
- (a) C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O [1 mark]
- (b) Muscle cells contract repeatedly and require large amounts of ATP [1 mark]; mitochondria are the site of aerobic respiration which produces most ATP; more mitochondria = more ATP production capacity [1 mark]
- (c)
During exercise: blood lactic acid increases [1 mark]; muscles respire anaerobically because oxygen delivery cannot meet demand; lactic acid is produced and enters the blood [1 mark]
After exercise: blood lactic acid decreases back toward resting level [1 mark]; the oxygen debt is repaid — extra oxygen is used for aerobic respiration of lactic acid in the liver [1 mark]
High-Frequency Mistakes — Topic 12 Overall
- 💨Confusing respiration with breathingRespiration is a chemical process in every cell that releases energy from glucose. Breathing (ventilation) is the mechanical movement of air in and out of the lungs. Respiration occurs in every living cell all the time — not just in the lungs.
- ❌Saying anaerobic respiration produces no energyAnaerobic respiration releases a relatively small amount of energy — not zero. The syllabus wording is important: "relatively small" compared to aerobic. The energy released is still used for ATP synthesis.
- 🍿Mixing up anaerobic products — lactic acid vs ethanolAnimals (including humans) produce lactic acid during anaerobic respiration. Yeast produces ethanol + CO₂. Never say humans produce ethanol, or that yeast produces lactic acid. These are fixed — learn them as two separate equations.
- 📈Saying aerobic respiration only occurs in animalsAerobic respiration occurs in virtually all living organisms — plants, fungi, animals, bacteria. Plants respire aerobically at all times (not just in the dark). Photosynthesis and respiration are separate processes.
- ⚖Ext: Saying lactic acid is "removed" by breathing aloneLactic acid is transported via the blood to the liver, where it is broken down by aerobic respiration using the extra oxygen supplied by elevated breathing. The lactic acid itself does not leave via the lungs — only the CO₂ produced from its breakdown does.
- 💻Ext: Forgetting that red blood cells have no mitochondriaRed blood cells are enucleate (no nucleus) and have no organelles including no mitochondria. They rely on anaerobic respiration (glycolysis) for their energy. This is a classic trap when questions ask which cells have many or few mitochondria.
- 🔥Writing the aerobic equation the wrong way roundReactants are glucose + oxygen (left). Products are carbon dioxide + water (right). A common error is writing CO₂ + H₂O on the left — that is the reverse (photosynthesis inputs).
Highest-yield Core items: the word equations for both aerobic (glucose + O₂ → CO₂ + H₂O) and both anaerobic equations (animals: glucose → lactic acid; yeast: glucose → ethanol + CO₂); the seven uses of energy (muscle contraction, protein synthesis, cell division, active transport, body temperature, growth, passage of nerve impulses); and aerobic vs anaerobic comparison (especially energy yield difference). For Extended: the balanced equation for aerobic respiration (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O); the balanced equation for fermentation (C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂); the oxygen debt mechanism (lactic acid in muscles → transported via blood to liver → extra O₂ used for aerobic respiration of lactic acid in liver); and the link between mitochondria number and energy demand in specific cell types.