Cellular Respiration

What is Cellular Respiration?

Cellular respiration is the process by which cells release energy from glucose. This energy, stored as ATP (adenosine triphosphate), powers every life process: muscle contraction, growth, active transport, protein synthesis, and maintaining body temperature.

Respiration is NOT the same as breathing. Breathing (ventilation) moves air in and out of the lungs. Cellular respiration is a chemical process occurring in every living cell — including plant, animal, fungal, and bacterial cells.

Aerobic Respiration

Aerobic respiration uses oxygen to completely break down glucose, releasing the maximum amount of energy.

Word equation: Glucose + Oxygen → Carbon dioxide + Water (+ energy as ATP)

Balanced symbol equation: C6H12O6 + 6O2 → 6CO2 + 6H2O

This is an exothermic reaction — it releases energy.

Where it occurs: mainly in the mitochondria — organelles with a folded inner membrane (cristae) that greatly increases the surface area for ATP production. Cells with high energy demands (e.g., muscle cells, liver cells) have more mitochondria.

Stages of aerobic respiration:

1. Glycolysis (in cytoplasm): glucose is split into two pyruvate molecules; small amount of ATP produced
2. Krebs cycle (in mitochondrial matrix): pyruvate is broken down; CO2 released; electron carriers loaded
3. Oxidative phosphorylation (on inner mitochondrial membrane): most ATP is produced using oxygen as the final electron acceptor

ATP — The Energy Currency

ATP (adenosine triphosphate) is the universal energy currency of cells. It cannot be stored in large quantities but is constantly produced and used:

• When ATP is broken down (ATP → ADP + phosphate), energy is released for cellular work
• Respiration continuously regenerates ATP from ADP

Uses of ATP in the body:

• Muscle contraction — myosin heads use ATP to pull actin filaments
• Active transport — pumping ions and molecules across membranes against concentration gradients
• Biosynthesis — building proteins, DNA, lipids
• Maintaining body temperature — heat released during respiration keeps warm-blooded animals at 37°C
• Nerve impulse transmission — ion pumps require ATP to reset after each impulse

Anaerobic Respiration

Anaerobic respiration occurs when oxygen supply is insufficient — for example, during intense exercise when muscles cannot receive oxygen fast enough.

It is faster than aerobic respiration (no need to wait for oxygen) but releases far less energy because glucose is only partially broken down.

In animals and humans:

Glucose → Lactic acid + (small amount of energy)

• Lactic acid accumulates in muscles, causing the burning sensation and fatigue felt during intense exercise
• Cannot be sustained for long without becoming painful

In yeast and plants:

Glucose → Ethanol + Carbon dioxide + (small amount of energy)

• Used in bread-making: CO2 produced by yeast makes dough rise; alcohol evaporates during baking
• Used in brewing and winemaking: ethanol is the alcohol in beer and wine
• Used in biofuel production: ethanol produced by fermentation can be used as fuel

Oxygen Debt

During intense exercise, lactic acid builds up in muscles. After exercise, the body needs extra oxygen to:

• Oxidise lactic acid in the liver (converting it back to glucose or CO2 and water)
• Repay the "oxygen debt"

This is why breathing rate and heart rate remain elevated for some time after exercise stops — the body continues supplying extra oxygen until all lactic acid is cleared.

Comparison of Aerobic vs Anaerobic Respiration

Feature	Aerobic	Anaerobic
Oxygen required	Yes	No
Energy (ATP) released	High (up to 38 ATP per glucose)	Low (2 ATP per glucose)
Products in animals	CO2 + H2O	Lactic acid
Products in yeast	CO2 + H2O	Ethanol + CO2
Location in cell	Mitochondria (mainly)	Cytoplasm
Duration	Sustained	Short-term only

Worked Example

A sprinter runs 100 m at maximum speed. Explain why their muscles respire anaerobically and what happens immediately after the race.

During the sprint, muscles demand energy faster than the circulatory system can deliver oxygen. Anaerobic respiration begins, producing ATP rapidly from glucose but generating lactic acid as a by-product. This causes muscle fatigue and the burning sensation.

After the race, the sprinter breathes heavily to increase oxygen supply. Extra oxygen is used to break down the accumulated lactic acid in the liver. This repays the oxygen debt and breathing rate returns to normal once lactic acid levels drop.

Key Terms

• Cellular respiration — process releasing energy from glucose in all living cells
• Aerobic respiration — respiration using oxygen; produces CO2, water, and large amounts of ATP
• Anaerobic respiration — respiration without oxygen; produces lactic acid (animals) or ethanol + CO2 (yeast)
• ATP — adenosine triphosphate; the energy currency of cells
• Mitochondria — organelle where aerobic respiration occurs
• Oxygen debt — extra oxygen needed after exercise to break down accumulated lactic acid
• Fermentation — anaerobic respiration in yeast; produces ethanol and CO2

Common Mistakes

• Confusing respiration (energy release in cells) with breathing (moving air in and out of lungs)
• Thinking plants only photosynthesise — plants respire continuously, day and night
• Saying anaerobic respiration is always "worse" — it is essential for short bursts of intense activity
• Confusing products: animals produce lactic acid; yeast produces ethanol + CO2

Tips and Tricks

• Aerobic = with oxygen = more energy = CO2 + H2O (complete combustion of glucose)
• Anaerobic = without oxygen = less energy = incomplete breakdown
• Remember fermentation products with "yeast makes bread and beer" — CO2 for bread rising, ethanol for alcohol
• After exercise, heavy breathing = repaying oxygen debt = breaking down lactic acid