Respiration is the process by which ALL living cells release energy from organic molecules — mainly GLUCOSE — to produce ATP (adenosine triphosphate).
ATENTION: Respiration is NOT the same as BREATHING. Breathing (ventilation) moves air in and out of the lungs. Respiration is a CHEMICAL PROCESS happening inside every cell.
ATP is the ENERGY CURRENCY of cells — it is the molecule that directly powers all cellular processes:
Muscle contraction.
Active transport.
Protein synthesis.
Cell division.
Maintaining body temperature.
Respiration is continuous — happening in every living cell, day and night, throughout your entire life.
The Equation for Aerobic Respiration
Aerobic respiration uses OXYGEN to completely break down glucose.
Word equation:
glucose + oxygen → carbon dioxide + water (+ energy released as ATP)
Symbol equation:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Key points:
Glucose is completely broken down — all the carbon atoms end up as CO₂.
Water is produced — the hydrogen from glucose combines with oxygen.
Large amount of ENERGY is released — approximately 36–38 ATP molecules per glucose.
This is the most efficient form of respiration.
Where Aerobic Respiration Occurs
Aerobic respiration takes place in the MITOCHONDRIA — the 'powerhouses' of the cell.
The mitochondria have a folded inner membrane (called cristae) which increases the surface area for the reactions.
Cells with HIGH energy demands have MORE mitochondria:
Muscle cells — need large amounts of ATP for contraction.
Liver cells — highly metabolically active, many chemical reactions.
Sperm cells — need ATP to power the flagellum for swimming.
Heart muscle cells — never stop contracting, need a constant ATP supply.
Cells with lower energy demands have fewer mitochondria.
Why Aerobic Respiration is so Efficient
Aerobic respiration releases approximately 36–38 ATP molecules per glucose molecule — far more than anaerobic respiration (which produces only about 2 ATP).
This is because oxygen allows the complete breakdown of glucose:
All six carbon atoms → 6 CO₂ (carbon fully released).
All hydrogen atoms → 6 H₂O (hydrogen fully removed).
All the chemical energy stored in the C–H bonds of glucose is harvested.
Without oxygen, only a small fraction of this energy can be released — the rest remains locked in the waste products (lactic acid or ethanol).
This is why we breathe — to supply the constant oxygen demand of aerobic respiration in every cell.
⚠️ Common Mistake
RESPIRATION is not the same as BREATHING. Breathing is ventilation — moving air in and out of lungs. Respiration is the CHEMICAL PROCESS in cells that releases energy from glucose. Also: aerobic respiration happens in MITOCHONDRIA — not in the nucleus, not in chloroplasts.
📐 Key Equations
glucose + oxygen → carbon dioxide + water
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
📌 Key Note
Aerobic respiration: glucose + oxygen → CO₂ + water. In mitochondria. Releases ~36–38 ATP. Continuous in all living cells. Exothermic — energy released.
🎯 Matching Activity — Aerobic Respiration — Match the Term
Match each term to its correct description. — drag the symbols on the right to match the component names on the left.
Glucose
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Oxygen
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ATP
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Carbon dioxide
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Mitochondria
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Water
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The energy molecule produced — directly powers all cell processes
Waste product — produced when glucose is completely broken down
Produced when hydrogen from glucose combines with oxygen
The organelle where aerobic respiration takes place
Needed for aerobic respiration — allows complete breakdown of glucose
The fuel molecule broken down in respiration
⭐ Higher Tier Only
ATP (adenosine triphosphate) is the immediate energy currency of cells — it directly powers muscle contraction, active transport, protein synthesis and cell division. Aerobic respiration produces ~36–38 ATP per glucose molecule. This compares to only ~2 ATP from anaerobic respiration — which is why aerobic respiration is much more efficient and is used for sustained activity.
🎯 Test Yourself
Question 1 of 3
1. Where in the cell does aerobic respiration take place?
2. What is the difference between respiration and breathing?
3. Why do muscle cells contain more mitochondria than skin cells?
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