Energy Metabolism: ATP & Cellular Respiration
How living cells harvest energy from food — Grade 9 Biology
🔋 What Is ATP?
ATP stands for adenosine triphosphate. It is the universal energy currency of all living organisms — from bacteria to blue whales.
Structure of ATP
ATP consists of three components:
- 1 Adenine — a nitrogen-containing base
- 2 Ribose — a five-carbon sugar
- 3 Three phosphate groups — linked in a chain (this is where the energy is!)
How ATP Releases Energy
When the terminal (third) phosphate group is removed by hydrolysis, the reaction releases approximately 7.3 kcal/mol of free energy:
ATP → ADP + Pi + Energy (≈ 7.3 kcal/mol)
This energy powers virtually everything your cells do: muscle contraction, nerve impulses, active transport, protein synthesis, and more.
⚠️ Common Misconception: Many students think "breaking the bond releases energy." In reality, breaking any bond requires energy input. The net energy release comes from the overall hydrolysis reaction because the products (ADP + Pi) are more stable than ATP. It's the formation of new bonds in the products that releases more energy than was needed to break the old ones.
🧬 3D Model: ATP Molecule
Interactive visualization — available in the full lesson
Drag to rotate • Scroll to zoom
🔥 Cellular Respiration — The Big Picture
Cellular respiration is the process by which cells convert glucose and oxygen into ATP (usable energy), water, and carbon dioxide.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP (energy)
Reactants (Inputs)
🍬 Glucose (C₆H₁₂O₆)
💨 Oxygen (O₂)
Process
⚙️ 4 Stages of
Cellular Respiration
Products (Outputs)
🔋 ATP (30–38)
💧 Water & CO₂
💡 Did you know? Plants do cellular respiration too! They photosynthesize to make glucose, then respire to use that glucose for energy — just like animals.