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IB DP Biology — C: Interaction and Interdependence | SL | C1.2 Cell Respiration
📋 What You Will Learn
- Identify the locations and substrates of glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation
- Explain how ATP, NADH, and FADH₂ are produced at each stage of aerobic respiration
- Analyse data from respirometer experiments and interpret graphs showing oxygen consumption and CO₂ production
- Apply the concept of substrate-level and oxidative phosphorylation to explain ATP yield differences
- Evaluate the relative ATP yield of aerobic versus anaerobic respiration and the role of the electron transport chain
🔍 Guiding Question How do cells transfer the chemical energy stored in organic molecules into a form that can drive cellular work, and why does this require so many sequential biochemical stages?
🌍 Why This Matters Every sprint, every heartbeat, every thought you have is powered by ATP produced through the pathways you are about to study. When elite athletes train at altitude, their mitochondria multiply and their electron transport chains become more efficient — a direct, measurable consequence of the biochemistry in this topic.
📋 What You Will Learn
The Problem the Cell Had to Solve
Glucose is a stable molecule. Its six carbon-carbon and carbon-hydrogen bonds hold enormous chemical potential energy, but that energy cannot be released in one explosive burst — the cell would be destroyed. The challenge of life is to extract that energy in small, controlled steps, capturing it in a usable currency before it dissipates as heat.
That currency is ATP — adenosine triphosphate. But ATP cannot be stored in bulk; it must be continuously regenerated. The solution evolution arrived at is a cascade of four linked stages, each stripping a little more energy from the original glucose molecule and funnelling it into ATP synthesis. Understanding these stages is understanding how life stays alive.
📖 Key Definition — Cell Respiration Cell respiration is the controlled release of energy from organic compounds to produce ATP. It occurs in all living cells and may be aerobic (requiring oxygen) or anaerobic (without oxygen). The process is enzymatically controlled and occurs in a precise sequence of reactions.
📖 Key Definition — ATP (Adenosine Triphosphate) ATP is the universal energy currency of the cell. It is a nucleotide consisting of adenosine bonded to three phosphate groups. Hydrolysis of the terminal phosphate bond releases approximately 30.5 kJ mol⁻¹ of free energy, which drives endergonic cellular processes.
📖 Key Definition — Substrate-Level Phosphorylation Substrate-level phosphorylation is the direct transfer of a phosphate group from a phosphorylated intermediate molecule to ADP, producing ATP. This occurs in both glycolysis and the Krebs cycle, independent of the electron transport chain.
Stage 1 — Glycolysis: Splitting Glucose in the Cytoplasm
Before a single mitochondrion is involved, glucose is already being dismantled. Glycolysis — from the Greek for "sugar splitting" — takes place entirely in the cytoplasm and requires no oxygen. It is the most ancient metabolic pathway, shared by virtually every