Respiration and Energy Transfer
Easy Overview
You eat food. But how does that food actually become energy your body can use? That's respiration — not just breathing, but the chemical process that turns glucose into ATP, the energy currency of life. This chapter follows the journey of a glucose molecule from start to finish.
The Big Picture — Why Respiration?
Respiration is how cells get energy from food. It's not the same as breathing (breathing is just gas exchange). Cellular respiration breaks down glucose to make ATP — the molecule your cells actually run on. Think of glucose as a battery. Respiration is how you drain that battery to power stuff.
Glycolysis — The First Step
Glycolysis happens in the cytoplasm. No oxygen needed. One glucose (6-carbon) splits into two pyruvate (3-carbon) molecules. You get a net gain of 2 ATP and 2 NADH. It's like stripping the simplest parts first. If no oxygen is available, this is where it stops in some organisms (fermentation).
Krebs Cycle — The Processing Plant
If oxygen is available, pyruvate enters the mitochondria and gets converted to acetyl-CoA, then enters the Krebs cycle. This cycle produces CO₂ (which you breathe out), ATP, and lots of NADH and FADH₂. It's like a roundabout where each turn extracts more energy.
Electron Transport Chain — The Power Station
This is where most of the ATP actually comes from. NADH and FADH₂ dump electrons into a chain of proteins in the inner mitochondrial membrane. The electrons flow through, pumping protons across the membrane. This drives ATP synthase — a tiny turbine that makes ATP. Oxygen is the final electron acceptor — that's why you need to breathe.
Fermentation — When Oxygen is Scarce
No oxygen? No problem. Yeast do alcoholic fermentation — making ethanol and CO₂ (that's how bread rises and beer is made). Our muscles do lactic acid fermentation when we exercise hard — that burning feeling is lactic acid building up. You get only 2 ATP per glucose instead of 38.
Aerobic vs. Anaerobic — The Comparison
Aerobic respiration (with oxygen): 1 glucose → ~38 ATP. Anaerobic respiration (without oxygen): 1 glucose → 2 ATP. Aerobic is way more efficient. But anaerobic is faster and doesn't need oxygen. It's like the difference between a long highway trip (aerobic) and a short sprint (anaerobic).
Key Points
- •Cellular respiration = making ATP from glucose.
- •Glycolysis: cytoplasm, no O₂ needed, 2 ATP net gain.
- •Krebs cycle: mitochondria, produces CO₂ and NADH.
- •ETC (electron transport chain): inner mitochondrial membrane, produces ~34 ATP.
- •Oxygen is the final electron acceptor in ETC.
- •Fermentation: anaerobic, only 2 ATP, produces lactic acid or ethanol.
- •Aerobic: ~38 ATP per glucose. Anaerobic: 2 ATP per glucose.
- •ATP = adenosine triphosphate — the energy currency of cells.
Practice Questions
- Explain the process of glycolysis.
- What is the Krebs cycle? Where does it occur?
- Describe the electron transport chain.
- Differentiate between aerobic and anaerobic respiration.
- What is fermentation? Explain alcoholic and lactic acid fermentation.
- Why is oxygen essential for aerobic respiration?