Comprehensive AQA A-Level Biology: Respiration Notes (3.5.2)

Glycolysis and the Link Reaction

Think of glycolysis as breaking a six-piece puzzle (glucose) into two three-piece puzzles (pyruvate) - and getting paid in energy for doing it! This happens in your cell's cytoplasm and doesn't need oxygen, making it incredibly versatile.

The process has two main stages: phosphorylation and oxidation. During phosphorylation, glucose gets two phosphate groups attached (using 2 ATP), then splits into two triose phosphate molecules. The oxidation stage then converts these into pyruvate, producing 4 ATP and 2 reduced NAD. With a net gain of 2 ATP, you're already ahead!

The link reaction acts like a preparation kitchen for the next stage. Each pyruvate molecule loses a carbon (forming CO₂), gets oxidised, and combines with coenzyme A to form acetyl coenzyme A. This happens twice per glucose molecule and produces 2 reduced NAD molecules.

Quick Tip: Remember that everything in the link reaction happens twice because you start with two pyruvate molecules from glycolysis!

The Krebs Cycle and Oxidative Phosphorylation

The Krebs cycle is like a recycling plant in your mitochondrial matrix that squeezes every bit of energy from acetyl CoA. It's a circular process that regenerates oxaloacetate to keep the cycle spinning.

Starting with acetyl CoA combining with oxaloacetate to form citrate, the cycle involves two decarboxylation steps (removing CO₂) and multiple dehydrogenation reactions. Each turn produces 1 ATP, 3 reduced NAD, and 1 reduced FAD. Since it runs twice per glucose molecule, you get double these amounts.

Oxidative phosphorylation is where the real energy jackpot happens. In the inner mitochondrial membrane, reduced NAD and FAD release hydrogen, which splits into protons and electrons. The electrons travel down the electron transport chain, losing energy that's used to pump protons into the intermembrane space.

This creates an electrochemical gradient that drives chemiosmosis - protons flow back through ATP synthase, producing ATP. Oxygen acts as the final electron acceptor, combining with protons and electrons to form water. The total yield? Up to 32 ATP molecules from one glucose!

Key Point: Oxygen is essential here - without it, the electron transport chain stops, and ATP production plummets.

Anaerobic Respiration

When oxygen runs out, cells don't just give up - they switch to anaerobic respiration! This backup system ensures glycolysis can continue by regenerating NAD from reduced NAD.

In alcoholic fermentation (plants and yeast), pyruvate converts to ethanol and CO₂. This is exactly how bread rises and beer ferments! The process recycles NAD, allowing glycolysis to keep producing ATP even without oxygen.

Lactate fermentation happens in animal cells, including your muscles during intense exercise. Pyruvate converts directly to lactate (lactic acid), which is why your muscles might feel sore after a tough workout. Again, this regenerates NAD to maintain energy production.

While anaerobic respiration produces far less ATP than aerobic respiration, it's a crucial survival mechanism. It keeps essential processes running when oxygen is scarce, though it can only sustain cells for limited periods.

Real-world connection: The "burn" you feel during exercise? That's lactate building up in your muscles during anaerobic respiration!