Cellular respiration is how your body breaks down glucose to...
Biology102 views·Updated 12 Sept 2026·2 pagesA-Level Biology: Clear Notes on Aerobic Respiration Processes
Isabelle @isabelle_study
1
of 2
Glycolysis and the Link Reaction
Ever wondered how your body turns that glucose from your breakfast into usable energy? Glycolysis kicks off this process right in your cell's cytoplasm, splitting one glucose molecule into two smaller pyruvate molecules. It's like breaking a six-carbon chain into two three-carbon pieces.
The process starts with phosphorylation, where glucose gets energised by adding two phosphate groups from ATP molecules. Think of it as an investment - you spend 2 ATP to get the process rolling. Next comes oxidation, where those triose phosphate molecules get converted into pyruvate, and NAD picks up hydrogen atoms to become reduced NAD.
Here's the brilliant bit: whilst you invested 2 ATP at the start, you actually produce 4 ATP during oxidation, giving you a net gain of 2 ATP. Plus, you've made 2 reduced NAD molecules that will be crucial later.
Quick Tip: Remember that glycolysis happens in both aerobic AND anaerobic respiration - it's the universal starting point for energy release.
After glycolysis, pyruvate heads into the link reaction in the mitochondrial matrix. Each pyruvate loses a carbon (released as CO₂) and gets converted to acetate, which then combines with coenzyme A to form acetyl CoA. Since you started with 2 pyruvate molecules, this happens twice per glucose molecule.
2
of 2
Krebs Cycle and Oxidative Phosphorylation
The Krebs cycle is where the real energy extraction happens, taking place in the mitochondrial matrix like a perfectly choreographed dance. Your acetyl CoA from the link reaction combines with oxaloacetate to form a 6-carbon citrate molecule, kicking off this cyclical process.
As citrate moves through the cycle, it undergoes decarboxylation (losing CO₂) and dehydrogenation (losing hydrogen atoms). These reactions gradually break down the molecule whilst producing reduced NAD, reduced FAD, and some ATP. The clever part? Oxaloacetate gets regenerated at the end, ready to accept another acetyl CoA.
Oxidative phosphorylation is the grand finale where most of your ATP gets made. All those reduced NAD and reduced FAD molecules from previous stages release their hydrogen atoms, which split into protons and electrons. The electrons travel down the electron transport chain, losing energy that's used to pump protons into the intermembrane space.
Remember: Oxygen is the final electron acceptor - without it, this whole process grinds to a halt!
This creates an electrochemical gradient that drives chemiosmosis. Protons flow back through ATP synthase like water through a turbine, powering the conversion of ADP + Pi into ATP. Meanwhile, protons, electrons, and oxygen combine to form water, completing the cycle that keeps your cells energised.
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Biology102 views·Updated 12 Sept 2026·2 pagesA-Level Biology: Clear Notes on Aerobic Respiration Processes
Isabelle @isabelle_study
Cellular respiration is how your body breaks down glucose to release energy for everything you do - from thinking to moving. This process happens in three main stages that work together like a well-oiled machine to produce ATP, the energy...
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Glycolysis and the Link Reaction
Ever wondered how your body turns that glucose from your breakfast into usable energy? Glycolysis kicks off this process right in your cell's cytoplasm, splitting one glucose molecule into two smaller pyruvate molecules. It's like breaking a six-carbon chain into two three-carbon pieces.
The process starts with phosphorylation, where glucose gets energised by adding two phosphate groups from ATP molecules. Think of it as an investment - you spend 2 ATP to get the process rolling. Next comes oxidation, where those triose phosphate molecules get converted into pyruvate, and NAD picks up hydrogen atoms to become reduced NAD.
Here's the brilliant bit: whilst you invested 2 ATP at the start, you actually produce 4 ATP during oxidation, giving you a net gain of 2 ATP. Plus, you've made 2 reduced NAD molecules that will be crucial later.
Quick Tip: Remember that glycolysis happens in both aerobic AND anaerobic respiration - it's the universal starting point for energy release.
After glycolysis, pyruvate heads into the link reaction in the mitochondrial matrix. Each pyruvate loses a carbon (released as CO₂) and gets converted to acetate, which then combines with coenzyme A to form acetyl CoA. Since you started with 2 pyruvate molecules, this happens twice per glucose molecule.
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Krebs Cycle and Oxidative Phosphorylation
The Krebs cycle is where the real energy extraction happens, taking place in the mitochondrial matrix like a perfectly choreographed dance. Your acetyl CoA from the link reaction combines with oxaloacetate to form a 6-carbon citrate molecule, kicking off this cyclical process.
As citrate moves through the cycle, it undergoes decarboxylation (losing CO₂) and dehydrogenation (losing hydrogen atoms). These reactions gradually break down the molecule whilst producing reduced NAD, reduced FAD, and some ATP. The clever part? Oxaloacetate gets regenerated at the end, ready to accept another acetyl CoA.
Oxidative phosphorylation is the grand finale where most of your ATP gets made. All those reduced NAD and reduced FAD molecules from previous stages release their hydrogen atoms, which split into protons and electrons. The electrons travel down the electron transport chain, losing energy that's used to pump protons into the intermembrane space.
Remember: Oxygen is the final electron acceptor - without it, this whole process grinds to a halt!
This creates an electrochemical gradient that drives chemiosmosis. Protons flow back through ATP synthase like water through a turbine, powering the conversion of ADP + Pi into ATP. Meanwhile, protons, electrons, and oxygen combine to form water, completing the cycle that keeps your cells energised.
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