Cellular Respiration

Cellular respiration is how cells extract energy from glucose. The process unfolds in three linked stages — glycolysis in the cytoplasm, the Krebs cycle in the mitochondrial matrix, and oxidative phosphorylation on the inner mitochondrial membrane — and yields roughly 30 to 32 ATP per glucose molecule. This note walks through each stage with stoichiometry, the electron transport chain, the difference between aerobic and anaerobic respiration, and how the whole sequence mirrors photosynthesis in reverse.

The Net Equation of Cellular Respiration

The summary fits in one line and is the exact reverse of photosynthesis:

$$ C_6H_{12}O_6 + 6\,O_2 \;\longrightarrow\; 6\,CO_2 + 6\,H_2O + \text{ATP} $$

Glucose plus oxygen yields carbon dioxide, water, and energy in the form of ATP. The energy is not released as heat in a single explosion; it is harvested step by step through controlled enzyme reactions so the cell can capture as much of it as possible in usable form.

Stage 1: Glycolysis

Glycolysis happens in the cytoplasm, not the mitochondrion. One glucose molecule (6 carbons) is broken into two molecules of pyruvate (3 carbons each). The cell invests 2 ATP to get the reaction started and then produces 4 ATP plus 2 NADH along the way. Net yield: 2 ATP and 2 NADH per glucose.

Glycolysis is anaerobic — it does not require oxygen. This matters because cells that lack mitochondria (red blood cells) or are temporarily oxygen-starved (muscle during a sprint) can still produce ATP this way, just less efficiently.

Stage 2: The Krebs Cycle

Each pyruvate enters the mitochondrial matrix and is converted to acetyl-CoA, releasing one CO₂ and producing one NADH. Acetyl-CoA then enters the Krebs cycle (also called the citric acid cycle or TCA cycle), where it joins oxaloacetate and runs through eight enzymatic steps that ultimately regenerate oxaloacetate while releasing more CO₂.

Per glucose (two pyruvates), the Krebs cycle produces 6 NADH, 2 FADH₂, 2 GTP/ATP, and releases 6 CO₂. Every breath of carbon dioxide you exhale came from the Krebs cycle running in your mitochondria right now.

Stage 3: Oxidative Phosphorylation

This is where the bulk of the ATP comes from. NADH and FADH₂ from stages 1 and 2 deliver electrons to the electron transport chain on the inner mitochondrial membrane. Electrons hop through four complexes (I, II, III, IV). At each hop, complexes I, III, and IV pump protons from the matrix into the intermembrane space, building a proton gradient.

The proton gradient drives ATP synthase (Complex V), which spins like a rotary motor and produces ATP. Oxygen is the final electron acceptor at Complex IV — without oxygen, the chain backs up, NADH and FADH₂ cannot deliver their electrons, and the whole stage stalls. That is why aerobic respiration requires oxygen.

Per glucose, oxidative phosphorylation produces roughly 26-28 ATP, depending on how efficiently the proton gradient is converted.

Total ATP Yield

Stage	Location	ATP	NADH	FADH₂	CO₂
Glycolysis	Cytoplasm	2 net	2	0	0
Pyruvate oxidation	Mitochondrial matrix	0	2	0	2
Krebs cycle	Mitochondrial matrix	2 (as GTP)	6	2	4
Oxidative phosphorylation	Inner membrane	~26-28	(uses NADH)	(uses FADH₂)	0
TOTAL per glucose	—	~30-32	10	2	6

Aerobic vs Anaerobic Respiration

When oxygen is unavailable, glycolysis continues but the Krebs cycle and oxidative phosphorylation cannot. The cell must regenerate NAD⁺ somehow so glycolysis can keep running.

• Lactic acid fermentation (animal muscle cells, some bacteria). Pyruvate is reduced to lactate, regenerating NAD⁺. This is why your muscles burn during intense sprinting — lactate accumulates faster than oxygen can clear it.
• Alcoholic fermentation (yeast, some bacteria). Pyruvate is converted to ethanol and CO₂. This is the entire basis of bread, beer, and wine.

Both anaerobic pathways yield only 2 ATP per glucose (the glycolysis net), versus 30-32 with oxygen. The 15x efficiency gap is why large active animals depend on aerobic respiration.

Cellular Respiration vs Photosynthesis

The two processes are mirror images. Photosynthesis builds glucose from CO₂ and water using light energy; cellular respiration breaks glucose back down using oxygen and stores the energy as ATP. Plants run both — photosynthesis in chloroplasts to make food, respiration in mitochondria to use it.

Related study notes: Photosynthesis, Mitochondria, Enzyme, Protein.

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