Cellular energy · Topic 10 of 17

Cellular Respiration

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Biology · cell energy

Cellular Respiration

Glucose and oxygen become CO₂, water and ATP in three stages: glycolysis (cytoplasm), Krebs cycle (matrix), electron transport chain (inner membrane).

01 · The equationGlucose + O₂ → CO₂ + H₂O + ATP

02 · Three stagesGlycolysis → Krebs → ETC

03 · WhereCytoplasm → matrix → inner membrane

What it is

Fermentation recycles NAD⁺ so glycolysis can keep going

Step by step

Glycolysis → Krebs cycle → electron transport chain

Stage 1 · Glycolysis

Step by step

Glycolysis: 1 glucose → 2 pyruvate, in the cytoplasm, no O₂ used

Glycolysis net: 2 ATP + 2 NADH per glucose

Stage 2 · Krebs cycle

Step by step

Pyruvate → acetyl CoA in the mitochondrion (CO₂ released, NADH made)

Stage 3 · Electron transport chain

Step by step

ETC: electrons from NADH/FADH₂ → H⁺ gradient → ATP synthase makes ATP

Oxygen is the final electron acceptor → forms water

Fermentation

Step by step

Lactic acid fermentation: oxygen-starved muscle, red blood cells, yogurt bacteria

Alcohol fermentation (yeast): pyruvate → ethanol + CO₂

Know by heart

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP) must know

Glycolysis: cytoplasm · Krebs: matrix · ETC: inner mitochondrial membrane must know

About 36–38 ATP per glucose (updated count ~30–32); most from the ETC must know

The rules

No oxygen → glycolysis + fermentation: only 2 ATP per glucose

Aerobic respiration or fermentation?

Commonly confused

	Aerobic respiration	Fermentation
Oxygen	Required	Not used
Where	Cytoplasm, then mitochondria	Cytoplasm only
ATP per glucose	About 36–38 (updated count 30–32)	2 (from glycolysis)
End products	CO₂ + water	Lactic acid, or ethanol + CO₂

Misconceptions

Trap: CO₂ comes from glucose's carbons; inhaled O₂ ends up in water

More detail

Show 6 more facts

Krebs, per glucose (2 turns): 2 ATP, 6 NADH, 2 FADH₂, 4 CO₂

Mature red blood cells: no mitochondria → glycolysis only

Chemiosmosis makes ~90% of the ATP in aerobic respiration

About 34% of glucose energy → ATP; the rest is heat

Prokaryotes: ETC in the plasma membrane (no mitochondria)

2 + 2 + 20 + 3 = 27, + 3–5 from glycolysis NADH = 30–32 ATP

Pearl

Split in the cytoplasm, stripped in the matrix, cashed in on the inner membrane, where oxygen catches the spent electrons.

Sources · OpenStax Concepts of Biology §4.1 Energy and Metabolism (respiration equation) · OpenStax Biology 2e §7.2 Glycolysis · OpenStax Biology 2e §7.3 Oxidation of Pyruvate and the Citric Acid Cycle · OpenStax Concepts of Biology §4.3 Citric Acid Cycle and Oxidative Phosphorylation · OpenStax Biology 2e §7.4 Oxidative Phosphorylation (electron transport chain, ATP yield) · OpenStax Biology 2e §7.5 Metabolism without Oxygen (fermentation) · OpenStax Biology 2e §6.1 Energy and Metabolism (one glucose yields 36 to 38 ATP) · LibreTexts Chemistry — Energy yield by complete oxidation of glucose (theoretical maximum 30–32 ATP) · OpenStax Concepts of Biology §4.4 Fermentation · Union Test Prep — HESI A2 Biology study guide, p. 1: cells, organelles, membrane, respiration, photosynthesis · Odessa College HESI A2 library guide — Biology content list (Metabolism, Cells, Cellular Respiration, Photosynthesis) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

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