Cell respiration transfers energy from organic compounds into ATP through glycolysis, anaerobic pathways and aerobic mitochondrial stages, supporting usable cellular work and measurable biological activity.
Cell respiration is the enzyme-controlled release of energy from organic compounds to produce ATP. Stepwise oxidation transfers energy in small amounts instead of releasing it suddenly as heat and light.
| In humans | Aerobic route | Anaerobic route |
|---|---|---|
| oxygen | required for continued electron transport | not required |
| location | glycolysis in cytoplasm, then mitochondria | cytoplasm only |
| glucose oxidation | complete | incomplete |
| products | CO₂ and H₂O | lactate |
| ATP yield | high | net 2 ATP per glucose |
The organism consumes O₂ and releases CO₂. Soda lime absorbs the CO₂, so total gas volume falls by the volume of O₂ taken up; reduced pressure moves the capillary fluid toward the organisms.
Use a control tube with an equal volume of inert beads, keep temperature constant in a water bath, allow equilibration before sealing, repeat measurements, and prevent direct contact between organisms and soda lime.
Two ATP phosphorylate and activate one glucose, producing an unstable six-carbon intermediate.
The six-carbon compound splits into two three-carbon triose phosphates.
Both triose phosphates are oxidized; NAD⁺ is reduced and inorganic phosphate is added.
Substrate-level phosphorylation forms four ATP as the two three-carbon molecules become two pyruvate. Four made minus two invested gives a net gain of two ATP per glucose.
Without oxygen, reduced NAD cannot be reoxidized through the electron transport chain. Transferring its hydrogen to a pyruvate-derived acceptor regenerates NAD⁺, which glycolysis needs for its oxidation step.
| Route | Hydrogen acceptor and products | Application or consequence |
|---|---|---|
| human lactate fermentation | pyruvate accepts hydrogen → lactate; no CO₂ | permits brief ATP production when oxygen delivery cannot meet demand |
| yeast alcoholic fermentation | pyruvate loses CO₂ to ethanal; ethanal accepts hydrogen → ethanol | CO₂ raises bread; ethanol and CO₂ are used in brewing |
Fermentation adds no ATP beyond glycolysis. Lactate and ethanol remain energy-rich because glucose has been incompletely oxidized.
Each 3C pyruvate enters the mitochondrial matrix and is decarboxylated, releasing one CO₂. The remaining 2C fragment is oxidized, reducing NAD⁺.
The 2C acetyl group attaches to coenzyme A to form acetyl-CoA. CoA carries the acetyl group into the Krebs cycle; the link reaction produces no ATP directly.
One glucose produces two pyruvate, so the link reaction happens twice per glucose: 2 acetyl-CoA + 2 CO₂ + 2 reduced NAD.
A 2C acetyl group combines with 4C oxaloacetate to form 6C citrate. Coenzyme A is released for reuse.
During one turn, citrate is rearranged and oxidized: 2 CO₂ are released, 3 NAD and 1 FAD are reduced, and 1 ATP is formed by substrate-level phosphorylation.
The remaining four-carbon skeleton is rebuilt as oxaloacetate, allowing another acetyl group to enter. Because two acetyl-CoA form per glucose, the cycle turns twice per glucose.
Reduced NAD and FAD donate electrons to carriers in the inner mitochondrial membrane and are reoxidized. Their electrons pass through a sequence of redox carriers, releasing energy in controlled steps.
Oxygen is the terminal electron acceptor. By removing electrons and matrix protons as water, it permits continued electron flow and continued reoxidation of NAD and FAD.
H⁺ flows down its electrochemical gradient from the intermembrane space to the matrix through ATP synthase. Proton movement drives conformational and rotational changes in the enzyme.
This coupling is oxidative phosphorylation: oxidation of reduced carriers powers the gradient, chemiosmosis powers ATP synthase, and oxygen removal of electrons and protons keeps the system from backing up.
| Property | Carbohydrate | Lipid |
|---|---|---|
| oxidation state | already contains more oxygen; less reduced | rich in C–H bonds; more reduced |
| energy and water per gram | lower | more than twice the energy and more metabolic water |
| oxygen demand | lower per unit energy pathway entry | higher because more electrons and hydrogen must reach oxygen |
| access | glucose enters glycolysis rapidly | fatty acids require mobilization and β-oxidation to acetyl-CoA |
| storage | glycogen is hydrated and affects mass | triglyceride is compact and osmotically inert |
Carbohydrate suits rapid, high-intensity ATP demand and can support anaerobic glycolysis. Lipid suits sustained aerobic demand and long-term storage. The best fuel depends on oxygen supply, intensity and duration—not one universal ranking.