Cell Respiration

ATP

The cell's main energy carrier; energy is released when the terminal phosphate bond is broken (ATP → ADP + Pi).

Glycolysis

Occurs in the cytoplasm; glucose is split into 2 pyruvate, producing 2 ATP (net) and 2 NADH. (Investment/Payoff)

Alcoholic fermentation

Anaerobic process converting pyruvate to ethanol and CO₂, regenerating NAD⁺. Example: yeast.

Lactic acid fermentation

Anaerobic process converting pyruvate to lactic acid, regenerating NAD⁺. Example: human muscle cells, lactic acid bacteria.

Citric Acid Cycle / Krebs Cycle

Occurs in the mitochondrial matrix; acetyl-CoA is oxidized to CO₂, producing NADH, FADH₂, and ATP.

Electron Transport Chain

Series of protein complexes in the inner mitochondrial membrane that transfer electrons from NADH and FADH₂ to oxygen, pumping H⁺ ions.

Oxidative phosphorylation

ATP production driven by the proton gradient; occurs on the inner mitochondrial membrane (cristae).

Reduction

Gain of electrons (often gains hydrogen).

Oxidation

Loss of electrons (often loses hydrogen).

Redox reaction

A paired reaction in which oxidation (loss of electrons) and reduction (gain of electrons) occur simultaneously.

Pyruvate

A 3-carbon molecule produced by glycolysis that is converted into acetyl-CoA before entering the Krebs cycle.

Substrate-level phosphorylation

ATP made by direct transfer of a phosphate group from a substrate to ADP, without the ETC. Examples: glycolysis and Krebs cycle.

Phosphofructokinase (PFK)

The rate-limiting enzyme of glycolysis; inhibited by high ATP and activated by ADP/AMP.

Cristae membrane

Folded inner mitochondrial membrane that increases surface area for the ETC and ATP synthase.

Outer compartment / Intermembrane space

Space between inner and outer membranes where H⁺ ions accumulate during the ETC.

Matrix

Innermost mitochondrial compartment where the Krebs cycle and pyruvate oxidation occur.

Acetyl-CoA

A 2-carbon molecule that delivers carbon from pyruvate into the Krebs cycle.

NADH and NAD⁺

Electron carriers; NAD⁺ accepts electrons, NADH transports high-energy electrons to the ETC.

FADH₂ and FAD⁺

Electron carriers; FADH₂ donates electrons to the ETC, producing less ATP than NADH.

Chemiosmotic theory

ATP is generated as H⁺ ions flow down their electrochemical gradient through ATP synthase.

ATP synthase channels

Protein channel that allows H⁺ ions to flow back into the matrix, using that energy to form ATP from ADP + Pi.

Anaerobic respiration (fermentation)

ATP production without oxygen, using glycolysis and fermentation to regenerate NAD⁺.

Facultative anaerobe

Can survive with or without oxygen. Found in an environment with oxygen. Example: yeast, E. coli.

Obligate anaerobe

Cannot survive in oxygen. Found in an environment without oxygen. Example: Clostridium.