Cellular Energy: ATP, Respiration, and Fermentation Key Concepts

ATP

The main energy molecule of the cell; energy is stored in the phosphate bonds, especially the last one.

ADP

Formed when ATP loses a phosphate; can be regenerated back into ATP.

ATP hydrolysis

ATP → ADP + Pi releasing energy (exergonic); used to power cellular work.

Energy coupling

Using energy from an exergonic reaction (ATP hydrolysis) to power an endergonic reaction.

Phosphorylated intermediate

A molecule that receives a phosphate from ATP and becomes more reactive.

Oxidation

Loss of electrons or hydrogen.

Reduction

Gain of electrons or hydrogen.

NAD+

Electron carrier that becomes NADH when reduced; carries electrons to the ETC.

FAD

Electron carrier that becomes FADH2 when reduced; carries electrons to the ETC.

Aerobic respiration

Cellular respiration that requires oxygen; produces the most ATP.

Anaerobic respiration

Respiration using an electron acceptor other than oxygen; still uses an ETC.

Fermentation

Allows glycolysis to continue without oxygen by regenerating NAD+; produces only 2 ATP.

Alcoholic fermentation

Pyruvate is converted to ethanol and CO2; regenerates NAD+.

Lactic acid fermentation

Pyruvate is converted to lactate; regenerates NAD+.

Obligate anaerobes

Organisms that cannot live in oxygen.

Facultative anaerobes

Organisms that can switch between aerobic respiration and fermentation.

Glycolysis

Occurs in the cytosol; breaks glucose into 2 pyruvate; produces net 2 ATP and 2 NADH.

Substrate-level phosphorylation

Direct formation of ATP from ADP by an enzyme (in glycolysis and Krebs cycle).

Pyruvate

End product of glycolysis; transported into the mitochondrion.

Pyruvate oxidation

Pyruvate → acetyl-CoA + CO2; produces 2 NADH per glucose.

Acetyl-CoA

2-carbon molecule that enters the Krebs cycle.

Citric acid cycle (Krebs cycle)

Occurs in the mitochondrial matrix; completes oxidation of acetyl-CoA; produces 6 NADH, 2 FADH2, 2 ATP, and 4 CO2 per glucose.

Mitochondrion

Organelle where aerobic respiration occurs.

Cristae

Folded inner mitochondrial membrane where the ETC and ATP synthase are located.

Electron transport chain (ETC)

Uses electrons from NADH and FADH2 to pump H+ into the intermembrane space; oxygen is the final electron acceptor.

Oxidative phosphorylation

ATP production using the ETC and chemiosmosis; makes the most ATP.

Chemiosmosis

Flow of H+ through ATP synthase that drives ATP production.

ATP synthase

Protein in the inner membrane; acts as an H+ channel and enzyme that makes ATP from ADP and Pi.

Proton-motive force

Potential energy stored in the H+ gradient across the membrane.

Glycolysis ATP yield

Net 2 ATP.

Glycolysis NADH yield

2 NADH.

Pyruvate oxidation NADH yield

2 NADH per glucose.

Krebs cycle ATP yield

2 ATP per glucose.

Krebs cycle NADH yield

6 NADH per glucose.

Krebs cycle FADH2 yield

2 FADH2 per glucose.

Total ATP from NADH

Each NADH produces about 2.5 ATP.

Total ATP from FADH2

Each FADH2 produces about 1.5 ATP.

Total ATP per glucose

Approximately 30-32 ATP.

Effect of no oxygen

ETC stops; H+ gradient collapses; ATP synthase stops; Krebs cycle stops; fermentation begins.

Purpose of fermentation

Regenerate NAD+ so glycolysis can continue.