Cell Respiration

ATP (adenosine triphosphate) = adenine + ribose + 3 phosphates. Hydrolysis of the terminal phosphate releases ~30 kJ/mol:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}.$

Used to drive active transport, muscle contraction, synthesis reactions, etc. ATP is constantly recycled — typical cells regenerate their entire ATP pool in seconds.

Aerobic respiration — complete oxidation of glucose using oxygen:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \sim 32\text{ ATP}.$

Four stages:

Stage	Location	Inputs → outputs
1. Glycolysis	Cytoplasm	Glucose → 2 pyruvate + 2 ATP + 2 NADH
2. Link reaction	Mitochondrial matrix	2 pyruvate → 2 acetyl CoA + 2 CO₂ + 2 NADH
3. Krebs cycle	Mitochondrial matrix	2 acetyl CoA → 4 CO₂ + 6 NADH + 2 FADH₂ + 2 ATP
4. Oxidative phosphorylation	Inner mitochondrial membrane	NADH + FADH₂ + O₂ → H₂O + ~26 ATP

Total ATP ≈ 2 (glycolysis) + 2 (Krebs) + 26 (oxidative) = ~32 ATP per glucose.

Oxygen is the final electron acceptor in the electron transport chain — forming water. Without O₂, the chain backs up and aerobic respiration halts.

Anaerobic respiration — partial breakdown of glucose without oxygen. Only glycolysis proceeds; the products of glycolysis must be processed differently to regenerate NAD⁺.

In animals (and some bacteria): $\text{glucose} \rightarrow 2\text{ lactate} + 2\text{ ATP}.$ Lactate (lactic acid) builds up in muscles during intense exercise, contributing to fatigue. Liver converts lactate back to glucose later (Cori cycle) when oxygen is available.

In yeast and some plants (alcoholic fermentation): $\text{glucose} \rightarrow 2\text{ ethanol} + 2\text{ CO}_2 + 2\text{ ATP}.$ Used in brewing (ethanol) and bread-making (CO₂ raises dough).

Comparison:

Feature	Aerobic	Anaerobic (animal)	Anaerobic (yeast)
O₂ needed	Yes	No	No
ATP per glucose	~32	2	2
Products	CO₂ + H₂O	Lactate	Ethanol + CO₂
Location	Cytoplasm + mito	Cytoplasm	Cytoplasm

Why so much less ATP? Anaerobic respiration only uses glycolysis; the Krebs cycle and oxidative phosphorylation (which produce most ATP) require oxygen.