Biology: Biochemistry and Plant Physiology - Aerobic and Anaerobic Respiration (Glycolysis, Krebs Cycle, Electron Transport, and ATP)

Cellular Respiration is the process by which cells break down glucose ($C_6H_{12}O_6$) to release energy in the form of Adenosine Triphosphate ($ATP$).

Aerobic Respiration occurs in the presence of oxygen ($O_2$) and is highly efficient, producing approximately $36$ to $38$ molecules of $ATP$ per molecule of glucose.

Glycolysis is the first stage of respiration, occurring in the cytoplasm. It breaks down one molecule of glucose ($6$-carbon) into two molecules of pyruvate ($3$-carbon), yielding a net gain of $2\ ATP$ and $2\ NADH$.

The Krebs Cycle (Citric Acid Cycle) takes place in the mitochondrial matrix. It processes pyruvate to produce $CO_2$, $ATP$, and electron carriers ($NADH$ and $FADH_2$).

The Electron Transport Chain (ETC) occurs in the inner mitochondrial membrane (cristae). It uses electrons from $NADH$ and $FADH_2$ to create a proton gradient that powers $ATP\ synthase$ to produce the bulk of the cell's $ATP$.

Anaerobic Respiration (Fermentation) occurs in the absence of $O_2$ in the cytoplasm. It produces only $2\ ATP$ per glucose molecule.

In animals, anaerobic respiration produces Lactic Acid ($C_3H_6O_3$), leading to muscle fatigue: $C_6H_{12}O_6 \rightarrow 2C_3H_6O_3 + 2\ ATP$.

In plants and yeast, anaerobic respiration (Alcoholic Fermentation) produces Ethanol ($C_2H_5OH$) and Carbon Dioxide ($CO_2$): $C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2 + 2\ ATP$.