Question

Why have we been discussing respiration at all – neither $O_2$ has come into the picture nor the promised large number of ATP has yet been synthesised till Kreb's cycle. Also what is the role of the $NADH$ + $H^{+}$ and $FAD{H}_2$ that is synthesised in Kreb's cycle?

Answer 1

Part 1: Introduction
All the energy required to perform varied life processes comes from respiratory breakdown of certain biomolecules, which are obtained in the process of nutrition. Respiration can occur both in the presence or in the absence of oxygen.

Part 2: Definition of aerobic respiration
Aerobic respiration is the process of complete oxidation of organic substances to release $C{O}_2$, water and a large amount of energy present in the substrate. This reaction occurs in the cytoplasm and mitochondria of the cell and $C{O}_2$, $H_{2}O$ and 38 molecules of ATP are produced.

Part 3: Steps of aerobic respiration
The complete process of aerobic respiration occurs in multiple stages: $\begin{array}{cccc}\text{S.no}& \text{Stages}& \text{Location}& \text{Net gain}\\ \text{1}& \text{Glycolysis}& \text{Cytoplasm}& \text{Two ATP and two NADH}\\ \text{2}& \text{Link reaction}& \text{Mitochondrial matrix}& \text{One NADH and Acetyl-CoA.}\\ \text{3}& \text{Citric Acid Cycle}& \text{Mitochondrial matrix}& \text{2 ATP and 6 NADH}\\ \text{4}& \text{Electron Transport Chain}& \text{Inner mitochondrial membrane}& \text{38 ATP}\end{array}$

Part 4:Electron transport chain
NADH and FADH2 formed in the various steps of aerobic respiration are redeemed to form ATP molecules in the electron transport chain. One $NADH$ and $FAD{H}_2$ give rise to three and two ATP molecules respectively.