Respiration in Plants: Krebs'Cycle or the citric acid cycle and The Respiratory Chain or Electron Transport Chain or Oxidative Phosphorylation

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In eukaryotes, the citric acid cycle takes place in the matrix of the mitochondria, just like the conversion of pyruvate to acetyl CoA. Details of this cycle were worked out by Sir Hans Krebs in the 1930s. It is also known as tricarboxylic acid cycle or TCA cycle.

Image showing Kreb’s Cycle.

Image Showing Kreb’S Cycle

Summary of this phase in respiration is

  • H-carrier NAD and FAD are derived from vitamin B complex and are known as coenzymes.

  • In the first step of the cycle, acetyl CoA combines with a four carbon acceptor molecule, oxaloacetate, to form a six carbon molecule called citrate. After a quick rearrangement, this six carbon molecule releases two of its carbons as carbon dioxide molecules in a pair of similar reactions, producing a molecule of NADH each time. The enzymes that catalyze these reactions are key regulators of the citric acid cycle, speeding it up or slowing it down based on the cell’s energy needs. The remaining four carbon molecule undergoes a series of additional reactions, first making an ATP molecule—then reducing the electron carrier FAD to FADH2 and finally generating another NADH. This set of reactions regenerates the starting molecule, oxaloacetate, so the cycle can repeat.

  • Overall, one turn of the citric acid cycle releases two carbon dioxide molecules and produces three NADH, one FADH2 and one ATP. The citric acid cycle goes around twice for each molecule of glucose that enters cellular respiration because there are two pyruvates—and thus, two acetyl CoA made per glucose. Thus, at the end of the Citric Acid Cycle, there are a total of 10NADH and 2FADH2 (2NADH from glycolysis). These hydrogen carriers enter the next phase known as the respiratory chain or

  • Electron-Transport-Chain (E.T.C.) for further release of energy.

The Respiratory Chain or Electron Transport Chain (E.T.C.) or Oxidative Phosphorylation

  • The hydrogen carriers now move to the inner membrane of the mitochondrion, the inner membrane is highly impermeable to ions and small molecules and contains many folds, called cristae, to increase its surface area. As electrons move down the chain, energy is released and used to pump protons out of the matrix, forming a gradient. Protons flow back into the matrix through an enzyme called ATP synthase, making ATP. This is called oxidative phosphorylation. Respiratory chain is a series of mitochondrial proteins that transport electrons of hydrogen, released in the Krebs cycle, from acetyl coenzyme A to inhaled oxygen to form H2O: the energy released in the process is conserved as ATP.

Image showing Respiratory Chain (Oxidative Phosphorylation).

Image Showing Respiratory Chain (Oxidative Phosphorylation)

  • During ETC step, the hydrogen ions (or a pair of electrons) are transported from one carrier to another and they are finally used to reduce oxygen to water. During this transfer of electrons, lot of energy is released which is in the form of ATP. ATP is thus an energy rich molecule and can be called the energy currency of the cell.

  • For each NADH2 that enters the respiratory chain, 3 ATP can be made but for each FADH2, only 2 ATP can be made.

Thus the overall equation for respiration is

Overall Budget for Aerobic Respiration of One Glucose Molecule

Table Showing Aerobic Respiration of One Glucose Molecule
Table showing aerobic respiration of one glucose molecule.










Pyruvate-> Acetyl CoA





Krebs cycle








10 NADH + H+ 10 3 =30 ATP

2FADH2 2 2 = 4 ATP

  • Two turns of the Krebs’ Cycle take place per glucose molecule as at the end of glycolysis two pyruvic acid molecules are formed each of which separately enters the Krebs’ Cycle.

  • Both glycolysis and TCA cycle produce 2 molecules of ATP per molecule of glucose (total 4 ATP). 10 NADH produced generate 30 ATP molecules (10 3) and FADH2 produced generate 4 ATP molecules. Thus a total of 38 ATP molecules are produced per glucose molecule oxidized. In most eukaryotic cells the net gain of ATP is 36 molecules since 2 molecules of ATP are required for transporting the NADH produced in glycolysis into the mitochondria for further oxidation. However, since bacteria do not have mitochondria; the number of ATP molecules produced per glucose molecule oxidized by prokaryotes, should be considered as 38.

Image showing Summary of Aerobic respiration.

Image Showing Summary of Aerobic Respiration

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