Question

Explain the chemiosmotic hypothesis for ATP synthesis.

Answer 1

Chemiosmotic hypothesis (Oxidative Phosphorylation):

It is the synthesis of energy-rich ATP from ADP and inorganic phosphate, that is connected to oxidation of reduced coenzymes produced in cellular respiration. ATP synthesis is explained by the chemiosmotic (coupling) theory given by British biochemist Peter Mitchell in 1961 (Nobel Prize in 1978). It is applicable to ATP synthesis both in respiration and photosynthesis. According to chemiosmotic theory, the energy liberated during electron transport performs the osmotic work of accumulating H⁺ ions, conserving energy in building a proton gradient or proton motive force (PMF) which is used to build ATP from ADP and inorganic phosphate.

(i) Development of Proton Gradient or Proton Motive Force: Oxidation of reduced coenzymes releases both electrons and protons. Electrons pass over an electron transport chain releasing energy at every step. The energy is employed in pushing not only the protons of their coenzymes but also other protons from the mitochondrial matrix to the intermembrane space or outer mitochondrial chamber. Oxidation of one NADH (+H⁺) is linked to pushing of three pairs of protons into the outer chamber while oxidation of FADH₂ is linked to sending out two pairs of protons. As a result, proton concentration increases in the outer chamber in the figure. The difference in the proton concentration on the two sides of a system is called the proton gradient. the pH of the outer chamber decreases as compared to the mitochondrial matrix.

Proton gradient in the mitochondrion.

An electrochemical potential difference occurs across the inner mitochondrial membrane that tends to push protons back towards the matrix. It is called a proton motive force (PMF).

(ii) Proton Driven ATP Synthesis: Proton gradient or proton motive force tends to push the protons from the outer mitochondrial chamber towards the mitochondrial matrix through the inner membrane. However, the inner membrane is permeable to protons only in the region of Fo or bases of elementary particles (FQ – F i particles). They function as proton tunnels or channels while F i headpiece has enzyme ATP-ase that catalyses ATP synthesis. Two protons are required for the synthesis of one molecule of ATP. They knock out one oxygen of inorganic phosphate and convert the latter into active phosphate (energy-rich phosphate).

The latter immediately combines with ADP to form ATP or ADP-P. Since NADH (+H⁺) drives out 3 pairs of protons to the outer chamber, its oxidation is linked to the synthesis of 3 ATP molecules. Similarly, oxidation of FADH₂ that causes pumping of two pairs of protons forms 2 ATP molecules. ATP molecules synthesised inside mitochondria come out of the latter through facilitated diffusion across the inner membrane and diffusion across the outer membrane.