Chemiosmotic Hypothesis


In 1961, British Biochemist Peter Dennis Mitchell coined the term chemiosmotic hypothesis. This is a biological process that explains the ATP synthesis by photosynthesis in the chloroplast. In this process, ATP and NADP both are generated by light reactions and used as a key component of dark reactions to produce the final product i.e., glucose by photosynthesis.

According to the hypothesis, ATP develops as a result of the proton gradient created in the thylakoid membrane. Proton pumps and ATP synthase are also important. The enzyme ATP synthase has two subunits- F0 and F1. When F1 activates the enzyme, it changes its configuration and the F0 acts as a transmembrane channel. The complex phosphorylates the ADP into ATP.

What is Photosynthesis?

Photosynthesis is a physiological and photochemical process used by various groups of plants, algae, and cyanobacteria by absorbing light energy, and carbon dioxide and converting them to chemical energy producing glucose, oxygen, and water as final products.

$$\mathrm{6CO_{2}+6H_{2}O\rightarrow C_{6}H_{12}O_{6}+6O_{2}}$$

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Features of Photosynthesis

  • It takes place in the chloroplast of plants and produces food and energy to survive.

  • They are the autotrophs and the ideal primary producers of our ecosystem.

  • The mesophyll tissue of the leaves that contains chloroplast having chlorophyll-containing disc-shaped thylakoid configurations.

  • Uses sunlight, water, and carbon dioxide as raw materials.

  • Produces oxygen and glucose by converting the light energy to chemical energy.

  • Stores energy as ATP during light reaction.

  • Runs Calvin cycle in stroma using the end products i.e., glucose.

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What Happens During Photosynthesis?

  • Plants have the pigments like carotene, chlorophyll a, chlorophyll b, xanthophylls, etc., which lead to photosynthesis in the thylakoid lumen of the chloroplast.

  • Autotrophic organisms are able to photosynthesize by using carbon dioxide, water, and sunlight and produce glucose and oxygen by the photochemical reaction.

  • These products are further used by the plant or by other autotrophic organism to produce the necessary metabolites, enzymes, hormones, and other factors needed for their growth.

  • They can be stored in the form of carbohydrates or starch and reconfigured into glucose when needed.

  • This reconstructed glucose can be used in cellular respiration and releases the stored energy.

Chemiosmotic Hypothesis: The Process

This theory possesses that glucose metabolizes energy-rich factor acetyl CoA as a byproduct. The mitochondrial matrix oxidized the acetyl CoA and was associated with the reduced version of NAD and FAD. These carrier molecules transport the electron to the electron transport system present in the inner mitochondrial membrane. Then the proteins of the electron transport chain are supplied. The electron energies to pump out the proton from the inner mitochondrial membrane matrix. It stores energy as a transmembrane electrochemical gradient. The ATP synthase helps to return the proton to the inner membrane and bear the proton flow across the matrix. They accumulate the energy to phosphorylate or integrate the ADP and inorganic phosphate to develop ATP.

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  • The main components are proton gradient, proton pump, ATP synthase, and carrier molecules like FAD and NAD.

  • The subunits (F0 and F1) of the enzyme ATP synthase involve proton transfer all over the membrane modifying the F1 pattern to activate the enzyme respectively.

  • The enzyme phosphorylates ADP to develop ATP.

  • In autotrophic organisms, mainly in plants, chlorophyll absorbs sunlight with the help of photosystems during light reactions.

  • The electrons and the protons formed by the hydrolysis get excited and jump to a higher energy level.

  • The electron transport system transports the electrons whereas the hydrogen ions that are released from the stroma get accumulated inside the membrane and develop a proton gradient by the electron transport chain.

  • Photosystem I uses some protons for the reduction of NADP+ to NADPH by the electrons gained from the photolysis of the water.

  • After the collapse, the proton gradient emits energy, heat, and protons.

  • The F0 carries the proton back to the stroma via the transmembrane channel.

  • The emitted energy changes the F1 configuration and targets the ATP synthase.

  • Hence, the enzyme converts the ADP to ATP.


Plants and other autotrophic organisms that bear chlorophyll are able to photosynthesize. They absorb sunlight, water, and carbon dioxide and convert them to glucose and oxygen by some photochemical reactions associated with the photosystems, pigments, and other complexes. In this procedure, the water splits to develop H+ and OH-. The electrons get excited and go to higher energy levels and protons integrate to form a proton gradient in the mitochondrial membrane by ETC. Some protons manipulate the NADP+ to NADPH. The proton gradient then falls and releases heat, energy, and protons that are carried back to the stroma by the F0 transmembrane channel and transform the F1 configuration by stimulating the ATP synthase and switching to ATP by ADP phosphorylation.


Q1. What is a light-harvesting complex?

Ans. Light-harvesting complex also known as antenna complex is a collection of membrane-associated proteins and photosensitive pigments i.e., chlorophyll a, b, carotenoids, etc., implanted on the thylakoid membrane of the chloroplast. It transfers light energy to the pigment molecule at the reaction center of the photosystem.

Q2. State the wavelength absorbing capacity of the photosystems.

Ans. The photosystems are of two types- photosystem I and II. Photosystem I is found at the outer membrane of the thylakoid of the grana and stroma lamellae whereas photosystem II locates in the inner grana membrane. The PS-I absorbs >680, probably 700 nm whilst the PS-II absorbs ≤ 680nm.

Q3. Define reaction center.

Ans. It is a complex integration of several pigments, proteins, and co-factors to execute the reaction of primary energy conversion by photosynthesis.

Q4. How antenna complex plays a vital role in noncyclic phosphorylation?

Ans. In the light-dependent reaction of noncyclic phosphorylation, the pigment molecule of the photosystem II antenna complex absorbs the photon. The primary electron acceptor of the photosystem II electron transport chain picks up the energized electron from the reaction center.

Q5. How much ATP is generated by the NADH in the electron transport chain? State the process.


  • NADH passes the electron to pump the proton in the complex.

  • It passes the electron to complex I and pumps 4 protons.

  • Then to complex III to pump 4 protons again.

  • Lastly it passes the electron to complex IV and pumps 2 protons. A total of 10 protons in a cycle are transported to intermembrane space from the matrix.

  • 4 protons can stimulate the ATP synthase to convert ADP by phosphorylation and produce 1 ATP.

So, for NADH= 10/4 = 2.5 ATP/ NADH produced.

Updated on: 02-Jan-2023


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