Which term describes ATP production resulting from the capture of light energy by chlorophyll?

A. Oxidative phosphorylation

B. Photophosphorylation

C. Dephosphorylation

D. Substrate-level phosphorylation.

The correct answer is B: Photophosphorylation.

The process by which light energy from the Sun is used to convert ADP into ATP is called photophosphorylation. It is the light dependent reaction of photosynthesis which occurs in the chloroplasts in the presence of chlorophyll.

On the other hand, oxidative phosphorylation is the process by which energy is produced in the form of ATP by the transfer of electrons from NADH or FADH2 to oxygen with the help of electron carriers. This process takes place in the mitochondria, is not light dependent and occurs in both plants and animals.

Dephosphorylation is the mechanism of by which the phosphate groups are removed from a molecule in the presence of enzyme called phosphatase. It is mostly carried on a protein molecule  to cause its deactivation.

It is the opposite of the phosphorylation reaction because in phosphorylation a phosphate group is added and in dephosphorylation a phosphate group is removed.

Lastly, substrate-level phosphorylation is the process in which a substrate is required for the formation of ATP. In this case, a phosphorylated compound transfers its phosphoryl group directly to ADP to result in the formation of ATP in the presence of enzymes.

Therefore, the process by which ATP is produced from the light energy captured by chlorophyl is called photophosphorylation.

What is Photophosphorylation?

Photophosphorylation is the light dependent process of formation of ATP from ADP. The light from the Sun is used to create a high-energy electron donor and a low-energy electron acceptor.

When the electrons move from the donor to the acceptor in an electron transport chain, energy is formed in the form of ATP.

It is different from oxidative phosphorylation and substrate level phosphorylation, wherein formation of energy is dependent on the oxidative process or on a substrate to supply the phosphoryl group.

ADP + Pi ———> ATP

Types of Photophosphorylation

Photophosphorylation is of two types. It can either be cyclic or non cyclic.

In the cyclic process, only the conversion of ADP to ATP takes place and immediate energy needs are supplied, whereas in non cyclic photophosphorylation, the conversion of ADP to ATP is accompanied by the splitting of water molecule to provide electron that helps in the formation of NADPH.

Cyclic Photophosphorylation

Cyclic Photophosphorylation is the process that occurs in prokaryotes and the unicellular organisms as they only require the formation of energy as ATP and do not require the formation of biological molecules like glucose. In this form of photophosphorylation, only the conversion of ADP to ATP takes place.

Photosystems play a key role in the light reactions of photosynthesis. A photosystem is formed by the grouping of pigments and proteins, of which the pigments absorb photons or bundles of light energy.

There are two photosystems embedded in the thylakoid membranes and they are designated as PS II and PS I. PS II has maximum absorption wavelength of 680 nm and is also known as PS 680 and PS I is also known as PS 700.

In cyclic photophosphorylation, only one photosystem, PS I, is used. The light is absorbed by chlorophyll and falls on the Photosystem I and excites an electron.

The excited electron enters into an electron transport chain and results in the formation of ATP. After getting de-energised, the electron comes back to the PS I, restoring its supply of electrons. It is because of this nature that the process is called cyclic.

The excited electron creates the proton gradient across the thylakoid membrane which is used by ATP synthase to form ATP.

Since the de-energised electron returns to the photosystem, NAD+ is not reduced to NADH and water is not required to provide continuous supply of electrons.

As a result, only ATP is formed from ADP, water is not required and light energy gets converted into chemical energy.

Non Cyclic Photophosphorylation

In the process of non cyclic photophosphorylation, ATP is formed from ADP, but at the same time water is required to provide a continuous source of electrons and NADH is also formed due to reduction of NAD+.

This system is more predominant in the green plants that perform photosynthesis in the presence of water and result in the formation of oxygen as a byproduct.

In non cyclic photophosphorylation, both the photosystems I and II are used. When the light molecule or photon falls on the Photosystem II, the electrons get excited and enter into an electron transport chain to form ATP. The proton gradient is used for the generation of ATP, similar to cyclic photophosphorylation.

However, another packet of photon is taken up by the Photosystem I and activates it to release an electron. This electron is used to reduce NAD+ to NADH. So, in non cyclic photophosphorylation, both ATP and NADH are formed.

Since the electrons are not returned to the photosystem, it becomes a non cyclic process. The continuous supply of electrons is maintained by the presence of water in the reaction.

Water molecules splits to give continuous supply of electrons to the photosystem II and the electron in photosystem I is replenished by electron released from photosystem II. Oxygen is released as a byproduct of the splitting of water molecule and the flow of electrons is unidirectional.

As a bottom line, for cyclic photophosphorylation only PS I is involved, water is not required, oxygen is not formed, NADH is not synthesised and it is used to form ATP for the cells to meet their energy requirements.

On the other hand, non-cyclic photophosphorylation involves both the use of PS I and PS II, water is required to maintain a steady flow of electrons, oxygen is released as a byproduct, NADH is formed and the products of the reaction are used for the light independent reactions of photosynthesis.

Thus, we can conclude that the process which involves the capture of light energy by chlorophyll for the production of ATP is called Photophosphorylation.

References

  1. M. Wink, Photophosphorylation: Encyclopaedia of Food Sciences and Nutrition (2003).
  2. What is the difference between Photophosphorylation and Dephosphorylation: Albert (September 2016).
  3. S. Saha, S. Izawa and N. E. Good, Electron Transport and Photophosphorylation in Chloroplasts as a function of the Electron Acceptor: The Journal of Biological Chemistry (October 26, 1970).
  4. Hans Lambers, James Alan Bassham, Encyclopaedia Britannica: Biology, Photosynthesis.
  5. M. Nozaki, K. Tagawa and Daniel Arnon, Non cyclic Photophosphorylation in Photosynthetic Bacteria: Biochemistry (August 1961).

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