The Difference between Cyclic and Noncyclic Photophosphorylation

Most of the organic materials required by organisms are created from the products of photosynthesis. Photosynthesis involves conversion of light energy into energy that can be used by the cell, most notably chemical energy. In plants and algae, photosynthesis occurs in an organelle called the chloroplast, which contain an outer membrane, an inner membrane and a thylakoid membrane (https://en.wikipedia.org/wiki/Chloroplast).

Photosynthesis can be broken down into two main parts: (1) the photosynthetic electron-transfer reactions (“light reactions”) and (2) the carbon fixation reactions (“dark reactions”). The “light reactions” involve sunlight energizing electrons in the photosynthetic pigment chlorophyll, which then travel along an electron transport chain in the thylakoid membrane, resulting in the formation of ATP and NADPH.  The “dark reactions” involve the production of organic compounds from CO2 using the ATP and NADPH produced by the “light reactions” and will not be discussed further in this article.

Photosynthesis involves the use of two photosystems (photosystem I and photosystem II) to harness the energy of light using electrons to produce ATP and NADPH, which can later be used by the cell as chemical energy to make organic compounds. Photosystems are large protein complexes that specialize in collecting light energy and converting it into chemical energy. Photosystems consist of two parts: an antenna complex and a photochemical reaction center. The antenna complex is important in capturing light energy and transmitting that energy to the photochemical reaction center, which then converts the energy into usable forms for the cell.

First, light excites an electron within a chlorophyll molecule in the antenna complex. This involves a photon of light causing an electron to move to an orbital of higher energy. When an electron in a chlorophyll molecule is excited, it is unstable in the higher energy orbital, and the energy is rapidly transferred from one chlorophyll molecule to another by resonance energy transfer until it reaches chlorophyll molecules in an area known as the photochemical reaction center. From here, the excited electrons are passed on to a chain of electron acceptors. Light energy causes the transfer of electrons from a weak electron donor (having a strong affinity for electrons) to a strong electron donor in its reduced form (carrying a high-energy electron). The specific electron donors used by a given organism or photosystem can vary and will be discussed further below for photosystems I and II in plants.

In plants, photosynthesis results in the production of ATP and NADPH by a two-step process known as noncyclic photophosphorylation. The first step of noncyclic photophosphorylation involves photosystem II. High-energy electrons (caused by light energy) from the chlorophyll molecules in the reaction center of photosystem II are transferred to quinone molecules (strong electron donors). Photosystem II uses water as a weak electron donor to replace electron deficiencies caused by transfer of high-energy electrons from chlorophyll molecules to quinone molecules. This is accomplished by a water-splitting enzyme that allows electrons to be removed from water molecules to replace the electrons transferred from the chlorophyll molecule. When 4 electrons are removed from two H2O molecules (corresponding to 4 photons), O2 is released. The reduced quinone molecules then pass the high-energy electrons to a proton (H+) pump known as the cytochrome b6-f complex. The cytochrome b6-f complex  pumps H+ into the thylakoid space, creating a concentration gradient across the thylakoid membrane.

This proton gradient then drives ATP synthesis by the enzyme ATP synthase (also called F0F1 ATPase). ATP synthase provides a means for H+ ions to travel through the thylakoid membrane, down their concentration gradient. The movement of H+ ions down their concentration gradient drives the formation of ATP from ADP and Pi (inorganic phosphate) by ATP synthase. ATP synthase is found in bacteria, archea, plants, algae, and animal cells and has a role in both respiration and photosynthesis (https://en.wikipedia.org/wiki/ATP_synthase).

The final electron transfer of photosystem II is the transfer of electrons to an electron deficient chlorophyll molecule in the reaction center of photosystem I. An excited electron (caused by light energy) from the chlorophyll molecule in the reaction center of photosystem I is transferred to a molecule called ferredoxin. From there, the electron is transferred to NADP+ to create NADPH.

Noncyclic photophosphorylation produces 1 molecule of ATP and 1 molecule of NADPH per electron pair; however carbon fixation requires 1.5 molecules of ATP per molecule of NADPH. To address this issue and produce more ATP molecules, some plant species use a process known as cyclic photophosphorylation. Cyclic photophosphorylation involves only photosystem I, not photosystem II, and does not form NADPH or O2. In cyclic phosphorylation, high-energy electrons from photosystem I are transferred to the cytochrome b6-f complex instead of being transferred to NADP+. The electrons lose energy as they are passed through the cytochrome b6-f complex back to the chlorophyll of photosystem I and H+ is pumped across the thylakoid membrane as a result. This increases the concentration of H+ in the thylakoid space, which drives the production of ATP by ATP synthase.

The level of noncyclic versus cyclic photophosphorylation that occurs in a given photosynthetic cell is regulated based on the cell’s needs. In this way, the cell can control how much light energy it converts into reducing power (fueled by NADPH) and how much is converted into high-energy phosphate bonds (ATP).

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