Most of the organic materials are required by organisms are created from the products of photosynthesis. It involves conversion of light energy into energy that can be used by the cell (chemical energy). Photosynthesis can be broken down into two main parts:

         i.            Photosynthetic electron transfer reaction (light reaction), the light reaction involves 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.

       ii.            Carbon fixation reaction (Dark reaction), it involves the production of organic compounds from CO2 using the ATP and NADPH produced by the light reaction.


Photosynthesis involves the use of two photosystems (PSI and PSII) to harness the energy of light using electron to produce ATP and NADPH2, which can later be used by the cells 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 consists of two parts: an antennae complex is important in capturing light energy and transmitting that energy to the photochemical reaction centre, which then converts the energy into usable forms for the cell.


First light excites an electron within a chlorophyll molecule in the antennae complex. This involves a proton of light causing an electron to move to an orbital of high 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 the chlorophyll molecule to another by resonance energy band transfer until it reaches chlorophyll molecule in an area known as the photochemical reaction centre. From here, the excited electrons are passed on to a chain of electron acceptors. Light energy causes the transfer of electron from a weak electron donor (having a strong affinity for electron) to a strong electron donor in its reduced form (carrying a high energy electron). The specific electron donor used by a given organisms or PS can vary.


In plants photosynthesis results in the production of ATP and NADPH by a two step process known as non- cyclic photophosphorylation. It is called non- cyclic photophosphorylation because in the light reaction of photosynthesis, there is one way flow of electron from chlorophyll to the electron carrier NADPH.  Both PSI and PSII are involved. This process takes place in green plants. DCMU inhibits this process. In this process electrons are released from PSII and later they are utilized in PSI and does not come back.


·         The first step of non- cyclic photophosphorylation involves PSII.

·         High energy electron (caused by light energy) from the chlorophyll molecule in the reaction centre of PSII uses water as a weak electron donor to replace electron deficiencies caused by transfer of high energy electron from chlorophyll molecule to quinine molecule.

·         This is accomplished by a water splitting enzyme that allows electron to be removed from water molecule to replace the electron transferred from the chlorophyll molecule. When four electrons removed from two water molecules, oxygen is released. The reduced quinine molecule then passes the high energy electron 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 F₀F1ATPase). 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. The movement of H+ ions down their concentration gradient derives the formation of ATP from ADP and Pi by ATP synthase.

·         ATP synthase found in bacteria, archae, algae, plants and animals (cells have a role in both respiration and photosynthesis).

·         The final electron transfer of PS II is the transfer of electrons to an electron deficient chlorophyll molecule in the reaction centre of PS I is transferred to a molecule called ferrodoxin. From there, the electron is transferred to NADPH to create NADPH.


The photophosphorylation occurring in a cyclic electron transport called cyclic photophosphorylation. Only PSI is involved. The active reaction centre is P700. Here electrons travel in a cyclic manner and gets back to PSI. Only ATP is produced in cyclic photophosphorylation. Photolysis or water splitting in presence of light is absent. Oxygen is not evolved. This system is predominant in bacteria and takes place in chloroplasts. No effect of DCMU.


·         When light is absorbed by PS I, the excited electron may enter into an electron transport chain to produce ATP.

·         Following this, the de- energized electron returns to the PS I, restoring its electron supply. Hence it is known as cyclic photophosphorylation.

·         As the electron returns to the PS NADP+ is not reduced and water is not needed to replenish the electron supply.

Cyclic photophosphorylation takes electron excited to the PSI electron acceptor and instead of sending them to NADPH, deposits them on Plastocyanin (PC) in the electron transport chain between PSII and PSI. These electrons then flow down in the cycle.  The pathway of electron as follows;

The Z- Scheme does not fact make enough ATP to power the Calvin cycle. But when the need for ATP exceeds the capacity of the tissue to make sugars, the photosynthetic apparatus can take a time out, resorting to cyclic photophosphorylation for a while.

So, some of the electron flow is directed away from producing NADPH and cycled back to the electron transport chain where more ATP can be produced satisfying the needs of the Calvin cycle.

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