Light dependent reactions | Non cycle and cycle phosphorylation | chemiosmosis

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Non-cyclic Phosphorylation
1. When photosystem II absorbs light, an electron excited to a higher energy level in the reaction
center chlorophyll P680 is captured by the primary electron acceptor of PS II. The oxidized
chlorophyll is now a very strong oxidizing agent; its electron “hole” must be illed.
2. This hole is illed by the electrons which are extracted, by an enzyme, from water. This reaction
splits a water molecules into two hydrogen ions and an oxygen atom, which immediately
combines with another oxygen atom to form O2
. This water splitting step of photosynthesis that
releases oxygen is called photolysis. The oxygen produced during photolysis is the main source
of replenishment of atmospheric oxygen.
3. Each photoexcited electron passes from the primary electron acceptor of photosystem II to
photosystem I via an electron transport chain. This chain consists of an electron carrier calledplastoquinone (Pq), a complex of two cytochromes and a copper containing protein called
plastocyanin (Pc).
4. As electrons move down the chain, their energy goes on decreasing and is used by thylakoid
membrane to produce ATP. This ATP synthesis is called photophosphorylation because it is
driven by light energy. Speciically, ATP synthesis during non-cyclic electron low is called non-
cyclic photophosphorylation. This ATP generated by the light reactions will provide chemical
energy for the synthesis of sugar during the Calvin cycle, the second major stage of photosynthesis.
5. The electron reaches the “bottom” of the electron transport chain and ills an electron “hole” in
P700, the chlorophyll a molecules in the reaction center of photosystem I. This hole is created
when light energy is absorbed by molecules of P700 and drives an electron from P700 to the
primary acceptor of photosystem I.
6. The primary electron acceptor of photosystem I passes the photoexcited electrons to a second
electron transport chain, which tmasmits them to ferredoxin (Fd), an iron containing protein.
An enzyme called NADP reductase then transfers the electrons from Fd to NADP. This is the
redox reaction that stores the high-energy electrons in NADPH. The NADPH molecule will provide
reducing power for the synthesis of sugar in the Calvin cycle.
The path of electrons through the two photosystems during non-cyclic photophosphorylation is
known as Z-scheme from its shape.

Chemiosmosis
In both cyclic and non-cyclic photophosphorylation, the mechanism for ATP synthesis is
chemiosmosis, the process that uses membranes to couple redox reactions to ATP production.
Electron transport chain pumps protons (H+
) across the membrane of thylakoids in case of
photosynthesis into the thylakoids space. The energy used for this pumping comes from the
electrons moving through the electron transport chain. This energy is transformed into potential
energy stored in the form of H+
gradient across the membrane. Next the hydrogen ions move down
their gradient through special complexes called ATP synthase which are built in the thylakoid
membrane. During this difusion of H+ the energy of electrons is used to make ATP (