Mechanism of photosynthesis | light reaction |class 11th chapter | Bioenergetics | скачать в хорошем качестве

Mechanism of photosynthesis | light reaction |class 11th chapter | Bioenergetics |

The light-dependent reactions (light reactions) which use light directly and The light-independent reactions (dark reactions) which do not use light directly. Light dependent reactions constitute that phase of photosynthesis during which light energy is absorbed by chlorophyll and other photosynthetic pigment molecules and converted into chemical energy. As a result of this energy conversion, reducing and assimilating power in the form of NADPH (NADPH + H+) and ATP, are formed, both temporarily storing energy to be carried alongwith H to the light independent reactions. NADPH provides energized electron (and H+), while ATP provides chemical energy for the synthesis of sugar by reducing CO2 , using reducing power and chemical energy of NADPH and ATP respectively, produced by light reactions. The energy is thus stored in the molecules of sugar. This phase of photosynthesis is also called dark reactions because these reactions do not use light directly and can take place equally well both in light and dark provided NADPH2 and ATP of light reactions are available. Light dependent Reactions (Energy-conversion phase; formation of ATP and NADPH) As has been described previously, sunlight energy which is absorbed by photosynthetic pigments drives the process of photosynthesis. Photosynthetic pigments are organized into clusters, called photosystems, for eicient absorption and utilization of solar energy in thylakoid membranes Each photosystem consists of a light-gathering ‘antenna complex’ and a ‘reaction center’. The antenna complex has many molecules of chlorophyll a, chlorophyll b and carotenoids, most of them channeling the energy to reaction center. Reaction center has one or more molecules of chlorophyll a along with a primary electron acceptor, and associated electron carriers of ‘electron transport system’. Chlorophyll a molecules of reaction center and associated proteins are closely linked to the nearby electron transport system. Electron transport system plays role in generation of ATP by chemiosmosis (which will be discussed in later section). Light energy absorbed by the pigment molecules of antenna complex is transferred ultimately to the reaction center. There the light energy is converted into chemical energy. There are two photosystems, photosystem I (PS I) and photosystem II (PS II). These are named so in order of their discovery. Photosystem I has chlorophyll a molecule which absorbs maximum light of 700 nm and is called P700 , whereas reaction center of photosystem II has P680 , the form of chlorophyll a which absorbs best the light of 680 nm. A specialized molecule called, primary electron acceptor is also associated nearby each reaction center. This acceptor traps the high energy electrons from the reaction center and then passes them on to the series of electron carriers. During this energy is used to generate ATP by chemiosmosis. In predominant type of electron transport called non-cyclic electron low, the electrons pass through the two photosystems. In less common type of path called cyclic electron low only photosystem I is involved. Formation of ATP during non-cyclic electron low is called non-cyclic phosphorylation while that during cyclic electron low is called cyclic phosphorylation. 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 called plastoquinone (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 #bioenergetic #lightreaction