Page 15 - 11-Bio-11 Kingdom plantae

11. Bioenergetics eLearn.Punjab

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 flow, the electrons pass
through the two photosystems. In less common type of path called cyclic electron flow only
photosystem I is involved. Formation of ATP during non-cyclic electron flow is called non-cyclic
phosphorylation while that during cyclic electron flow 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 filled.

2. This hole is filled 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

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