Some of the processes involved in photosynthesis require light while others do not. Therefore, there are two basic parts to photosynthesis, the light reactions (which are light dependent) and the light independent reactions (or dark reactions).

One can think of the light dependent reactions as a way to increase the free energy of the system and the light independent reactions as a way to convert that new free energy into the bonds of glucose.

The light-absorbing pigments of thylakoid membranes and their associated electron carriers are arranged in functional sets or clusters. These clusters are called photosystems. For example: in spinach chloroplasts these photosystems contain about 200 chlorophyll molecules and about 50 carotinoids, arranged in what are called light harvesting antenna. These photosystems can absorb light over the entire visible spectrum but especially well between 400 to 500 nm and 600 to 700 nm.

When a chlorophyll molecule in the thylakoid membrane is excited by light, the energy level of an electron in its structure is boosted by an amount equivalent to the energy of the absorbed light and the chlorophyll becomes excited. The packet of excitation energy (The Exciton) now migrated rapidly through the light harvesting pigment molecules to the reaction centre of the photosystem where it causes an electron to acquire the large amount of energy.

The photochemical reaction centre of photosystems II is called P680. The photochemical reaction centre of photosystems I is called P700.

The thylakoid membranes of plant chloroplasts have two different kinds of photosystems each with its own set of light harvesting chlorophyll and carotenoid molecules and the photochemical reaction centre.

Photosystem I - is maximally excited by light at longer wavelengths. (P700)

Photosystems II - is maximally excited by shorter wavelengths. (Less the 680). (P680)

NB: Generalisation

All oxygen-evolving photosynthetic cells. (E.g. those of higher plants and cyanobacteria) contain both photosystems I and II, whereas all other species of photosynthetic bacteria, which do not evolve oxygen only contain photosystem I.

These increase the free energy made available to the system. This free energy can be used in three ways:

1) To build the chemiosmotic or proton gradient.

2) Generate ATP.

3) Reduce NADP⁺ to NADPH.

There are two ways to generate ATP

1) Non cyclic photophosphoraltion.

2) Cyclic photophosphoralation.

These two systems differ in the route taken by the "light activated" electrons and in some of the products formed.

Non Cyclic Photophosphoralation

These events begin with photons being absorbed in photosystem II and its energy is shunted into P680 and the chlorophyll a of this photoactivation centre passes the energy rich electron on into the electron transport system. Once in the electron transport system of the thylakoid membrane the electron is passed from electron carrier to electron carrier eventually entering photosystem I. In the "tumbling" down of the electron transport chain the electron gradually loses energy. Some of that energy will be used to "pump protons" across the thylakoid membrane into the "lumen" (thylakoid compartment).

NB: Each light activated electron allows the "proton pump" to pump one proton across the membrane. These partly spent electrons then pass into the next photosystem I P700 where they receive another "boost" to their highest energy level yet. But these electrons do not pump protons they use their energy to reduce NADP⁺ to NADPH.

 

Chemiosmotic Phosphoralation

The cyclic and non-cyclic events of the electron carriers and photosystems of the thylakoid membrane serve to pump protons into the lumen of the thylakoids.

Just as in the mitochondria outer compartment the protons are allowed to flow back into the stroma (which has a high OH^- (hydroxide) content (i.e. very basic). The controlled flow of H⁺ down this concentration gradient occurs through the so-called CFi particles. Which turn out to be ATP synthase complexes.

These systems therefore contribute ATP and high-energy electrons NADPH (reducing power) to the next series of reaction in photosynthesis. (i.e. the dark reaction the Calvin cycle).