Two Pigments Systems (Photosystems) | Photosynthesis

The below mentioned article provides a study-note on the two pigments systems (Photosystems).

The discovery of red drop and the Emerson’s enhancement effect has led scientists to suggest that photosynthesis is driven by two photochemical processes. These processes are associated with two groups of photosynthetic pigments called as pigment system I and pigment system II. Wavelength of light shorter than 680 mµ affect both the pigment systems while wavelengths longer than 680 mµ affect only pigment system I.

i. Pigment system I is relatively very weakly fluorescent while pigment system II strongly fluorescent.

ii. In green plants, pigment system I contains chlorophyll-b, different forms of chlorophyll-a such as chl.-a 670, chl.-a 680, chl.-a 695 or chl.-677, chl.-a 684, chl.-a 692 or chl. 673, chl. a 683 (these differ according to different workers). A very small amount of a special form of chlorophyll-a absorbing at 700 mµ which is known as P 700 however, constitutes the reaction centre (or trap 1) of pigment system I.

iii. The pigment system II contains chlorophyll-b and some forms of chlorophyll-a (such as chl.-a 662, chl.-a 670, chl.-a 677). A very small amount of special form of chlorophyll- a called P 680 constitutes the reaction centre (trap II) of pigment system II. In some plants P 690 constitutes the reaction centre of pigment system II.

iv. Carotenoids are present in both the pigment systems.

v. In case of the red and blue-green algae the pigments systems contain phycobilins in place of chlorophyll-b.

vi. The relative proportion of chlorophyll-b and different forms of the chlorophyll-a is different in the two pigment systems.

vii. Each pigment system (or photo-system) consists of a central core complex (CC) and a light harvesting complex (LHC). In pigment system I and II, these are called as CCI, LHCI and CCII, LHCII respectively. The light harvesting complexes contain antenna pigments associated with proteins. Their main function is to harvest light energy and transfer it to their respective reaction centres.

The core complexes consist of their respective reaction centres associated with proteins and also electron donors and acceptors.

I. The two photo-systems are spatially separated in thylakoid membranes. Photo-system II is predominantly localised in stacked regions of thylakoid membranes (Fig. 11.6).

II. The two pigment systems I and II are believed to be interconnected by a third inte­gral protein complex called cytochrome b₆-f complex (also called as Rieske Centre). The other intermediate components of electron transport chain viz., plastoquinone (PQ) and plastoyanin (PC) act as mobile electron carriers between this complex and either of the two pigment sys­tems.

Plastoquinone which infact is a hydrogen carrier acts between pigment system II and cytochrome b₆-f complex. Plastocyanin acts between cytochrome b₆-f complex and pigment sys­tem I.

The ATPase or ATP-synthase, the enzymic protein complexes responsible for photophos- phorylation are found in thylakoid membranes in those regions which are in direct contact with stroma of chloroplast. ATP-synthase consists of two subunits called CF₀ and CF₁ (CF = coupling factor). CF₀ is an integral protein complex across the thylakoid membrane and consists of a cluster of at least four different types of polypetides named a, b, b’ and c with stoichiometry of a, b, b’ and C₁₂). It forms a channel for movement of proton across the thylakoid membrane. The head piece CF, consists of five different polypeptides (3 cop­ies each of α, β and one copy each of, δ and ɛ (Fig. 11.11). The catalytic sites are situ­ated largely on α and β subunits.

Apart from ATP-synthase, structure and composition of other complexes and compo­nents of photosynthetic electron transfer are now known in great detail which have been illustrated in Fig. 11.12.

i. The four complexes (PSI, PSII, Cyt b₆f and ATPase) are integral membrane proteins with a substantial portion of their structure embedded in the lipid bilayer.

ii. The orientation of these four complexes is not random but is vectorial. This is essen­tial feature of all energy transducing membranes and a prerequisite for their capacity to con­serve energy through chemiosmosis.

iii. The two large multi-molecular protein-pigment complexes in PSI and PSII consist of about 18 and over 30 distinct subunits respectively. In PSI, CCI contains two large proteins called PsaA and PsaB with mol. mass ranging from 66 to 70 k Da. In PSII, CCII contains two large membrane proteins called D₁ and D₂ (so called because they were first identified as two diffuse bands by gel electrophoresis and staining).

Deisenhofer, J., Huber, R., and Michael, H. were jointly awarded Nobel Prize of 1988 in Chemistry for the first successful crystallization, X-ray diffraction and determination of precise three- dimensional structure of photosynthetic reaction centre in purple bacterium (Rhodopseudomonas viridis)

i. Photosystem, I and II can be isolated from chloroplasts of green algae and higher plants by means of physical and chemical methods.

ii. Light energy absorbed by pigments in the two systems is ultimately trapped by the P 700 and P 680 (or in some plants P 690) forms of chlorophyll-a which alone can take part in further photochemical reaction.