Introduction
Plants, just like other photosynthetic organisms, are capable of capturing light energy and using it to produce their food by photosynthesis. This may sound simple, but in fact it is a very complex process. However, the process of converting light energy can only be performed in the green parts of the plants, like leaves. The reason behind this is that not all cells of plants have chloroplasts, which are the site of photosynthesis. The non-green parts, like roots and stems, have other forms of plastids, modified either to store molecules (leucoplasts) or to provide colour (chromoplasts).
The ground tissue of leaves, called mesophyll, is the main site of photosynthesis in plants. Each of these cells contains as many as 30–40 chloroplasts capable of producing food. A chloroplast is a cell organelle of variable shapes and sizes, surrounded externally by two membranes. The outer membrane and the inner membrane are separated by a small space called the intermembrane space. The fluid part enclosed inside the inner membrane is called the stroma, which contains several components, like the circular double-stranded DNA of the chloroplast (which makes it semi-autonomous), all major types of RNA, ribosomes (70S type), and many proteins and enzymes. Along with these, the stroma also contains membrane-bound, sac-like structures called thylakoids. In higher plants, the thylakoids are stacked over each other to form grana, and the granal thylakoids are connected through membranous extensions called stroma lamellae.
Photosystems
The light-capturing pigments are present in the thylakoids and stroma lamellae in the form of groups called photosystems. Each photosystem is a group of 200–300 pigments called accessory pigments (forming the light harvesting complex) and also includes a special type of chlorophyll a (dimeric) as its reaction centre. On the basis of the type of reaction centre, the photosystems are divided into two types – photosystem I and photosystem II. The names I and II are given in the order of their discovery.
1) Photosystem I
- Photosystem I is present on the non-appressed parts of the grana as well as the stroma lamellae.
- The ratio of chlorophyll a to chlorophyll b is high in the LHC of photosystem I.
- The reaction centre chlorophyll is designated as P700, as its peak absorption occurs at the 700 nm wavelength of light.
2) Photosystem II
- Photosystem II is present on the appressed parts of the grana only.
- The ratio of chlorophyll a to chlorophyll b is almost equal in the LHC of photosystem II.
- The reaction centre here is called P680, due to its peak absorption at the 680 nm wavelength.
- Photosystem II is also associated with a water-splitting complex, also called the oxygen evolving complex (towards the thylakoid lumen), capable of splitting water molecules into protons, electrons and oxygen. The OEC uses Ca, Mn and Cl ions as cofactors and releases one electron at a time to P680.
The names I and II follow the order of discovery, not the order of working: in the flow of electrons, PS II acts first, then PS I. The splitting of water by PS II can be written as 2H2O → 4H+ + O2 + 4e–.
Cytochrome complex
Besides the photosystems, the thylakoids and stroma lamellae also contain a cytochrome complex called the cytochrome b6f complex (similar to complex III of the mitochondrial ETS). This complex is equally distributed in both the appressed and non-appressed parts of the grana. The cytochrome b6f complex contains a b-type cytochrome with two heme groups (designated bH and bL), a Rieske iron-sulfur protein and cytochrome f.
Electron carriers
Several electron carriers are also present in the membranes to help in the flow of electrons from one photosystem to the other. These electron carriers are of two types:
1) Plastoquinone
Plastoquinone or PQ is a lipid-soluble electron carrier (it can freely diffuse in the lipid bilayer), which receives the electrons coming from photosystem II and transfers them to the cytochrome b6f complex. It is also responsible for pumping protons from the stroma side to the lumen, thus creating the proton gradient required for ATP formation.
2) Plastocyanin
Plastocyanin or PC is another electron carrier of the thylakoid membrane. It is a copper-containing, water-soluble electron carrier present towards the lumen side of the membrane. Plastocyanin receives electrons from the cytochrome b6f complex and transfers them to photosystem I.
Enzymes
1) ATP synthase
The thylakoids and stroma lamellae also contain an enzyme having ATP synthase activity for photophosphorylation. The activity of this ATP synthase is similar to that of the one found in the inner mitochondrial membrane. It also consists of two parts – CF1 and CF0 – where CF1 has the main catalytic activity and undergoes conformational changes to make ATP, while CF0 simply acts as an ion channel for the facilitated diffusion of protons. The CF1 part is located towards the stroma side.
2) Ferredoxin: NADP+ oxidoreductase
Ferredoxin: NADP+ oxidoreductase is a flavoprotein present on the thylakoid membrane. It reduces NADP+ to NADPH by capturing electrons from reduced ferredoxin and transferring them to NADP+.
Electron transport and photophosphorylation
Photophosphorylation is the process of formation of ATP by using light energy. Just like oxidative phosphorylation, photophosphorylation also depends on a proton gradient to produce ATP from ADP and inorganic phosphate.
The process starts as soon as light strikes the leaves. The light harvesting complex of each photosystem absorbs light of different wavelengths and transfers this energy to the reaction centre of the respective photosystem. As a result, the reaction centre acquires sufficient light energy, its electrons reach an excited state, and they are captured by the respective associated electron carriers.
The photoexcited electrons from the photosystem II reaction centre are captured by the nearest plastoquinone, which is tightly bound (called PQA), and then by a loosely bound PQB. PQB also acquires protons from the stroma side and gets reduced to plastoquinol (PQBH2). The loss of electrons from PS II is compensated by the splitting of water molecules in the lumen by the OEC.
The electrons are then transferred from PQBH2 to the cytochrome b6f complex, and the protons are released inside the lumen. The cytochrome complex receives the electrons and passes them along the cyt b6 – Fe-S – cyt f chain, and then these electrons are captured by plastocyanin (PC), located towards the lumen side of the membrane.
Photoexcitation of electrons also occurs in PS I, creating a deficiency in the PS I reaction centre. PC makes up this deficiency by releasing the electrons it captured from the cytochrome b6f complex. Electrons released from the PS I reaction centre are transferred to an iron-sulfur protein and then to ferredoxin (Fd). These electrons from reduced Fd are then captured by the enzyme ferredoxin: NADP+ oxidoreductase, located towards the stroma side of the thylakoid membrane. This enzyme transfers electrons from reduced Fd to NADP+, thus reducing it to NADPH.
The proton gradient created by the flow of electrons from PS II to PS I creates a P-side in the lumen and an N-side in the stroma. This proton gradient breaks down by the facilitated diffusion of protons from the lumen to the stroma through CF0 of the ATP synthase complex. The CF1 part of ATP synthase undergoes conformational changes and phosphorylates ADP to form ATP.
NCERT lists three causes of the proton gradient: splitting of water releases H+ inside the lumen, PQ carries H+ from the stroma into the lumen, and NADP+ reduction removes H+ from the stroma.
The process above, using both photosystems, is non-cyclic photophosphorylation (the Z-scheme): it makes ATP and NADPH and releases O2. In cyclic photophosphorylation, only PS I works and the electrons return to it, so only ATP is formed – no NADPH and no O2. It occurs in the stroma lamellae, which lack PS II and NADP reductase.