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There are two stages of photosynthesis: the light-dependent and the light-independent stage. The former can only occur in the presence of light, whereas the latter is possible either in the presence or absence of light. The site for the light-dependent stage of photosynthesis is the thylakoid membrane. The following things happen during the light-dependent stage of the photosynthesis:
- Water is broken down (photolysis) using the light energy to generate hydrogen ions, electrons, and oxygen in the thylakoid lumen
- Because of the photolysis of water which results in the high concentration of hydrogen ions in the thylakoid lumen, a proton gradient is created
- Electrons move through an electron transport chain of proteins inside the membrane
- When hydrogen ions in the stroma and electrons from the electron transport chain unite with the carrier molecule NADP, reduced NADP (NADPH) is produced
- A process called photophosphorylation produces ATP (ADP + Pi → ATP) by employing the proton gradient between the thylakoid lumen and the stroma to drive the enzyme known as ATP synthase
- Based on the pattern of electron flow in the photosystem I or photosystem II or both, the photophosphorylation of ADP to ATP can be either:
- Cyclic: In this photophosphorylation, photosystem I is involved only
- Non-cyclic: Unlike cyclic photophosphorylation, in this photophosphorylation, both photosystem I and photosystem II are involved.
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What are Photosystems?
Photosystems refer to the collection of photosynthetic pigments that absorb light energy and transfer it onto the electrons. Each photosystem entails a primary pigment. For instance:
- Photosystem I contains a primary pigment that absorbs light at a 700 nm wavelength and is hence known as P700. Remember that the photosystem I is in the middle of the electron transport chain.
- Photosystem II contains the primary pigment that absorbs light at the 680 nm wavelength and is hence known as P680. Photosystem II is the start of the electron transport chain and is the place where photolysis of water occurs
The energy which is carried by ATP is then employed during the light-independent reactions of photosynthesis.
Types of Photophosphorylation
Cyclic Photophosphorylation
- In cyclic photophosphorylation, the photosystem I is involved only
- Photosystem I (present in the thylakoid membrane) absorbs the light which is then transferred to the primary pigment (P700) of photosystem I
- The electron in the primary pigment molecule gets excited to a higher energy level. The chlorophyll molecule then emits it in a process called photoactivation
- An electron receptor captures the excited electron which is then transferred through a chain of electron carriers, referred to as an electron transport chain, before being moved back to the chlorophyll molecule in the photosystem I
- When electrons move through the electron transport chain, they give energy to the transport protons (H+) from the stroma to the thylakoid lumen through a proton pump
- A build-up of protons in the thylakoid lumen can then be employed to instigate the ATP synthesis from ADP and an inorganic phosphate group (Pi) via a process known as chemiosmosis
- Chemiosmosis refers to the movement of chemicals, i.e. protons down their concentration gradient. The energy that is released from this is used by the enzyme ATP synthase to synthesize ATP
- The ATP is then transferred to the light-independent reactions.
Non-Cyclic Photophosphorylation
Unlike cyclic phosphorylation, the non-cyclic phosphorylation involves photosystem I (PSI) and photosystem II (PSII)
Photosystem II
- Photosystem II (PSII) (found in the thylakoid membrane) absorbs light which is then passed to the primary pigment (P680) of the photosystem II (PSII)
- An electron in the primary pigment molecule gets excited to a higher energy level which is then emitted from the chlorophyll molecule in the process called photoactivation
- This energetic (excited) electron then travels down a chain of electron carriers called an electron transport chain, before being passed on to the photosystem I
- In this process, the synthesis of ATP takes place from ADP and an inorganic phosphate group through a process of chemiosmosis
- The ATP is then transferred to the light-independent reactions
- Photosystem II has a water-splitting enzyme known as the oxygen-evolving complex that catalyzes (speeds up) the breakdown of water (photolysis) by light
H2O → 2H+ + 2e- + ½O2
- When the excited electrons exit the primary pigment of the photosystem II and are transferred to the photosystem I, they get replaced by the electrons from the breakdown (photolysis) of water
Photosystem I
- Photoactivation of electrons in the photosystem II and the photosystem I take place simultaneously
- The excited electrons from the photosystem I also transfer along an electron transport chain
- These electrons unite with the hydrogen ions (generated by the photolysis of water) and the carrier molecule NADP to produce reduced NADP
2H+ + 2e- + NADP → reduced NADP
- The reduced NADP (NADPH) is then transferred to the light-independent reaction to be employed in the carbohydrate synthesis
Photophosphorylation and Chemiosmosis
- In photophosphorylation, the electron receptor traps the excited electrons
- These energetic electrons are then transferred along a chain of electron carriers (called the electron transport chain)
- The electron carriers are reduced alternatively (when they gain an electron) and then oxidized (when they lose the electron by transferring it to the next carrier)
- The excited electrons slowly release their energy as they move through the electron transport chain
- The released energy is employed to transport protons actively (H+ions) across the thylakoid membrane, from the stroma (the fluid inside the chloroplasts) to the thylakoid lumen which is the space inside the thylakoids
- A proton pump transfers the protons across the thylakoid membrane from the stroma to the thylakoid lumen
- It forms a proton gradient in which there is a high concentration of protons in the thylakoid lumen and a low concentration of protons in the stroma
- Protons then move back to the stroma by facilitated diffusion via transmembrane ATP synthase enzymes in a process called chemiosmosis
- This process provides the energy required for the synthesis of ATP by adding an inorganic phosphate group to ADP
- The entire process is referred to as photophosphorylation because the light provides the initial energy source for the synthesis of ATP
- After being transported down the electron transport chain, the de-energized electrons from the photosystem II (PSII) move to the photosystem I (PSI)
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