C1.3.14 Thylakoids as systems for performing the light-dependent photosynthesis reactions (HL) Notes

Thylakoids Are the Sites of Light-Dependent Reactions in Photosynthesis

1. Thylakoids are the site of light-dependent reactions, where sunlight is transformed into chemical energy in the form of ATP and reduced NADP.
2. These reactions occur in three main steps:
   1. Photolysis of Water
   2. Synthesis of ATP by Chemiosmosis
   3. Reduction of NADP

Photolysis of Water Provides Electrons for PSII and Releases Oxygen

The process begins with photosystem II (PSII), a complex of proteins and pigments embedded in the thylakoid membrane.

How Photolysis Works

1. Light Absorption: PSII absorbs photons of light, exciting electrons in a special chlorophyll molecule called P680.
2. Electron Replacement: The excited electrons are transferred to an electron transport chain (ETC), leaving P680 oxidized.
3. Water Splitting: To replace these electrons, water molecules are split in a process called photolysis: $$2\text{H}_2\text{O} \rightarrow \text{O}_2 + 4\text{H}^+ + 4\text{e}^-$$
4. Oxygen Release: Oxygen is released as a by-product, while the electrons and protons are used in later stages.

Where It Happens

1. Photolysis occurs in the oxygen-evolving complex of PSII, located on the inner surface of the thylakoid membrane.
2. The protons (H⁺) are released into the thylakoid lumen, contributing to a proton gradient.

Chemiosmosis Powers ATP Synthesis as Electrons Flow Through the ETC

Once the electrons leave PSII, they travel through a series of proteins in the electron transport chain, including plastoquinone (PQ), cytochrome b6f, and plastocyanin (PC).

How Chemiosmosis Works

1. Proton Pumping: As electrons move through the ETC, energy is released and used by cytochrome b6f to pump protons from the stroma into the thylakoid lumen.
2. Proton Gradient: This creates a high concentration of protons inside the lumen compared to the stroma.
3. ATP Synthase: Protons flow back into the stroma through ATP synthase, a protein complex that uses this flow to convert ADP and inorganic phosphate (Pi) into ATP.

Where It Happens

1. The proton gradient is established across the thylakoid membrane.
2. ATP synthase is embedded in the membrane, with its active site facing the stroma, where ATP is produced.

Reduction of NADP in PSI Stores Energy for the Calvin Cycle

1. The final step of the light-dependent reactions occurs in photosystem I (PSI).
2. The reduction of NADP occurs on the stroma side of the thylakoid membrane, ensuring that NADPH is readily available for the Calvin cycle.

How NADP is Reduced

1. Light Absorption: PSI absorbs light, exciting electrons in a chlorophyll molecule called P700.
2. Electron Transfer: The excited electrons are passed to a series of carriers and eventually to NADP reductase, an enzyme located on the stroma side of the thylakoid membrane.
3. Formation of Reduced NADP: NADP reductase uses the electrons, along with protons from the stroma, to convert NADP+ into reduced NADP (NADPH).

Thylakoid Structure Optimizes Light-Dependent Reactions

The structure of thylakoids is finely tuned to optimize the light-dependent reactions:

1. Large Surface Area: The extensive membrane system provides ample space for photosystems, electron carriers, and ATP synthase.
2. Compartmentalization: The separation between the thylakoid lumen and stroma allows for the establishment of a proton gradient.
3. Strategic Distribution: Photosystem II and cytochrome b6f are concentrated in the stacked thylakoids (grana), while photosystem I and ATP synthase are found in the unstacked regions (stroma lamellae).