Overview of the Thylakoid Membrane
Photosynthesis is split into two main stages: the Light-Dependent Reaction (which occurs in the **thylakoid membranes** of chloroplasts) and the Light-Independent Reaction (occurring in the **stroma**).
The main purpose of the light-dependent stage is to harvest light energy to produce **ATP** and **reduced NADP** ($NADPH$), which are required to fuel the subsequent synthesis of glucose in the Calvin cycle.
Step-by-Step Molecular Stages
To secure top marks on long-answer questions, you must explain this chemical process using the following five sequence stages:
Photoionisation of Chlorophyll
Light energy is absorbed by photosystem II (PSII). This excites a pair of electrons inside chlorophyll to a higher energy level, causing them to leave the molecule. The chlorophyll becomes oxidised ($\text{Chl} \rightarrow \text{Chl}^{2+} + 2\text{e}^-$).
The Electron Transport Chain (ETC)
The excited electrons are captured by an electron acceptor and passed along a series of electron carrier proteins embedded in the thylakoid membrane. As the electrons move, they lose energy at each carrier.
Chemiosmosis and Proton Gradient
The energy released by the ETC is used to actively pump protons ($\text{H}^+$) from the stroma across the thylakoid membrane into the thylakoid space. This establishes a high concentration gradient of protons inside the thylakoid.
Photophosphorylation via ATP Synthase
Protons diffuse back down their electrochemical gradient into the stroma through the channel protein **ATP Synthase**. This proton flow causes the enzyme to rotate, driving the synthesis of ATP: $$\text{ADP} + \text{P}_i \rightarrow \text{ATP}$$
Reduction of NADP and Photolysis of Water
Light excites electrons in photosystem I (PSI). These electrons, combined with protons in the stroma, reduce the coenzyme NADP: $$\text{NADP} + 2\text{H}^+ + 2\text{e}^- \rightarrow \text{reduced NADP}$$ Meanwhile, an enzyme in PSII splits water to replace the lost chlorophyll electrons: $$2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + 4\text{e}^- + \text{O}_2$$
Worked Exam Example
Worked Exam Example
Question: Describe the role of water in the light-dependent reaction of photosynthesis.
Solution:
- Water is split by light energy in reproductive photosystem II, a process called **photolysis** ($2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + 4\text{e}^- + \text{O}_2$).
- The **electrons ($\text{e}^-$)** produced replace those lost from chlorophyll in photosystem II during photoionisation, keeping the process continuous.
- The **protons ($\text{H}^+$)** accumulate in the thylakoid space to maintain the proton concentration gradient for ATP synthesis, and are eventually used to reduce NADP.
- **Oxygen ($\text{O}_2$)** is released as a waste product of photolysis.
Frequently Asked Questions
In non-cyclic photophosphorylation, electrons flow from water through PSII and PSI to NADP, producing both ATP and reduced NADP. In cyclic photophosphorylation, electrons from PSI are recycled back through the ETC, producing ATP only, and no reduced NADP or oxygen is generated.
The thylakoid space accumulates a high concentration of protons ($\text{H}^+$) due to both active pumping by the ETC and the photolysis of water. A higher concentration of hydrogen ions corresponds to a lower pH (more acidic).
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