Photosynthesis: Converting Light Energy to Chemical Energy

Pigments are molecules that absorb specific wavelengths of light while reflecting others. For example, leaves appear green because they absorb red and violet wavelengths but reflect green light. When a pigment absorbs a photon, an electron is boosted to an excited state, an unstable high-energy orbital. To return to a stable ground state, the electron may release energy as heat, light (fluorescence), or transfer its energy to a nearby molecule via resonance energy transfer.

Plants utilize a variety of pigments to maximize energy capture, including chlorophyll $a$, chlorophyll $b$, and carotenoids.

The Light-Dependent Reactions

The first stage of photosynthesis takes place in the thylakoid membranes and transforms light energy into chemical energy in the form of ATP and NADPH.

Photosystem II (PSII) and Water Splitting

The process begins at Photosystem II (PSII), which contains the pigment P680. Light energy is captured by an antenna complex and transferred via resonance energy transfer to P680. To prevent the excited electron from simply losing its energy as heat, PSII transfers the electron to a stable primary acceptor called pheophytin (Pp).

To replace the lost electron, water is oxidized, a process that releases oxygen ($\text{O}_{2}$) as a byproduct and contributes $\text{H}^{+}$ ions to the thylakoid lumen.

Photosystem I (PSI) and NADPH Production

Electrons travel from PSII through an Electron Transport Chain (ETC) to Photosystem I (PSI). At PSI, the pigment P700 absorbs light to re-energize the electrons. These high-energy electrons are then passed to NADP$^{+}$ reductase, which facilitates the formation of NADPH by combining $\text{NADP}^{+}$, electrons, and $\text{H}^{+}$.

Chemiosmosis and ATP Synthesis

As electrons move through the ETC, they help pump $\text{H}^{+}$ ions into the thylakoid lumen, creating an electrochemical gradient. This gradient drives ATP synthase, a molecular motor that produces ATP as $\text{H}^{+}$ ions flow back into the stroma. This specific process of using light to drive ATP synthesis is called photophosphorylation.

The Calvin Cycle (Light-Independent Reactions)

The second stage of photosynthesis occurs in the stroma and uses the ATP and NADPH produced in the light reactions to fix $\mathrm{CO}_{2}$ into carbohydrates. This cycle proceeds in three distinct phases:

1. Carbon Fixation: The enzyme Rubisco incorporates $\mathrm{CO}_{2}$ into a five-carbon sugar called ribulose bisphosphate (RuBP). This produces an unstable intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).
2. Reduction: Using energy from ATP and electrons from NADPH, 3-PGA is converted into glyceraldehyde-3-phosphate (G3P), a high-energy three-carbon sugar.
3. Regeneration: While some G3P molecules exit the cycle to form glucose and other carbohydrates, the majority are used to regenerate RuBP using additional ATP, allowing the cycle to continue.

Environmental Adaptations and Photorespiration

Photosynthetic efficiency is heavily influenced by light intensity, temperature, and water availability.

The Challenge of Photorespiration

In many plants (known as $\text{C}_{3}$ plants), Rubisco can mistakenly bind to $\text{O}_{2}$ instead of $\mathrm{CO}_{2}$ when $\text{O}_{2}$ levels are high or $\mathrm{CO}_{2}$ levels are low (often due to closed stomata in hot/dry conditions). This process, called photorespiration, consumes energy and reduces the efficiency of carbohydrate production, though it may serve to protect the plant from oxidative damage.

Evolutionary Solutions: $\text{C}_{4}$ and CAM Plants

To minimize photorespiration, some plants have evolved specialized pathways:

• $\text{C}_{4}$ Plants: These plants (e.g., corn) physically separate carbon fixation from the Calvin cycle. They use the enzyme PEP carboxylase to fix $\mathrm{CO}_{2}$ into a four-carbon molecule in the mesophyll cells, which is then transported to bundle-sheath cells where $\mathrm{CO}_{2}$ is released at high concentrations for Rubisco to use.

  

• CAM Plants: These plants (e.g., cacti) separate the processes temporally. They open their stomata at night to take up $\mathrm{CO}_{2}$ and store it as organic acids, then close their stomata during the day to conserve water, releasing the stored $\mathrm{CO}_{2}$ for use in the Calvin cycle.