5.3 Photosynthesis: Light Reactions and Calvin Cycle

Photosynthesis converts light energy into chemical energy stored in glucose. The light-dependent reactions capture solar energy and produce ATP and NADPH, which then fuel the Calvin cycle to build sugar from carbon dioxide.

This process is the foundation of nearly all life on Earth. Plants use it to make their own food, and almost every other organism depends on that food either directly or indirectly. Understanding photosynthesis connects major themes in biology: energy flow, ecosystem structure, and the carbon cycle.

Light-Dependent Reactions

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Photosystems and Electron Transport

The light-dependent reactions take place in the thylakoid membranes of chloroplasts. Their job is to convert light energy into two chemical products: ATP and NADPH. Both of these molecules carry energy that the Calvin cycle will use later.

Chloroplasts contain stacks of flattened membrane sacs called thylakoids. Embedded in those membranes are two key protein complexes: Photosystem II (PSII) and Photosystem I (PSI).

• Photosystem II absorbs light best at a wavelength of 680 nm (its reaction center chlorophyll is called P680)
• Photosystem I absorbs light best at 700 nm (reaction center chlorophyll called P700)
• Each photosystem contains a reaction center chlorophyll molecule that gets excited by light energy and kicks off the chain of reactions

A common point of confusion: Photosystem II acts first in the sequence, even though it's numbered "II." The numbering reflects the order of discovery, not the order of events.

Electron Flow and Energy Conversion

Here's how the light reactions proceed, step by step:

1. Light hits Photosystem II. The P680 chlorophyll absorbs a photon, exciting an electron to a higher energy level.

2. Water is split (photolysis). To replace the lost electron, PSII splits a water molecule: $2H_2O \rightarrow 4H^+ + 4e^- + O_2$. The oxygen is released as a byproduct.

3. Electrons travel through the electron transport chain (ETC). A series of protein complexes and carriers pass the excited electron from PSII toward PSI. As electrons move through the chain, they lose energy at each step.

4. Protons are pumped. That released energy is used to pump $H^+$ ions from the stroma into the thylakoid lumen, building up a proton gradient (high $H^+$ concentration inside the thylakoid, low outside).

5. ATP is made via chemiosmosis. The $H^+$ ions flow back down their concentration gradient through ATP synthase, an enzyme that harnesses that flow to combine ADP + inorganic phosphate into ATP.

6. Light hits Photosystem I. The electron arriving from the ETC is re-energized by a second photon at P700.

7. NADPH is produced. The re-energized electron is transferred to $NADP^+$, reducing it to NADPH. This molecule acts as an electron carrier, delivering high-energy electrons to the Calvin cycle.

Calvin Cycle

Carbon Fixation and Reduction

The Calvin cycle (also called the light-independent reactions) occurs in the stroma of the chloroplast. It doesn't use light directly, but it depends entirely on the ATP and NADPH generated by the light reactions.

The cycle has three main phases:

1. Carbon fixation. The enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase) attaches a molecule of $CO_2$ to a 5-carbon sugar called RuBP (ribulose bisphosphate). This unstable 6-carbon intermediate immediately splits into two molecules of the 3-carbon compound 3-PGA (3-phosphoglycerate).
2. Reduction. ATP and NADPH from the light reactions are used to convert 3-PGA into G3P (glyceraldehyde 3-phosphate), a 3-carbon sugar. This is where the energy from sunlight actually gets stored in the bonds of an organic molecule.
3. Regeneration of RuBP. Most of the G3P molecules are rearranged and combined, using more ATP, to regenerate RuBP so the cycle can keep running.

RuBisCO is worth remembering: it's the most abundant enzyme on Earth, and it's the entry point for carbon into the living world.

Glucose Synthesis and Regeneration

For every three turns of the Calvin cycle, three $CO_2$ molecules are fixed, and one net molecule of G3P is produced. That means it takes six turns (fixing six $CO_2$) to produce enough G3P to assemble one glucose molecule ($C_6H_{12}O_6$).

• Of the six G3P molecules produced after six turns, one represents the net gain and can be used to build glucose or other organic compounds
• The other five G3P molecules are recycled to regenerate three molecules of RuBP, keeping the cycle going

The glucose and other organic molecules produced serve multiple purposes for the plant:

• Immediate energy through cellular respiration
• Energy storage as starch
• Structural material like cellulose in cell walls
• Building blocks for amino acids, lipids, and other biomolecules