Photosynthesis: chloroplast structure, light reactions and the Calvin cycle

What the exam asks

• Tracking inputs, outputs, and locations: Connecting the two stages (light-dependent reactions in the thylakoid membrane/grana and the Calvin cycle in the stroma) via their shared energy shuttle molecules: $\text{ATP}/\text{ADP} + \text{P}_i$ and $\text{NADPH}/\text{NADP}^+$.
• Disruption and inhibitor scenarios (FRQ Q1, Q2, and Q4): Predicting the immediate effect on the proton gradient, $\text{ATP}$ synthesis, $\text{NADPH}$ production, or carbon fixation if a membrane becomes permeable to $\text{H}^+$, if light is turned off, or if a specific photosystem is blocked.
• Model analysis (FRQ Q5 and MC sets): Interpreting diagrams of the thylakoid membrane to identify the direction of electron flow, proton pumping, and chemiosmosis.
• Evolutionary context: Using evidence to support claims about the prokaryotic origins of photosynthesis and the oxygenation of Earth’s early atmosphere.

Core ideas

1. Overview and evolutionary origins

Photosynthesis captures light energy to convert carbon dioxide ($\text{CO}_2$) and water ($\text{H}_2\text{O}$) into carbohydrates and oxygen ($\text{O}_2$):

Photosynthesis first evolved in prokaryotes billions of years ago; the oxygen-releasing kind that plants use today arose in the ancestors of modern cyanobacteria. Because prokaryotes lack membrane-bound organelles, photosynthetic bacteria carry out the light-dependent reactions on internal membranes (in cyanobacteria, thylakoid membranes) or infoldings of the plasma membrane, and fix carbon in their cytoplasm. Scientific evidence supports the claim that cyanobacterial photosynthesis produced Earth’s oxygenated atmosphere (the Great Oxidation Event, about 2.4 billion years ago): rocks record oxygen building up in the atmosphere at that time, and cyanobacteria are the only prokaryotes whose photosynthesis releases oxygen. That oxygen later allowed aerobic cellular respiration and eukaryotic life to evolve. Through endosymbiosis, an ancestral cyanobacterium was engulfed by an early eukaryote, giving rise to modern chloroplasts, which still resemble cyanobacteria (they have their own circular DNA, bacteria-like ribosomes and a double membrane).

2. Chloroplast structure and compartmentalization

In eukaryotes, photosynthesis is compartmentalized within the chloroplast, which is surrounded by two membranes (an outer and an inner membrane); the inner membrane encloses a fluid-filled space called the stroma.

• Thylakoids: Flattened, interconnected membrane sacs suspended in the stroma and stacked into grana (singular: granum). Folding so much membrane into stacks packs a large membrane surface area into a small space, with room for many photosystems, electron transport proteins and ATP synthase molecules.
• Thylakoid lumen (space): The tiny aqueous compartment inside the thylakoid. Its small volume allows pumped protons ($\text{H}^+$) to rapidly build up a steep electrochemical gradient (low pH).
• Stroma: The fluid surrounding the thylakoids (higher pH when illuminated), where the enzymes of the Calvin cycle synthesize carbohydrates.