IB Biology Topic B2.2.5: Adaptations of the Chloroplast for Photosynthesis Notes

The Chloroplast: Where Photosynthesis Happens

1. The chloroplast is a double-membrane organelle found in the cells of plants and algae.
2. It is the site of photosynthesis, which converts light energy into chemical energy stored in organic molecules.
   1. Like the mitochondrion, every structural feature of the chloroplast relates directly to its function.
3. The chloroplast has three distinct compartments.
   1. The thylakoid lumen is the space inside the thylakoid membranes.
   2. The stroma is the fluid between the thylakoids and the inner membrane.
   3. The intermembrane space lies between the outer and inner membranes.
4. This compartmentalisation lets different stages of photosynthesis run at the same time under optimal conditions.

Thylakoid Membranes and Grana: Maximising Light Absorption

1. The thylakoid membranes are the site of the light-dependent reactions.
   1. Embedded in them are photosystems (clusters of chlorophyll and accessory pigments that absorb light and excite electrons) and an electron transport chain that uses the energy of those electrons to pump H+ ions into the thylakoid lumen.
2. Thylakoids are stacked into columns called grana (singular: granum).
   1. Stacking increases the total membrane surface area in a small volume, so more photosystems and electron transport chains fit inside the chloroplast.
   2. The result is a higher rate of light absorption and ATP/NADPH production.
   3. Individual grana are connected by unstacked regions called intergranal lamellae (or stroma lamellae), which link the thylakoid compartments.

The Thylakoid Lumen: A Small Space for a Steep Gradient

1. The thylakoid lumen is the narrow fluid-filled space inside each thylakoid.
   1. Its small volume matters: when H+ ions are pumped in during the light-dependent reactions, the concentration rises quickly because there is so little space to fill.
   2. This creates a steep proton gradient across the thylakoid membrane.
2. H+ ions then flow back out of the lumen into the stroma through ATP synthase in the thylakoid membrane, and the energy of this flow drives the phosphorylation of ADP to ATP.
   1. This is chemiosmosis, the same principle as in mitochondria, but here the gradient is across the thylakoid membrane rather than the inner mitochondrial membrane.

The Stroma: Site of the Calvin Cycle

1. The stroma is the fluid region surrounding the thylakoids, enclosed by the inner membrane.
   1. It contains the enzymes for the Calvin cycle (light-independent reactions), including RuBisCO, which fixes CO2 by combining it with RuBP (ribulose bisphosphate).
2. Compartmentalisation helps here in two ways.
   1. The ATP and NADPH made in the thylakoid membranes are released directly into the stroma, so they are immediately available for the Calvin cycle.
   2. The enclosed stroma keeps optimal pH and ion concentrations for Calvin cycle enzymes.
3. The stroma also contains 70S ribosomes and circular DNA, like the mitochondrial matrix.
   1. This supports the endosymbiotic theory: chloroplasts are thought to have evolved from free-living photosynthetic cyanobacteria engulfed by an ancestral eukaryotic cell.