Chloroplast Structure

Study Notes

Chloroplast Structure

Chloroplasts have three distinct membranes: outer, inner, and thylakoid membranes.
The thylakoid membrane forms flattened disc-like sacs called thylakoids, which contain energy-generating systems, including chlorophyll.
Individual thylakoids are interconnected via lamellae and tend to stack to form grana.

Chloroplast Compartments

Chloroplasts have three internal compartments: intermembrane space, stroma, and thylakoid space.
The stroma contains photosynthetic metabolic enzymes, ribosomes, DNA, and RNA.

Photosynthesis

Both stages of photosynthesis depend on the chloroplast.
Stage 1: Photolysis of H2O produces O2, ATP, and NADPH.
Stage 2: ATP and NADPH are used to reduce CO2, producing sugar.

Light-Capturing Systems

Light-absorbing chlorophyll, photosystem protein complexes, electron-transport chains, and ATP synthase are located on the thylakoid membrane.
Water is split by photosystem II on the side of the membrane facing the thylakoid space.

ATP Synthesis

The diffusion of H+ from the thylakoid space back to the stroma powers ATP synthase.
ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place.
Chloroplasts and mitochondria generate ATP by the same mechanism: chemiosmosis/chemiosmotic coupling.

Chemiosmosis

Chemiosmosis is a mechanism that uses energy stored in a transmembrane proton gradient to drive an energy-requiring process, such as ATP synthesis.
An electron transport chain pumps protons across a membrane as electrons are passed along a series of more electronegative carriers, creating a proton gradient.
ATP synthase harnesses the proton-motive force to generate ATP as H+ diffuses back across the membrane.

Comparison of Chemiosmosis

Chemiosmotic coupling is used in both mitochondria and chloroplasts to synthesize ATP.
The proton gradient produced in one phase serves as an energy store that can be used to drive various energy-requiring reactions.

Description

Test your knowledge of the basic structure of chloroplasts, including the different membranes and internal compartments. Learn about thylakoids, grana, and lamellae. Identify the components that make up this vital plant cell organelle.