5.1.1 Chloroplast Structures & their Functions

Structure of Chloroplasts and Their Adaptations

1. Thylakoid Membranes:

• Organised into stacks called grana, providing a large surface area for the attachment of chlorophyll, electron carriers, and enzymes involved in the light-dependent reactions.
• A network of proteins within the grana holds chlorophyll in a precise arrangement to maximise light absorption.

2. Granal Membrane Features:

• Embedded with ATP synthase channels, which allow the synthesis of ATP.
• Selectively permeable to facilitate the establishment of a proton gradient, essential for ATP production via chemiosmosis.

3. Stroma:

• The fluid-filled matrix surrounding the grana, where the light-independent reactions (Calvin Cycle) occur.
• Contains enzymes essential for the conversion of CO2 into organic molecules like glucose.

4. Chloroplast DNA and Ribosomes:

• Enable the synthesis of proteins required for the light-dependent reactions directly within the chloroplast.

5. Intergranal Lamellae:

• Thin, flat extensions connecting grana to facilitate the exchange of products between grana.

Key Concepts

• Photosynthesis occurs in two stages:
  • The light-dependent reactions in the grana.
  • The light-independent reactions (Calvin Cycle) in the stroma.

• Chloroplasts' structural adaptations ensure efficient capture of light energy, production of ATP, and synthesis of carbohydrates.