5.2.1(b) - Chloroplast structure and photosynthesis sites
A chloroplast separates light capture from carbon fixation. Its internal membranes hold the light-dependent machinery, while the surrounding stroma contains enzymes that use its products to build organic molecules.
Two connected reaction sites
A chloroplast is a photosynthetic organelle found in plants and algae. An envelope of outer and inner membranes surrounds an internal membrane system immersed in the stroma. The envelope is distinct from the thylakoid membranes inside it.
The light-dependent reactions occur at thylakoid membranes and produce ATP and reduced NADP. The Calvin cycle occurs in the stroma and uses these products to fix carbon dioxide. Keeping components in different compartments allows the two stages to have different chemical conditions while exchanging the molecules they need.
Named Chloroplast Parts
The main structures to recognise are outer membrane, lamellae, grana, thylakoid, stroma and DNA.
Thylakoid
A thylakoid is a flattened membrane-bound sac inside a chloroplast.
A granum is a stack of thylakoids. Grana is the plural. Stacking gives a large membrane surface area for reactions associated with light absorption.
Lamellae are membrane regions that link grana. You may see the term inter-granal lamellae for lamellae between grana. They are part of the thylakoid membrane system, not separate fluid spaces.
The stroma is the fluid-filled region around the thylakoid membrane system. It contains enzymes needed for the light-independent stage. It is not the same as stomata, which are pores in the leaf epidermis.
The chloroplast envelope has an inner and an outer membrane; the outer membrane is its outermost boundary. The chloroplast contains circular DNA and small ribosomes in the stroma. These enable synthesis of some chloroplast proteins; other proteins are encoded by nuclear DNA and imported.
Use the diagram as a map: membrane structures are where the light-dependent stage is located; the surrounding fluid stroma is where the light-independent stage is located.
[DIAGRAM: chloroplast_structure: Chloroplast compartments and photosynthesis sites; asset_slug: 122_m05_2_1_chloroplast_structure_and_photosynthesis_sites__diagram_01; recommended_method: drawn_biology; description: Double chloroplast envelope enclosing stroma, circular DNA and membrane-connected grana, with individual thylakoid membranes and lumen distinguished.]
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