Chloroplasts in Plant Cells

Overview

This lecture covers the structure, functions, and significance of chloroplasts in plant cells, emphasizing their key role in photosynthesis.

Chloroplasts are organelles in plant and algal cells where photosynthesis occurs.
The name comes from "chloros" (green) and "plast" (organelle).
Chloroplasts contain chlorophyll, a pigment that absorbs light energy.
They are believed to have evolved from free-living photosynthetic cyanobacteria via endosymbiosis.

Chloroplasts are generally round, oval, or disk-shaped, and are 4-6 μm in diameter and 1-3 μm thick.
They are distinguished by their green color due to chlorophyll a and b pigments.
Present mainly in the mesophyll cells of leaves.
Chloroplasts capture sunlight and use it to produce energy (ATP and NADPH).
Shapes vary across species; in algae, they can form large, network-like structures.

Outer Membrane: Semi-porous, allows small molecules and ions to pass.
Intermembrane Space: Thin space between inner and outer membranes.
Inner Membrane: Regulates passage of materials and is involved in lipid/carotenoid synthesis.
Stroma: Alkaline, protein-rich fluid containing DNA, ribosomes, starch granules, and enzymes.
Thylakoid System: Membranous sacs stacked into grana, where light-dependent reactions occur.

Absorbs light energy and converts it into chemical energy during photosynthesis.
Produces ATP and NADPH2 via photophosphorylation.
Generates oxygen through the photolysis (splitting) of water.
Facilitates carbon fixation and sugar synthesis in dark reactions (Calvin cycle in stroma).
Stores food as starch.
Plays a role in plant immune responses alongside other organelles.

Chloroplasts contain all components required for photosynthesis.
Light reactions take place on thylakoid membranes; dark reactions in the stroma.
Chloroplasts act as the site for all stages of photosynthesis, converting raw materials into sugars and oxygen.

Chloroplast — Plant cell organelle for photosynthesis.
Plastid — A group of plant organelles including chloroplasts.
Chlorophyll — Light-absorbing green pigment in chloroplasts.
Thylakoid — Membranous sac within chloroplasts, site of light reactions.
Stroma — Fluid inside chloroplast where dark reactions occur.
Grana — Stacks of thylakoids.
Photosynthesis — Process converting sunlight, water, and CO₂ into sugars and oxygen.
Endosymbiotic Theory — Theory that chloroplasts originated from ancient cyanobacteria.