The chloroplast is a crucial, oval-shaped, double-membrane-bound organelle found in plant and algal cells, the primary site of photosynthesis.

Structure:

Outer Membrane: Permeable, enclosing the organelle.

Inner Membrane: Less permeable, regulating molecule passage.

Intermembrane Space: Between the two membranes.

Stroma: Gel-like matrix filling the inner membrane, containing enzymes (e.g., RuBisCO for Calvin cycle), ribosomes, circular DNA, starch, and lipid droplets.

Thylakoids: Flattened, sac-like membranes within the stroma; site of light-dependent reactions.

Granum (plural: Grana): Stacks of 10-20 thylakoids, interconnected by stromal lamellae.

Thylakoid Lumen: Space inside the thylakoid, where the proton gradient is established.

Functions:

Photosynthesis: Primary function. Converts light energy into chemical energy (ATP, NADPH), then fixes atmospheric CO2 into glucose.

Light-Dependent Reactions: On thylakoid membranes. Chlorophyll captures light, splitting water, releasing O2, and synthesizing ATP/NADPH.

Light-Independent Reactions (Calvin Cycle): In the stroma. ATP/NADPH convert CO2 into sugars.

Starch Synthesis & Storage: Synthesizes and temporarily stores starch.

Fatty Acid & Amino Acid Synthesis: Involved in synthesizing some fatty acids and amino acids.

Immune Response: Plays roles in plant defense signaling.

Its intricate, compartmentalized structure allows efficient execution of photosynthesis, vital for plant life and global ecosystems.