Chloroplast
Overview
The chloroplast is an organelle found in plant cells and eukaryotic algae that conduct photosynthesis. They absorb sunlight and use it in conjunction with water and carbon dioxide to produce glucose for the plant. Chloroplasts are a type of plastid, distinguished by their green color, the result of specialized chlorophyll pigments.
Structure
Chloroplasts are among the largest organelles, typically 5 to 10 micrometers in diameter and 1 to 3 micrometers in thickness. They are enclosed in a chloroplast envelope, which consists of an inner and an outer membrane with an intermembrane space in between.
Inside the chloroplast is the stroma, a dense fluid-filled inner compartment, which contains the chloroplast DNA and various enzymes. Within the stroma are stacks of thylakoids, the sub-organelles, the site of photosynthesis, which are arranged in stacks known as grana.
Function
The primary role of chloroplasts is to conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight and stores it in the energy storage molecules ATP and NADPH while freeing oxygen from water. They then use the ATP and NADPH to make glucose from carbon dioxide, in a process known as the Calvin cycle.
Origin
Chloroplasts are believed to have originated from cyanobacteria through the process of endosymbiosis. This is where a larger cell engulfed a free-living cyanobacterium, which then formed a symbiotic relationship with the host, eventually losing its independence and becoming an integral part of the cell as a chloroplast.
Evolution
The evolution of chloroplasts is closely tied to the evolution of photosynthetic organisms, particularly plants and algae. The first photosynthetic organisms were likely aquatic bacteria, some of which are thought to have undergone endosymbiosis to form the first chloroplasts.