20.2: The Anatomy of Chloroplasts
Green algae and plants, including green stems and unripe fruit, harbor specialized organelles called chloroplasts to carry out photosynthesis. They coordinate both stages of photosynthesis — the light-dependent reactions and the light-independent reactions. The light-dependent reactions use sunlight to release oxygen and produce chemical energy in the form of ATP and NADPH, and the light-independent reactions capture CO2 and use ATP and NADPH to produce sugar.
Structure of Chloroplasts
A chloroplast is surrounded by a double membrane. The outer membrane faces the cytoplasm of the plant cell on one side and the intermembrane space of the chloroplast on the other. The inner membrane separates the narrow intermembrane space from the aqueous interior of the chloroplast called the stroma.
Within the stroma reside another set of membrane-bound disk-shaped compartments known as thylakoids. The interior of a thylakoid is called the thylakoid lumen. In most plant species, the thylakoids are interconnected and form stacks called grana. Embedded in the thylakoid membranes are multi-protein light-harvesting complexes that consist of proteins and pigments, such as chlorophyll.
Evolution of Chloroplasts
According to the endosymbiosis theory, chloroplasts evolved when an endosymbiont cyanobacterium was engulfed by a primitive eukaryotic cell. During the course of evolution, they slowly acquired new attributes within the eukaryotic cells while retaining some characteristics of their ancestral prokaryotes. For example, chloroplast genomes neither bind with histone-like proteins nor show complex packaging into chromosome-like structures similar to prokaryotic cells. In addition, chloroplasts undergo binary fission and equally separate their DNA into the daughter organelles, as observed in prokaryotes.
