9.3: Anatomy of Chloroplasts

Green algae and plants, including green stems and unripe fruit, harbor chloroplasts—the vital organelles where photosynthesis takes place. In plants, the highest density of chloroplasts is found in the mesophyll cells of leaves.

A double membrane surrounds chloroplasts. 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, another set of membranes form 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 (or antenna) complexes. These complexes consist of proteins and pigments, such as chlorophyll, that capture light energy to perform the light-dependent reactions of photosynthesis. These processes release oxygen and produce chemical energy in the form of ATP and NADPH.

The second part of photosynthesis—the Calvin cycle—is light-independent and takes place in the stroma of the chloroplast. The Calvin cycle captures CO₂ and uses the ATP and NADPH to ultimately produce sugar.

Chloroplasts coordinate the two stages of photosynthesis. Photosynthesis releases oxygen and sugars—the basis of plant biomass which directly or indirectly feeds most life on Earth.

In plants, photosynthesis takes place within the leaf's dense mesophyll cell layers where the highest number of chloroplasts is contained.

Scattered throughout these specialized double membrane organelles are another set of compartments. Fluid filled membranous sacs called thylakoids, which are interconnected and form into multiple stacks called grana.

On the outer edges of each granum, embedded within the thylakoid membranes, are multiprotein complexes such as the photosystems. These structures contain the antenna proteins bound with numerous pigment molecules, like chlorophylls, to absorb light and begin the first stage of light dependent reactions.

Meanwhile the second stage, the Calvin cycle, takes place in the stroma, the aqueous cavity outside of the thylakoid's lipid bi-layer. With both processes working together, plants produce their own food thanks to the biochemical factories found in the chloroplast.