Chloroplasts are organelles found in plant cells, algae, and certain protists. Mesophyll cells contain dozens to hundreds of oval-shaped chloroplasts, ranging from 2 to 10 micrometers in length. They are the site of photosynthesis in plant cells, where light energy, carbon dioxide, and water are converted into glucose for energy storage and release oxygen.
Chloroplast Structure: Membrane, Thylakoid, Stroma
The internal structure of chloroplasts is not discernible under an optical microscope but can be visualized using an electron microscope. They consist of three main components: the chloroplast membranes, thylakoids, and stroma.
Chloroplast Membrane: Consisting of an inner and outer membrane, they separate the cytoplasm from the chloroplast stroma. There is a 20nm-wide gap between the inner and outer membranes. The molecules below a certain threshold (usually around 10 kDa) were allowed to pass the outer membrane via protein carriers. The inner membrane exhibits stronger selective permeability, controlling the movement of metabolites in and out of the chloroplast to maintain stable environment.
Thylakoid: Numerous disc-shaped vesicles called thylakoids are present in the stroma. They stack each other like coins to form cylindrical grana that are also connected by tubular structures called stromal thylakoids, and the space in all thylakoids are interconnected. Pigments and proteins on the thylakoid membranes participate in the light reactions of photosynthesis to convert the energy from sunlight into electron potential energy. This connected closed membrane increases the area to raise the photosynthetic rate and is also advantageous for creating a proton concentration gradient. The primary pigment on the thylakoid membrane is chlorophyll, responsible for giving leaves their green color.
Stroma: The space between the chloroplast membranes and thylakoids is filled with a gel-like substance called the stroma where the Calvin cycle of photosynthesis occurs. In addition to enzymes and metabolites, the stroma contains circular chloroplast DNA and ribosomes. This means that chloroplasts can autonomously synthesize some proteins they require. However, chloroplasts still depend on DNA within the cell nucleus and other organelles in the cytoplasm to complete the synthesis of other proteins. Therefore, they are referred to as semi-autonomous organelles, similar to chromoplasts and leucoplasts.