Within the cells of plants and green algae are specialized organelles known as chloroplasts, the engines of photosynthesis. This process converts sunlight into chemical energy, producing organic compounds and oxygen. Found in all green tissues of a plant, they are particularly concentrated in the cells that make up the internal layers of leaves, where they sustain the plant and contribute to the energy base for many ecosystems.
The Chloroplast Envelope
A chloroplast is separated from the cell’s cytoplasm by a double-membrane structure called the chloroplast envelope. This envelope consists of an outer membrane and an inner membrane, separated by a narrow intermembrane space. This multi-layered boundary regulates the passage of materials between the chloroplast and its surrounding cellular environment.
The two membranes of the envelope have different permeability characteristics. The outer membrane is permeable to small molecules and ions because it contains channel-forming proteins called porins. In contrast, the inner membrane is highly selective and acts as the primary permeability barrier for the chloroplast. It is largely impermeable to ions and metabolites, which can only cross it with the help of specific transport proteins embedded within it.
The Stroma
Enclosed by the inner membrane is a large, fluid-filled space called the stroma. This gel-like matrix contains a rich mixture of enzymes, dissolved ions, and other molecules. The stroma is the site where carbon dioxide from the atmosphere is converted into carbohydrates through a series of biochemical reactions known as the Calvin cycle.
The stroma also houses the chloroplast’s own genetic system. This includes multiple copies of a circular DNA molecule, often called cpDNA, which contains between 60 and 200 genes. These genes code for some of the proteins and RNA molecules needed for photosynthesis. Also suspended in the stroma are ribosomes, which synthesize proteins from the instructions in the cpDNA, and starch grains where sugars may be stored temporarily.
The Thylakoid System
Embedded within the stroma is a third, internal membrane system known as the thylakoid membrane. This system is composed of numerous flattened, sac-like compartments called thylakoids. The thylakoid membrane encloses a single, continuous aqueous space known as the thylakoid lumen. This membrane network is the site of the light-dependent reactions of photosynthesis.
In many chloroplasts, the thylakoids are organized into neat stacks that resemble a pile of coins; these stacks are called grana. A chloroplast can contain anywhere from 10 to 100 grana. The grana are interconnected by a network of membrane bridges called stroma lamellae, or intergranal thylakoids. These connections ensure the entire thylakoid system forms a single, continuous functional compartment.
Advanced imaging has revealed that the stroma lamellae are organized as wide sheets that wind around the grana stacks in a helical pattern. This complex, three-dimensional arrangement creates an enormous surface area for housing the chlorophyll pigments and protein complexes that capture light energy. The specific organization into stacked and unstacked regions also helps to segregate different parts of the photosynthetic machinery.
Structural Roles in Photosynthesis
The internal organization of the chloroplast is directly tied to its function, as the distinct compartments provide the specific environments needed for each stage of photosynthesis. The chloroplast envelope maintains this controlled environment, ensuring the stroma has the required substrates and enzymes. It also allows the final products of photosynthesis to be exported to the rest of the cell.