A chloroplast is a type of structure, called an organelle, that is found in plants and is where photosynthesis occurs. Normally present in plant leaves, chloroplasts contain all of the components that allow the plant to convert sunlight into usable energy. The main components of chloroplasts are the membranes, chlorophyll and other pigments, grana and stroma.
Chloroplasts are one of the most important components of a plant because the entire photosynthetic process takes place in them. Each cell in a plant leaf can have 50 of these organelles. Chloroplasts appear only in eukaryokic organisms, which are primarily non-animal.
There are three types of membranes in chloroplasts: the outer membrane, the inner membrane and the thylakoid membranes. The outer membrane surrounds the chloroplast and allows molecules to move into and out of the organelle without discretion. The inner membrane is located underneath the outer and is more discriminatory about what it allows in and out of the chloroplast. The thylakoid membranes lie inside the inner membrane and are arranged in stacks that are connected together by stromal lamellae. These lamellae serve as a framework or skeleton for each chloroplast.
Chlorophyll is a green pigment that gathers the sunlight needed for photosynthesis. Located in the thylakoid membranes, chlorophyll is what causes the green coloration of leaves. Other pigments, such as carotenoids, which make carrots orange, are also found in the thylakoid membranes.
Usually, these other pigments are found in much smaller quantities than chlorophyll. Each pigment absorbs different wavelengths of light. For example, chlorophyll absorbs every wavelength but green, which is why the pigment appears green to the eye.
The grana are stacks of thylakoid membranes. Each granum stores the sunlight obtained by the chlorophyll and obtains water (Hsub2O) and carbon dioxide (CO2) from other parts of the leaves to form a type of sugar (C6H12O6) that the plant uses for food. This is the light-dependent process of photosynthesis. Sugar that is not immediately used by the plant is converted into adenosine triphosphate (ATP) and is stored for later use. This process also occurs in the grana.
The stroma is a gel-like substance that surrounds the thylakoid membranes in each chloroplast. Enzymes in the stroma take the ATP and convert it back into sugars that are then used by the plants. This process is called a dark reaction because, unlike the light-dependent reactions, it does not rely on sunlight to complete. The conversion of ATP to sugar is known as the Calvin cycle.