Photosynthetic organisms, such as plants and algae, contain chloroplasts. A chloroplast is a membrane-bound organelle found in plants and some other organisms that captures energy from light and converts it into chemical energy. Chloroplasts are green because they contain chlorophyll, a green pigment used in photosynthesis to capture energy from light. Chloroplasts function in similar ways, regardless of the species of the organism.All green parts of plants contain chloroplasts, but the main sites of photosynthesis are the leaves. This is the reason that leaves are green, while other parts of the plant may be other colors. Leaves contain cells known as mesophyll cells, which are found in the interior of the leaf, and stomata on the exterior of the leaf. A mesophyll cell is the cell in a leaf that contains chloroplasts, where photosynthesis takes place, or where carbon fixation occurs in C4 plants. A stoma (plural, stomata) is a small opening mostly found on the undersides of leaves that allows for gas exchange between the plant and the external environment. Chloroplasts consist of two membranes (an outer membrane and an inner membrane) surrounding stroma, a fluid inside chloroplasts that contains thylakoids. A thylakoid is a membrane-bound sac inside the stroma that may be stacked into columns. Stacked thylakoid sacs where the light reaction of photosynthesis takes place is a granum (plural: grana). The space within each thylakoid sac is called lumen.
Other pigments besides chlorophyll may be involved in photosynthesis as well. Carotenoids may be red, orange, yellow, or brown. They cannot transfer captured light energy directly to the reactions that make up photosynthesis but must instead transfer their energy to chlorophyll. For this reason, they are known as accessory pigments. Phycobilins, another type of photosynthetic pigment, are not found in plants but are found in algae and cyanobacteria. These pigments may be red or blue. Unlike carotenoids, phycobilins may be directly involved in photosynthesis.