Organisms are often divided into three main groups: producers, consumers, and decomposers. Each group plays a fundamental role in the food chain and without any of them life would not exist as it does today. Producers are also known as autotrophs or primary producers while consumers are known as heterotrophs or secondary producers. Autotrophs can produce their own food from materials in the environment using light or chemical energy. On the other hand, heterotrophs rely on other organisms, plants or animals, as a food source since they cannot synthesize their own food. Autotrophic organisms, such as plants, algae, and cyanobacteria, use photosynthesis or chemosynthesis to produce energy. The best-known organisms of these autotrophs In plants, proteins called photosynthetic reaction centers contain green chlorophyll that absorbs light energy. These proteins are contained within organelles called chloroplasts, which are abundant in leaf cells. In contrast, bacteria house proteins in the plasma membrane. Chloroplasts are found in the cells of green plants and photosynthetic algae where photosynthesis occurs. Inside the chloroplast there are folded, disc-shaped structures called thylakoids, which enclose chlorophyll in their membrane. Only some portions of the light spectrum can be absorbed and the spectrum of photosynthetic action depends on the type of accessory pigment present. Green plants mainly absorb red and blue wavelengths because the action spectrum corresponds to the absorption spectrum of chlorophylls and carotenoids. The color of the pigment comes from the wavelengths of the reflected light. Plants appear green because they reflect yellow and green wavelengths of light. Photosynthesis involves two sets of chemical events, called light-independent reactions that occur in the stroma and light-dependent reactions that occur in the lumen. They are also known as light and dark reactions; this terminology is somewhat ambiguous, because the entire process of photosynthesis is regulated so that it occurs when an organism absorbs visible light. Organized clusters of chlorophyll and beta-carotene in the thylakoid membrane are present for light-independent reactions that do not require light to occur, however, products of the light-dependent reaction such as ATP and NADPH are required to function. Within photosystem I, low-energy electrons are reenergized and passed through an electron transport chain where they are used to reduce the electron carrier NADP+ to NADPH. When the chloroplast receives a constant supply of photons, NADPH and ATP molecules are rapidly supplied to metabolic pathways in the stroma. Therefore ATP and NADPH formed during light-dependent reactions are used in the stroma to fuel the Calvin cycle reactions. Light-independent reactions use specific molecules to temporarily house energy. These are also called energy carriers and move energy from light-dependent to light-independent reactions. Once the energy is released, the energy carriers transmit it back to the light-dependent reaction to get more