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Stored Sunlight
Light is a form of electromagnetic radiation, which is energy that propagates as a wave of alternating electric and magnetic fields. Other types of electromagnetic radiation include radio waves, ultraviolet light and x-rays. The energy in light is associated with particles known as photons, which transfer some of their energy to other substances when they come in contact. In plants, a specialized substance called chlorophyll absorbs energy from photons and initiates a process that results in stored energy. The site of this energy transformation is the chloroplast, which contains chlorophyll.
Cellular Food
The basic form of stored cellular energy is a molecule known as adenosine triphosphate, abbreviated ATP. In the cells of non-photosynthetic organisms ATP is produced through a complex chemical reaction involving glucose and oxygen. This reaction converts energy stored in glucose molecules into energy stored in ATP molecules. The cells of photosynthetic organisms such as plants also produce ATP, but the process is fundamentally different because plant cells create molecular energy from the energy found in sunlight.
Cells and Organelles
Plant cells, like animal cells, contain a variety of specialized structures known as organelles. Examples include the nucleus, which governs cellular activity, and ribosomes, which synthesize proteins. Chloroplasts are also organelles. They have a specific purpose -- to convert sunlight into energy stored in the molecular bonds of ATP -- and they interact with the other organelles to support the overall biological activity of the cell. For example, the ATP produced in chloroplasts powers the protein synthesis that occurs in ribosomes.
From Photons to ATP
Chloroplasts are a component of the cell, but chloroplasts also have their own components. The internal structure of chloroplasts is so complex that they can be compared to a cell within a cell. They contain stroma, a semi-fluid substance analogous to cytoplasm; their own genetic information in the form of deoxyribonucleic acid, or DNA; and their own specialized ribosomes. The primary functional components of a chloroplast are thylakoids, which are arranged in stacks throughout the stroma. Chlorophyll contained in the thylakoids absorbs energy from photons and releases this energy in the form of electrons projected out of the thylakoids. When these electrons pass into the stroma, their energy fuels the production of ATP.