Photosynthesis

Photosynthesis

Photosynthesis converts light energy into the chemical energy of sugars and other organic compounds. This process consists of a series of chemical reactions that require carbon dioxide (CO₂) and water (H₂O) and store chemical energy in the form of sugar. Light energy from light drives the reactions. Oxygen (O₂) is a byproduct of photosynthesis and is released into the atmosphere. The following equation summarizes photosynthesis:

6 CO₂ + 6 H₂O → 6(CH₂O) + 6 O₂

sugar

Photosynthesis transfers electrons from water to energy-poor CO₂ molecules, forming energy-rich sugar molecules. This electron transfer is an example of an oxidation-reduction process: the water is oxidized (loses electrons) and the CO₂ is reduced (gains electrons). Photosynthesis uses light energy to drive the electrons from water to their more energetic states in the sugar products, thus converting solar energy into chemical energy.

Photochemical Reaction, a chemical reaction initiated by the absorption of energy in the form of light. The consequence of molecules’ absorbing light is the creation of transient excited states whose chemical and physical properties differ greatly from the original molecules. These new chemical species can fall apart, change to new structures, combine with each other or other molecules, or transfer electrons, hydrogen atoms, protons, or their electronic excitation energy to other molecules. Excited states are stronger acids and stronger reductants than the original ground states.

It is this last property that is crucial in the most important of all photochemical processes, photosynthesis, upon which almost all life on Earth depends. Through photosynthesis, plants convert the energy of sunlight into stored chemical energy by forming carbohydrates from atmospheric carbon dioxide and water and releasing molecular oxygen as a byproduct. Both carbohydrates and oxygen are needed to sustain animal life. Many other processes in nature are photochemical. The ability to see the world starts with a photochemical reaction in the eye, in which retinal, a molecule in the photoreceptor cell rhodopsin, isomerizes (or changes shape) about a double bond after absorbing light. Vitamin D, essential for normal bone and teeth development and kidney function, is formed in the skin of animals after exposure of the chemical 7-dehydrocholesterol to sunlight. Ozone protects Earth’s surface from intense, deep ultraviolet (UV) irradiation, which is damaging to DNA and is formed in the stratosphere by a photochemical dissociation(separation) of molecular oxygen (O2) into individual oxygen atoms, followed by subsequent reaction of those oxygen atoms with molecular oxygen to produce ozone (O3). UV radiation that does get through the ozone layer photochemically damages DNA, which in turn introduces mutations on its replication that can lead to skin cancer.

Photochemical reactions and the properties of excited states are also critical in many commercial

processes and devices. Photography and xerography are both based upon photochemical processes, while the manufacture of semiconductor chips or the preparation of masks for printing newspapers relies on UV light to destroy molecules in selected regions of polymer masks.

For more information:
http://www.wonderville.ca/asset/photosynthesis
http://wizznotes.com/biology/nutrition-in-plants/introduction