Photosynthesis - Nature's green energy

Justin Kander Published 12/31/1969 Science

Photosynthesis is one of the most important processes on Earth, if not the most important. Without it, life could not have thrived and evolved into what it is today, and the biotic organisms that inhabit the planet today would be very different, possibly inexistent. Also, some of the abiotic factors of the planet would also be different, mainly the atmosphere.

By definition, photosynthesis is the process by which plants use light energy to make food (glucose) from carbon dioxide and water. The chemical equation for photosynthesis is 6 CO2 + 12H2O ---> C6H12O6 (glucose) + 6 H2O + 6 O2. Not seen in the equation is light (which would normally go above the arrow). The light from the sun provides energy for the plant to change water and carbon dioxide into glucose. However, it's not that simple, in fact, photosynthesis is probably the most complicated process on Earth.

Photosynthesis comprises of two stages. The first stage of photosynthesis is called the Light Reactions, because that is of photosynthesis where the plant incorporates light. In the light reactions, light energy is converted to chemical energy and O2 is produced as a waste product. The light reaction takes place in the chloroplast of the plant cell. Light is absorbed by the chlorophyll and is later used to power photosynthesis. The light is actually used to make ATP, the chemical energy molecule, from ADP and phosphate. Light is also used to excite electrons, which are used to reduce NADP+ to NADPH, with the assistance of enzymes and an extra H+ molecule. So, the purpose of the light reactions in photosynthesis is to use solar energy to make the energy molecules ATP and NADPH. The final product of photosynthesis, sugar, is not made in this step, and carbon dioxide is not used until the next step, the "Dark Reactions", more commonly known as the Calvin Cycle.

The Calvin Cycle occurs in the chloroplast too, but in a different part. The Calvin Cycle is the part of photosynthesis which actually uses the carbon dioxide and makes glucose. Using the carbon from carbon dioxide, the energy provided by ATP, and the excited electrons carried by NADPH, the Calvin Cycle produces a sugar molecule called glyceraldehyde 3-phosphate, which later is broken down into glucose.

The Light Reactions are actually broken down into two processes, called Photosystem I and Photosystem II. Photosystem II actually occurs first, but they are named in the order of which they were discovered. Both photosystems absorb light energy. In Photosystem II, light energy is used to break apart H2O and create 2 H+ molecules (used in NADPH) and 1/2 an O2 molecule, or an oxygen atom. However, that oxygen atom immediately combines with another oxygen atom, and diffuses out of the plant as an oxygen molecule. The 2 H+ molecules and the 2 electrons are used in Photosystem I to create NADPH, which of course moves onto the Calvin Cycle.

Photosynthesis does have additional molecules that are involved other then the ones listed above, and although they are crucial to the process, it is not imperative that they be known.

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