Photosynthesis and the Calvin Cycle
An anabolic process is one half of the idea we call metabolism. Anabolism is the building up of molecules using some kind of energy (for example the conversion of ATP to ADP in human cells to build up organelles). Another anabolic process is a process called photosynthesis.
Photosynthesis is a process that occurs only in plants. It is the way that sunlight is converted into stored energy to be used later to perform work by living things. Because plants can feed themselves, they are called autotrophs (auto meaning 'self', and troph meaning 'nourishing'). In other words, they use the process of photosynthesis to feed themselves using sun, air and earth. Animals are ultimately nourished by consuming autotrophs or some creature that does.
Most plants contain green organelles called chloroplasts. A chloroplast is made up of thousands of chlorophyll molecules. These molecules use light (photo) to make (synthesis) various chemical reactions. In most cases the first reaction involves the absorption of energy from the light of the sun (or even an artificial light) by the chloroplasts. The chlorophyll molecules' electrons are agitated and release an electron, with water present a Hydrogen ion is also split from H2O and together they attach to an electron acceptor molecule. This is the point at which plants release the oxygen which we breath as a leftover product from the water being split.
The energy released by the breakdown of the water molecule is used to create ATP from ADP. Some of the created ATP (with its stored, potential energy) is used in the second stage of photosynthesis. This is called the Calvin Cycle, named for the American scientist who discovered it, Melvin Calvin1. In this cycle the ATP releases energy which acts on Carbon Dioxide (CO2) and the electron receptor molecule mentioned above to create sugar molecules containing three Carbons. (The Calvin Cycle actually must go round six times just to make one sugar molecule.)
Some of these sugars enter the cytoplasm of the plant cell. They are subsequently converted to glucose, fats, and amino acids. These then serve various functions within the cell and as a source of stored energy for the plant. Animals eat the plants and also make use of this stored energy. Animals also benefit greatly from the fact that this process frees up considerable oxygen for respiration. The equation looks like this:
6CO2 + 6H2O = C6H12O6 + 6O2
The ADP and the electron receptors are recycled.
Interestingly enough the respiration (taking in of Carbon Dioxide and expelling of oxygen) occurs through microscopic pores in the leaves called stomata. Because the process of photosynthesis consumes water, on dry days the stomata may close down, not allowing the passage of moisture, or the gasses passing both ways. Without the Carbon Dioxide, the Calvin Cycle cannot occur. The plant's metabolic processes then slow down and even cease for a time2.
1. Melvin Calvin (1911-1997)
2. Origin of the Organic Soup