The Light and Dark Reactions of Photosynthesis

Photosynthesis is the process by which preen plants and other microorganisms use the energy inherent in sunlight to convert carbon dioxide and water into the simple sugar known as glucose. Photosynthesis therefore provides the basic energy source for all organisms since plants are essentially sugar factories that create millions of glucose molecules per second.

Glucose is a compound that is the end product of starch metabolism and is used by the plants as an energy source to build leaves, fruits, flowers and seed. Photosynthesis is localized in most plants and generally occurs in the leaves and green stems in specialized structures called chloroplasts. These chloroplasts contain a thylakoid membrane that contains hundreds of molecules of chlorophyll, this is a pigment that traps light and is essential for photosynthesis. Other light trapping pigments, enzymes and other compounds and molecules needed for photosynthesis are located in the thylakoid membrane. Photosynthesis involves two complex processes occurring sequentially, they are termed the light dependent and light independent reactions.

In the light dependent reactions, light is absorbed by chlorophyll which then excites the electrons in the chlorophyll molecule. The light reactions generally occur in the thylakoid membranes which contain the light harvesting proteins, reaction centers, electron transport chains and ATP synthase proteins. The electrons are passed through a series of electron carriers and ATP is produced along with the splitting of water that produces oxygen.

In the light independent stage, the energy that is required for the light independent reactions is supplied by ATP and NADPH molecules that are produced in the light dependent reactions. The light independent stage or dark reactions of photosynthesis takes place in the stroma within the chloroplast converts carbon dioxide into sugar.

The rapid conversion of sugar into starch (a storage polysaccharide) enables researchers to use iodine to test for photosynthesis. In many leaves, excess sugar is rapidly converted to starch and gives an intense blue-black color with contact with iodine.

Plants break down these sugar molecules during respiration to produce energy in the form of the molecule ATP, the rate of respiration is generally relative to that of respiration. A plant in the dark will respire but will not carry out all the stages of photosynthesis. The carbon dioxide produced by respiration will not be used in photosynthesis resulting in a net accumulation of carbon dioxide in its general area. A plant in light however is able to carry out all the stages of photosynthesis and results in a net loss of carbon dioxide from its surroundings.

References
1. http://www.wiziq.com/tutorial/6486-Light-and-Dark-reactions-in-photosynthesis
2. http://biology.about.com/od/plantbiology/a/aa050605a.htm
3. http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/dark.htm