"Photosynthesis" comes from two Greek words meaning "light" and "putting together." In photosynthesis, plants use sunlight energy to put together carbon dioxide from the air and water from the soil to make sugar.
The net process of photosynthesis is described by the following equation:
6CO2 + 6H2O + Light Energy = C6H12O6 + 6O2
This equation simply means that carbon dioxide from the air and water combine in the presence of sunlight to form sugars; oxygen is released as a by-product of this reaction.
Photosynthesis is not a very efficient process. Of the sunlight reaching the surface of a leaf, approximately:
75% is evaporated
15% is reflected
5% is transmitted through the leaf
4% is converted to heat energy
1% is used in photosynthesis
In plants, algae, and cyanobacteria, pigments are the means by which the energy of sunlight is captured for photosynthesis. However, since each pigment reacts with only a narrow range of the spectrum, there is usually a need to produce several kinds of pigments, each of a different color, to capture more of the sun's energy.
The food we eat and the oxygen we breathe are both formed by plants (including algae) through photosynthesis. The power to drive this reaction comes from sunlight absorbed by chlorophyll in the chloroplasts of plants. At the present time, no known chemical system can be made to serve as a substitute for this process.
It is convenient to divide photosynthesis into four distinct phases, which together make up the complete process... The four phases are: (1) light absorption and energy delivery by antenna systems, (2) primary electron transfer in reaction centers, (3) energy stabilization by secondary processes, and (4) synthesis and export of stable products.
The terms light reactions and dark reactions have traditionally been used to describe the different phases of photosynthetic energy storage. The first three phases that we have identified make up the light reactions, and the fourth encompasses the dark reactions.
Light-Independent reactions (the Calvin Cycle) incorporate CO2 into sugar, the basic food source for all organisms. Thylakoid membranes are the sites of the Light-Dependent reactions, whereas the Calvin cycle occurs in the stroma.
Sugar production occurs in the light-independent reactions which are also called "dark" reactions (this is misleading because they require NADPH and ATP from the light reactions and do not occur in the dark).
In photosynthesis we will follow energy from light as it is converted through another electron transport chain via ATP and NADPH back into chemical energy in carbohydrate. Many of the energy yielding reactions in respiration are irreversible; the pathways cannot simply be run backwards.