Photosynthesis — Definition, Reactants, Process, and More Explained

Photosynthesis- The conversion of Solar energy into Chemical energy

Photosynthesis is the procedure used by plants, algae, and certain bacteria (cyanobacteria or blue green algae) to convert light energy into chemical energy that can later be released to sustain the organisms’ activities.

The first photosynthetic organisms probably evolved early in the evolutionary history of life and probably utilized reducing agents such as hydrogen or hydrogen sulphide, instead of water, as sources of electrons. Cyanobacteria appeared later; the excess oxygen they produced contributed directly to the oxygenation of the Earth, which made the development of intricate life possible.

The structure and breaking of carbon-based material– from carbon dioxide to complex hydrocarbons- sugars (photosynthesis) then back to carbon dioxide (respiration)– becomes part of what is commonly called the global carbon cycle. Undoubtedly, the non-renewable fuel sources (fossils) we use to power our world today are the ancient remains of once-living organisms, and they offer a dramatic example of this cycle. The carbon cycle would not be possible without photosynthesis, because this process accounts for the “building” part of the cycle.

However, photosynthesis does not simply drive the carbon cycle– it likewise produces the oxygen essential for respiring organisms. Remarkably, although green plants contribute much of the oxygen in the air we breathe, phytoplankton and cyanobacteria world wide’s oceans are believed to produce between one-third and half of the climatic oxygen on Earth.


Photosynthesis can be defined as the process in which energy-poor inorganic oxidised compounds of carbon (i.e. C02) and hydrogen (i.e. generally water) are minimized to energy-rich carbohydrate (i.e. sugar-glucose) using the light energy that is taken in and converted into chemical energy by chlorophyll and some other photosynthetic pigments in green plants.

Formula for Chemical Reaction


Photosynthetic Reactants and Products

From the above total reaction of photosynthesis, it becomes apparent that CO2, water, and light are the reactants while glucose and oxygen are the products. Water appears on both sides of the equation since water is utilized as a reactant in some reactions and released as a product in others. However, because there is no net yield of H20, we can simplify the summary formula of photosynthesis for further discussion:


Photosynthesis utilizes the products of respiration and respiration uses the products of photosynthesis. There is another important difference between the two processes: Photosynthesis occurs only during the daytime, whereas respiration goes on day and night. During darkness, leaves (and other actively metabolizing cells) respire and utilize oxygen and release carbon dioxide.

At dawn and sunset, when light intensity is low, the rate of photosynthesis and respiration may, for a short time, equal one another. Thus, the oxygen released from photosynthesis is simply the quantity required for cellular respiration.

Compensation Point

The CO2 released by respiration simply equates to the amount required by photosynthesizing cells. At this moment there is no net gas exchange between the leaves and the atmosphere. This is called as compensation point.

As the light intensity increases, so does the rate of photosynthesis and for this reason, the requirement for more CO2 boosts which respiration alone can not provide. Similarly, the oxygen produced throughout photosynthesis is more than the requirement of the respiring cells, so the result is the net release of oxygen paired with the uptake of carbon dioxide.

Water and Photosynthesis

Oxygen produced throughout photosynthesis comes from water and is an important source of atmospheric oxygen which most organisms require for aerobic respiration and therefore for acquiring energy to live. In the 1930s, Van Niel assumed that plants divided water as a source of hydrogen, producing oxygen as a by-product. Niel’s hypothesis was based on his investigations on photosynthesis in bacteria that make carbohydrate from carbon dioxide, however, do not produce oxygen.

Niel’s hypothesis that the source of oxygen produced throughout photosynthesis is water and not CO2, was later confirmed by researchers throughout the 1940s when the first use of an isotopic tracer (O18) in the biological research study was made. Water and CO2 containing heavy-oxygen isotope O18 were prepared in the laboratory. Experimental green plants in one group were provided with H20 consisting of O18 and with CO2 including only common oxygen O16. Plants in the second group were supplied with H2O consisting of typical oxygen O16 however with CO2 containing O18.

It was found that plants of the first group produced O18 however the plants of the second group did not.


Water is therefore among the raw products of photosynthesis, other being CO2. Hydrogen produced by the splitting of water decreases NADP to NADPH2 (NADPH + H+).

NADPH is the “reducing agent” which, in addition to ATP likewise formed throughout ‘light reactions’, is used to lower CO2 to form sugar throughout ‘dark reactions.