Photosynthesis is a complex process that involves a series of reactions. It can be summarized into two main reactions.
a) Light reaction/Light stage
This is the first stage of photosynthesis. It occurs in the presence of light. Without light it cannot take place.
Light stage occurs in the grana of the chloroplasts.
During light stage, two fundamental processes occur:i. Photolysis of water
This refers to the splitting of water molecules using sunlight energy to give hydrogen ions and oxygen gas.
This is aided by the fact that the grana contain chlorophyll molecules that trap sunlight energy for photolysis.
The oxygen gas produced can either be released into the atmosphere or be utilized by the plant for respiration.Water=Hydrogen atoms + Oxygen gas
ii. Formation of adenosine triphosphate (ATP)
Some of the sun light energy is used to combine Adenosine Diphospate molecule in the plant tissues with a phosphate molecule to form Adenosine Triphosphate (ATP). ATP is an energy rich molecule that stores energy for use in the dark stage when sunlight energy could be unavailable.ADP + P = ATP
The hydrogen ions and ATP formed during light stage are later used in dark stage.
b) Dark reaction/Dark stage
These reactions are light independent. The energy that propels these reactions are derived from the ATP formed during light stage.
Also known as carbon (IV) oxide fixation, dark stage involves combination of carbon (IV) oxide molecule with hydrogen ions to form a simple carbohydrate and a water molecule.
Dark reactions take place in the stroma.
CO2 + 4H+----(CH2O)n + H2O
Other food materials are then synthesized from the simple sugars through complex synthesis reactions. The simple sugar formed in dark stage is quickly converted to starch which is osmotically inactive.
When a lot of simple sugars accumulate in the chloroplasts, osmotic pressure of the guard cells would increase causing the guard cells to draw a lot of water through osmosis. This makes the guard cells to bulge and open the stomata. This can result into excessive water loss.
To prevent, this, the simple sugars are quickly converted to starch. To test whether photosynthesis has taken place in a leaf, therefore, a test for presence of starch and not simple sugars is carried out.