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PHOTOSYNTHESIS PROCESS

• Photosynthesis is a complex process that involves a series of reactions. It can be summarized into two main reactions.

1. 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:

1. 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

1. 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 à ATP

• The hydrogen ions and ATP formed during light stage are later used in dark stage.

1. 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.

SUMMERIZE THE LIGHT AND DARK STAGE OF PHOTOSYNTHESIS