Mechanism of Photosynthesis:Nutrition:Biology Chapter 6

 

Sructure Of Chloroplast

Granum/grana

• membranous structures that are piled up into stacks like a stack of pancakes
• contain chlorophyll to trap sunlight situated in a gel-like matrix called stroma
• site where light reaction occur

Stroma

• colourless gel-like matrix
• enzymes responsible for dark reaction are found here
• starch grains are also found here
• site where dark reaction occur

First Stage of Photosynthesis

Light Reaction

1.Chlorophyll captures light energy

2.Light energy is used to split the water molecules into hydrogen ions and hydroxyl ions

3.Excites the electrons of chlorophyll molecules to higher level energy so that it can leave the chlorophyll molecules

4.The electrons that leave the chlorophyll molecules then combine with the hydrogen ion to produce hydrogen atom which will be used in the dark reaction

5.The hydroxyl ion then donates its electrons to the chlorophyll that had loses its electron just now to become hydroxyl groups

6.This hydroxyl groups will combine each other to form water and oxygen

Second Stage Of Photosynthesis

Dark Reaction

1.ATP and hydrogen atoms that are produced in the light reaction just now then move to the stroma to provide energy and reducing power for the dark reaction

2.Carbon dioxide is absorb through the stomata

3.Hydrogen atoms are used to fix carbon dioxide in a series of reactions catalysed by photosynthesis enzymes

4.Reduction of carbon dioxide into glucose occur

5.Glucose is the condensed into starch to be stored temporarily in the starch grain until it is needed.

6.Formation of starch is necessary:

- Precense of glucose will cause high level of cell osmosis pressure

- Starch is insoluble,thus didn't effect the osmosis of water

Photolysis Of Water 

                       light                   

24H2O   ------------------->  24H+   +  24OH

                  chlorophyll

(splitting of water molecules into ion H+   and  ion OH- )

Formation of Hydrogen Atom

24H+  + 24e --------> 24H

(by combination of hydrogen ions with the released electron)

Formation of Hydroxyl Group

24OH-  -  24e --------> 24OH

(by losing one electron from the hydroxyl ion)

Formation of Water And Oxygen Gas

24OH ------> 12H2O + 6CO2

(by combination of many hydroxyl groups)

Reduction of Carbon Dioxide Into Glucose

6CO2 + 24H --------> 6(CH2O) + 6H20

(by the help of hydrogen atom)

Overall Photosynthesis Equation

                             sunlight

6CO2 + 6H2O ------------------>  C6H12O6  + 6H20

                           Chlorophyll

Comparison of Light Reaction and Dark Reaction

Light Reaction

time of reaction - day

product of reaction - oxygen and water

site of reaction - grana

substances required - water

light energy - needed

Dark Reaction

time of reaction - day and night

product of reaction - glucose and water

site of reaction - stroma

substances required - carbon dioxide

light energy - no need