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Photosynthesis - Lesson Summary

Life processes
The processes performed by living organisms in order to maintain and continue life are called as life processes.  Life processes include nutrition, respiration, circulation or transport, excretion, and reproduction.

It is the process by which organisms can assimilate and utilise food for their basic needs. Nutrition is of two different types, namely, autotrophic and heterotrophic.

Photosynthesis is an autotrophic mode of nutrition by plants and some bacteria. Photosynthesis is the physico-chemical process by which plants can convert light energy into chemical energy, in the form of carbohydrate from simple inorganic substances like atmospheric carbon dioxide and water.

Basic raw materials for photosynthesis
Photosynthesis requires carbon dioxide, sunlight, water and chlorophyll as its basic raw materials.

Sites of photosynthesis
  • Leaves are considered to be the sites of photosynthesis. Hence, they are called as food factories of the plant.
  • Leaves possess small pores called as stomata on both their surfaces. Stomata are the structures which help in the exchange of gases. Opening and closing of the stomata are brought about by the guard cells present in them.
  • Leaves are rich in plastids. Green coloured plastids are chloroplasts rich in chlorophyll pigment. Chlorophyll  is responsible for trapping the energy from sunlight.

Overall reaction of photosynthesis

                                 Light Energy
6CO 2 + 12H 2O     Chlorophyll →       C 6H 12O 6 + 6O 2 + 6H 2O  
Photosynthesis involves two types of reactions namely, light reactions and dark reactions.

a) Light reactions: Light reactions are light dependent reactions. These reactions happen only in the presence of sunlight. The photosynthetic pigments trap the energy from the sunlight. PS-I and PS-II collectively bring in the cyclic photophosphorylation and non-cyclic photophosphorylation reactions. As a result of these reactions energy rich molecules like ATP and NADPH 2 are synthesised. Photolysis of water molecules results in the release of oxygen as a by-product.  
          H2O → 2H + + 2e - + ½O 2

b) Dark reactions: Dark reactions are independent of light. Energy rich molecules like ATP andNADPH2 are utilised in these reactions. Dark reactions involve Calvin cycle during which carbon dioxide is reduced to carbohydrate.
C3 pathway is also known as Calvin cycle.C3 pathway involves set of carbon reactions which are catalysed by the enzyme Rubisco to synthesise 3 carbon compound, 3-phosphoglycerate from 5-carbon compound, Ribulose bisphosphate.
  • Ribulose bisphosphate is the primary acceptor of CO2.
  • Chloroplasts present are only of one type.
  • Anatomy of leaves does not resemble Kranz anatomy.
  • Phosphoglyceric acid is synthesised as the product.
  • Mesophyll cells exhibit Calvin cycle.
  • Photorespiration is observed.
  • Optimum temperature required for photosynthesis is between 20 degree celsius to 25 degree celsius.
  • This cycle is less efficient in utilising CO2.
               LIGHT REACTIONS           DARK REACTIONS These are light dependent reactions. These are light independent reactions. Splitting of water molecules releases oxygen as a by-product. Carbon dioxide is reduced to carbohydrates. Grana of chloroplasts are the sites of these reactions. Stroma of chloroplasts are the sites of these reactions. ATP and NADPH 2 are the energy rich compunds synthesised in these reactions. Energy rich compounds are utilised in the synthesis of carbohydrates.
Events in photosynthesis
Photosynthesis can be split into three basic events.

Factors affecting photosynthesis
Rate of photosynthesis depends on many factors like light, carbon dioxide, water and chlorophyll.  

a) Light: Visible spectrum of the sunlight ranges from 380 to 750 nanometres in wavelength.
  • Quality of the light also influences the rate of photosynthesis. Photosynthetic rate is higher in red and blue light. It is very poor in green light.
  • Intensity of light also determines the rate of photosynthesis.
  • Day-length of plants is also an important factor for photosynthesis to be effective. 
  • Essentiality of sunlight and chlorophyll are demonstrated using starch test.
  • Essentiality of carbon dioxide is demonstrated by bell jar experiment.
  • Plants obtain water from the soil by the process of absorption performed by roots.

b) Carbon dioxide: Carbon dioxide occupies 0.04% of the total atmosphere.
  • Carbon dioxide plays an important role in providing carbon for the process of photosynthesis.
  • Increased concentration of carbon dioxide content enhances the rate of photosynthesis.
  • But too much of its concentration proves to be toxic to the plants.
  • Carbon dioxide is reduced to carbohydrate in the dark reaction.

c) Water: Water being a universal solvent, almost all the minerals present in the soil dissolve in it.
  • It plays a vital role in the process of photosynthesis.
  • Water along with minerals is absorbed by roots and is carried to the sites of photosynthesis through xylem tissue.
  • 1% of absorbed water is utilized and the remaining water is released during photosynthesis.
  • Water serves as a source for oxygen which is released as a by-product. 

d) Chlorophyll: Chlorophyll, green coloured pigment, present in the chloroplasts plays a vital role in the process of photosynthesis.
  • Chlorophyll absorbs blue light effectively and then the red light, but proves to be a poor absorber of green light.
  • It traps the solar energy and converts into chemical energy which is utilised in the dark reaction of photosynthesis to form glucose.
  • Chlorophyll a and chlorophyll b are most prominent forms of chlorophyll found in plants.


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