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12. Photosynthesis

-Created by-

Shri. Deshmukh A. B.

Asst. Teacher,

Agasti Art’s, Commerce and Dadasaheb Rupwate Science College,

Akole, Dist. Ahmednagar

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Part -I

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  • Photosynthesis is the only process on earth by which solar energy is trapped by green plants and converted into food.
  • Photosynthesis may be defined as synthesis of carbohydrates ( glucose) from inorganic materials like CO2 and H2O with the help of solar energy trapped by pigments like chlorophyll.

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6CO2

12H2O

C6H12O6

+

+

+

6O2

6H2O

Light

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Chlorophyll

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  • This process is unique to green plants.
  • It is the final light energy trapping process on which all life ultimately depends.
  • It is one of the most massive chemical processes going on earth.
  • Atmosphere contains only about 0.03 % CO2 by volume.
  • This small percentage represents 2200 billion tons of CO2 in the atmosphere.
  • The oceans contain over 50 times by amount of atmospheric CO2 in the form of dissolved gas or carbonates.
  • From these two sources, about 70 billion tons of carbon is fixed by the green plants annually.

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12.1 Chloroplasts :

  • These are mainly located in the mesophyll cells of leaves.
  • The CO2 reaches them through the stomata and water reaches them through veins.

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  • In higher plants, the chloroplasts are discoid or lens-shaped.
  • Each chloroplast is bounded by double membrane.

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  • Inside the membranes is found a ground substance, the stroma.
  • Inside the stroma is found a system of chlorophyll bearing double membrane sacs or lamellae.
  • These are stacked one above the other to form grana (singular, granum).
  • Individual sacs in each granum are known as thylakoids.

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  • All the pigments such as chlorophylls, carotenes and xanthophylls are located in the thylakoid membranes.
  • These pigments absorb light of a specific spectrum in the visible region.
  • The pigments are fat soluble and located in the lipid part of the membrane.
  • With the help of certain enzymes, they participate in the conversion of solar energy into ATP and NADPH.
  • The enzymes of stroma utilize ATP and NADPH to produce carbohydrates.

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    • Pigments
      • Chlorophylls
        • 1. Chl.-a
        • 2. Chl.-b
        • 3. Chl.-c
        • 4. Chl.-d
        • 5. Chl.-e
        • 6. Bacteriochlorophyll
        • 7. Bacterioviridin

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      • Carotenoids
        • 1. Carotenes
        • 2. Xanthophylls

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      • Phycobilins
        • 1. Phycocyanins
        • 2. Phycoerythrins

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Two predominant types of chlorophylls :

  • Chlorophyll a and b differ in the nature of groups.
  • Chlorophyll a has a methyl group (-CH3) while chlorophyll b has an aldehyde group (-CHO).
  • Chemically chlorophyll molecule consists of two parts
    • Head of tetrapyrrol the Porphyrin ring and
    • A long hydrocarbon tail called phytol attached to the porphyrin group.

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Phytol Tail

In case of Chl. b

Porphyrin ring

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  • Carotenoids are lipid compound present universally in almost all the higher plants and several micro-organisms.
  • They are usually red, orange, yellow, brown, and are associated with chlorophyll.
  • They are of two types - the carotenes and xanthophylls.
    • Carotenes (C40H56) - orange red
      • Major carotene found plant is β carotene.
    • Xanthophylls (C40H56O2) - yellow.
      • Lutein is the major xanthophyll present in plants.
  • They absorb primarily in the violet to blue regions of the spectrum.
  • The light energy absorbed by the carotenoids is transferred to Chl.-a to be utilized in photosynthesis.

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Phycobilins:

  • present only in BGA and red algae.
  • 2 types- 1. Phycocyanins (Blue)

2. Phycoerythrins (Red)

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In higher plants, there are chlorophyll-a, chlorophyll-b, carotene and xanthophyll as main photosynthetic pigments.

Anthocyanin, purple coloured pigment present in flower is not photosynthetic.

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  • All photosynthetic plants have these pigments.
  • They absorb light between the red and blue region of the spectrum.
  • They also protect the chlorophyll molecule from photo-oxidation.

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12.2 Nature of Light :

  • Light is a form of energy.
  • It travels as stream of tiny particles called photons.
  • A photon contains a quantum of light.
  • Light has different wavelengths having different colors.
  • One can see electromagnetic radiation with wavelengths ranging from 390nm to 730nm.
  • This part of the spectrum is called the Visible light.
  • It lies between wavelengths of ultraviolet and infra-red.

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Violet

Indigo

Blue

Green

Yellow

Orange

Red

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  • Absorption and Action spectrum :
  • All the pigments of the chloroplast absorb light quanta or photons and transfer the absorbed energy to chlorophyll a.
  • The amount of light absorbed at each wavelength can be shown in the form of a graph.
  • It shows different curves at different wavelengths.
  • Such a curve which shows the amount of light absorbed at each wavelength is termed as Absorption spectrum.

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  • The absorption spectrum of chlorophyll a and b clearly shows that
    • more light energy is absorbed at blue, violet and red wavelengths of the visible spectrum.
  • The relative rate of photosynthesis at different wavelengths indicates close relationship with absorption spectrum of chlorophyll a and b.
  • This curve that shows the rate of photosynthesis at different wavelengths is called Action Spectrum.

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  • Action spectrum of photosynthesis differs from the absorption spectrum.
  • There is quite a lot of photosynthetic activity even in parts of the spectrum where chlorophyll a absorb little light.
  • This infers that the light energy absorbed by other pigments is transferred to chlorophyll a.

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12.3 Mechanism of Photosynthesis :

  • In 1931, Van Neil proved that bacteria used H2S and CO2 to synthesize carbohydrates as follows :
  • 6CO2 + 12H2S C6H12O6 + 6H2O + 12S ↓
  • This led Van Neil to postulate that in green plants, water is utilized in place of H2S and O2 is evolved in place of sulphur.
  • Ruben (in 1941) confirmed it in Chlorella.
  • He used water labeled with heavy oxygen (18O2) i.e. H218O.

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  • The oxygen evolved contain 18O2 thereby proving Van Neil’s hypothesis that oxygen evolved in photosynthesis comes from water.
  • This leads to the currently accepted general equation of photosynthesis –

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  • 6CO 2 + 12H218O C6H12O6+ 6H2O + 618O2

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Light

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  • In 1937, R. Hill demonstrated that isolated chloroplasts evolved oxygen when they were illuminated in the presence of a suitable electron acceptor such as ferricyanide.
  • Ferricyanide is reduced to ferrocyanide by photolysis of water.
  • This is called Hill reaction.

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  • Thus Hill reaction proves that :
  • i. In photosynthesis, oxygen is released from water.
  • ii. Electrons for the reduction of CO2 are obtained from water.
  • According to Arnon, in this process light energy is converted to chemical energy.
  • This energy is stored in ATP and NADPH is formed as hydrogen donor.
  • This ATP formation is known as photophosphorylation.

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  • In modern concept, the process of photosynthesis is an oxidation and reduction process in which
    • water is oxidized (to release O2) and CO2 is reduced to form sugar.
  • It consists of two successive series of reactions.
    • The first reaction requires light and is called Light or Hill reaction.
    • Second reaction does not require light and is called Dark or Blackman reaction.
  • Of the two reactions, the former is a photochemical reaction, while the latter is a biochemical reaction.

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