11 Science
Ch. 12 Photosynthesis
Notes
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.
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.
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.
In higher plants, the chloroplasts are discoid or lens-shaped.
Each chloroplast is bounded by double membrane.
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.
All the pigments such as chlorophylls, carotenes and xanthophylls are located in the thylakoid membranes.
Pigments
Chlorophylls
1. Chl.-a
2. Chl.-b
3. Chl.-c
4. Chl.-d
5. Chl.-e
6. Bacteriochlorophyll
7. Bacterioviridin
Carotenoids
1. Carotenes
2. Xanthophylls
Phycobilins
1. Phycocyanins
2. Phycoerythrins
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.
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.
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.
Phycobilins:
present only in BGA and red algae.
2 types- 1. Phycocyanins (Blue)
2. Phycoerythrins (Red)
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.
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.
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.
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.
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.
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.
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.
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
6CO 2 + 12H218O -----------------> C6H12O6+ 6H2O + 618O2
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.
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.
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.
