Photosynthesis

1. Photosynthesis

2. • In 1881, Theodor Engelmann discovered that
photosynthesis occurs in the chloroplast.
• Until the 1930s, the O2 produced from photosynthesis
was believed to originate from CO2. L.B. Niel, a graduate
student at Stanford University at the time, hypothesized
that the O2 produced during photosynthesis came from
H2O rather than from CO2.
• In 1941, two scientists named Samuel Ruben and Martin
Kamen did just that and showed that the O2 released
during photosynthesis came from H2O. Used isotopes!!!
• Melvin Calvin (1954) Traced the path of CO2 assimilation
in photosynthesis and awarded the Nobel Prize in 1961.

11. • (i) Chlorophylls It is a green pigment which traps
solar radiation and convert light energy to the
chemical energy.
• (ii) Carotenoids These are yellow, brown and
orange pigments, which absorb light strongly in
blue-violet range. These are called shield
pigments, because they protect chlorophyll from
photo oxidation by light intensity and also from
oxygen produced during photosynthesis. Along
with chlorophyll-b, the cartenoids are also called
as accessory pigments, because they absorb
energy and give it to chlorophyll-a. carotenoids are
two types:

12. • (a) Carotenes: Carotenes consists of an open chain
conjugated double bond system ending on both the
sides with ionone rings. They are hydrocarbons with
molecular formula C40H56 carotenes are orange in
colour. The red colour of tomato and chillies is,
because of carotene called lycopene. The common
carotene is β-carotene which is converted to
vitamine-A by animals and humans
• (b) Xanthophylls: Also known as carotenols. These
are similar to carbon, but differ in having two oxygen
atoms is the form of hydroxyl, carboxyl group
attached to the ionone rings. Their molecular
formula is C40H56O2. The yellow colour of autumn
leaves is due to lutein and a characteristics
xanthophylls of brown algae is fucoxanthin.

13. • (iii) Phycobilins: Phycobilins consist of four pyrrol rings
and lack Mg and phytol tail. The phycobilin pigments are
of two types.
• (a) Blue – Phycocyanin, allophycocyanin
• (b) Red – phycoerythein These pigments are useful in
chromatic adaptations.
• Phycoerytherin transfer energy to phycocyanin which in
turn transfer energy to carotenoids which is ultimately
received by chlorophyll –a. The chlorophylls, carotenoids
and phycobilins together form a complex of pigment in
thylakoid membrane. These complexes work for the
absorption of light and its transfer to a reaction center.
These complexes are called photosynthetic unit or
photosystem or pigment system.

35. Nutrient assimilation
• Higher plants are autotrophic organisms that can synthesize all of their
organic molecular components out of inorganic nutrients obtained from
their surroundings. For many inorganic nutrients, this process involves
absorption from the soil by the roots and incorporation into the organic
compounds that are essential for growth and development. This
incorporation of inorganic nutrients into organic substances such as
pigments, enzyme cofactors, lipids, nucleic acids, and amino acids is
termed nutrient assimilation.

36. Nitrogen cycle
• Nitrogen is present in many forms in the biosphere. The atmosphere
contains vast quantities (about 78% by volume) of molecular nitrogen
(N2).
• For the most part, this large reservoir of nitrogen is not directly
available to living organisms. Acquisition of nitrogen from the
atmosphere requires the breaking of an exceptionally stable triple
covalent bond between two nitrogen atoms (N≡N) to produce ammonia
(NH3) or nitrate (NO3 – ). These reactions, known as nitrogen fixation,
occur through both industrial and natural processes.

37. • Today, the Earth’s atmosphere is about 78% nitrogen, about 21%
oxygen, and about 1% other gases. This is an ideal balance
because too much oxygen can actually be toxic to cells. In
addition, oxygen is flammable. Nitrogen, on the other hand, is
inert and harmless in its gaseous form. However, nitrogen gas is
not accessible to plants and animals for use in their cells.

39. Nitrogen fixation
• The following natural processes fix about 190 million metric tons per year of
nitrogen:
• Lightning. Lightning is responsible for about 8% of the nitrogen fixed by
natural processes. Lightning converts water vapor and oxygen into highly
reactive hydroxyl free radicals, free hydrogen atoms, and free oxygen
atoms that attack molecular nitrogen (N2) to form nitric acid (HNO3). This
nitric acid subsequently falls to Earth with rain.
• Photochemical reactions. Approximately 2% of the nitrogen fixed derives from
photochemical reactions between gaseous nitric oxide (NO) and ozone (O3)
that produce nitric acid (HNO3).

40. • Biological nitrogen fixation. The remaining 90% results from
biological nitrogen fixation, in which bacteria or cyanobacteria
(blue-green algae) fix N2 into ammonia (NH3). This ammonia
dissolves in water to form ammonium (NH4+):
NH 3 + H2O → NH4+ + OH–
• Once fixed into ammonia or nitrate, nitrogen enters a
biogeochemical cycle and passes through several organic or
inorganic forms before it eventually returns to molecular nitrogen.

41. Biological nitrogen fixation
• Certain bacteria or prokaryotes are capable of converting atmospheric nitrogen to ammonia.
This process is called biological nitrogen fixation. The enzyme nitrogenase converts
dinitrogen to ammonia. Nitrogen-fixing bacteria may be free-living or symbiotic. Some of the
free-living nitrogen fixers are Azotobacter, Beijernickia, Rhodospirillum, cyanobacteria, etc.
Examples of symbiotic nitrogen fixers are Rhizobium (in the root nodules of legumes)
and Frankia (in the root nodules of non-leguminous plants), etc.
• Symbiotic Nitrogen Fixation
• A species of bacteria called Rhizobium, help in nitrogen fixation. These bacteria live in the
roots of leguminous plants (e.g., pea and beans plants) and using certain types of enzymes,
they help in fixing nitrogen in the soil. During this biological process, they convert the non-
absorbable nitrogen form into a usable form. This form of nitrogen gets dissolved in the soil,
and plants absorb the modified nitrogen from the soil. This is the reason behind farmers
implementing crop rotation, where leguminous plants help to replenish nitrogen content in
the soil without the necessity of fertilizers.

44. • Ammonification:- Ammonification happens when microbes act on
organic material, such as animal manure or decomposing plant or animal
material and begin to convert it to a form of nitrogen that can be used by plants.
All plants under cultivation, except legumes get the nitrogen they require
through the soil. The first form of nitrogen produced by the process of
mineralization is ammonia, NH3. The NH3 in the soil then reacts with water to
form ammonium, NH4. This ammonium is held in the soils and is available for
use by plants.

45. • Nitrification :- The third stage, nitrification, also occurs in soils. During
nitrification the ammonia in the soils, produced during mineralization, is
converted into compounds called nitrites, NO2
−, and nitrates, NO3
−. Nitrates
can be used by plants and animals that consume the plants.
• Immobilization :- The fourth stage of the nitrogen cycle is immobilization,
sometimes described as the reverse of mineralization. These two processes
together control the amount of nitrogen in soils. Just like plants,
microorganisms living in the soil require nitrogen as an energy source. These
soil microorganisms pull nitrogen from the soil when the residues of
decomposing plants do not contain enough nitrogen. When microorganisms
take in ammonium (NH4
+) and nitrate (NO3
−), these forms of nitrogen are no
longer available to the plants and may cause nitrogen deficiency, or a lack of
nitrogen.

46. • Denitrification :- nitrogen returns to the air as nitrates are
converted to atmospheric nitrogen (N2) by bacteria through the
process we call Denitrification. This results in an overall loss of
nitrogen from soils, as the gaseous form of nitrogen moves into
the atmosphere, back where we began our story.
• Volatilisation
• Leaching