Photosynthesis is a oxidation reduction process in which water is oxidized and carbon dioxide is reduced to carbohydrate level, the water and oxygen being by product.

1. PRESENTATION ON
TOPIC : PHOTOSYNTHESIS
PRESENTED BY
BAJRANG BALI
Dept. of Vegetable Science
M.Sc. Previous Year.

2. PHOTOSYNTHESIS
 Autotrophic Process: Plants and plant-like organisms
make their energy (glucose) from sunlight.
 Stored as carbohydrate in their bodies.
CO2 + 6H2O + sunlight  C6H12O6 + 6O2
 Occurs in chloroplast.
Photo means ‘light’ and synthesis means ‘to make’

3. OXIDATION REACTION
 The loss of electrons from a substance.
 Or the gain of oxygen.
glucose
6CO2 + 6H2O  C6H12O6 + 6O2
Oxidation
“Photosynthesis is a oxidation reduction process in which water is
oxidized and carbon dioxide is reduced to carbohydrate level, the water
and oxygen being by product.”

4. REDUCTION REACTION
 The gain of electrons to a substance.
 Or addition of H ion.
glucose
6CO2 + 6H2O  C6H12O6 + 6O2
Reduction

5.  Since Ruben Kamen (1941) demonstrated that the source
of liberated oxygen is water, the equation may be
corrected as
6CO2 + 12H2O C6H12O6 + 6H2 O + 6O2

6. CHLOROPLAST
GranumThylakoid
Stroma
Outer Membrane
Inner Membrane
 First observed by Antony Von leewenhock (1679).
 Small, green, discoid or ellipsoidal, 4-10 ưm dia or 1-3 ưm thick
 Lipo-proteinous two unit of membrane

7. IMPORTANCE IN BIO PRODUCTIVITY

8. Photosynthesis-starts to ecological food webs!

9. MECHANISM OF PHOTOSYNTHESIS
1. Light Reactions
(Occur inon the Grana or Thylakoids)
 Converts light energy into chemical energy; produces ATP
molecules to be used to fuel light-independent reaction.
Until 1930s it was thought that photosynthetic reaction is
reverse of respiration and oxygen evolved during the process
comes from CO2 and water combines with CO2 to produces
carbohydrate.
Photosynthesis
6 CO2 + 6 H2O C6 H12 O6 + 6 O2
In 1937, Robert hill demonstrated that isolated
chloroplasts evolved O2 when they were illuminated in the
presence of a suitable electron acceptor, such as ferricyanide.
The ferricyanide is reduced to ferrocyanide by photolysis of
water. This reaction is now called Hill reaction and it explains
that water is used as a source of electron for CO2 fixation and
O2 is evolved as a by product.

10. Ruben, Randall and Kamen (1941) using heavy isotope of
oxygen (O18) in their experiments provide the direct prove that
the oxygen evolved in photosynthesis comes from water and
not from carbon dioxide. When photosynthesis is allowed to
proceed in presence of H2O18 and normal CO2 the evolved
oxygen contains the heavy isotope. .
Green plants
2 H2O18 + CO2 (CH2O) + H2O + O2
18
Light
And if photosynthesis is allowed to proceed in presence of
CO2
18 and normal water (H2O), heavy oxygen is not evolved.
Green plants
2 H2O + CO2
18 O2 + (CH2O18) + HO2
18
Light
Thus the fate of different molecules can be summarized as
follows:
6CO2 + 12H2O C6H12O6 + 6H2 O + 6O2

11. Only two general types are used in green plant
photosynthesis
1. Chlorophylls
 Chl a (C55H72O5N4Mg) (Blue-Green)
 Main pigment in plants and cyanobacteria
 Only pigment that can act directly to convert light energy to
chemical energy
 Absorbs violet-blue (420 nm) and red light (660 nm)
 Chlorophyll have Mg+ in the center to capture the light
energy.
 Chl b (C55H70O6N4Mg) (Yellow-Green)
 Accessory pigment or secondary pigment absorbing light
wavelengths that chlorophyll a does not absorb (absorption
peak 452 and 642 nm).
Photosynthetic pigments

13. Evidences for the existence of PSI and PSII
 Red drop
 If the quantum yield of photosynthesis is measured at
different range of wave length most of the ranges are
remarkably constant however at the extreme red edge of
chlorophyll absorption (>680) the yield is drops drastically.
The phenomenon is called red drops.
Conclusion: It is proposed that at this wave length only
one photosystem remain in operation. As another
photosystem is not able function at this wave length
quantum yield drops.
 Quantum yield is as the no. of O2 molecules released per
light quanta absorbed. In the process, one molecule of O2 is
evolved utilizing 8 quanta energy. i.e. Quantum yield is 1/8 or
12%.
The two Photosystems

14.  Emerson enhancement effect
 The rate of photosynthesis was measured separately with
light of two different wave lengths and then the two beams
were used simultaneously. When exposed to a wavelength
more than 680 nm (far red region) a specific rate of
photosynthesis was observed. Likewise when the exposure
was given at wavelength less than 680 nm some other effect
was observed. When the system was exposed to the light of
both wavelengths simultaneously, the effect on
photosynthesis exceeded the sum of the two effects caused
separately. This provided evidences that the two pigment
systems worked in co-operation with each other and the
increase in photosynthesis was due to synergism. This
phenomenon is known as Emerson enhancement effect.

15. PS I
 The reaction centre chlorophyll of PS I absorb maximum at
700nm in its reduced state. It is named P700.
 PS I contain large amount of chl a, small amount of chl b
and some B carotene.
 It control the process of producing a strong reductant to
reduce NADP into NADH+H+.
PS II
 The reaction centre chlorophyll of PS II absorb maximum at
680nm in its reduced state. It is named P680.
 PS II contain large amount of chl b, some amount of chl a
and B carotene.
 It concerned with generation of strong oxidant and weak
reduction coupled with the release of oxygen.
Photosystem I and II

16.  The Z scheme illustrated electron transport and the
production of NADPH and ATP in chloroplast.
 The phenomenon of synthesis of ATP in light reaction of
photosynthesis is called photophosphorylation.
Light
ADP + Pi ATP
Chlorophyll
1. Non-cyclic photophosphorylation
2. Cyclic photophosphorylation
Electron transport chain and Photophosphorylation

17. 1. Non-cyclic photophosphorylation

18. 18
2. Cyclic photophosphorylation

19. 2. Dark Reactions
(Occur in Stroma)
 Light-independent: uses ATP produced to make simple sugars.
 The CO2 combines which compounds in sequential steps to form
intermediate compounds ultimately leading to the formation of sugars
and starch. This occurs by three pathways
1. Calvin cycle (C3 cycle)
2. Hatch-slack cycle (C4 cycle)
3. CAM cycle (CAM plants)
Calvin cycle (C3 cycle)
1. Carbon fixation/ Carboxylation
RuBP + CO2 → PGA
2. Reduction:
PGA is reduced to Glyceraldehyde 3 Phosphate
3. Regeneration of RuBP
 PGA is used to regenerate RuBP
.

21. Energy Requirement:
 In order to synthesized the equivalent of one molecule of hexose sugar,
Six molecules of CO2 are fixed at the expense of 18 ATP and 12
NADPH. In other words, the C3 cycle consumes two molecules of
NADPH and three molecules of ATP for every CO2 fixed.
6CO2 + 18ATP + 12NADPH C6H12O6+12NADP+ 6H++ 18ADP +17Pi
Hatch-slack cycle (C4 cycle)
1. Carboxylation
2. Breakdown
3. Splitting/Decarboxylation
4. Phosphorylation

22. Energy Requirement:
 Total energy requirement for fixing one molecule of CO2 in C4 cycle is 5 ATP plus 2
NADPH. This include the cost of concentrating CO2 with the bundle sheath cell i.e. 5 ATP
per CO2.

23. CAM CYCLE
 Fix CO2 at night and store as a 4 carbon molecule
 Keep stomata closed during day to prevent water loss.
 Acidification: (Day) and Deacidification: (Night)

25. LAW OF LIMITING FACTORS
 Liebig (1843) “When a process is governed by a number of
separate factors the rate of the process is limited by the pace
of the slowest factor.”
.

26.  Three factors limit photosynthesis from going any
faster: Light level, carbon dioxide level, and
temperature.
 Sometimes photosynthesis is limited by the level of
carbon dioxide. Even if there is plenty of light a plant
cannot photosynthesize if it has run out of carbon
dioxide.
 Temperature can be a limiting factor too. If it gets too
cold the rate of photosynthesis will slow right down;
equally, plants cease to be able to photosynthesize if it
gets.

27. THANK YOU…