Photosynthesis has two types of reaction, first one is light reaction (Hill's reaction) and the other one is dark reaction (Blackman's reaction). In this presentation you learn full mechanism of how plants produce energy for their survival by photosynthesis.

1. PHOTOSYNTHESIS
MECHANISM

4. Photophosphorylation
mechanism

5. PHOTOSYNTHESIS
MECHANISM
LIGHT REACTION
(HILL’S REACTION)
NON CYCLIC
PHOSPHORYLATION
CYCLIC
PHOSPHORYLATION
DARK REACTION
(BLACKMAN’S
REACTION)
CALVIN CYCLE
(C3 CYCLE)
HATCH SLACK
PATHWAY
(C4 CYCLE)

6. LIGHT DEPENDENT REACTION
Photophosphorylation process
Hill’s reaction
Light reaction
Primary photochemical
reaction

7. PHOTOPHOSPHORYLATION
 Takes place in grana of chloroplast.
 STEPS
A. Absorption of light energy by chloroplast pigment
B. Transfer of light energy from accessory pigments to
chlorophyll a
C. Activation of chlorophyll molecules
D. Photolysis of water and evolution of oxygen
E. Electron transport and production of assimilatory
powers

8. A. ABSORPTION OF LIGHT BY
CHLOROPLAST
Different chloroplast pigments absorb
light in different regions of the visible
spectrum.

9. B. TRANSFER OF LIGHT ENERGY FROM
ACCESORY PIGMENT TO CHL - a
 Accessory & antenna pigments – all pigments
except chlorophyll – a.
 These absorb light and transfer to chlorophyll – a
to start the light reaction.
 Pigment system (I) - Photoreaction centre - P680
 Pigment system (II) – Photoreaction centre – P700

10. C. ACTIVATION OF CHLOROPHYLL
MOLECULES
 ON RECEIVING PHOTONS BY PS I & II, CHLOROPHYLL
MOLECULES GET EXCITED. THEY HAVE MORE ENERGY THAN
GROUND STATE ENERGY.
 AS A RESULT CHLOROPHLLY EXPELS AN ELECTRON AND
BECOMES POSITIVELY CHARGED ALONG WITH SOME
ENERGY.
Chlorophyll – a light excited triplet state of chlorophyll – a
Excited triplet state of chlorophyll – a (chl-a)+ + e-

11. D. PHOTOLYSIS OF WATER AND
EVOLUTION OF OXYGEN
 Occurs in Oxygen Evolving Complex (OEC) of PS –
II in presence of Mn++ & Cl- ions.
 When PS – II is active it receives light.
 Water molecules splits into H+ & OH- ions, this is
photolysis of water.
H2O H+ + OH-
2OH- + 2OH- 2H2O + O2 + e-
 Strong oxidant = Z

12. E. ELECTRON TRANSPORT SYSTEM AND
PRODUCTION OF ASSIMILATORY POWERS
 Released electron travel through number of electron
carrier.
 These again cycled back or used in reduction of NADP+
(Nicotinamide Adenine Dinucleotide phosphate) to NADPH+
H+.
 Extra light energy is used in ATP formation from ADP &
inorganic P.
 Photophosphorylation is of two types :-
Non cyclic
Cyclic

13. DIFFERENCE BETWEEN PS – I & II

14. NON – CYCLIC
PHOTOPHOSPHORYLATION

15. NON - CYCLIC PHOTOPHOSPHORYLATION

16. NON – CYCLIC PHOTOPHOSPHORYLATION

17. CYCLIC
PHOTOPHOSPHORYLATION

19. CYCLIC PHOTOPHOSPHORYLATION

21. CYCLIC PHOTOPHOSPHORYLATION

22. PHOTOPHOSPHORYLATION
Cyclic
photophosphorylation
1. Bundle sheath cells – C4
plants & photosynthetic
bacteria.
2. PS – I only.
3. Electron cycle around PS –
I.
4. Electron completes and
then repeats its cycle.
5. No oxygen evolution
Non cyclic
photophosphorylation
1. All photosynthetic
plants.
2. Both PS – I & II.
3. Electron moves zig-zag
from PS-II to PS – I.
4. Electron is drained Into
NADPH2 for dark
reaction.
5. Oxygen is evolved.

24. LIGHT REACTION AND DARK REACTION

25. CALVIN CYCLE

26. DARK REACTION
The synthesis of glucose in chloroplast by the way of CO2
reduction without the direct influence of light is called Dark
reaction. This is the second step in the mechanisms of
photosynthesis. It take place in the stroma of the chloroplast.
The dark reaction of photosynthesis is purely enzymatic and it
is slower than the light reaction. The dark reaction occurs
both in the day and night.
In Dark reaction, glucose is synthesised from CO2 by using
ATP and the assimilatory power(NADPH2) generated in the
light reaction. This process is called Carbon fixation or
Carbon reduction. In dark reaction, 2 types of cyclic
reactions occurs.

27. i) CALVIN CYCLE OR C3 CYCLE
ii) HATCH-SLACK CYCLE OR C4 CYCLE
CALVIN CYCLE
The synthesis of simple sugars in the dark reaction of
photosynthesis discovered by Calvin is called Calvin cycle.
Calvin and Benson traced the path of carbon during the
synthesis of sugars in a unicellular green alga chlorella using
radioactive C14 labelled CO2. For this discovery Calvin was
awarded the Nobel prize for the year 1961.

28.  The Calvin cycle is a Dark reaction because it does not need sunlight.
 CALVIN-BENSON CYCLE- refers to the set of light independent redox
reaction that takes place in the chloroplast during photosynthesis and
carbon fixation that would convert carbon dioxide into the glucose.
 The carbon and oxygen required for this process are obtained from the
ATP and NADPH produced during the photosynthesis process. The
conversion of CO2 to carbohydrate is called Calvin cycle or C3 cycle and
is named after Melvin Calvin who discovered. The plant undergo Calvin
cycle for carbon fixation are known as C3 plant.
 In C3 cycle, five carbon compound(ribulose 1,5-biphosphate) is reduced
by CO2 so that it is also known as reductive pentose phosphate
pathway(RPP).
OTHER NAMES FOR CALVIN CYCLE:

29.  Moreover Calvin cycle is also known as Calvin-Benson-Bassham(CBB) cycle, an
attribute to its discoverers: Melvin Calvin, James Bassham and Andrew Benson.
Calvin, Bassham and Benson discovered the cycle in 1960s at the university of
California in order. They used the radioactive carbon-14 in order to trace the path
of the carbon atoms in carbon fixation. They were able to trace the carbon-14 atom
soaking up its atmospheric carbon dioxide to is conversion into organic compounds
such as carbohydrate.

30. STAGES OF CALVIN CYCLE

31.  The Calvin cycle can described under three stages:
I) CARBOXYLATION
II) REDUCTION
III) REGENERATION
I) CARBOXYALTION
Carboxylation is the fixation of CO2 into a stable organic intermediate.
Carboxylation is the most crucial step of the Calvin cycle where CO2 is utilised for
the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBP
carboxylase which results in the formation of two -

32. -molecules of 3-PGA. Since this enzyme also has an oxygenation activity it
would be more correct to call it RuBP carboxylase-oxygenase or RuBisCO.
II) REDUCTION
These are a series of reaction that lead to the formation of glucose. The step
involve utilisation of 2 molecules of ATP for phosphorylation and two of NADPH
for reduction per CO2 molecule fixed. The fixation of six molecules of CO2 and
6 turns of the cycle are required for the removal of one molecule of glucose
from the pathway.
III) REGENERATION
Regeneration of the CO2 acceptor molecule RuBP is crucial if the cycle is to
continue uninterrupted. The regeneration steps require one ATP for
phosphorylation to form RuBP.

33. CALVIN CYCLE

34. The synthesis of simple sugars in the dark reaction of photosynthesis
discovered by CALVIN is called Calvin cycle. The first stable product of the
dark reaction is a 3-carbon compound( 3-phosphoglycerate). Hence, this dark
reaction is known as C3 cycle.
In the first step of C3 cycle, a five carbon compound(ribulose 1,5-
biphosphate) is reduced by CO2 so that it is also known as reductive pentose
pathway (RPP).
The C3 cycle occurs in all green plants.
Since the path of carbon in the C3 cycle was elucidated by Melvin Calvin in
1957, the C3 cycle is called Calvin cycle. Calvin first named this process as
carbon assimilation.

35. The Calvin cycle involves the following:
1. The CO2 is accepted by ribulose biphosphate( a 5-carbon compound present in
the stroma) to form an unstable intermediate 6- carbon compound.
Ribulose biphosphate + CO2 Rubisco 6-carbon compound
2. This 6-carbon compound reacts with one H2O molecule and splits into two
molecules of 3-phosphoglyceric acid( 3-phosphoglycerate). Both of these reaction
are catalysed by the enzyme Ribulose biphosphate carboxylase( Rubisco).
6 carbon compound + H2O Rubisco 3-phosphoglyceric acid + 2H+
3- phosphoglyceric acid is the first stable product of dark reaction of
photosynthesis. Thus 12 molecules of 3-phosphoglyceric acid and 12H+

36. are formed from 6CO2,6H2O and 6 RuBP molecules.
3. Each 3-phosphoglyceric acid molecules is phosphorylated by an ATP to produce a
1,3-diphosphoglyceric acid and ADP. This reaction is catalysed by the enzyme
phosphoglycerate kinase.
3-phosphoglyceric acid + ATP phosphoglycerate 1,3-diphosphoglyceric acid +
ADP kinase
4. The 1,3diphosphoglyceric acid is reduced by an NADPH2 to form a 3-
phosphoglyceraldhyde molecule and NADP. One H3PO4 molecule is released free.
This reaction is catalysed by the enzyme triose phosphate dehydrogenase.
1,3-diphosphoglyceric acid Triose phosphate 3-phosphoglyceraldehyde
dehydrogenase
NADP + H3PO4

37. Thus 12 molecules of 3-phosphoglyceraldehyde are formed from the 12 3-
phosphoglyceraldehyde molecules by consuming 12 ATP and 12 NADPH2.
5. In this step, the simple sugar called fructose 6 phosphate is synthesised from
the 3-phosphoglyceraldehyde. It involves the following steps
 Five molecules of 3-phosphoglyceraldehyde isomerise into dihydroxyacetone
phosphates.
3-phosphoglyceraldehyde Triose phosphate Dihydroxyacetone
isomerase
phosphate

38.  Three 3-phosphoglyceraldehyde molecules and three dihydroxyacetone
phosphate then unite in the presence of the enzyme aldolase to form
fructose 1,6-diphosphate.
3-phosphoglyceraldehyde + Dihydroxyacetone phosphate aldolase
Fructose 1,6 diphosphate
 The fructose 1,6-diohosphate is converted into fructose 6-phosphate by
the activity of the enzyme Fructose 1,6-phosphatase. Thus three fructose
6-phosphate molecules are formed.
Fructose 1,6-diphosphate Fructose 1,6 phosphatase fructose 6-phosphate
 Of these one molecule is used to produce glucose 6-phosphate which in
turn converted into sucrose while the other two molecules are use

39. in the regeneration of RuBP.
Fructose 6 phosphate phosphohexose Glucose 6-phosphate
isomerase
Glucose 6 phosphate + H2O phosphatase Glucose + Phosphate
6. Two molecules of 3-phosphoglyceraldyde react with two fructose-6-
phosphate in the presence of enzyme transketolase to form erythrose 4-
phosphate and xylulose-5-phosphate
3-phosphoglyceraldehyde + fructose-6-phosphate Transketolase
Erythrose 4-phosphate + xylulose-5-phosphate
7. Two Erythrose 4-phosphate molecules combine with two
dihydroxyacetone phosphate in the presence of the enzyme aldose to
form 2 Sedoheptulose-7-phosphate.

40. Erythrose-4-phosphate + Dihydroxyacetone phosphate Aldolase
sedoheptulos1,7- diphosphate
8. Sedoheptulose1,7-diphosphate loses one phosphate group by the action
of the enzyme Sedoheptulose1,7-phosphatase to form sedoheptulose-7-
phosphate.
Sedoheptulose1,7-diphosphate+2H2O sedoheptulose1,7-phosphatase + iP
9. Two sedoheptulose-7-phosphate molecules react with two molecules of 3-
phosphoglyceraldehyde in the presence of transketolase to form two
xylulose—phosphate and two ribose-5-phosphate molecules.
Sedoheptulose-7-phosphate + 3-phosphoglyceraldehyde Transketolase
Xylulose-5-phosphate + Ribose5-phosphate

41. 10. Two ribose 5-phosphate molecules are converted into
ribulose5-phosphate by the enzyme ribulose 5-phosphate
isomerase. Similarly, the four xylulose 5-phosphate molecules
are converted into ribulose 5-phosphate by the enzyme
ribulose 50phosphate epimerase.
Ribose 5-phosphate Ribulose 5-phosphate Ribulose 5-phosphate
isomerase
Xylulose 5-phosphate Ribulose 5-phosphate Ribulose 5-phosphate
epimerase
11. The six Ribulose 5-phosphate molecules undergo
phosphorylation with 6ATP to form six molecules of ribulose
1,5-biphosphate and 6ADP. The reaction is catalysed by the
enzyme phosphor ribulose kinase.

42. Thus,the Calvin cycle is completed.
Ribulose 5-phosphate + ATP phosphor ribulose kinase ribulose 1,5-diphosphate
+ ADP
SIGNIFICANCE OF CALVIN CYCLE
1. Calvin cycle enables the plant to accumulate food material in them.
2. Plant growth and yield are determined by the rate of dark reaction.
3. The vast reserve of energy in the form of coal, oil, peat and dung are
rely the outcomes of Calvin cycle in plants.
4. The entire living world depends on food synthesised in plants via
Calvin cycle

43. Hatch and slack pathway
( C4 pathway )

44. HATCH AND SLACK PATHWAY

45. DISCOVERY
 The first experiments indicating that some plants
do not use C3 carbon fixation but instead produce
malate and aspartate in the first step of carbon
fixation were done in the 1950s and early 1960s
by Hugo Peter Kortschak and Yuri Karpilov.
 The C4 pathway was elucidated by Marshall
Davidson Hatch and Charles Roger Slack, in
Australia, in 1966; it is sometimes called the
Hatch–Slack pathway.

46. HATCH AND SLACK

47.  PEP carboxylase is located in the mesophyll cells, on the leaf exterior near the
stomata.
 There is no rubisco in the mesophyll cells. CO2 entering the stomata is rapidly
fixed by PEP carboxylase into a 4-carbon compound, called malate, by attaching
the CO2 to PEP.
 The malate is then transported deeper into the leaf tissue to the bundle sheath
cells, which are both far away from the stomata (and thus far away from
oxygen) and contain rubisco.
 Once inside the bundle sheath cells, malate is decarboxylated to release
pyruvate and CO2; the CO2 is then fixed by rubisco as part of the Calvin cycle,
just like in C3 plants.
 Pyruvate then returns to the mesophyll cells, where a phosphate from ATP is
used to regenerate PEP.
 Thus in C4 plants, C4 carbon fixation has a net added cost of 1 ATP for every
CO2 delivered to rubisco; however, C4 plants are less likely to die of dehydration
compared to C3 plants in dry conditions.

48. C4 PATHWAY
 Inseveral plants like Sugarcane , Maize ,Euphorbia ,
Amaranthus , Sorghum , first stable product in dark
reaction is oxaloacetic acid (4C).
 First Co2 fixation occur in mesophyll chloroplast , while
second in bundle sheath .
 Two aspect of c4 model are:
 (a) Asparate formers
 (b) Malate formers

49.  There are two important adaptations that allow C4 plants
to do this:
=> First, C4 plants use an alternate enzyme for the first
step of carbon fixation. This enzyme is called
phosphoenolpyruvate (PEP) carboxylase, and it has no
oxygenase activity and has a much higher affinity for CO2
than rubisco. As the name “PEP carboxylase” suggests, the
enzyme attaches CO2 to a compound called
phosphoenolpyruvate (PEP).
=>Second, C4 plants have specialized leaf anatomy with
two different types of photosynthetic cells: mesophyll cells
(on the exterior of the leaf, near stomata) and bundle
sheath cells (in the interior of the leaf, far away from
stomata). Rubisco is located in bundle sheath cells, but
not in mesophyll cells.(KRANZ ANATOMY)

51. KRANZ ANATOMY

52. KRANZ ANATOMY
 The word Kranz means “wreath” or “ring”. Kranz anatomy is a
specialized structure in C4 plants where the mesophyll cells are
clustered around the bundle-sheath cells in a ring-like fashion.
 The number of chloroplasts in the bundle-sheath cells is more than
that in the mesophyll cells. This is found in C4 grasses such as maize
and a few dicots. The Kranz anatomy is developed in three different
steps:
 Initiation of procambium
 Bundle sheath and mesophyll cells specification
 Chloroplast development and integration of the C4 cycle

53. DIFFERENCE BETWEEN C3 AND C4
PATHWAY

54. PHOTOSYNTHESIS VIDEO
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55. THANK YOU
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