different methods in plants to conserve energy for their essential life process.

1. ENERGY
CONSERVATION
IN PLANTS

2. INTRODUCTION
• It is the synthesis of carbohydrates by green plants in the
presence of light by using CO2 and water.
• In this process light energy is converted into chemical
energy.
6CO2 12H2O
+
Sunlight
C6H12O6 6H2O 6O2
+ +
• Photosynthesis is an anabloic, endergonic and reductive
process in which CO2 is reduced to carbohydrates.
• 90% of total photosynthesis in world is carried out by the
algae present in ocean and fresh water.

3. PHOTOSYNTHETIC APPARATUS
• Photosynthesis takes place in chloroplast.

4. HISTORY OF PHOTOSYNTHESIS
• First of all Stephen Hales pointed out that air and light
are necessary for nourishment.
• DeSausser observed that CO2 is absorbed by plants
and O2 is evolved.
• Mayer recognized that green plants utilize light energy
and convert it into chemical energy.
• Blackmann established photosynthesis as two step
process having light and dark reactions.
• Robert Hill discovered light reaction of photosynthesis.
• Arnon discovered photo systems I and II.

5. WHY ARE PLANTS GREEN?
Plant Cells
have Green
Chloroplasts
The thylakoid
membrane of the
chloroplast is
impregnated with
photosynthetic
pigments (i.e.,
chlorophylls,
carotenoids).

6. • Chloroplasts
absorb light
energy and
convert it to
chemical energy
Light
Reflected
light
Absorbed
light
Transmitted
light
Chloroplast
THE COLOR OF LIGHT SEEN IS THE
COLOR NOT ABSORBED

7. • The location and structure of chloroplasts
LEAF CROSS SECTION MESOPHYLL CELL
LEAF
Chloroplast
Mesophyll
CHLOROPLAST Intermembrane space
Outer
membrane
Inner
membrane
Thylakoid
compartment
Thylakoid
Stroma
Granum
Stroma
Grana

8. • The Calvin cycle makes
sugar from carbon
dioxide
– ATP generated by the light
reactions provides the energy
for sugar synthesis
– The NADPH produced by the
light reactions provides the
electrons for the reduction of
carbon dioxide to glucose
Light
Chloroplast
Light
reactions
Calvin
cycle
NADP
ADP
+ P
• The light reactions
convert solar
energy to chemical
energy
– Produce ATP & NADPH
AN OVERVIEW OF PHOTOSYNTHESIS

9. PHOTOSYNTHETIC PIGMENTS
• Three types of pigments are involved in
photosynthesis, namely
• Chlorophylls
• Carotenoids, and
• Phycobellins.

10. Chlorophylls
• Is the green pigment in
plants, it is insoluble in
water but soluble in
plants.
• It is a magnesium
porphyrin compound.
• Chlorophyll a can be
emperically represented
as C55H72O5N4Mg and
chlorophyll b as
C55H70O6N4Mg

11. Types of chlorophyll
• Chlorophyll a – found in all oxygen evolving green
plants from algae to angiosperms.
• Chlorophyll b – present in chlorophyceae (green algae)
and from bryophytes to angiosperms
• Chlorophyll c – found in algal family Phaeophyceae
(brown algae)
• Chlorophyll d – found in algal family Rhodophyceae
(red algae)
• Chlorophyll e – found in algal family Xanthophyceae
(yellow green algae)

12. Carotenoids
• Insoluble in water but
soluble in organic
solvents.
• These are of two types:
– Carotenes: orange coloured.
Have general formula C40H56
– Xanthophyll: have general
formula C40H50O2.

13. Location and function of
carotenoids
• Carotenoids are present in
chloroplast around chlorophyll
molecules.
• They regulate transfer of light
energy to chlorophyll.
• They protect chlorophyll
molecule from photooxidation/
solarization.
• Carotenoids absorb blue-green
light.

14. Phycobellins
• These are water soluble
and consist of four pyrrol
rings.
• These occur mainly in
cyanobacteria and red
algae.

15. Utilization of light energy by
photosynthetic pigments
• Chief source of light is sun.
• Earth receives only 40% of total solar energy rest is
absorbed or scattered in space.
• Only 0.2% of this light is absorbed by green plants.
• Green plants absorb only visible light.

16. Absorption spectrum
• It is the graph showing the absorption of light by at
different wave lengths of light.
• This graph shows that absorption is maximum in blue
light followed by red light.

17. Action spectrum
• It the graph showing the rate of photosynthesis at
different wave lengths of light.
• Action spectrum of photosynthesis shows that rate is
maximum in red light followed by blue light.

18. LIGHT REACTION
• Discovered by Robert Hill
• It takes place in thalakoid membrane.
• It can occur only in presence of light so it is called as
light reaction.
• Light reaction can be studied under the following
heads:
A. Red drop and emerson effect
B. Photo systems I and II
C. Photophosphorylation.

19. Fig: Basic concept of energy transfer during photosynthetic light reaction.

20. A. Red drop and Emerson effect
• It was discovered by Emerson
• Red drop can be explained with the help of quantum
yield (defined as number of oxygen molecules evolved
per light quanta absorbed).
• Red drop can be explained as sharp decline of quantum
yield when chloroplasts are illuminated with far red light.
• Because this decline in QY takes place in red part of the
spectrum it is called as red drop.

21. Fig: Red drop effect. The quantum yield of photosynthesis (black curve) falls off drastically
for far-red light of wavelengths greater than 680 nm, indicating that far-red light alone is
inefficient in driving photosynthesis.

22. • Emerson found that QY can be increased to full
efficiency if supplementary red light of shorter
wavelength is provided simultaneously.
• This increase in QY when both red and far red light are
given is called as Emerson Enhancement Effect.
• The QY in combined beam is higher than sum total of
QY in separate beams.

23. Fig: Enhancement effect. The rate of photosynthesis when red and far-red light
are given together is greater than the sum of the rates when they are given
apart.

24. B. Photo Systems
• Discovery of red drop and enhancement effect led to the
discovery of two pigment systems.
• These are called as photo system I and photo system
II.
• PS I is mainly present in stroma lamellae.
– It is a pigments which contains carotenes, chlorophyll b and
different forms of chlorophyll a like Chl a 670, Chl a 680, Chl a
695, Chl a 677, Chl a 692, etc.
– Energy trapped by antenna molecules is transferred to reaction
centre (P-700).

26. • PS II is mainly present in grana lamellae.
– It contains xanthophyll, chlorophyll b and different
forms of chlorophyll a.
– Reaction centre of PS II is P-680.
– Reaction centre is a special type of chlorophyll a
molecule and it was discovered by KOK.

28. C. Photophosphorylation
• Discovered by Arnon
• It is the formation of ATP in the presence of light.
• It takes place in thalakoid membrane.
• It is of three types:
• Non-cyclic photophosphorylation
• Cyclic photophosphorylation, and
• Pseudocyclic photophosphorylation.

29. Non cyclic photophosphorylation
Fig: Z-scheme of photosynthesis.

30. Oxygen evolution cycle

31. • It takes place in thalakoid membrane.
• Two light quanta are required to move each electron.
• 8 light quanta move 4 electrons, which are then used to
reduce 2 NADP molecules which are further required to
reduce 1 CO2 molecule.
• Each water molecule gives 2 electrons.
• Excitation of chlorophyll in the first reaction of this
process after this photolysis of water takes place.
• Protons released after the photolysis of water
accumulate in intra-thalakoid space resulting in a proton
gradient. When these protons diffuse across the
thalakoid membrane into the stroma along proton
gradient, they release energy which is then used for the
synthesis of ATP.

32. • Photolysis of water occur with the help of PS II.
• Terminal electron acceptor is NADP.
• The non-cyclic electron transport takes the shape of “Z”
so it was called Z-scheme by Hill and Bendal.
• Arnon used the term assimilatory power for NADPH
and ATP.
• NADPH is also called reducing power.

33. Fig: Detailed Z scheme for O2-evolving photosynthetic organisms.

38. Synthesis of ATP
• Photophosphorylation – light dependent
synthesis of ATP – ARNON and co-workers.
• Very similar to oxidative phosphorylation in
mitochondria.
• Major feature – ATP synthesis is coupled with
electron transport.
• This means that
• No ATP in absence of electron transport
• No electron transfer in absence of ATP synthesis

39. Chemiosmotic mechanism
• In 1960s Peter Mitchell gave
chemiosmotic model.
• According to this model, the main deriving
force for the synthesis of ATP is an ion
gradient across a selectively permeable
membrane – proton gradient.
• The gradient creates a concentration
difference of protons across the
membrane.

41. • These sources of potential energy can be
used for the phosphorylation of ADP by
ATP synthase.
• Some important features of chemisomotic
model:
• Intact membrane system with low intrinsic
permeability to protons.
• Electron transport components in the membrane
arranged vectorially – to give proton motive force.
• Final element of chemiosmotic model is ATP
synthase system.

45. DARK REACTION
• It is also known as carbon fixation, Blackmann’s
reaction or light independent reaction.
• It can take place in the presence or absence of light.
• It is supported by light reaction because products of light
reaction are consumed here.
• In this reaction CO2 is reduced to glucose.
• It takes place in the stroma of chloroplast.

46. The C3 cycle
• First of all studied by Calvin and Benson (1954) in
Chlorella. BASHAM
• For this they got nobel prize in 1961.
• C3 cycle can be studied in three steps:
• Carboxylation
• Glycolytic reversal
• Regeneration of RUBP

48. • First stable product of this cycle is phosphoglyceric acid
(PGA) which is a 3-C compound so this cycle is also
called as C3-cycle.
• 1 RUBP except 1 CO2 molecule at a time so 6 turns of
this cycle are required to produce 1 glucose molecule.
• In this cycle 18 ATP and 12 NADPH are required for the
synthesis of 1 glucose molecule from 6 CO2.
• C3-cycle generally operates in temperate plants.
• Optimum temperature for C3-cycle is 10-25°C.

50. Photorespiration
• Term given by KrotKov and Decker.
• It is a special type of respiration that occurs in green
parts of C3 plants in the presence of light.
• It takes place in three organelles chloroplasts,
peroxisomes and mitochondria.
• Under high O2 and low CO2 concentration RUBP-
carboxylase shows oxygenase activity.
• Then it binds with O2 instead of CO2 and forms
phosphoglycolic acid (2C) and PGA.
• It is a wasteful process and results in loss of 25% of the
total photosynthetically fixed carbon.

52. C4 Cycle
• Kortshek reported the formation of dicarboxylic acid in
sugarcane and gave the idea of C4-Cycle.
• Later on it was confirmed by Hatch and Slack and they
proposed C4 cycle.
• Leaves in C4 plants show a special type of anatomy
known as Kranz anatomy, i.e. vascular bundle of leaves
are covered by bundle sheath cells.
• Bundle sheath chloroplast depend upon mesophyll
chloroplasts for their CO2 supply.

54. • In C4 plants primary CO2 acceptor is phospho enol
pyruvate (PEP).
• In C4 plants enzymes for both C3 and C4 cycle are
present.
• Enzymes for C3 cycle are present in bundle sheath cells
and that of C4 cycle are present in mesophyll cells.
• In C4 plants RUBP is the secondary CO2 acceptor.
• In C4 cycle every CO2 molecule had to be fixed twice, so
C4 pathway is more energy consuming than C3 pathway.
• In C4 plants photosynthetic rate is higher because
photorespiration does not take place.
• In C4 plants optimum temperature for the process is 30-45
°C.
• Examples of C4 plants: maize, sugarcane, sorghum, etc.

56. CAM cycle
• It is a type of dark reaction which takes place in
succulent xerophytes, first off all discovered in
Crassulaceae family.
• In CAM plants stomata open at night and are usually
closed during the day time, so CAM plants accept CO2
during the night.
• In CAM plants enzymes for both C3 and C4 cycle are
present in mesophyll cells.
• In C4 plants CO2 fixation and calvin cycle are separated
in space while in CAM plants they are separated in time.