SSC Andhra Pradesh Board

1. • Almost all plants are photosynthetic autotrophs,
– Autotrophs generate their own organic matter through
photosynthesis
– Sunlight energy is transformed to energy stored in the
form of chemical bonds
(a) Mosses, ferns, and
flowering plants
(b) Kelp
(c) Euglena (d) Cyanobacteria
THE BASICS OF PHOTOSYNTHESIS

2. 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2

3. WHYWHY AREARE PLAPLANTSNTS GREGREEN?EN?
Plant Cells
have Green
Chloroplasts
The thylakoid
membrane of the
chloroplast is
impregnated with
photosynthetic
pigments (i.e.,
chlorophylls,
carotenoids).

4. • Photosynthesis is the process by which
autotrophic organisms use light energy to
make sugar and oxygen gas from carbon
dioxide and water
AN OVERVIEW OF PHOTOSYNTHESIS
Carbon
dioxide
Water Glucose Oxygen
gas
PHOTOSYNTHESIS

5. • Chloroplasts absorb
light energy and
convert it to
chemical energy
through a serious of
reactions that occur
in chloroplast.
• The reactions of
photosynthesis or
two types.
Light
Reflected
light
Absorbed
light
Transmitted
light
Chloroplast
1.3 Mechanism of Photosynthesis

6. Types of reactions
Light Reaction
(Hill reaction)
Dark Reaction
The reactions of
photosynthesis that occur
in the presence of light is
called light reaction
During this phase the
energy required for Carbon
fixation is generated.
The reactions of
photosynthesis that occur
in the absence of light is
called dark reaction
.

7. • To understand photosynthesis let us know
the reactions that occur in the presence of
light. i:e Oxidation and reduction process
Oxidation:
1. Addition of oxygen atom
2. removal of hydrogen atom
3. removal of electrons from atom
Reduction:
1. Removal of oxygen atom
2. Addition of hydrogen atom
3. Addition of electrons from atom
1.Light Reactions

8. • When a substance is oxidised the hydrogen and
electrons removed from this substances are
transformed to another substance.
• The substance which accepts hydrogen and
electrons get reduced.
• The substances which gives electrons are called
DONAR and accepts electrons are called
ACCEPTOR.
• DH2 + A D + AH2
Reduced
Donar
Oxidized
acceptor
Oxidized
Donar
Reduced
Acceptor

9. • These acceptors which accept electrons
are called ELECTRON ACCEPTORS.
• Eg:
Ferridoxins
Adenine dinucleotide ( NAD)
Nicotinamide adenine dinucleotide phosphate ( NADP)
Plastoquinones
Cytochromes
(Fan in Personal computer)
When an electron acceptor is reduced it may transfer the Hydrogen and
electrons to another acceptor molecule

10. Mechanism of photosynthesis
1.During Ps Chlorophyll molecule traps solar energy and gets oxidized
• When light falls on the chlorophyll, energy present in the photons is
absorbed by the chlorophyll molecule.
• This energy pushes an electron in the chlorophyll molecule to a
higher energy level.
• This electron from chlorophyll is transferred to an electron acceptor
as a result chlorophyll gets oxidized by loosing an electron and the
acceptor gets reduced by accepting the electron from chlorophyll.
• Thus the energy present in the photon of the sun light is used to
eject an electron from chlorophyll. This is the basic mechanism by
which solar energy is trapped by chlorophyll.
• Chlorophyll Chlorophyll* chlorophyll + ẽ
• A + ẽ A--
+ ve
(Low energy state ) (High energy state) (Oxidized) ( Electron)
(Acceptor oxidized) (electron) (Acceptor reduced

11. 2.Chlorophyll splits water molecule and gets back its electron from water
• With the removal of electron, chlorophyll is in oxidation state.
• This oxidising power of chlorophyll is used for splitting of water
molecule to liberate electrons.
• By accepting these electrons, chlorophyll returns to its original state.
• Oxygen is formed when the chlorophyll splits water molecule.
• This oxygen escapes in to atmosphere.
• 2 H2O 4 H+ + 4 ẽ - + O2
• 4 ẽ - + 4 chl 4 chl
• 2 H2O + 4 Chl 4 Chl + 4H+ O2
• In this reaction water molecule is split by light activated chlorophyll . Hence this process is called PHOTOLYSIS
of water
• Light does not split water molecule directly it acts through chlorophyll molecule in PS II
(Water) (Protons) (Electrons) (oxygen)
(Water) (Chlorophyll oxidised) (Chlorophyll reduced)

12. 3.ATP AND NADPH are formed during photosynthesis
• Protons (derived from splitting of water molecule) are left behind are
accumulate in thylokoids.
• When their concentrations becomes sufficiently high they are
transported across the thylokoid membrane in to stroma.
• The energy in the movement of protons is used to produce ATP.
• The electrons from PSII are taken up by PS I through a serious of
other acceptor molecules in the PS I, these electrons are
transferred to NADP to produce NADPH.
• Up to this stage all the reactions occur only when light is present.
There fore all these reactions are called LIGHT REACTIONS.
• Oxygen , ATP and NADPH are the end products of light reactions.

13. Light
NADPH
OH-
2 H+
CARBOHYDRATES
Co2
½ O2NADP
Formation of carbohydrates and NADPH

14. 1.3.2 The Dark reaction or
Carbon fixation
• Co2 Glucose in a series of
reactions that occur in stroma(Chloroplast).
• ATP & NADPH produced in the light
reactions are used in these reactions.
• The entire reaction from Co2 to productions
of glucose is observed by American scientist
Melvin Kelvin.
• So it is called “Calvin Cycle. Calvin received
Nobel prize fro this work.

15. Calvin Cycle
6 Co2 ( 1 mol)
6 Ribulose 1, 5 Di phosphate
6 Hexose Sugar 1,5 Diphosphate
2 PGA
Glyceraldehyde 3 – Phosphate
Glucose
Corbon Sugar (6 mol) + 2 Phosphates
Attached to it
6 molecules of 6 carbon sugar phosphate
Is formed
6 HS 1,5 D is highly unstable compound and it
Breaks down in to 2 Phosphoglyceric Acid
(Each PGA has 3 Carbon atoms)
ATP & NADPH are produced in Light reaction
Are used up at this stage
(3 Carbon atoms)
2 mol of G 3 P (2 x 3=6 C atoms are used to produce
1 mol Glucose ( 6 c atoms)
(Glucose is converted to Starch)
10 mol of G 3 P (10 x 3 = 30 C ) are used to regenerate
6 mol of R 1, 5 D ( 6 x 5 = 30 C atoms)

16. 2 H+
+ OH - 1
/2
Water-splitting
photosystem
Reaction-
center
chlorophyll
Light
Primary
electron
acceptor
Energy
to make
Electron transport chain
Primary
electron
acceptor
Primary
electron
acceptor
NADPH-producing
photosystem
Light
NADP+
1
2
3
How the Light Reactions Generate ATP and NADPH

18. • The production of ATP by chemiosmosis in
photosynthesis
Thylakoid
compartment
(high H+
)
Thylakoid
membrane
Stroma
(low H+
)
Light
Antenna
molecules
Light
ELECTRON TRANSPORT
CHAIN
PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE

19. • 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

20. PHOTOSYNTHESIS
• Sunlight provides
ENERGY
CO2 + H2O produces
Glucose + Oxygen
6CO2 + 6H2O
C6H12O6 + 6O2

21. Steps of Photosynthesis
• Light hits reaction centers of
chlorophyll, found in chloroplasts
• Chlorophyll vibrates and causes water
to break apart.
• Oxygen is released into air
• Hydrogen remains in chloroplast
attached to NADPH
• “THE LIGHT REACTION”

22. Steps of Photosynthesis
• The DARK Reactions= Calvin Cycle
• CO2 from atmosphere is joined to H
from water molecules (NADPH) to form
glucose
• Glucose can be converted into other
molecules with yummy flavors!

23. • In most plants, photosynthesis occurs
primarily in the leaves, in the chloroplasts
• A chloroplast contains:
– stroma, a fluid
– grana, stacks of thylakoids
• The thylakoids contain chlorophyll
– Chlorophyll is the green pigment that captures
light for photosynthesis
Photosynthesis occurs in chloroplasts

24. • The location and structure of chloroplasts
LEAF CROSS SECTION MESOPHYLL CELL
LEAF
Chloroplast
Mesophyll
CHLOROPLAST Intermembrane space
Outer
membrane
Inner
membrane
Thylakoid
compartmentThylakoidStroma
Granum
StromaGrana

25. • Chloroplasts contain several pigments
Chloroplast Pigments
– Chlorophyll a
– Chlorophyll b
– Carotenoids
– Xanthophyll
Figure 7.7

26. Chlorophyll a & b
•Chl a has a methyl
group
•Chl b has a carbonyl
group
Porphyrin ring
delocalized e-
Phytol tail

27. Different pigments absorb light
differently

28. Cyclic Photophosphorylation
• Process for ATP generation associated with
some Photosynthetic Bacteria
• Reaction Center => 700 nm

29. Photon
Photon
Water-splitting
photosystem
NADPH-producing
photosystem
ATP
mill
• Two types of
photosystems
cooperate in the
light reactions

30. Primary
electron acceptor
Primary
electron acceptor
Electron
transport chain
Electron
transport
Photons
PHOTOSYSTEM I
PHOTOSYSTEM II
Energy for
synthesis of
by chemiosmosis
Noncyclic Photophosphorylation
• Photosystem II regains electrons by splitting
water, leaving O2 gas as a by-product

31. • The O2 liberated by photosynthesis is made
from the oxygen in water (H+
and e-
)
Plants produce OPlants produce O22 gas by splitting Hgas by splitting H22OO

32. • Two connected photosystems collect
photons of light and transfer the energy to
chlorophyll electrons
• The excited electrons are passed from the
primary electron acceptor to electron
transport chains
– Their energy ends up in ATP and NADPH
In the light reactions, electron transportIn the light reactions, electron transport
chains generate ATP, NADPH, & Ochains generate ATP, NADPH, & O22

33. • The electron transport chains are arranged
with the photosystems in the thylakoid
membranes and pump H+
through that
membrane
– The flow of H+
back through the membrane is
harnessed by ATP synthase to make ATP
– In the stroma, the H+
ions combine with NADP+
to
form NADPH
Chemiosmosis powers ATP
synthesis in the light reactions

34. Summary—Light Dependent
Reactions
a. Overall input
light energy, H2
O.
b. Overall output
ATP, NADPH, O2
.

36. • Animation is of the Calvin Cycle
Note what happens to the carbon dioxide
and what the end product is.
• Second animation of the Calvin Cycle
is very clear and even does the
molecular bookkeeping for you.

37. Light Independent Reactions
aka Calvin Cycle
Carbon from CO2 is
converted to glucose
(ATP and NADPH
drive the reduction
of CO2 to C6H12O6.)

38. Light Independent Reactions
aka Calvin Cycle
CO2 is added to the 5-C sugar RuBP by the
enzyme rubisco.
This unstable 6-C compound splits to two
molecules of PGA or 3-phosphoglyceric acid.
PGA is converted to Glyceraldehyde 3-phosphate
(G3P), two of which bond to form glucose.
G3P is the 3-C sugar formed by three turns of the
cycle.

39. Summary—Light Independent
Reactions
a. Overall input
CO2
, ATP, NADPH.
b. Overall output
glucose.

40. Review: Photosynthesis uses light
energy to make food molecules
Light
Chloroplast
Photosystem II
Electron
transport
chains
Photosystem I
CALVIN
CYCLE Stroma
Electrons
LIGHT REACTIONS CALVIN CYCLE
Cellular
respiration
Cellulose
Starch
Other
organic
compounds
• A summary of
the chemical
processes of
photosynthesis

41. Types of Photosynthesis
C3
C4
CAM
Rubisco: the world’s busiest enzyme!

42. Competing Reactions
• Rubisco grabs CO2, “fixing” it into a
carbohydrate in the light independent
reactions.
• O2 can also react with rubisco, inhibiting its
active site
– not good for glucose output
– wastes time and energy (occupies
Rubisco)

43. Photorespiration
• When Rubisco reacts with O2 instead of
CO2
• Occurs under the following conditions:
– Intense Light (high O2 concentrations)
– High heat
• Photorespiration is estimated to reduce
photosynthetic efficiency by 25%

44. Why high heat?
• When it is hot, plants close their
stomata to conserve water
• They continue to do photosynthesis 
use up CO2 and produce O2  creates
high O2 concentrations inside the plant
 photorespiration occurs

45. C4 Photosynthesis
• Certain plants have developed ways to
limit the amount of photorespiration
– C4 Pathway*
– CAM Pathway*
* Both convert CO2 into a 4 carbon
intermediate  C4 Photosynthesis

46. Leaf Anatomy
• In C3 plants (those that do C3
photosynthesis), all processes occur in the
mesophyll cells.
Image taken without permission from http://bcs.whfreeman.com/thelifewire|
Mesophyll cells
Bundle sheath
cells

47. C4 Pathway
• In C4 plants
photosynthesis occurs
in both the mesophyll
and the bundle sheath
cells.
Image taken without permission from

48. C4 Pathway
• CO2
is fixed into a 4-
carbon intermediate
• Has an extra
enzyme– PEP
Carboxylase that
initially traps CO2
instead of Rubisco–
makes a 4 carbon
intermediate

49. C4 Pathway
• The 4 carbon intermediate
is “smuggled” into the
bundle sheath cell
• The bundle sheath cell is
not very permeable to CO2
• CO2 is released from the
4C malate  goes through
the Calvin Cycle
C3 Pathway

50. How does the C4 Pathway
limit photorespiration?
• Bundle sheath cells are far from the
surface– less O2 access
• PEP Carboxylase doesn’t have an
affinity for O2  allows plant to collect a
lot of CO2 and concentrate it in the
bundle sheath cells (where Rubisco is)

51. CAM Pathway
• Fix CO2
at night and
store as a 4 carbon
molecule
• Keep stomates
closed during day to
prevent water loss
• Same general
process as C4
Pathway

52. How does the CAM Pathway
limit photorespiration?
• Collects CO2 at night so that it can be
more concentrated during the day
• Plant can still do the calvin cycle during
the day without losing water

53. Summary of C4
Photosynthesis
• C4 Pathway
– Separates by
space (different
locations)
• CAM Pathway
– Separates
reactions by
time (night
versus day)