Photosynthesis is the process by which plants, algae, and some bacteria use sunlight, water and carbon dioxide to produce oxygen and energy in the form of glucose. It occurs in two phases - the light-dependent reactions where ATP and NADPH are produced to store energy, and the light-independent Calvin cycle where carbon is fixed from carbon dioxide into organic compounds like glucose using the ATP and NADPH produced in the light reactions. Environmental factors like light intensity, temperature and oxygen concentration can affect the rate of photosynthesis. Photorespiration occurs when oxygen is used instead of carbon dioxide in the Calvin cycle, preventing glucose production.

1. 4. Photosynthesis
Metabolism
• Anabolism- Building up larger molecule from
smaller molecules
–Energy is stored in chemical bonds
–Eg. photosynthesis
• Catabolism -Breaking down of larger molecules
to small
–Energy released
–Eg. Respiration
1

2. Photosynthesis
• Both anabolism and catabolism involve oxidation
reduction.
• Oxidation- loss of one or more electrons or
removal of hydrogen
• Reduction- gain of one or more electrons or
addition of H to a substance
• N.B: Hydrogen atom is often removed during
oxidation and adding during reduction
2

3. Photosynthesis
Photosynthesis
• literally means “synthesis using light.”
• Takes place in chloroplast
• Light energy is converted to chemical energy
and stored in chemical bonds
• Photosynthetic organisms use solar energy to
synthesize carbon compounds that cannot be
formed without the input of energy.
3

4. Photosynthesis
• Light energy drives the synthesis of
carbohydrates from carbon dioxide and water
with the generation of oxygen:
• Energy stored in these molecules can be used
later to power cellular processes in the plant
and can serve as the energy source for all forms
of life.
4

5. Photosynthesis
Light
• Light Has Characteristics of Both a Particle and a
Wave
• A wave is characterized by a wavelength,
lambda ( ), which is the distance between
successive wave crests.
• The frequency, (V), is the number of wave crests
that pass an observer in a given time.
5

6. Photosynthesis
• A simple equation relates the wavelength, the
frequency, and the speed of any wave:
• Where c is the speed of the wave—in the
present case, the speed of light (3.0 × 108 ms–1)
6

7. Photosynthesis
• Light is also a particle, which we call a photon. Each
photon contains an amount of energy that is called a
quantum (plural quanta).
• The energy content of light is not continuous but rather
is delivered in these discrete packets, the quanta.
7

8. Photosynthesis
• The energy (E) of a photon depends on the
frequency of the light according to a relation
known as Planck’s law:
• where h is Planck’s constant (6.626 × 10–34 J s).
• Energy of photon depends on wave length
• Light with different wave length reach the surface
of the earth
8

9. Photosynthesis
• Longest wave length (radio wave) to the shortest
wavelength (gamma rays)
• 40% of radiation energy that the earth receives
visible light (violet to far red)
9

10. Structure of chloroplast
• Comprises three major components
• The Chloroplast is the Site of Photosynthesis
– Envelope(membrane)
– Stroma
– Thylakoid
• The most striking aspect of the structure of the
chloroplast is the extensive system of internal
membranes known as thylakoids.
• All the chlorophyll is contained within this membrane
system, which is the site of the light reactions of
photosynthesis.
10

11. Structure of chloroplast
• The carbon reduction reactions, which are
catalyzed by water-soluble enzymes, take place
in the stroma (plural stromata), the region of the
chloroplast outside the thylakoids.
• Most of the thylakoids appear to be very closely
associated with each other.
11

12. Structure of chloroplast
 These stacked membranes are known as grana lamellae
(singular lamella; each stack is called a granum), and the
exposed membranes that lack stack are stroma lamellae.
 Two separate membranes, each composed of a lipid
bilayer and together known as the envelope, surround
most types of chloroplasts
12

13. Structure of chloroplast
13

14. Thylakoid membrane
• It has 30-50 polypeptides (proteins)
• Grouped in to 5 major complexes
Photosystem I
• Reciting center for P700
• Several polypeptides
• Low chlorophyll a: chl. b ratio than PSII
• To trap light energy & induce reductions of
NADP+to NADPH
14

15. Thylakoid membrane
• Photosystem II
• Reactive center P 680
• Higher chlorophyll a: chl. b ratio than PSI
• To trap light and releasing of oxygen from water
• Cytocromes
• Cytochrome b6f receives electrons from PSII and
delivers them to PSI.
• It also transports additional protons into the
lumen from the stroma.
15

16. Thylakoid membrane
 ATP synthase -Produce ATP as protons and diffuse
back through it from the lumen into the stroma.
 Makes channel for proton to produce ATP
 Light harvesting complex
 Contain chl. a,b & other pigments (Xantophyll &
carotenoids)
 Functionally, used to capture solar energy (photon)
16

17. Photosynthesis: An Overview
• The net overall equation for photosynthesis is:
• Photosynthesis occurs in 2 “stages”:
1. The Light Reactions (or Light-Dependent Reactions)
2. The Calvin Cycle or
 Calvin-Benson Cycle or
 Dark Reactions or
 Light-Independent Reactions)
17
6 CO2 + 6 H2O C6H12O6 + 6 O2
light

18. Phase 1: Light dependent reaction
• Initiated when photons strike chlorophyll
molecules in thylakoid membrane
• During LDR:
Water molecules splitted in to H+ and O2
2 H2O → O2 + 4 H+ + 4 e–
e–from splited water pass to e-carriers (ETS)
Energy storing ATP molecules are produced
(chemosmosis)
Some H+ from splited water reduce NADP to
NADPH 18

19. Z scheme
19

20. Light reaction
• Detailed Z scheme for O2-evolving photosynthetic
organisms.
(1) The vertical arrows represent photon absorption
by the reaction center chlorophylls:
– P680 for photosystem II (PSII) and
– P700 for photosystem I (PSI).
• The excited PSII reaction center chlorophyll,
P680*, transfers an electron to pheophytin
(Pheo).
20

21. Light reaction
(2) On the oxidizing side of PSII (to the left of the arrow
joining P680 with P680*), P680 oxidized by light is re-
reduced by Yz , that has received electrons from oxidation
of water.
(3) On the reducing side of PSII (to the right of the arrow
joining P680 with P680*), pheophytin transfers electrons
to the acceptors QA and QB, which are plastoquinones
21

22. Light reaction
(4) The cytochrome b6 f complex transfers electrons to
plastocyanin (PC), a soluble protein, which in turn reduces
P700+ (oxidized P700).
(5) The acceptor of electrons from P700* (A0) is thought to
be a chlorophyll, and the next acceptor (A1) is a quinone.
• A series of membrane-bound iron–sulfur proteins (FeSX,
FeSA, and FeSB) transfers electrons to soluble ferredoxin
(Fd).
• The dashed line indicates cyclic electron flow around PSI.
22

23. Light reaction
• The over all movement of e-s from H2O →PSII →e-
carriers →PSI →e-carriers →NADP to form NADPH
Non cyclic e- flow
• When e- passing ETS protons (H+) subsequently move
across the thylakoid membrane to stroma
(Chemosmosis) → ATP formation
 The production of ATP inLDR→Photophosphorylation
• ATP also be produced when e-s released from high
energy to low energy level (release energy)→ produce
ATP
Cyclic photophosphorylation
23

24. Photosynthesis
Phase II: Light Independent Reaction
• The Calvin Cycle, Environmental Conditions,
& Preventing Photorespiration
24

25. Photosynthesis: An Overview
• To follow the energy in photosynthesis,
25
light
light ATP
NADPH
Light
Reactions
thylakoids
Calvin
Cycle
stroma
Organic
compounds
(carbs)

26. Phase 2: The Calvin Cycle
• In the Calvin Cycle, chemical energy (from the light
reactions) and CO2 (from the atmosphere) are used to
produce organic compounds (like glucose).
• The Calvin Cycle occurs in the stroma of chloroplasts.
26

27. Phase 2: The Calvin Cycle
• The Calvin Cycle involves the process of carbon fixation.
• This is the process of assimilating carbon from a non-
organic compound (ie. CO2) and incorporating it into
an organic compound (ie. carbohydrates).
27
CARBON FIXATION

28. Phase 2: The Calvin Cycle
Step 1: Carbon Fixation
• 3 molecules of CO2 (from the atmosphere) are joined to 3
molecules of RuBP (a 5-carbon sugar) by Rubisco (an enzyme
also known as RuBP carboxylase)
28
C CC CC
C CC CC
C CC CC
C
C
C
3 carbon dioxide
molecules
3 RuBP molecules
Rubisco
This forms 3
molecules
which each
have 6 carbons
(for a total of 18
carbons!)

29. Phase 2: The Calvin Cycle
Step 2: Reduction
• The three 6-carbon molecules (very unstable) split in half, forming
six 3-carbon molecules.
• Then they are reduced by gaining electrons from NADPH.
• ATP is required for this molecular rearranging
29
C CC CC C
C CC CC C
C CC CC C
C CC
C CC
C CC
CC C
CC C
CC C
NADPH
NADP+
ATP ADP P

30. The normal RuBP Carboxylase product, 3-phospho-
glycerate is converted to glyceraldehyde-3-P.
Phosphoglycerate Kinase catalyzes transfer of Pi from ATP
to the carboxyl of 3-phosphoglycerate (RuBP Carboxylase
product) to yield 1,3-bisphosphoglycerate.
OH
H2C
CH
C
OO
OPO3
2

OH
H2C
CH
C
OPO3
2
O
OPO3
2
OH
H2C
CH
CHO
OPO3
2
ATP ADP NADPH NADP+
Pi
1,3-bisphospho-
glycerate
3-phospho-
glycerate
glyceraldehyde-
3-phosphate
Phosphoglycerate
Kinase
Glyceraldehyde-3-phosphate
Dehydrogenase

31. Phase 2: The Calvin Cycle
• There are now six 3-carbon molecules, which are known
as G3P or PGAL.
• Since the Calvin Cycle started with 15 carbons (three 5-
carbon molecules) and there are now 18 carbons, we
have a net gain of 3 carbons.
31
C CC
C CC
C CC
CC C
CC C
CC C
• One of these “extra” 3-carbon
G3P/PGAL molecules will exit
the cycle and be used to form ½
a glucose molecule.

32. Phase 2: The Calvin Cycle
• Once the Calvin Cycle “turns” twice (well, actually 6
times), those 2 molecules of G3P (a 3-carbon
carbohydrate) will combine to form 1 molecule of glucose
(a 6-carbon carbohydrate molecule) OR another organic
compound.
32
CC C
G3P
(from 3 turns of
the Calvin Cycle)
C CC
G3P
(from 3 turns of
the Calvin Cycle)
CC C C CC
glucose

33. Phase 2: The Calvin Cycle
Step 3: Regeneration of RuBP
• Since this is the Calvin Cycle, we must end up back at
the beginning.
• The remaining 5 G3P molecules (3-carbons each!) get
rearranged (using ATP) to form 3 RuBP molecules (5-
carbons each).
33
C CC
C CC
C CC
C C C
CC C
5 G3P molecules
Total: 15 carbons
3 RuBP molecules
Total: 15 carbons
ATP
ADP
P

34. Phase 2: The Calvin Cycle
ORGANIC
COMPOUND
NADPH
NADP+
ATP
ADP
P
RuBP
CO2

35. Phase 2: The Calvin Cycle
35

36. Phase 2: The Calvin Cycle
Quick recap:
•In the Calvin Cycle, energy and electrons from the Light
Reactions (in the form of ATP and NADPH) and carbon
dioxide from the atmosphere are used to produce organic
compounds.
•The Calvin Cycle occurs in the stroma inside the
chloroplasts (inside the cells…).
•Carbon dioxide, ATP, and NADPH are required as a
(reactants).
• Organic compounds (G3P) are produced (products).
36

37. Photosynthesis: A Recap
• So, as a broad overview of photosynthesis,
 The Light Reactions (Phase 1) capture the energy in sunlight
and convert it to chemical energy in the form of ATP and
NADPH through the use of photosystems, electron
transport chains, and chemiosmosis.
 The Calvin Cycle (Phase 2) uses the energy transformed by
the light reactions along with carbon dioxide to produce
organic compounds.
37

38. Photosynthesis: A Recap
38
The photosynthetic equation:
light
Excites
electrons
during the
light
reactions
6 H2O
Split during the
light reactions
to replace
electrons lost
from
Photosystem II
6 CO2
Provides the carbon to
produce organic
compounds during the
Calvin Cycle
Produced as a
byproduct of the
splitting of
water during the
light reactions
6 O2 C6H12O6
The organic compound
ultimately produced
during the Calvin Cycle

39. Most realistically, the rate of photosynthesis could be
measured by using the:
- Decrease in environmental CO2 (in a closed system)
- Increase in environmental O2 (in a closed system)
- Increase in glucose (perhaps measured using radioactive
carbon)
Environmental Factors & Photosynthesis
• The rate (or speed) of photosynthesis can vary, based on
environmental conditions.
• Light intensity
• Temperature
• Oxygen concentration 39

40. Environmental Factors & Photosynthesis
• Light intensity
• As light intensity increases, so too does the rate of
photosynthesis.
40
• This occurs due to increased excitation
of electrons in the photosystems.
• However, the photosystems will
eventually become saturated.
• Above this limiting level, no
further increase in photosynthetic
rate will occur.
light
saturation
point

41. Environmental Factors & Photosynthesis
• Temperature
• The effect of temperature on the rate of
photosynthesis is linked to the action of enzymes.
• As the temperature increases up to a certain point,
the rate of photosynthesis increases.
• Molecules are moving faster &
colliding with enzymes more
frequently, facilitating chemical
reactions.
• However, at temperatures higher
than this point, the rate of
photosynthesis decreases.
• Enzymes are denatured.

42. Environmental Factors & Photosynthesis
• Oxygen concentration
• As the concentration of oxygen increases, the rate
of photosynthesis decreases.
• This occurs due to the phenomenon of
photorespiration.
42

43. Photorespiration
• Photorespiration occurs when Rubisco (RuBP
carboxylase) joins oxygen to RuBP in the first step
of the Calvin Cycle rather than carbon dioxide.
• Whichever compound (O2 or CO2) is present in higher
concentration will be joined by Rubisco to RuBP.
• Photorespiration prevents the synthesis of glucose AND utilizes
the plant’s ATP.
43
More CO2
More O2
Rubisco joins
CO2 to RuBP
Rubisco joins
O2 to RuBP
Photosynthesis
occurs; glucose is
produced
Photorespiration
occurs; glucose is
NOT produced

44. Photorespiration
• Photorespiration is primarily a problem for plants under water
stress.
• When plants are under water stress, their stomata close to
prevent water loss through transpiration.
• However, this also limits gas exchange.
• O2 is still being produced (through the light reactions).
44
• Thus, the concentration of O2 is
increasing.
• CO2 is not entering the leaf since the
stomata are closed.
• Thus, as the CO2 is being used up (in the
Calvin Cycle) and not replenished, the
concentration of CO2 is decreasing.

45. Photorespiration
• As the concentration of O2 increases and the concentration
of CO2 decreases (due to the closure of the stomata to
prevent excessive water loss), photorespiration is favored
over photosynthesis.
• Some plant species that live in hot, dry climates (where
photorespiration is an especially big problem) have
developed mechanisms through natural selection to prevent
photorespiration.
• C4 plants
• CAM plants
45

46. C3 Plants
• C3 plants, which are “normal” plants, perform
the light reactions and the Calvin Cycle in the
mesophyll cells of the leaves.
46
• The bundle sheath cells of
C3 plants do not contain
chloroplasts
palisade mesophyll
spongy mesophyll
bundle sheath cells

47. C4 and CAM Plants
• C4 plants and CAM plants modify the process of
C3 photosynthesis to prevent photorespiration.
• Overview:
• C4 plants perform the Calvin Cycle in a different
location within the leaf than C3 plants.
• CAM plants obtain CO2 at a different time than C3
plants.
• Both C4 and CAM plants separate the initial fixing
of CO2 (carbon fixation) from the using of CO2 in
the Calvin Cycle.
47

48. C4 Plants: Preventing Photorespiration
• Plants that use C4 photosynthesis include corn,
sugar cane, and sorghum.
• In this process, CO2 is transferred from the
mesophyll cells into the bundle-sheath cells,
which are impermeable to CO2.
48
• This increases the concentration of
CO2.
• Thus, the Calvin Cycle is favored
over photorespiration.
• The bundle-sheath cells of C4
plants do contain chloroplasts.

49. C4 Plants: Preventing Photorespiration
• C4 plants use the Hatch-Slack
pathway prior to the Calvin Cycle:
• PEP carboxylase adds carbon dioxide
to PEP, a 3-carbon compound, in the
mesophyll cells.
• This produces a 4-carbon
compound (which is why it’s
known as C4 photosynthesis).
• This 4-carbon molecule then moves
into the bundle-sheath cells via
plasmodesmata.
49
• In the bundle sheath cells, the CO2 is
released and the Calvin Cycle begins.

50. C4 Plants: Preventing Photorespiration
50
If the Hatch-Slack
pathway helps to
prevent
photorespiration,
why wouldn’t ALL
plants have this
adaptation?

51. C3 vs C4 Plants
A Lesson in Photoefficiency
• CO2 directly
• RuBP recipient
• RUBISCO open
• O2 can interfere
• Photorespiration
likely
51
• CO2 indirectly
• PEP recipient
• RUBISCO shielded
• O2 cannot interfere
• No photorespriation
C3 C4

52. C3 and C4 Plants
52
C4
C3
• soybean
• wheat
• rice
• sugar beet
• alfalfa
• spinach
• tobacco
• sunflower
• corn
• sorghum
• sugar cane
• millet
• crab grass
• Bermuda grass
• pigweed

53. CAM Plants: Preventing Photorespiration
• Plants that use CAM photosynthesis include
succulent plants (like cacti) and pineapples.
• In CAM (crassulacean acid metabolism)
photosynthesis, plants open their stomata at
night to obtain CO2 and release O2.
• This prevents them from drying out by keeping
their stomata closed during the hottest & driest
part of the day.

54. CAM Plants: Preventing Photorespiration
• When the stomata are opened at night, the CO2 is
converted to an organic acid (via the C4 pathway) and
stored overnight.
• During the day – when light is present to drive the Light
Reactions to power the Calvin Cycle – carbon dioxide is
released from the organic acid and used in the Calvin
Cycle to produce organic compounds.
• Remember:
54
• Even though the CO2 is
taken in at night, the Calvin
Cycle cannot occur because
the Light Reactions can’t
occur in the dark!

55. 55

56. Avoiding Photorespiration
• Both C4 and CAM plants – which are primarily found in hot,
dry climates – have evolutionary adaptations which help
prevent photorespiration.
• C4 plants perform the Calvin Cycle in the bundle-
56
sheath cells.
• CAM plants
open their
stomata at
night and store
the CO2 until
morning.

57. 5. Translocation in the phloem
57

58. 5.1 Phloem anatomy
• Phloem is composed of two types of cells
I. Sieve tube members : relatively large more or
less cylindrical
II. Companion cells: narrow and more or less
tapered
o Companion cells are associated with sieve tube
members; metabolically very active cells
58

59. Phloem anatomy
• Phloem is derived from the plant cell of cambium
which also produces xylem
• Also includes fibers, parenchyma and ray cells
• Sieve tube members are laid end to end and
form sieve tube
• Porous regions of sieve tube members is called
sieve plate
• Have no nucleus at maturity
Although their cytoplasm is very active in conduction
59

60. Phloem anatomy
• Associated with companion cells which aid in
conduction by providing energy
• Sieve cells and sieve tubes are the chief food
conducting tissues
• Numerous plasmodesmata occurs between
companion cells and sieve tube elements
• Direction of translocation of organic molecule :
– Predominantly down ward or up ward direction
– Radial translocation also common
60

61. 5.2 Phloem sap composition
• Phloem sap contains:
• Large amount of sugars ( of which more than 90% is
sucrose= a non reducing sugar)
• Amino acids and amines, and certain amounts of salts
• However, composition of sap vary from plant to plant
• The most common transported plant is sucrose
(glucose + fructose)
• Carbohydrates transported in phloem are almost
all non reducing sugars. This is because they are
less reactive
61

62. Phloem sap composition
• Reducing sugars such as Glucose, Maltose and
Fructose contain an exposed aldehyde or
keton group; too chemically reactive to be
transported in the phloem
• Thus sucrose is the most common sugar
transported in sieve tube of phloem
62

63. 63

64. Translocation in sieve tubes
• Sugar produced by photosynthesis moves
through one to several paranchyma cells to
reach sieve elements In phloem
• Movement of sugar from photosynthetic cells
to sieve tube elements called vein loading while
the reverse is vein unloading
• Region that supplies sugar is called source and
that where it is utilized is known as sink
64

65. Translocation in sieve tubes
Sources are green leaves
Sinks are growing points of roots and shoots,
and storage organs such as fruit and seeds,
stem, rhizome, tuber, roots etc.
• Sugar concentration in sieve tube (phloem) is
the highest at the source and the lowest near
the sink
65

66. Mechanism of Translocation
A) Phloem loading
• There are four steps in phloem loading
I. Diffusion of triose phosphate (GA3P) from
stroma to cytoplasm where it is converted to
sucrose
II. Sucrose travels from mesophyll cells to near
sieve elements (short distance transport)
III. Sucrose enter sieve elements -companion cell
complex
66

67. Phloem loading
IV. Sugars are then transported to sink (long
distance transport) through vascular system
(export)
• It is believed that:
• Mass flow of sugar is passive process (long
distance transport)
• Phloem loading and unloading are active
proceses (consume ATP)
67

68. Phloem unloading
• i.e sieve element companion cell complex has
high concentration of sucrose and sugars
enters these cells against concentration
gradient (at cost of energy)
B. Phloem unloading
• Transport of sugar from sieve elements in to
cells of sink is generally apoplastic and to
some extent symplastic
68

69. Phloem unloading
• Symplastic unloading is down concentration
gradient
• Appoplastic transport may be energy
consuming
• Phloem unloading occurs in three steps
I. Transport of sugar out of sieve elements
II. The sugar diffuses in to the storage (utilizing)
tissue (sink)
III. The sugars are hydrolysed and converted in to
starch for storage
69

70. Mechanism…
• The most widely accepted theory about mechanism of
phloem transport is the pressure flow (mass flow)
hypothesis
• In sieve elements near to source tissue , energy driven
phloem loading leads to a build of sugars in sieve
elements
 This results in low solute potential in sieve elements
→causes a step drop in water potential →water enters
sieve elements from xylem →thus phloem turger
pressure increases →increase pressure in phloem
sieve elements (near source)
70

71. Mechanism…
• In sieve elements near the sink tissue, phloem
unloading leads to lower sugar concentration →results
in hign solute potential →water potential increases
→water leaves phloem and enters sink sieve elements
and xylem →thus phloem turger pressure decreses
→decreses pressure in the phloem sieve elements near
the sink.
• Pressure difference b/n phloem sieve elements near
the source (highpressure) and sink (low pressure)
results in mass flow of sugar down pressure gradient.
71

72. 72

73. 6. Nitrogen metabolism
• METABOLISM
• Metabolism is the sum total of all chemical reactions
occurring in living organisms.
• Metabolic reactions that synthesize compounds are referred
to as anabolic reactions and are generally endergonic,
requiring an input of energy.
• In contrast, catabolic reactions, which breakdown compounds,
are usually exergonic reactions, which release energy.
• Many of these reactions also involve the conversion of energy
from one form to another.
73

74. Nitrogen fixation
• Nitrogen is the most abundant element in the
atmosphere.„
• Nitrogen makes up 78% of the troposphere.„
• Nitrogen can not be absorbed directly by the
plants and animals until it is converted in to
compounds they can use. This process is called
the Nitrogen Cycle.
• The process of nitrogen cycle include nitrogen
fixation, Nitrification, Ammonification,
Assimilation and Denitrification.
74

75. 7. Growth and development
• Growth:
– Apical
– Lateral
– intercallary
• Growth curve
– Lag phase
– Log exponential
– Stationary phase
75

76. Plant growth hormons
• Auxins
• Gibbrerelines
• Cytokinins
• Ethylene
• Abscisic acid
• Auxins
• Synthesis: shoot tip (appical meristem)
76