This document summarizes key aspects of photosynthesis. It describes that photosynthesis occurs in plants, algae, and photosynthetic bacteria. Light energy is captured and used to fix carbon from carbon dioxide into sugars, with oxygen as a byproduct. The process takes place in chloroplasts and involves two stages - the light reactions where ATP and NADPH are produced, and the dark reactions where CO2 is fixed into sugars. Pigments like chlorophyll and accessory pigments harvest light energy which is used to power electron transport and produce chemical energy carriers.

2.  Green plants, green, red and brown algae, blue green algae &
photosynthetic bacteria
 0.2 % of the light is used up
 90 % of the photosynthesis is in the oceans
 70 billion tons of CO2 is annually fixed
 Carbon reduction/C-assimilation

3.  Mesophyll cells
 Double membraned fluid filled bags
 Gelatinous proteinaceous core- Stroma (site of CO2
fixation or dark reaction)
 Periplastidial space
 Thylakoid system
 Grana and stromal thylakoids – contain pigments –
light reaction

5.  Present in thylakoid membranes
 Chlorophylls – Chl a and Chl bare most abundant. Chl a
only participates in photochemical reactions. All others
are accessory pigments.
 Crotenoids – fat soluble, accessory pigments. Include
carotenes & xanthophylls
 Phycobilins – Red or blue accessory pigments. Water
soluble.- Phycoerythrin and Phycocyanin

6. Chlorophyll
 Principal pigment. Porphyrin head & Phytol tail
 Porphyrin Head- 4 Tetrapyrrole rings, arranged cyclic
 Mg atom is chelated to the 4 Nitrogen atoms of the Head
 Phytol (20 C) alcohol, derivative of isoprene
 Different species – a, b, c, d and e
 Except chl a, all are accessory pigments
 Different pigments are closely packed, some absorb shorter and
others longer wavelengths

8. CHLOROPHYLL A CHLOROPHYLL B
 Principal pigment
 Blue green in colour
 Empirical formula is
C55H77O5N4Mg
 Functional group is methyl
 Present in all phototrophs
 Absorbs light between 430
to 660
 Accessory pigment
 Olive or yellow green in colour
 Empirical formula is
C55H77O5N4Mg
 Functional group is aldehyde
 Present in all plants & green
algae
 Absorbs light between 450 to
650

9.  Sun is the primary source of natural radiations
 Solar radiations are electromagnetic
 Electro-magnetic spectrum – x-rays, gamma rays,
cosmic rays, UV rays, IR radiations and radio waves
 Visible spectrum bet 390-760 nm (VIBGYOR)
 PAR (Photosythetically active radiation)

10.  Light travels as waves of tiny particles- photons
 Energy contained in a photon- Quantum
 Quantum requirement- The quanta required to produce
one o2 molecule in Photosynthesis
 Quantasome – The no. of chlorophyll molecules involved
in the absorption of one quantum of light (200-400)

12.  Photoexcitation occurs when a molecule absorbs light
energy
 Ground state and excited state
 Excited state is short lived

13.  Energy can be lost from an excited molecule by
 Heat loss
 Resonance – a system vibrates with maximum amplitude
in response to excitations from a closeby vibrating
system
 Fluorescence – Immediate emission of EM radiation
followed by energy absorption. Short lived and stops
when source is removed
 Phosphorescence – Delayed and long-lasting emission
 Metastable Triplet state- Photochemical reaction

15. ABSORPTION SPECTRUM ACTION SPECTRUM
 Graphical repesentation
of light absorbed by a
pigment as a function of
wavelength
 Graphical repesentation
of rate of photosynthesis
as a function of
wavelength

18.  Photosynthetic electron transport
 Multimolecular aggregates in thylakoid membrane –
Photosystems (Pigment systems) & Reaction Centre
 Two large complexes- PS I & PS II, linked by a third
complex – Cytochrome complex
 Low energy electrons from water are energised by light
energy and produce a strong reductant - NADPH

19.  Photosystems contain different proteins, chlorphyll and
carotenoids
 Antenna chlorophyll & Reaction centre chlorophyll
 CP (Chlorophyll-protein) complexes to harvest light
 PS II- 2 complexes, CP 43 & CP47 (20-25 Chl a mols)
 RC consists of 4-6 Chl a mols. P700 for PS I & P680 for PS II
 Antenna absorbs and funnel the excitation energy to RC
where photochemical oxidation-reduction takes place

21.  2 addidtional CP
complexes, closely
associated with
photosystems are
LHC I & LHC II
 Extended antenna
system to ensure
efficient light
harvesting

23. PS II PS ICyt
H20
½ o2+2H+
NADPH+H+
NADP+2H+
Two Photosystems operating in series
light
light

24. Organization of photosynthetic electron
transport in the thylakoid membrane

25.  Quantum yield is the rate
of photosynthesis
measured as the no. of O2
molecules evolved per
quantum of light absorbed
 Quantum requirement is
the no of light quanta
required for the reduction
of one molecule of CO2 or
evolution of one molecule
of O2

26.  Chlorella suspension is exposed to monochromatic light of
different wavelengths
 Sudden decrease in quantum yield at the red part of spectrum
 Enhancement of photosynthetic rate when longer wavelengths
supplemented by shorter wavelengths
 Photosynthesis involves two photochemical process, one
driven by longer wavelengths & the other by shorter
 There are 2 photosystems, each absorbing longer and shorter
wavelength

28. Redox reaction, water is oxidised, CO2 is
reduced
 Light reaction(Photochemical reaction)
Occurs in thylakoid membranes, light energy is
traped and used for synthsising ATP & NADPH
 Dark reaction (Thermochemical reaction)
Occurs in stroma, products of light reaction are used
for reduceing CO2 to sugar. Biosythetic phase or Calvin
cycle

29.  Photosynythesis in intermittent light
 CO2 reduction in darkness

32.  Light dependent
 Thylakoid membrane
 Absorption of light energy
 Photolysis of water
 Cyclic & Non-cyclic phtophosphorylation (Synthesis of
ATP)
 Cyclic & Non-cyclic electron transport
 Reduction of NADP to NADPH+H+

33.  Photoexcitation of chlorophyll AND Reaction Centre
Chl a (Reduced) Chl b (Oxidised)+ e- (energised electron)
Conversion of light energy to chemical energy, travel downhill through
a series of oxidation reduction reactions, coupled to synthesis of ATP
 Electron transport and photophosphorylation
Cyclic and Non-cyclic
 Photolysis of water (Photo-oxidation) of water
Light dependent oxidation of water
Mn-protein system
 Photoreduction of NADP to form NADPH
Assimilatory power to reduce CO2
4 MAJOR
STAGES

34. CYCLIC ELECTRON
TRANSPORT
NON-CYCLIC ELECTRON
TRANSPORT
 PS I operates independently of
PS II
 NADPH is not produced
 Energy is conserved as ATP by
cyclic phtophosphorylation
 Oxygen is not evolved
 Driven by longer wavelenths of
light
 PS I and PS II are involved
 NADPH is produced
 Energy is conserved as ATP by
non-cyclic phtophosphorylation
 Photolysis of water
 Oxygen is evolved
 Driven by longer & shorter
wavelenths of light

37.  Phaeophytin – Form of Chl a where Mg is replaced by two
H-atoms
 Ferredoxin - Iron-sulfur proteins that mediate electron
transfer in a range of metabolic reactions
 Cytochrome complex - Multiprotein membrane spanning
complex. Consists of Cyt b6 and Cyt f and Fe-S proteins
 Cytochromes - Iron containing hemeproteins central to
which are heme groups. Primarily responsible for the
generation of ATP via electron transport and catalyse
redox reactions

38. Plastoquinone (PQ) - isoprenoid quinone molecule involved
in the electron transport chain in the light-dependent
reactions of photosynthesis
Plastocyanin (PC) - copper-containing protein involved in
electron-transfer. Small peripheral protein that diffuses
along the lumen side of thylakoid membrane
NADPH – Strong reluctant, mobile electron carrier, used to
reduce CO2 to carbohydrate

39. Oxygen evolving complex (OEC)
Splitting of water & the consequent
evolution of O2
Small complex of protein & 4 Mn ions
Lumen side of thylakoid membrane, bound
to PS II
Also binds Cl- needed for photolysis

40. Non-cyclic electron transport
PS I, PS II and Cyt complex are 3 major complexes linked by 2
mobile carriers –
PQ and PC – Freely diffuse in the membrane

42. Electron Carriers as complexes

43.  Energy conservation in electron transport
 Light driven accumulation of protons in the lumen
1. Oxidation of water (2H+ are deposited)
2. PQ- Cytochrome pump
 Energy of proton gradient is used yto drive ATP
synthesis
 Electrons are pumped from stroma to lumen side

44. Photosynthetic carbon reduction cycle
Occurs stroma
Studies in Chlorella using 14CO2
3 major phases Carboxylation of RuBP
Glycolytic Reversal
Regeneration of RuBP

45.  Carboxylation of RuBP
6 mol. of RuBP reacts with 6 mols of CO2 to form 6 mols of
an unstable compound
Unstable compound splits into 12 mols of 3-
Phosphoglyceric acid
Cataysed by Rubisco (RuBP Carboxylase Oxygenase)

46.  Glycolytic reversal
2 mols of 3-Phosphoglyceraldehyde is converted to 1 mol
of glucose
Utilizing 12 mols each of ATP and NADPH produced in the
light reaction
Reaction chain is a reversal of glycolysis

47.  Regeneration of RuBP
6 mols of RuBP are regenerated for the continous turning
around the cycle
Complex series of reactions using 6 ATP molecules

50. 6 RuBP (6X5C)
6 mols of unstable
compound
(6X6C)
12 PGA
(12X3C)
1,3- DPGA
(12X3C)
12 PGAL
10 PGAL
6 CO2
12 ATP
12 ADP
12 NADPH
12 NADP
6 ATP
6 ADP
Phosphoglycero kinase
Triose phosphate dehydrogenase
CARBOXYLATION OF RuBP
REGENERATION OF RuBP

51. PGAL (1X3C)DHAP (1X3C)
1 Fructose DP
(1X6C)
1 Fructose 6-P
(1X6C)
1 Glucose 6-P
(1X6C)
Triose phosphate isomerase
Aldolase
Phosphatase
Hexose isomerase
GLYCOLYTIC REVERSAL

53.  Light dependent respiration
 Green cells
 Oxygen is consumed & CO2 is evolved
 No energy rich compound are produced
 Fixed carbon is lost as CO2
 Wasteful process
 High temp, high light intensity & high O2
concentration

55.  Rubisco acts as an Oxygenase
 Oxidation of RuBP
 Glycollate (2C)
 Glycollate cycle/C-2 Cycle
 Chloroplast, peroxisomes & Mitochondria
 Interferes with C-3 cycle and affects yield

58. PHOTORESPIRATION TRUE RESPIRATION
 Only in green cells
 Found in C3 plants
 Presence of light
 Chloroplast, mitochondria and
peroxisomes
 Rubisco is involved
 NH3 is formed
 Substrate is glycolate
 End products are CO2 and PGA
 Affected by O2 level
 ATP and NADKH are consumed
 Occurs in all cells
 Found in all plants
 Presence of light is not essential
 Only mitochondria
 Rubisco is not involved
 NH3 is not formed
 Substrate is glucose
 End products are CO2 and H2O
 Not affected by O2 level
 ATP and NADPH are formed

59.  Alternative pathway of C-fixation
 First stable product is C4 dicarboxylic acid- Oxaloacetic
acid
 Tropical grasses – Sugarcane, Maize, Sorghum,
Amaranthus, Atriplex
 Mesophyll and Bundle sheath cells
 Photorespiration is absent

60.  C4 plants are found in hot tropics n sub tropics
 Very special type of leaf anatomy
 Wreath like arrangement of mesophyll & bundle sheath
cells (ring like, radial or concentric arrangement)
 Dimorphic chloroplasts
 Bundle sheath cells have large, agranal, cenripetally
arranged chloroplasts
 Calvin cycle enzymes confined to bundle sheath cells

61. KRANTZ ANATOMY

63. PEP OAA
CO2
PEP Case
Malic
dehydrogenase
NADPH
NADP
Malic acid
Malic acid
CO2
NADPH
NADP
Pyruvic acid
CALVIN
CYCLE
Glucosee
Pyruvic acid
ATP
AMP
Rubisco
BUNDLE SHEATH CELL
MESOPHYLL CELL

64. MESOPHYLL CELLS BUNDLE SHEATH CELLS
 First carboxylation
 PEP combines with CO2 to
form OAA
 PEP Carboxylase
 OAA is reduced to Malic
acid using NADPH
 Malic acid is transported to
bundle sheath cells
 Second carboxylation
 Malic acid is oxidatively
decarboxylated to Pyruvic acid,
NADPH and CO2
 CO2 enters Calvin cycle to produce
sugar
 Pruvic acid returns to mesophyll
cells & is phosporylated to
regenerate PEP

65.  Energetically expensive, but efficient mechanism of C-
fixation
 Absence of photorespiration
 Acts as a CO2 pump to concentrate CO2 at the site of C3
cycle
 Can absorb CO2 even from a much low concentration
 PEP Case is more active in CO2 fixation
 Better adapted to tropical and desert areas

66. C3 PLANTS C4 PLANTS
 Most crop plants
 Only C3 pathway is present
 Don’t possess Krantz
anatomy
 Monomorphic & Granal
 Primary CO2 acceptor is
RuBP
 Enzyme is Rubisco
 Moderate affinity for CO2
 First stable product is 3-
PGA
 Optimum temp is lower
 Photorespiration is present
 Maize, Sorghum, Sugarcane
 Both C3 & C4 pathways
present
 Possess Krantz anatomy
 Dimorphic chloroplasts,
granal & agranal
 Primary CO2 acceptor is
PEP
 Enzyme is PEP case
 High affinity for CO2
 First stable product is OAA
(4C)
 Optimum temp is higher
 Photorespiration is minimal

67.  Seen in succulent plants
 Initially in Crassulaceae
 Euphorbiaceae, Cactaceae, Polypodiacea
 Modified photosynthetic pathway
 Scotoactive stomata

68.  External CO2 fixation during night into organic acids
 During day time acids are decarboxylated releasing CO2
 Final incorporation into carbohydrate
 Stoma are closed during day time, preventing
transpiration
 CO2 is stored as malic acid
 During day time CO2 is released from malic acid

69. DARK PERIOD LIGHT PERIOD
 Acidification phase
 Starch is broken down to
form PEP
 PEP reacts with CO2 to form
OAA
 OAA is reduced malic acid
 End product of CO2 fixation
in night
 Stored in vacoules
 Deacidification Phase
 Malic acid diffuses out of
vacoules
 Maleic acid is
decarboxylated to pyruvate
and CO2
 CO2 is utilized by Rubisco to
run Calvin cycle

70.  Stomatal movements prevent water loss. Helps in
effective usage of water in succulents
 Helps to carry out photosynthesis without water loss
 Adaptation to extreme hot climates
 Tools in genetic attempts of plant improving
programmes

71. CO2
CO2
PEP
OAA Malic acid
Malic
acid
Vacoule
Malic acidPyruvic acidStarch
Starch
CO2
Calvin
cycle
Sugar
D
A
R
K
L
I
G
H
T

72.  The rate of the process is limited by the pace of
the slowest factor
 The slowest factor is the one present in relatively
lesser amounts than what is actually required
 The increase in only that factor which is limiting
will bring about an increase in the rate of the
process

74.  Redox potential – the tendency to accept electrons
from or donate electrons to another couple
 Redox potential allows the feasibility and directions
electron transfers
 Defined against the arbitrary standard , a hydrogen
half cell
 Negative potential- donate electrons
Positive potential- accept electrons
 Direction of electron transfer by comparing their redox
potentials
Redox potential

75.  Inhibitors of photosynthetic electron transport are effective
herbicides
 Two major classes- Derivatives of Urea (Monuron, Diuron) &
Triazine derivatives(Atrazine, Simazine)
 Bind with the QB site of D1 protein, interfering the binding of
PQ, blocking electron transport
 Another class – Bipyridylium Viologen dyes (Diquat &
Paraquat), obstruct electrons near PS I. They also produce
superoxide radicals