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.
TEST BANK For Radiologic Science for Technologists, 12th Edition by Stewart C...
Photosynthesis: Light Reactions and Carbon Fixation
1.
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
4.
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
7.
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)
11.
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
14.
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
16.
17.
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
20.
21. 2 addidtional CP
complexes, closely
associated with
photosystems are
LHC I & LHC II
Extended antenna
system to ensure
efficient light
harvesting
22.
23. PS II PS ICyt
H20
½ o2+2H+
NADPH+H+
NADP+2H+
Two Photosystems operating in series
light
light
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
27.
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
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
35.
36.
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
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
48.
49.
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
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
54.
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
56.
57.
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
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
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
73.
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