16Photosynthesis

📚
PHOTOSYNTHESIS
▢ Definition: Formation of carbohydrates from \(CO_2\) and \(H_2O\) by illuminated green cells of plants
▢ Overall Reaction: \(6CO_2 + 12H_2O^{18} \xrightarrow[Chlorophyll]{Light} C_6H_{12}O_6 + 6O_2^{18} + 6H_2O\)
▢ Reaction Components:

Table 1: Photosynthesis reaction components

Component
Role
\(CO_2\)
Raw material
\(H_2O\)
Raw material
\(C_6H_{12}O_6\)
Required product
\(O_2\)
By-product
\(H_2O\)
By-product
Sunlight
Main / ultimate energy source
▢ Light Spectrum:

Table 1: Light and photosynthesis

Point
Order
Maximum photosynthesis
White light / sunlight
Best effective spectrum
White light > Red > Blue > Green
Best absorption spectrum
White light > Blue > Red > Green
▢ Oxygen Source:
  • •
    \(O_2\) evolved during photosynthesis comes from \(H_2O\), not \(CO_2\)
  • •
    Experimentally proved by Ruben and Kamen
  • •
    Material used → Chlorella
  • •
    Radioactive isotope used → \(O^{18}\)
  • •
    Oxygen in glucose + water comes from \(CO_2\)
▢ Global Photosynthesis:
    Image 1
  • •
    About \(90\%\) of total photosynthesis → algae, mainly BGA, in oceans + freshwater
  • •
    Volume of \(CO_2\) absorbed ≈ volume of \(O_2\) liberated
📚
NATURE OF PHOTOSYNTHESIS
▢ Process Type:
  • •
    Reductive
  • •
    Oxidation-reduction / redox process
  • •
    Endergonic / energy requiring
  • •
    Anabolic
  • •
    Photochemical process
▢ Best Answer Point: Photosynthesis is best called reduction process because glucose forms by reduction of \(CO_2\)
▢ Energy Conversion: Light energy → chemical energy in presence of chloroplast
▢ MCQ Point:
Q1.
Photosynthesis is best described as
📚
CHLOROPLAST AND PHOTOSYNTHETIC SITE
Image 1
Image 1
▢ Chloroplast Number: Leaf parenchyma cell contains about 25–50 chloroplasts
▢ Pigment Site: Photosynthetic pigments mostly in grana, associated with thylakoid membrane
▢ Light and Dark Reaction:

Table 1: Main reactions of photosynthesis

Reaction
Process
Site / Time
Light reaction
Oxidation of \(H_2O\) to \(O_2\)
In light
Dark reaction
Reduction of \(CO_2\) to glucose
In light or dark
\(O_2\) produced only during day; glucose can be produced during day and night if products of light reaction are available
▢ Bacterial Photosynthesis:
  • •
    \(H_2S\) oxidized
  • •
    Hydrogen donor → \(H_2S\)
  • •
    Sulphur produced instead of \(O_2\)
  • •
    No oxygen liberation
📚
PHOTOSYNTHETIC PIGMENTS
📖
Chlorophyll
Image 1
Image 1
▢ Types:
  1. Chlorophyll-a
  2. Chlorophyll-b
  3. Chlorophyll-c
  4. Chlorophyll-d
  5. Chlorophyll-e
▢ Structure:
  • •
    Magnesium porphyrin compound
  • •
    Hydrophilic porphyrin head
  • •
    Lipophilic phytol tail
  • •
    Whole structure resembles tadpole larva
▢ Special Points:
  • •
    Burnt chlorophyll leaves Mg
  • •
    Mg-porphyrin head contains 4 pyrrole rings
  • •
    Phytol tail located at 7th carbon of 4th pyrrole ring
  • •
    Bacteriochlorophyll has 1 pyrrole ring with 2 hydrogen compared to chlorophyll-a
  • •
    Bacterial photosynthesis does not liberate oxygen
▢ Chlorophyll-a vs Chlorophyll-b:

Table 1: Chlorophyll-a vs Chlorophyll-b

Feature
Chlorophyll-a
Chlorophyll-b
Formula
\(C_{55}H_{72}O_5N_4Mg\)
\(C_{55}H_{70}O_6N_4Mg\)
Group at 3rd carbon of 2nd pyrrole ring
\(-CH_3\)
\(-CHO\)
Role
Primary / universal photosynthetic pigment
Accessory pigment
▢ Universal Pigment:
  • •
    Chlorophyll-a found in all photosynthesizing plants except bacteria
  • •
    Primary / universal photosynthetic pigment
  • •
    All algae contain chlorophyll-a + carotene
📖
Photosynthetic unit / quantasome
📝
Quantasome
  • •
    Photosynthetic pigments present in thylakoid membrane at quantasomes
  • •
    One quantasome has about 280 pigments
  • •
    Eukaryotic quantasome: 230 chlorophyll molecules + 50 carotenoids
  • •
    230 chlorophyll = 160 chlorophyll-a + 70 chlorophyll-b
  • •
    Prokaryotic quantasome: about 50 chlorophyll molecules

Table 1: Photosystem II vs Photosystem I

Feature
PS-II
PS-I
Location
Only on grana lamella
Grana + stroma lamella
Reaction centre
\(P_{680}\)
\(P_{700}\)
Pigment dominance
Chlorophyll-b > Chlorophyll-a
Chlorophyll-a > Chlorophyll-b
Function
Receives electrons from photolysis of water; non-cyclic photophosphorylation
Performs cyclic photophosphorylation independently
Fluorescence
More strongly fluorescent
Less fluorescent
📖
Other pigments

Table 1: Other photosynthetic pigments

Pigment
Formula / Type
Colour / Solubility
Bacteriochlorophyll
\(C_{55}H_{74}O_6N_4Mg\)
Bacterial photosynthesis
Carotene
Carotenoid
Orange; organic solvent soluble
Xanthophyll / carotenol
\(C_{40}H_{56}O_2\)
Yellow; organic solvent soluble
Phycoerythrin
Phycobilin
Red; water soluble
Phycocyanin
Phycobilin
Blue; water soluble
Allophycocyanin
Phycobilin
Bluish green; water soluble
Anthocyanin
Vacuolar pigment
Water soluble; bacterial chameleon
📝
Solubility
  • •
    Chlorophyll + carotenoids → soluble in organic solvents like acetone
  • •
    Phycobilins → water soluble
  • •
    Anthocyanin → water soluble; found in vacuole
📝
Accessory Pigments
  • •
    All pigments except chlorophyll-a
  • •
    Carotenoids protect chlorophyll from photo-oxidation
  • •
    Carotenoids transfer light energy to chlorophyll
📚
LIGHT REACTION / LIGHT DEPENDENT REACTION / HILL REACTION / OXIDATION REACTION
▢ Photolysis of Water:
❖ Equation: \(4H_2O \xrightarrow[Mn^{2+},\ Cl^-]{h\nu,\ pigment} 2H_2O + O_2 + 4H^+ + 4e^-\)
❖ Water Molecules:

Table 1: Water in photolysis

Point
Number
Water molecules required
4
Water molecules used
2
▢ Non-cyclic vs Cyclic Photophosphorylation:

Table 1: Non-cyclic vs cyclic photophosphorylation

Feature
Non-cyclic photophosphorylation
Cyclic photophosphorylation
Found in
Higher plants + BGA / cyanobacteria
Bacteria + few red algae
Wavelength
Common visible rays
Infrared / IR
Primary acceptor
2
1
Pigment system
PS-I + PS-II
PS-I only
Source of electrons + protons
\(H_2O\)
PS-I
Terminal acceptor of proton
NADP
PS-I
Water production
2
0
Oxygen production
1; oxygenic
0; anoxygenic
ATP production
1
2
\(NADPH_2\) production
2
0
Significance
\(H_2O\), \(O_2\), \(NADPH_2\)
ATP
Universal H acceptor = NAD; natural H acceptor in photosynthesis = NADP
Image 1
Image 1
▢ Oxygen Contribution:
  • •
    About \(83\%\) \(O_2\) evolved by BGA
  • •
    About \(95\%\) \(O_2\) comes from sea
▢ Quantum Requirement and Quantum Yield:

Table 1: Quantum terms

Term
Meaning / Value
Quantum yield
Number of \(O_2\) molecules evolved per quantum of light absorbed
Quantum requirement
Number of quanta required for evolution of 1 molecule of \(O_2\)
Quantum requirement value
8 quanta
Quantum yield value
\(\frac{1}{8} = 0.125\)
📚
DARK REACTION / LIGHT INDEPENDENT REACTION / BLACKMAN REACTION / REDUCTION REACTION
📖
Photorespiration / c2 cycle / warburg effect
Image 1
▢ Definition: Light-dependent wasteful oxidation pathway involving chloroplast, peroxisome and mitochondria; \(O_2\) consumed and \(CO_2\) released
▢ Key Enzyme: RuBisCO acts as carboxylase or oxygenase
▢ Chloroplast Events:
  • •
    \(RuBP + CO_2 \xrightarrow{RuBP\ carboxylase} 2\ molecules\ of\ 3-PGA\)
  • •
    \(RuBP + O_2 \xrightarrow{RuBP\ oxygenase} 1\ molecule\ 3-PGA + phosphoglycolate\)
  • •
    3-PGA enters \(C_3\) cycle
  • •
    Phosphoglycolate → glycolate / glycolic acid
▢ Peroxisome Events:
  • •
    Glycolic acid → glyoxylate
  • •
    Glyoxylate → glycine
  • •
    \(H_2O_2\) + \(\frac{1}{2}O_2\) involved
▢ Mitochondria Events:
  • •
    2 glycine \((C_2)\) → serine \((C_3)\)
  • •
    \(CO_2\), \(NH_3\) and energy lost
▢ Result:
  • •
    Photosynthetic efficiency decreases
  • •
    \(CO_2\) released
  • •
    \(O_2\) consumed
  • •
    No sugar/ATP production
📖
C3 cycle / calvin cycle
Image 1
▢ Discovery:
  • •
    Discovered by Melvin Calvin
  • •
    Nobel Prize awarded
  • •
    Method used → autoradiography
  • •
    Plant used → Chlorella
▢ Occurrence: About \(83\%\) plants
▢ Main Reaction Flow: \(6CO_2 + 6RuBP \xrightarrow{RuBisCO} 12\ 3-PGA \xrightarrow[12NADPH_2]{12ATP} 12\ PGAL \rightarrow Glucose + Regeneration\ of\ RuBP\)
▢ Phases:

Table 1: Calvin cycle phases

Phase
Event
CO₂ fixation
\(6CO_2 + 6RuBP\) → \(12\ 3-PGA\)
Reduction
\(12\ 3-PGA\) → \(12\ PGAL\) using \(12ATP + 12NADPH_2\)
Glycolytic reversal
2 molecules of PGAL → glucose
Regeneration
10 molecules of PGAL → \(6RuMP\) → \(6RuBP\) using \(6ATP\)
▢ Requirement per Glucose: \(6CO_2 : 12NADPH_2 : 18ATP = 1 : 2 : 3\)
📖
C4 cycle / hatch and slack pathway
Image 1
▢ Occurrence: About \(13\%\) plants
▢ Nature: Additive pathway
▢ Special Point: Herbicide kills grasses by inhibiting \(C_4\) pathway
▢ Cellular Separation:

Table 1: C4 pathway cells

Cell
Event
Mesophyll cell
\(CO_2\) fixation by PEP carboxylase
Bundle sheath cell
\(CO_2\) released and enters \(C_3\) cycle
▢ Main Flow: \(6CO_2 + 6PEP \xrightarrow{PEP\ carboxylase} 6OAA \rightarrow 6Malic\ acid \rightarrow 6CO_2 + 6Pyruvic\ acid\); \(CO_2\) enters \(C_3\) cycle
▢ Energy: Extra \(12ATP\) used for regeneration of PEP
▢ Requirement per Glucose: \(6CO_2 : 12NADPH_2 : 30ATP = 1 : 2 : 5\)
📖
Cam cycle
Image 1
▢ Occurrence: About \(4\%\) plants
▢ Night:
  • •
    Acidification
  • •
    \(CO_2\) fixed by PEP
  • •
    Malic acid accumulates
  • •
    Glucose decreases
▢ Day:
  • •
    Deacidification
  • •
    Malic acid releases \(CO_2\)
  • •
    \(CO_2\) enters \(C_3\) cycle
  • •
    Glucose increases
▢ Requirement per Glucose: \(6CO_2 : 12NADPH_2 : 30ATP = 1 : 2 : 5\)
📚
COMPARISON OF C3, C4 AND CAM PLANTS

Table 1: C3, C4 and CAM plants

Character
C3 plants / Calvin cycle
C4 plants / Hatch-Slack pathway
CAM plants
Percentage of plants
\(83\%\)
\(13\%\)
\(4\%\)
CO₂ acceptor
RuBP \((5C)\)
Primary: PEP in mesophyll; secondary: RuBP in bundle sheath
Primary: PEP at night; secondary: RuBP at day
First enzyme
RuBisCO
PEP carboxylase
PEP carboxylase
First stable compound
3-PGA \((3C)\)
OAA / oxaloacetic acid \((4C)\)
Malic acid \((4C)\) > OAA \((4C)\)
CO₂ compensation point
45–100 ppm
15–25 ppm
5–10 ppm
Leaf anatomy
Kranz anatomy absent
Kranz anatomy present
Kranz anatomy absent
Types of chloroplast
Single type
Two types: granal + agranal chloroplast
Single type
C3 cycle occurs in
All photosynthetic cells
Bundle sheath cells
All photosynthetic cells
Requirement per glucose
\(6CO_2 : 12NADPH_2 : 18ATP = 1:2:3\)
\(6CO_2 : 12NADPH_2 : 30ATP = 1:2:5\)
\(6CO_2 : 12NADPH_2 : 30ATP = 1:2:5\)
Optimum temperature
\(25-30^\circ C\)
\(35-45^\circ C\)
\(>45^\circ C\)
Photorespiration
May occur
No chance
May occur
Common plant group
Common in dicots; also algae, bryophytes, pteridophytes, gymnosperms
Common in monocots + tropical plants
Succulents + desert plants
📚
EXAMPLES OF C3, C4 AND CAM PLANTS

Table 1: Examples

Type
Examples
C3 plants
Algae e.g., Chlorella; bryophytes; pteridophytes; gymnosperms; common dicots; rare monocots e.g., rice, wheat, barley, oat
C4 plants
Maize, sugarcane, grasses, tropical plants; rare dicots e.g., Amaranthus, Alternaria, Atriplex, Chenopodium, Euphorbia
Both C3 and C4
Atriplex
CAM plants
Bryophyllum, Crassulaceae, Orchidaceae, desert plants, Agave, Aloe, cactus, Opuntia, Sedum, Kalanchoe, Portulaca, pineapple
Q1.
The first stable compound of the Calvin cycle in C₃ plants is:
📅CEE MBBS 2025
Q2.
During non-cyclic photophosphorylation, the light reactions produce:
📅CEE MBBS 2024a
Q3.
During photosynthesis, oxygen is evolved from water by:
📅CEE MBBS 2024a
Q4.
Which region of visible light is typically most effective for photosynthesis?
📅CEE MBBS 2023
Q5.
The metal ion at the centre of chlorophyll is:
📅CEE BDS 2024
Q6.
The first CO₂ acceptor in the Calvin cycle is:
📅CEE Nursing 2024
Q7.
ATP synthesis linked to PSII electron transport is driven immediately by:
📅IOM 2007
Q8.
For simple oxygenic photosynthesis, CO₂ uptake and O₂ evolution are approximately:
📅IOM 2006
Q9.
C₄ photosynthesis is an alternative carbon-fixation adaptation found in:
📅BPKIHS 2000
Q10.
Some herbicides such as atrazine inhibit photosynthesis by blocking:
📅BPKIHS 2000
Q11.
Photorespiration is promoted by:
📅BPKIHS 2001
Q12.
The Calvin cycle requires per 3 CO₂ fixed:
📅BPKIHS 2001
Q13.
Which statement about chloroplast photophosphorylation is correct?
📅BPKIHS 2002
Q14.
CO₂ release during photorespiration occurs chiefly in the:
📅BPKIHS 2002
Q15.
The primary CO₂ acceptor in C₄ mesophyll is:
📅BPKIHS 2002
Q16.
In typical C₄ leaves, the Calvin cycle occurs mainly in:
📅BPKIHS 2002
Q17.
C₄ plants are especially efficient under:
📅BPKIHS 2002
Q18.
Oxygenic photosynthesis converts:
Q19.
A short light pulse interrupting the dark period of photoperiodism is a:
📅BPKIHS 2004
Q20.
The molecular formula of chlorophyll a is:
📅BPKIHS 2005
Q21.
Chlorophyll b differs from chlorophyll a by having:
Q22.
The initial CO₂ acceptor of the C₄ pathway is:
📅MOE
Q23.
Light reactions occur across the:
📅BPKIHS 1994
Q24.
The final electron acceptor in linear photosynthetic electron transport is:
📅BPKIHS 1995
Q25.
Calvin cycle reactions take place in the:
📅BPKIHS 1996
Q26.
The main final hydrogen/electron acceptor in oxygenic photosynthesis is:
📅BPKIHS 1996
Q27.
A chlorophyll-free plant cell generally cannot:
📅BPKIHS 1997
Q28.
Photorespiration is promoted by:
📅BPKIHS 1997
Q29.
Which light quality is least effective for chlorophyll-driven photosynthesis?
📅BPKIHS 1997
Q30.
PEP is the primary CO₂ acceptor in:
📅BPKIHS 1998
Q31.
Light energy is converted to chemical energy during:
📅MOE 2008
Q32.
Moll's half-leaf experiment demonstrates the requirement of:
📅MOE 2063
Q33.
Chemoautotrophs derive energy for carbon fixation by oxidation of:
📅MOE 2063
Q34.
The balanced overall equation for oxygenic photosynthesis may be written as:
📅MOE 2063
Q35.
Approximately what fraction of incident solar energy is captured in global net primary production?
📅MOE 2062
Q36.
Hatch–Slack–Kortschak (HSK) pathway is also called:
📅MOE
Q37.
To test a destarched leaf for starch, chlorophyll is removed using:
📅MOE 2062
Q38.
The isotope used by Calvin to trace carbon fixation was:
Q39.
Thylakoids are directly associated with:
📅MOE 2002
Q40.
The O₂ released in photosynthesis originates from:
📅MOE 2000
Q41.
Photosynthetic green plants are predominantly:
📅MOE 2056
Q42.
The first stable product of C₃ carbon assimilation is:
📅MOE 2056
Q43.
Some anoxygenic photosynthetic bacteria can use:
📅MOE 1997
Q44.
The major products of the light reactions are:
Q45.
The central metal atom of chlorophyll is:
📅IE 2001
Q46.
Plants commonly store excess photosynthate as:
📅IE 2002
Q47.
ATP formation driven by light is called:
📅IE 2003
Q48.
The Hill reaction with isolated chloroplasts is demonstrated using an artificial electron acceptor such as:
📅IE 2004
Q49.
Photosynthesis in green plant cells occurs in:
📅IE 2005
Q50.
Plants store carbohydrate as starch rather than free glucose largely because starch:
📅IE 2006
Q51.
In C₄ plants the Calvin cycle takes place mainly in:
📅BPKIHS 2014
Q52.
Inhibition of photosynthesis by elevated O₂ is:
📅BPKIHS 2014
Q53.
Which wavelengths are generally absorbed most strongly by chlorophyll?
Q54.
Cytochromes are:
Q55.
The Calvin cycle is also called the:
📅BPKIHS 2010
Q56.
The initial photochemical event at a reaction centre is:
📅IOM 2016
Q57.
Cytochromes mainly participate in:
Q58.
High O₂ inhibition of C₃ photosynthesis is the:
📅IOM 2014
Q59.
Photolysis of water occurs during the:
📅IOM 2013
Q60.
Light-driven ATP synthesis occurs during the:
📅IOM 2012
Q61.
The primary CO₂ acceptor of the Calvin cycle is:
📅MOE 2014
Q62.
Stomata open at night and close during day typically in:
📅MOE 2013
Q63.
Initial carbon fixation in C₄ mesophyll is catalysed by:
📅MOE 2013
Q64.
Kranz anatomy is characteristic of:
📅MOE 2012
Q65.
The first stable compound of the C₃ cycle is:
📅MOE 2068
Q66.
The plastid containing grana and stroma is:
Q67.
The core metal of chlorophyll is:
📅IE 2008
Q68.
The plant that uses crassulacean acid metabolism is:
Q69.
Photorespiration involves chloroplasts, peroxisomes, and:
📅IOM 2014
Q70.
Only ATP, not NADPH, is generated by:
📅MOE 2010