17Photosynthesis

📚
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.
Energy for the ATP formation in PS-II comes directly from: [IOM 2007]
📅IOM 2007
Q2.
Volume of CO_2 absorbed in photosynthesis is almost: [IOM 2006]
📅IOM 2006
Q3.
In some grasses C_4 pathway occurs. It is:    [BPKIHS 2000]
📅BPKIHS 2000
Q4.
Herbicide kills grasses (weeds) by:  [BPKIHS 2000]
📅BPKIHS 2000
Q5.
Which of the following induces photo-respiration?  [BPKIHS 2001]
📅BPKIHS 2001
Q6.
The no. of ATP and NADPH2 required to fix 3CO_2 molecules in Calvin cycle: [BPKIHS 2001]
📅BPKIHS 2001
Q7.
Which is wrong?      [BPKIHS 2002]
📅BPKIHS 2002
Q8.
In Photo-respiration, CO_2 loss takes place in [BPKIHS 2002]
📅BPKIHS 2002
Q9.
In C_4 cycle, in mesophyll cells, CO_2 is picked up by: [BPKIHS 2002]
📅BPKIHS 2002
Q10.
In C_4 plants, Calvin cycle takes place in: [BPKIHS 2002]
📅BPKIHS 2002
Q11.
Which is correct about C_4 plants?   [BPKIHS 2002]
📅BPKIHS 2002
Q12.
Photosynthesis is:
Q13.
Light break reaction means:  [BPKIHS 2004]
📅BPKIHS 2004
Q14.
Chemical formula of 'chlorophyll' is:
📅BPKIHS 2005
Q15.
In chlorophyll-b, −CHO (aldehyde) group is attached to the 3^rd carbon atom of:
Q16.
The first acceptor of CO_2 in C_4 pathway is:  [MOE]
📅MOE
Q17.
Light reaction of photosynthesis occurs in:  [BPKIHS 1994]
📅BPKIHS 1994
Q18.
The electron acceptor during photosynthesis is:  [BPKIHS 1995]
📅BPKIHS 1995
Q19.
Dark reaction of photosynthesis occurs in  [BPKIHS 1996]
📅BPKIHS 1996
Q20.
Universal hydrogen acceptor   [BPKIHS 1996]
📅BPKIHS 1996
Q21.
A cell devoid of chlorophyll will not  [BPKIHS 1997]
📅BPKIHS 1997
Q22.
Photorespiration is due to:  [BPKIHS 1997]
📅BPKIHS 1997
Q23.
The rate of photosynthesis is independent of  [BPKIHS 1997]
📅BPKIHS 1997
Q24.
Phosphoenol pyruvate acts as CO_2 acceptor in: [BPKIHS 1998]
📅BPKIHS 1998
Q25.
Light energy is converted into chemical energy in:  [MOE 2008]
📅MOE 2008
Q26.
Moll's half leaf experiment can prove:  [MOE 2063]
📅MOE 2063
Q27.
Chemoautotrophs obtain energy for photosynthesis by the oxidation of: [MOE 2063]
📅MOE 2063
Q28.
Which of the following represent photosynthetic equation:  [MOE 2063]
📅MOE 2063
Q29.
The % of sunlight utilized by the green plant in the net plant productivity during photosynthesis?  [MOE 2062]
📅MOE 2062
Q30.
HSK pathway is also called as  [MOE]
📅MOE
Q31.
To demonstrate there is no starch formation in leaves of a green plant kept in dark for 1 or 2 days the leaves are tested by:   [MOE 2062]
📅MOE 2062
Q32.
During photosynthesis to trace out the path of carbon fixation, the isotope used is:
Q33.
Thylakoids are associated with:  [MOE 2002]
📅MOE 2002
Q34.
In photosynthesis O2 is liberated due to: [MOE 2000]
📅MOE 2000
Q35.
Photosynthetic plants are:  [MOE 2056]
📅MOE 2056
Q36.
The 1^st stable product in carbon assimilation is: [MOE 2056]
📅MOE 2056
Q37.
Which wavelength of light carries out photosynthesis in bacteria?  [MOE 1997]
📅MOE 1997
Q38.
During photosynthesis the product of the light reaction is:
Q39.
The core metal of chlorophyll molecule is: [IE-2001]
📅IE-2001
Q40.
In plant food is stored in the form of:  [IE-2002]
📅IE-2002
Q41.
ATP formation during photosynthesis is  [IE-2003]
📅IE-2003
Q42.
Hill  reaction occurs in   [IE-2004]
📅IE-2004
Q43.
Photosynthesis occurs in  [IE-2005]
📅IE-2005
Q44.
Plants don't store carbohydrate as glucose because:  [IE-2006]
📅IE-2006
Q45.
Calvin cycle in \(C_4\) plants occurs in  [BPKIHS 2014]
📅BPKIHS 2014
Q46.
The effect of excess O_2 concentration causing decrease in photosynthsis is known as:  [BPKIHS 2014]
📅BPKIHS 2014
Q47.
Rate of photosynthesis will be higher in
Q48.
Cytochrome is
Q49.
'Calvin cycle' is synonymous with  [BPKIHS 2010]
📅BPKIHS 2010
Q50.
The first step involved in the photosynthesis  [IOM 2016]
📅IOM 2016
Q51.
Cytochrome helps in
Q52.
Inhibition of photosynthesis with the increase in oxygen concentration is  [IOM 2014]
📅IOM 2014
Q53.
The process of photolysis of water takes place during  [IOM 2013]
📅IOM 2013
Q54.
ATP is formed in triphosphate in a large number during  [IOM 2012]
📅IOM 2012
Q55.
Primary acceptor of \(CO_2\) in photosynthesis is  [MOE 2014]
📅MOE 2014
Q56.
Stomata open at nigh and close during day in  [MOE 2013]
📅MOE 2013
Q57.
Carbon fixation in \(C_4\) plants is catalyzed by [MOE 2013]
📅MOE 2013
Q58.
Kranz anatomy is found in  [MOE 2012]
📅MOE 2012
Q59.
First stable compound in \(C_3\) cycle is  [MOE 2068]
📅MOE 2068
Q60.
Which organelle in cell division consists of grana and stroma?