The document summarizes key aspects of photosynthesis including the structure and function of the cytochrome b6f complex and photosystem I. It discusses: 1) The cytochrome b6f complex transfers electrons from photosystem II to photosystem I while pumping protons across the thylakoid membrane. It is composed of four large subunits including cytochrome f and b6 and four small subunits. 2) Photosystem I contains a reaction center called P700 and associated antenna pigments that absorb light and transfer energy to P700. It is a multi-subunit protein complex located in the stroma lamellae. 3) Both complexes play important roles in the light-dependent reactions of

1. Structure and function of Cytochrome
b6f complex and Photosystem-I
Under the Guidance
of
Prof. S. M. Prasad
Presented By
Rajnish Kumar
M.Sc-3rd Sem.(Botany)
Department of botany

2. Photosynthesis
 Photosynthesis is a biochemical reaction which convert solar
energy to chemical energy.
 Photosynthesis is anabolic process.
6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O
 In photosynthesis, plants are taken CO2 from atmosphere and
H2O from soil to formed carbohydrate and Oxygen. In the
presence of sunlight and chlorophyll.
Sun light
Chlorophyll

3. Photosynthesis in higher plants
 In plants light energy convert into chemical energy or
photosynthesis are takes place in chloroplasts.
 In higher plants the most active photo -synthetic tissue is
mesophyll of leaves.
 Mesophyll cells have many chloroplast, which contain the
specialized light-absorbing green pigments. i.e.-chlorophyll.

4.  In photosynthesis plant use solar energy to oxidize water,
thereby releasing oxygen, and to reduce carbon dioxide,
thereby forming large carbon compounds, primarily sugars.

5. The location and structure of -
chloroplasts
LEAF CROSS SECTION MESOPHYLL CELL
LEAF
Chloroplast
Mesophyll
CHLOROPLAST
Intermembrane space
Outer
membrane
Inner
membrane
Thylakoid
compartmentThylakoidStroma
Granum
StromaGrana

6. Structure of Chloroplast
 Chloroplast has two membranes-
1-outer membrane – permeable.
2-inner membrane - enclosed inner compartment.
 Each membrane is made up of lipoproteins.
 The width of each membrane is 50Å.
 Inner compartment-
This membrane enclosed sacs called Thylakoids involving
paired folds (lamellae) by stacking form "Grana".

7.  The soluble portion of the chloroplast is the "Stroma".
 The aqueous compartment (Stroma) contained several
enzymes, So it is the site of carbon fixation (synthesis of
carbohydrate - Dark reaction).
 The interior of the thylakoid vesicles is the "Thylakoid space"
or "Thylakoid lumen".
 Thylakoid membrane contains Pigments like chlorophyll and
carotenoids, for Photophosphorelation and the enzymes for
ATP synthesis.
Chlorophylls - primary light-absorbing pigments.
Carotenoids - accessory pigments.

8. Ultrastructure of chloroplast

9. CHLOROPLAST PIGMENTS
 A Chloroplast pigment (accessory pigment;
photosynthetic pigment; antenna pigment) is a pigment that is
present in chloroplasts or photosynthetic bacteria and captures
the light energy necessary for photosynthesis.
 Chloroplasts contain several pigments -
Chlorophyll a - Blue-green pigment.
Chlorophyll b - Yellow-green pigment.
Carotenoids - An orange pigment.
Xanthophyll - An Yellow pigment.

10. Chlorophyll
 Chlorophyll is a light absorbing green pigment.
 Chlorophyll is a lipid soluble pigment.
 Chlorophyll contains a polycyclic, planar tetrapyrrole ring
structure.
 The Central metal ion in chlorophyll is Mg2+ .
 Chlorophyll has a cyclopentanone ring (ring-V) fused to
pyrrole ring III.

11.  Chlorophyll has a long
Phytol chain on Pyrrole
ring-IV
 Chl-a & chl-b are mostly
found in green plants.
 In Chlorophyll-a
(C55H72O5N4Mg)-present
CH3 Group on II Pyrrole
ring.
 In Chlorophyll-b
(C55H70O6N4Mg)-CH3
Group is replaced by CHO
Group.

12.  Carotenoids are linear molecules with
multiple conjugated double bonds.
 Carotenoids are lipid soluble pigment.
 Carotenoids are Orange pigment
(400-500 nm).
 Carotenoids are accessory pigment.
 Carotenoids are linear polyenes that
serve as both antenna pigments and
photo protective agents.
Carotenoids

13. Nature of light
 Sunlight is a type of energy, which are called as radiant
energy or electromagnetic energy.
 Light has characteristics of both a wave and particle.
 A wave is characterized by a wavelength.
 The distance between two successive wave crests is known as
wavelength.
 Wavelength is denoted by the Greek letter lambda(𝝀).
 Wavelength is measure in nanometer(nm).

14.  Frequency is the number of wave crests that pass an observer
in a given time.
 Frequency is denoted by Greek letter nu(𝝂).
 Frequency is measure in Hertz(Hz).
 The relationship between wavelength and frequency is-
Where, C = Speed of light.(3.8×10 8 ms -1)
𝝀 = Wavelength.
𝝂 = Frequency.
c = 𝝀𝝂

15.  Light is also travels a particles. These particles or packets are
called Photon.
 Each photon contains an amount of energy that is called a
Quantum(pl.-Quanta).
 The energy (E) of photon depends on the frequency of the
light according to a relation known as Planks law-
Where, E = Energy.
H = Planks constant(6.626×10-34 js).
𝝂 = 𝐅𝐫𝐞𝐪𝐮𝐞𝐧𝐜𝐲.
E = h𝝂

16.  When sunlight is pass through a prism then will we found
found different colour of rays which are called as visible
spectrum.
 The complete visible spectrum is made up of seven colour.
Which are found in form of VIBGYOR .

17. ROLE OF PHOTOSYNTHETIC -
PIGMENTS IN PHOTOSYNTHESIS
 When sunlight is falls on
leaves. In which 83% is
absorbed by leaves, 12% is
reflected & 5% is transmitted.
 In photosynthesis only 4%
sunlight is used by
chlorophyll and 79 %
sunlight is diffused in
atmosphere in the form of
heat.

18.  The light absorbing complex is called as photosynthetic
pigment. for example-
Chl-a, Chl-b, Carotenoids etc.
 These photosynthetic pigment are found in membrane of
thylakoid and mainly absorbed Blue, Violet, Red & Orange
rays.
 Chlorophyll-b, carotenoids & some kinds of Chl-a are play
the important role in Antenna molecule.
 These antenna molecule absorb light energy and transfer to
reaction center.

19.  After absorb the light energy pigment molecules are excited or
unstable.
chl. + h𝝂 chl.*
 Excited chlorophyll can re-emit a photon in the form of heat
and thereby returns to its ground state. These process is
known as fluorescence.

20. Mechanism of Photosynthesis -
Source of Librated Oxygen In Photosynthesis
 Before 1930, Scientists consider, the one molecule of
CO2 and one molecule of H2O are formed one
molecule of formaldehyde in the presence of sunlight.
 These formaldehyde give rise to glucose molecule after
Polymerization.
CO2 + H2O CH2O + O2
6CH2O C6H12O6
Sunlight
Polymerization
Formaldehyde
Hexose sugar

21.  But formaldehyde are poisonous compound, which causes
death of plants. Hence in photosynthesis the formation of
formaldehyde is not possible.
 In 1930, Cornelius B. Van Niel proved that sulfur bacteria are
formed carbohydrate from H2S and CO2 in the presence of
light and produced sulfur.
6CO2 +12H2S C6H12O6 + 6H20 +12 S
Light
 Hence Van Niel suggest that sulfur are separated by synthesis
of sulfur bacteria. therefore in photosynthesis oxygen is
separated by decomposition of water.

22. 6CO2 + 12H2O C6H12O6 + 6H2O + 6O2
 The above hypothesis was proved by Ruben in 1941 on his
experiment by using O18 atomic weight of isotopes and he
concluded oxygen are come out from water molecule in
photosynthesis.
 So the complete equation of photosynthesis is written
fallowing-
Light
Chlorophyll

23. Oxygenic and An-oxygenic
photosynthesis
 Oxygenic photosynthesis produce oxygen and it is found in
green plants and cyanobacteria.
CO2 + H2O SUGAR + OXYGEN
 An-oxygenic photosynthesis does not produce oxygen and it is
found in green and purple photosynthetic bacteria.
CO2 + H2S SUGAR + SULPHUR

24. Modern concept of mechanism of photosynthesis
 Photosynthesis is a oxidation-reduction process. In which
water oxidize, thereby releasing oxygen, and carbon dioxide
reduce, thereby forming large carbon compounds, primarily
sugars.
 Photosynthesis are completed in two phases -
1. Photocemical reaction / light dependent reaction.
2. Chemical dark reaction / light independent reaction.

25. Light dependent reaction
 It is occurs in grana of chloroplast.
 In the chloroplast, light energy is converted into chemical
energy by two different functional units called photosystems.
 Photosystem are two type -
1. Photosystem-I / Photo-act-I
2. Photosystem-II / Photo-act-II
 Each photosystem has over 200 molecules of chlorophylls and
about 50 molecules of carotenoids.

26. LIGHT HARVESTING COMPLEXES (LHCs)
 The light harvesting
complex (antenna
complex) is an complex of
protein and chlorophyll
molecules embedded in the
thylakoid membrane of
plants, which transfer light
energy to one chlorophyll a
molecule at the reaction
center of photosystem.
 Those LHC is associated
with PS-I is called LHC-I
protein.
 Those LHC is associated
with PS-II is called LHC-II
protein.

27. DISTRIBUTION OF PHOTOSYNTHETIC
PROTEINS
 The thylakoid membrane of chloroplast are found in two
forms stacked (appressed) and un-stacked (non-appressed)
region.
 PS-II are found in mainly stacked region.
 PS-I & ATP Synthase are found in mainly un-stacked region.
which are attached to stroma.
 Cyt-b6-f complex are similarily found in both stacked & un-
stacked region.

29. Cytochrome b6f Complex
 The cytochrome b6f complex is an enzyme found in
the thylakoid membrane in chloroplasts of plants,
cyanobacteria, and green algae, that catalyze the transfer of
electrons from plastoquinol to plastocyanin.
 During photosynthesis, the cytochrome b6f complex
transfers electrons from Photosystem II to Photosystem I,
whereby pumping protons into the thylakoid space and
creating an electrochemical (energy) gradient where it is later
used to create adenosine triphosphate (ATP).

30.  The cytochrome b6f complex is a large multi -subunit protein
with Seven prosthetic groups.
 The cytochrome b6f complex is a dimer, with each monomer
composed of eight subunits.
 The cytochrome b6f complex is consist of four large subunits-

31.  cytochrome f (PetA )
with a c-type
cytochrome,
 cytochrome b6 (PetB)
with a low- and high-
potential heme group,
 Rieske iron-sulfur
protein (PetC)
containing a [2Fe-2S]
cluster, and
 subunit IV(PetD); along
with four small
subunits : PetG, PetL,
PetM, and PetN.

32. H2O →
photosy
stem II
→ QH2 → Cyt b6f → Pc →
photosy
stem I
→ NADPH (1)
QH2 → Cyt b6f → Pc →
photosyst
em I
→ Q (2)
Reaction mechanism
The cytochrome b6f complex is responsible for "non-
cyclic" (1) and "cyclic" (2) electron transfer between two mobile
redox carriers, plastoquinone (QH2) and plastocyanin (PC).
Cytochrome b6f catalyzes the transfer of electrons from
plastoquinol to plastocyanin, while pumping two protons from the
stroma into the thylakoid lumen:
QH2 + 2Pc(Cu2+) + 2H+ (stroma) → Q + 2Pc(Cu+) + 4H+ (lumen)

33. This reaction occurs through the Q cycle as in Complex
III. Plastoquinone acts as the electron carrier, transferring its
two electrons to high- and low-potential electron transport
chains (ETC) via a mechanism called electron bifurcation.
Q - Cycle
(PQH₂) is oxidised & one of 2 e⁻ is passed a linear e⁻ transport
chain toward PS-I & other e⁻ goes through a cyclic process that
increases the number of protons pumped across the membrane.

34. (A) The non-cyclic or linear process
PQH₂ Produced by PS-II is oxidised near the lumenal side of
the complex, transfering its 1e⁻ to Rieske Fe-S protein & 1e⁻ to
b-type Cyt. & expelling 2 protons to lumen. The e⁻ transferred to
cyt f & to PC which reduces P700 of PS-I. The reduced b-type cyt
transfers e⁻ to other b-type cyt which reduces PQ to
plastosemiquinone (PQ⁻) state.

35. (B) The cyclic process
A second PQH₂ is oxidised , with 1 e⁻ going from FeSR to
pc & finally to P700. The 2nd e⁻ goes through 2 b-type cyt &
reduces the plastohydroquinone, at the same time picking
up two protons from stroma ,Overall, 4 protons are
transported across the membrane for every 2 e⁻ delivered
to P700.

36. Biological function
 In photosynthesis, the cytochrome b6f complex functions to
mediate the transfer of electrons between the two
photosynthetic reaction center complexes, from Photosystem
II to Photosystem I, while transferring protons from the
chloroplast stroma across the thylakoid membrane into
the lumen.
 Electron transport via cytochrome b6f is responsible for
creating the proton gradient that drives the synthesis
of ATP in chloroplasts.
 In a separate reaction, the cytochrome b6f complex plays a
central role in cyclic photophosphorylation, when NADP+ is
not available to accept electrons from reduced ferredoxin.

37.  This cycle results in the creation of a proton gradient by
cytochrome b6f, which can be used to drive ATP synthesis.
 It has also been shown that this cycle is essential for
photosynthesis in which it is proposed to help maintain the
proper ratio of ATP/NADPH production for carbon fixation.
PHOTOSYSTEM-I (PS-I)
Photochemical events are similar to those in PS-II.
PS-I is composed of a heterodimer of proteins that act as
ligands for most of the electron carriers.

38.  light is absorbed by antenna molecules and the energy is
transferred to P700 (reaction center) by resonance energy
transfer.
Chlorophyll-a & Chlorophyll-b
 The PS-I reaction center and its associated antenna pigments
and electron transfer proteins, as well as the coupling-factor
enzyme that catalyzes the formation of ATP, are found almost
exclusively in the Stroma lamellae and at the edges of the
grana lamellae.

39. PHOTOSYSTEM-I
• Multi-subunit protein complex.
• Contains about 14 different proteins:
a) Core proteins –PsaA , PsaB.
b) Peripheral proteins on stromatal side-PsaC, PsaD, PsaE ,
c) Integral membrane proteins-PsaF, PsaG, PsaH, PsaI, PsaJ,
PsaK, PsaL, PsaM.
d) Lumenal protein- PsaN.

40.  Approx. 100 chlorophyll
molecules.
 12-16 carotenes.
 Phylloquinone.
 Additional chain of 5
electrons acceptors:A0,
A1and 3(4fe-4s) centers
:fx, FA, FB.

41.  The excited reaction center P700* loses an electron to an
electron acceptor, A0 (like pheophytin in PS-II) creating A0
-
and P700+.
 This results in charge separation at the photochemical
reaction center. P700+ is a strong oxidizing agent ,
 It acquires an electron from plastocyanin, a soluble cu-
containing electron transfer protein.
 A0
- is a strong reducing agent. It passes its electrons through
a chain of carriers leading to NADP+.
ELECTRON TRANSFER PATHWAY IN PS-I

42.  A0 passes its electrons to
Phylloquinone(A1)
 A1 passes it to an fe-s protein.
 Fe-s protein passes the
electron to ferredoxin(fd)
(another fe-s protein).
 The electron is then
transferred to a ferredoxin
NADP reductase.
(flavoprotein) The electron is
transferred from reduced fd to
NADP+.

44. USE OF PHOTOSYSTEM - I
 Photosystem-I used in cyclic and noncyclic
photophosphorylation.
 While photosystem-II used in only
noncyclic photophosphorylation.

45. Cyclic Photophosphorylation
 In cyclic electron transfer, the electron begins in a pigment
complex called Photosystem-I.
 Then passes from the primary acceptor to ferredoxin, then
to cytochrome b6f and then to plastocyanin before returning to
chlorophyll.
 This transport chain produces a proton-motive force, pumping
H+ ions across the membrane; this produces a concentration
gradient that can be used to power ATP synthase during
chemiosmosis.
 This pathway is known as cyclic photophosphorylation, and it
produces neither O2 nor NADPH.

46.  In bacterial photosynthesis, a single photosystem is used, and
therefore is involved in cyclic photophosphorylation. It is
favored in anaerobic conditions and conditions of high
irradiance and CO2 compensation points.

48.  In noncyclic photophosphorylation, is a two-stage process
involving two different chlorophyll photosystems. Being a
light reaction, non-cyclic photophosphorylation occurs in
the frets or stroma lamellae.
 First, a water molecule is broken down into 2H+ + 1/2 O2 +
2e− by a process called photolysis (or water-splitting). The
two electrons from the water molecule are kept in
photosystem II, while the 2H+ and 1/2O2 are left out for
further use.
 Then a photon is absorbed by chlorophyll pigments
surrounding the reaction core center of the photosystem.
Noncyclic Photophosphorylation

49.  The light excites the electrons of each pigment, causing a
chain reaction that eventually transfers energy to the core of
photosystem II, exciting the two electrons that are transferred
to the primary electron acceptor, pheophytin.
 The deficit of electrons is replenished by taking electrons
from another molecule of water.
 The electrons transfer from pheophytin to plastoquinone,
which takes the 2e− from Pheophytin, and two H+ atoms from
the stroma and forms PQH2, which later is broken into PQ,
the 2e− is released to Cytochrome b6f complex and the two
H+ ions are released into thylakoid lumen.

50.  The electrons then pass through the Cyt b6and Cyt f. Then
they are passed to plastocyanin, providing the energy for
hydrogen ions (H+) to be pumped into the thylakoid space.
 This creates a gradient, making H+ ions flow back into the
stroma of the chloroplast, providing the energy for the
regeneration of ATP.
 The photosystem -II complex replaced its lost electrons from
an external source; however, the two other electrons are not
returned to photosystem -II as they would in the analogous
cyclic pathway.

51.  Instead, the still-excited electrons are transferred to a
photosystem I complex, which boosts their energy level to a
higher level using a second solar photon.
 The highly excited electrons are transferred to the acceptor
molecule, but this time are passed on to an enzyme
called Ferredoxin-NADP+ reductase which uses them to
catalyse the reaction (as shown):
NADP+ + 2H+ + 2e− → NADPH + H+
 This consumes the H+ ions produced by the splitting of water,
leading to a net production of ½ O2, ATP, and NADPH+H+ with
the consumption of solar photons and water.

53. Effect of herbicides or weedicides on ETC
 Herbicides are classified into three categories-
1. Urea Herbicides-Monuron And Diuron(DCMU)
2. Triazine Herbicides-Atrazine And Simazine.
3. Bypyridilium Herbicides-Diquat And Paraquat.

54.  Urea and Triazine herbicides are absorbed in plants
through root and then transfer to leaves. and it block the
electron flow in plastoquinone. Therefore the electron
transport will be stopped in PS-II to PS-I.
 Bypyridilium herbicides are also known as Viologen dyes. It
blocks the electron flow in Fe-S to Fd in PS-I. These
herbicides are taken electron from PS-I and performed
reaction with O2 and give rise to O-
2 (super-oxide
ion).which are harmful to chloroplast component specially
lipids. Therefore it is also harmful to chlorophyll.

56. Fe-S
Fd

57. Plant Physiology - Taiz & Zeiger.
Plant Physiology - B. Salisburg & C. W. Ross.
Life Science F & P-I: Pranav Kumar & Usha -
Mina.
Wikipedia, the Free Encyclopedia.
REFERENCES

58. THANKS