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Photosynthesis• An Overview of Photosynthesis
• How Plants Capture Energy from Sunlight
• Organizing Pigments into Photosystems
• Light Reaction of Photosysnthesis
Arba Minch University Dr. Chinthapalli Bhaskar Rao
An Overview of Photosynthesis
• Photosynthesis is the process that captures light energy and transforms into the chemical energy of carbohydrates
• It occurs in the– Plasma Membranes of Some Bacteria– Cells of Algae– Leaves of Plants
How Plants Capture Energy from Sunlight• Light has characteristic of both wave and
particle• Wave: wavelength and frequency
• Light is also a particle, which we call a photon.
• Each photon contains an amount of energy that is called a quantum (plural quanta).
• Its not continuous but rather is delivered in these discrete packets, the quanta. – High energy photons have shorter
wavelengths than low energy photons• The full range of these photons is called the
electromagnetic spectrum
Photons of different energy: the electromagnetic spectrum
GB OY RV I
Light absorption and emission by chlorophyll1. Excited chlorophyll can re-emit a
photon and thereby return to its ground state a process known as fluorescence.
2. The excited chlorophyll can return to its ground state by directly converting its excitation energy into heat, with no emission of a photon.
3.Chlorophyll may participate in energy transfer, from one molecule to another molecule.
4. A fourth process is photochemistry, in which the energy of the excited state causes chemical reactions to occur.
The photochemical reactions of photosynthesis are among the fastest known chemical reactions. This extreme speed is necessary for photochemistry to compete with the three other possible reactions of the excited state just described.
Absorption spectra of Chlorophylls and Carotenoids
Pigment Plant Light absorbedChlorophyll a All green plants Red and blue violetChlorophyll b Green plants excluding red and
blue green algaeRed and blue violet
Chlorophyll c Brown algae, diatoms Red and blue – violetChlorophyll d Red algae Red and blue – violetProtochlorophyll Etiolated plants Near red and blue violetBacterio chlorophyll Purple bacteria Near red and blue violetBacterioviridin Green, sulphur bacteria Near red and blue violetPhycocyanin Blue green algae Orange redPhyco erythrin Red, algae GreenCarotenoids Most plants, bacteria Blue, blue green
List of photosynthetic pigments
What is a Chloroplast?
Organizing Pigments into Photosystems
This pigment-protein complex forms the photosystem
The protein components of thylakoid membrane are represented by 30 to 50 polypeptides disposed in different supramolecular complexes.
Pigments PS I complex:
Small and densely packed particles.
It consists of ~200 chlorophyll a, ~50 carotenoids.
The reaction centre is called P700, maximum absorption at 700 nm.
Energy funneling into P700 is responsible for the ejection of an election from the chlorophyll.
PS II complex:
Its consists of ~200 molecules of chlorophyll a, ~200 molecules of carotenoids, chlorophyll b and chlorophyll c, depending upon the species.
Its reaction centre as P680 or shorter wavelength trap.
PS I and PS II are arranged near one another because they are functionally related.
Excitation energy originating from one system is shunted to another system.
Two photosystems are coupled chemically rather than through direct energy transfer.
Cytochrome 559 and cytochrome 553:
This complex contains
one cytochrome f,
two cytochromes of b553,
one FeS center, and a polypeptide.
This system is uniformly distributed in the grana region.
coupling factor I or CF I:
Synthesize ATP from ADP and Pi using the proton gradient.
Light harvesting complex (LHC):
It contains two main polypeptides and both chlorophylls a and b.
The system remains mainly associated with PSII .
but may also be related to PSI.
This is mainly located in the stacked membranes.
Pigments contin…
Photosynthesis takes place in three stages
– 1. Capturing energy from sunlight
– 2. Using energy to make ATP and NADPH
– 3. Using ATP and NADPH to power the synthesis of carbohydrates from CO2
Light-dependent reactions
Light-independent reactions (Dark
Reaction)
The Calvin Cycle or
6 CO2
carbondioxide
+ 12 H2Owater
+ Light energy C6H12O6
glucose
+ 6 O2
oxygen
+ 6 H2Owater
Evidences from temperature coefficient
Evidences from intermittent light
Evidences from carbon dioxide reduction in dark
Evidences in Support of Light and Dark Reaction
Evidences in Support of Light and Dark Reaction
Until 1930s it was thought that photosynthetic reaction is reverse of respiration
Though O2 evolved from CO2
Mechanism of PhotosynthesisMechanism of Photosynthesis
6 CO2
carbondioxide
+ 12 H2Owater
+ Light energy C6H12O6
glucose
+ 6 O2
oxygen
+ 6 H2Owater
Photosynthesis
Respiration
In 1937 Robert Hill demonstrated that isolated chloroplasts evolved Oxygen, when illuminated with suitable electron acceptor Ferricyanide.
This is called hill reaction.2H2O
O2 + 4H+
4e-
4Fe3+
Election acceptor
4Fe2+
Reduced Product
Mechanism of Photosynthesis continu…Mechanism of Photosynthesis continu…
Ruben, Randall and Kamen (1941) using heavy isotope of oxygen (O18) in their experiments provide direct proof.
Oxygen evolved in photosynthesis comes from water.
Oxygen-Evolving Organisms Have Two Photosystems That Operate in Series
Oxygen-Evolving Organisms Have Two Photosystems That Operate in Series
Photosynthesis is considered as a two quanta process
Two light quanta energy to drive one e- Since 4e- are required, so eight quanta
required to reduced and evolve one O2
Number of O2 molecules released is called Quantum yield. (1/8 or 12%)
Red drop and Emerson Effect:
Emerson and Lewis worked on Photosynthesis in monochromatic light
After 8 years Emerson and Chalmers measured the rate of photosynthesis separately with light of two different wavelengths and then used the two beams simultaneously
Light-Dependent Reactions
• The light-dependent reactions take place in five stages
– 1. Capturing light– 2. Exciting an electron– 3. Electron transport– 4. Making ATP– 5. Making NADPH
Production of Assimilatory Powers in Photosynthesis
Production of Assimilatory Powers in Photosynthesis
Reduction of NADP or electron transport system. Phosphorylation or Formation of ATP from ADP and Pi.
PhotophosphorylationPhotophosphorylation
Arnon and his associates (1954) first showed that isolated chloroplast can produce ATP when exposed to light.
This is phosphorylation or Photophosphorylation
The role of this ATP in two ways:
First, it suppliments the energy for the reduction of CO2 utilizing NADPH + H+ (end product of light reaction).
Secondly, this ATP is used in the phosphorylation of RUBP during its regeneration in Calvin cycle.
There are two different types of phosphorylation present.
Non-cyclic Photophosphorylation
Cyclic Photophosphorylation
How a Photosystem Works
Excitation energy is transferred
between molecules
Lost electron is replaced by one from
water breakdown
Non-cyclic PhotophosphorylationNon-cyclic Photophosphorylation
PS II (P680)
PS I (P700)
Cyt b6
FRS
Cyt f
PC
PQ2e-
2e-2e-
2e-
2e-
2e-
2e-
Fd
NADP
NADP + H+
2H2O 2OH + 2H+
O2 + H2O
Cl- Mn++
2e-
ADP + Pi
ATP
-0.6
-0.4
-0.2
-0.0
+0.2
+0.4
+0.6
+0.8
Difference in redox potential of two cytochromes amounts to 0.33 eV, it is more than enough to accommodate phosphorylation of ADP
P680
Reactioncenter
Water-splittingenzyme
Photosystem IPhotosystem II
Ene
rgy
of e
lect
rons
P700
Reactioncenter
Electron transport system
Electron transport system
Photon
Excitedreactioncenter
e–
Electron transport systemH+
Protongradientformed forATP synthesis
ATP
Photon
Excitedreactioncenter
e– NADP+ + H+ NADPH
e- transport system
Electron Transport System in Non-cyclic Photophosphorylation
Electron Transport System in Non-cyclic Photophosphorylation
Light Reactions and Non-Cyclic Photophosphorylation
Hmmmm…
Try to interpret this diagram in laymen’s terms.
Non-cyclic
photophosphorylation
The Photosynthetic Electron Transport System
Stroma
Thylakoidspace
Thylakoidmembrane
Photosystem IPhotosystem IIElectron transport
systemElectron transport
system
Water-splittingenzyme
H2O
1/2 O2 2H+
Antennacomplex
e-
Photon
H+
e-
e-
Photon
e-
H+ NADP++NADPH
Protongradient
Light-dependentreactions
Thylakoid space
NADPHATP
Calvincycle
NADP+ picks up two electrons and a proton
to become NADPH
Thylakoidspace
Photosystem II ATP synthaseElectron transport system
Photon
H2O
½O2
e–
H+2
H+
H+
H+
H+
H+
H+
H+
ADP ATP
Light-dependentreactions
Thylakoid space
NADPHATP
Calvincycle
Membrane is impermeable
to protons
Chemiosmosis in a Chloroplast
Cyclic Photophosphorylation
PS I (P700)
Cyt b6
Fd
Cyt f
PC
e-
e- e-
e-
e-
ADP + Pi
ATP
ADP + Pi
ATP
-0.4
-0.2
-0.0
+0.2
+0.4
Difference in redox potential of two cytochromes amounts to 0.36 eV, and ferredoxin and cytochrome b6 is 0.32 eV
Cyclic Non-cyclic1. In this process PSI is involved2. Electron moves in closed circle3. Reduced NADPH2 is not
formed and assimilation of CO2 is slow down.
4. Oxygen is not evolved.5. The system is found dominantly
in photosynthetic bacteria6. The process is not inhibited by
DCMU
1. Both PSI and PSII are involved2. Not closed circle, water is the
ultimate sources of electrons.3. NADPH2 is formed which is used
in assimilation of carbon dioxide4. Oxygen as by produced is
evolved5. The system is dominant is green
plants6. The process is stopped by use of
DCMU
Differences between cyclic and non-cyclic photophosphorylation
ATP RequirementIn C3 plants:
18 ATP molecules are required to synthesize one glucose molecule.
2 photons are required to drive 1e-. Four electrons removed from water.
Eight quanta (photons) are required (4 at PSI and 4 at PSII)
Only 18 ATP are generated in generation of 6O2.
18 ATP are required. Where additional 6 ATP come?
Assumed that 2 additional quanta (photons) are required to generate 6 ATP molecules. i.e. 3 ATP +2NADPH for fixation of one molecule of CO2
6CO2 + 12NADPH + H+ + 18ATP C6H12O6 + 6H2O + 12NADP + 18ADP +18Pi
C4 Plants:
30 ATP molecules are required to produce one molecule of glucose. Hatch and Slack (1970) proposed that C4 plants have higher capacity for photophosphorylation.
They have higher chlorophyll ration of a/b ratio. But PSI component of chlorophyll a is also greater.
Thus cyclic photophosphorylation supply abundant ATP molecules.
Part 2Mechanism of Dark Reaction
Recent estimates indicate that about 200 billion tons of CO2 are converted to biomass each year.
About 40% of this mass originates from the activities of marine phytoplankton.
The bulk of the carbon is incorporated into organic compounds by the carbon reduction reactions associated with photosynthesis.
First time Blackman (1905) established that non-photochemical process (dark reaction) is involved in photosynthesis.
In 1946 using radioactive materials and sophisticated techniques elucidate CO2 reduction .
Such techiques are done by Calvin and his coworkers.
THE LIGHT INDEPENDENT REACTION OR DARK REACTION
• Enzyme controlled
• Located in the stroma of the chloroplast
• Occurs simultaneously with the light dependent reaction
• It can continue in the dark provided the necessary raw materials are available (CO2, NADPH + H+ and ATP)
Enzyme controlled reaction pathways
To find out the sequence of the reactions and the point at which X is added in, two approaches can be used:
1. Label and trace the products formed through time2. Cut the supply of X and observe what happens to
the intermediates in the pathway e.g. in studying photosynthesis,
cut the CO2 supply or switch off the lightso cutting the supply of ATP and NADPH+H+
Calvin and Benson 1946 to 1953• Used 14C radioisotope for labelling
• Unicellular algae: Chlorella and Scenedesmus
• Simple plants which respond quickly to changes in the environment
• So little time lagImage Credit Scenedesmus
A flat-sided, round flask containing the culture of algae
This shape:- provided even illumination of all the cells- permitted careful control of environmental conditions (e.g. pH, temperature)- permitted rapid mixing of contents- precise sampling time
The “Lollipop” vessel
Labelling and tracing carbon using 14C
• Add NaH14CO3 solution
• At timed intervals the algae are sampled and killed by dropping in hot methanol
• Two-way (2-dimensional) chromatography used to separate the compounds
• Identify radioactively labelled compounds by autoradiography
A. Mixture placed at the origin
B.1st run
D. 2nd run
E. Autoradiograph reveals the compound/s which are labelled with
14C
C. Rotate the paper 90°
C3 Cycle
A RBP PGA GP EHexoses
Light Independent Pathway
Ea RUBISCO Ec Ed Ee
CO2
12 ATP12 NADPH + H+
Building New Molecules
• In hot weather, plants have trouble with C3 photosynthesis– This leads to
photorespiration
– O2 is now consumed and CO2 is produced as a by-product
– This decreases the photosynthetic yields
C4 Pathway– Some plants decrease
photorespiration by performing C4 photosynthesis
– CO2 is fixed initially into a four-carbon molecule
– It is later broken down to regenerate CO2
Crassulacean acid metabolism(CAM) Pathway
– C4 plants • Examples: Sugarcane, corn
• CO2 fixation and the Calvin cycle are separated in space, occurring in two different cells
– CAM plants• Examples: Cacti, pineapples
• Initial CO2 fixation is called crassulacean acid metabolism (CAM)
• CO2 fixation and the Calvin cycle are separated in time, occurring in two different parts of the day
• The C4 pathway is used by two types of plants
CAM plant pathways are separated
temporally
C4 plant pathways are separated
spatially
Any Question?
