The Fit for Passkeys for Employee and Consumer Sign-ins: FIDO Paris Seminar.pptx
Chapter 3-photosynthesis aa
1. 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
2. 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
3. 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
5. Light absorption and emission by chlorophyll
1. 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.
8. List of photosynthetic pigments
Pigment
Chlorophyll a
Chlorophyll b
Chlorophyll c
Chlorophyll d
Protochlorophyll
Bacterio chlorophyll
Bacterioviridin
Phycocyanin
Phyco erythrin
Carotenoids
Plant
All green plants
Green plants excluding red and
blue green algae
Brown algae, diatoms
Red algae
Etiolated plants
Purple bacteria
Green, sulphur bacteria
Blue green algae
Red, algae
Most plants, bacteria
Light absorbed
Red and blue violet
Red and blue violet
Red and blue – violet
Red and blue – violet
Near red and blue violet
Near red and blue violet
Near red and blue violet
Orange red
Green
Blue, blue green
10. Organizing Pigments into Photosystems
The protein components of thylakoid membrane are
represented by 30 to 50 polypeptides disposed in different
supramolecular complexes.
This pigment-protein complex
forms the photosystem
11. PS I complex:
Pigments
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.
12. Pigments contin…
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.
13. Photosynthesis takes place in
three stages
Light-dependent
reactions
The Calvin Cycle
or
Light-independent
reactions (Dark
Reaction)
– 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
6 CO2 + 12 H2O + Light energy
C6H12O6
carbon
dioxide
glucose
water
+ 6 H2 O + 6 O 2
water
oxygen
14. Evidences in Support of Light
Evidences in Support of Light
and Dark Reaction
and Dark Reaction
Evidences
coefficient
from
temperature
Evidences from intermittent light
Evidences from carbon dioxide
reduction in dark
15. Mechanism of Photosynthesis
Mechanism of Photosynthesis
Until 1930s it was thought that photosynthetic reaction is reverse
of respiration
Though O2 evolved from CO2
Photosynthesis
6 CO2 + 12 H2O + Light energy
carbon
dioxide
water
C6H12O6
Respiration
glucose
+ 6 H2 O + 6 O 2
water
oxygen
In 1937 Robert Hill demonstrated that isolated chloroplasts
evolved Oxygen, when illuminated with suitable electron
acceptor Ferricyanide.
This is called hill reaction.
4Fe3+
2H2O
Election acceptor
4eO2 + 4H+
4Fe2+
Reduced Product
16. 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.
17. Oxygen-Evolving Organisms Have Two
Oxygen-Evolving Organisms Have Two
Photosystems That Operate in Series
Photosystems That Operate in Series
Red drop and Emerson Effect:
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%)
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
18. Light-Dependent Reactions
• The light-dependent reactions take place in
five stages
– 1.
– 2.
– 3.
– 4.
– 5.
Capturing light
Exciting an electron
Electron transport
Making ATP
Making NADPH
19. Production of Assimilatory Powers in
Production of Assimilatory Powers in
Photosynthesis
Photosynthesis
Reduction of NADP or electron transport system.
Phosphorylation or Formation of ATP from ADP and Pi.
20. Photophosphorylation
Photophosphorylation
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
21. How a Photosystem Works
Lost electron is
replaced by one from
water breakdown
Excitation energy
is transferred
between molecules
22. Non-cyclic Photophosphorylation
Non-cyclic Photophosphorylation
-0.6
-0.4
-0.2
Difference in redox potential of two
cytochromes amounts to 0.33 eV, it is more
than enough to accommodate
phosphorylation of ADP
FRS
2e-
Fd
NADP + H+
2e-
-0.0
PQ
2eCyt b6
2eCyt f
+0.2
+0.4
ATP
2e+0.6
+0.8
2e-
PC 2e- PS I (P700)
ADP + Pi
2H2O
ClMn++
PS II (P680)
2e-
O2 + H2O
2OH + 2H+
NADP
23. Electron Transport System in
Electron Transport System in
Non-cyclic Photophosphorylation
Non-cyclic Photophosphorylation
Excited
reaction
center
Energy of electrons
Excited
reaction
center
Ele
e–
Reaction
center
Photon
P680
Photosystem II
e- t
ra
ATP
c t ro
n tr
Watersplitting
enzyme
ans
p
sys
yst
e
m
NADPH
Reaction
center
Photon
tem
H+
Proton
gradient
formed for
ATP synthesis
Electron transport
system
ort
s
NADP+ + H+
e–
o rt
ns p
P700
Photosystem I
Electron transport
system
24. Light Reactions and Non-Cyclic Photophosphorylation
Hmmmm…
Try to
interpret this
diagram in
laymen’s
terms.
Non-cyclic
photophosphorylation
25. The Photosynthetic Electron
Transport System
NADP+ picks up two
electrons and a proton
to become NADPH
Calvin
cycle
ATP
Photon
Antenna
complex
Thylakoid
membrane
NADPH
Photon
H+ + NADP+
e-
e-
Stroma
NADPH
e-
e-
Light-dependent
reactions
Proton
gradient
H2O
Thylakoid space
Water-splitting
enzyme
1/2 O2
H+
Thylakoid
space
2H+
Photosystem II
Electron transport
system
Photosystem I
Electron transport
system
26. Chemiosmosis in a Chloroplast
H+
Photon
Calvin
cycle
ATP
H2 O
H+
Thylakoid space
ATP
H+
e–
NADPH
Light-dependent
reactions
Membrane is
impermeable
to protons
ADP
H+
H+
+
Thylakoid ½O2 2 H
space
Photosystem II
H+
H+
Electron transport system
ATP synthase
27.
28. Cyclic Photophosphorylation
-0.4
Difference in redox potential of two
cytochromes amounts to 0.36 eV, and
ferredoxin and cytochrome b6 is 0.32 eV
Fd
-0.2
e-
ADP + Pi
-0.0
+0.2
Cyt b6
ATP
eCyt f
e-
PC
ADP + Pi
+0.4
e-
ATP
e-
PS I (P700)
29. Differences between cyclic and noncyclic photophosphorylation
1.
2.
3.
4.
5.
6.
Cyclic
In this process PSI is involved
Electron moves in closed circle
Reduced NADPH2 is not
formed and assimilation of CO2
is slow down.
Oxygen is not evolved.
The system is found dominantly
in photosynthetic bacteria
The process is not inhibited by
DCMU
1.
2.
3.
4.
5.
6.
Non-cyclic
Both PSI and PSII are involved
Not closed circle, water is the
ultimate sources of electrons.
NADPH2 is formed which is used
in assimilation of carbon dioxide
Oxygen as by produced is
evolved
The system is dominant is green
plants
The process is stopped by use of
DCMU
30. In C3 plants:
ATP Requirement
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.
31. Part 2
Mechanism 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 nonphotochemical 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.
32. 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)
33. 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 time
2. 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 light
so cutting the supply of ATP and NADPH+H+
34. 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 lag
Image 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
35. Labelling and tracing carbon using 14C
A. Mixture
placed at
the origin
D. 2nd
run
B.1st run
C. Rotate the paper 90°
E. Autoradiograph reveals
the compound/s which
are labelled with 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
39. 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
40.
41. 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
43. • The C4 pathway is used by two types of plants
– 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
Some of the photons in sunlight carry a great deal of energy (xrays, uv light)
Other carry very little energy (radio waves)
High energy photons have shorter wavelength than low energy photons.
The full range of these photons is called the electromagnetic spectrum.
Our eyes perceive photons carrying intermediate amounts of energy as visible light. Why?
Because our eyes can only absorb photons of intermediate wavelengths.
How do we, or for that matter plants, absorb these wavelengths? Through molecules called pigments.
The cells of plant leaves contain organelles called chloroplasts that carry out photosynthesis
The internal membranes of chloroplasts are organized into flattened sacs called thylakoids.
Often, numerous thylakoids are stacked on top of one another in columns called grana (singular granum).
Surrounding the thylakoid membrane system is a semi liquid substance called stroma.
Plants absorb mainly blue and red light and reflect back what is left of the visible light, which is why they appear green.
Chlorophyll a and b similar in structure, but have differences in absorption spectra
An absorption spectra is a graph indicating how effectively a pigment absorbs different wavelengths of visible light.
The first two stages take place only in the presence of light and are commonly called light-dependent reactions.
The third stage, the formation of organic molecules from atmospheric CO2 is called the Calvin Cycle. It is also referred to as light-independent or dark reactions because they do not require direct light.
These reactions do depend indirectly on the light dependent reactions because they require the ATP and NADPH produced by the light-dependent reactions.
We can sum up the overall process of photosynthesis by the following simple equation.
Electron Transport: the excited electron is then shuttled along a series of electron-carrier molecules embedded in the membrane.
This is called the electron transport system.
As the electron passes along the electron transport system, the energy from the electron is siphoned out in small amounts. This energy is used to pump hydrogen ions (protons) across the membrane, building up a high concentration of protons on one side of the membrane.
Making ATP: the high concentration of protons can be used as an energy source to make ATP molecules (remember what an ATP molecule is?).
Protons are only able to move back across the membrane via special channels. The kinetic energy that is released by the movement of protons is transferred to potential energy in the building of ATP molecules from ADP. This process is called chemiosmosis and makes the ATP that will be used in the Calvin Cycle to make carbohydrates.
Making NADPH: the electron leaves the transport system and enters another photosystem where it is reenergized by the absorption of another photon of light. This energized electron enters another electron transport system where it is again shuttled along a series of electron-carrier molecules. The result of this electron transport system is not the synthesis of ATP, but the formation of NADPH (a coenzyme). The electron is transferred to a molecule (NADP) and a hydrogen ion that from NADPH. This molecule is important in the synthesis of carbohydrates in the Calvin Cycle.
If we break in down, photosynthesis is just a way of making organic molecules from carbon dioxide. To build organic molecules, cells use raw materials provided by the light-dependent reactions.
Energy. ATP (provided by photosystem II) drives endergonic reactions
Reducing Power. NADPH (provided by photosystem I) provides a source of hydrogens and energetic electrons needed to bind them to carbon atoms
The actual assembly of new molecule employs a complex battery of enzymes in what is called the Calvin Cycle or C3 photsynthesis.
Many plants have trouble carrying out C3 photosynthesis when the weather is hot. (click)
As temperature increases, plants partially close their leaf openings (called stomata) to conserve water. (Click)
As a result, CO2 and O2 are not able to enetr and exit the leaves through these openings. (click)
The concentration of CO2 falls, while the concentration of O2 in the leaves rises.
Under these conditions rubisco, the enzyme that carries out the first step of the calvin cycle engages in photorespiration (click)
Where the enzyme incorporates O2 not CO2 into the cycle and when this occurs, CO2 is ultimately released as a by-product.
Photorespiration short circuits the successful performance of the calvin cycle.
Some plants are able to adapt to climate with higher temperatures by performing C4 photosysnthesis.
In this process, plants such as sugar cane, corn, and many grasses are able to fix carbon using different types of cells and chemical reactions within their leaves and thereby avoiding a reduction in photosynthesis due to higher temperatures.
C4 plants fix CO2 first as the four carbon molecule oxaloacetate (Hence name C4) rather than as the three carbon molecule phosphoglycerate of C3 photosynthesis.
C4 plants carry out this process in the mesophyll cells of their leaves using a different enzyme.
The oxaloacetate is then converted to malate which is transferred to the bundle-sheath cells of the leaf and there broken down to regenerate CO2 which then eneters the calvin cycle.
The bundle sheath cells are impermeable to CO2 and therefore hold it within them
The concentration of CO2 increases and thus decreases the occurrence of photorespiration.
A second strategy to decrease photorespiration is used by many succulent (water storing) plants such as cacti and pineapples.
The mode of initial carbon fixation is called crassulacean acid metabolism (CAM) after the plant family Crassulaceae in which it was first discovered.
In these plants, the stomata open during night when it’s cooler and close during the day.
CAM plants initially fix CO2 into organic compounds at night, using the C4 pathway.
These organic compounds accumulate at night and are broken down during the day, releasing CO2.
These high levels of CO2 drive the Calvin Cycle and decrease photorespiration.
CAM plants differ from C4 plants in that the C4 pathway anf the Calvin Cycle occur in the same cell, a mesophyll cell, but they occur at different times of the day, the C4 cycle at night and the Calvin cycle during the day.