1. Photosynthesis Light Reaction
Dr. Abdul Latif Khan

2. 1) mid-rib branches into a network of veins
two layers of tissues: palisade mesophyll & spongy mesophyll

3. 2) Palisade mesophyll
 made up of tightly packed cylinder-shaped cells
 contain many chloroplasts

4. 3) Spongy mesophyll
 cells not regular in shape
 fewer chloroplasts
 many air spaces

5. 4) Epidermis
 cover both surfaces
 protects cells against mechanical injuries and infection

6. 4)  Epidermal cells have no chloroplasts, except guard cells

7. 5) Cuticle
 cover the epidermis
 reduces loss of water

8.  guard cells have many chloroplasts
6) Stoma
 surrounded by guard cells which control the opening and closing of the stoma
 guard cells have many chloroplasts
stoma
guard cell

9. 6)  There are far fewer stomata on the upper surface of a leaf to reduce water loss.
guard cell

10. 7) Vascular bundles
 for transport solutions containing chemical which plants need.
 contains xylem and phloem

11. Xylem
 transports water and minerals from the roots to the leaves
Phloem
 transports nutrients from the leaves

13. Overall Organization of the Chloroplast

15. Photosynthesis: The Light Reactions
Role of light in photosynthesis
Current understanding of the structure and function of the photosynthetic apparatus
Photochemical reaction: Processes that begin with the excitation of chlorophyll by light and culminate in the synthesis of ATP and NADPH.

16. Photosynthesis: “Synthesis using Light”.
Light energy drives the synthesis of carbohydrates and generation of oxygen from carbon dioxide and water:
Energy stored in the carbohydrates molecules can be used to power cellular processes in the plant and can serve as the energy source for all forms of life.

17. Photosynthesis in Higher Plants
Mesophyll of leaves is the most active photosynthesic tissue. It has many chloroplasts (chlorophylls).
The plant uses solar energy to oxidize water, thereby releasing oxygen, and to reduce CO2, thereby forming large carbon compounds, primarily sugars.
The thylakoid (light) reactions, in the membranes of thylakoids.
The carbon fixation reactions, in the stroma of the chloroplasts.

19. Light reactions
Light energy is converted into chemical energy by two photosystems.
The absorbed light energy is used to power the transfer of electrons through a series of compounds that act as electron donors and electron acceptors.
The majority of electrons reduce NADP+ to NADPH and oxidize H2O to O2.
Light energy is also used to generate a proton motive force (used to synthesize ATP).

20. Photosynthesis in Higher Plants
Oxidation of H2O
Reduction of CO2 to glucose
Thylakoid reactions (light reactions)
- light energy to chemical energy
- reduction of NADP to NADPH
- oxidation of H2O to O2
- generation of a proton motive force
- synthesis of ATP
ATP & NADPH are high energy compounds
Carbon fixation reactions
Takes place in the stroma of the chloroplasts.

21. General Concepts Nature of light
Properties of particles and waves (fig. 7.1)
As a wave, light is characterized by wavelength, frequency and speed (c = λν)
The light wave is a transverse (side-to-side) electromagnetic wave.
As a particle, light is called Photon with energy (quantum)
Energy content of light is delivered in discrete packets, the quanta.
E = hv (Plank’s law)
Photons are of different frequencies.

23. Sunlight: rain of photons of different frequencies
Visible light (fig. 7.2)
Absorption spectrum: amount of light energy absorbed by a molecule as a function of the wavelength of the light.

25. Solar and visible light spectrum

26. Curve A: energy output of the sun.
Curve B: energy that strikes the surface of the Earth.
Curve C: Absorption spectrum of chlorophyll. Strong absorption the blue and red portions of the spectrum.
Green light not efficiently absorbed, most of it reflected into our eyes, and gives the plants their green color.

27. Properties of pigments
Molecules change their electronic state when they absorb or emit light.
Chl + hv → Chl*
Chl = Ground state chlorophyll
Chl* = Excited state chlorophyll
Distribution of electrons in the excited molecule is different (fig. 7.5)
Higher excited state: Chl unstable (heat loss)
Lowest excited state: pathways for disposing available energy (fluorescence).

29. Four alternative pathways for the chlorophyll molecule in its lowest excited state to dispose its available energy of excitation:
Flourescence process (re-emission of a photon)
Heat process (no emission of a photon)
Energy Transfer process
Photochemistry process (photochemical reactions of photosynthesis)

30. Roles of photosynthetic pigments
They absorb sunlight energy.
Chlorophylls and bacteriochlorophylls.
More than one kind of pigment in an organism.
Loosely bound electrons in the ring structure.
Carotenoids (orange color) are accessory pigments: light absorbed is transferred to chlorophyll for photosynthesis.
They also help protect the organism from damage caused by light.

33. Understanding Photosynthesis
The balanced overall chemical reaction for photosynthesis :
6CO2 + 6H2O → C6H12O6 + 6O2
The chemical reactions of photosynthesis are complex. At least 50 intermediate reaction steps have been identified.
Photosynthesis is a redox (reduction-oxidation) process.

34. Action spectra
They relate light absorption to photo-synthetic activity.
An action spectrum depicts the magnitude of a response of a biological system to light, as a function of wavelength.
In photosynthesis: evolution / production of oxygen at different wavelengths.
It can often identify the chromophore (pigment) responsible for a particular light-induced phenomenon.

36. The action spectrum is measured by plotting oxygen evolution, as a function of wavelength.
If the pigment used to obtain the absorption spectrum is the same as those that cause the response, the absorption and action spectra will match.
The action spectrum for oxygen evolution matches the absorption spectrum of intact chloroplasts quite well, indicating that light absorption by the chlorophylls mediates oxygen evolution.

37. Light-harvesting antennas and photochemical reaction centers
The conversion of energy from one form to another is a complex process that depends on cooperation between many pigment molecules and a group of electron transfer proteins.
The majority of pigments serve as an antenna complex, collecting light and transferring energy to the reaction center complex (redox reactions).

38. Basic concept of energy transfer during photosynthesis
Basic concept of energy transfer during photosynthesis. Many pigments together serve as an antenna, collecting light and transferring its energy to the reaction
center, where chemical reactions store some of the energy by transferring electrons from a chlorophyll pigment to an electron acceptor molecule. An electron donor then reduces the chlorophyll again. The transfer of energy in the antenna is a purely physical phenomenon and involves no chemical changes.

39. No molecule of glucose has formed spontaneously from H2O and CO2 without external energy being provided. The energy needed to drive the photosynthetic reaction comes from light.
About 9-10 photons of light are required to drive the reaction of the following equation (formation of one-sixth of glucose molecule):
CO2 + H2O → (CH2O) + O2

40. The quantum efficiency is a measure of the fraction of absorbed photons that engage in photochemistry.
The quantum efficiency (quantum yield) under optimum conditions is nearly 100% (i.e. almost all the absorbed photons engage in photochemistry).
However, the efficiency of the conversion of light into chemical energy is much less (about 27%).

41. The Red Drop Effect
In an experiment, the quantum yield of photosynthesis (oxygen evolution) for the light wavelengths absorbed by chlorophyll was measured. It was found constant throughout most of the range 400 nm-680 nm.
Constant values of the quantum yield of photosynthesis indicate that any photon absorbed by chlorophyll or other pigments is as effective as any other photon in driving photosynthesis.

42. However, the quantum yield drops dramatically in the far-red region of chlorophyll absorption (greater than 680 nm) ─ known as red drop effect.
The red drop effect indicates that far-red light alone is inefficient in driving photosynthesis.
Thus, light with a wavelength greater than 680 nm is much less efficient than light of shorter wavelengths.

43. The Enhancement Effect
In another experiment, the rate of photosynthesis was measured separately with two light beams of two different wavelengths (red and far-red light), and then was measured with the two light beams simultaneously.
It was found that the rate of photosynthesis when red and far-red light are given together is greater than the sum of the individual rates when they are given separately.

44. These observations and other experimental observations led to the discovery that two photochemical complexes, now known as photosystems I and II (PSI and PSII), operate in series, to carry out the early energy storage reactions of photosynthesis.
The two systems are linked by an electron transport chain (Fig. 7.14).

45. - Photosystem I:
- mainly absorbs far-red light of wavelengths greater than 680 nm
- produces a strong reductant (a strong electron donor, P700*) capable of reducing NADP+
- produces a weak oxidant (a weak electron acceptor, P700).
- Photosystem II:
- mainly absorbs red light of 680 nm and is driven very poorly by far-red light.
- produces a very strong oxidant (a strong electron acceptor, P680) capable of oxidizing water
- produces a weak reductant (P680*).

46. Photosynthetic Apparatus
- The chloroplast is the site of photosynthesis.
- It has an extensive system of internal membranes known as thylakoids (Fig. 7.15).
- All the chlorophyll is contained within this membrane system which is the site of the light reactions of photosynthesis.
- The carbon reduction reactions, which are catalyzed by water-soluble enzymes, take place in the stroma.
- The chloroplast contains its own DNA, RNA, and ribosomes.

50. Mechanisms of Electron Transport
- Excitation of chlorophyll by light
- Reduction of the first electron acceptor
- Flow of electrons through photosystems II and I
- Oxidation of water as the primary source of electrons
- Reduction of the final electron acceptor (NADP+)
- Chemiosmotic mechanism (coupling proton transport to ATP synthesis).

51. Electrons from chlorophyll travel through the carriers organized in the “Z scheme” (Fig. 7.21)
1- The specialized chlorophyll of the reaction centers (P680 for PSII, and P700 for PSI) are excited at the same time by absorbed light photons.
As a result, an electron from each of P680 and P700 is ejected.
2- The ejected electron then passes through a series of electron carriers. The electron ejected from P680 eventually reduces P700. The electron ejected from P700 eventually reduces NADP+ to NADPH.

53. The photochemical reactions are carried out by four integral, transmembrane protein complexes, in the thylakoid membrane:
1- Photosystem II:
--- oxidizes water and releases O2 and protons in the thylakoid lumen
--- the oxidized P680 (P680+) by light is re-reduced by receiving electrons from oxidation of water
--- transfers electrons of the oxidized P680 by light (P680+) to pheophytin, then to plasto-quinones
--- reduces plastoquinones “PQ” to plasto-hydroquinones “PQH2”).

56. 2- Cytochrome b6f:
--- evenly distributed in the thylakoid membrane system
--- oxidizes the reduced form of plasto-hydroquinone, PQH2
--- transfers electrons to plastocyanin (PC)
--- plastocyanin (PC) delivers electrons to PSI, thus, reduces the oxidized P700 (P700+) by light
--- the oxidation of PQH2 is coupled to proton transfer from the stroma into the lumen, generating a proton motive force.

59. 3- Photosystem I: --- found in the unstacked thylakoid lamellae --- reduces NADP+ to NADPH, by the action of ferredoxin “Fd” and flavoprotein ferredoxin-NADP reductase “FNR” --- transfers electrons of the oxidized P700 by light (P700+) to a series of electron acceptors: chlorophyll, a quinone, then to a series of membrane-bound iron-sulfur proteins, to soluble ferredoxin (Fe), then to soluble flavoprotein ferredoxin-NADP reductase (FNR) --- FNR reduces NADP+ to NADPH.

60. 4- ATP synthase enzyme: (Fig. 7.22)
--- found in the unstacked thylakoid lamellae protruding into the stroma
--- produces ATP as protons diffuse back (from the lumen to the stroma down the electrochemical potential gradient) through the ATP synthase enzyme.

61. Energy is captured when an excited chlorophyll reduces an electron acceptor
- Light excites a specialized chlorophyll molecule in the reaction center.
- The excitation process is a promotion of an electron from the highest-energy filled orbital of the chlorophyll to the lowest-energy unfilled orbital.
- The electron in the upper orbital is only loosely bound to the chlorophyll and is easily lost if a molecule that can accept electrons is nearby.

62. - The first reaction that converts electron energy into chemical energy - primary photochemical event- is the transfer of an electron from the excited reaction center chlorophyll to an electron acceptor molecule.
- The acceptor, then, transfers its extra electron to a secondary acceptor and so on down the electron transport chain.
- Each of the secondary electron transfer is accompanied by a loss of some energy, thus making the process effectively irreversible.

63. - The ultimate electron donor is H2O, and the ultimate electron acceptor is NADP+.
- The reaction center chlorophylls of the two photosystems absorb at different wavelengths (P680, P700).
- PS II reaction center is a multi-subunit protein supercomplex (two complete reaction centers and some antenna complexes). The core of the reaction center consists mainly of two trans-membrane proteins (D1 and D2) and two antennae proteins (CP43 and CP47).

64. Water is oxidized to oxygen by PSII
2H2O → O2 + 4H+ + 4e-
- 4 electrons are removed from 2H2O generating an oxygen molecule and four H+.
- Water is a very stable molecule. Oxidation of water to form molecular oxygen is very difficult.
- The photosynthetic oxygen-evolving complex is the only known biochemical system that carries out this reaction.

65. - The oxygen-evolving complex is localized on the interior surface of the thylakoid.
- Four manganese ions (Mn2+) are associated with each oxygen-evolving complex; suggesting that Mn is an essential cofactor in the water-oxidizing process.
- Cl- and Ca2+ ions are essential as well for O2 evolution.
- Yz is an electron carrier that functions between the oxygen-evolving complex and P680.

66. Pheophytin and two Quinones accept electrons from PS II
- Pheophytin is the first electron acceptor in PSII.
- Pheophytin is a chlorophyll in which the central Mg atom has been replaced by two H atoms.
- Two plastoquinones (PQA and PQB), bound to the reaction center of PSII, receive electrons from pheophytin in a sequential fashion.
- The reduced plastoquinone takes two protons from the stroma side, yielding a fully reduced plastohydroquinone (PQH2).

68. \figures\ch07\pp07291.jpg

69. \figures\ch07\pp07292.jpg

70. - PQH2 dissociates from the reaction center complex and enters the hydrophobic portion of the thylakoid membrane.
- Then, PQH2 transfers its two electrons to the cytochrome b6f complex.

71. Electron flow through the cytochrome b6f complex also transports protons
- The cytochrome b6f complex is a large multisubunit protein complex with several groups of cofactors.
- It contains two b-type cytochromes (cyt b) and one c-type cytochrome (cyt c or cyt f).
- The prosthetic (cofactor) groups the cytochrome b6f complex includes: Rieske iron-sulfur protein (FeSR), PQ, PQH2 and plastocyanin (PC).

72. \figures\ch07\pp07371.jpg

73. \figures\ch07\pp07372.jpg

74. \figures\ch07\pp07373.jpg

75. \figures\ch07\pp07374.jpg

76. Some herbicides block photosynthetic electron flow
- Many different classes of herbicides have been developed, they act by blocking amino acid, carotenoid, or lipid biosynthesis or by disrupting cell division.
- Other herbicides, such as DCMU (dichlorophenyldimethylurea) and paraquat, block photosynthetic electron flow. DCMU is also known as diuron.

78. - Many herbicides, DCMU among them, act by blocking electron flow at the quinone acceptors of photosystem II, by competing for the binding site of plastoquinone that isnormally occupied by QB.
- Other herbicides, such asparaquat, act by accepting electrons from the early acceptors of photosystem I and then reacting with oxygen to form superoxide, O2–, very damaging to chloroplast components, especially lipids.