Photosynthesis: The Light Reactions



Photosynthesis: The Light Reactions

- In photosynthesis “synthesis using light” light energy drives the synthesis of carbohydrates and generation of oxygen from carbon dioxide and water:

6CO2 + 6H2O = C6H12O6 + 6O2

- Energy stored in carbohydrates molecules can be used to power cellular processes in the plant and can serve as the energy source for all forms of life.

Objectives:

- The basic physical principles that underlie photosynthetic energy storage (the role of light in photosynthesis)

- The current understanding of the structure and function of the photosynthetic apparatus

- The processes that begin with the excitation of chlorophyll by light and culminate in the synthesis of ATP and NADPH.

Photosynthesis in Higher Plants

- mesophyll tissue of leaves; chloroplast; chlorophylls;

- Photosynthesis (the plant uses solar energy to oxidize water, thereby releasing oxygen, and to reduce carbon dioxide, thereby forming large compounds, primarily sugars)

Photosynthesis:

- the thylakoid (light) reactions, take place in the membranes of thylakoids

- the carbon fixation reactions, take place in the stroma of the chloroplasts

The thylakoid (light) reactions:

- 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 (which is used to turns synthesize ATP)

- the and products are ATP and NADPH

General Concepts

Characteristics of light

Light absorption by molecules and its effect on their electronic states

Photosynthetic pigments and the light of photosynthesis

Key Experiments in Understanding Photosynthesis

- the balanced overall chemical reaction for photosynthesis can be as follow:

6CO2 + 6H2O → C6H12O6 + 6O2

- glucose is not the actual product of the carbon fixation reactions, but the energetics for the actual products is approximately the same

- at least 50 intermediate reaction steps have now been identified

Action spectra relate light absorption to photosynthetic activity

- light absorption spectra

- action spectra (biological processes induced by light; e.g. evolution/production of oxygen as a function of light absorption spectra)

Photosynthesis takes place in complexes containing light-harvesting antennas and photochemical reaction centers.

- photosystems (PS1 & PS2); pigments (antenna complex), reaction center complex

- energy transfer and energy conversion during photosynthesis

- relationship of oxygen production to flash energy (flash intensity)

- quantum yield

The chemical reaction of photosynthesis is driven by light

- It is confident that in the entire history of the universe no molecule of glucose has formed spontaneously from H2O and CO2 without external energy being provided.

- About 9-10 photons of light are required to derive the reaction of the following equation (formation of one-sixth of glucose molecule):

CO2 + H2O → (CH2O) + O2

- the photochemical quantum efficiency (quantum yield) under optimum conditions is nearly 100% (i.e. almost all the absorbed photons engage in photochemistry).

- The quantum efficiency is a measure of the fraction of absorbed photons that engage in photochemistry.

- however, the efficiency of the conversion of light into chemical energy is much less (about 27%; the difference between the total energy input and the free energy change)

- The energy efficiency is a measure of how much energy in the absorbed photons is stored as chemical products.

- most of the stored energy (27%) is used for cellular maintenance processes

- much less amount of the stored energy is diverted to the formation of biomass

Light drives the reduction of NADP and the formation of ATP

- the overall process of photosynthesis is a redox chemical reactions

- In the light, isolated chloroplast thylakoids reduce a variety of compounds, such as iron salts, which serve as oxidants” oxidizing agents” in place of CO2:

4Fe3 + 2H2O → 4Fe2+ + O2 + 4H+

- the previous equation provides the first evidence that oxygen evolution could occur in the absence of CO2

- the oxygen in photosynthesis originates from water, not from carbon dioxide.

- In the photosynthesis system, light reduces NADP+ ( to NADPH) which in turn serves as the reducing agent for carbon fixation in the Calvin cycle.

- During the electron flow from water to NADP+, ATP is formed

- ATP is also used in carbon reduction.

- thylakoid reactions: take place within the thylakoids

- in the thylakoid reactions: water is oxidized to oxygen, NADP+ is reduced to NADPH, and ATP is formed.

- stroma reactions: take place in the aqueous region of the chloroplast (the stroma)

- in the stroma reactions: carbon dioxide is fixed into organic compounds, and NADPH is oxidized to NADP+

Oxygen-evolving organisms have two photosystems that operate in series

- The red drop effect:

- In an experiment, the quantum yield of photosynthesis as a function of wavelength was measured.

- The quantum yield of photosynthesis (i.e. the quantum yield of oxygen evolution) for the light wavelengths absorbed by chlorophyll was measured and found constant throughout most of the range, 400 nm-680 nm (see Fig. 7.12)

- 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 deriving photosynthesis.

- However, the quantum yield of photosynthesis 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. This drop cannot be explained (caused) by a decrease in chlorophyll absorption (see Fig. 7.12).

- Thus, light with a wavelength greater than 680 nm is much less efficient than light of shorter wavelengths.

- 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 (see Fig. 7.13)

- this surprising observation 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.

- Photosystems I and II (see Fig. 7.14):

- Photosystem I:

- mainly absorbs far-red light of wavelengths greater than 680 nm

- produces a strong reductant (a strong electron donnar, 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*)

Organization of the Photosyntheic Apparatus

(The architecture of the photosynthetic apparatus and the structure of its components)

The chloroplast is the site of photosynthesis:

- chloroplast has an extensive system of internal membranes known as thylakoids

-this internal membrane system contains all the chloroplast and is the site of the light reactions of photosynthesis

- (see fig. 7.15: Grana lamellae; Stroma lamellae; Thylakoid; Stroma; Lumen; Envelope)

- the chloroplast contains its own DNA, RNA, and ribosomes.

Mechanisms of Electron Transport

(studying in detail the chemical reactions involved in electron transfer during photosynthesis)

- the excitation of chlorophyll by light

- the reduction of the first electron acceptor

- the flow of electrons through photosystems II and I

- the oxidation of water as the primary source of electrons

- the reduction of the final electron acceptor (NADP+)

- the chemiosmotic mechanism (coupling proton transport to ATP synthesis)

Electrons from chlorophyll travel through the carriers organized in the “Z scheme”

- the “Z-arrangement” of all known protein molecules/complexes that involve in carrying electrons from H2O to NADP+ is really a synthesis of both kinetic and thermodynamic information (see Fig. 7.21)

- the photochemical reactions in brief (Fig. 7.21):

1- the specialized chlorophyll molecules 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 (protein compounds)

- the electron ejected from P680 eventually reduces P700

- the electron ejected from P700 eventually reduces NADP+ to NADPH

- thus, the photochemical reactions are carried out by four integral, transmembrane protein complexes, which are vectorially oriented in the thylakoid membrane:

- photosystem II:

--- found mainly in the stacked thylakoid lamellae;

--- 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 plastoquinones

--- reduces plastoquinones “PQ” to plastohydroquinones “PQH2”)

- the cytochrom b6f :

--- evenly distributed in the thylakoid membrane system;

--- oxidizes the reduced form of plastohydroquinone, 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

- photosystem I:

--- found in the unstacked thylakoid lamellae;

--- reduces NADP+ to NADPH, the reduction of NADP+ is carried out 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, then to a quinone, then to a series of membrane-bound iron-sulfur proteins, then to soluble ferredoxin (Fe), then to soluble flavoprotein ferredoxin-NADP reductase (FNR);

--- FNR reduces NADP+ to NADPH

- the ATP synthase enzyme:

--- 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

Energy is captured when an excited chlorophyll reduces an electron acceptor molecule

- the ground-state chlorophyll is a poor reducing agent and is a poor oxidizing agent

- light excites the specialized, reaction center chlorophyll molecule

- the excitation process is a promotion of an electron from the highest-energy filled orbital of the chlorophyll to the lowest-energy unfilled orbital (see Fig. 7.23).

- the excited-state chlorophyll is a strong reducing agent (can lose an electron from the highest-energy orbital) and is a strong oxidizing agent (can accept an electron into the lower-energy orbital)

- the primary photochemical event (the first reaction that converts electron energy into chemical energy) 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 transfers is accompanied by a loss of some energy, thus making the process effectively irreversible

(i.e. the acceptor molecule does not donate its electron back to the reaction center chlorophyll)

- the oxidized reaction center chlorophyll is re-reduced by a secondary donor (Yz), which in turn is reduced by a tertiary donor; H2O is the ultimate electron donor

The reaction center chlorophylls of the two photosystems absorb at different wavelengths

The photosystem II reaction center is a multi-subunit pigment-protein complex

- phorosystem II is a multisubunit protein supercomplex includes two complete reaction centers and some antennae protein complexes.

- the core of the reaction center consists mainly of two trans-membrane proteins (D1 and D2) and two antennae proteins (CP43 and CP47)

Water is oxidized to oxygen by photosystem II

- 2H2O → O2 + 4H+ + 4e-

- (i.e. 4 electrons are removed from 2H2O generating an oxygen molecule and four hydrogen ions)

- the photosynthetic oxygen-evolving complex is located on the interior surface of the thylakoids, and it is the only known biochemical system that carries out the oxidation of water to molecular oxygen.

- 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

Pheophytin and two quinones accept electrons from photosystem II

- pheophytin is the first (earliest) electron acceptor in PSII

- two plasoquinones (PQA and PQB), which are bound to the reaction center of PSII, receive electrons from pheophytin in a sequential fashion

- the reduced plastoquinone (PQ2-) takes two protons from the stroma side, yielding a fully reduced plastohydroquinone (PQH2)

- plastohydroquinone dissociates from the reaction center complex and enters the hydrocarbon (hydrophobic) portion of the thylakoid membrane, then transfers its two electrons to the cytochrome b6f complex, (and releases its 2H+ into the lumen?)

.

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

- the large multisubunit protein complex 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), plastoquinone (PQ), plastohydroquinone (PQH2), plastocyanin (PC)

- the mechanism of the Q cycle of electrons and protons flow through the cytochrome b6f complex:

--- plastohydroquinone (PQH2) is oxidized, by transferring one of its electron along a linear transport chain to FeSR, and its second electron through a cyclic process to one of the cyt b

--- 2H+ (due to oxidation of PQH2) are released into the lumen side of the membrane, thus increases the number of protons pumped across the membrane

--- in the linear process of electron transport chain, FeSR transfers its electron to cyt c (cyt f), then to PC, which in turn reduces oxidized P700 of PSI.

--- in the cyclic process, cyt b transfers its electron to the other (second) cyt b, which in turn reduces an oxidized plastoquinone molecule to the plastosemiquinone form (or state)

Photosynthesis blocking chemicals

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.

- 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.

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