|Plant Physiology (Biology 327) - Dr. Stephen G. Saupe; College of St. Benedict/ St. John's University; Biology Department; Collegeville, MN 56321; (320) 363 - 2782; (320) 363 - 3202, fax; firstname.lastname@example.org|
Photosynthesis: Light Dependent Reactions
I. Overview of photosynthesis:
Photosynthesis can be defined as the light-driven synthesis of carbohydrate. The equation for this reaction, that youve seen many times is:
CO2 + H2O + light + chloroplast → (CH20)n + O2
From this simple equation we can make some elegant conclusions:
A. Photosynthesis is a redox reaction.
NAD(P) + (ox) + 2e- + 2H+ → NAD(P)H (red) + H+
FAD(ox) + 2e- + 2H+ → FADH2 (red)
B. CO2 is reduced to a carbohydrate.
C. Water is oxidized (to oxygen).
D. Water supplies the electrons for the reduction; water is cleaved in the process yielding oxygen as a byproduct.
E. Light provides the energy for the reduction.
F. Photosynthesis is an energy conversion process that ultimately converts light energy to chemical energy (carbohydrate). In a broad sense, it is an example of the 1st Law of Thermodynamics - energy cannot be created nor destroyed, but it can be changed from one form to another.
G. BLACK BOX summary model for photosynthesis. Diagram in class that shows two boxes (light dependent & light independent reactions). This model further shows that during the light-dependent and light-independent reactions that there are three major types of energy conversions during photosynthesis:
Conversion 1: Radiant energy (sunlight) → electrical energy (passage of electrons via a series of carrier). This reaction series is part of the light-dependent reactions (z-scheme, non-cyclic electron flow)
Conversion 2: Electrical energy → "Labile" chemical energy (ATP, NADPH; unstable, not readily stored). During this step, ATP and NADPH are produced as the end result of non-cyclic electron flow.
Conversion 3: "Labile" chemical energy → Stable chemical energy (carbohydrate). This last step is the light-independent reactions or Calvin-Benson cycle. This process requires ATP and NADPH.
II. Chloroplasts - specialized organelles that carry out the process of photosynthesis
Remember the cell unit? To jog your memory, reread Chapter 1. Terms that you should know are thylakoid (or lamellae), lumen (intermembrane space), envelope, double membrane, stroma, granum, granal thylakoids (or lamellae), stromal thylakoids (lamellae), and starch grains. Chloroplasts may contain fat globules (plastoglobuli). Stacked (or appressed) regions - portion of granum in which thylakoids are adjacent to one another. Non-stacked (non-appressed) regions - regions of the chloroplast where the thylakoids are not adjacent to another.
B. Ontogeny and phylogeny - recall the cell unit?
C. Chemistry - Chloroplasts contain:
These molecules look like a tennis racket. The head of the racket is a porphyrin ring system, made of four pyrolle units linked together (tetrapyrolle). It has a long hydrocarbon tail, called phytol (C-20), that is derived from the terpene pathway (diterpene), built from the isoprene skeleton. Magnesium is chelated in the ring. The tail is important for orienting the molecule in the membrane. The interaction of the chlorophyll with the membrane is non-covalent and is important because it ultimately determines the physical properties of the chlorophyll.
- chlorophyll a - methyl group
- chlorophyll b - formyl group
- phaeophytin - chlorophyll without the magnesium
- chlorophyllide - chlorophyll without the tail
Both are terpenoid pigments, tetraterpenoids (C-40). Carotenes are hydrocarbons, xanthophylls are oxygenated. These pigments are orange and yellow in color.
3. Chlorophyll biosynthesis - Some take-home-lessons:
- ALA (Δ-aminolevulinic acid) is the first well-established precursor
- ALA is derived from α-ketoglutarate (or glutamate) (a Kreb's cycle intermediate, from the mitochondrion)
- 2 ALA condense to form a unit of pyrolle
- 4 pyrolles condense to form porphyrin (tetrapyrolle)
- Magnesium is inserted
- A photoreduction step occurs (converts protochlorophyllide → chlorophyllide)
- the tail is added
4. Light and the Greening Process
Recall that etiolated plants (grown in the dark) are yellowish but turn green rapidly when placed in the light. Light is required, among reasons, to:
- convert etioplasts → chloroplasts;
- photo-reduce protochlorophyllide to chlorophyllide; and
- activate enzymes for ALA synthesis.
III. Conversion 1: Photons to electrons
A. Nature of light
Light is part of the electromagnetic spectrum - radiation emitted by sun. Acts as discrete particles (called photons) traveling as waves. Wavelength - distance between any two crests (or troughs). Symbolized by lambda (λ); frequency - number of waves passing a point in one second (υ). Frequency is inversely related to wavelength υ = c/λ where c = speed of light (3 x 1010 cm sec-1). The energy of a photon is a quantum.
B. Which photons are important in photosynthesis?
Run an action spectrum (plot of a physiological process vs. wavelength).
insert action spectrum of photosynthesis here
Conclusion: radiations between 400-700 nm are photosynthetically active (termed PAR). Specifically, red (600s) and blue (400s) light are important.
C. Photons must be absorbed to be used in a photochemical reaction.
In other words, only those molecules that absorb quanta participate in photosynthesis. So, which molecules absorb the red and blue light? Run an absorption spectrum of potential pigment candidates (plot of light absorption vs. wavelength) and compare it to the action spectrum.
insert absorption spectrum of photosynthetic pigments here
Chlorophyll a & b absorb light in the red and blue regions of the visible spectrum. Note that the absorption spectra match the action spectrum of photosynthesis and hence, implicates (though doesnt prove) that they are involved in the process. (Subsequent work has shown the chlorophylls to be the major photosynthetic pigments).
D. Quantity vs. Quality
E. What happens when chlorophyll absorbs light?
The chlorophyll molecule becomes excited (this takes only 10-15 sec = femptosec) and an electron moves to an outer energy level. This is diagrammed:
CHL (ground state) → CHL* (excited state)
Blue light excites an electron to a higher energy level than red light. Imagine the "bell ringer" at a carnival. The electrons change spin at the first (S1) and second (S2) excited singlet states. Electrons dont stay excited long (10-9 sec), because they either:
F. Why excite electrons?
The ultimate purpose of exciting electrons from chlorophyll is to provide the energy needed to transfer electrons from water to NADP+. Recall that spontaneous electron transfers proceed from a carrier with a more negative redox potential to a more positive one. The redox potential of water/oxygen = +0.82 eV while for NADP/H = -0.32 eV. Thus, photosynthetic electron flow is not a spontaneous process and requires an energy.
G. How much energy is required to transfer electrons from water to NADP+?
First, let's calculate the actual redox difference (
ΔEm = Em (acceptor) - Em (donor). Or, ΔEm = -0.320 - (0.820) = -1.14 = ca. -1.2 eV.
The actual amount of energy involved is calculated from the equation:
ΔG = -n F Em
where F = Faraday constant = 96,000 J/coulombs, and n = number of electrons involved in the reaction (which equals one for each photon). Substituting in the equation:
ΔG = - (1) x 96000 x (-1.14) = 109440 J mol-1 (=109.4 kJ mol-1 )
To summarize, approx. 110 kJ mol-1 is required to reduce NADPH from water.
H. Do red and blue photons have enough energy?
Let's calculate the energy in red photons. Assume red photons have a wavelength of 660 nm = 6.6x10-5 cm.
The energy of a photon is expressed by the following equation:
E = hυ
where h = Plancks constant which relates energy to frequency of oscillation and is 6.6255 x 10-34 J sec photon-1; and υ = pulses sec-1.
Since υ = c/λ (see A above), we can substitute back in original equation:
E = hc/λ
Take home lesson #1: the energy of a photon is inversely proportional to its wavelength. Thus, blue light has more energy than red light.
E = ((6.625x10-34 j sec photon-1)(3x1010 cm sec-1))/6.6x10-5 cm
= 3.01x10-19 j photon-1
multiply by Avogadros number
= 3.01x10-19 j photon-1 x 6.02 x 1023 photon mol-1
= 181,000 j mol-1
= 181 kj mol-1
Take home lesson #2: Red light has more than enough energy to do the job.
IV. Chloroplast complexes:
A. Photosystem II (PSII) Complex
B. Cytochrome b/f Complex
1. occurs in stacked and non-stacked regions
2. cytochrome b (b-type cytochrome, not associated with protein)
3. cytochrome f (c-type cytochrome, associated with protein)
4. non heme iron-sulfur protein (Fe-SR)
C. Photosystem I (PSI) Complex
D. ATP synthase/Coupling Factor Complex
1. occurs in non-stacked regions
2. stalk - CFo (4 polypeptides)
3. head - CF1 (5 polypeptides)
4. nine polypeptides, some nuclear, some chloroplastic
V. The Z-Scheme (Or, the Light-Dependent Reactions; Or, Non-cyclic photophosphorylation).
During the light-dependent reactions of photosynthesis, electrons are transferred from water to NADP+. This reaction is depicted as follows:
H2O → NADP+
As the electrons move from water to NADP+, they pass through three of the four complexes described above - Photosystem II (PSII), a cytochrome b/f complex (cyt b/f), and Photosystem I (PSI). After electrons are removed from water, they are sequentially shuttled from PSII to the cyto b-f complex to PSI and then finally to NADP+. Thus:
H2O → PSII → Cytb/f → PSI → NADP+
Since PSII, cyt b/f, and PSI are physically separated from one another, there must be a means to transfer electrons between the complexes. A mobile form of plastoquinone (PQ) transfers electrons from PSII to cyt b-f. A copper-containing protein, plastocyanin (PC), transfers electrons from the cytochrome b-f complex to PSI. Thus, the reaction sequence is modified as follows:
H2O → PSII → PQ → Cytb/f → PC → PSI → NADP+
The transfer of electrons from PSI to NADP+ is mediated by a soluble complex found in the stroma, ferredoxin (Fd). Thus our revised equation:
H2O → PSII → PQ → Cytb/f → PC → PSI → Fd → NADP+
The transfer of electrons from water to PSII involves an "oxygen evolving complex" (OEC), part of PSII, that is rich in chloride and manganese ions. Thus,
H2O → OEC → PSII → PQ → Cytb/f → PC → PSI → Fd → NADP+
B. Origin of the name
Derived from the zig-zag arrangement of components with regard to redox potential. But, why dont we call it the N-scheme?
C. Oxygen evolving complex
The energy of a single photon is not sufficient to split water. Experiments suggest that 4 photons are required to split two water molecules. Since only one electron can be excited at a time (Einstein Law of Photochemical equivalents), this presents a minor problem.
The solution a water oxidizing "clock". Single electrons are transferred through a series of intermediate stages sequentially increasing the electron deficit to a total of four. At this point the original oxidation state is restored by extracting four electrons from water.
Diagram of the water-oxidizing clock - in class
- A series of five intermediate states, S0 - S4 are postulated;
- Initially the clock is in the So state, and may be associated with Mn II
- S1, which may be associated with Mn III, is the most stable form;
- S2 may be associated with Mn IV;
- S3 may be associated with a histidine (one of the amino acids in the D1 protein);
- The nature of S4 isnt clear;
- Conversion from one state to the next requires one photon and results in the loss of one electron to P680; and
- the loss of 4 total electrons generates a strong enough potential to split water.
- after a dark equilibration period, oxygen is released after the third light flash and then after every fourth flash;
- explains occurrence of Mn in photosystem II.
D. PQ Shuttle (Q cycle)
Take-Home-Lessons: for every two electrons shuttled to PSI, four protons are moved across the membrane! The stoichiometry requires more than 2 protons per electron pair to account for ATP synthesis.
E. Herbicides and electron transport
Electrons are passed in a single direction, one-way, no backtracking, from water to NADP+.
Electrons cycle through PSI. This occurs when carriers get backed up with electrons but still want to get rid of electrons. This is a mechanism for generating additional ATP.
VII. How many photosystems? Two
Emerson observed that the rate of photosynthesis was greater than the sum of the rates when red light (660 nm) and far red light (710 nm) were given separately. This synergistic effect, called the Emerson enhancement Effect, suggested two cooperating systems which has been the conventional wisdom for a long time.
01/07/2009 � Copyright by SG