4 Energy conservation in photosynthesis: Harvesting Sunlight Fig. 1 2 3 4 5 6 7 8 9 10 11 12 13 16 14 15 17 18 19 20
Focus of this chapter (1) • The structure of higher plant leaves with respect to the interception of light • Photosynthesis as the reduction of carbon dioxide to carbohydrate • The photosynthetic electron transport chain, its organization in the thylakoid membrane, and its role in generating reducing potential and ATP • Problems encounters by chloroplasts when they are subjected to varying amount of light
Focus of this chapter (2) • The dynamic nature of the thylakoid membrane, showing how changes in the organization of light-harvesting apparatus influence the absorption and distribution of light energy • The role of carotenoids as accessory pigments and in photoprotection of chlorophyll and • The use of herbicides that specifically interact with photosynthetic electron transport
The structure of the leaf • The architecture of a typical higher plant leaf is particularly well suited to absorb light. • The photosynthetic tissues (mesophyll) are located between the two epidermal layers. • Dicotyledonous leaf is structurally different from monocotyledonous leaf.
The structure of dicotyledonous leaf • One-to-three layers of palisade mesophyll cells forms the upper photosynthetic tissue. • Below is the spongy mesophyll cells.
The structure of dicotyledonous leaf • Palisade mesophyll cells are elongated, cylindrical cells with the long axis perpendicular to the surface of the leaf. • Spongy mesophyll cells are irregular with lots of air spaces between the cells.
The structure of monocotyledonous leaf • Monocotyledonous leaf lack the distinction between palisade and spongy mesophyll cells.
Comparison between mesophyll cells • Palisade mesophyll generally have larger numbers of chloroplasts than spongy mesophyll.
Sieve effect • When light passes through the first layer of cells (palisade mesophyll cells) without being absorbed, we call this sieve effect. • The sieve effect is due to the fact that chlorophyll is not uniformly distributed throughout cells but instead is confined to the chloroplasts.
Sieve effect • To reduce sieve effect, plant develops multiple layers of photosynthetic cells. • The reflection, refraction, and scattering of light inside leaf may also reduce sieve effect.
Photosynthesis • Photosynthesis can be viewed as a photochemical reduction of CO2. • In the 1920s, C.B. van Niel discovered the O2 produced from photosynthesis is from water. • In 1939 Robert Hill found light reaction still can happen in isolated chloroplast when no CO2 is consumed and no carbohydrate was produced. • In the early 1940s S. Ruben and M. Kamen showed O2 produced from photosynthesis is from water by using O18 labeled water.
Photosynthetic electron transport The principle function of the light-dependent reactions of photosynthesis is therefore to generate the NADPH and ATP required for carbon reduction.
Photosynthetic electron transport • The effect of photosynthetic electron transport chain is to extract low-energy electrons from water and raise the energy level of those electrons to produce a strong reductant NADPH. • The energy plant used to raise the energy level of those electrons is the light energy trapped by chlorophyll.
Photosynthetic electron-transport chain • Two large, multimolecular complexes, photosystem I (PSI) and photosystem II (PSII), linked with a third multiprotein aggregate called the cytochrome complex, form the photosynthetic electron-transport chain.
Photosystems • Photosystems contain several different proteins together with a collection of chlorophyll and carotenoid molecules that absorb photons. • Most of the chlorophyll in the photosystem functions as antenna chlorophyll.
Photosystems • The antenna chlorophyll absorb light but do not participate in photochemical reactions. It pass its energy to the next chlorophyll by either inductive resonance or radiationless energy transfer.
Reaction center of photosystem • For PSII, each reaction center consisted of two chlorophyll a called reaction center chlorophyll. • Reaction center chlorophyll is the lowest-energy absorbing chlorophyll in the PSII complex (energy sink).
Energy transfer efficiency of Photosystem • The design of photosystems ensure efficient energy transfer. Only about 10% of the energy is lost during the whole transfer process (from antenna to reaction center chlorophyll).
Why photosystems? • The principle advantage of associating a single reaction center with a large number of antenna chlorophyll molecules is to increase efficiency in the collection and utilization of light energy.
Why photosystems? • Even in bright sunlight, an individual chlorophyll will only be struck not more than a few times per second. However, energy transfer only takes ms. So it is more economical not to make every chlorophyll into reaction center.
Light-harvesting complexes (LHC) • Light harvesting complex (also consisted of chlorophyll and proteins) serves as extended antenna systems for harvesting additional light energy. • In chloroplast, there are two LHCs. The one associated with PSI is named LHCI and the one associated with PSII is named LHCII, accordingly.
Light-harvesting complexes (LHC) • All the chlorophyll b are contained in LHCs. Most of the chloroplast pigments (70%) are in LHCs. • LHCI has a chlorophyll a/b ratio about 4 and it is tightly bound to PSI. • LHCII has a chlorophyll a/b ratio about 1.2. Besides owning most of the chloroplast chlorophyll (50~60%), LHCII also contains most of the chlorophyll b and xanthophyll.
PSII pheophytin • P680 is located at the lumenal side of reaction center. • When excited, the excited P680 (P680*) is rapidly (10-12s) photooxidized as it passes an electron to pheophytin (primary electron acceptor).
pheophytin • Pheophytin is a form of chlorophyll a with the Mg2+ replaced by two hydrogens. • The photo-oxidation of P680 is then followed by charge separation (P680+Pheo-).
phytyl Pheophytin Pheophytin a R1 =-CH3; R2 = phytyl Pheophytin b R1 = -CHO; R2 = phytyl
P680 pheophytin • Noted the direction of electron movement in PSII. P680 is located at the lumen side of PSII, then the electron is transferred to pheophytin, which is more towards the stromal side, so electron will not recombine with P680+.
Pheophytin QA PQ • Reaction proteins D1 and D2 orient specific redox carriers of the PSII reaction center so the probability of charge recombination is further reduced.
Pheophytin QA PQ • D2 contains QA (quinone electron acceptor) which will accept electrons from pheophytin within picoseconds. • Then electron from QA will be passed to plastoquinone (PQ), a quinone bound transiently to the binding site on D1 protein (QB).
Plastoquinone (PQ) • The reduction of plastoquinone (PQ) to plastoquinol (PQH2) lowering the affinity of this molecule for the binding site. • After plastoquinol is released from the reaction center, another molecule of PQ will occupy the empty space.
Oxygen-evolving complex (OEC) • P680+ got its electron directly from a cluster of four Mn2+ associated with a small complex of proteins. • OEC is located on the lumen side of the thylakoid membrane and bound to the D1 and D2 proteins of PSII reaction center.
Oxygen-evolving complex (OEC) • The OEC proteins functions to stabilize the Mn2+ cluster. • Chloride ion (Cl-) is also required for the water splitting function.
Oxygen-evolving complex (OEC) • To generate one molecule of O2, four electrons must be withdrawn from two molecule of H2O. This suggest that OEC should be able to store charges (and experiment results agree with this).
PQ cyt b6f complex • After plastoquinol is released from PSII, it diffuses through the membrane until reaches cytochrome b6f complex. • Because plastoquinol has to reach cyt b6f by diffusion, this is the slowest step in photosynthetic electron transport (milliseconds).
Cytb6f complex • Electron is then transferred from plastoquinol to Rieske iron-sulfur (FeS) protein cytochrome f (all on the lumenal side). • Then electrons are picked up by plastocyanin (PC).
Plastocyanin (PC) • Plastocyanin is a small peripheral protein that is able to diffuse freely along the lumenal surface of the thylakoid membrane.
PC PSI • PC is then transfer electron to PSI. • The reaction center chlorophyll (P700) first become P700*, then photooxidized to P700+ and give its electron to a molecule of chlorophyll a.
Photosystem I • The electron is then passed to a quinone (phylloquinone). • Electron transfer then proceeds through a series of Fe-S centers and ultimately to the soluble iron-sulfur protein, ferredoxin (Fdx).
Ferredoxin NADP+ • Ferredoxin-NADP+ reductase (Fd-NADP+ reductase) then uses ferredoxin to reduce NADP+.
…can also be activated by light. • When PSI is directly activated by light, the electron it lost is satisfied by withdrawing an electron from reduced PC.
Photosynthetic efficiency • The efficiency of photosynthesis can be expressed as quantum yield (). • Quantum yield is number of photochemical product produced per photon absorbed.
Noncyclic electron transport • When electron transport is operating according to the figure above, electrons are continuously supplied from water and withdrawn as NADPH. This flow-through form of electron transport is known as noncyclic or linear electron transport.
Cyclic electron transport • Cyclic electron transport is referring to a condition when electrons from PSI is transported not to Fd-NADP+-reductase but to a Fd-PQ reductase.
Photophosphorylation The light-driven production of ATP by chloroplasts is known as photophosphorylation.
How is ATP generated? The light-driven accumulation of protons in the lumen by oxidation of water and PQ-cytochrome proton pump is the energy source of ATP production.
How cytb6f complex moves protons (H+) across the membrane • The most widely accepted model for this question is known as the Q-cycle.