Light and photosynthesis

1. Light and photosynthesis Rost et al., “Plant Biology”, 2nd ed. Plant Phys and Biotech Biol 3470 Lecture 6, Tues. 24 Jan. 2006 Chps. 3 & 4

2. What is light? Is it a Particle – quantifiable in discreet units (mol, photons) or a Wave – has a frequency (s-1) and wavelength (l) frequency (v) = c/l and is proportional to 1/l light energy is proportional to v is proportional to 1/l higher wavelength  less energy Fig. 3.1

3. Light interactions with plant pigments power photosynthesis • Light can interact with matter • i.e., with plant pigments in photosynthesis • Pigments absorb the energy in light and use it to excite electrons • These electrons then flow through electron transport chains in the thylakoid membranes and are used to make ATP

4. ENERGY Pigments are required to convert radiant energy into chemical potential energy • They accept energy from triplet state electrons (are reduced) then pass on electrons to photosynthetic electron transport chain (are oxidized) SHORT LIFETIME Excited/singlet state  Triplet state  Pigment  e- transport LONGER LIFETIME Make ATP, NADPH Fuel the plant! Heat (entropy) Ground State Light (hv)

5. Pigments define the portion of the electromagnetic spectrum useable by plants • Pigments have action spectra • are able to absorb energy from only certain wavelengths of light • Should be same as (light) absorption spectrum • These are unique to each pigment • i.e., chlorophyll • Pigments must be able to absorb light energy in order to excite electrons and drive the photosynthetic E.T.C. Photosynthesis action spectrum More light absorbed Plant pigment extract absorption spectrum (mostly chlorophyll) Less light absorbed Green Red Blue Fig 3.4 • Since the pigments useful in photosynthesis interact with light, they are photoreceptors (perception of information) • Typical pigment = chlorophyll

6. Chlorophyll’s structure permits its role in absorbing light energy • Chlorophyll has a porphyrin ring (like heme) • Long hydrocarbon tail: hydro_________ • So it is membrane-bound in thylakoids • Almost all chlorophyll is also bound to membrane proteins involved in harvesting light energy • These are the photosystems Fig. 3.7

7. Other pigments also contribute to the collection and transduction of light energy Compare the absorption spectra of these different plant pigments • Light absorption is not just dependent on chlorophyll • Accessory light-harvesting pigments collect light energy and pass it to chlorophyll to excite electrons for entry into photosynthetic E.T.C. • Note how many of these have absorption spectra that complement that of chlorophyll! Fig. 3.8: Chlorophyll Fig. 3.10: Phycocyanin and phycoerythrin Fig. 3.12: Carotenes Fig. 3.15: Pelargonin

8. These accessory pigments are often called “antenna pigments” • Fill in action spectrum of photosynthesis  more light useable for E.T.C. (not just red and blue of chlorophyll) • Are important in human nutrition • Directly – b-carotene – Vitamin A (metabolic cofactor) • Indirectly – as antioxidants – quench excess photosynthetically generated electrons in chlorophyll and reactive O2 (singlet excited O2)

9. Photosynthesis is composed of light-dependent and –independent reactions (see chapter 4) • As we have discussed, this process conserves energy found in light-excited electrons as ATP and NADPH • Involved in the conversion of less stable energy  more stable energy • Metabolically useful • Leaves are structured to maximize light absorption

10. Chloroplasts are predominantly found in the leaf palisade mesophyll cells Location is just underneath the top surface of leaf The structure of leaves favors light absorption • Incoming photons directly excite chlorophyll in chloroplasts • Sieve effect – epidermis does not absorb light  passes through layers until absorbed • Epidermal cells  lenses redirecting light to palisade • Palisade cells are light guides – allocate photosynthesis to spongy mesophyll  save energy! A B C D Fig. 4.2

11. The light-dependent reactions of photosynthesis generate oxygen and chemical energy • These reactions take place in the autotrophic tissues of the plant • Again, predominantly (80%+) in chloroplasts of mesophyll cells of leaves Light energy + H2O  O2 + ATP + NADPH • These products are then used in the light-independent (dark) reactions to convert CO2 to sugar Note that the equation is not stoichiometric! Chemiosmotic synthesis

12. Like oxidative phosphorylation, the light-dependent reactions are a series of redox reactions • It thus involves giving up (oxidation) and accepting (reduction) electrons • Defined as the “photochemical reduction of CO2” • CO2 accepts electrons and thus H+ and forms the reducing sugar glucose • CO2 C6H12O6 – not energetically favoured • Highly positive DG°’ • Use the photosynthetic E.T.C to generate ATP and NADPH

13. The structure of the photosynthetic electron transport chain can be conceptualized linearly Rost et al., “Plant Biology”, 2nd ed. • 3 major multiprotein complexes embedded in thylakoid membranes • The electrons flow noncyclically from an e- source (water) to an e- sink (oxidized reducing power, NADP+) • The electrons pass through • Photosystem II • Cytochrome complex • Photosystem I Fig. 4.3

14. The electrons pass through the “Z-scheme” from water to the final e- acceptor NADP+ Fig. 4.8 • So-named because of the shape • The photosystems are excited by light energy • This increases their energy level and redox potential • + = accepts e- • - = passes on e- • Thus they become very good at passing on their e- to other e- carriers Cytochrome complex The Z-scheme shows the order of e- excitation and flow in the light-dependent reactions of photosynthesis

15. First, PSII oxidizes water to produce oxygen and electrons Rost et al., “Plant Biology”, 2nd ed. • H2O is oxidized on inside the thylakoid (in the lumen) • This produces O2 + H+ important to generate proton gradient • e- pass through the cytochrome complex on their way to PSI • PSII needs energy from 8 photons to evolve one O2 1

16. Second, PSI re-excites electrons in the transport chain, allowing them to reduce NADP+ Rost et al., “Plant Biology”, 2nd ed. • More light is used to increase energy level of electrons in PSI at P700 (reaction centre) • Excited reaction centre P700 reduces ferridoxin (Fd) • Fd reduces NADP+ NADPH 2

17. E.T.C function is to extract low energy electrons from water and use light to convert them to high energy electrons Rost et al., “Plant Biology”, 2nd ed. • These are then used to generate a strong reductant (NADPH) that is used in the subsequent light-independent reactions of photosynthesis • The efficiency of the light dependent reactions is “only” 32% • 32% of energy from excited photons is conserved in reducing NADP+  NADPH • However, the proton gradient generated in lumen drives chemiosmotic ATP synthesis (photophosphorylation) • This substantially increases the energy yield!

18. The photosystem physical structure is designed to maximize its ability to extract energy from light • Light excites electrons in antenna chlorophyll • The antenna chlorophyll surrounds the reaction centre • The reaction centre is the site where e- are swapped between donor (A) and acceptor (Q) proteins • Recall that only the more stable triplet state electron is passed to the e- acceptor protein, reducing it • This occurs in both photosystems! • P700 (PSI) • P680 (PSII) Fig. 4.4 The sum total of these components is called a light harvesting complex

19. The PS design allows a high efficiency of energy collection from light • 90%+ thanks to large number of antenna chlorophylls per reaction centre • Both LHCs together contain ~70% of total chlorophyll in plant • Do not memorize the names of the electron carrier proteins in the membrane • Do note the large number of proteins involved in this process! Fig. 4.6

20. ATP can also be generated by cyclic electron flow Fig. 4.9 • This process involves passing e- between PSI and certain e- carrier proteins • Generates more H+ ! • Chemiosmotically synthesize more ATP • Purpose: supplies energy for chloroplast metabolism beyond that needed for dark reactions (oxygen fixation) NADP+

21. Cyclic electron flow is possible because plants can adapt their metabolic needs to their environment • ATP is synthesized when H+ built up inside the lumen by the splitting of water pass through ATP synthases in thylakoid membrane • Precise stoichiometry difficult to calculate because plant metabolism is very plastic • Cyclic and noncyclic transport importance vary • Electrons supply can be adjusted to modulate ATP and NADPH levels • Generally • 2 ATP per NADPH • 3 H+ through ATP synthase per ATP

22. The plasticity of the light-dependent reactions extends to their location • The three main E.T.C. components (2 PS, 1 cytochrome complex) are NOT: • Fixed in membrane (movement) • Present in equal (stoichiometric) numbers • Physically next to each other – spatial segregation • All of these factors affect ATP and NADPH yield

23. The light harvesting complexes are essential parts of the photosystems • We have seen that they are required to gather energy from light and transduce it to E.T.C. • The amount and activity of the LHC proteins can be varied to modulate the electron flow through the photosystems • Sun plants: less • Shade plants: more • Long-term adjustment: changes [protein] • Sun stress via sunflecks • Reversible phosphorylation of LHCII: +PO4, ↓ ability of LHCII to pass energy to PSII therefore downregulate E.T.C. • Short-term adjustment: changes protein activity

24. Photoinhibition occurs when the supply of light-excited e- exceeds the demand from the E.T.C. • Too much light damages PSII reaction centre (P680) = photoinhibition • Electron transport saturated  excess energy damages reaction centre protein • Electrons passed to oxygen  forms reactive oxygen species (ROS) • Carotenoids – accessory pigments • Photoprotect by dissipating this energy (b-carotene) • Activated forms release energy as heat

25. Herbicides act by stealing e- away from the photosynthetic E.T.C. • Herbicides are thus alternative electron acceptors in photosynthetic E.T.C. (viologen dyes) • They can then pass these e- to oxygen to form superoxide and other ROS • Damage chlorophyll and membranes • Block protein binding in reaction centres necessary for electron transfer We will see next lecture how the products of the light-dependent reactions (NADPH and ATP) are used to fix atmospheric carbon in the light-independent reactions