Plant Physiology talk Seven Photosynthesis

1. Plant Physiology talk SevenPhotosynthesis

2. Photosynthesis • Oneof the most important biochemical process in plants. • Let’s not forget cell wall biosynthesis and adaptation during plant development, growth, interaction with the environment, and disease defense. • Among the most expensive biochemical processes in plant in terms of investment • The biochemical process that has driven plant form and function

3. Photosynthesis • Via photosynthesis, over 100 billion metric tons of CO2 and H2O are converted into cellulose and other plant products. • The term carbohydrate is a generic one that refers primarily to carbon-containing compounds that contain hydroxyl, keto, or aldehydic functionalities. • Carbohydrates can range in sizes, from simple monosaccharides (sugars) to oligosaccharides, to polysaccharides. From the wikimedia free licensed media file repository

4. Photosynthesis • There are two parts to this process: The light reactionsand the Carbon reactions. • The main components required by any photosynthetic organisms for this process to occur are: • H20 MW 18 • CO2 MW 44 • Sunlight (Sorry everyone – couldn’t resist!)

5. Photosynthesis • The effectiveness of controlling water loss and allowing CO2 uptake for photosynthesis is called the transpiration ratio. • There is a large ratio of water efflux and CO2 influx. The concentration ratio driving water loss is 50 times larger than that driving CO2 influx. • Furthermore, CO2 diffuses 1.6 times slower than water,due to CO2 being a larger molecule than water (Sorry everyone – couldn’t resist!)

6. Photosynthesis is an endergonic reaction because it takes in energy. Energy is captured from sunlight. Oxygen is released. Sugar is synthesized and used in plant tissues. Carbon dioxide is absorbed from the air. Carbon for making carbon compounds (such as sugar) comes from the atmosphere. photosynthesis Oxygen, hydrogen, and minerals are needed also. Oxygen and hydrogen come from water. Minerals comes from the soil Water is absorbed from soil, used in photosynthesis, and stored in cells. Inorganic mineral nutrients (nitrate, phosphate) are absorbed from soil and used in plant tissues. From the wikimedia free licensed media file repository

7. General overall reaction Carbondioxide Water Glucose Oxygengas PHOTOSYNTHESIS Photosynthetic organisms use solar energy to synthesize carbon compounds that cannot be formed without the input of energy. More specifically, light energy drives the synthesis of carbohydrates from carbon dioxide and water with the generation of oxygen. Carbondioxide Water Glucose Oxygengas PHOTOSYNTHESIS

8. In a nutshell! • The overall process of photosynthesis is a redox chemical reaction: • Electrons are removed from one chemical species (oxidation) and added to another (reduction) • Light reduces NADP, which serves as the reducing agent for carbon fixation during the dark reactions • ATP also formed during electron flow from water to NADH and is also used during carbon fixation • Thylakoid (Light) reactions: water oxidized to oxygen, NADP reduced and ATP is formed • Stroma (Carbon) reactions: carbon fixation and reduction reactions

9. Overall Perspective • Carbon Reactions: • Expend chemical energy • Fix Carbon [convert CO2 to organic form] • Light reactions: • Harvest light energy • Convert light energy to chemical energy

10. Only a small fraction of the sun’s energy reaches the surface • Solar spectrum and its relation to the absorption spectrum of chlorophyll • Very little of the Sun’s energy gets to the ground (red line). • gets absorbed by water vapor in the atmosphere • The absorbance spectra of chlorophyll (green line). • Absorbs strongly in the blue and red portion of the spectrum • Green light is reflected and gives plants their color.

11. Photosynthetic pigments • Two types in plants: • Chlorophyll- a • Chlorophyll –b • Structure almost identical, • Differ in the composition of a sidechain • In a it is -CH3, in b it is CHO • The different sidegroups 'tune' the absorption spectrum to slightly different wavelengths • light that is not significantly absorbed by chlorophyll a, will instead be captured by chlorophyll b

12. Photosynthetic pigments • Chlorophyll has a complex ring structure • The basic structure is a porphyrin ring, co-coordinated to a central atom. • This is very similar to the heme group of hemoglobin • Ring contains loosely bound electrons • It is the part of the molecule involved in electron transitions and redox reactions of photosynthesis

13. Photosynthetic pigments • When chlorophyll absorbs a light particle (Proton) • Enters a higher excitation state • Becomes unstable, gives up energy as heat • Enters lower excited state • can be stable for a few nanoseconds • This energy causes chemical reactions to occur • These reactions are the fastest known to science!!!!

14. Different pigments absorb light differently • In addition to the chlorophyll pigments, there are other pigments present • During the fall, thegreen chlorophyll pigments are greatly reducedrevealing the other pigments • Carotenoidsare pigments that are either red, orange, or yellow From the wikimedia free licensed media file repository

15. Fall Colors • Chlorophylls degrade into colorless non-fluorescent chlorophyll catabolites (NCCs). • As the chlorophylls degrade, the hidden pigments of Carotenoids are revealed. • Photoprotection: • Carotenoids protect the leaf against the harmful effects of light and low temps. • By shielding the leaf with Carotenoids the tree manages to reabsorb nutrients (especially nitrogen) more efficiently, including the components of chlorophyll. • These are stored within the phloem for the next growing season. From the wikimedia free licensed media file repository Hortensteiner, S. (2006). Annual Review of Plant Biology. 57: 55–77

16. Photosynthesis • Takes place in complexes containing light-harvesting antennas & photochemical reaction centers • Antenna complex • Chemical oxidation & reduction reactions leading to long term energy storage take place • Antenna collects light and transfers its energy to the reaction center • Chemical reactions store some of the energy by transferring electrons from chlorophyll to an electron acceptor molecule

17. Photosynthesis • An electron donor then reduces the chlorophyll again • The transfer of energy to the antenna is a purely physical phenomenon and involves no chemical changes • Even in bright sunlight, a chlorophyll molecule absorbs only a few photons each second • Therefore, many chlorophyll molecules send energy into a common reaction center • The whole system is kept active most of the time

18. The light reactions • Plants have two reaction centers: • PS-II - chlorophyll a • Absorbs Red light – 680mn • makes strong oxidant (Weak reductant) • oxidizes 2 H2O molecules to 4 electrons, 4 protons & 1 O2 molecule • Mostly found in Granum • PS-I - chlorophyll b • Absorbs Far-Red light – 700nm • strong reductant (Weak Oxidant) • PS-I reduces NADP to NADPH • Mostly found in Stroma

19. Oxygen-evolving organisms have two photosystems (PS) that operate in series • Z (zigzag) scheme – the basis of understanding O2-evolution • Red light absorbed by PS-II makes strong oxidant (& weak reductant) • Far-Red light absorbed by PS-I makes strong reductant (& weak oxidant) • PS-II oxidizes water and PS-I reduces NADP - P680 and P700 refers to wavelength of max absorption of reaction center chlorophylls

20. The Chloroplast • Membranes contain chlophyll and it’s associated proteins • Site of photosynthesis • Have inner & outer membranes • 3rd membrane system • Thylakoids • Stack of Thylakoids = Granum • Surrounded by Stroma • Works like mitochondria • During photosynthesis, ATP from stroma provide the energy for the production of sugar molecules

21. PS-II and PS-I are spatially separated in the thylakoid membrane • The PS-II reaction center is located mostly in Granum. • Stack of Thylakoids • The PS-I reaction center is located in the Stroma& the edges of the Granum. • There is a cytochrome b6f complex that connects the two photosystems that is evenly distributed between Granum and Stroma • One or more of the electron carriers that function between the photosystems diffuses from the from the Granum to the Stroma.

22. Electron transfer A step by step look

23. Oxygen-evolving organisms have two photosystems (PS) that operate in series What is an electron transport chain? A series of coupled oxidation/reduction reactions where electrons are passed from one membrane-bound protein/enzyme to another before being finally attached to a terminal electron acceptor (usually oxygen or NADPH).

24. Mechanisms of electron transfer • The energy changes of electrons as they flow through the light reactions are analogous to the cartoon. • The light reactions use solar power to generate ATP and NADPH which provide chemical energy and reducing power to the sugar making reactions

25. The transport chain

26. The transport chain • PS-II oxidizes water to O2 in the thylakoid lumen • Releases protons into the lumen • Cytochrome b6f receives electrons from PS-II & delivers them to PS-I • Also transports additional protons into lumen from stroma

27. The transport chain • PS-I reduces NADP to NADPH in the stroma • Uses the action of: • Ferredoxin (Fd) • Ferredoxin-NADP reductase (FNR) • ATP synthesis produces ATP as protons diffuse back through it from the lumen into the stroma

28. Summary of light reactions • Photosynthesis (light reactions): • Storage of solar energy carried out by plants • Absorbed photons excite chlorophyll molecules • can dispose of this energy as: • Heat • Fluorescence • Energy transfer • Photochemistry – the light reactions of photosynthesis • Absorption of light occurs in the thylakoid membranes of the chloroplast by chlorophyll a & b

29. Summary of light reactions 2 H2O + 2 NADP+ + 3 ADP + 3 Pi O2 + 2 NADPH + 3 ATP + 4 e- + 2 H+ (gas) Light reactions: Chemical energy compounds are made from light energy, water is split into O2 and protons

30. Summary of light reactions • Excess light energy can damage photosynthetic systems • Several mechanisms occur to minimize such damage • Some proteins made in chloroplasts act as photoprotective agents to control excited state of chlorophyll molecules • Chloroplasts contain DNA and encode and synthesize most of the proteins essential for photosynthesis • Others encoded by nuclear DNA

31. At the end of the light reactions • The reactions catalyzing the reduction of CO2 to carbohydrates are coupled to the consumption of NADPH and ATP by enzymes found in the stroma • fluid environment • These reactions were thought to be independent of the light reactions • So the name “dark reactions”stuck • However, these chemical reactions are regulated by light • So are called the “carbon reactions” of photosynthesis

32. Overview of the carbon reactions • The Calvin cycle: • Stage 1: • CO2 accepted by Ribulose-1,5-bisphosphate. • This undergoes carboxylation • Has a carboxyl group (-COOH) attached to it • At the end of stage 1, CO2 covalently linked to a carbon skeleton forming two 3-phosphycerate molecules.

33. Carboxylation: The first step is the most important • Step 1: The enzyme RUBISCO (Ribulose bis-phosphate carboxylase oxygenase) carries out this conversion • Rubisco accounts for 40% of the protein content of chloroplasts • is likely the most abundant protein on Earth • Rubisco is, in fact, very inefficient, and that a mechanism has evolved to deal with this handicap

34. Overview of the carbon reactions • The Calvin cycle: • Stage 2: • Each of the two 3-phosphycerate molecules are altered. • First phosphorylated through the use of the 3 ATPs generated during the light reaction. • Then reduced through the use of the 2 NADPHs generated during the light reaction. • Forms a carbohydrate • glyceraldehyde-3-phosphate

35. 3-phosphycerate molecules are altered • First phosphorylated through the use of the 3 ATP molecules generated during the light reaction • Forms 1,3-bisphosphoglycerate • Then reduced through the use of the 2 NADPH molecules generated during the light reaction • Forms glyceraldehyde-3-phosphate • Note the formation of triose phosphate

36. Overview of the carbon reactions • The Calvin cycle: • Stage 3: • Regeneration of Ribulose-1,5-bisphosphate. • This requires the coordinated action of eight reaction steps • And thus eight specific enzymes • Three molecules of Ribulose-1,5-bisphosphate are formed from the reshuffling of carbon atoms from triose phosphate.

37. Regeneration of Ribulose-1,5-bisphosphate • The Calvin cycle reactions regenerate the biochemical intermediates needed for operation • More importantly, the cycle is Autocatalytic • Rate of operation can be enhanced by increasing the concentration of the intermediates in the cycle • So, Calvin cycle has the metabolically desirable of producing more substrate than is consumed • Works as long as the produced triose phosphate is NOT diverted elsewhere (as in times of stress or disease)

38. Overview of the carbon reactions • The Calvin cycle: • The cycle runs six times: • Each time incorporating a new carbon . Those six carbon dioxides are reduced to glucose: • Glucose can now serve as a building block to make: • polysaccharides • other monosaccharides • fats • amino acids • nucleotides

39. Only one-sixth of the triose phosphate is used for polysaccharide production • Synthesis of polysaccharides, such as starch and sucrose, provide a sink • Ensures an adequate flow of carbon atoms through the cycleIFCO2 is constantly available • During a steady rate of photosynthesis 5/6 of the triose phosphates are used for the regeneration of Ribulose-1,5-bisphosphate • 1/6 is transported to the cytosol for the synthesis of sucrose or other metabolites that are converted to starch in the chloroplast

40. Regulation of the Calvin cycle • The high energy efficiency of the Calvin cycle indicates that some form of regulation ensures that all intermediates in cycle: • Are present at adequate concentrations • The cycle is turned off when it is not needed in the dark • Remember: • These are the “carbon reactions”, NOT the “dark reactions” • Many factors regulate the Calvin cycle

41. Regulation of the Calvin cycle • 1: The pH of the stroma increases as protons are pumped out of it through the membrane assembly of the light reactions. • The enzymes of the Calvin Cycle function better at this higher pH. • 2: The reactions of the Calvin cycle have to stop when they run out of substrate • as photosynthesis stops, there is no more ATP or NADPH in the stroma for the dark reactions to take place.

42. Regulation of the Calvin cycle • 3: The light reactions increase the permeability of the stromal membrane to required cofactors • Mg ions are required for the Calvin Cycle. • 4: Several enzymes of the Calvin Cycle are activated by the breaking of disulphide bridges of enzymes involved in the working of the cycle. • the activity of the light reactions is communicated to the dark reactions by an enzyme intermediate

43. When conditions are not optimum

44. Photorespiration • Occurs when the CO2 levels inside a leaf become low • This happens on hot dry days when a plant is forced to close its stomata to prevent excess water loss • If the plant continues to attempt to fix CO2 when its stomata are closed • CO2 will get used up and the O2 ratio in the leaf will increase relative to CO2 concentrations • When the CO2 levels inside the leaf drop to around 50 ppm, • Rubisco starts to combine O2 with Ribulose-1,5-bisphosphate instead of CO2

45. Photorespiration • Instead of producing 2 3C PGA molecules, only one molecule of PGA is produced and a toxic 2C molecule called phosphoglycolateis produced • The plant must get rid of the phosphoglycolate • The plant immediately gets rid of the phosphate group • converting the molecule to glycolic acid

46. Photorespiration • Theglycolic acid is then transported to the peroxisome and there converted to glycine • Peroxisomes are ubiquitous organelles that function to rid cells of toxic substances • The glycine (4 carbons) is then transported into a mitochondria where it is converted into serine(3 carbons) • Releases CO2

47. Photorespiration • The serine is then used to make other organic molecules • All these conversions cost the plant energy and results in the net lost of CO2 from the plant • 75% of the carbon lost during the oxygenation of Rubisco is recovered during photorespiration and is returned to the Calvin cycle

48. The C4 Carbon cycle

49. The C4 carbon Cycle • The C4 carbon Cycle occurs in 16 families of both monocots and dicots. • Corn • Millet • Sugarcane • Maize • There are three variations of the basic C4 carbon Cycle • Due to the different four carbon molecule used

50. The C4 carbon Cycle • This is a biochemical pathway that prevents photorespiration • C4 leaves have TWO chloroplast containing cells • Mesophyll cells • Bundle sheath (deep in the leaf so atmospheric oxygen cannot diffuse easily to them) • C3 plants only have Mesophyll cells • Operation of the C4 cycle requires the coordinated effort of both cell types • No mesophyll cells is more than three cells away from a bundle sheath cells • Many plasmodesmata for communication