Photosynthesis

1. Photosynthesis

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. The Chloroplast

15. 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.

16. 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).

17. 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

18. 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

19. 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.

20. 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

21. 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.

22. 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

23. Summary • The Calvin cycle requires the joint action of several light-dependant systems • Changes in ions (Mg+ and H+) • Changes in effector metabolites (enzyme substrates) • Changes in protein-mediated systems (rubisco activase) • Rubisco can also act as an oxygenase • The carboxylation & oxygenation reactions take place at the active sites of rubisco.

24. The C4 Carbon cycle

25. 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

26. 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

27. The C4 carbon Cycle • Four stages: • Stage 1: • In Mesophyll cell • Fixation of CO2 by the carboxylation of phosphenol-pyruvate (primary acceptor molecule) • forms a C4 acid molecule • Malic acid and/or aspartate • Stage 2: • Transport of the C4 acid molecule to the bundle sheath cell

28. The C4 carbon Cycle • Stage 3: • Decarboxylationof theC4 acid molecule (in bundle sheath) • Makes a C3 acid molecule • This generates CO2 • This CO2 is reduced to carbohydrate by the Calvin cycle • Stage 4: • The C3 acid molecule (pryuvate) is transported back to mesophyll cells • Regeneration of phosphenol-pyruvate

29. The C4 carbon Cycle • Regeneration of phosphenol-pyruvate consumestwo high energy bonds from ATP • Movement between cells is by diffusion via plasmodesmata • Movement within cells is regulated by concentration gradients • This system generates a higher CO2 conc in bundle sheath cells than would occur by equilibrium with the atmosphere • Prevents photorespiration!!!!!!!!!!

30. The C4 carbon Cycle • The net effect of the C4 carbon Cycle is to convert a dilute CO2 solution in the mesophyll into a concentrated solution in the bundle sheath cells • This requires more energy than C3 carbon plants • BUT – This energy requirement is constant no matter what the environmental conditions • Allows more efficient photosynthesis in hotter conditions

31. Photorespiration

32. Photorespiration • Excess light energy 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

33. 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

34. 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

35. 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

36. 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

37. Any Questions?