Photosynthesis overview

1. Photosynthesis overview Ch 10

2. What is the equation for photosynthesis?

3. 6CO2 + 6H2O + light energy •  C6H12O6 + 6O2

5. What does each part do for the plant? • Epidermis – protects leaf tissues • Cuticle – waxy covering, resists water, dirt • Stoma- pore used for gaseous exchange (water, CO2, O2) (stomata – plural) • guard cell – open & close stomata

6. Vein – vascular tissue • Xylem – moves water & minerals from roots to leaf • Phloem – moves sap from photosynthesis to other parts of plant • palisade layer (mesophyll) – dense upper middle layer of dicot leaf, where photosynthesis takes place • spongy mesophyll – lower middle layer of dicot leaf, has lots of air spaces, where photosynthesis takes place

7. Chloroplast Inner membrane Stroma Thylakoid (thylakoid space inside) Outer membrane Granum (stack of thylakoids)

9. 2 stages of photosynthesis

10. Tracking atoms in photosynthesis - Redox reaction: H2O is oxidized, CO2 is reduced - Endergonic reaction – requires energy - Experiments done with heavy water to determine where oxygen comes from – it comes from water

11. Capturing energy from light

12. Plant Pigments • Pigments – a substance that absorbs light • Plant pigments absorb light in the blue/violet and red region of the spectrum • Plant pigments reflect light in the yellow/green region of the spectrum

13. Absorbance vs. Action spectrum

14. Absorbance spectra = range of wavelengths absorbed by a particular pigment

15. Chlorophyll A & B

16. Chlorophyll a – main photosynthetic pigment • Accessory pigments: help broaden spectrum • Chlorophyll b • Carotenoids

17. Action spectra = range of wavelengths capable of driving a particular biological process

18. Chlorophyll excited by light • When pigment gets excited by light it goes from ground state to excited state • When electron goes back to ground state, it gives off energy

19. Englemann’s experiment - first action spectrum - 1883 - Used filamentous algae - Put on flat surface w/water & CO2 - Solution of aerobic bacteria Looked to see where bacteria built up

20. Photosystem Light gathering complex – various pigment molecules bound to proteins Reaction center – has special chlorophyll A associated with a primary electron acceptor Photon excites pigments – they transfer electrons until they get to the chlorophyll a in reaction center

21. Non cyclic photophosphorylation

22. Cyclic Phosphorylation

23. Cyclic photophosphorylation • Alternate electron path • Photosystem 1 transfers electrons back to the ETC from PS II. This generates ATP through photophosphorylation, rather than NADPH. • - some bacteria only have PS 1, some plants have cells that only have PS1

24. Calvin Cycle(equations for 3 turns of cycle) 1) Fixation of carbon dioxide: CO2 reacts with RuBP, produces 2 molecules of 3 PGA. (catalyzed by rubisco) 3 RuBP + 3 CO2 6 3PGA 2) Reduction of 3PG to form glyceraldehyde-3-phosphate, or G3P 6 3PGA + 6 ATP + 6 NADPH  (5 G3P to step 3, 1 G3P yield) 3) Regeneration of the CO2 acceptor, RuBP, ribulosebiphosphate 5 G3P + 3 ATP  3 RuBP

26. Rubisco is the key enzyme

28. One complete turn of the Calvin Cycle (with one CO2): • 1 CO2+ 2 NADPH + 3 ATP  • (CH2O) + 2 NADP+ + 3 ADP + 3 Pi • For each turn, one CO2 is converted into one (CH2O) unit • It takes 3 turns to produce one net G3P • It takes 6 turns to produce one 6 carbon carbohydrate, such as glucose

29. Calvin cycle animation • http://www.science.smith.edu/departments/Biology/Bio231/calvin.html

30. Rubisco • slow: Catalyzes 3 reactions per second vs. thousands per second for other enzymes • Inefficient: catalyzes addition of either CO2or oxygen. CO2and oxygen compete at the enzyme’s active sites.

31. In most plants (C3plants), • initial fixation of CO2(via rubisco), • forms a three-carbon compound (3-phosphoglycerate) • In photorespiration, • rubiscoadds O2 instead of CO2 in the Calvin cycle, • producing a two-carbon compound

32. Photorespiration

33. Photorespiration • The reaction of oxygen and RuBP results in photorespiration, which consumes ATP. • Photorespiration consumes energy and releases fixed CO2 , so it “undoes” photosynthesis. • Photorespiration consumes O2 and organic fuel, and releases CO2 without producing ATP or sugar

34. Alternate pathways for photosynthesis • Problem - Dehydration is a problem • This can result in trade-offs with photosynthesis • On hot, dry days, plants close stomata, - conserves H2O • - limits photosynthesis • - reduces access to CO2 • - causes O2 to build up • These conditions favor an apparently wasteful process called photorespiration

35. Photorespiration • may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2 • limits damaging products of light reactions that build up in the absence of the Calvin cycle • - a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle

36. C4 plants C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells This step requires the enzyme PEP carboxylase PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle

38. CAM plants Other plants also use PEP carboxylase to fix and accumulate CO2. Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon CAM plants open their stomata at night, incorporating CO2 into organic acids (oxaloacetate – 4 C, then converted to malic acid) Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle (malic acid goes to chloroplasts)

40. Plants can make everything they need from CO2, H2O, sulfate, phosphate & ammonium. - Some G3P can enter glycolysis cycle & be converted to pyruvate - Some G3P can enter gluconeogenesis pathway and form 6 carbon sugars, and then sucrose

41. Chemiosmosis in Mitochondria vs. chloroplast