Photosynthesis: Capturing Energy

1. Photosynthesis: Capturing Energy Chapter 8

2. Carbon Fixation • Carbon fixation is the process of building complex carbon compounds from simple carbon compounds. • Organisms that fix carbon are autotrophs – they use carbon dioxide as a carbon source, and combine it with water

3. Name the type of organism that… autotroph • Uses C02 as a carbon source • Cannot fix carbon • Uses light energy as an energy source • Uses organic compounds as sources of energy • Absorbs and converts light energy into chemical energy • Uses light energy, but can’t do carbon fixation • Obtain energy from H2S, N02 or NH2 • Use organic molecules as a source of energy and carbon heterotroph phototroph chemotroph photoautotroph photoheterotroph chemoautotroph chemoheterotroph

4. Photoautotrophs, Heterotrophs, and Chemoautotrophs • Photoautotrophs: Organisms that gain energy from light in order to carry out carbon fixation • Photosynthetic plants, algae and bacteria • Use light energy to make ATP and carbohydrate • Chemoautotrophs: Organisms that use chemical energy only to cause carbon fixation and to build structure • Certain bacteria • Heterotrophs: Organisms that gain energy by eating other organisms, including autotrophs • Animals, nonphotosynthetic plants, nonphotosynthetic unicellular organisms(such as protists), bacteria, and fungi

5. What is the Electromagnetic Spectrum? • Complete range of radiant energy longer wavelengths shorter wavelengths

6. Characteristics of Visible Light • Order is ROYGBIV • Range is 380 – 760 nm • Violet has a shorter wavelength than red • Tiny packets of this energy is called a photon • Photons of light can energize an electron

7. The Relationship Between Photon Energy Capture and Electrons • Electrons can absorb photon energy. • When they do so, they hop to a higher shell. • When they release energy, they release a photon, and drop back to the lower shell. • The released photon has less energy than the one that was absorbed • The released photon has a longer wavelength • Light released this way is fluorescence.

8. The Sun • The sun is a star • It has very high surface temperatures • It produces a vast amount of electromagnetic radiation of widely varying frequencies • Gamma-rays, which have very short wavelengths • To the far-infrared (much longer )

9. Absorption of Light Energy • Light energy is absorbed by electrons in energy-absorbing atoms • The energy causes electrons to change shells; the more energy absorbed, the further electrons move fromthe nucleus • The energy may be shed as fluorescence ... • Or transferred in the form of an electron to another molecule

10. Leaf Structure • Leaves have a layered organization • The mesophyll tissue (middle layers of cells) is the main site of photosynthesis

11. The Chloroplast • The site of light harvesting & carbohydrate synthesis

12. Parts of a Chloroplast • Thylakoid - Disc like structure that contains the chlorophyll which absorbs the light • Grana – stacks of thylakoids • Purpose is to increase the amount of light that can be absorbed • Stroma – middle fluid part that contains the enzymes needed for photosynthesis

13. Chlorophyll • Chlorophyll collects light energy (absorbs it) in a resonant porphyrin group that hangs out like a kite on the surface of the thylakoid • Chlorophyll a initiates the light-dependent reactions • Bright green • Chlorophyll b is an accessory pigment • Duller green • Carotenoids are yellow and orange pigments that capture light energy and pass electrons to chlorophyll • Xanthophyll (yellow) • Carotene (orange)

14. Chlorophyll Structure • Note the double bonds and the Mg2+ ion • The positive charges on the Mg2+ ion attract electrons • The electrons become delocalized over the porphyrin ring (so they can reside there for some time, then can move easily on)

15. Two types of Chlorophyll • Light antenna • These chlorophyll’s are the ones that gather the light energy • When the light hits it, the chlorophyll vibrates and the molecules move rapidly to the reaction center • Reaction Center • These chlorphylls are the ones that boost the electrons to a higher energy state

17. Reaction center hn The Antenna-Complex Energy Funnel • About 200 chlorophylls ‘inhabit’ an antenna complex • Electrons move from one chlorophyll to another, losing energy (depicted by a red-shift in the electrons in this diagram)

18. Each photosynthetic Unit contains a slightly different type of antenna and reaction center There are 2 types of photosynthetic units, or photosystems

19. Photosystem I Also called P700 Absorbs light that has a 700 nm wavelength Can operate independently so probably evolved first Photosystem II Also called P680 Absorbs light that has a 680 nm wavelength Photosystems Both photosystems are required for photosynthesis

20. Overview of Photosynthesis • Photosynthesis is a redox reaction: • Carbon dioxide is reduced to sugar • Water is oxidized to molecular oxygen

21. Function in the Chloroplast • The light-dependent reactions (the harvesting of light) occur on thylakoid membranes • The carbon fixation reactions (formation of carbohydrate) occur in the stroma

22. Two Energy Carriers of Photosynthesis • ATP, and… • NADPH: • Much like NADH except that it bears a phosphate • Phosphate is attached to the sugar group • NADPH is characteristic of anabolic redox processes

23. Two Different Photosynthesis Reaction Sets 1. Cyclic photophosphorylation • e- run in a cycle • (photophosphorylation of ADP to make ATP) by chemiosmosis • No carbohydrate made • Uses only P700 2. Noncyclic photophosphorylation • e- derived from splitting of water • Releases molecular oxygen • Makes ATP • Makes carbohydrate b/c NADPH (terminal e- acceptor) passes to the Calvin Cycle • Uses P700 and P680

24. Summary Equation for light dependent reactions of photosynthesis 12H20 + 12 NADP+ + 18 ADP + 18Pi 602 + 12 NADPH + 18ATP

25. How Photosynthesis Starts • Sunlight hits chloroplast in a leaf (Remember the sun is the ultimate source of energy on earth) • Automatically, water goes through photolysis and splits • H20  02 + H+ + e- • This provides O2 for the atmosphere and H+ ions and electron’s for photosystem II • Antenna chlorophyll A associated with PII absorbs the photon • This absorbed energy moves from one chlorphyll to another and eventually falls into the reaction center

26. In the reaction center, an electron gets energized and moves to a higher energy level • Energized electrons get accepted to a primary electron acceptor and moved through the electron transport chain

28. The primary electron acceptor passes the electron to plastoquinone then to cytochrome complex than to plastocyanin. Each step of the electron transport chain creates ATP by chemiosmosis

30. Now the “tired” electron needs more energy, so it travels to Photosystem I and received more sunlight • It gets energized and accepted by a primary electron acceptor • Primary electron acceptor passes the electron to 6 different electron acceptors, with the final acceptor as ferredoxin • Ferredoxin transfers the electron to NADP+ forming NADPH which is released into the stroma **Sometimes Ferredoxin transfers the electron back to PI, so the light dependent reactions can be referred to as being cyclic

31. Noncyclic Photophosphorylation

32. Protons Build up Inside Thylakoids • The activity of the ETC causes a gradient of protons across the thylakoid membrane

33. The Chloroplast ATP Synthase • Protons fall back through the chloroplast ATP synthase • Makes ATP by combining ADP and phosphate in a process called chemiosmosis • Much like the mitochondrial ATP synthase

34. The Electron Transport Chain and Chemiosmosis

35. Ok so now what • All the light dependent phase did was transfer the energy from the sun into ATP and NADPH. • The carbon fixation stage is next and will use this energy to make glucose!

36. The Dark Reactions • So-called because they do not directly need light • They occur in the stromaof the chloroplast • They fix carbon to make carbohydrate • They are the Calvin-Benson Cycle reactions

37. Calvin Cycle • Melvin Calvin discovered it and received the nobel prize in 1961 • Calvin Cycle means “fixing carbon” or putting together carbons • (not as difficult and complex as the Krebs)

38. Summary Equation for the Dark Phase (i.e. Calvin Cycle) 12 NADPH + 18 ATP + 6C02 C6H1206 + 12 NADP+ + 18 ADP + 18Pi + 6H20

39. 3 phases of Calvin Cycle • Carbon dioxide Uptake • Carbon reduction • RuBP regeneration

40. Carbon Dioxide Uptake • Carbon dioxide reacts with a 5C molecule called RuBP (ribulose biphosphate) • This reaction is catalyzed by the enzyme Rubisco. (ribulose bisphosphate carboxylase) This enzyme is present in the chloroplast in large amounts • The product is an unstable 6C molecule which immediately breaks down to 2 molecules of PGA (stands for phosphoglycerate). Each of these PGA molecules have 3C each.

42. Carbon Reduction • PGA is converted to G3P (glyceraldehyde 3 phosphate) – which is also nicknamed PGAL • This reaction requires a lot of ATP and NADPH • G3P is then converted to sugars like glucose! (and other carbohydrates) • Not all G3P’s make sugar some go on to phase 3

44. Regeneration of RuBP • G3P converts to RP (ribulose phosphate) • RP then converts to RuBP using ATP • Now RuBP is available to join again with CO2

45. The Calvin Cycle

47. Calvin Cycle = C3 Pathway • Its named this way because the intermediate product G3P has 3 carbons • This pathway is very inefficient; less than 1% of the light energy that reaches the chloroplast is found in glucose • It’s all Rubisco’s fault!

48. When RUBISCO Doesn’t Work • In high light and temperature: • Photosynthesis is very active • Water is easily lost • Leaf stomata close (small pores on the underside of the leaf) to protect against water loss • Oxygen builds up • Ribulose bisphosphate carboxylase (= RUBISCO) (it’s a carboxylase because it adds a -COOH) has mixed affinity for oxygen and CO2 • It binds to O2 when O2 is abundant • G3P is NOT produced

49. Photorespiration • And as a result, photorespiration occurs: • In the presence of oxygen and light, carbohydrates are oxidized and therefore No carbohydrate is produced • Instead, CO2 and H2O are produced • In C3 plants, 50% of the glucose made may be reoxidized back into Co2 due to this • So photorespiration reduces the efficiency of C3 plants because Rubisco can bind to O2 as much as it binds to C02 • In C4, Co2 is very concentrated, so photorespiration is limited

50. Evolutionary “Work-Arounds” Avoid Photorespiration 1. The C4 pathway • Physically sequesters the carbon dioxide-requiring RUBISCO (carbon fixation) away from high oxygen • Uses compartmentation with biological membranes (in different cells) • In crabgrass, corn, and sugar cane 2. Crassulacean acid metabolism (CAM) • Carries out C fixation separated in time from high temperature and high oxygen • In desert plants: succulents and cacti (also lilies and orchids)