PHOTOSYNTHESIS

1. PHOTOSYNTHESIS

2. Photosynthesis • process by which green plants and some organisms • seaweed, algae & certain bacteria • use light energy to convert CO2 + water glucose • all life on Earth, directly or indirectly, depends on photosynthesis as source of food, energy & O2

4. Autotrophs • self feeders • organisms that make their own organic matter from inorganic matter • producers • need inorganic molecules such as CO2, H2O & minerals to make organic molecules

5. Heterotrophs • consumers • other feeders • depend on glucose as an energy source • cannot produce it • obtained by eating plants or animals that have eaten plants

6. Carbon and Energy Flow Heat energy CO2 + H2O Light energy Photosynthesis Carbs Proteins Lipids + O2 Cellular (Aerobic) Respiration (ATP Produced)

7. Food Chain • byproduct of photosynthesis is O2 • humans & other animals breathe in oxygen • used in cellular respiration

8. Other Benefits of Photosynthesis • humans also dependent on ancient products of photosynthesis • fossil fuels • natural gas, coal & petroleum • needed for modern industrial energy • complex mix of hydrocarbons • represent remains of organisms that relied on photosynthesis millions of years ago

9. Photosynthesis • plants produce more glucose than they can use • stored as starch & other carbohydrates in roots, stems & leaves • can draw on these reserves for extra energy or building materials as needed

10. Sites of Photosynthesis • leaves & green stems • in cell organelles • chloroplasts • concentrated in green tissue in interior of leaf • mesophyll • green due to presence of green pigment chlorophyll

11. Chloroplasts • each cell has 40-50 chloroplasts • oval-shaped structures with double membrane • inner membrane encloses compartment filled with stroma • suspended in stroma are disk-shaped compartments-thylakoids • arranged vertically like stack of plates • one stack-granum (plural, grana) • embedded in membranes of thylakoids are hundreds of chlorophyll molecules

12. Chlorophyll • light-trapping pigment • other light-trapping pigments, enzymes & other molecules needed for photosynthesis are also found in thylakoid membranes

13. How Photosynthesis Works • Requires • CO2 • Water • Sunlight • Makes • O2 • Glucose

14. How Photosynthesis Works • CO2 enters plant via pores-stomata in leaves • water-absorbed by roots from soil • membranes in chloroplasts provide sites for reactions of photosynthesis • chlorophyll molecules in thylakoids capture energy from sunlight • chloroplasts rearrange atoms of inorganic molecules into sugars & other organic molecules

15. Photosynthesis • redox reaction • 6CO2 + 12H2OC6H12O6 + 6O2 + 6H2O in presence of light • must be anoxidation&areduction • water is oxidized • loses electrons & hydrogen ions • carbon dioxide is reduced • gains electrons & hydrogens

16. Photosynthesis • relies on a flow of energy & electrons initiated by light energy • light energy causes electrons in chlorophyll pigments to boost electrons up & out of their orbit • hydrogens along with electrons are transferred to CO2sugar • requires that H2O is split into H & O2 • O2 escapes to air • light drives electrons from H2O to NADP+ which is oxidized NADPH which is reduced

18. Photosynthesis • 2 stages • light-dependent reactions • chloroplasts trap light energy • convert it to chemical energy • contained in nicotinamide adenine dinucleotide phosphate-(NADPH) & ATP • used in second stage • light-independent reactions • Calvin cycle • formerly called dark reactions • NADPH (electron carrier) provides hydrogens to form glucose • ATP provides energy

19. Light Dependent Reactions • convert light energy to chemical energy & produce oxygen • takes place in thylakoid membranes • solar energy absorbed by chlorophyllATP + NADPH

20. Light Energy for Photosynthesis • sun energy is radiation • electromagnetic energy • travels as waves • distance between 2 waves- wavelength • light contains many colors • each has defined range of wavelengths measured in nanometers • range of wavelengths is electromagnetic spectrum • part can be seen by humans • visible light

21. Pigments • light absorbing molecules • built into thylakoid membranes • absorb some wavelengths & reflect others • plants appear green because chlorophyll-does not absorb green light • reflected back. • as light is absorbedenergy is absorbed • chloroplasts contain several kinds of pigments • different pigments absorb different wavelengths of light • red & blue wavelengths are most effective in photosynthesis • other pigments are accessory pigments • absorb different wavelengths • enhance light-absorbing capacity of a leaf by capturing a broader spectrum of blue & red wavelengths along with yellow and orange wavelengths

22. Pigment Color & Maximum Absoption • Violet:   400 - 420 nm • Indigo:   420 - 440 nm • Blue:   440 - 490 nm • Green:   490 - 570 nm • Yellow:   570 - 585 nm • Orange:   585 - 620 nm • Red:   620 - 780 nm

23. Chlorophylls • Chlorophyll A • absorbs blue-violet & red light • reflects green • participates in light reactions • Chlorophyll B • absorbs blue & orange light • reflects yellow-green • does not directly participate in light reactions • broadens range of light plant can use by sending its absorbed energy to chlorophyll A

24. Carotenoids • yellow-orange pigments • absorb blue-green wavelengths • reflect yellow-orange • pass absorbed energy to chlorophyll A • have protective function • absorb & dissipate excessive light energy that would damage chlorophylls

25. Light Energy • light behaves as discrete packages of energycalled photons • fixed quantity of energy • shorter wavelengths have greater energy • violet light has 2X as much energy as red

26. Light Energy • when pigment absorbs a photon • pigment’s electrons gains energy • electrons are excited • unstable • electrons do not stay in unstable state • fall back to original orbits • as electrons fall back to ground state heat is released • absorbed energy is passed to neighboring molecules

27. Photosynthesis • Pigments • Absorb light • Excites electrons • Energy passed to sites in the cell • Energy used to make glucose

28. Photosystems • chlorophyll & other pigments are found clustered next to one another in a photosystem • energy passes rapidly from one chlorophyll pigment molecule to another

29. Photosystems • two photosystems participate in light reactions • photosystem I &II • each has a specific chlorophyll at reaction center • photosystem II • chlorophyll P680 • photosystem I • chlorophyll P700 • named for type of light they absorb best • P700 absorbs light in far red region of electromagnetic spectrum

30. Reaction Center • when photon strikes one pigment molecule • energy jumps from pigment to pigment until arrives at reaction center • electron acceptor traps a light excited electron from reaction center chlorophyll • passes it to electron transport chain which uses energy to make ATP & NADPH

31. Reaction Center

32. Light Reactions • during process of making ATP & NADPH • electrons are removed from molecules of water • passed from photosystem II to photosystem I to NADP+

33. Photosystem II • water is split • oxygen atom combines with oxygen from another split water forming molecular oxygen-O2 • each excited electron passes from photosystem II to photosystem I via electron transport chain

35. Photosystem I • primary electron acceptor captures an excited electron • excited electrons are passed through a short electron transport chain to NADP+ reducing it to NADPH • NADP+ is final electron acceptor • electrons are stored in a high state of potential energy in NADPH molecule • NADPH, ATP and O2 are products of light reactions

36. ATP Formation-Chemiosmosis • uses potential energy of hydrogen ion concentration gradient across membrane • gradient forms when electron transport chain pumps hydrogen ions across thylakoid membrane as it passes electrons down chain that connects two photosystems

37. ATP Formation-Chemiosmosis • ATP synthase (enzyme) uses energy stored by H gradient to make ATP • ATP is produced from ADP & Pi when hydrogen ions pass out of thylakoid through ATP synthase • photophosphorylation

38. Chemiosmosis H+ H+ pH 7 pH 8 Chemiosmosis

39. Substrate-level Phosphorylation

40. Calvin Cycle • light independent reactions • depend on light indirectly to obtain inputs for cycle-ATP & NADPH • takes place in stroma of chloroplast • each step controlled by different enzyme • cycle of reactions • makes sugar from CO2 & energy • ATP provides chemical energy • NADPH provides high energy electrons for reduction of CO2 to sugar

41. Steps of Calvin Cycle • starting material-ribulose bisphosphate (RuBP) • first step-carbon fixation • rubisco (an enzyme) attaches CO2 to RuBP • Next-reduction reaction takes place • NADPH reduces 3-phosphoglyceric acid (3-PGA) to glyceraldehye 3-phosphate (G3P) with assistance of ATP • to do this cycle uses carbons from 3 CO2 molecules • to complete cycle must regenerate beginning component-RuBP • for every 3 molecules of CO2 fixed, one G3P molecule leaves cycle as product of cycle • remaining 5 G3P molecules are rearranged using ATP to make 3 RuBP molecules

43. Calvin Cycle • regenerated RuBP is used to start Calvin cycle again • process occurs repeatedly in each chloroplast as long as CO2, ATP & NADPH are available • thousands of glucose molecules are produced • used by plants to produce energy in aerobic respiration • used as structural materials • stored

44. Photosynthesis Variations • plants vary in the way they produce glucose and when

45. C3 Plants • use CO2 directly from air • first organic compound produced is a 3 carbon compound 3-PGA • reduce rate of photosynthesis in dry weather • CO2 enters plants through pores in leaves • on hot days stomata in leaves close partially to prevent escape of water • with pores slightly open, adequate amounts of CO2 cannot enter leaf • Calvin cycle comes to a halt • no sugar is made • in this situation rubisco adds O2 to RuBP • 2-carbon product of this reaction is broken down by plant cells to CO2 + H20 • Photorespiration • provides neither sugar nor ATP

46. C4 Plants • have special adaptations allowing them to save water without shutting down photosynthesis • corn, sugar cane & crabgrass • evolved in hot, dry environments • when hot & dry stomata are closed • saves water • sugar is made via another route • developed way to keep CO2 flowing without capturing it directly from air

47. C4 Plants • have enzymes that incorporate carbon from CO2 into 4-C compound • enzyme has an intense desire for CO2 • can obtain it from air spaces even when levels are very low • 4-C compound acts as a shuttle • transfers CO2 to nearby cells -bundle-sheath cells • found in vast quantities around veins of leaves • CO2 levels in these cells remain high enough for Calvin cycle to produce sugar

48. CAM Plants • pineapple, some cacti & succulent plants • conserve water by opening stomata & letting CO2 in at night • CO2 is fixed into a 4-C compound • saves CO2 at night & releases it in the day • photosynthesis can take place without CO2 needing to be admitted during the day when conditions are hot and dry

49. Environmental Consequences of Photosynthesis • CO2 makes up 0.03% of air • provides plants withCO2 to make sugars • important in climates • retains heat from sun thatwould otherwise radiate from Earth • warms the Earth • greenhouse effect

50. Global Warming • CO2 traps heatwarms air • maintains average temperature on Earth about 10oC warmer than without it. • Earth may be in danger of overheating because of this greenhouse effect • CO2 in air is increasing because of industrialization • when oil, gas and coal are burned CO2 is released • levels in atmosphere have increased 30% since 1850 • increasing concentrations have been linked to global warming • slow & steady rise in surface temperature of Earth • could have dire consequences for all life forms on Earth