1. Chapter 10 Photosynthesis

2. Importance of Photosynthesis
Plant:
Glucose for respiration
Cellulose
Global:
O2 Production
Food source

3. Photosynthesis Overview
Energy for all life on Earth ultimately comes from photosynthesis.
6CO2 + 12H2O C6H12O6 + 6H2O + 6O2
Photoautotrophs: use light energy to make organic molecules
Heterotrophs: consume organic molecules from other organisms for energy and carbon
Redox Reaction:
water is split  e- transferred with H+ to CO2  sugar Remember: OILRIG Oxidation: lose e-
Reduction: gain e-

4. Photoautotrophs Oxygenic photosynthesis is carried out by:
cyanobacteria, 7 groups of algae, all land plants

5. Chloroplasts: sites of photosynthesis in plants
thylakoid membrane – internal membrane arranged in flattened sacs
contain chlorophyll and other pigments
grana – stacks of thylakoid membranes
stroma – semiliquid substance surrounding thylakoid membranes

7. Sites of Photosynthesis
mesophyll: chloroplasts mainly found in these cells of leaf
stomata: pores in leaf (CO2 enter/O2 exits)
chlorophyll: green pigment in thylakoid membranes of chloroplasts

8. Discovery of Photosynthesis
Jan Baptista van Helmont – mass not completely from soil
Joseph Priestly – plants “renew” air
Jan Ingenhousz – leaves use sunlight to restore air, give off oxygen
F.F. Blackman – multistage process

9. C. B. van Niel, 1930’s
-proposed a general formula:
CO2+H2A + light energy CH2O + H2O + 2A
where H2A is the electron donor
-van Niel identified water as the source of the O2 released from photosynthesis
-Robin Hill confirmed van Niel’s proposal that energy from the light reactions fuels carbon fixation

10. Photosynthesis = Light Reactions + Calvin Cycle

11. photon: a particle of light
Nature of sunlight
Light = Energy = electromagnetic radiation
Shorter wavelength (λ): higher energy
Visible light - detected by human eye
Light: reflected, transmitted or absorbed
photon: a particle of light
acts as a discrete bundle of energy
energy content of a photon is inversely proportional to the wavelength of the light

12. Electromagnetic Spectrum

13. Pigments Pigments absorb different λ of light
Each pigment has a characteristic absorption spectrum, the range and efficiency of photons it is capable of absorbing.
photoelectric effect: removal of an electron from a molecule by light
occurs when photons transfer energy to electrons
Interaction of light with chloroplasts

14. Chlorophyll Pigments Chlorophyll a Chlorophyll b
The main photosynthetic pigment
Primarily absorb red and blue light (reflect green light)
Chlorophyll b
The accessory photosynthetic pigment

15. Photosynthetic pigments
chlorophyll – absorb violet-blue/red light, reflect green
chlorophyll a (blue-green): light reaction, converts solar to chemical E
chlorophyll b (yellow-green): conveys E to chlorophyll a
carotenoids (yellow, orange): photoprotection, broaden color spectrum for photosynthesis
Types: xanthophyll (yellow) & carotenes (orange)
anthocyanin (red, purple, blue): photoprotection, antioxidants

16. Electrons in chlorophyll molecules are excited by absorption of light

17. Photosynthesis Broken into 2 phases Phase I : light-dependent reaction
Light energy splits H2O to O2 releasing high energy electrons (e-)
Movement of e- used to generate ATP
Electrons end up on NADP+, reducing it to NADPH
Phase II: light-independent reactions
ATP and NADPH that were formed in phase I are used to make glucose

18. Photosystem: reaction center & light-harvesting complexes (pigment + protein)

19. In chloroplasts, two linked photosystems are used in noncyclic (linear) photophosphorylation 1. photosystem I -reaction center pigment (P700) with a peak absorption at 700nm 2. photosystem II -reaction center pigment (P680) has a peak absorption at 680nm

20. Light Dependent Reactions
Photosystem II acts first
Reaction center of PSII absorbs a photon
Electron in P680 is excited which transfers this electron to an acceptor
Electrons are removed from oxygen in water molecules (water is split)
Produces 1 O2 in the process
Electrons replace those lost when excited by light

21. Electron Acceptors 3. Primary initial acceptor is quinone molecule
Turns into plastiquinone – an electron donor
4. Transfers electrons to b6-f complex embedded within thylakoid membrane
Resembles bc1 from respiration
Series of electron carriers
5. b6-f complex pumps a proton into the thylakoid space
6. Plastocyanin then transfers electron to photosystem I

22. 7. e- passed to Photosystem I via ETC
Ferredoxin is used to transfer electron (iron sulfur compound)
8. Electrons are used to change NADP+ reduced to NADPH
NADP reductase catalyzes reaction
MAIN IDEA: Use solar energy to generate ATP & NADPH to provide energy for Calvin cycle

23. Light-Dependent Reactions
ATP is produced via chemiosmosis.
ATP synthase is embedded in the thylakoid membrane
protons have accumulated in the thylakoid space
protons move into the stroma only through ATP synthase
ATP is produced from ADP + Pi

26. Cyclic Electron Flow: uses PS I only; produces ATP for Calvin Cycle (no O2 or NADPH produced) - electron creates ATP instead of NADPH - goes into the b6-f complex

27. Both respiration and photosynthesis use chemiosmosis to generate ATP

28. Chemiosmosis
Protons accumulate in the thylakoid space, creating a concentration gradient
Electrons flow down the concentration gradient to produce ATP
The breakdown of water to start PHOTOSYSTEM II  is essential for providing the e- that initiate the ETC and providing the protons necessary to drive the ATP synthesis for chemiosmosis

29. Proton motive force generated by:
H+ from water
H+ pumped across by cytochrome
Removal of H+ from stroma when NADP+ is reduced

30. Calvin Cycle: Uses ATP and NADPH to convert CO2 to sugar
Occurs in the stroma
Uses ATP, NADPH, CO2
Produces 3-C sugar G3P (glyceraldehyde-3-phosphate)
Three phases:
Carbon fixation
Reduction
Regeneration of RuBP (CO2 acceptor)

31. Phase 1: 3 CO2 + RuBP (5-C sugar ribulose bisphosphate)
Catalyzed by enzyme rubisco (RuBP carboxylase)

32. Phase 2: Use 6 ATP and 6 NADPH to produce 1 net G3P

33. Phase 3: Use 3 ATP to regenerate RuBP

34. Alternative mechanisms of carbon fixation have evolved in hot, arid climates
Photorespiration
Metabolic pathway which:
Uses O2 & produces CO2
Uses ATP
No sugar production (rubisco binds O2  breakdown of RuBP)
Occurs on hot, dry bright days when stomata close (conserve H2O)
Why? Early atmosphere: low O2, high CO2?

35. Evolutionary Adaptations
Problem with C3 Plants:
CO2 fixed to 3-C compound in Calvin cycle
Ex. Rice, wheat, soybeans
Hot, dry days:
partially close stomata, ↓CO2
Photorespiration
↓ photosynthetic output (no sugars made)

36. C4 Plants: CO2 fixed to 4-C compound Ex. corn, sugarcane, grass
Hot, dry days  stomata close
2 cell types = mesophyll & bundle sheath cells
mesophyll : PEP carboxylase fixes CO2 (4-C), pump CO2 to bundle sheath
bundle sheath: CO2 used in Calvin cycle
↓photorespiration, ↑sugar production
WHY? Advantage in hot, sunny areas

37. C4 Leaf Anatomy

38. CAM Plants: Crassulacean acid metabolism (CAM)
NIGHT: stomata open  CO2 enters  converts to organic acid, stored in mesophyll cells
DAY: stomata closed  light reactions supply ATP, NADPH; CO2 released from organic acids for Calvin cycle
Ex. cacti, pineapples, succulent (H2O-storing) plants
WHY? Advantage in arid conditions

41. Comparison C3 C4 CAM C fixation & Calvin together
C fixation & Calvin in different cells
C fixation & Calvin at different TIMES
Rubisco
PEP carboxylase
Organic acid

42. Review of Photosynthesis

43. Comparison RESPIRATION PHOTOSYNTHESIS Plants + Animals
Needs O2 and food
Produces CO2, H2O and ATP, NADH
Occurs in mitochondria membrane & matrix
Oxidative phosphorylation
Proton gradient across membrane
Plants
Needs CO2, H2O, sunlight
Produces glucose, O2 and ATP, NADPH
Occurs in chloroplast thylakoid membrane & stroma
Photorespiration
Proton gradient across membrane