PHOTOSYNTHESIS

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RESOURCES

1. Textbook website:
   1. outline, quiz, and flash cards: select from resources for chapter 8.
2. STARR and TAGGART 9th edition website:
   1. outline, quiz, and flash cards: select from resources for chapter 7.
3. STARR AN TAGGART 10th edition website:
   1. outline, quiz, and flash cards: select from resources for chapter 7.
4. On-Line Biology Book by M. J. Farabee 
   1. Photosynthesis
5. Kimball's Biology Pages
   1. Photosynthesis: The Role of Light from Kimball's Biology Pages
   2. Photosynthesis: Pathway of Carbon Fixation from Kimball's Biology Pages
   3. Photorespiration and C4 Plants from Kimball's Biology Pages
6. The Biology Project  an interactive online resource for learning biology, developed at The University of Arizona
   1. Photosynthesis I
   2. Photosynthesis II
7. MIT Biology Hypertext Photosynthesis

Sunlight and Survival 

1. photoautotrophs 
   1. convert solar energy into stored chemical energy 
   2. synthesize energy-rich organic molecules from energy poor inorganic molecules
2. Englemann's observations on bacteria and Spirogyra [Fig. 7.1] 
3. Links between photosynthesis and aerobic respiration [Fig. 7.2]

7.1 -- PHOTOSYNTHESIS -- AN OVERVIEW

1. chloroplasts (fig 7.3)
   1.  double membrane
   2. grana: stacks of membranous disks 
      1.  thylakoid membrane system: site of light dependent reactions 
   3. Stroma:  contains enzymes for light independent reactions

7.2 – SUNLIGHT AS AN ENERGY SOURCE 

1. photons: energy packets that travel as waves 
2. wavelength and the electromagnetic spectrum (fig 7.4) 
3. photosynthetic pigments and absorption spectra [Figs. 7.5, 7.6]

7.3 – THE RAINBOW CATCHERS 

1. Photosynthetic Pigments 
   1. Chlorophyll a: [fig 7.7]
   2. Chlorophyll b
   3. Carotenids:  visible in fall following breakdown of chlorophyll [Fig. 7.8] 
   4. Phycobilins
2. Where are the Photosynthetic Pigments Located 
   1. Photosystems of the thylakoid membrane [Fig. 7.9]

7.4 – LIGHT-DEPENDENT REACTIONS

1. take place in thylakoids
2. split H₂O (photolysis) to form O₂
3. reduce ADP and NADP⁺ to provide ATP and NADPH.

photosystems [Fig. 7.10] 

1. contain 200-300 pigment molecules (e.g. chlorophyll) 
2. reaction center 
3. Photosystem I -- see below
4. Photosystem II -- see below

cyclic electron pathway [Fig. 7.11]

1. produces ATP only 
2. most primitive pathway, found in certain anaerobic prokaryotes

noncyclic electron pathway [fig 7.12] 

1. produces both ATP and NADPH 
2. Photosystem II 
   1. photolysis
   2. see below for more detail
3. Photosystem I 
   1. produces NADPH 
   2. H⁺ flows to stroma  to produce ATP
   3. H is transported to Calvin Benson Cycle via NADPH
   4. see below for more detail

7.5 -- A CLOSER LOOK AT ATP FORMATION IN CHLOROPLASTS

1. H ions stay in thylakoid space. [Fig. 7.5a]
2. ejected electron travels down electric transport chain [cytochrome system] 
3. energy is used to transport more H⁺ from stroma into thylakoid space [Fig. 7,5b]
4. chemiosmotic phosphorylation (fig 7.14c): ATP is produced as H⁺ flows out of thylakoid 

7.6 THE LIGHT INDEPENDENT REACTIONS

The Calvin-Benson Cycle [Fig. 7.15]

1. RuBP
2. Rubisco
3. PGA
4. carbon fixation
5. PGAL

7.7 -- FIXING CARBON

• C3 plants - see below
• C4 plants - see below
  • stomata
• CAM plants

REACTIONS OF PHOTOSYNTHESIS

LIGHT DEPENDENT REACTIONS

PHOTOSYSTEM II

1. sunlight energy + chlorophyll --> activated chlorophyll + e^-
   • e^- is ejected to an electron acceptor and then to an electron transport system
2. PHOTOLYSIS: splitting of water in thylakoid compartment by activated chlorophyll
   • 2H₂O --> 4H⁺ + O₂ + 4e^-
3. e^-'s generated by photolysis enter Photosystem II to replace ejected e^-'s

ELECTRON TRANSPORT SYSTEM OF PHOTOSYSTEM II

4. e^- ejected from photosystem II is transferred down transport system
5. released energy from electron transfers is used to pump more H⁺ into thylakoid compartment
6. flow of H⁺ down a concentration gradient across membrane channel --> ATP
7. e^- enters photosystem I to replace another ejected electron

PHOTOSYSTEM I

8. sunlight energy + chlorophyll --> activated chlorophyll + e^-
9. e^- is ejected to an electron acceptor and then to an electron transport system

ELECTRON TRANSPORT SYSTEM OF PHOTOSYSTEM I

10. coenzyme helper (NADP⁺) picks up two electrons and a hydrogen ion
    • NADP⁺ + 2e^- + H⁺ --> NADPH
11. NADPH enters the Calvin-Benson Cycle

LIGHT-INDEPENDENT REACTIONS

CALVIN-BENSON Cycle for C₃ plants

Six turns of the cycle ---> 1 glucose

1. Carbon dioxide fixation
   • takes place in stroma
   • utilizes ATP and NADPH to reduce CO₂ to carbohydrate
   • 6 CO₂ + 6 RuBP --> 6 [unstable 6 carbon sugar]
2. 6 [unstable 6 carbon sugar] --> 12 PGA molecules
3. 12 PGA + 12 NADPH + 12 H⁺ + 12 ATP --> 12 PGAL [a 3 carbon compound]
4. 10 PGAL + 6 ATP --> 6 RuBP [reenter Calvin Cycle]
5. 2 PGAL --> carbohydrate (GLUCOSE phosphate)

photorespiration:  

O₂ will be attached to RuBP by Rubisco when stomata close and O₂ levels rise and CO₂ levels fall.  Only one PGA will form. [Fig. 7.16a]

CALVIN-BENSON Cycle for C₄ plants

1. CO₂ is fixed twice
2. photorespiration will not occur
3. CO₂ is first fixed in the mesophyll cells
   1. CO₂ + PEP --> oxaloacetate [a 4-carbon compound]
4. oxaloacetate --> malate
5. malate releases CO₂ to the Calvin Cycle [in the bundle sheaf cells]

 CAM plants

1. stomata are open during the night
2. CO₂ is fixed at night
3. stomata are closed during the day; CO₂ is released to Calvin cycle

KEY TERMS FOR CHAPTER 7

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