1. CHAPTER 7
Photosynthesis

2. I. Photosynthesis
A. Process by which plants use light energy to make chemical energy of food molecules (carbohydrates).
1. Only 42% of Sun’s energy that is directed towards Earth actually reaches the surface.
a. Of that, only 2% is captured by photosynthesis.
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3. B. Summary Equation for Photosynthesis
6CO H2O ---> C6H12O6 + 6O2
Light energy
However, carbon dioxide and water do NOT directly combine in this process.
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4. a. Make organic food molecules from simple raw materials
C. Autotrophs
1. Make their own food
2. Producers
a. Make organic food molecules from simple raw materials
b. Make biosphere’s food supply
3. Examples of autotrophs
a. Plants, bacteria, algae, protists
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5. a. Cells that make up most of the interior of leaves
II. Leaf Structure
A. While all green parts of plants have chloroplasts, leaves are the major sites of photosynthesis
B. Leaf Structure
1. Mesophyll cells -
a. Cells that make up most of the interior of leaves
b. Where most chloroplasts are located
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6. a. Tiny pores, or openings, into the interior of the leaf
2. Stomata
a. Tiny pores, or openings, into the interior of the leaf
b. This is where CO2 enters the leaf and O2 will leave
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7. Leaves and Photosynthesis

8. III. Structure of Chloroplasts
A. Outer membrane - outside
B. Inner membrane - inside of that
1. Outer compartment is between these two membranes. Also called intermembrane space.
C. Inner compartment - inside inner membrane
1. Filled with stroma
a. Thick fluid where sugars will be made
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9. a. Disk-like membranous sacs
2. Thylakoids
a. Disk-like membranous sacs
b. Organized into stacks that are called grana
c. Inside the thylakoids is the third chloroplast compartment, the thylakoid space
d. The molecular machinery that will convert light energy into chemical energy is built into thylakoid membranes
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10. IV. Photosynthesis Experiments
See transparencies
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11. V. Photosynthesis is redox process
A. When water molecules are split, yielding O2, they are oxidized. They lose electrons & H+
B. CO2 is reduced to glucose as electrons and H+ are added to it
reduction
6CO H2O ---> C6H12O6 + 6O H2O
oxidation
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12. VI. Photosynthesis occurs in 2 steps
A. Light Reactions
1. Reactions that convert light energy to chemical energy & produce O2 as a waste product
2. Occurs in thylakoid membranes
3. Light energy used to make:
a. ATP
b. NADPH (similar to NADH)
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13. Photosynthesis Overview

14. B. Calvin Cycle (old name = Dark Reaction)
1. Series of reactions that assemble sugar molecules using CO2 & energy from light reactions
2. Occurs in stroma of chloroplast
3. Carbon fixation occurs:
a. CO2 gets incorporated into organic compounds
4. Uses ATP & NADPH from light reaction
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15. Photosynthesis Overview

16. VII. Solar radiation
A. Visible light is only a small part of the entire electromagnetic spectrum
B. Consists of different wavelengths; our eyes see as different colors
1. Some wavelengths are absorbed into plant (Don’t see)
2. Some are reflected back or transmitted through (Do see)
C. Pigments are molecules that absorb certain wavelengths of light.
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17. VIII. Photosynthetic pigments
A. Different pigments absorb different wavelengths of light.
1. Chlorophyll a –
Absorbs mostly blue-violet & red
2. Chlorophyll b - absorbs more of the blue & orange wavelengths
Does not participate as directly; broadens range of light that plants can use by conveying energy to chlorophyll a
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18. a. Yellow-orange pigments; accessory pigments
3. Carotenoids
a. Yellow-orange pigments; accessory pigments
b. Mainly absorb blue-green light
c. Some carotenoids pass energy to chlorophyll a
d. Others are protective:
 Absorb light energy that might damage chlorophyll
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19. Action Spectrum
Absorption Spectrum

20. IX. Capturing solar power
A. Light consists of photons, discrete packets of energy
1. The shorter the wavelength, the greater the energy
a. Violet twice as much as red
B. When pigment molecule absorbs photon, one of pigment’s electrons gains energy (now excited state)
C. Electron falls back to ground state
1. Released as heat or light
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21. D. Intact chloroplasts
1. Chlorophyll passes excited electron to neighboring molecule, the primary electron acceptor
a. Chlorophyll oxidized
b. Primary acceptor is reduced
2. This is first step in light reaction
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22. 3. Whole thing is a photosystem
E. Photosystems
1. Chlorophyll a & b & carotenoids are clustered in thylakoid membrane in groups of molecules.
2. Chlorophyll a & primary electron acceptor make up the reaction center
a. Outside pigments act as light gathering antenna that shuttle energy to the reaction center
3. Whole thing is a photosystem
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23. F. Two types of photosystems
1. Photosystem I (P700)
a. Chlorophyll a absorbs best up to 700 nm wavelengths (red light)
2. Photosystem II (P680)
a. Absorbs best up to 680 nm (orange red light)
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24. X. Cyclic electron pathway (photophosphorylation)
A. Photophosphorylation
1. Using light to add a phosphate group to a molecule
B. The cyclic pathway involves only photosystem I (PS I)
C. First form of photosynthesis that evolved.
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25. D. Steps involved in cyclic pathway:
1. Light stimulates PS I unit
2. Electrons escape the reaction center
3. Acceptor molecule takes electron
4. Electron passed along a series of carriers which are membrane-bound enzymes. Make up the electron transport chain (ETC).
5. Energy is captured in form of a H+ gradient
6. Electron reaches final acceptor molecule (PC = plastocyanin)
7. Electron waits in PC until a new electron “hole” appears in PS I
8. Electron fills hole in PS I
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26. 1. Energy is discharged in a series of small steps
E. Results:
1. Energy is discharged in a series of small steps
2. Some of this energy is used to make ATP by substrate-level phosphorylation
3. Only releases a small amount of ATP (about 3.4 kcal/mole worth)
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27. XI. Modern light reaction = Noncyclic photophosphorylation
A. Involves 2 different photosystems & 2 different electron transport chains
B. Both photosystems will be stimulated by light, absorb energy & pass electrons down chains to produce ATP and NADPH
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28. Noncyclic Electron Pathway

29. 1. PS I excited ---> electron sent to acceptor molecules in a chain
C. Photosystem I (P700)
1. PS I excited ---> electron sent to acceptor molecules in a chain
2. As electrons move some energy is harnessed to produce ATP by phosphorylation
3. Some energy used to pump H into the interior of the thylakoid disk
4. Eventually the electrons reach NADP+ which is reduced to NADPH
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30. 1. There are now “electron holes” in Photosystem I.
D. Electron “holes”
1. There are now “electron holes” in Photosystem I.
2.These need to be replaced before Photosystem I can respond to light once again
3.They will be replaced by electrons from Photosystem II
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31. Noncyclic Electron Pathway

32. 1. PS II excited --> electron sent to acceptor molecules
E. Photosystem II (P680)
1. PS II excited --> electron sent to acceptor molecules
2. Some energy is harnessed to produce ATP indirectly
3. Eventually electron reaches end of chain and is passed to the “hole” in PS I
4. The “electron hole” is now in PS II
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33. 1. Where do electrons come from to fill the “hole” in PS II?
F. Production of Oxygen
1. Where do electrons come from to fill the “hole” in PS II?
2. Water splits apart:
a. 2 of its electrons replace the missing electrons in PS II
b. 2 H+ remain behind in the thylakoid compartment. This raises the H+ concentration
c. An oxygen atom combines with another to form O2
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34. XII. Chemiosmosis makes ATP
A. H+ has accumulated inside the thylakoid compartment
B.This concentration gradient came from the splitting of water & pumping of H+ across membrane
C. An ATP synthase enzyme complex will produce ATP as H+ flow out of the thylakoid interior & into the stroma
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35. Thylakoid Organization
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36. ATP Synthase Action in Chloroplasts

37. XIII. Calvin Cycle = Carbon Fixation
A. CO2 enters leaves; gets into the stroma
B. It combines with a 5-carbon sugar, called RuBP, ribulose bisphosphate
C. Catalyzed by the enzyme RuBP carboxylase (rubisco)
D. Produces unstable intermediate that splits into two 3PG molecules
(3-C)
E. Each 3PG is phosphorylated by ATP & NADPH to G3P (3-C).
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38. Calvin Cycle

39. Reduction of CO2
InLine Figure p125

40. F. 5 of the G3Ps remain in cycle & are used to reform RuBP
1. This uses up ATP
G. The one remaining G3P must combine with another in order to form one molecule of glucose
H. It will take 6 complete turns of the Calvin Cycle to produce the 2 PG3P molecules needed to manufacture 1 glucose molecule
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41. Calvin Cycle

42. Regeneration of RuBP

43. I. Importance of Calvin Cycle
1. G3P can be converted to many other molecules:
a. Fatty acids & glycerol to make plant oils
b. Glucose phosphate (sugar)
c. Fructose
d. Starch & cellulose
e. Amino acids
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44. A. Use Calvin Cycle exclusively B. Produce 3PG first (3-C)
XIV. C3 plants
A. Use Calvin Cycle exclusively
B. Produce 3PG first (3-C)
C. Examples:
1. Soybeans
2. Oats
3. Wheat
4. Rice
5. Maples
6. Tulips
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45. a. This prevents CO2 from entering leaf and O2 from leaving
D. Photorespiration
1. In hot dry weather, plants will close their stomata to prevent water loss
a. This prevents CO2 from entering leaf and O2 from leaving
b. CO2 levels can get very low while O2 levels build up
c. When this happens rubisco will incorporate O2 instead of CO2 & produces 1 molecule of 3PG and releases CO2.
d. This is a very wasteful process.
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46. XV. C4 Plants & Kranz Anatomy
A. Some plants in hot dry climates solve this problem with a unique anatomy and physiology.
B. This unique anatomy is called Kranz Anatomy & consists of:
1. Bundle sheath cells which have many chloroplasts that are bigger
2. Mesophyll cells that cluster in a ring around bundle sheath cells Contain many small chloroplasts
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47. Comparison of C3 and C4 Plant Anatomy

48. 3. Examples of C4 plants: a. Corn b. Bermuda grass c. Sugarcane
4. These tropical plants have a unique physiology to go along with their unique anatomy that helps them to produce carbohydrates in a hot, dry climate.
C4 plants are 2-3 times as efficient as C3 plants in hot dry weather.
In cool, moist climates, however, they are much LESS efficient than C3 plants.
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49. C. Special physiology of C4 plants
1. When weather is hot & dry, C plants keep stomata closed most of time
2. They will then undergo a different process to fix their carbon into the cell than a C3 plant
a. This process can occur even in very low concentrations of CO2
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50.  The enzyme that catalyzes this is NOT affected by O2
3. Hatch-Slack Pathway
a. CO2 combines with PEP (3-C) in mesophyll cells and forms oxaloacetate (4-C)
 The enzyme that catalyzes this is NOT affected by O2
b. 4-C product is passed to bundle- sheath cells
c. It is decarboxylated (CO2removed)
d. CO2 enters Calvin Cycle
e. 3-C product returns to mesophyll
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51. Carbon fixation in C3 vs C4 plants

52. XVI. CAM Photosynthesis
A. Crassulacean-Acid Metabolism
1. Plants partition carbon fixation by time
a. During the night:
CAM plants fix CO2
Forms C4 molecules,
Stored in large vacuoles
b. During daylight
NADPH and ATP are available
Stomata closed for water conservation
C4 molecules release CO2 to Calvin cycle
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53. CAM Plant

54. XVII. Climatic Adaptations of C3, C4, CAM Plants
A. Advantages and disadvantages of each type:
1. Depends on the climate
a. C4 plants most adapted to high light intensities, high temperatures, & limited rainfall
b. C3 plants better adapted to cold (below 25oC) & high moisture
c. CAM plants better adapted to extreme aridity
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55. XVIII. Tropical Rain Forests
A. Contribute greatly to uptake of CO2
1. Helping to slow down global warming
B. Development has reduced them from 14% to 6% of Earth’s surface
1. Deforestation adds 20-30% of atmospheric CO2 while removing CO2 sinks from world
2. Increasing temperatures reduce plant productivity.
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56. Global Warming and Tropical Rain Forests
ecology Focus