Photosynthesis
AP Biology 2015 - 2016
Campbell Biology in Focus: Chapter 8
I. Photosynthesis and the Biosphere
A. Plants and other autotrophs are producers of biosphere
B. Photoautotrophs: use light energy to make organic molecules
C. Heterotrophs: consume organic molecules from other organisms for energy and carbon
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II. Overview - Photosynthesis converts light energy to chemical energy of food
A. Chloroplasts: photosynthetic organelles
B. Leaf anatomy & photosynthesis
i. mesophyll: chloroplasts mainly
found in these cells of leaf
ii. stomata: pores in leaf
(CO2 enter/O2 exits)
iii. chlorophyll: green pigment in
thylakoid membranes of
chloroplasts
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C. Net Chemical Reaction: 6CO2 + 6H2O + Light Energy → C6H12O6 + 6O2
i. Redox Reaction: water is split → e- transferred with H+ to CO2 → sugar
ii. Observations in bacteria and experiments preformed with isotopes allowed
scientists to trace the flow of atoms in photosynthesis
D. Photosynthesis consists of two series of chemical reactions:
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III. The Light Reactions: solar energy converted to short-term chemical energy
A. Sunlight
i. Light = energy = electromagnetic radiation
ii. Shorter wavelength (λ) = higher energy
iii. Visible light is detected by human eye
iv. Light: reflected, transmitted or absorbed
Electromagnetic Spectrum
B. Pigments: molecules that absorb specific λ of light
i. Photosynthetic pigments absorb violet-blue/red light and reflect green
ii. Chlorophyll a (reflects blue-green): converts solar to chemical energy
iii. Accessory pigments absorb light missed by chlorophyll a
a. Chlorophyll b (reflects yellow-green)
b. carotenoids (reflects yellow and orange and is also involved in
photoprotection)
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iv.
iv. Action Spectrum: plots rate of
photosynthesis vs. wavelength
(absorption of chlorophylls a, b, &
carotenoids combined)
Engelmann in 1882: used bacteria
seeking oxygen to measure rate of
photosynthesis in algae; established
action spectrum
C. First Step of Light Reactions: Light Absorption
i. Electrons in pigments are excited by absorption of light
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ii. Pigments are organized into photosystems along with proteins on the
thylakoid membrane
D. Second Step of Light Reactions: Photosystem II → e- → ETC and ATP Production
i. Energy is passed to reaction
center of Photosystem II
ii. e- captured by primary electron acceptor
iii. Water is split to replace e- → O2 formed
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iv. e- passed through Electron Transport Chain
(ETC)
v. ETC transfer pumps H+ to thylakoid space
vi. ATP produced by chemiosmosis
vii. e- passed to Photosystem I
E. Third Step of Light Reactions: Photosystem I → e- → ETC and NADPH Production
i. Photosystem I receives an electron that still contains some potential energy
ii. more light energy is absorbed and the electron has even more potential energy
iii. e- moves from PS I’s primary electron acceptor to 2nd ETC
iv. NADP+ reduced to NADPH
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Analogy:
F. Chemiosmosis: both respiration and photosynthesis use chemiosmosis to generate ATP
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Proton motive force is generated by:
1. H+ from water
2. H+ pumped across by cytochrome of the ETC
3. Removal of H+ from stroma when NADP+ is reduced
IV. The Calvin Cycle - uses ATP and NADPH to convert CO2 to sugar
A. Uses ATP, NADPH, CO2
B. Produces 3-C sugar G3P (glyceraldehyde-3-phosphate)
C. Three phases:
1. Carbon fixation
2. Reduction
3. Regeneration of RuBP (CO2 acceptor)
Phase 1: Carbon Fixation - 3 CO2 + RuBP (5-C sugar ribulose bisphosphate)
Catalyzed by enzyme Rubisco (RuBP carboxylase)
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Phase 2: Reduction - Use 6 ATP and 6 NADPH to produce 1 net G3P
Phase 3: Regeneration of RuBP (CO2 acceptor) - Use 3 ATP to regenerate RuBP
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V. Alternative mechanisms of carbon fixation have evolved in hot, arid climates
A. Photorespiration - metabolic pathway which:
i. Uses O2 & produces CO2
ii. Uses ATP
iii. No sugar production (Rubisco binds O2 which leads to breakdown of RuBP)
iv. Occurs on hot, dry bright days when stomata close (conserve H2O)
v. Why? Early atmosphere: low O2, high CO2?
B. Evolutionary Adaptations
i. C3 Plants: CO2 fixed to 3-C compound → Calvin cycle
a. Ex. Rice, wheat, soybeans
b. Hot, dry days: partially close stomata, ↓CO2
Photorespiration: ↓ photosynthetic output (no sugars)
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ii. C4 Plants: CO2 fixed to 4-C compound
a. Ex. corn, sugarcane, grass
b. Hot, dry days → stomata close
c. 2 cell types = mesophyll & bundle sheath cells
1. mesophyll : PEP carboxylase fixes CO2 (4-C)
2. mesophyll pumps CO2 to bundle sheath cells
3. bundle sheath: CO2 used in Calvin cycle
4. ↓photorespiration, ↑sugar production
d. WHY? Advantage in hot, sunny areas
C4 Leaf Anatomy
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iii. CAM Plants: Crassulacean acid metabolism (CAM)
a. NIGHT: stomata open → CO2 enters → converts to organic acid,
stored in mesophyll cells
b. DAY: stomata closed → light reactions supply ATP, NADPH;
CO2 released from organic acids for Calvin cycle
c. Ex. cacti, pineapples, succulent (H2O-storing) plants
d. WHY? Advantage in arid conditions
C4
vs.
CAM
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Three Types of Carbon Metabolism:
Photosynthesis Review
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Comparison or Cellular Respiration to Photosynthesis:
RESPIRATION
PHOTOSYNTHESIS
• Plants + Animals
• Plants
• Needs O2 and food
• Needs CO2, H2O, sunlight
• Produces CO2, H2O and ATP, NADH
• Produces glucose, O2 and ATP, NADPH
• Occurs in mitochondria membrane &
• Occurs in chloroplast thylakoid
matrix
membrane & stroma
• Proton gradient across membrane
• Proton gradient across membrane
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