Photosynthesis

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Photosynthesis

• How Plants Make Food from Sunlight and Low Energy
  Molecules

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Photoautotrophs
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Carbon and Energy Sources

• Photoautotrophs
• Carbon source is carbon dioxide
• Energy source is sunlight
• Heterotrophs
• Get carbon and energy by eating autotrophs or one
  another

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Photoautotrophs

• Capture sunlight energy and use it to carry out
  photosynthesis
• Plants
• Some bacteria
• Many protistans

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T.E. Englemanns Experiment

• Background
• Certain bacterial cells will move toward places
  where oxygen concentration is high
• Photosynthesis produces oxygen

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T.E. Englemanns Experiment

• Hypothesis
• Movement of bacteria can be used to determine
  optimal light wavelengths for photosynthesis

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T.E. Englemanns Experiment

• Method
• Algal strand placed on microscope slide and
  illuminated by light of varying wavelengths
• Oxygen-requiring bacteria placed on same slide

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T.E. Englemanns Experiment
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T.E. Englemanns Experiment

• Results
• Bacteria congregated where red and violet
  wavelengths illuminated alga
• Conclusion
• Bacteria moved to where algal cells released
  more oxygen--areas illuminated by the most
  effective light for photosynthesis

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Linked Processes

• Photosynthesis
• Energy-storing pathway
• Releases oxygen
• Requires carbon dioxide

• Aerobic Respiration
• Energy-releasing pathway
• Requires oxygen
• Releases carbon dioxide

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Focusing in on the location of photosynthesis in
a plant
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Location and structure of chlorophyll molecules
in plants
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Photosynthesis Equation
LIGHT ENERGY
12H2O 6CO2
6O2 C6H12O6 6H2O
water
carbon dioxide
oxygen
glucose
water
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Two Stages of Photosynthesis
sunlight
water uptake
carbon dioxide uptake
ATP
ADP Pi
LIGHT INDEPENDENT-REACTIONS
LIGHT DEPENDENT-REACTIONS
NADPH
NADP
glucose
P
oxygen release
new water
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Sunlight Energy

• Continual input of solar energy into Earths
  atmosphere
• Almost 1/3 is reflected back into space
• Of the energy that reaches Earths surface, about
  1 is intercepted by photoautotrophs

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Electromagnetic Spectrum

• Shortest Gamma rays
• wavelength X-rays
• UV radiation
• Visible light
• Infrared radiation
• Microwaves
• Longest Radio waves
• wavelength

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Visible Light

• Wavelengths humans perceive as different colors
• Violet (380 nm) to red (750 nm)
• Longer wavelengths, lower energy

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Photons

• Packets of light energy
• Each type of photon has fixed amount of energy
• Photons having most energy travel as shortest
  wavelength (blue-green light)

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Pigments

• Light-absorbing molecules
• Absorb some wavelengths and transmit others
• Color you see are the wavelengths NOT absorbed

chlorophyll a
chlorophyll b
Wavelength (nanometers)
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Pigments in Photosynthesis

• Bacteria
• Pigments in plasma membranes
• Plants
• Pigments embedded in thylakoid membrane system
• Pigments and proteins organized into photosystems
• Photosystems located next to electron transport
  systems

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Pigments in a Photosystem
reaction center (a specialized chlorophyll a
molecule)
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Light-Dependent Reactions

• Pigments absorb light energy, give up e- which
  enter electron transport systems
• Water molecules are split, ATP and NADH are
  formed, and oxygen is released
• Pigments that gave up electrons get replacements

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Photosystem Function Harvester Pigments

• Most pigments in photosystem are harvester
  pigments
• When excited by light energy, these pigments
  transfer energy to adjacent pigment molecules
• Each transfer involves energy loss

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Photosystem Function Reaction Center

• Energy is reduced to level that can be captured
  by molecule of chlorophyll a
• This molecule (P700 or P680) is the reaction
  center of a photosystem
• Reaction center accepts energy and donates
  electron to acceptor molecule

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Cyclic Electron Flow
e
electron acceptor
electron transport system
e
e
ATP
e
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Electron Transport System

• Adjacent to photosystem
• Acceptor molecule donates electrons from reaction
  center
• As electrons flow through system, energy they
  release is used to produce ATP and, in some
  cases, NADPH

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Cyclic Electron Flow

• Electrons
• are donated by P700 in photosystem I to acceptor
  molecule
• flow through electron transport system and back
  to P700
• Electron flow drives ATP formation
• No NADPH is formed

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Energy Changes
second
transport
system
e
NADPH
e
first
transport
system
e
Potential to transfer energy (voids)
e
(PHOTOSYSTEM I)
(PHOTOSYSTEM II)
1/2 O2 2H
H2O
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Noncyclic Electron Flow

• Two-step pathway for light absorption and
  electron excitation
• Uses two photosystems type I and type II
• Produces ATP and NADPH
• Involves photolysis - splitting of water

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Figure 10.4 An overview of photosynthesis
cooperation of the light reactions and the Calvin
cycle (Layer 1)
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Figure 10.4 An overview of photosynthesis
cooperation of the light reactions and the Calvin
cycle (Layer 2)
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Light-Independent Reactions

• Synthesis part of photosynthesis
• Can proceed in the dark
• Take place in the stroma
• Calvin-Benson cycle

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Calvin-Benson Cycle

• Overall reactants
• Carbon dioxide
• ATP
• NADPH

• Overall products
• Glucose
• ADP
• NADP

Reaction pathway is cyclic and RuBP (ribulose
bisphosphate) is regenerated
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Melvin Calvin
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The Calvin cycle (Layer 1)
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The Calvin cycle (Layer 2)
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The Calvin cycle (Layer 3)
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Using the Products of Photosynthesis

• Phosphorylated glucose is the building block for
• sucrose
• The most easily transported plant carbohydrate
• starch
• The most common storage form

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The C3 Pathway

• In Calvin-Benson cycle, the first stable
  intermediate is a three-carbon PGA
• Because the first intermediate has three carbons,
  the pathway is called the C3 pathway

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Photorespiration in C3 Plants

• On hot, dry days stomata close
• Inside leaf
• Oxygen levels rise
• Carbon dioxide levels drop
• Rubisco attaches RuBP to oxygen instead of carbon
  dioxide
• Only one PGAL forms instead of two

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C4 Plants

• Carbon dioxide is fixed twice
• In mesophyll cells, carbon dioxide is fixed to
  form four-carbon oxaloacetate
• Oxaloacetate is transferred to bundle-sheath
  cells
• Carbon dioxide is released and fixed again in
  Calvin-Benson cycle

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Figure 10.18 C4 leaf anatomy and the C4 pathway
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CAM Plants

• Carbon is fixed twice (in same cells)
• Night
• Carbon dioxide is fixed to form organic acids
• Day
• Carbon dioxide is released and fixed in
  Calvin-Benson cycle

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Figure 10.19 C4 and CAM photosynthesis compared
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Figure 10.20 A review of photosynthesis