I' Introduction to Photosynthesis

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I. Introduction to Photosynthesis
A. Types of anabolic metabolism
B. Overview of photosynthesis
C. Early experiments w/ photosynthesis
D. Anatomy of photosynthesis
E. The Light Reactions
1. Harnessing light
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Photosynthesis
Where does the energy come from that sustains all
life?
The Sun!!
Well most of it anyway
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Plants and other photosynthetic organisms
(cyanobacteria, algae) harness the suns energy
to make organic molecules from inorganic sources.
photoautotrophs
They utilize sunlight for energy!
They take in CO2 and other inorganic nutrients
and build complex organic molecules.
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Some organisms create organic molecules from
inorganic sources, but they dont utilize
sunlight!
Chemoautotrophic bacteria
Obtain energy through the oxidation of inorganic
molecules.
They take in CO2 and other nutrients and build
complex organic molecules.
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Chemoautotrophic bacteria can be found around
hydrothermal vents.
Chemoautotrophic bacteria oxidize H2S which
provides the energy for building organic
molecules.
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Some hydrothermal vent fauna
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Cellular respiration -vs- Photosynthesis
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Overview of Photosynthesis
6CO2 12H2O ? C6H12O6 6O2 6H2O DG 686
kcal/mol

• Photosynthesis, like cellular respiration, is a
  series of redox reactions!

What is being oxidized and what is being reduced
in the summarizing equation above?
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Historical perspective
Early thinking O2 released came from the CO2
CO2 ? C O2
sugar
C H2O ? CH2O
Water was added to the carbon to make sugar
van Neils experiments with the sulfur bacteria
showed that the bacteria used CO2 but did NOT
release O2 ...he suggested that plants split
water as a source of e- and H
Scientists used radioactive isotopes to test this
idea
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18Oxygen (Heavy oxygen) isotope experiment
Experiment 1
Experiment 2
CO2
CO2
C18O2
C18O2
CO2
C18O2
CO2
CO2
C18O2
CO2
C18O2
CO2
C18O2
C18O2
H218O
H2O
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18O isotope experiment
Hypothesis If CO2 is being oxidized
C18O2
H218O
6CO2 12H2O ? C6H12O6 6O2 6H2O
6CO2 12H2O ? C6H12O6 6O2 6H2O
18O
18O
Can you predict where the heavy oxygen will end
up in each case?
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18O isotope experiment
Hypothesis If H2O is being oxidized
C18O2
H218O
6CO2 12H2O ? C6H12O6 6O2 6H2O
6CO2 12H2O ? C6H12O6 6O2 6H2O
18O
18O
Can you predict where the heavy oxygen will end
up in each case?
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The actual results!!
C18O2
H218O
6CO2 12H2O ? C6H12O6 6O2 6H2O
6CO2 12H2O ? C6H12O6 6O2 6H2O
18O
18O
So..its the water thats being oxidized!
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Tracking atoms in photosynthesis
6 CO2
12 H2O
Reactants
Products
C6H12O6 6H2O 6O2
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What does this suggest about photosynthesis?
The sugars that result from photosynthesis are
produced by adding the hydrogen ions and
electrons from water to carbon dioxide, NOT by
splitting CO2 and adding water!
oxidized
H and e-
6CO2 12H2O ? C6H12O6 6O2 6H2O
reduced
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Anatomy of Photosynthesis
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The stages of photosynthesis
Light Dependent Reactions

• Occurs along the thylakoid membrane

• this is where H2O is oxidized.

• chemiosmotic production of ATP!

• electron carrier (NADPH) is synthesized

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Light reactions
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The stages of photosynthesis
Light Independent Reactions

• Occurs in the stroma of the chloroplast.

• Where CO2 is fixed into sugars!

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Light independent
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How plants harness energy from sunlight
Photosynthetic pigments
Chlorophylls a the main photosynthetic pigment
Accessory pigments..
Chlorophyll b
Carotenoids
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A little bit about light
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What wavelengths of light do you think plants use
the least in photosynthesis?
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Engelmanns experiment
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Photosystems use some wavelengths of light but
reflect others
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Photosynthetic pigments are arranged as
photosystems
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Aphoto-system
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Photon is passed to reaction center Chl. a
Photon excites Chl. electron e- captured by
primary acceptor
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Water split 2 electrons given to 2 P680 Chl a. O
combines with another O O2. Electron passes
down ETC to PSII, ATP is made
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Photon excites P700 e-, the e- captured by
acceptor. The e- hole in P700 is filled by e-
coming from ETC
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Excited e- from primary acceptor passes down
second ETC, e- added to NADP by reductase
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Non-cyclic photophosphorylation
Functions

• produces ATP!

• produces NADPH an electron carrier

Used during the light independent reactions to
make sugar