Photosynthesis

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Photosynthesis
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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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Respiration vs. Photosynthesis
Photosynthesis and respiration as complementary
processes in the living world. Photosynthesis
uses the energy of sunlight to produce sugars and
other organic molecules. These molecules in
turn serve as food. Respiration is a process
that uses O2 and forms CO2 from the same carbon
atoms that had been taken up as CO2 and
converted into sugars by photosynthesis. In
respiration, organisms obtain the energy that
they need to survive. Photosynthesis preceded
respiration on the earth for probably billions
of years before enough O2 was released to create
an atmosphere rich in oxygen. (The earth's
atmosphere presently contains 20 O2.)
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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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Plant Photosynthesis
internal leaf structure
chloroplasts
outer membrane
inner membrane
thylakoid
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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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The stages of photosynthesis
Light Independent Reactions

• Occurs in the stroma of the chloroplast.

• Where CO2 is fixed into sugars!

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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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Photosynthetic pigments are arranged as
photosystems
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What wavelengths of light do you think plants use
the least in photosynthesis?
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Photosystems use some wavelengths of light but
reflect others
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From the Photosystem, e- are passed along an
Electron Transport Chain..The Photosynthetic
Electron Transport Chain (PETC)
Photosystem
Photon
Reaction Center
Light Harvesting Pigments
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Photosystem
Photon
PETC
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Photosynthetic Electron Transfer Chain (PETC)

• series of electron carriers which take electrons
  from photosystem, and..
• ultimately carry electrons to NADP

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Photosystems

• Experiments in the 1940s suggested that light
  photons are absorbed at 2 different points along
  the same PETC.
• In fact, there are two Photosystems in operation

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