supramolecular chemistry, uaf, 2005 photosynthesis

Supramolecular Chemistry, UAF, 2005

PhotosynthesisPhotosynthesis


PhotosynthesisPhotosynthesis

Where does the energy come from that sustains all life?

The Sun!!Well most of it anyway……


Respiration vs. PhotosynthesisRespiration 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.)


Cellular Respiration -vs- Cellular Respiration -vs- PhotosynthesisPhotosynthesis

Exergonic and endergonicreactions of metabolism…Exergonic:

C6H12O6 + 6O2 6CO2 + 6H2O G = -686 kcal/mole

Cellular respiration!

Endergonic:

6CO2 + 6H2O C6H12O6 + 6O2 G = +686 kcal/mole

Photosynthesis!

Provides the energy to run cell processes

Allows plants to make energy rich sugars fromenergy poor molecules


Overview of PhotosynthesisOverview of Photosynthesis

6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O G = + 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?


Early thinking: O2 released came from the CO2

CO2 C + O2

C + H2O CH2O

sugar

Water was added to the carbon to make sugar

van Neil’s 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+

Historical perspectiveHistorical perspective

Scientists used radioactive isotopes to test this idea


Experiment #1

H218O

CO2CO2

CO2

CO2

CO2

CO2

CO2

H2O

C18O2

C18O2

C18O2 C18O2C18O2

C18O2

C18O2

Experiment #2

1818Oxygen (Heavy Oxygen) Isotope Oxygen (Heavy Oxygen) Isotope Experiment…Experiment…


1818O Isotope ExperimentO Isotope Experiment

6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O

C18O2

Hypothesis: If CO2 is being oxidized

H218O

18OCan you predict where the heavy oxygen will end up in each case?

6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O

18O


6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O

C18O2

Hypothesis: If H2O is being oxidized

H218O

18OCan you predict where the heavy oxygen will end up in each case?

6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O

18O

1818O Isotope ExperimentO Isotope Experiment


6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O

C18O2

H218O

18O

6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O

18O

The Actual Results!!The Actual Results!!

So..it’s the water that’s being oxidized!


6 CO2 12 H2OReactants

Products C6H12O6 6H2O 6O2

Tracking Atoms in Photosynthesis…Tracking Atoms in Photosynthesis…


What does this suggest about What does this suggest about photosynthesis?photosynthesis?

6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O

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

reduced

H+ and e-


internal leaf structure

chloroplastsouter membrane

inner membrane

thylakoid

Plant PhotosynthesisPlant Photosynthesis


The stages of photosynthesis…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


Light Independent Reactions

• Occurs in the stroma of the chloroplast.

• Where CO2 is “fixed” into sugars!

The stages of photosynthesis…The stages of photosynthesis…


How plants harness energy from sunlight…How plants harness energy from sunlight…

Photosynthetic pigments

Chlorophylls a – the “main” photosynthetic pigment

Chlorophyll b

Accessory pigments..

Carotenoids


Photosynthetic pigments are arranged as Photosynthetic pigments are arranged as “photosystems”“photosystems”


What wavelengths of light do you think plants use the least in photosynthesis?


Photosystems use some wavelengths of Photosystems use some wavelengths of light but reflect others…light but reflect others…


Light Harvesting Pigments

Reaction Center

Photon

PhotosystemPhotosystem

From the Photosystem, e- are passed along an Electron Transport Chain..

The Photosynthetic Electron Transport Chain (PETC)


PETCPhoton

PhotosystemPhotosystem


Photosynthetic Electron Transfer Chain Photosynthetic Electron Transfer Chain (PETC)(PETC)

• series of electron carriers which take electrons from photosystem, and..

• ultimately carry electrons to NADP+


PhotosystemsPhotosystems

• Experiments in the 1940’s 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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supramolecular chemistry, uaf, 2005 photosynthesis

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