Photosynthesis

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Slide 1

Photosynthesis

Slide 2

Photosynthesis

Where does the energy come from that sustains all life?

The Sun!!

Well most of it anyway……

Slide 3

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.)

Slide 4

Cellular Respiration

-vs- Photosynthesis

Slide 5

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?

Slide 6

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 perspective

Scientists used radioactive isotopes to test this idea

Slide 7

Experiment #1

H218O

CO2

CO2

CO2

CO2

CO2

CO2

CO2

H2O

C18O2

C18O2

C18O2

C18O2

C18O2

C18O2

C18O2

Experiment #2

18Oxygen (Heavy Oxygen) Isotope Experiment…

Slide 8

18O Isotope Experiment

6CO2 + 12H2O  C6H12O6 + 6O2 + 6H2O

C18O2

Hypothesis: If CO2 is being oxidized

H218O

18O

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

6CO2 + 12H2O  C6H12O6 + 6O2 + 6H2O

18O

Slide 9