Photosynthesis Where does the energy come from that sustains all life? The Sun!! Well most of it anyway……
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.)
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?
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 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+ Scientists used radioactive isotopes to test this idea
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
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?
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?
The Actual Results!! C18O2 H218O 6CO2 + 12H2O C6H12O6+ 6O2 + 6H2O 6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O 18O 18O So..it’s the water that’s being oxidized!
Tracking Atoms in Photosynthesis… Reactants 6 CO2 12 H2O Products C6H12O6 6H2O 6O2
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
Plant Photosynthesis internal leaf structure chloroplasts outer membrane inner membrane thylakoid
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
The stages of photosynthesis… Light Independent Reactions • Occurs in the stroma of the chloroplast. • Where CO2 is “fixed” into sugars!
How plants harness energy from sunlight… Photosynthetic pigments Chlorophylls a – the “main” photosynthetic pigment Accessory pigments.. Chlorophyll b Carotenoids
What wavelengths of light do you think plants use the least in photosynthesis?
Photosystem From the Photosystem, e- are passed along an Electron Transport Chain..The Photosynthetic Electron Transport Chain (PETC) Photon Reaction Center Light Harvesting Pigments
Photosystem Photon PETC
Photosynthetic Electron Transfer Chain (PETC) • series of electron carriers which take electrons from photosystem, and.. • ultimately carry electrons to NADP+
Photosystems • 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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