Photosynthesis

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Photosynthesis
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Origin of plant matter

• Van Helmont (early 1600s)
• Grew a tree for 5 years in a tub with 200 lbs.
  of soil
• Covered tub only added distilled or rain water
• After 5 years plant material weighed 169 lbs.
  (doesnt count leaf fall each autumn).
• Soil weighed a few ounces under 200 lbs.
• Conclusion
• Plant material had to come from the water

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Later work

• Experiments over the centuries began to throw
  more light on the subject.
• People like Priestly and Ingenhouz began to show
  the importance of gases and light for plant
  matter production.

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Overall photosynthesis reaction

• Van Niel found that the oxygen produced by the
  reaction is from water (H2O)
• Overall equation
• 6CO2 12 H20 -----gt C6H12O6 6H20 6O2

light
6 carbon sugar (glucose)
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Uses of carbohydrate product

• The sugar produced is used for various purposes
• Respiration
• Disaccharides (eg. sucrose)
• Polysaccharides (eg. starch, cellulose)
• Many other products (eg. nucleic acids)

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Energy transformations

• Light energy is absorbed by the rock
• Light energy is transformed to the energy of an
  excited electron
• Electron energy is transformed to heat energy and
  radiated

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Photosynthesis process

• Three phases
• Absorption and retention of light energy
• Conversion of light energy into chemical
  potential
• Stabilization and storage of chemical potential

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Visable light spectrum

• We see a very small portion of the
  electromagnetic spectrum

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Pigments

• These substances selectively absorb light
• They are colored because of the wavelengths they
  do not absorb
• Plant pigments
• Carotenoids carotene and xanthophyll
• absorbs blue, bluegreen wavelengths
• Chlorophylls Chlorophyll a Chlorophyll b
• absorbs red and blue wavelengths

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Chlorophyll molecule

• Small changes in the bonds form the different
  chlorophylls

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Abosorption spectrum of chlorophyll

• This explains why leaves are green - - the
  leftover wavelengths

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Light and CO2 Amounts
Light
CO2
Depth
Increasing amount
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Chloroplasts within cells
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Cholorplast structure
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Energy transformations

• Light energy is absorbed by the rock
• Light energy is transformed to the energy of an
  excited electron
• Electron energy is transformed to heat energy and
  radiated

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Energy transfer process

• Light energy excites an electron, allowing it
  to be lost to another molecule.

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Process occurs in the chloroplast
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Temperature experiments

• With excess CO2
• Temperature is not limiting

O2 production
20oC
0oC
0
light
O2 uptake
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Temperature experiments

• With excess light
• Temperature is limiting
• This turns out to be light independent

O2 production
20oC
0oC
0
CO2
O2 uptake
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Light Dark reactions summary
photons
ATP
NADPHH
6 O2

• Products of the light reaction

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Light Dark reactions summary
6CO2
ATP
NADPH
NADP
ADP
Pi
Glucose C6H12O6

• Dark reaction products

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Light Dark reactions summary
photons
6CO2
ATP
NADPre
NADPox
ADP
Pi
Glucose C6H12O6
6 O2

• Note the ultimate products

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Overall photosynthesis reaction

• Overall equation
• 6CO2 12 H20 -----gt C6H12O6 6H20 6O2
• Most of the equation is from the dark reaction
• Light reaction provides the energy

light
From the splitting of water
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Source of atoms in products
6CO2
12H2O
Reactants
Products
C6H12O6
6H2O
6O2
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CAM carbon fixation

• Found in many plants living in desert conditions
  (cacti for example)
• CO2 is fixed in chemical form and can be
  liberated later for use in the dark reaction
• Stomata can be closed in day and opened at night
• Conserves water loss

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C4 metabolism

• Specialized leaf structure helps to provide for
  efficient transport of sugars
• CO2 is taken up to form a 4 C organic acid

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