D’YOUVILLE COLLEGE
BIOLOGY 102 - INTRODUCTORY BIOLOGY II
LECTURE # 16
PHOTOSYNTHESIS I
1.
Basic Outline: review chloroplast structure (including thylakoids, grana,
stroma) (fig. 10 – 4 & ppt. 1)
• overall reaction in photosynthesis:
- carbon dioxide + water -----------------------> sugar + oxygen
- 6 CO2 + 6 H2O -----------------------> C6H12O6 + 6 O2
- carbon dioxide enters leaf through pores (stomata) on underside of leaf;
water reaches leaf from roots via stem and leaf veins (fig. 10 - 3 & ppt. 1)
- former interpretation: oxygen derived from splitting of carbon dioxide;
waters + carbons formed sugar
• revised chemical reaction (fig. 10 – 6 & ppt. 2): oxygen is derived from splitting
of water; this was determined by tracking oxygen isotope from water or from carbon
dioxide
- 6 CO2 + 12 H2O -----------------------> C6H12O6 + 6 O2 + 6 H2O
• two major groups of reactions (fig. 10 – 6 & ppt. 3):
- light dependent reactions (occur in thylakoids of chloroplast; depend on
light energy captured by chlorophyll photosystems):
- photophosphorylation involves ATP production driven by light energy;
also NADPH production (storage of reducing power)
- photolysis of water (source of oxygen & electrons)
Biology 102, lec 16 - Spring ‘13
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- light independent reactions (occur in stroma of chloroplast)
- Calvin cycle: carbon dioxide fixation & sugar production
• overall process: approx. 2000 kcal of photon energy; 686 captured in
glucose; approx. 1300 lost to surroundings; highly exergonic
2.
Photosystems & Light Capture:
• pigments & light absorption: chlorophylls & carotenoids are the
photosynthetic pigments (fig. 10 – 11 & ppt. 4)
- absorption occurs at red wavelengths & blue-violet wavelengths (figs. 10
– 7, 10 – 10, & ppts. 5 & 6); absorbance or transmittance measured by colorimeter
(fig. 10 – 9 & ppt. 7)
- green wavelengths are reflected or transmitted (fig. 10 – 8 & ppt. 8)
- light excites electrons of pigment to higher energy levels; return to
resting level with emission of light (fluorescence), if no acceptor of excited electrons
present (fig. 10 – 12 & ppt. 9)
- photosystem organization: numerous “antenna” molecules (include
chlorophylls, carotenoids) + reaction center (chlorophyll a linked to primary electron
acceptor of an electron transport chain) (fig. 10 – 13 & ppt. 10)
- photosystem I (pigment reaction center “P700”)
- photosystem II (pigment reaction center “P680”)
Biology 102, lec 16 - Spring ‘13
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