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Lecture 8

BIOL 2P03 Lecture Notes - Lecture 8: Starch, Cytosol, Glyceraldehyde 3-Phosphate

Biological Sciences
Course Code
Jeff Stuart

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Lecture 8 Photosynthesis
During photosynthesis, chloroplasts capture the energy of sunlight, convert it into
chemical energy in the form of ATP and NADPH, and then use this energy to make
complex carbohydrates out of carbon dioxide and water.
Chemical equation of photosynthesis
6 CO2 + 6 H2O → 6 O2 + C6H12O6
Occurs in the thylakoid membranes within chloroplasts
Photosynthetic machinery – a close-up look:
Two photosystems (PSI and PSII)
Light harvesting complexes (LHCI and LHCII)
Cytochrome bf complex
Quinone (Q)
ATP synthase
NAPDH, ATP and CO2 from atmosphere are then used to make carbohydrates
(sugar) in the chloroplast stroma
This second set of reactions requires no light, just the NADPH and ATP made
by the light-requiring reactions
The Calvin cycle is in the stroma of the chloroplasts
Uses the ATP and NADPH from light reactions, and CO2 from the atmosphere,
to produce 3 carbon glyceraldehyde-3-phosphate
Glyceraldehyde-3-P is exported from chloroplasts to cytosol, where it is made
into sugars
Photosynthesis in plants – summary
Light photons captured by PSI and PSII
PSII makes O2 from water
PSII and cytochrome bf complex make H+ gradient (used to drive the ATP
PSI reduces NADP+ to NADPH
Products NADPH, ATP combine with CO2 to make glyceraldehyde-3-phosphate
(G-3-P) - stimulated by photosynthetic activity
G-3-P used to make starch (in stroma) or exported to cytosol
In cytosol G-3-P converted to fructose & glucose = sucrose
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