Photosynthesis: using light energy to reduce CO 2 to make CH 2 O (carbohydrate)
Feb 23, 2016
Photosynthesis light reactions
• How do photosynthetic organisms capture light energy?
• What reactions convert light energy to chemical energy?
Light reaction take-home points• Light energy is used to generate a proton gradient
across a membrane for chemiosmotic ATP synthesis.• Light energy is used to reduce electron carriers that are
used, in turn, to reduce inorganic carbon (CO2) to make carbohydrate (CH2O).
• A key innovation enabled cyanobacteria to take electrons from water molecules instead of organic molecules or sulfur and changed the surface of the planet forever.
Oxygenic Photosynthesis
• 6 CO2 + 12 H2O C6H12O6 + 6 H2O + 6 O2
• Reverse of aerobic respiration:C6H12O6 + 6 H2O + 6 O2 6 CO2 + 12 H2O
Evolution of chloroplasts from cyanobacterial endosymbiont?
http://evolution.berkeley.edu/evosite/history/endosym.shtml
Bacteria and chloroplasts use chlorophyll to capture light energy
Xiong et al. 2000Science 289:1724-1730
Light-harvesting complexes funnel light energy to a photosystem reaction center
Light-activated reaction center chlorophyll molecules give up electrons (are oxidized)
Noncyclic photophosphorylation (Z-scheme) uses two photosystems: PSI and PSII
Light + H2O + NADP+ + ADP + Pi NADPH + H+ + ATP + 1/2O2
Chemiosmotic ATP synthesis in chloroplasts resembles that in mitochondria
Electron transport chains are located on the thylakoid membrane and inner mitochondrial membranes.
Campbell & Reece, Biology 8th ed.
Summary of Z-scheme (non-cyclic)
• PS II– P680– Gets electrons
from:
– Gives electrons to:
– Products:
• PS I– P700 – Gets electrons
from:
– Gives electrons to:
– Products: