Chapter 22 – The Light Reactions
Chapter 22 ? The Light Reactions
(Problems 1,2,4,5,8-10,13-19)
Energy + 6H2O + 6CO2
Light
Photosynthesis
C6H12O6 + O2
ETC/OX-Phos
C6H12O6 + O2
Glycolysis/CAC
6H2O
+
6CO2
+
Energy
ATP
? Photosynthesis: a process that converts
atmospheric CO2 and H2O to carbohydrates
? Solar energy is captured in chemical form as ATP and NADPH
? ATP and NADPH are used to convert CO2 to hexose phosphates
? Phototrophs: photosynthetic organisms (some bacteria, algae, higher plants)
Respiratory ETC
Photosynthesis ETC
Downhill Energy Process Uphill Energy Process
Energy + 6H2O + 6CO2
Light
Photosynthesis
C6H12O6 + O2
C6H12O6 + O2
Glycolysis/CAC
ETC/OX-Phos
6H2O
+
6CO2
+
Energy
ATP
Light and dark reactions
? Both processes can occur simultaneously
Reactions that require light (light reactions):
H2O + ADP + Pi + NADP+
O2 + ATP + NADPH + H+
Reactions which do not require light (dark reactions):
CO2 + ATP + NADPH + H+ Sum: CO2 + H2O
(CH2O) + ADP + Pi + NADP+ (CH2O) + O2
22.1 Photosynthesis Takes Place in Chloroplasts
Three membranes, and three "spaces".
H2O + ADP + Pi + NADP+ O2 + ATP + NADPH + H+ CO2 + ATP + NADPH + H+ (CH2O) + ADP + Pi + NADP+
22.2 Photosynthesis Transforms Light Energy into Chemical Energy
I II
I II
I II
I II
Structures of Chlorophyll and bacteriochlorophyll
? Chlorophylls - usually most abundant and most important pigments in light harvesting
? Contain tetrapyrrole ring (chlorin) similar to heme, but contains Mg2+
? Chlorophylls a (Chl a) and b (Chl b) in plants ? Bacteriochlorophylls a (BChla) and b (BChlb) are major pigments
in bacteria
Light harvesting complexes enhance the efficiency of photosynthesis
Accessory pigments
Reaction centers of the photosystems
? PSI and PSII each contain a reaction center (site of the photochemical reaction)
? Special pair: two chlorophylls in each reaction center that are energized by light
? In PSI special pair is: P700 (absorb light maximally at 700nm)
? In PSII the special pair is: P680 (absorb light maximally at 680nm)
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