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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