Chapter 8
Photosynthesis
Trophic RolesAutotrophs
produce organic molecules from CO2 & inorganic raw materials “producers” plants
Heterotrophs “consumers”
Chloroplast StructureFound in
mesophyll 1 mesophyll cell
may have 30 chloroplasts
Stomata regulate passage of CO2, O2 and H2O
Chloroplast Structure
Chloroplast Structure (cont.) Pigments light receptors
Absorb light energy Boost e-
Chlorophyll Chlorophyll “a” main
pigment blue-green Chlorophyll “b” accessory
pigment yellow-green Accessory pigments
absorb different wavelengths of light Carotenoids yellow-
orange
Photosystems Pigment molecules absorb energy boost electrons unstable
Passes energy to reaction center of antenna complex (chlorophyll “a” molecule) transfers energy to primary electron acceptor
Photosynthesis Light reactions Cyclic
photophosphorylation Non-cyclic
photophosphorylation Photolysis (breaking
water up with light)
Dark reactionsCalvin Cycle
Photosystems (cont.) Located in thylakoid
membrane Photosystem I
P700 absorbs light of 700 nm
Photosystem II P680 absorbs light of 680 nm
Both are primarily chlorophyll “a”
Electron acceptor NADP+ NADPH
Non-cyclic photophosphorylation
LIGHTREACTOR
NADP+
ADP
ATP
NADPH
CALVINCYCLE
[CH2O] (sugar)STROMA(Low H+ concentration)
Photosystem II
LIGHT
H2O CO2
Cytochromecomplex
O2
H2OO2
1
1⁄2
2
Photosystem I
Light
THYLAKOID SPACE(High H+ concentration)
STROMA(Low H+ concentration)
Thylakoidmembrane
ATPsynthase
PqPc
Fd
NADP+
reductase
NADPH + H+
NADP+ + 2H+
ToCalvincycle
ADP
P
ATP
3
H+
2 H++2 H+
2 H+
Cyclic PhotophosphorylationPrimitive used by
bacteria only generates energy--no glucose
Electron “boosted” out of PI ETC returned to PI
Electron drives proton pump chemiosmosis ATP
Calvin Cycleaka Dark Reactions
Occur in the dark or the lightLight independent reactions3 “steps”
Carbon fixationReductionRegeneration of RuBP
Step 1:Carbon Fixation RuBP (ribulose
bisphosphate) 5 C sugar catalyzed by RuBP carboxylase (Rubisco) unstable compound (splits) PGA
Step 2:Reduction Phosphorylated
by ATPReduced by
NADPHProduces
pyruvatesome pyruvate
glucosemost pyruvate
regenerate RuBP
Step 3:Regeneration of RuBP
Pyruvate rearranged into RuBP
Requires input of 3 ATP
Takes 12 turns of cycle 1 glucose
http://www.science.smith.edu/departments/Biology/Bio231/calvin.html
Alternative mechanisms:Photorespiration
Competitive reaction between RuBP, CO2 and O2
Rubisco substitutes O2 for CO2 2 C compound (phosphoglycerate)
Eventually broken down releases CO2
Non-productive
C3 plants rice, wheat, soybeans (hot, bright days)
Photo (light) respiration( releases CO2/consumes O2)
Alternative mechanisms:C4 Plants Sunny
ecosystems Carbon “fixed”
outside cells (in bundle sheath cells very efficient requires extra ATP Balances out photorespiration
Corn is a C4 plant
Alternative mechanisms:CAM Plants Crassulacean acid
metabolism Hot/dry climates
Orchids, cacti, etc.
Stomates open at night to reduce water loss evaporation
CO2 is fixed, used later