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