Photosynthesis: using light energy to reduce CO2 to make CH2O (carbohydrate)

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

Visible light is a portion of the EM spectrum

Light is energy: E = h

Bacteria and chloroplasts use chlorophyll to capture light energy

Xiong et al. 2000Science 289:1724-1730

Chlorophylls and other pigments form a light-harvesting complex in photosynthetic membranes

Absorption spectra for chlorophylls match photosynthetic action spectrum

Campbell & Reece, 7th ed.

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

Energy diagram of non-cyclic electron flow

Protons are pumped into the thylakoid interior

Campbell & Reece, Biology 8th ed.

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:

Cyclic photophosphorylation by PSI makes ATP but not NADPH

Fd(red) can reduce NADP+, or PQ (electron transport chain)

Campbell & Reece, Biology 8th ed.