Cell Bio: Chapter 14- Photosynthesis Part 1

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Heterotrophs v Autotrophs

Heterotrophs: Obtain energy and reduced carbon by consuming other organisms

Autotrophs: Obtain energy from inorganic sources and use to make their own reduced carbon molecules

• Chemoautotroph: Obtain energy from reduced inorganic sources and incorporate it through chemosynthesis

• Photoautotroph: Obtain energy from sunlight and incoporate it through photosynthesis

Photosynthesis equation and phases

6CO2 + 12H2O + sunlight —> C6H12O6 + 6O2 + 6H2O

Energy transduction (light dependent reaction): chlorophyl molecules use energy from photons to make ATP and NADPH

Carbon assimilation (Calvin Cycle): uses ATP and NADPH to to join carbon atoms into carbs

Inorganic v Organic Carbon in Biological Terms

*Organic usually means something that is containing carbon

Inorganic Carbon: just Co2

Organic Carbon: Co2 w/energy ex. glucose

Biologists use that term to differentiate co2 and sugar

Where does energy transduction and carbon assimilation take place in eukaryotic organisms?

• ET: thylakoid membrane

• CA: stroma (aqueous environment surrounded by inner membrane)

Where does energy transduction and carbon assimilation take place in prokaryotic organisms?

• ET: plasma membrane

• CA: carboxysomes (enzymes)

Pigments

• Molecules that absorb energy

  • Most important: chlorophylls a and b

  • Accessory pigment: carotenoids

• Different pigments absorb different wavelengths

  • Chlorophylls absorb higher wavelengths and reflect green

What happens in the photosystem and ETC?

Photosystems: photons energize the electrons in p680, the light harvesting complex

• p680: pigment composed of chlorophyll a

ETC: give electrons to NADP+ to make NADPH; ATP is made thru chemiosmosis of H+

Parts of light harvesting complexes

Antenna complex: Composed of chlorophyll and other pigments that capture energy from sunlight

Reaction center: a specialized chlorophyll that accepts excited electrons

Photoexcitation

• A photon strikes the antenna complex and the electrons become “excited” (ground state to excited state)

• Excited electrons are unstable and either

  • Return to their ground state and emit energy as heat (only 2% of electrons do this)

  • Resonance Energy Transfer: Transfer the energy to another chlorophyll molecule in the antenna complex

• Electrons eventually reach the reaction center and are given to a specialized chrolophyll that acts as an electron acceptor

  • Electron acceptor gets reduced and the electromagnetic energy —> chemical energy

Photosystem 2

Electron acceptor = P680

• Electrons are transported to plastoquinone (reducing it to plastoquinol) and then to the cytochrome b6/f complex

• Cytochrome passes electrons to plastocyanin and it simultaneously pumps H+ ions from the stroma into the lumen to make a gradient

• Gradient used for ATP synthesis

Photosystem 1

• Electrons are accepted from plastocyanin by P700 and are reexcited (the same electrons from P680)

• Transports electron through many chlorophyll molecules

• Final step: transfer of electrons from ferredoxin to NADP+ to make NADPH

How is ATP synthesized in chloroplast?

• H+ gradient power CF0/F1 ATP synthase (very similar to the F0/F1 synthase in cell respiration) = chemiosmosis

• Photophosphorylation = using light to make ATP

How does each photosystem replace lost electrons?

Photosystem 2: splits 2 water molecules to obtain 4 electrons in a process called oxygenic photosynthesis

Photosystem 1: accepts electron from plastocyanin

Summary of energy transduction

8 photons + 2 H2O + 2 ADP + 2 Pi + 2 NADP+ —> O2 + 2 ATP + 2 NADPH

*4 photons for each photosystem

*2 H2O produces 4 electrons

What happens when the cell requires more ATP than NADPH?

• Ferredoxin will randomly give electrons back to cytochrome b6/f instead of NADP+

• Allows more H+ to be transferred out of the lumen and into the stroma to produce more ATP, resulting in less NADPH production = cyclic photophosphorylation