Photosynthesis Notes

Photosynthesis

Overview

• Photosynthesis completes the carbon cycle by converting $CO_2$ to sugar.
• The light reactions are similar to a reverse mitochondrial ETC.
• Energy from light absorbed by chlorophyll drives photosynthesis.
• A proton gradient across the thylakoid membrane drives ATP synthesis.
• Overall pathway: $CO<em>2 + H</em>2O \rightarrow (CH<em>2O) + O</em>2$

Light Reactions

• Driven by sunlight.
• Take place in the thylakoid membrane.
• High-energy electrons move through protein complexes.
• Protons are transported across the membrane into the thylakoid lumen, building a proton gradient.
• Mobile electron carriers: plastoquinone (similar to ubiquinone), plastocyanin (like cytochrome c).
• The proton gradient is used to synthesize ATP via $CF<em>0CF</em>1$ ATP synthase (analogous to mitochondrial $F<em>0F</em>1$ ATP synthase).

Photosystems (PSI and PSII)

• Light reactions energize electrons using photosystems.
• Proteins in the complexes contain porphyrin rings with $Mg^{2+}$ instead of $Fe^{2+}$ (as in heme groups).
• Chlorophyll molecules capture light energy.
• Antenna complexes capture photons, exciting electrons in chlorophyll.
• Energy is transferred to a reaction center: P700 in PSI and P680 in PSII.
• In PSII, energy strips electrons from $H<em>2O$, releasing protons and $O</em>2$.

Electron Flow

• Electrons pass from PSII to PSI, generating a proton gradient.
• PSI reexcites electrons, which are then used to reduce $NADP^+$ to NADPH.
• ATP and NADPH fuel the dark reactions.

Chemiosmotic Theory

• Explains energy extraction by mitochondria and energy capture by chloroplasts.
• In chloroplasts, most of the $H^+$ gradient exists as a chemical (pH) gradient.

Dark Reactions

• Do not require light; involved in carbon assimilation.
• Key enzyme: ribulose bisphosphate carboxylase (rubisco).
• Reaction: ribulose-1,5-bisphosphate + $CO_2 \rightarrow$ (2) glyceraldehyde-3-P
• Use NADPH rather than NADH.