Photosynthesis is the opposite of cellular respiration → takes in carbon dioxide and water, takes out carbohydrates and oxygen

Photosystem 2 Photoexcitation

• photosystem 2 starts the light dependent reactions

• the light antenna complex absorbs a photon and transfers it to p680 chlorophyll molecule which excites an electron

• the electron gets transferred to the acceptor which makes the p680 positively charged

• p680 gets its electron back from the water splitting complex

• each water molecule donates 2 electrons so the process happens twice to produce oxygen

• the acceptor molecules that took p680’s electron passes it to a molecule of plastiquinone (PQ)

Electron Transport Shuttles

• the PQ molecule accepts an electron from photosystem 2 as well as protons (H⁺) from the stroma

• the electron then travels through the thylakoid membrane and donates the electron to the cytochrome complex

• at the same time, it releases the H⁺ into the lumen which increases the proton complex

• the cytochrome complex passes the electron to another shuttle molecule PQ which carries it to photosystem 1

  

Photosystem 1 and Photoexcitation

• when a photon of light strikes photosystem 1, an electron in the p700 molecule is excited

• this excited electron is transferred to another acceptor leaving p700 with a positive charge

• the electron transferred from photosystem 2 is delivered by PQ to neutralize p700

Ferredoxin and NADPH

• the electron from photosystem 1 gets transferred to ferredoxin (an iron sulfur protein)

• oxidation of ferredoxin transfers the electron to NADP+ making NADP

• a second ferredoxin adds another electron along with H⁺ from the stroma to make NADPH

• all of this happens with an enzyme called NADP+ reductase

Proton Gradient

• all this activity has increased the proton concentration in the thylakoid lumen and decreased it in the stroma

  • splitting of water produces protons in lumen

  • PQ and cytochrome complex transport protons from storm to lumen

  • NADPH production removes protons from stroma solution

• this proton gradient can be used for chemiosmosis

ATP Synthase

• identical to ATP synthase in mitochondria

• this time app is produced by photophosphorylation (uses light energy, no oxidation occurred to produce the proton gradient)

Electron Energy

• the boost in energy provided by the photons in sunlight gradually decreases as the electrons move through ETCs of photosystem 1 and 2

Cyclic Electron Transport

• photosystem 1 works independently of photosystem 2 if the electron on ferredoxin is transferred to PQ rather than NADP+ reductase

• this continuously pumps protons into the lumen, thus producing more ATP

• more ATP is needed than NADPH in the Calvin cycle → making this process very useful

  • 9 ATP and 6 NADPH are needed to make 1 G3P (half a glucose)