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LIGHT DEPENDENT REACTIONS

LIGHT DEPENDENT REACTIONS

In the thylakoid membrane

ADP + Phosphate à ATP

Produce reduced NADP, ATP and waste oxygen

  • Light energy provided by sun
  • Light absorbed by chlorophyll pigment in chloroplasts
  • Light energy excites electrons in the chlorophyll so they get boosted up to a higher energy level
  • Electrons are accepted and pass along protein carriers in an electron transport chain
  • Energy released from oxidation and reduction of carriers enables ATP to form ADP and a phosphate (photophosphorylation).
  • Electrons and hydrogen ions from the water move along carriers and are accepted by the coenzyme NADP. NADP becomes reduced NADP (NADPH) through photolysis.
  • As well as two hydrogen ions being taken from the water, two electrons are taken to replace those from the chlorophyll. The electron transport chain splits water (photolysis) using energy from sunlight.
  • The oxygen from the water is released through the leaf (waste product).

 

Light dependent reactions include cyclic and non-cyclic photophosphorylation:

Non-cyclic photophosphorylation:

  • Photosystem II absorbs light energy
  • Excites chlorophyll a (p680)
  • Chlorophyll promotes 2e- to higher energy level
  • e- picked up by electron carriers, releasing energy to produce ATP
  • Chlorophyll II p680 and p200 are unstable because of lost electrons
  • e- from photosystem II replace lost ones in photosystem I
  • Photosystem II gets a replacement of e- from photolysis of water
  • Photolysis produces oxygen which is released as a waste gas and 2H+ one of which reduces the NADP+ and NADPH
  • Electrons held by electron carrier also get passed to NADP+

Products are oxygen as waste gas, NADPH and ATP (for light independent reaction)

 

Cyclic-photophosphorylation

No NADPH or oxygen, just ATP

  • Only involves photosystem I
  • Light absorbed by chlorophyll a
  • Chlorophyll a excited and promotes electron to higher energy level
  • e- picked up by electron carrier
  • Enough energy released to photophosphorylate
  • Electrons aren’t passed on to NADP, but are passed back into photosystem I via electron carriers – electrons repeatedly flow through PSI (recycled).