Light Dependent Reaction

Cyclic photophosphorylation

  • Photophosphorylation is the process by which ATP is formed using light energy

  • Cyclic photophosphorylation uses light energy to produce small amounts of ATP, and typically occurs when NADP is not available

  • It does not produce reduced NADP or oxygen.

Key stages:

  1. Light energy is absorbed by pigments and excites electrons in the pigments in PS1

  2. Electrons are lost from the pigments in PS1

  3. Elextrons are transferred to an electron carrier molecule and passed along an electron transport chain (ETC) releasing energy

  4. This energy powers active Proton transport across the thylakoid membrane from the stroma into the thylakoid space

  5. ATP is produced as protons flow back into the stroma through ATP synthase

  6. Electrons are returned to PS1 so no NADP is reduced

Non-Cylic Photophosphorylation

  • Non-Cylic Photophosphorylation uses water and light energy

  • It produces ATP, reduced NADP and oxygen as by product

Key stages

Absorption of Light energy by pigments

  • Light negery is absorbed by pigments (e.g chlorophyll a)

  • This excites electrons in the pigments in photosystem 1 (PS1) and photosystem 2 (PS2) reaction centers

  • Electrons are lost from the pigments (photoactivation)

  • Electrons are transferred to an electron carrier molecule

  • Electrons are passed along the ETC realseing energy as they go

  • The electron from PS2 replaces the lost electron from PS1

Electron transfer along the electron transport chain

Photolysis of water

  • Light is used to split water into electrons, protons, and oxygen: 2H2O → 4H+ + 4e- + O2

  • The electron replace those lost from PS2 during photo activation

  • The protons are used from ATP production and combine withe lectorns to reduce NADP

  • Oxygen gas is released as a by-product

Chemiosmosis

  • Generates ATP and reduced NADP

  1. The energy lost by electrons along the electron transport chain