Light dependent reaction

Light dependent

  • light energy converted to chemical energy

  • water molecules split into H+ and O2

    • photolysis

  • reduced NADP and ATP produced from chemiosmosis

  • takes place on the thylakoid membrane

Aim of light dependent stage

  • produce a supply of ATP and electrons that can be used to reduced carbon dioxide

  • the electrons are carried by NADP

    • called reduced NADP when is carrying electrons

  • oxygen is released as waste product

light dependent reaction (clear photo taken 24/09/2025)LDR summarised

Cyclic photophosphorylation

  • uses light energy to produce small amounts of ATP

  • typically occurs when NADP is not available

  • does not produce reduced NADP or oxygen

Cyclic photophosphorylation key stages

  1. light energy is absorbed by pigment and excites electrons in the pigments in PSI only

  2. electrons are lost from the pigments in PSI

  3. electrons are transferred to an electron carrier molecule and passed along an electron transport chain, releasing energy

  4. this energy powers active 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 PSI so no NADP is reduced

Non-cyclic photophosphorylation

  • uses water and light energy

  • produces ATP, reduced NADP, and oxygen as a by-product

Non cyclic photophosphorylation key stages

  1. absorption of light energy by pigments

  2. electron transfer along the electron transport chain

  3. photolysis of water

  4. chemiosmosis

Absorption of light and the electron transport chain

  1. light energy is absorbed by pigments

  2. excites electrons in the pigments in PSI and PSII

  3. electrons are lost from the pigments - photoactivation

  4. electrons are transferred to an electron carrier molecule

  5. electrons are passed along the electron transport chain, releasing energy as they go

  6. the electron from PSII replaces the lost electron in PSI

Photolysis of water

  1. light is used to split water into electrons, protons, and oxygen

  2. the electrons replace those lost from PSII during photoactivation

  3. the protons are used for ATP production and combine with electrons to reduce NADP

  4. oxygen gas is released as a by-product

photolysis of water equation

Chemiosmosis

  1. the energy lost by electrons along the electron transport chain is used to pump protons across the thylakoid membrane into the thylakoid space

  2. produces a proton gradient, where they are in a higher concentration in the thylakoid space

  3. the protons diffuse through ATP synthase into the stroma

  4. this movement powers up ATP synthase to produce ATP from ADP and an inorganic phosphate

  5. NADP takes up protons and electrons from PSI into the stroma and is reduced

  6. reduced NADP is carried into the light-independent reaction

non-cyclic photophosphorylation

light dependent stage (Miss Clark help session)

Pathway of electrons

Non-cyclic

Cyclic

Photosystem involved

1 & 2

1

Photolysis

Y

N

1st electron donor

Water

Photosystem 1

Last electron donor

NADP

Photosystem 1

Products

ATP

Reduced NADP

Oxygen

ATP

Describe how the light energy absorbed by photosystems is converted into chemical energy in the light dependent stage of photosynthesis

  • light is absorbed by PSII in the thylakoid membrane where the light energy is absorbed by pigments

  • this excites electrons and are released along the electron carrier chain

  • the energy lost by electrons along the transport chain is used to pump protons via electron carriers through the thylakoid membrane and into the thylakoid space

  • produces a proton gradient where more protons are in the thylakoid space

  • protons diffuse through ATP synthase in the stroma

  • promotes the reaction of ADP plus an inorganic phosphate to convert into ATP