Light Dependent Reactions
Light Dependent Reactions
The light dependent reaction of photosynthesis involves the transfer of sunlight energy into two things:
The production of ATP
Reduction of coenzyme NADP to NADPH
NADP – Nicotinamide adenine dinucleotide phosphate
NADPH – Reduced NADP
The energy from sunlight is used to excite an electron, within a primary pigment, to a higher energy level. The excited electron is then donated to an electron acceptor leaving the chlorophyll molecule oxidised and reducing the acceptor.
Chlorophyll a ----------------chlorophyll a+ (oxidised state) + e-
The electron is then passed along a series of electron acceptor, each with a lower energy level than the one before as the electron is passed along each carrier becomes sequentially reduced and then re-oxidised (loses electron) releasing enough energy to pump H+ ions from the stroma into the thylakoid lumen in order to create an electrochemical gradient which is used to convert ADP + Phosphate to ATP. This phosphorylation is called photophosphorylation because the energy originally came from light. (Read ATP synthesis in the chloroplast box 2.3 on hand out)
Non-cyclic Photophosphorylation (diagram above ATP synthesis in the chloroplast box 2.3 on hand out)
1- Light causes and electron to be lost from P680 and P700 and then accepted by and electron acceptor.
2- Both primary pigments are now in the oxidised form and must gain an electron to replace the lost one if photosynthesis is to continue.
3- Photolysis of water- an enzyme associated with photosystem II( PS II) catalyses the splitting of water 2H2O O2 + 4H+ + 4e-
This enzyme is located on the lumen side of the thylakoid membrane. The electrons produced replace the ones lost from P680.
4- The electron lost from PS II is passed from its electron acceptor down an electron transport chain to P700 of PS I replacing its lost electron. The transfer of electrons down the electron transport chain is coupled to the synthesis of ATP by
The light dependent reaction of photosynthesis involves the transfer of sunlight energy into two things:
The production of ATP
Reduction of coenzyme NADP to NADPH
NADP – Nicotinamide adenine dinucleotide phosphate
NADPH – Reduced NADP
The energy from sunlight is used to excite an electron, within a primary pigment, to a higher energy level. The excited electron is then donated to an electron acceptor leaving the chlorophyll molecule oxidised and reducing the acceptor.
Chlorophyll a ----------------chlorophyll a+ (oxidised state) + e-
The electron is then passed along a series of electron acceptor, each with a lower energy level than the one before as the electron is passed along each carrier becomes sequentially reduced and then re-oxidised (loses electron) releasing enough energy to pump H+ ions from the stroma into the thylakoid lumen in order to create an electrochemical gradient which is used to convert ADP + Phosphate to ATP. This phosphorylation is called photophosphorylation because the energy originally came from light. (Read ATP synthesis in the chloroplast box 2.3 on hand out)
Non-cyclic Photophosphorylation (diagram above ATP synthesis in the chloroplast box 2.3 on hand out)
1- Light causes and electron to be lost from P680 and P700 and then accepted by and electron acceptor.
2- Both primary pigments are now in the oxidised form and must gain an electron to replace the lost one if photosynthesis is to continue.
3- Photolysis of water- an enzyme associated with photosystem II( PS II) catalyses the splitting of water 2H2O O2 + 4H+ + 4e-
This enzyme is located on the lumen side of the thylakoid membrane. The electrons produced replace the ones lost from P680.
4- The electron lost from PS II is passed from its electron acceptor down an electron transport chain to P700 of PS I replacing its lost electron. The transfer of electrons down the electron transport chain is coupled to the synthesis of ATP by