13.1.2
Light-Dependent Reactions
Non-Cyclic Photophosphorylation
Non-Cyclic Photophosphorylation
There are two types of photophosphorylation that take place during the light-dependent reaction: cyclic and non-cyclic photophosphorylation. Here, we will look at non-cyclic photophosphorylation:


Light absorption by PSII
Light absorption by PSII
- The light-dependent reactions occur in the thylakoid membranes of chloroplasts.
- Embedded in the thylakoid membranes are two photosystems (PSI and PSII) which are complexes of pigments that capture light energy (light harvesting systems).
- Non-cyclic photophosphorylation involves both PSII and PSI.
- The process is initiated when light energy is absorbed by PSII.
- Light energy excites electrons in the chlorophyll of PSII and the electrons are moved to a higher energy level (they are high-energy electrons).
- This is photoionisation of chlorophyll.


Electron transport chain
Electron transport chain
- High-energy electrons are released from the chlorophyll and transferred to an electron carrier.
- Electron carriers are proteins located in the thylakoid membranes. They are tightly linked to PSI and PSII and transfer electrons.
- When high-energy electrons are released from PSII they are transferred along a chain of electron carriers to PSI.
- The series of electron carriers is called the electron transport chain (ETC).


The proton gradient
The proton gradient
- As the electrons move down the ETC, they lose energy.
- This energy pumps protons from the stroma into the thylakoids. The protons are being transported against their concentration gradient and this requires energy.
- As protons build up inside the thylakoids, a proton gradient forms across the thylakoid membrane because the concentration of protons inside the thylakoids is greater than in the stroma.


Chemiosmosis
Chemiosmosis
- The protons diffuse down the concentration gradient across the thylakoid membrane through the ATP synthase enzyme.
- As protons diffuse through the ATP synthase, energy is released.
- This energy converts ADP and inorganic phosphate to ATP.
- This process is called chemiosmosis.


Reduced NADP
Reduced NADP
- Non-cyclic photophosphorylation produces reduced NADP and ATP.
- When light energy is absorbed by PSI, high-energy electrons are released.
- The electrons are transferred directly to NADP. They are not passed along the ETC.
- The electrons react with a proton in the stroma to produce reduced NADP.


Photolysis
Photolysis
- An important feature of non-cyclic photophosphorylation is that the electrons are NOT recycled through the photosystems.
- Instead, the electrons are replaced in PSII by photolysis.
- In this process, light energy splits water into protons, electrons and oxygen.
- The electrons can then replace those released when PSII absorbs light.
Cyclic Photophosphorylation
Cyclic Photophosphorylation
There are two types of photophosphorylation that take place during the light-dependent reaction: cyclic and non-cyclic photophosphorylation. Here, we will look at cyclic photophosphorylation.


Absorption of light by PSI
Absorption of light by PSI
- Cyclic photophosphorylation only involves PSI.
- The process is initiated when light energy is absorbed by PSI.
- Light energy excites electrons in the chlorophyll of PSI and the electrons are moved to a higher energy level (they are high-energy electrons).
- This is photoionisation of chlorophyll.


Electron transport chain
Electron transport chain
- High-energy electrons are released from the chlorophyll and transferred to an electron carrier.
- The electrons are transferred along a chain of electron carriers in the electron transport chain (ETC).
- Unlike in non-cyclic photophosphorylation, the electrons are not transferred from PSII to PSI.
- Instead the electrons cycle continuously through the electron carriers to PSI.


The proton gradient
The proton gradient
- As the electrons move down the ETC, they lose energy.
- This energy pumps protons from the stroma into the thylakoids.
- This is the same as in non-cyclic photophosphorylation.
- As protons build up inside the thylakoids, a proton gradient forms across the thylakoid membrane.


Chemiosmosis
Chemiosmosis
- The protons diffuse down the concentration gradient across the thylakoid membrane through the ATP synthase enzyme.
- As protons diffuse through the ATP synthase, energy is released.
- This energy converts ADP and inorganic phosphate to ATP.
- This process is called chemiosmosis.


Comparing cyclic and non-cyclic
Comparing cyclic and non-cyclic
- In cyclic photophosphorylation:
- ATP is produced.
- No reduced NADP is produced.
- Electrons are continuously recycled.
- Photolysis does not take place.
- In non-cyclic photophosphorylation:
- ATP and reduced NADP are produced.
- Electrons in PSII are replaced by photolysis.
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
Jump to other topics
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
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