7.3.2.1. Photochemical or light phase

Photochemical or light phase of photosynthesis

The photochemical phase light phase, clear phase or Hill reaction is the first step or phase of photosynthesis, which depends directly on the light or light energy to obtain chemical energy in the form of ATP and NADPH, from the dissociation of water molecules, forming oxygen and hydrogen. The energy created in this phase will be used during the dark phase to continue photosynthesis.

This process is performed in the electron transport chain of the chloroplast in chlorophyll protein-complexes which are grouped into units called photosystems are in thylakoid (internal membranes) of chloroplasts.

The light

Light is formed by electromagnetic waves from the Sun, which arrive in the form of small energy packets (quanta or photons), whose energy depends on the wavelength (λ), or the frequency of the radiation emitted (γ).

The wavelength (λ) of the visible spectrum is between 400 and 700 nanometers (from violet to red and with low energy content).

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Capture of energy from light radiation

In the membranes of the thylakoids of the chloroplasts, there is a set of photosensitive pigments (chlorophylls, carotenes, xanthophylls, ...) associated with proteins, forming the light collector complex (CCL), a system that acts as a solar antenna.

Each light-collecting complex is made up of proteins and hundreds of photosensitive pigments that, when they absorb light energy, channel it to a special molecule, the so-called chlorophyll in the reaction center. Thus, each light collector complex collects the energy of the photons of light and carries it to the photoactive components of the reaction center, which are dimers of chlorophyll a, called P₇₀₀ and P₆₈₀ (depending on the wavelength at which their maximum absorption occurs), and are part of photosystems I and II, respectively.

Photosynthetic pigments

The photosynthetic pigments are located in the thylakoid membranes of the chloroplast, and are responsible for absorbing the light energy that can be transformed into chemical energy. Some examples are chlorophylls (chlorophyll a, b and bacteriochlorophyll), xanthophyll, and carotenoids. Photosynthetic pigments form the functional unit called the photosystem.

Photosynthetic organisms have several types of pigments with different molecular structures. Eukaryotes use chlorophyll a as a pigment responsible for transforming the energy of light into chemical energy. But in addition, photosynthetic cells usually have other photosynthetic pigments, such as plants and green algae, which have chlorophyll b and carotenoids, and diatoms and some protozoa, which have chlorophyll c

Each pigment absorbs light with certain wavelengths:

The chlorophyll b absorb wavelengths corresponding to red violet light, blue, and orange.
The carotenoids absorb the lengths of violet, blue and green wavelengths.

When these photosynthetic pigments capture the photons, they are excited, and when they return to their original state they give up an energy that excites, in turn, the neighboring molecule. Thus, the excitation passes from one molecule to another.

The concept of an excited molecule should not be confused with that of an oxidized molecule. An excited molecule has undergone a change in the distribution of its electrons after receiving energy, but when it returns to its primitive state, it gives off less energy than it absorbed to become excited.

The sunlight that reaches a photosynthetic organism is composed of many wavelengths, so the existence of different types of pigments ensures that photons can excite these pigments and begin photosynthesis.

Fundamental ideas about the photochemical phase of photosynthesis

Photochemical or light phase of photosynthesis

Reducing power (NADPH) and ATP are produced.
The pigments of the thylakoid membrane capture sunlight, light or light energy in order to obtain chemical energy in the form of ATP and NADPH, from the dissociation of water molecules, forming oxygen and hydrogen.
The energy obtained in the photochemical phase will be used during the dark phase to continue photosynthesis.
The photosystems are protein complexes located in thylakoid membranes of chloroplasts where grouped photosynthetic pigments such as chlorophyll, able to capture the light energy from the sun and transform it into chemical energy. Photosystems consist of:
- Antenna complex: they capture light energy, transform it into chemical energy and transmit it to other pigments to the photochemical reaction center.
- The photochemical reaction center, made up of three molecules:
  - A target chlorophyll molecule, which captures the excited electrons coming from the antenna, and yields to the primary electron acceptor .
  - The primary electron acceptor that is reduced with the electron that comes from chlorophyll.
  - The final electron donor, a molecule that gives electrons to the target molecule so that it can recover the lost electron. This electron donor is water, which oxidizes and gives oxygen as a by-product.
  - The transfer of electrons leaves the pigments in the reaction centers with a net positive charge,
    generating an electronic flux. The water, present in the chloroplasts, through an enzymatic process,
    breaks down, generates protons (which will participate in the synthesis of ATP), electrons (which are taken up by
    photosystem II).
There are two photosystems, which have different absorption maxima. Both are linked by a chain of electron transporters giving rise to a Z scheme.
- Photosystem II.
  - It has a maximum absorption at 680 nm.
  - It is located in the thylakoid membranes that pile up to form the grana.
  - It captures a photon, chlorophyll is excited and gives an electron to a chain of transporters, the primary acceptor being pheophytin, and from here to a chain of electron transporters: quinone, plastoquinone, cytochromes b6-f and plastocyanin.
  - As they pass through the b6-f complex, protons are pumped from the stroma into the em thylakoid against the electrochemical gradient. As there is an excess of H+ in the thylakoid space, they pass through ATPsynthetase and ATP (photophosphorylation) occurs.
  - The electron is finally transferred to the chlorophyll in the photosystem I reaction center.
- Photosystem I.
  - Absorption maximum at 700 nm, known as P-700.
  - It captures a photon and the chlorophyll is excited by giving up an electron. In this case there are two possible ways, one non-cyclical and the other cyclical.
Types of photophosphorylation:
- Non-cyclic photophosphorylation
  - Photosystems II and I are involved.
  - Photosystem II
    - It is called a Z-schema.
    - A photon of light is captured by photosystem II, chlorophyll P ₆₈₀ emits an electron, and it oxidizes.
    - The photolysis of water produces protons, oxygen and electrons, so the water will be the electron donor for the chlorophyll P ₆₈₀to recover the electron.
    - The electron emitted by chlorophyll P₆₈₀ is captured by pheophytin, which in turn transfers it to an electron transport chain until it ends up in a final acceptor, plastocyanin.
    - A proton gradient similar to that seen in mitochondria when they produced ATP from ADP is produced in this pathway. This process is called photophosphorylation.
    - The electrons from the photolysis of water will be used to reduce a molecule of NADP⁺and obtain NADPH.
  - Photosystem I
    - The photosystem I can accept the electron from the plastocyanin, that comes from photosystem II, because another photon had allowed the release of an electron from the chlorophyll P₇₀₀. This electron will pass to phylloquinone and ferredoxin, which gives it to the enzyme NADP-reductase that reduces NADP⁺ and NADPH is obtained.
- Cyclic photophosphorylation
  - Without the participation of photosystem II .
  - It does not form NADPH and O₂, and the electron flow only generates ATP.
  - In photosystem I, a photon is captured that allows an electron to be released from the P ₇₀₀ reaction center, yielding it to chlorophyll A₀, then to phylloquinone (Q), and passes it to ferredoxin (Fd). This gives it to the b₆ -f complex, where enough energy is produced to transport the H⁺against the electrochemical gradient, and produce ATP as in non-cyclic photophosphorylation. The b₆ -f complex gives the electrons to plastocyanin (PC), which carries them to the hole left in the P₇₀₀, so that they can be excited again by a photon and restart cyclic photophosphorylation.
  - No photolysis of water occurs and, therefore, NADP⁺ is not reduced or oxygen is released, obtaining only ATP.

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Skip navigation Biology 2nd Baccalaureate Biology 2nd Bac. 1. Bioelements and biomolecules. Water and mineral salts 1.1. Chemical bonds in biology 1.1.1. Intramolecular bonds 1.1.1.1. Covalent bond 1.1.1.2. Ionic bond 1.1.2. Intermolecular bonds 1.1.2.1. Van der Waals forces 1.1.2.2. Hydrogen bonds 1.2. The chemical basis of life: inorganic and organic components 1.2.1. Primary bioelements 1.2.2. Secondary bioelements 1.2.3. Trace elements 1.3. The immediate principles or biomolecules 1.3.1. Water 1.3.1.1. Chemical structure of water 1.3.1.2. Properties and functions of water 1.3.2. Mineral salts 1.3.2.1. Functions of salts in solution 1.3.2.2. Colloidal solutions and dispersions 1.3.2.3. Properties of colloidal dispersions 1.3.2.4. Diffusion, osmosis and dialysis 1.3.2.4.1. Diffusion 1.3.2.4.2. Osmosis 1.3.2.4.3. Dialysis 1.4. Bibliography 2. Glucides 2.1. Glucid concept 2.2. Classification of carbohydrates 2.3. Monosaccharides 2.3.1. Trioses 2.3.1.1. Spatial isomerism or stereoisomerism 2.3.2. Tetroses 2.3.3. Pentoses 2.3.3.1. Monosaccharide cyclization 2.3.4. Hexoses 2.4. The N-glucosidic and O-glucosidic bonds 2.4.1. Substances derived from monosaccharides 2.5. Disaccharides 2.6. Polysaccharides 2.6.1. Homopolysaccharides 2.6.1.1. Reserve homopolysaccharides 2.6.1.2. Structural homopolysaccharides 2.6.2. Heteropolysaccharides 2.6.3. Heterosides 2.7. General functions of glucides 2.8. Bibliography 3. Lipids 3.1. Lipid concept 3.2. Classification of lipids 3.2.1. Fatty acids 3.2.1.1. Saturated and unsaturated fatty acids 3.2.1.2. Chemical properties of fatty acids 3.2.1.3. Physical properties of fatty acids 3.2.1.4. Fatty Acid Functions 3.2.2. Saponifiable lipids or with fatty acids 3.2.2.1. Simple lipids 3.2.2.1.1. Acylglycerides 3.2.2.1.2. Cerides 3.2.2.2. Complex lipids 3.2.2.2.1. Phospholipids 3.2.2.2.2. Glycolipids 3.2.3. Unsaponifiable lipids or without fatty acids 3.3. Lipid functions 3.4. Bibliography 4. Proteins 4.1. Introduction to proteins 4.2. Amino acids 4.2.1. Classification of amino acids 4.2.2. Properties of amino acids 4.3. The peptide bond 4.4. Protein 4.4.1. Protein structure 4.4.1.1. Primary structure of proteins 4.4.1.2. Secondary structure of proteins 4.4.1.3. Tertiary structure of proteins 4.4.1.4. Quaternary structure of proteins 4.4.2. Protein properties 4.4.3. Classification of proteins 4.4.3.1. Holoproteins 4.4.3.2. Heteroproteins 4.4.4. Protein functions 4.5. Enzymes 4.5.1. Structure of enzymes 4.5.2. The active center of enzymes 4.5.3. General Mechanism of Enzyme Catalysis 4.5.4. Regulation of enzyme activity 4.5.5. Allosteric enzymes 4.5.6. Nomenclature and classification of enzymes 4.6. The vitamins 4.6.1. Fat-soluble vitamins 4.6.2. Water soluble vitamins 4.7. Bibliography 5. Nucleotides and nucleic acids 5.1. The chemical composition of nucleic acids 5.1.1. Components of nucleic acids 5.1.2. Concept, types and functions of nucleic acids 5.1.3. Nucleosides 5.1.4. Nucleotides 5.1.4.1. Nucleotide functions 5.1.5. Nucleic acids 5.2. Deoxyribonucleic acid (DNA) 5.2.1. DNA structure 5.2.1.1. Primary structure of DNA 5.2.1.2. Secondary structure of DNA 5.2.1.3. Tertiary structure of DNA 5.2.1.3.1. Tertiary structure of DNA in prokaryotes 5.2.1.3.2. Tertiary structure of DNA in eukaryotes 5.2.1.4. Quaternary structure of DNA 5.2.2. DNA types 5.2.3. Genetic material: chromatin and chromosomes 5.2.3.1. Karyotype and karyogram 5.3. Ribonucleic acid (RNA) 5.3.1. RNA types 5.3.1.1. Transfer RNA 5.3.1.2. Messenger RNA 5.3.1.3. Ribosomal RNA 5.3.1.4. Nucleolar RNA 5.3.1.5. Small nuclear RNA 5.3.2. Functions of ribonucleic acids 5.4. Bibliography 6. Cell morphology 6.1. Cell concept 6.1.1. Cell history 6.1.2. Cell theory 6.2. Levels of cellular organization 6.2.1. General characteristics of cells 6.2.2. Prokaryotic cells 6.2.2.1. Prokaryotic cell morphology 6.2.2.2. Structure of prokaryotes 6.2.2.2.1. Bacterial capsule 6.2.2.2.2. Bacterial wall 6.2.2.2.3. Plasma membrane 6.2.2.2.4. Cytoplasm-Ribosomes-Inclusions 6.2.2.2.5. Bacterial DNA 6.2.2.2.6. Bacterial appendages 6.2.2.3. Bacteria Physiology 6.2.3. Eukaryotic cells 6.2.3.1. Animal and plant cell 6.2.3.1.1. Animal eukaryotic cell 6.2.3.1.2. Plant eukaryotic cell 6.2.3.2. Cell Evolution: Endosymbiont Theory 6.3. Size, shape and quantity 6.4. The plasma membrane 6.4.1. Chemical composition of the membrane 6.4.2. The structure of the membrane. The fluid mosaic model 6.4.3. Functions of the plasma membrane 6.4.4. Transport across the membrane 6.4.4.1. Small molecule transport 6.4.4.2. Macromolecule transport 6.4.5. Plasma membrane differentiations 6.5. The cell wall of plant cells 6.5.1. Chemical composition of the cell wall 6.5.2. Cell wall structure 6.5.3. Cell wall differentiations 6.5.4. Cell wall functions 6.6. Extracellular matrix or glucocalyx 6.7. Cytosol or hyaloplasm 6.8. The cytoskeleton 6.8.1. Microfilaments or actin filaments 6.8.2. Intermediate filaments 6.8.3. Microtubules 6.9. Cell membrane and organelle systems 6.9.1. Membrane systems 6.9.1.1. The endoplasmic reticulum 6.9.1.2. Golgi apparatus 6.9.2. Cell organelles 6.9.2.1. Lysosomes 6.9.2.2. Peroxisomes 6.9.2.3. Vacuoles 6.9.2.4. Mitochondria 6.9.2.4.1. Functions of the mitochondria 6.9.2.5. Plastids 6.9.2.5.1. Chloroplasts 6.9.2.6. Ribosomes 6.9.2.7. Centrosome 6.9.2.8. Cilia and flagella 6.10. Cell motility 6.11. The cell nucleus 6.11.1. Characteristics of the cell nucleus 6.11.2. The interphase nucleus 6.11.3. The mitotic nucleus or nucleus in division 6.12. Bibliography 7. Metabolism 7.1. Metabolism 7.1.1. Energetic Aspects of Cellular Chemical Reactions 7.2. Catabolism 7.2.1. Catabolism and obtaining energy 7.2.2. Carbohydrate catabolism 7.2.2.1. Glycolysis 7.2.2.2. Cellular respiration 7.2.2.2.1. Oxidative decarboxylation 7.2.2.2.2. Krebs cycle 7.2.2.2.3. Electron transport chain 7.2.2.2.4. Energy balance of cellular respiration 7.2.2.3. Fermentations 7.2.2.3.1. Alcoholic or ethyl fermentation 7.2.2.3.2. Lactic fermentation 7.2.2.3.3. Other types of fermentations 7.3. Anabolism 7.3.1. Autotrophic anabolism 7.3.2. Photosynthesis 7.3.2.1. Photochemical or light phase 7.3.2.1.1. Photosystems 7.3.2.1.2. Photophosphorylation 7.3.2.1.2.1. Non cyclic photophosphorylation 7.3.2.1.2.2. Cyclic photophosphorylation 7.3.2.2. Biosynthetic or dark phase 7.3.2.3. Factors influencing photosynthesis 7.3.2.4. Products of photosynthesis 7.3.3. Chemosynthesis 7.4. Bibliography 8. Cell reproduction 8.1. The cell cycle 8.1.1. Interphase 8.1.2. Cellular division 8.2. Cellular division 8.2.1. Mitosis 8.2.1.1. Prophase 8.2.1.2. Metaphase 8.2.1.3. Anaphase 8.2.1.4. Telophase 8.2.1.5. Cytokinesis 8.2.1.6. More on mitosis 8.2.2. Meiosis 8.2.2.1. Meiosis I 8.2.2.2. Meiosis II 8.2.3. Meiosis and sexual reproduction 8.2.4. Meiosis and life cycle 8.2.5. Biological importance of meiosis 8.3. Differences and similarities between mitosis and meiosis 8.4. Bibliography 9. Classical genetics 9.1. Genetics basics 9.2. Mendelian genetics 9.2.1. The method 9.2.2. Mendel's Laws 9.2.2.1. Mendel's first law or principle of uniformity in the first generation 9.2.2.2. Mendel's second law or principle of segregation 9.2.2.2.1. Backcross and test crossover 9.2.2.3. Mendel's third law or principle of independent combination 9.3. Gene interaction 9.3.1. Gene interaction between allelic genes 9.3.2. Gene interaction between non-allelic genes that influence for the same trait 9.4. Chromosomal theory of heredity 9.5. Ligation and recombination 9.6. Complex mendelism 9.6.1. Multiple allelism 9.6.1.1. Lethal genes 9.6.2. Pleiotropy 9.6.3. Expressiveness and genetic penetrance 9.7. Sex-linked inheritance 9.7.1. Y-linked inheritance 9.7.2. X-linked inheritance 9.8. Heredity influenced by sex 9.9. Bibliography 10. Molecular genetics 10.1. DNA as hereditary material 10.2. DNA replication 10.2.1. The need for DNA replication 10.2.2. First hypotheses about DNA duplication 10.2.3. Meselson and Stahl experiment 10.2.4. The sense of growth of the new strands 10.2.5. Mechanism of DNA replication 10.2.5.1. Replication in prokaryotes 10.2.5.2. Replication in eukaryotes 10.2.6. Error correction 10.2.7. Enzymes involved in replication 10.3. Gene concept 10.4. "One gene-one enzyme" theory 10.5. Transcription 10.5.1. Mechanism of transcription 10.5.1.1. Transcription in prokaryotes 10.5.1.2. Eukaryotic transcription 10.6. Translation or protein biosynthesis 10.6.1. The genetic code 10.6.2. Mechanism of translation of RNA to proteins 10.6.2.1. Translation in eukaryotes 10.7. Regulation of gene expression 10.8. Bibliography 11. Genetics and evolution 11.1. Mutations 11.2. Types of mutations 11.2.1. Gene or point mutation 11.2.2. Chromosomal or structural mutation 11.2.3. Mutation at the genomic level 11.3. Causes of mutations 11.4. Evolutionary implications of mutations 11.5. Metabolic implications of mutations 11.6. Bibliography 12. Microbiology and biotechnology 12.1. Microbiology 12.2. Viruses 12.2.1. Viral morphology 12.2.2. Viral physiology 12.2.2.1. Lytic cycle 12.2.2.2. Lysogenic cycle 12.2.2.3. Cycles of viruses of special interest 12.2.2.3.1. Flu virus life cycle 12.2.2.3.2. Life cycle of the AIDS virus 12.3. Other acellular forms 12.3.1. Viroids 12.3.2. Prions 12.4. Biotechnology 12.5. Bibliography 13. Immunology 13.1. Infection concept 13.2. Defense mechanisms against infections 13.2.1. Nonspecific mechanisms 13.2.1.1. Primary barriers: external defenses 13.2.1.2. Secondary barriers: internal defenses 13.2.2. Specific mechanisms 13.3. Immunity and immune system 13.3.1. Components of the immune system 13.3.2. Concept and nature of antigens 13.4. The immune response 13.4.1. Humoral immune response: the antibodies 13.4.1.1. Antibody-producing cells: B lymphocytes 13.4.1.2. Antibodies 13.4.1.2.1. Types of antibodies 13.4.1.2.2. Antigen-antibody reaction 13.4.1.3. Immune memory: primary and secondary responses 13.4.2. Cellular immune response: T lymphocytes 13.5. Types of immunity 13.5.1. Natural immunity 13.5.2. Artificial immunity 13.6. Immune system disorders 13.6.1. Autoimmunity 13.6.2. Hypersensitivity 13.6.3. Immunodeficiency 13.6.4. Transplants and the phenomenon of rejections 13.7. Bibliography « Previous \| Next » 7.3.2.1. Photochemical or light phase Photochemical or light phase of photosynthesis The photochemical phase light phase, clear phase or Hill reaction is the first step or phase of photosynthesis, which depends directly on the light or light energy to obtain chemical energy in the form of ATP and NADPH, from the dissociation of water molecules, forming oxygen and hydrogen. The energy created in this phase will be used during the dark phase to continue photosynthesis. This process is performed in the electron transport chain of the chloroplast in chlorophyll protein-complexes which are grouped into units called photosystems are in thylakoid (internal membranes) of chloroplasts. The light Light is formed by electromagnetic waves from the Sun, which arrive in the form of small energy packets (quanta or photons), whose energy depends on the wavelength (λ), or the frequency of the radiation emitted (γ). The wavelength (λ) of the visible spectrum is between 400 and 700 nanometers (from violet to red and with low energy content). CC BY-SA 3.0, Enlace Capture of energy from light radiation In the membranes of the thylakoids of the chloroplasts, there is a set of photosensitive pigments (chlorophylls, carotenes, xanthophylls, ...) associated with proteins, forming the light collector complex (CCL), a system that acts as a solar antenna. Each light-collecting complex is made up of proteins and hundreds of photosensitive pigments that, when they absorb light energy, channel it to a special molecule, the so-called chlorophyll in the reaction center. Thus, each light collector complex collects the energy of the photons of light and carries it to the photoactive components of the reaction center, which are dimers of chlorophyll a, called P₇₀₀ and P₆₈₀ (depending on the wavelength at which their maximum absorption occurs), and are part of photosystems I and II, respectively. Photosynthetic pigments The photosynthetic pigments are located in the thylakoid membranes of the chloroplast, and are responsible for absorbing the light energy that can be transformed into chemical energy. Some examples are chlorophylls (chlorophyll a, b and bacteriochlorophyll), xanthophyll, and carotenoids. Photosynthetic pigments form the functional unit called the photosystem. Photosynthetic organisms have several types of pigments with different molecular structures. Eukaryotes use chlorophyll a as a pigment responsible for transforming the energy of light into chemical energy. But in addition, photosynthetic cells usually have other photosynthetic pigments, such as plants and green algae, which have chlorophyll b and carotenoids, and diatoms and some protozoa, which have chlorophyll c Each pigment absorbs light with certain wavelengths: The chlorophyll b absorb wavelengths corresponding to red violet light, blue, and orange. The carotenoids absorb the lengths of violet, blue and green wavelengths. When these photosynthetic pigments capture the photons, they are excited, and when they return to their original state they give up an energy that excites, in turn, the neighboring molecule. Thus, the excitation passes from one molecule to another. The concept of an excited molecule should not be confused with that of an oxidized molecule. An excited molecule has undergone a change in the distribution of its electrons after receiving energy, but when it returns to its primitive state, it gives off less energy than it absorbed to become excited. The sunlight that reaches a photosynthetic organism is composed of many wavelengths, so the existence of different types of pigments ensures that photons can excite these pigments and begin photosynthesis. Fundamental ideas about the photochemical phase of photosynthesis Photochemical or light phase of photosynthesis Reducing power (NADPH) and ATP are produced. The pigments of the thylakoid membrane capture sunlight, light or light energy in order to obtain chemical energy in the form of ATP and NADPH, from the dissociation of water molecules, forming oxygen and hydrogen. The energy obtained in the photochemical phase will be used during the dark phase to continue photosynthesis. The photosystems are protein complexes located in thylakoid membranes of chloroplasts where grouped photosynthetic pigments such as chlorophyll, able to capture the light energy from the sun and transform it into chemical energy. Photosystems consist of: Antenna complex: they capture light energy, transform it into chemical energy and transmit it to other pigments to the photochemical reaction center. The photochemical reaction center, made up of three molecules: A target chlorophyll molecule, which captures the excited electrons coming from the antenna, and yields to the primary electron acceptor . The primary electron acceptor that is reduced with the electron that comes from chlorophyll. The final electron donor, a molecule that gives electrons to the target molecule so that it can recover the lost electron. This electron donor is water, which oxidizes and gives oxygen as a by-product. The transfer of electrons leaves the pigments in the reaction centers with a net positive charge, generating an electronic flux. The water, present in the chloroplasts, through an enzymatic process, breaks down, generates protons (which will participate in the synthesis of ATP), electrons (which are taken up by photosystem II). There are two photosystems, which have different absorption maxima. Both are linked by a chain of electron transporters giving rise to a Z scheme. Photosystem II. It has a maximum absorption at 680 nm. It is located in the thylakoid membranes that pile up to form the grana. It captures a photon, chlorophyll is excited and gives an electron to a chain of transporters, the primary acceptor being pheophytin, and from here to a chain of electron transporters: quinone, plastoquinone, cytochromes b6-f and plastocyanin. As they pass through the b6-f complex, protons are pumped from the stroma into the em thylakoid against the electrochemical gradient. As there is an excess of H+ in the thylakoid space, they pass through ATPsynthetase and ATP (photophosphorylation) occurs. The electron is finally transferred to the chlorophyll in the photosystem I reaction center. Photosystem I. Absorption maximum at 700 nm, known as P-700. It captures a photon and the chlorophyll is excited by giving up an electron. In this case there are two possible ways, one non-cyclical and the other cyclical. Types of photophosphorylation: Non-cyclic photophosphorylation Photosystems II and I are involved. Photosystem II It is called a Z-schema. A photon of light is captured by photosystem II, chlorophyll P ₆₈₀ emits an electron, and it oxidizes. The photolysis of water produces protons, oxygen and electrons, so the water will be the electron donor for the chlorophyll P ₆₈₀to recover the electron. The electron emitted by chlorophyll P₆₈₀ is captured by pheophytin, which in turn transfers it to an electron transport chain until it ends up in a final acceptor, plastocyanin. A proton gradient similar to that seen in mitochondria when they produced ATP from ADP is produced in this pathway. This process is called photophosphorylation. The electrons from the photolysis of water will be used to reduce a molecule of NADP⁺and obtain NADPH. Photosystem I The photosystem I can accept the electron from the plastocyanin, that comes from photosystem II, because another photon had allowed the release of an electron from the chlorophyll P₇₀₀. This electron will pass to phylloquinone and ferredoxin, which gives it to the enzyme NADP-reductase that reduces NADP⁺ and NADPH is obtained. Cyclic photophosphorylation Without the participation of photosystem II . It does not form NADPH and O₂, and the electron flow only generates ATP. In photosystem I, a photon is captured that allows an electron to be released from the P ₇₀₀ reaction center, yielding it to chlorophyll A₀, then to phylloquinone (Q), and passes it to ferredoxin (Fd). This gives it to the b₆ -f complex, where enough energy is produced to transport the H⁺against the electrochemical gradient, and produce ATP as in non-cyclic photophosphorylation. The b₆ -f complex gives the electrons to plastocyanin (PC), which carries them to the hole left in the P₇₀₀, so that they can be excited again by a photon and restart cyclic photophosphorylation. No photolysis of water occurs and, therefore, NADP⁺ is not reduced or oxygen is released, obtaining only ATP. Licensed under the Creative Commons Attribution Share Alike License 4.0 « Previous \| Next » These pages are semi-automatically translated from the web Biology 2nd Baccalaureate. Biología de 2º de Bachillerato (biologia-geologia.com). 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Photochemical or light phase of photosynthesis

The light

Capture of energy from light radiation

Photosynthetic pigments