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7.3.2.1. Photochemical or light phase

Photochemical or light phase of photosynthesis

The photochemical phase light phaseclear 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).

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:
      • 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 or acyclic 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 680 to recover the electron .  
        • The electron emitted by  chlorophyll P 680  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 A 0 , 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 Hagainst 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.