LIGHT PHASE OF PHOTOSYNTHESIS: MECHANISM AND PRODUCTS - BIOLOGY - 2024

The light phase of photosynthesis is that part of the photosynthetic process that requires the presence of light. Thus, light initiates reactions that result in the transformation of part of the light energy into chemical energy.

Biochemical reactions occur in the chloroplast thylakoids, where photosynthetic pigments are found that are excited by light. These are chlorophyll a, chlorophyll b, and carotenoids.

Light phase and dark phase. Maulucioni, from Wikimedia Commons

Several elements are required for light-dependent reactions to occur. A light source within the visible spectrum is necessary. Likewise, the presence of water is needed.

The final product of the light phase of photosynthesis is the formation of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). These molecules are used as a source of energy for the fixation of CO ₂ in the dark phase. Likewise, during this phase, O ₂ is released, a product of the breakdown of the H ₂ O molecule.

Requirements

In order for light-dependent reactions in photosynthesis to occur, an understanding of the properties of light is required. Likewise, it is necessary to know the structure of the pigments involved.

The light

Light has both wave and particle properties. Energy comes to Earth from the sun in the form of waves of different lengths, known as the electromagnetic spectrum.

About 40% of the light that reaches the planet is visible light. This is found in wavelengths between 380-760 nm. It includes all the colors of the rainbow, each with a characteristic wavelength.

The most efficient wavelengths for photosynthesis are those from violet to blue (380-470 nm) and from red-orange to red (650-780 nm).

Light also has particle properties. These particles are called photons and they are associated with a specific wavelength. The energy of each photon is inversely proportional to its wavelength. The shorter the wavelength, the higher the energy.

When a molecule absorbs a photon of light energy, one of its electrons is energized. The electron can leave the atom and be received by an acceptor molecule. This process occurs in the light phase of photosynthesis.

Pigments

In the thylakoid membrane (structure of the chloroplast) there are various pigments with the ability to absorb visible light. Different pigments absorb different wavelengths. These pigments are chlorophyll, carotenoids, and phycobilins.

Carotenoids give the yellow and orange colors present in plants. Phycobilins are found in cyanobacteria and red algae.

Chlorophyll is considered the main photosynthetic pigment. This molecule has a long hydrophobic hydrocarbon tail, which keeps it attached to the thylakoid membrane. In addition, it has a porphyrin ring that contains a magnesium atom. Light energy is absorbed in this ring.

There are different types of chlorophyll. Chlorophyll a is the pigment that most directly intervenes in light reactions. Chlorophyll b absorbs light at a different wavelength and transfers this energy to chlorophyll a.

In the chloroplast, approximately three times more chlorophyll a is found than chlorophyll b.

Mechanism

-Photosystems

Chlorophyll molecules and the other pigments are organized within the thylakoid into photosynthetic units.

Each photosynthetic unit is made up of 200-300 chlorophyll a molecules, small amounts of chlorophyll b, carotenoids, and proteins. There is an area called the reaction center, which is the site that uses light energy.

Image: Light phase of photosynthesis. Author: Somepics.

The other pigments present are called antenna complexes. They have the function of capturing and passing light to the reaction center.

There are two types of photosynthetic units, called photosystems. They differ in that their reaction centers are associated with different proteins. They cause a slight shift in their absorption spectra.

In photosystem I, the chlorophyll a associated with the reaction center has an absorption peak of 700 nm (P ₇₀₀). In photosystem II the absorption peak occurs at 680 nm (P ₆₈₀).

-Photolysis

During this process the breakdown of the water molecule occurs. Photosystem II participates. A photon of light strikes the P ₆₈₀ molecule and drives an electron to a higher energy level.

The excited electrons are received by a molecule of pheophytin, which is an intermediate acceptor. Subsequently, they cross the thylakoid membrane where they are accepted by a plastoquinone molecule. The electrons are finally transferred to the P ₇₀₀ of photosystem I.

The electrons that were given up by P ₆₈₀ are replaced by others from the water. A manganese-containing protein (protein Z) is required to break down the water molecule.

When H ₂ O is broken, two protons (H ⁺) and oxygen are released. Two molecules of water are required to be cleaved for one molecule of O _{2 to be released}.

-Photophosphorylation

There are two types of photophosphorylation, depending on the direction of the electron flow.

Non-cyclic photophosphorylation

Both photosystem I and II are involved in it. It is called non-cyclical because the flow of electrons goes in only one direction.

When the excitation of the chlorophyll molecules occurs, the electrons move through an electron transport chain.

It begins in photosystem I when a photon of light is absorbed by a P ₇₀₀ molecule. The excited electron is transferred to a primary acceptor (Fe-S) containing iron and sulfide.

Then it goes on to a molecule of ferredoxin. Subsequently, the electron goes to a transport molecule (FAD). This gives it to a molecule of NADP ⁺ that reduces it to NADPH.

The electrons transferred by photosystem II in photolysis will replace those transferred by P ₇₀₀. This occurs through a transport chain made up of iron-containing pigments (cytochromes). In addition, plastocyanins (proteins that present copper) are involved.

During this process, both NADPH and ATP molecules are produced. For the formation of ATP, the enzyme ATPsyntetase intervenes.

Cyclic photophosphorylation

It occurs only in photosystem I. When the molecules of the P ₇₀₀ reaction center are excited, the electrons are received by a P ₄₃₀ molecule.

Subsequently, the electrons are incorporated into the transport chain between the two photosystems. In the process ATP molecules are produced. Unlike non-cyclic photophosphorylation, NADPH is not produced and O ₂ is not released.

At the end of the electron transport process, they return to the reaction center of photosystem I. For this reason, it is called cyclic photophosphorylation.

Final products

At the end of the light phase, O ₂ is released into the environment as a by-product of photolysis. This oxygen goes out into the atmosphere and is used in the respiration of aerobic organisms.

Another end product of the light phase is NADPH, a coenzyme (part of a non-protein enzyme) that will participate in the fixation of CO ₂ during the Calvin cycle (dark phase of photosynthesis).

ATP is a nucleotide used to obtain the necessary energy required in the metabolic processes of living beings. This is consumed in the synthesis of glucose.

References

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