Metabolism and Energy Transformation in Body Cells - Renata Armenakovna Petrosova 2004

Primary Synthesis of Organic Compounds. Photosynthesis
Light-dependent phase

The first phase of Photosynthesis is called the light-dependent phase because it proceeds only under the action of solar energy. The Reactions of the light phase occur on the thylakoid membranes, which contain the photosynthetic pigment chlorophyll (Fig. 11).

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Fig. 11. General scheme of the processes of the light phase of photosynthesis occurring in the thylakoid: 1 — chlorophyll; 2 — thylakoid membrane; 3 — thylakoid lumen; 4 — Electron Transport Chain Enzymes; 5 — channel with ATP synthase enzyme

Several processes take place during the light phase:

1) Excitation of chlorophyll by light quanta and movement of excited electrons;

2) photolysis of Water under the action of light, leading to The formation of oxygen and hydrogen protons;

3) synthesis of ATP molecules driven by the energy of excited electrons;

4) reduction of the carrier NADP⁺ (nicotinamide adenine dinucleotide phosphate) by hydrogen, forming NADPH · H⁺ (or NADPH · 2H).

Let us examine each of these processes in detail.

Under the action of light, water molecules undergo splitting, known as photolysis. This yields two protons and free oxygen, which is released into the atmosphere as a byproduct. Consequently, the light phase is also referred to as the water photolysis stage:

₊ 2H₂O

2H⁺ + 2OH^-

2OH^-

H₂O + ¹/₂ O₂

+ 2ē

H₂O

2H⁺ + ¹/₂ O₂

+ 2ē. (1)

Water photolysis occurs on the inner surface of the thylakoid membrane, which is impermeable to hydrogen ions. They accumulate within the thylakoid lumen, in the so-called H⁺ reservoir.

Simultaneously, light quanta striking chlorophyll located in the thylakoid membrane raise it to an excited state, transforming it into an electron donor:

хлорофиллсвет (hv)

хлорофилл⁺² + 2ē^[1]. (2)

Two Types of Photosynthetic Systems containing chlorophyll are known. Photosystem I (PS I) consists of a chlorophyll 700 variety, whereas Photosystem II (PS II) consists of a chlorophyll 690 variety. Each photosystem is capable of absorbing light quanta and emitting excited electrons, which enter The electron transport chain of enzymes embedded in the thylakoid membrane. These processes occur synchronously (Fig. 12).

Fig. 12. Scheme of chlorophyll electron transport during ATP synthesis (phosphorylation). In non-cyclic phosphorylation, ATP synthesis is driven by the energy of PS II, while NADPH · 2H synthesis is driven by PS I. In cyclic phosphorylation, only PS I operates, resulting in ATP synthesis

Under the action of light, chlorophyll molecules of PS I and PS II become excited and, upon absorbing light quanta, emit excited electrons. The energy of electrons moving along the enzyme chain from PS II to PS I drives the synthesis of ATP molecules. This process is called phosphorylation and takes place in the presence of the enzyme ATP synthase.

АДФ + Ф_н

АТФ

where Ф_н is inorganic phosphate H₃PO₄.

The PS II electrons, having lost their energy, arrive at PS I, which serves as their final acceptor.

Electrons leaving the chlorophyll of Photosystem I upon Light absorption also enter the electron transport chain. However, their energy is used to combine protons H⁺ with the carrier NADP⁺.

The electrons lost by PS I are replenished by electrons from PS II, whereas electrons released by the water molecule during photolysis replenish those lost by the PS II chlorophyll, thereby reducing it back:

хлорофилл⁺² + 2ē

хлорофилл.

The membrane features specialized proton channels through which hydrogen ions can pass at a specific moment from the thylakoid H⁺ reservoir into the chloroplast stroma. These channels are associated with the enzyme ATP synthase. When the excited electrons of PS I, moving along the membrane, reach the proton channel, it opens, and hydrogen ions rush into it. This process is coupled with ATP synthesis and occurs synchronously.

On the outer side of the thylakoid membrane—that is, in the chloroplast stroma—molecules of the hydrogen carrier NADP⁺ accumulate in an oxidized state. They accept electrons from PS I, which triggers their combination with hydrogen ions H⁺ to form NADPH · 2H:

НАДФ⁺ + 2H⁺ + 2ē

НАДФ · 2Н. (4)

The synthesis of ATP and NADPH · 2H occurs on the thylakoid membranes and is coupled with The transfer of excited electrons along the electron transport chain. Thus, solar energy is converted into the energy of excited electrons and subsequently stored via synthesis in ATP and NADPH · 2H molecules.

Overall equation of the light-dependent reactions:

H₂O + NADP⁺ + 2ADP + 2P_i

NADPH · H⁺ + 2ATP + ¹/₂O₂

.