Fundamentals of Molecular Biology. Part 1: Molecular Cell Biology - A. N. Oagurtsov 2011

Photosynthetic Systems
Stages of Photosynthesis

Photosynthesis takes place in the METABOLISM/14.html">Chloroplasts of plants. The main products of photosynthesis are:

✵ sucrose - the disaccharide glucose{a1→2}fructose,

✵ starch - [poly{a1→4glucose}] - an insoluble polymer of glucose monomers.

Starch is synthesized in plant leaves and stored within the chloroplasts (see Figure 21 and Section 4.3).

Sucrose is synthesized in the Cytosol of leaf Cells from tricarbon precursors, which in turn are produced in the chloroplasts.

Sucrose is transported to non-photosynthetic (non-green) plant Tissues—such as roots or fruits—where it serves as an energy source for ATP synthesis.

In plants, eukaryotic Algae, and certain photosynthetic Bacteria (e.g., cyanobacteria or prochlorophytes), molecular oxygen is also produced during photosynthesis. The overall reaction for oxygenic photosynthesis

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is simply the reverse of carbohydrate oxidation to CO₂ and H₂O.

Green and purple bacteria also perform photosynthesis, but they do not produce oxygen.

The photosynthetic light-absorption and ATP-synthesis reactions take place on the thylakoid membranes located within the chloroplast stroma (Figure 21). The two membranes enclosing the chloroplast—a permeable outer membrane and an inner membrane—do not participate in photosynthesis.

The process of photosynthesis can be divided into four stages, each occurring in a specific region of the chloroplast:

1) Light absorption,

2) electron transfer, which results in the generation of O₂ from H₂O, the reduction of NADP⁺ to NADPH, and the establishment of a proton-motive force,

3) ATP synthesis,

4) The conversion of CO₂ into CARBOHYDRATES, commonly referred to as carbon fixation.

All four stages of photosynthesis are coupled, interconnected, and interdependent to ensure that the plant produces precisely The amount of carbohydrates it needs. All reactions in stages 1–3 are catalyzed by Proteins of the thylakoid membrane.

The Enzymes that convert CO₂ into carbohydrates and subsequently into starch are Water-soluble and located in the chloroplast stroma.

The enzymes responsible for synthesizing sucrose from tricarbon precursors are located in The Cell cytosol.

9.1.1. Light absorption. Photosynthesis begins with the absorption of light by chlorophyll molecules associated with proteins in the thylakoid membrane.

Chlorophyll molecules resemble Hemes in that they contain a porphyrin ring (a cyclic tetrapyrrole) (Figure 164) attached to a long hydrocarbon tail (Figure 174).

Unlike hemes, chlorophyll contains a magnesium ion, Mg²⁺, instead of an iron ion, along with an additional five-carbon ring (Figure 174(3)).

The energy of the absorbed light is used to remove an electron from a donor (which, in green plants, is a water molecule) and generate molecular oxygen

and subsequently to transfer the electron to the primary electron acceptor, a quinone designated as Q, which is analogous to CoQ. The quinone found in chloroplast thylakoids is called plastoquinone.

Plastoquinone carries the electron away from positively charged chlorophyll a—a process known as charge Separation.

Figure 174 - Structure OF THE chlorophyll a molecule: 1 - CH₃ group, replaced by an -HC=O group in chlorophyll b; 2 - porphyrin ring; 3 - additional five-membered ring compared to heme. The hydrophobic phytol tail ensures the embedding of chlorophyll into the hydrophobic protein core

Following this, chlorophyll is reduced by an electron that (in plant chloroplasts) is "extracted" from a water molecule, ultimately resulting in The formation of molecular oxygen.

9.1.2. Electron Transfer and Generation of Proton-Motive Force.

Electrons are transferred from the primary electron acceptor—plastoquinone—along a chain of electron carriers to the final electron acceptor, typically nicotinamide adenine dinucleotide phosphate (in its oxidized form, NADP⁺), reducing it to NADPH (Figure 175).

Electron Transport Across the thylakoid membrane is coupled with the translocation of protons from the stroma into the thylakoid lumen. As a result, a pH gradient is established across the thylakoid membrane (pH_lumen < pH_stroma).

This process is analogous to the generation of a proton-motive force across The inner mitochondrial membrane driven by Electron transport along the Respiratory Chain (Figure 144).

The overall reaction for stages 1 and 2 is as follows:

Figure 175 - Oxidized and reduced forms of the nicotinamide adenine dinucleotide phosphate (NADP) molecule: a - oxidized form NADP⁺, b - reduced form NADPH

9.1.3. ATP Synthesis. The flow of protons from the thylakoid lumen into the stroma, driven by the proton concentration gradient, passes through F₀F₁-complexes (ATP synthases). Here, similarly to Mitochondria, proton movement is coupled with the synthesis of ATP from ADP and P_i. The Mechanism of ATP synthesis in chloroplasts is identical to that in mitochondrial membranes and The Plasma Membrane of bacteria (Figures 170 and 171).

9.1.4. Carbon Fixation. The energy and electrons stored in ATP and NADPH molecules during the second and third stages of photosynthesis are utilized to synthesize six-carbon sugar polymers from CO₂ and H₂O molecules.

The overall reaction is written as

The reactions responsible for the formation of ATP and NADPH directly depend on light absorption. Therefore, stages 1–3 are referred to as the light reactions of photosynthesis.

The reactions taking place in stage 4 do not directly depend on light absorption; hence, they are called dark reactions.

However, it should be kept in mind that the reactions of stage 4 are not actually driven by darkness; under physiological conditions, they occur primarily during the daytime under natural illumination.

The reactions of stage 4 take place in the chloroplast stroma and the cell cytosol.