PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

5. PHOTOSYNTHESIS

Endogenous mechanisms of photosynthetic regulation

Photosynthesis is regulated at the level of Enzymes, membranes, METABOLISM/14.html">Chloroplasts, Cells, Tissues, Organs, and the entire plant.

Regulation at the leaf level

The primary Functions of the leaf are photosynthesis, Transpiration, and the synthesis of Organic compounds.

The laminar shape of the leaf and its internal Structure facilitate aerial Nutrition. The fundamental tissue of the leaf (mesophyll) consists of spongy and palisade parenchyma. This is where photosynthesis occurs and where chloroplasts are localized.

Chloroplasts in most plants are capable of moving within The Cell depending on light intensity. This is driven by The activity of contractile Proteins associated with the chloroplast membrane. Upon the transition to photosynthesis, a series of changes occurs within the chloroplasts. Within a minute of illumination, they change volume and become flatter. Thylakoids and grana shift and condense, triggering the chloroplast Electron Transport Chain (ETC), which is linked to transmembrane proton transfer and the synthesis of ATP, NADPH, and O₂.

Proton transport into the thylakoids leads to the acidification of their internal content to pH 5.0, while simultaneously increasing the stromal pH to 8.0.

The appearance of NADPH, ATP, O₂, Mg²⁺, and The change in pH have both Direct and Indirect effects on photosynthetic reactions, most of which are inactive at pH<7.2 or in the absence of specific substances or ions. The increase in pH also leads to significant CO₂ fixation. CO₂ fixation is primarily regulated by light, which activates the enzymes of The Calvin Cycle. The key enzyme of the Calvin cycle, RuBisCO, is activated not only by light but also by fructose-6-phosphate, and is inhibited by 6-phosphogluconate and fructose-1,6-diphosphate.

An increase in O₂ concentration in the stroma can lead to a decrease in The rate of CO₂ fixation due to the enhancement of Photorespiration.

ATP, NADPH, and assimilates produced in the chloroplasts have a major impact on the cell and its metabolism. Upon illumination, the ATP/ADP ratio rises sharply in the chloroplasts and subsequently in the Cytoplasm. This is mediated by a system of complex transporters. Export occurs in the form of PGA, G3P, and DHAP; fructose diphosphate also enters the cytoplasm. Here, the synthesis of sucrose, starch, and other organic substances takes place. During intensive photosynthesis, starch is also synthesized and accumulated within the chloroplasts.

When leaf tissues are overloaded with assimilates, the intensity of photosynthesis decreases. Furthermore, the accumulation of excess starch in Plastids causes reversible, and eventually irreversible, Changes in the chloroplast stroma.

However, the enrichment of leaf tissues with sugars increases the functional activity of the vascular bundles. Theoretically, There are two ways for assimilates to be transported to the vascular bundles: via the symplast (through plasmodesmata and the cytoplasm of a series of cells) and via the apoplast (through cell walls). In most plants, this transport occurs via the apoplast, where up to 1/5 of the leaf's sugars can accumulate. Cells of the phloem endings are capable of actively absorbing Amino Acids and sugars against a concentration gradient using energy-dependent transporters.

Under The Influence of light, changes occur not only in mesophyll cells but also in the Stomata. In the light, the stomata of most plants are wide open, while they close in the dark (with the exception of Crassulacean plants). This is related to the operation of the photosynthetic apparatus in the guard cells. Upon illumination, an H⁺-pump mechanism is triggered, enhancing K+ uptake and malate synthesis, which results in an increase in intracellular osmotic pressure, causing the stomata to open.

The width of the stomatal aperture also increases when the CO₂ content in the intercellular spaces decreases. This is due to a mechanism that ensures a sufficient supply of substrate (CO₂) for the Dark Phase of photosynthesis.

Increased stomatal opening leads to enhanced transpiration and a more intensive influx of Water and nutrients via the xylem from the roots to the leaves, which is necessary for the normal functioning of mesophyll cells.

Regulation at the whole-plant level

The intensity of photosynthesis depends primarily on environmental conditions, as well as the physiological state of the plant, the stage of ontogenesis, and the demand for photoassimilates. In the plant Organism, There is a system that integrates the complex relationships of photosynthesis with all other plant functions.

Ontogenetic processes ensure the constant existence of attracting zones. In these attracting centers, either the formation and growth of new structures occur, or there is intensive organ-directed synthesis of storage substances. In both cases, the state of the attracting centers determines the magnitude of the demand for photosynthetic products. If external conditions do not limit photosynthesis, the leading role in its determination belongs to these attracting centers.

Phytohormones and inhibitors play an important role in all these processes. Phytohormones act on photosynthesis both remotely (As a result of regulating GROWTH AND DEVELOPMENT processes → epigenesis) and directly (through Changes in membrane states). The Role of cytokinin in chloroplast biogenesis, chlorophyll synthesis, and the synthesis of Calvin cycle enzymes has been proven.