PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

10. BIOGENESIS OF CELLULAR STRUCTURES AND PLANT CELL ONTOGENESIS

Class="right">"What we call Structure is a slow process of long duration;

what we call function is a rapid process of short duration."

L. von Bertalanffy

Water and nutrients transported to Cells via vascular bundles are utilized for the synthesis of various substances that constitute the cells themselves. NUCLEIC ACIDS and Proteins play a central role in these processes. Proteins form the foundation of all cellular activity, including the Biogenesis of Cellular structures.

Self-assembly and Biogenesis

Self-assembly of cellular structures

The First stage in The formation of intracellular structures involves the processes of METABOLISM/31.html">Transcription and Translation.

The subsequent stages involve the assembly of supramolecular complexes and their transport to specific sites within The Cell.

All stages of protein molecule transformation—starting from the synthesis of its ribonucleic template, through the Formation of primary, secondary, tertiary, and quaternary structures, to its integration with Other Compounds into specific cell components—are governed by self-assembly processes. These processes underpin the formation and biogenesis of cellular structures.

Self-assembly is a process of spontaneous aggregation of homogeneous or heterogeneous molecules, leading to their Organization and the growth of multicomponent complexes.

Self-assembly can be viewed as a crystallization process if the aggregation of molecules does not involve the formation of covalent bonds. When covalent bonds are formed, the process is referred to as polymerization.

Self-assembly mechanisms are based on weak interactions. Primarily, long-range electrostatic forces exert an orienting influence on molecules (at a distance of 0.7 nm). Subsequently, the mutual attraction of molecules is supplemented by Hydrogen Bonds, and finally, at a distance of 0.1 nm, Van der Waals forces (arising between neutral atoms and molecules due to their polarization) and hydrophobic interactions begin to manifest.

The selectivity of the self-assembly mechanism is ensured by the existence of recognition sites on biopolymer molecules that are complementary to specific loci on partner molecules. Complementary structures are defined as steric configurations capable of contacting several atoms or groups of atoms and engaging in pairwise non-covalent interactions. This ensures high affinity and Specificity in the formation of such complexes. Self-assembly of molecules occurs with a decrease in Free energy and is therefore a spontaneous process. Given that it involves only weak forces, it is also a reversible process. The Nature of self-assembly is determined by the Specific features of the polymer's Primary Structure. However, in many cases, additional correction of the aggregation process occurs. Physicochemical environmental conditions, regulatory molecules, and other factors can serve as regulators.

An important element of recognition is the adjustment of The structure of one polymer to the binding site of another, which creates more precise steric correspondence between the interacting regions (RNA polymerase→promoter).

Self-assembly in biological systems is manifested in the bilayer arrangement of Phospholipids in membranes, the complementary sequence of nitrogenous bases in nucleic acids, the interaction between enzyme and substrate, receptor protein and effector, and the assembly of multicomponent enzymatic complexes, among others. For instance, ribulose bisphosphate carboxylase in Chloroplasts assembles from 8 large (catalytic Functions) and 8 small (regulatory functions) subunits.

Multi-enzyme complexes are mounted onto membranes and Cytoskeleton elements via self-assembly. It is hypothesized that a living cell contains complementarily linked blocks of Enzymes and other Biopolymers. These blocks, in turn, are complementarily joined to one another, thereby creating a unified, interconnected system. The aggregation of molecules in the aqueous environment of the Cytoplasm leads to the formation of a liquid-crystalline state of substances within such a system.

The liquid-crystalline state can be considered the fourth state of matter. Liquid crystals are more structured than liquids, yet less so than the same substances in a solid state. Organic substances form liquid-crystalline structures in water. A key feature of liquid crystals is their structural order combined with molecular mobility. Such liquid crystals "respond" to various environmental influences (light, Temperature, pressure, fields), meaning they possess properties characteristic of living cells.

Self-assembly of membranes. Proteins and Lipids that constitute membranes are capable of self-assembly. Hydrophobic Membrane Proteins associate with one another. Structural membrane proteins determine the orientation of other membrane proteins. Membrane Lipids are synthesized in various PARTS OF THE cell and, like Glycolipids and Glycoproteins, are delivered in vesicles to the assembly site, where they associate with each other and with the hydrophobic parts of proteins.

The assembly process occurs in several stages, following THE PRINCIPLE OF mutual "recognition" of constituent parts and lipid-lipid, protein-protein, and Lipid-Protein Interactions. Hydrophobic bonds provide strength to membranes. Furthermore, pre-formed membrane blocks from Golgi vesicles participate in the Formation of the Plasmalemma, integrating into it during the secretion of Cell wall components. The qualitative composition of membranes, and thus their functional characteristics, is determined by integral protein globules, whose primary structure is encoded in the cell's genome.

Self-assembly of Polysomes. The assembly of ribosomal subunits occurs in stages. Initially, ribosomal RNA and proteins are synthesized through transcription and translation, and later, mature ribosomal biopolymers are formed through Processing. Subsequently, with the involvement of 28S and 18S rRNA structures, proteins specific to each subunit are sequentially incorporated. The small ribosomal subunit interacts with initiator tRNA in the presence of GTP, ATP, and protein initiation factors. This entire complex binds to mRNA. Mg²⁺ ions are required for this process. The large ribosomal subunit is the last to attach to the mRNA.

Analogously, other Ribosomes attach to the mRNA, giving rise to polyribosomal complexes. Another type of polyribosome is associated with the granular ER and the outer nuclear membrane.

Self-assembly of microtubules and microfilaments. Microtubules of flagella, the cortical layer of the cytoplasm, and the mitotic apparatus are built according to a single blueprint from the globular protein tubulin. Microtubule assembly requires: acidic pH, the presence of Mg, tubulin, ATP, GTP, and regulatory proteins. The process is sensitive to Ca ions: their excess (0.02 mmol/L and above) promotes the disassembly of microtubules.

Assembly occurs in two stages. First, a seed (Nucleus) is formed, and then the microtubule grows through The addition of subunits. There is a critical concentration of tubulin monomers, above which microtubule assembly is induced.

Microtubules are polarized structures. Their assembly is initiated at microtubule-organizing centers (MTOCs). Such centers include clusters of ER membranes at the spindle poles and the kinetochores of Chromosomes. When one end of a microtubule is anchored at an organizing center, its elongation occurs at the free (distal) end.

Microfilaments. Non-Muscle Actin has been identified in the cytoplasm of plant cells. The assembly of globular G-actin monomers into double-helical fibrillar F-actin is an ATP-dependent process requiring Mg²⁺. Fibrillar actin forms bundles of microfilaments that participate in cytoplasmic streaming. Furthermore, actin can form thin fibrils capable of cross-linking, creating a reticular cytoplasmic structure. These cross-linked structures induce localized, reversible gelation of the cytoplasm and a local increase in Ca²⁺ concentration.

Biogenesis of Organelles

Chloroplast biogenesis. During plant cell ontogeny, chloroplasts develop from Plastids with a poorly developed internal membrane system, known as proplastids, which are found in meristematic cells. The Development of a chloroplast from a proplastid involves the Differentiation of the membrane system (formation of lamellae and grana) through a series of plastid transformations. Concurrently, the synthesis and Spatial Organization of pigments, Light-Harvesting Complexes (LHCs), Photosystem I and II proteins, and other components take place. The remodeling of the membrane system in chloroplasts and other plastids is accompanied by the constant renewal of Membrane Structure, the degradation of lipids and proteins, and the incorporation of new constituents into the membranes.

Chloroplast biogenesis occurs only in the presence of light. This is clearly observed during The conversion of etioplasts, formed in the dark, into chloroplasts. In the light, the protochlorophyllide of etioplasts is rapidly converted into chlorophyll a. Simultaneously, the synthesis of mRNA, rRNA, structural proteins, and other compounds occurs. This is followed by a phase of slow chlorophyll accumulation lasting about two hours, after which The rate of synthesis increases significantly. By this time, the lamellar STRUCTURE OF THE chloroplast is already formed, but grana are not yet present. The Protein Complexes of the Photosystems act as the organizers of the grana.

The number of chloroplasts increases during cell growth through the division of proplastids or existing chloroplasts during the developmental phase. During division, the lamellar system is constricted by a partition across the organelle. Occasionally, chloroplast budding is observed. Chloroplast division occurs every 6–20 hours. It is regulated by red light (660 nm), inhibited by far-red light (730 nm), and also by low temperatures.

Mitochondrial biogenesis. Possessing their own genetic system and the ability of Mitochondrial DNA to replicate, Mitochondria are capable of independent reproduction. Consequently, mitochondria in the cell arise from pre-existing mitochondria and, possibly, from promitochondria. In meristematic cells, mitochondria undergoing fission (constriction) are frequently observed. As a meristematic cell transitions to elongation growth, the number of mitochondria increases 3–8 fold, accompanied by structural changes.

The growth of mitochondrial membranes during functional demand or following division occurs through accretion. The process of mitochondrial membrane biogenesis is not yet fully understood.

Most proteins that constitute mitochondria are synthesized in the cytoplasm. Only 5–15% of proteins are products of mitochondrial polysome translation, and these proteins are exclusively incorporated into The inner mitochondrial membrane. Proteins of mitochondrial origin are hydrophobic Polypeptides, and their self-assembly in the presence of phospholipids serves as the foundation for the formation of the inner membrane.

The half-life of mitochondria depends on the coordinated activity of The Nucleus, the cytoplasm, and the mitochondria themselves, typically ranging from 5 to 10 days in various organisms; notably, the outer membrane is renewed significantly faster than the inner one.

Membrane biogenesis. Cell membranes are in a state of constant renewal. The genetic relationship between cellular membrane components highlights the leading role of the rough ER in the biogenesis of cell membranes. The ER is the primary site for the synthesis of cellular membrane proteins and lipids. The final stages of glycerolipid and membrane phospholipid synthesis (which are critical for the assembly of Mitochondrial and Chloroplast membranes), as well as sterol Biosynthesis, the synthesis of all saturated Fatty acids, and the systems for desaturating fatty acids, are localized in the ER membranes. Polyunsaturated fatty acids characteristic of plant membranes (linoleic, linolenic, and arachidonic acids) are synthesized here.

The membranes of vacuoles, Microbodies, and spherosomes, and possibly the outer membranes of plastids and mitochondria, are derivatives of the reticulum membranes. The reticulum is directly connected to the nuclear envelope. Through the Golgi apparatus membrane system, it participates in the Synthesis of the plasmalemma.

The transition of membranes into one another is termed membrane "flow" (or "flux"). Currently, the process of membrane flow is viewed as a component of a broader concept: the endomembrane system theory.

The multi-step processes of cellular structure biogenesis discussed above can be schematically represented as follows:

DNA → RNA → proteins → self-assembly of supramolecular functionally active units → functional activity of metabolic cycles and cellular organelles.