PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

2. STRUCTURE AND FUNCTIONS OF THE PLANT CELL

Subcellular Structures of Plant Cells

Plastids

These are double-membrane-bound Organelles, typically spherical or oval in shape, containing an internal system of membranes of varying complexity. Plastids are sites for the synthesis and storage of organic substances. They can be colorless (proplastids, leucoplasts, etioplasts) or pigmented (METABOLISM/14.html">Chloroplasts, chromoplasts).

Proplastids are found in Meristems. These are small (up to 1 µm), undifferentiated plastids of various shapes, whose internal membrane exhibits only slight invaginations. If this Structure is retained in mature Cells, it is referred to as a leucoplast. Leucoplasts are most commonly found in the cells of ROOT apical meristems, tuber cells, rhizomes, and leaf epidermis. Being small in size, they contain isolated thylakoids and perform diverse Functions. They serve as storage sites for reserve nutrients: starch (amyloplasts), protein (proteinoplasts), fats (oleoplasts), etc.

Etioplasts form when green plants are grown in the dark. Under these conditions, a prolamellar body develops within the proplastids, transforming them into etioplasts. Etioplasts can be viewed as a developmental stage of chloroplasts. They form in the primary leaves or cotyledons of seedlings before they emerge from the soil into the light. Upon exposure to light, the internal Membrane Structure of etioplasts undergoes complex remodeling, converting them into chloroplasts. In some gymnosperms, this transformation can occur even in the dark. Photosynthesis takes place in chloroplasts. They are primarily found in leaf parenchyma cells and are absent in meristems. These are oval bodies 5-10 µm in length and 2-3 µm in diameter. A single Cell typically contains 15-20 chloroplasts or more (up to several hundred). They possess both outer and inner membranes. The inner membrane encloses a homogeneous internal medium (stroma) and forms dense, flat, wide invaginations known as stroma thylakoids, as well as small, dense, disc-like structures called grana thylakoids. Several such thylakoids stacked on top of one another form a stack called a granum. A chloroplast may contain 40-60 grana, and typically 5 to 20 thylakoids per granum. Perforations have been identified in the grana, through which the grana membranes connect, and the intrathylakoid space communicates via narrow tubules known as frets.

The thylakoid system is a single compartment separated from the stroma by a thylakoid membrane system. There are three important chloroplast compartments:

✵ the intermembrane space between the outer and inner membranes;

✵ the stromal compartment;

✵ the intrathylakoid space.

The outer membrane is permeable to metabolites. This permeability is facilitated by specific Membrane Proteins called porins, which form pores that allow the free passage of substances with a Molecular Weight of up to 10 kDa.

The stromal compartment Supports the essential metabolic cycles of photosynthesis. Enzyme systems for the synthesis and conversion of CARBOHYDRATES are localized here. The intrathylakoid space generates a proton gradient during the light reactions of photosynthesis. These reactions involve chlorophylls, carotenoids, and components of redox and energy-storage chains localized within the thylakoid membranes.

Chloroplasts possess their own protein-synthesizing system, including DNA, rRNA, and 70S Ribosomes. Chloroplast DNA is inherited maternally (via proplastids). Circular chloroplast DNA differs from nuclear DNA both in the information it carries and in its physicochemical properties. Depending on the plant species, the plastid genome (plastome) accounts for only 0.001-0.1% of the nuclear genome. However, The plant cell contains many copies of this genome, allowing plastid DNA content to reach 5-10% of the total DNA pool in The Cell.

The chloroplast is the primary cellular organelle responsible for converting solar energy and synthesizing the energy and structural resources of the biosphere through phototrophic Nutrition. Chloroplasts are also sites for the synthesis of pigments, chloroplast Cytochromes, Fatty acids (palmitic, oleic, linoleic), and galactolipids.

During cell growth, the number of chloroplasts increases through division (either by forming a partition across the organelle or via budding). Chromoplasts are non-photosynthetic, pigmented plastids that contain primarily red, orange, and yellow pigments. They typically develop from chloroplasts, have similar dimensions and shapes, and are quite similar in structure. However, they lack a photosynthetic membrane system. Based on their internal structure, chromoplasts are divided into five types: globular, membranous, tubular, reticulotubular, and crystalline. The physiological function of chromoplasts has not been studied in detail. Their bright coloration likely attracts insects for pollination and seed dispersal.

Circular DNA is present in the stroma of all plastids.

Microbodies

Spherical organelles 0.2-1.5 µm in diameter, bounded by an elementary membrane and containing a granular matrix. They sometimes contain a protein crystalloid. Various types of microbodies with specific functions are found in cells.

Peroxisomes are found in leaves, where they are associated with chloroplasts and Mitochondria. They contain a variety of oxidative Enzymes and are considered compartments for the detoxification of H₂O₂. Peroxisomes participate in specific stages of Photorespiration. They facilitate The oxidation of glycolic acid, which enters from the chloroplasts, into Glycine, which is then converted into Serine in the mitochondria.

Glyoxysomes are present in endosperm cells and cotyledons only during the germination of seeds containing reserve fats. Once the cotyledons emerge above the soil surface, fat utilization ceases, glyoxysomes disappear, and peroxisomes appear. This process is light-regulated. Glyoxysomes contain enzymes necessary for converting fatty acids into sugars: β-oxidation systems for Fatty Acids and The Glyoxylate cycle. Thanks to these, acetyl-CoA is not fully oxidized but is instead used for carbohydrate synthesis. During the operation of peroxisomal and glyoxysomal enzyme systems, hydrogen peroxide is produced, which is then degraded by catalase.

Sphaerosomes (oleosomes) are spherical structures 0.5 µm in diameter that strongly refract light. They contain Lipids and the enzymes lipase and esterase. During the germination of oilseeds, sphaerosomes function in complex with glyoxysomes. They participate in Gluconeogenesis processes.

Vacuolar system

A vacuole is a compartment within the protoplast filled with an aqueous solution (cell sap) and separated from the Cytoplasm by a membrane called the tonoplast. The vacuole is a typical organelle of the plant cell. In meristematic cells, vacuoles are small vesicles; in mature cells, There is a large central vacuole that can occupy up to 90% of the cell volume.

The Vacuolar System is formed through several pathways:

1. Through the expansion of ER cisternae. Initially, individual fragments of the lamellar or tubular membrane system swell, increasing in volume, and then pinch off. This process creates provacuoles, which subsequently fuse to form the tonoplast.

2. Through tonoplast invagination. In this case, portions of the cytoplasm, along with macromolecules and even organelles, are engulfed. Once inside the enlarged vacuole, they are broken down by a system of Hydrolases.

3. Through autophagy. The process begins when The Endoplasmic reticulum membrane surrounds a specific region of the cytoplasm. Subsequently, the outer membrane and enzymatic systems undergo remodeling. The activity of hydrolases leads to the degradation of the cytoplasmic polymer content, which, in turn, facilitates the influx of Water.

Vacuoles formed via different pathways can fuse. Water accounts for 98% of the vacuolar content. Vacuolar sap has a specific composition, containing organic substances (sugars, acids, enzymes), secondary metabolites (phenols, Tannins, Alkaloids, etc.), and mineral salts (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^-, sulfate residues SO₄^2-, and phosphate residues PO₃^3-). The concentration of dissolved substances reaches 0.4–0.6 M. Most vacuolar enzymes are acid hydrolases. The pH of the vacuolar sap is generally between 5 and 6.5, though it can be as low as 1 (in begonias) or 2.0 (in lemons). Some vacuoles contain insoluble compounds (crystals, amorphous deposits, inclusions). The composition of vacuolar sap varies not only between different plants but also between cells of different Tissues. However, water remains its primary component. Substances enter the vacuole through various transport systems: ATP-dependent H⁺-pumps (transporting H⁺, sugars, and organic acids inward, and K⁺ both inward and outward), and tonoplast protein carriers (for Amino Acids and other substances). Vacuoles serve as storage sites for reserve substances (aleurone grains). Pigments are also frequently found here. Plant cells acquire blue, violet, or dark red hues due to anthocyanins, which are highly water-soluble. These pigments determine the coloration of many vegetables, fruits, and flowers.

Vacuoles perform a structural role by maintaining turgor, a storage function (for Reserve Proteins, seeds, etc.), and a role in accumulating cytoplasmic waste products. The osmotic Properties of the cell depend significantly on the concentration of the vacuolar sap. Due to the presence and activity of hydrolases, they also perform a lysosomal function.

The process of vacuolization is a necessary prerequisite for cell growth by elongation.

Microtubules and microfilaments (Cytoskeleton)

Microtubules are localized in the outer cortical layer of the cytoplasm in mature cells. They have an outer diameter of 30 nm and an inner diameter of 14 nm, and are oriented parallel to one another along the cell axis.

The functions of microtubules are quite diverse. During Cell Division, they form The basis of the spindle apparatus, participate in Cell wall formation (controlling the deposition of Cellulose microfibrils), and play a significant role in The Development of the cell plate. In monad Algae, microtubules are components of flagella.

All microtubules are composed of the globular acidic protein tubulin. It is believed that the microtubule wall is formed by 13 protofilaments of tubulin protein arranged in a spiral. Cytoplasmic microtubules are dynamic structures that easily dissociate into subunits and reassemble. Microtubule assembly requires acidic pH, magnesium ions, GTP, and ATP. Dissociation is accelerated by high calcium ion concentrations and low temperatures.

Plant Cell Cytoplasm also contains microfilaments made of non-Muscle Actin. This contractile protein can exist in a monomeric form (globular, G-actin) or a polymeric double-helical form (fibrillar, F-actin). The assembly of globular monomers into The Double Helix of fibrillar actin requires energy and the presence of magnesium ions. Microfilaments can create a reticular cytoplasmic structure, forming a network directly beneath the Plasmalemma.

Actin microfilaments participate in the Spatial Organization of metabolic processes and serve as the basis for cytoplasmic movement, known as cyclosis. They interact with microtubules and the plasmalemma to form a flexible cytoskeletal structure. The cytoskeleton is linked to changes in cell shape, the movement of intracellular structures, their orientation, and the specific localization of organelles and most molecules (enzymes) within the cytoplasm.

Cell wall

Plant cells are surrounded by a dense polysaccharide envelope, which is lined internally by the plasmalemma. In higher plants, The cell wall consists of three layers: the middle lamella, the primary wall, and the secondary wall. During cell division at the telophase stage, the middle lamella is the first to form. It binds the walls of adjacent daughter cells and consists of sticky, gelatinous pectic substances.

The basis of the middle lamella is a network structure formed by chains of pectic acid (synthesized from polygalacturonic acid) that are tightly intertwined with each other and with hemicellulose molecules. Individual molecules are linked through The formation of calcium and magnesium double salts and Esterification. In the absence of calcium, pectic substances become mucilaginous, causing cells to separate (tissue maceration).

Primary cell wall. Cells that are dividing or growing by elongation possess a primary cell wall (e.g., meristems, leaf mesophyll). Once growth is complete, new layers are deposited on the inside of the cell wall, resulting in the formation of a rigid secondary wall.

The cell wall is composed of:

1. structural components (cellulose in plants, Chitin in Fungi);

2. matrix components (hemicellulose, pectin, proteins);

3. encrusting components (Lignin, suberin);

4. substances deposited On the surface of the wall (cutin, wax).

Cellulose (a polymer of β-D-glucose), hemicellulose (polymers of hexoses and pentoses), and pectic substances (derivatives of uronic acids) are the carbohydrate components of cell walls. They adsorb water, bind Metal Ions, and determine the cation-exchange capacity of cell walls.

In addition to carbohydrates, the matrix contains proteins (enzymes, Lectins, extensin, protein granules). The primary wall protein extensin is a glycoprotein similar to Collagen that performs a structural function. It contains up to 90% of the cell's total hydroxyproline and is crucial for cell wall elongation. Carbohydrate-containing lectins facilitate identification and interaction between different cells, protect against infection, and perform receptor functions. Enzymes facilitate wall expansion and the integration of new components.

Lignin—a polymer with an unbranched chain of aromatic alcohols (p-coumaryl and sinapyl alcohols)—and suberin render cell walls impermeable to water (found in the endodermis and periderm; during lignification and suberization of cells). The surface of epidermal cells is protected by cutin and Waxes. A layer of cutin, permeated with polysaccharide wall components, forms the cuticle.

Cellulose molecules are assembled into microfibrils and held together by Hydrogen Bonds. Hydrogen bonds also exist between cellulose and hemicellulose microfibrils. Almost all other bonds in the cell wall are covalent: those between hemicelluloses and Pectins, between pectins and the protein extensin, and between lignin and extensin or cellulose.

The primary cell wall reaches a thickness of 0.1–1 μm and consists of cellulose micro- and macrofibrils embedded in a matrix. Each microfibril is 1–5 μm long and 4–10 nm in diameter. 100 cellulose molecules form a micelle, 20 micelles form a microfibril, and 250 microfibrils form a macrofibril.

The primary cell wall contains up to 80% water.

Waxes are esters of high-molecular-weight alcohols and fatty acids that form long-chain lipid compounds.

Lignin. Chemically, it is a group of related high-molecular-weight polymers. Its main building block is the phenylpropane unit (C₆-C₃).

Cutin is the primary polymer of the cuticle, consisting of a mixture of fatty hydroxy acids linked by ester bonds into a three-dimensional structure. Cutin, together with embedded waxes, forms the cuticle.

Suberin is structured similarly to cutin but differs in The Nature of its fatty acids.

Secondary cell wall. During its formation, multiple secondary layers are deposited on the inside of the primary wall. The cellulose content reaches 60%; its fibrils in each layer lie parallel, while those in adjacent layers are oriented at an angle to one another. Various inclusions, such as lignin, suberin, cutin, and wax, appear in the matrix. This provides the secondary wall with significant strength and rigidity, causing it to lose elasticity and the cell to lose its ability to grow further. The main component of this rigid wall is lignin, which is typical for the secondary walls of wood cells. Lignin binds cellulose fibers together and holds them in place. Xylem elements and mechanical sclerenchyma cells undergo intense lignification. Suberin, in combination with wax, causes the suberization of the cell wall, which reduces its permeability. Cutin forms a cuticular layer on the outside of the wall, making it nearly impermeable.

Functions of the cell wall. The cell wall is a secretory product of the protoplast and performs a variety of functions.

The cell wall protects the cell contents from damage and excessive water loss, maintains cell shape (due to turgor), protects the plasmalemma from rupture under high hydrostatic pressure, and serves as an essential component for Ion Exchange and the Transport of substances between cells.

The cell wall provides individual cells and the plant as a whole with mechanical strength and support. It determines the size, shape, and Stability of the cell. At the same time, it can change its dimensions depending on conditions: contracting or stretching. Stretching can be reversible (elastic) or partially irreversible (plastic).

The cell wall acts as an anti-infective barrier because it contains lectins, protective substances (Glycoproteins, elicitors, callose), and enzymes (oxidases, hydrolases) that neutralize foreign organisms or isolate them. It contains enzymatically active proteins, which enables the cell wall to participate in tissue metabolic processes.

The structural features and Formation of the cell wall provide one of the unique properties of the plant Organism — The ability to grow by elongation.

The cell wall participates in the absorption, transport, and secretion of substances.

The wall of root hairs is the first barrier to the penetration of substances from the soil. The MAIN MECHANISMS OF passive ion absorption are adsorption, and to a lesser extent, diffusion and mass flow. Due to the cation- and anion-exchange properties of cell walls, the plant forms its own absorption complex. As a result of contact between adjacent cells, the apoplast is formed—a unified system of cell walls. Substances move through the apoplast, bypassing membrane barriers.

Cell walls are perforated by pores through which plasmodesmata extend. A plasmodesma is a channel lined by the plasmalemma and filled with cytoplasm. The central part of the pore is occupied by a desmotubule, which consists of helically arranged protein subunits. The desmotubule connects to the ER membranes of adjacent cells.

The aggregate of interconnected protoplasts of a plant organism forms the symplast.

Thus, connections between adjacent cells can be established through the plasmalemma, cytoplasm, endoplasmic reticulum, and the cell wall.

There is evidence that carbohydrate Components of the cell wall, by interacting with phytohormones, induce physiological Changes in the cell.