BOTANY: Lecture Course for Bachelor Degree Students in Agronomy - 2016

LECTURE 2. Cytoplasm, Nucleus

Structure, physical properties, and chemical composition. Plastids. Origin, structure, and types of plastids. Structural Organization, physical properties, chemical composition, and Functions of METABOLISM/14.html">Chloroplasts, chromoplasts, and leucoplasts. Endoplasmic reticulum (ER). Structure, physical properties, chemical composition, and functions. Cell/35.html">Mitochondria. Origin, structural organization, physical properties, chemical composition, and functions. Ribosomes. Structure, physical properties, and chemical composition. Golgi apparatus. Origin, structural elements, physical properties, chemical composition, functions, and significance. Nucleus. Shape, size, and number. Structural organization, physical properties, and chemical composition. Nuclear envelope. Karyoplasm. Nucleolus. Chromonema. Chromosome structure. The Nucleus as the basis for the preservation and transmission of heredity. Nuclear and Cell Division.

Cytoplasm is a semi-fluid colloidal structure with a membranous organization where the most vital life processes take place (synthesis, Respiration, growth, movement, etc.). It determines such properties as irritability, movement, growth, and selective permeability.

It has a membranous organization based on Introduction/36.html">Biological Membranes consisting of Lipids and Proteins. They divide the cytoplasm into isolated compartments where biochemical processes occur simultaneously and independently of one another. The membranes determine The chemical composition of the cytoplasm.

It consists of three layers:

- Plasmalemma: a thin outer layer separated from The Cell wall, which regulates the Metabolic exchange between The Cell and the environment and participates in the synthesis of substances.

- Tonoplast or vacuolar system: separates the cytoplasm from internal cavities.

- Mesoplasm: located between the plasmalemma and the tonoplast; numerous Organelles are formed here, consisting of a uniform, structureless hyaloplasm and structural organelles.

Hyaloplasm, or matrix, is the fluid continuous medium in which organelles are suspended.

Chemical composition: Water (up to 80-85%), proteins, fats, CARBOHYDRATES, micro- and macroelements. Altogether, they account for 95-98% of the total mass of a living Organism.

Types of movement: vibrational, circulatory, rotational (or circular), and fountain-like.

The ability to withstand environmental changes and maintain a dynamic relative constancy is called Homeostasis. In plants, the Transport of substances across the plasmalemma and tonoplast plays the primary role in maintaining homeostasis. Known Mechanisms of Substance transport across membranes are divided into two categories: passive and Active Transport. Passive transport is the movement of substances According to the laws of diffusion and osmosis, which does not require Energy Expenditure.

Diffusion is the movement of molecules or ions along a concentration gradient: from an area of higher concentration to an area of lower concentration.

Osmosis is the Diffusion of Water through a semipermeable membrane from an area of low solute concentration to an area of high concentration. The uptake of water by the cell leads to an increase in the volume of the cell contents, generating hydrostatic pressure on the cell wall, which is called turgor pressure, while the tensed state of the cell wall is referred to as turgor.

In a hypertonic solution, water begins to leave the cell through the plasmalemma. The volume of the protoplast starts to decrease, and it gradually detaches from the cell wall. This phenomenon is called plasmolysis, which can be reversible or irreversible. The process reverse to plasmolysis is called deplasmolysis.

The NUCLEUS was discovered in 1832 by R. Brown. Cells contain one or several nuclei (Algae, Fungi). The nucleus has a colloidal nature, but is more viscous than the cytoplasm.

Shape: spherical or oval.

Size: 1 µm - 1.5 cm, depending on the number and size of Chromosomes. The volume changes throughout the day. THE POSITION OF the nucleus within the cell is variable: central in meristematic cells, and parietal in mature ones.

Chemical composition: proteins (73.9%), Nucleic Acids (26.1%), lipids, and mineral salts. 20% of proteins form NUCLEOTIDES with nucleic acids (nitrogenous base + sugars + phosphoric acid). Two Types of Nucleic acids are distinguished (DNA and RNA).

There are three states of the nucleus in the cell, which correspond to its three functions.

1. Interphase nucleus, in which DNA Replication takes place, resulting in The formation of a second chromatid and The conversion of a single-chromatid chromosome into a two-chromatid one.

2. Dividing nucleus, in which the two-chromatid chromosome splits into single-chromatid ones and they are distributed between the daughter nuclei, ensuring the transmission of hereditary information.

3. Working nucleus, in which genes localized in the chromosomes regulate cellular activity.

Structure:

The nuclear envelope surrounds the nucleus, separating its contents from the cytoplasm. It consists of two membranes with a perinuclear space between them. The inner membrane is agranular, while ribosomes are attached to the outer membrane. It features pores, controls the metabolic exchange between the nucleus and cytoplasm, and is capable of synthesizing proteins and lipids.

Nuclear sap, or nucleoplasm (karyoplasm), is a colloidal solution containing chromosomes and one or more nucleoli. It contains DNA molecules bound to specific proteins (Histones) as well as nuclear Enzymes. Function: connects with nuclear organelles and facilitates The transport of substances.

Chromatin (the main nuclear substance) is the site of RNA Transcription. During mitosis, it condenses to form chromosomes.

Chromosomes ensure the preservation of hereditary information, its replication, and its transmission to daughter cells during cell division. They exist in two states: as decondensed, thin (10 nm) filamentous structures (visible under an Electron microscope), and as condensed, short, and thick structures visible under a Light Microscope. They are Nucleoproteins consisting of DNA—nucleotides (phosphoric acid residue + deoxyribose + nitrogenous bases: adenine, guanine, thymine, cytosine)—and protein.

The nucleolus is a spherical body composed of Protein and RNA; it synthesizes rRNA and binds it to proteins. RNA consists of phosphoric acid + ribose + nitrogenous bases (adenine, guanine, uracil, cytosine), and lies adjacent to the secondary chromosomal constriction.

Class="center">DOUBLE-MEMBRANE ORGANELLES

Plastids are well-defined, viscous bodies of specialized Structure and function that can change their shape. In lower plants, plastids may be absent or number 1–2 per cell. In higher plant cells, they are numerous. They are classified according to the presence and type of pigments: green pigments are chloroplasts, other colors are chromoplasts, and colorless ones are leucoplasts. All plastids share a proteinaceous basis, or stroma.

Semiautonomous double-membrane organelles responsible for the Primary and secondary synthesis of carbohydrates.

They develop from proplastids—spherical bodies surrounded by a double membrane and filled with a matrix—from which all Other types of plastids are formed.

Proplastids are a type of plastid with a relatively simple internal structure, capable of developing into other plastid types; they are found in embryonic cells and the Meristems of mature plants. Like all members of this organelle class, proplastids possess their own circular DNA and protein-synthesizing apparatus, and are bounded externally by two membranes, with the inner membrane forming no invaginations. Proplastids lack chlorophyll and most of the enzymes required for Photosynthesis.

During the division and differentiation of embryonic or meristematic cells, proplastids can transform into various types of plastids. This process is regulated by the nuclear genome and depends on the type of tissue being formed: for instance, in epidermal cells,

proplastids develop into underdeveloped, unpigmented leucoplasts, whereas in mesophyll cells they form large green chloroplasts.

The transformation of proplastids into chloroplasts in angiosperms and some gymnosperms occurs exclusively under The Influence of light. During this process, some of the necessary proteins are synthesized within the organelle itself while others are imported from the Cytosol, an internal membrane system develops, and photosensitive pigments accumulate.

Since seeds mostly germinate in the soil out of reach of light, chloroplasts only begin to appear after the young SHOOT emerges onto the surface. If seeds are grown in the dark, proplastids differentiate into etioplasts, which feature an underdeveloped, semi-crystalline internal membrane system composed of so-called prolamellar bodies. Instead of chlorophyll, etioplasts contain the yellow-green precursor pigment protochlorophyll. Nevertheless, just a few minutes of light stimulation are sufficient for an etioplast to begin differentiating into a chloroplast.

Chloroplasts contain chlorophyll formed on the internal membrane, alongside chromoplasts located in the matrix (stroma).

Leucoplasts are colorless double-membrane plastids that store nutrient reserves: proteins in proteoplasts, oils in oleoplasts, and carbohydrates in amyloplasts.

Chromoplasts are orange, red, and yellow plastids that develop from leucoplasts and chloroplasts.

Origin of plastids.

1. All plastids are genetically interrelated, allowing transitions between different plastid types. During the Aging of leaves and stems, or the ripening of fruits, chlorophyll in chloroplasts can break down, The structure of the internal membrane system becomes simpler, and they transform into chromoplasts. Chromoplasts represent The final stage of plastid development and do not convert into other types of plastids.

2. They develop from proplastids.

3. They multiply during cell division through the division of maternal plastids.

Mitochondria are semiautonomous organelles that appear as small

granules, rods, or filaments visible at a magnification of 1000–1200x. They serve as the centers of intracellular oxidation and contain Enzymes for the di- and tricarboxylic acid cycles, the Electron Transport Chain, and Oxidative Phosphorylation. A typical Eukaryotic Cell contains about 2,000 mitochondria, which occupy approximately one-fifth of its total volume. Mitochondria possess their own Mitochondrial DNA, which is independent of the DNA located in the Cell Nucleus. According to the widely accepted endosymbiotic theory, mitochondria originated from free-living Prokaryotic Cells that were relatives of modern proteobacteria.

Main function: meeting the cell's energy demands and regulating the concentration and qualitative composition of ions in the cytoplasm.

Structure: the inner membrane forms cristae, and the internal space is filled with a matrix containing ribosomes and DNA.

Origin — formed from initial particles separated from the nucleus and through the division of maternal mitochondria.

SINGLE-MEMBRANE ORGANELLES

ER (endoplasmic reticulum) is a dynamically changing, branched network of ultramicroscopic tubules, vesicles, and cisternae. It is bounded by a unit membrane and filled with a structureless matrix distinct from the hyaloplasm. The ER tubules connect with the outer nuclear envelope, linking the nucleus to the cytoplasm, and facilitate communication between neighboring cells as well as with other cellular organelles.

Two Types of reticulum:

granular (rough), studded with ribosomes, which is responsible for protein and enzyme synthesis, substance transport, intercellular connection, and the formation of new membranes and vacuoles.

agranular (smooth), responsible for lipid synthesis, as well as the synthesis and transport of Essential Oils, resins, and rubbers.

Golgi apparatus or dictyosome system — parallel groups of 5–8 flattened cisternae bounded by an agranular membrane (dictyosomes). Small vesicles are found at the edges of the cisternae. These cisternae can enlarge to form large vacuoles.

They accumulate Metabolic waste products—foreign inclusions and toxic substances—which are subsequently transferred to the vacuole, contribute to Cell wall formation, and store and facilitate the transport of proteins, fats, and carbohydrates.

Microbodies — spherical bodies with a diameter of 0.2–1.5 µm, surrounded by a membrane. Their matrix contains the enzyme catalase and various oxidases.

Peroxisomes are found in the photosynthetic cells of higher plants in close contact with chloroplasts and mitochondria, participating in Photorespiration.

Glyoxysomes occur in the endosperm or cotyledon cells of fat-storing seeds, such as castor bean and sunflower.

Spherosomes — rounded, single-membrane bodies located in the hyaloplasm. Enzymes are contained within the matrix beneath the membrane. Their function is the synthesis and storage of plant fats, which are converted into carbohydrates during germination.

Lysosomes — rounded, single-membrane bodies with a diameter of 0.5–2.0 µm. Lysosomes perform several vital functions within the cell: breaking down intra- and extracellular waste and aged organelles, destroying pathogens, and supplying the cell with nutrients. They contain over 40 different acidic Hydrolases, including proteases, Nucleases, lipases, phospholipases, Phosphatases, and sulfatases. The optimal pH for these enzymes ranges from 4.5 to 5, which is the acidity maintained inside lysosomes. Furthermore, proteases exhibit maximum activity only after Limited proteolysis. The Biological Significance of these properties lies in protecting the cell's cytoplasm from degradation by lysosomal enzymes. Even if the membrane bounding this compartment loses its integrity for any reason, the hydrolases will remain inactive in the cytosol, which has a pH of approximately 7.2.

Microtubules. Microtubules are hollow cylinders with a diameter of 25 nm. Their length can range from a few micrometers to presumably several millimeters (in nerve cell axons). Their walls are formed by tubulin dimers. Like Actin microfilaments, microtubules are polar: self-assembly occurs at one end, while disassembly takes place at the other. Within cells, microtubules act as structural components and participate in numerous cellular processes, including mitosis, cytokinesis, and Vesicular Transport. They are structured with 13 α- and β-tubulin heterodimers arranged in a circle around a hollow cylinder. The outer diameter of the cylinder is approximately 25 nm, and the inner diameter is about 15 nm. One end of the microtubule, termed the plus-end, continuously adds free tubulin, whereas tubulin subunits dissociate from the opposite, minus-end.

Three phases are distinguished in microtubule formation in vitro:

1. lag phase, or nucleation. This is the initiation stage of microtubule formation, where tubulin molecules begin to associate into larger structures. This association occurs more slowly than The addition of tubulin to an already assembled microtubule, which is why it is called the lag phase.

2. polymerization phase, or elongation. When the concentration of free tubulin is high, its polymerization outpaces depolymerization at the minus-end, causing the microtubule to elongate. As it grows, the tubulin concentration drops to a critical level, and the growth rate slows down before entering the next phase;

3. steady-state phase. Depolymerization balances polymerization, and microtubule growth halts. Laboratory studies show that microtubule assembly from tubulin occurs only in the presence of guanosine triphosphate and magnesium ions, with an optimum Temperature of 37 °C.

Types of microtubules:

- form the mitotic spindle during cell division;

- found in the cytoplasm;

- serve as Structural elements of flagella and cilia.

NON-MEMBRANOUS ORGANELLES

Ribosomes are non-membranous cellular organelles composed of rRNA and ribosomal proteins. They carry out Protein Biosynthesis by translating mRNA into a polypeptide chain. Thus, the ribosome can be considered a factory producing proteins based on available Genetic information. In the cell, mature ribosomes are predominantly located in compartments undergoing active Protein Synthesis. They may float freely in the cytoplasm or attach to the cytoplasmic side of The endoplasmic reticulum or nuclear membranes. Active ribosomes (those currently engaged in Translation) are mostly found as Polysomes. There is considerable evidence indicating that the ribosome is a ribozyme. These essential, tiny, nearly spherical cellular organelles are visible under an electron microscope and function to synthesize proteins. They are located in mitochondria, plastids, and the hyaloplasm, and are attached to The surface of ER membranes. Ribosomes were discovered in the early 1950s. The first thorough study and description of ribosomes as cellular organelles was conducted by George E. Palade. Consequently, they were initially named "Palade particles," but were later renamed "ribosomes" in 1958 due to their high RNA content. Their role in protein biosynthesis was established more than a decade later.

They are arranged in groups known as polysomes and are formed in the nucleus.