Observing the diversity of living and non-living things prompts the fundamental question: what distinguishes a living organism from an inanimate object? The answer lies in the presence of the cell, the basic unit of life, in all living organisms.

All organisms are made up of cells. Organisms consisting of a single cell are termed unicellular, while those composed of many cells are called multicellular.

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What Is A Cell?

Unicellular organisms demonstrate the ability to exist independently and perform all the essential life functions. This highlights that a complete cellular structure is necessary for independent living.

Therefore, the cell is the fundamental structural and functional unit of all living organisms.

• The first observation and description of a live cell were made by Anton Von Leeuwenhoek.
• Later, Robert Brown discovered the nucleus within the cell.

The development and refinement of the microscope, especially the invention of the electron microscope, have allowed for detailed exploration of cellular structures.

Cell Theory

The cell theory, a cornerstone of biology, was developed through the observations of several scientists:

• In 1838, Matthias Schleiden, a German botanist, observed that all plants he studied were composed of different types of cells that formed their tissues.
• Around the same time (1839), Theodore Schwann, a British zoologist, studied animal cells and noted the presence of a thin outer layer (now known as the plasma membrane). Based on his plant studies, he concluded that the cell wall is a characteristic unique to plant cells.

Schwann then proposed the hypothesis that the bodies of both animals and plants are made up of cells and the products derived from cells.

Schleiden and Schwann jointly formulated the initial cell theory. However, their theory did not explain how new cells come into existence.

• In 1855, Rudolf Virchow provided the explanation. He stated that cells divide and that new cells arise only from pre-existing cells. His famous phrase is "Omnis cellula-e cellula" (all cells from cells).

Virchow modified the cell theory formulated by Schleiden and Schwann, giving it its final shape as understood today.

The modern Cell Theory states:

1. All living organisms are composed of cells and products of cells.
2. All cells arise from pre-existing cells.

An Overview Of Cell

Observing typical plant cells (like onion peel cells) and animal cells (like human cheek cells) reveals common and distinct features.

• A typical plant cell has a prominent cell wall as its outer boundary, with a cell membrane just inside.
• Animal cells have a cell membrane as their outer boundary.

Inside most cells is a dense, membrane-bound structure called the nucleus. The nucleus contains the chromosomes, which carry the genetic material, DNA.

• Cells possessing a membrane-bound nucleus are called eukaryotic cells.
• Cells that lack a membrane-bound nucleus are called prokaryotic cells.

In both prokaryotic and eukaryotic cells, the space within the cell membrane is filled with a semi-fluid substance called cytoplasm. The cytoplasm is the primary site of cellular activities and chemical reactions necessary to keep the cell alive.

Eukaryotic cells have additional distinct, membrane-bound structures within the cytoplasm called organelles. These include the endoplasmic reticulum (ER), Golgi complex, lysosomes, mitochondria, microbodies, and vacuoles. Prokaryotic cells lack these membrane-bound organelles.

Ribosomes are unique non-membrane bound organelles found in all cells, both prokaryotic and eukaryotic. In eukaryotic cells, ribosomes are present in the cytoplasm, and also within mitochondria, chloroplasts (in plant cells), and attached to the surface of the rough ER.

Animal cells also contain a non-membrane bound organelle called the centrosome, which plays a role in cell division.

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Cells vary significantly in their size, shape, and activities (Figure 8.1):

• Size:
  • Smallest cells: Mycoplasmas ($\sim 0.3 \mu\textsf{m}$ in length).
  • Bacteria: Typically $3$ to $5 \mu\textsf{m}$.
  • Largest isolated single cell: Egg of an ostrich.
  • Human red blood cells: About $7.0 \mu\textsf{m}$ in diameter.
• Shape: Can be diverse, including disc-like, polygonal, columnar, cuboid, thread-like, or irregular. The shape of a cell is often related to its specific function. Nerve cells are among the longest cells.

Prokaryotic Cells

Prokaryotic cells are represented by organisms such as bacteria, blue-green algae (cyanobacteria), mycoplasma, and PPLO (Pleuro Pneumonia Like Organisms).

Compared to eukaryotic cells, prokaryotes are generally smaller and multiply more rapidly.

They exhibit variety in shape and size (Figure 8.2).

The four basic shapes of bacteria are:

• Bacillus: Rod-like
• Coccus: Spherical
• Vibrio: Comma-shaped
• Spirillum: Spiral

Basic Organisation:

Despite the diversity in form and function, the fundamental organization of prokaryotic cells is similar. All prokaryotes (except Mycoplasma) have a cell wall surrounding the cell membrane.

The cell is filled with cytoplasm, which is the fluid matrix.

Nucleus: There is no well-defined, membrane-bound nucleus.

Genetic Material: The genetic material (DNA) is typically naked (not enclosed by a nuclear membrane). It consists of a single, usually circular chromosome (genomic DNA) located in a region called the nucleoid.

Plasmids: Many bacteria contain smaller, circular DNA molecules located outside the genomic DNA. These are called plasmids. Plasmid DNA can confer unique characteristics, such as antibiotic resistance. Plasmids are important tools in genetic engineering.

Organelles: Prokaryotic cells lack membrane-bound organelles found in eukaryotes (ER, Golgi, lysosomes, mitochondria, etc.). The only common organelle is the ribosome.

Inclusions: Prokaryotes contain unique structures called inclusion bodies, which store reserve materials and are not bound by membranes.

Mesosome: A specialized, differentiated structure formed by infoldings of the plasma membrane into the cell. It is characteristic of prokaryotes and serves various functions.

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Cell Envelope And Its Modifications

Most prokaryotic cells, especially bacteria, have a complex cell envelope consisting of three tightly linked layers:

1. Glycocalyx: Outermost layer. Varies in composition and thickness. Can be a loose slime layer or a thick, tough capsule.
2. Cell Wall: Located beneath the glycocalyx. Provides structural support, determines cell shape, and prevents bursting or collapsing.
3. Plasma Membrane: Innermost layer. Selectively permeable, regulating the passage of substances. Structurally similar to the eukaryotic plasma membrane.

These three layers function together as a single protective unit.

Gram Staining: Bacteria can be classified into two groups based on their cell envelope structure and their response to Gram staining:

• Gram positive: Take up the Gram stain (appear purple/blue).
• Gram negative: Do not retain the Gram stain (appear pink/red after counterstaining).

Mesosome: A unique membranous structure formed by invaginations (extensions) of the plasma membrane into the cytoplasm. These extensions can be in the form of vesicles, tubules, or lamellae. Mesosomes are involved in several functions:

• Cell wall formation
• DNA replication and distribution to daughter cells
• Respiration
• Secretion processes
• Increasing the surface area of the plasma membrane and its enzymatic content

Chromatophores: In some prokaryotes like cyanobacteria, other membranous extensions into the cytoplasm called chromatophores contain photosynthetic pigments.

Motility: Bacterial cells may be motile or non-motile. Motile bacteria have filamentous extensions from their cell wall called flagella.

Bacterial Flagellum: Composed of three parts: filament, hook, and basal body. The filament is the longest part, extending outside the cell.

Other Surface Structures (not involved in motility):

• Pili: Elongated, tubular structures made of a special protein.
• Fimbriae: Small, bristle-like fibers projecting from the cell surface.

Both pili and fimbriae can help bacteria attach to surfaces (like rocks in streams) or to host tissues.

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Ribosomes And Inclusion Bodies

Ribosomes: In prokaryotes, ribosomes are associated with the plasma membrane. They are smaller than eukaryotic ribosomes, with a size of about $15 \times 20 \textsf{ nm}$. Prokaryotic ribosomes are 70S, composed of two subunits: a larger 50S subunit and a smaller 30S subunit. Ribosomes are the cellular machinery for protein synthesis.

Polyribosomes (Polysomes): Several ribosomes can attach to a single messenger RNA (mRNA) molecule, forming a chain called a polyribosome or polysome. Ribosomes on the polysome translate the mRNA into proteins.

Inclusion bodies: These are reserve storage structures in the cytoplasm of prokaryotic cells. They are not enclosed by any membrane and lie freely in the cytoplasm. Examples include:

• Phosphate granules
• Cyanophycean granules
• Glycogen granules

Gas vacuoles are found in some photosynthetic bacteria (blue-green, purple, and green photosynthetic bacteria), providing buoyancy.

Eukaryotic Cells

Eukaryotes encompass all protists, plants, animals, and fungi.

Characteristic Features:

• Membrane-bound organelles: Cytoplasm is extensively compartmentalized by the presence of various membrane-bound organelles (ER, Golgi complex, lysosomes, mitochondria, plastids, vacuoles, etc.).
• Organised nucleus: Possess a distinct nucleus enclosed by a nuclear envelope.
• Complex structures: Have intricate locomotory structures (cilia, flagella) and a cytoskeleton.
• Chromosomes: Genetic material (DNA) is organized into multiple linear chromosomes associated with proteins.

Not all eukaryotic cells are identical. There are significant differences between plant and animal cells (Figure 8.3):

Plant cells possess:

• Cell walls (outside the cell membrane)
• Plastids (chloroplasts, chromoplasts, leucoplasts)
• A large central vacuole (often occupying a significant volume of the cell)

These structures are generally absent in animal cells (except for a few lower forms that might have some plastids or temporary vacuoles).

Animal cells possess:

• Centrioles (involved in cell division and formation of cilia/flagella basal bodies)

Centrioles are absent in almost all plant cells.

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Cell Membrane

Detailed study of the cell membrane structure became possible after the advent of the electron microscope in the 1950s. Chemical studies, particularly on human red blood cells (RBCs), provided initial insights.

Composition: The cell membrane is primarily composed of lipids and proteins. Carbohydrates are also present, often attached to lipids (glycolipids) or proteins (glycoproteins).

Lipid Bilayer: The major lipids are phospholipids, arranged in a bilayer. The phospholipids are oriented with their polar (hydrophilic, water-attracting) heads facing outwards towards the aqueous environment (both outside the cell and inside the cell, within the cytoplasm), and their nonpolar (hydrophobic, water-repelling) tails facing inwards towards each other. This creates a hydrophobic interior (Figure 8.4).

Cholesterol is also present in the membrane, contributing to its fluidity and stability.

Proteins: The ratio of proteins and lipids varies between cell types (e.g., human erythrocyte membrane is ~52% protein, 40% lipid). Membrane proteins are classified based on their location and ease of extraction:

• Peripheral proteins: Located on the surface of the membrane (either outer or inner).
• Integral proteins: Partially or completely buried within the lipid bilayer.

Fluid Mosaic Model: Proposed by Singer and Nicolson in 1972, this model is widely accepted. It describes the membrane as a "fluid mosaic".

• The quasi-fluid nature of the lipid bilayer allows for the lateral movement of proteins within the membrane. This ability of components to move laterally is referred to as membrane fluidity.
• Proteins are embedded in this fluid lipid bilayer like a mosaic.

Importance of Fluidity: Membrane fluidity is essential for various cellular functions, including:

• Cell growth
• Formation of intercellular junctions
• Secretion of substances
• Endocytosis (engulfing materials from outside)
• Cell division

Transport across the Membrane: The plasma membrane is selectively permeable, controlling which substances enter or leave the cell.

• Passive Transport: Movement of molecules across the membrane without requiring cellular energy (ATP).
  • Simple Diffusion: Neutral solutes move across the membrane down their concentration gradient (from high to low concentration). Water also moves by diffusion (osmosis).
  • Facilitated Diffusion: Polar molecules cannot easily pass through the hydrophobic lipid bilayer. Their transport is facilitated by carrier proteins embedded in the membrane, still following the concentration gradient.
• Active Transport: Transport of molecules across the membrane against their concentration gradient (from low to high concentration). This process requires energy in the form of ATP. Examples include the sodium-potassium pump (Na$^+$/K$^+$ Pump).

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Cell Wall

In plants and fungi, a non-living, rigid structure called the cell wall forms an outer covering outside the plasma membrane.

Functions:

• Provides shape to the cell.
• Offers mechanical support and protects the cell from mechanical damage and infection.
• Helps in cell-to-cell interaction.
• Acts as a barrier to undesirable macromolecules.

Composition:

• Algae: Cell wall made of cellulose, galactans, mannans, and minerals like calcium carbonate.
• Other plants: Cell wall primarily composed of cellulose, hemicellulose, pectins, and proteins.

Layers in Plant Cell Wall:

• Primary wall: Formed in young plant cells, capable of growth.
• Secondary wall: As the cell matures, growth diminishes, and a secondary wall is laid down on the inner side of the primary wall (towards the plasma membrane).

Middle Lamella: A layer, mainly of calcium pectate, that acts as a cementing layer, holding (gluing) adjacent plant cells together.

Plasmodesmata: The cell wall and middle lamellae are perforated by cytoplasmic connections between neighboring cells called plasmodesmata, allowing communication and transport between cells.

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Endomembrane System

Several membrane-bound organelles within a eukaryotic cell function in a coordinated manner, forming the endomembrane system. These organelles are considered together because their functions are integrated.

The endomembrane system includes:

• Endoplasmic Reticulum (ER)
• Golgi complex
• Lysosomes
• Vacuoles

Organelles like mitochondria, chloroplasts, and peroxisomes are membrane-bound but are *not* part of the endomembrane system as their functions are not directly coordinated with the functions of the ER, Golgi, lysosomes, and vacuoles.

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The Endoplasmic Reticulum (ER)

Observed under electron microscope as a network or reticulum of tiny tubular structures distributed throughout the cytoplasm (Figure 8.5).

Function in Compartmentalization: ER divides the intracellular space into two distinct compartments:

• Luminal compartment: The space inside the ER tubules/cisternae.
• Extra-luminal compartment: The cytoplasm outside the ER.

Types of ER based on Ribosome Presence:

• Rough Endoplasmic Reticulum (RER): ER that has ribosomes attached to its outer surface. RER is commonly found in cells actively involved in protein synthesis and secretion. It is often extensive and continuous with the outer membrane of the nuclear envelope.
• Smooth Endoplasmic Reticulum (SER): ER that lacks ribosomes on its surface, giving it a smooth appearance. SER is the primary site for lipid synthesis. In animal cells, steroid hormones (lipid-like molecules) are synthesized in the SER.

Functions of ER:

• Protein synthesis and modification (RER)
• Lipid synthesis (SER)
• Transport of substances within the cell

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Golgi Apparatus

First observed by Camillo Golgi in 1898 near the nucleus. Named after him as Golgi bodies or Golgi complex (Figure 8.6).

Structure: Consists of many flat, disc-shaped sacs called cisternae, typically $0.5 \textsf{ µm}$ to $1.0 \textsf{ µm}$ in diameter. Cisternae are stacked parallel to each other. A Golgi complex has a variable number of cisternae.

Orientation: The cisternae are arranged concentrically near the nucleus with two distinct faces:

• Cis face (forming face): Convex, facing the ER.
• Trans face (maturing face): Concave, facing away from the ER.

The cis and trans faces are functionally different but interconnected.

Functions:

• Packaging of materials: Principal function is to package materials received from the ER into vesicles for transport within the cell or secretion outside the cell.
• Modification of proteins: Proteins synthesized on the RER are often modified (e.g., glycosylation) in the cisternae of the Golgi apparatus before being released from the trans face.
• Formation of glycoproteins and glycolipids: The Golgi apparatus is a key site for the synthesis of these complex molecules.

The close association between the ER and Golgi reflects the flow of materials from the ER to the Golgi for processing and packaging.

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Lysosomes

These are membrane-bound vesicular structures formed by the process of packaging within the Golgi apparatus.

Enzymes: Lysosomal vesicles contain a wide variety of hydrolytic enzymes (hydrolases), including lipases, proteases, and carbohydrases. These enzymes are optimally active at an acidic pH.

Function: The hydrolytic enzymes are capable of digesting or breaking down carbohydrates, proteins, lipids, nucleic acids, and cellular debris. Lysosomes are sometimes called "suicidal bags" because they can digest the cell itself if their membrane ruptures.

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Vacuoles

Vacuoles are membrane-bound spaces within the cytoplasm. They contain various substances, such as water, sap, excretory products, and other materials that are not useful to the cell.

Membrane: The vacuole is enclosed by a single membrane called the tonoplast.

In plant cells, vacuoles can be very large, sometimes occupying up to 90% of the cell volume. The tonoplast in plant cells actively transports ions and other materials into the vacuole, often against concentration gradients, resulting in a significantly higher concentration of these substances inside the vacuole than in the cytoplasm.

Other types of vacuoles:

• Contractile vacuole: Found in some protists (like Amoeba). Important for osmoregulation (maintaining water balance) and excretion.
• Food vacuoles: Formed in many protists by engulfing food particles. Involved in digestion of the ingested food.

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Mitochondria

Mitochondria (singular: mitochondrion) are generally not visible without specific staining under a light microscope.

Number and Shape: The number of mitochondria per cell varies widely depending on the cell's metabolic activity. They are typically sausage-shaped or cylindrical, with a diameter of $0.2 \textsf{-} 1.0 \textsf{ µm}$ and length of $1.0 \textsf{-} 4.1 \textsf{ µm}$.

Structure: Each mitochondrion is enclosed by two membranes (a double membrane-bound structure): an outer membrane and an inner membrane (Figure 8.7).

• Outer membrane: Smooth and forms the continuous outer boundary.
• Inner membrane: Forms numerous inward folds called cristae (singular: crista) into the inner compartment (matrix). Cristae increase the surface area for chemical reactions.
• Compartments: The two membranes divide the mitochondrion into two aqueous compartments: the outer compartment (space between the outer and inner membranes, the inter-membrane space) and the inner compartment, which is filled with a dense, homogeneous substance called the matrix.

Functions:

• Aerobic respiration: Mitochondria are the primary sites of aerobic respiration in eukaryotic cells.
• ATP production: They produce cellular energy in the form of ATP (adenosine triphosphate) through oxidative phosphorylation. Due to this function, they are commonly called the 'power houses' of the cell.

Mitochondrial Matrix Contents: The matrix contains enzymes for the Krebs cycle (part of aerobic respiration). It also possesses:

• A single, circular DNA molecule (like in prokaryotes)
• A few RNA molecules
• Ribosomes (70S), similar to prokaryotic ribosomes
• Components required for protein synthesis

Mitochondria are semi-autonomous organelles due to the presence of their own DNA and ribosomes.

Division: Mitochondria divide by fission.

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Plastids

Plastids are large organelles found in all plant cells and in euglenoids. They contain specific pigments, giving plants their characteristic colours.

Classification based on Pigments:

1. Chloroplasts: Contain chlorophyll and carotenoid pigments. Responsible for trapping light energy essential for photosynthesis. Typically green.
2. Chromoplasts: Contain fat-soluble carotenoid pigments like carotene and xanthophylls. Impart yellow, orange, or red colours to parts of the plant (e.g., flowers, fruits).
3. Leucoplasts: Colourless plastids. Store various nutrients and vary in shape and size.
   • Amyloplasts: Store carbohydrates (starch) (e.g., in potato tubers).
   • Elaioplasts: Store oils and fats.
   • Aleuroplasts: Store proteins.

Chloroplast Structure (Figure 8.8):

• Location: Most chloroplasts in green plants are found in the mesophyll cells of leaves.
• Shape and Size: Lens-shaped, oval, spherical, discoid, or ribbon-like. Variable length ($5 \textsf{-} 10 \textsf{ µm}$), width ($2 \textsf{-} 4 \textsf{ µm}$). Number per cell varies (e.g., 1 in *Chlamydomonas*, 20-40 in mesophyll cells).
• Membranes: Double membrane-bound. Inner membrane is less permeable than the outer.
• Stroma: The space enclosed by the inner membrane. Contains enzymes for carbohydrate and protein synthesis, small double-stranded circular DNA, and ribosomes (70S).
• Thylakoids: Flattened membranous sacs present in the stroma. Contain chlorophyll pigments.
• Grana (singular: granum): Stacks of thylakoids.
• Stroma lamellae: Flat membranous tubules connecting thylakoids of different grana.
• Lumen: The space enclosed within the membrane of a thylakoid.

Chloroplasts are also semi-autonomous organelles.

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Ribosomes

Ribosomes are granular structures first observed by George Palade in 1953. They are composed of ribonucleic acid (RNA) and proteins.

Unlike other organelles discussed so far (except centrosome and microbodies), ribosomes are not surrounded by any membrane.

Types based on Sedimentation Coefficient ('S' unit):

• Eukaryotic ribosomes: 80S.
• Prokaryotic ribosomes: 70S.

'S' (Svedberg's Unit) is a measure of sedimentation rate in a centrifuge, indirectly related to size and density.

Subunits: Each ribosome consists of two subunits, a larger and a smaller one (Figure 8.9).

• 80S ribosome: Composed of 60S (larger) and 40S (smaller) subunits.
• 70S ribosome: Composed of 50S (larger) and 30S (smaller) subunits.

Function: Ribosomes are the primary sites for protein synthesis, where the genetic information from mRNA is translated into amino acid sequences to build proteins.

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Cytoskeleton

The cytoplasm of eukaryotic cells contains an elaborate network of filamentous proteinaceous structures. This network is collectively called the cytoskeleton.

Components: The cytoskeleton includes microtubules, microfilaments, and intermediate filaments.

Functions:

• Provides mechanical support to the cell.
• Involved in cell motility (movement).
• Helps maintain the shape of the cell.

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Cilia And Flagella

Cilia and flagella are hair-like outgrowths of the cell membrane found on the surface of some eukaryotic cells.

• Cilia: Shorter structures that function like oars, causing movement of either the cell itself or the fluid surrounding the cell.
• Flagella: Comparatively longer structures responsible for cell movement.

Note: Prokaryotic bacteria also have flagella, but they are structurally different from eukaryotic flagella.

Structure (Eukaryotic Cilium/Flagellum):

• Covering: Covered by the plasma membrane.
• Core (Axoneme): The central core contains a specific arrangement of microtubules running parallel to the long axis.
• Microtubule Arrangement: Typically consists of nine doublets of microtubules arranged radially around the periphery and a pair of centrally located microtubules. This arrangement is referred to as the 9+2 array (Figure 8.10 b).
• Connecting Structures:
  • Central microtubules are connected by bridges and enclosed by a central sheath.
  • Radial spokes connect the central sheath to one microtubule of each peripheral doublet.
  • Peripheral doublets are interconnected by linkers.
• Basal Body: Both cilia and flagella emerge from a centriole-like structure at the base called the basal body.

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Centrosome And Centrioles

The centrosome is an organelle usually containing two cylindrical structures called centrioles.

Location and Arrangement: Centrioles are surrounded by amorphous pericentriolar materials. The two centrioles in a centrosome are typically arranged perpendicular to each other.

Structure: Each centriole has an internal organization resembling a cartwheel. It is made up of nine evenly spaced peripheral fibrils composed of tubulin protein. Each of these peripheral fibrils is a triplet of microtubules (9 triplets in total). Adjacent triplets are also linked.

Central Structure: The central proteinaceous part of the proximal region of the centriole is called the hub. The hub is connected to the tubules of the peripheral triplets by radial spokes made of protein.

Functions:

• Form the basal bodies of cilia or flagella.
• In animal cells, centrioles give rise to the spindle fibres that form the spindle apparatus during cell division.

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Nucleus

The nucleus was first described as a cell organelle by Robert Brown in 1831.

The material within the nucleus that stained with basic dyes was later termed chromatin by Flemming.

Structure of Interphase Nucleus (Figure 8.11):

• Nuclear Envelope: A double membrane structure (two parallel membranes) enclosing the nucleus. The space between the two membranes ($10 \textsf{-} 50 \textsf{ nm}$) is called the perinuclear space. The nuclear envelope separates the nuclear contents from the cytoplasm. The outer membrane is often continuous with the ER and may have ribosomes attached.
• Nuclear Pores: Minute openings in the nuclear envelope formed by the fusion of the two membranes. These pores regulate the passage of molecules (like RNA and proteins) between the nucleus and cytoplasm in both directions.
• Nucleoplasm (Nuclear Matrix): The fluid substance within the nucleus, enclosed by the nuclear envelope. It contains the nucleolus and chromatin.
• Nucleolus (plural: nucleoli): One or more spherical structures present within the nucleoplasm. It is not membrane-bound. The nucleolus is the primary site for the synthesis of ribosomal RNA (rRNA) and ribosome biogenesis. Cells actively synthesizing proteins usually have larger and more numerous nucleoli.
• Chromatin: In the interphase nucleus, the chromatin is a loose, indistinct network of nucleoprotein fibers. During cell division, the chromatin condenses to form structured entities called chromosomes.

Chromatin Composition: Chromatin is made of DNA and some basic proteins called histones, as well as some non-histone proteins and RNA. The DNA in a single human cell is approximately 2 meters long and is packaged into 46 chromosomes (23 pairs).

Number of Nuclei: Normally, a cell has a single nucleus. However, variations occur; some organisms or tissues have multiple nuclei (e.g., some fungi, muscle cells), while some mature cells lack a nucleus (e.g., mammalian erythrocytes, sieve tube cells of plants - raising the question of their 'living' status).

Chromosome: Visible as condensed structures during cell division (Figure 8.12). Each chromosome has a primary constriction called the centromere.

Kinetochores: Disc-shaped structures located on the sides of the centromere. They serve as the attachment points for spindle fibers during cell division.

Centromere Function: The centromere holds the two chromatids (replicated DNA strands) of a chromosome together.

Classification of Chromosomes based on Centromere Position (Figure 8.13):

• Metacentric: Centromere is in the middle, forming two equal arms.
• Sub-metacentric: Centromere is slightly away from the middle, resulting in one shorter arm and one longer arm.
• Acrocentric: Centromere is situated close to one end, forming one very short arm and one very long arm.
• Telocentric: Centromere is located at the terminal end, resulting in only one arm.

Satellite: Some chromosomes have non-staining secondary constrictions at a constant location, giving rise to a small fragment called the satellite.

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Microbodies

Microbodies are small, membrane-bound vesicles found in both plant and animal cells. They contain various enzymes and are involved in metabolic reactions, such as those in peroxisomes (involved in oxidative reactions).

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