Biotechnology: Class 11 — Chapter 2: Cellular Organelles

• Prokaryotic Cells: Characterized by the absence of a membrane-bound nucleus and internal organelles. Genetic material is localized in an unstructured nucleoid. Features specialized structures like mesosomes (invaginations of the plasma membrane that assist in respiration and cell wall synthesis) and locomotory flagella.
• Eukaryotic Cells: Contain a highly compartmentalized interior with a well-defined, membrane-enclosed nucleus and diverse specialized organelles suspended within the cytoplasm.
• Common Features: Both cellular designs share a selective plasma membrane, a semi-fluid cytoplasm, genetic material in the form of DNA, and protein-synthesizing ribosomes.

Proposed by Seymour Jonathan Singer and Garth L. Nicolson in 1972, this model describes the membrane as a fluid lipid bilayer interspersed with a mosaic of proteins.

• Lipid Bilayer: Composed primarily of amphipathic phospholipids with hydrophilic polar heads facing outward and hydrophobic fatty acid tails oriented inward.
• Membrane Proteins: Classified into peripheral membrane proteins (superficially attached, involved in cell signaling) and integral membrane proteins (partially or fully embedded, including transmembrane channels).
• Chemical Composition: Varies by cell type; for example, human erythrocyte membranes consist of approximately 52% protein and 40% lipid.

Transport across the plasma membrane occurs via passive, facilitated, or active mechanisms depending on concentration gradients and energy requirements.

• Peptidoglycan: The structural backbone of bacterial cell walls, consisting of linear polysaccharide chains cross-linked by short tetrapeptides.
• Glycocalyx: An outer gelatinous layer that can exist as a loose slime layer or a thick, structured capsule, offering protection against host immune systems and environmental stress.

• Plant Cell Wall: Composed of cellulose (a linear polymer of $\beta$-D-glucose linked via $\beta$-1,4-glycosidic bonds). Growing cells have a flexible primary cell wall, which transitions to a rigid secondary cell wall reinforced with lignin upon maturity.
• Fungal Cell Wall: Composed of chitin (a polymer of N-acetylglucosamine linked via $\beta$-1,4-glycosidic bonds).
• Intercellular Connections: The middle lamella (rich in calcium pectate) glues adjacent plant cells together, while plasmodesmata form cytoplasmic bridges for intercellular transport.

An extensive network of membrane-enclosed tubules and flattened sacs (cisternae) continuous with the outer nuclear membrane.

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes on its cytosolic face; serves as the primary site for protein synthesis, folding, and initial sorting.
• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; functions in lipid metabolism, steroid hormone synthesis, calcium storage, and detoxification of drugs/toxins.

First described by Camillo Golgi in 1898, this organelle consists of a series of flattened, curved membranous sacs called cisternae.

• Polarity: Possesses a distinct cis face (forming face, oriented toward the ER to receive transport vesicles) and a trans face (maturing face, oriented toward the plasma membrane to release secretory vesicles).
• Functions: Post-translational modification of proteins and lipids (e.g., glycosylation to form glycoproteins and glycolipids), sorting, packaging, and dispatching cargo to intracellular or extracellular destinations.

• Lysosomes: Single membrane-bound vesicles containing hydrolytic enzymes (acid hydrolases) active at acidic pH. They mediate intracellular digestion, waste degradation, and autophagy. Deficiencies in lysosomal enzymes lead to storage disorders like Tay-Sachs disease.
• Vacuoles: Membrane-bound spaces enclosed by a selective membrane called the tonoplast. In mature plant cells, a large central vacuole can occupy up to 90% of the volume, regulating turgor pressure, storing nutrients, and managing waste. Specialized forms include contractile vacuoles (for osmoregulation in amoeba) and food vacuoles.

The powerhouses of the eukaryotic cell, responsible for aerobic respiration and ATP generation via oxidative phosphorylation.

• Double-Membrane Structure: Consists of a smooth outer membrane and a highly folded inner membrane. The folds, known as cristae, maximize surface area for electron transport chain proteins.
• Compartments: The narrow peri-mitochondrial space lies between the membranes, while the innermost compartment is the mitochondrial matrix, containing enzymes for the tricarboxylic acid (TCA) cycle, circular DNA, RNA, and 70S ribosomes.

Double-membrane organelles found in plants and algae, classified based on their pigment content.

• Chromoplasts: Contain fat-soluble carotenoid pigments (carotenes, xanthophylls), imparting yellow, orange, or red colors to flowers and fruits.
• Leucoplasts: Colorless plastids specialized for nutrient storage, including amyloplasts (starch), aleuroplasts (proteins), and elaioplasts (lipids/oils).
• Chloroplasts: Green plastids containing chlorophyll and carotenoids, serving as the site of photosynthesis.

Chloroplast Internal Architecture:

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The interior is divided into the fluid-filled stroma (containing circular DNA, 70S ribosomes, and carbon-fixation enzymes) and a system of membrane-bound sacs called thylakoids. Thylakoids are stacked like coins into grana, interconnected by stroma lamellae. The thylakoid membrane houses light-harvesting complexes and ATP synthase.

Discovered by George Palade in 1955, these are non-membranous ribonucleoprotein complexes composed of ribosomal RNA (rRNA) and proteins.

• Subunit Structure: Consist of a large and a small subunit, which associate during protein translation.
• Sedimentation Coefficients: Measured in Svedberg units ($S$). Prokaryotes, mitochondria, and chloroplasts contain 70S ribosomes ($50S + 30S$ subunits). Eukaryotic cytoplasm contains 80S ribosomes ($60S + 40S$ subunits).
• Ribozymes: Catalytic RNA molecules within the ribosome that facilitate peptide bond formation.

Small, single-membrane vesicles containing specialized enzymes for metabolic reactions.

• Peroxisomes: Contain oxidases (producing hydrogen peroxide) and catalase (degrading toxic peroxide into water and oxygen). They participate in photorespiration in plants and lipid oxidation/alcohol detoxification in animals.
• Glyoxysomes: Specialized peroxisomes found in germinating fatty seeds; they contain enzymes for the glyoxylate cycle, converting stored lipids into carbohydrates (gluconeogenesis).

A dynamic network of protein filaments extending throughout the cytoplasm to provide structural support, facilitate cell motility, and coordinate intracellular transport.

• Microtubules: Hollow cylinders ($25\text{ nm}$ diameter) composed of polymerized $\alpha$- and $\beta$-tubulin dimers. They form the mitotic spindle, cilia, and flagella.
• Actin Filaments (Microfilaments): Solid rods ($6\text{ nm}$ diameter) composed of actin; essential for muscle contraction, amoeboid movement, and cytokinesis.
• Intermediate Filaments: Fibrous proteins ($10\text{ nm}$ diameter) resembling ropes; provide mechanical strength to prevent cellular deformation.

Hair-like projections from the cell surface that facilitate locomotion or fluid movement.

• Axoneme: The core structure, consisting of microtubules arranged in a characteristic eukaryotic 9+2 array (nine outer doublets surrounding a central pair). Prokaryotic flagella differ structurally, lacking this arrangement ($9+0$ array).
• Basal Body: The centriole-like structure from which the axoneme emerges.

Found primarily in animal cells near the nucleus, the centrosome contains two cylindrical centrioles oriented perpendicular to each other.

• Centriole Structure: Composed of nine triplets of microtubules arranged in a ring ($9+0$ pattern) around a central hub, resembling a cartwheel.
• Mitotic Spindle: Centrioles duplicate during the S-phase of the cell cycle and migrate to opposite poles during mitosis to organize the spindle apparatus.

• Nuclear Envelope: A double lipid bilayer separating the nucleoplasm from the cytoplasm. The outer membrane is continuous with the rough ER.
• Nuclear Pores: Complex protein channels composed of nucleoporins that regulate the bidirectional transport of macromolecules (RNA and proteins).
• Nuclear Lamina: A fibrous network of lamin proteins lining the inner nuclear membrane, providing structural support.
• Nucleolus: A non-membrane-bound region within the nucleoplasm dedicated to transcription of rRNA and assembly of ribosomal subunits.

Genetic material exists as diffuse chromatin during interphase, condensing into distinct chromosomes during cell division.

• Nucleosomes: The basic structural unit of chromatin, consisting of double-stranded DNA wrapped around octamers of basic histone proteins.
• Chromosome Classification: Divided into autosomes (non-sex chromosomes) and allosomes (sex chromosomes). Humans possess 23 pairs of chromosomes (22 pairs of autosomes and 1 pair of allosomes), totaling 46.