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ICSE Class X Notes 2026 : Biology

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Ultra-detailed notes on the cell complete, refined, and paragraph form Overview and fundamental principles The cell is the fundamental structural and functional unit of life, an integrated chemical factory and information processor bounded by a selectively permeable membrane that maintains an internal milieu distinct from the external environment. Cells demonstrate organizational hierarchies that range from macromolecules to supramolecular complexes and organelles, and they perform energy transduction, information storage and processing, biosynthesis, transport, communication, motility, and regulated self-replication. Classical cell theory asserts that every living organism is composed of one or more cells, that the cell is the basic unit of structure and function in organisms, and that all cells arise from preexisting cells; modern refinements add that genetic information flows through nucleic acids and proteins within cells and that emergent cellular properties arise from dynamic networks of interactions rather than from single parts acting alone. Cells are broadly categorized into two principal organizational types: prokaryotic cells, which lack membrane-bound organelles and a nucleus and typically possess a single circular chromosome, and eukaryotic cells, which compartmentalize functions within membrane-bound organelles, package their genomic information in linear chromosomes, and use complex mechanisms for intracellular trafficking and regulation. Plasma membrane: architecture, physical chemistry, and transport functions The plasma membrane is a fluid mosaic composed primarily of a bilayer of amphipathic lipids interspersed with proteins that are integral, peripheral, or lipid-anchored; lipid diversity includes phospholipids, sphingolipids, and sterols, and lateral heterogeneity creates microdomains such as lipid rafts that concentrate signaling molecules. Membrane asymmetry is a defining feature, with particular lipids preferentially localized to the inner or outer leaflet and with glycoconjugates facing the extracellular space where they contribute to the glycocalyx and cell recognition. Integral membrane proteins perform selective transport through channels and carriers, transduce signals as receptors, anchor the cytoskeleton, and catalyze reactions at the surface. Membrane physical properties such as fluidity, curvature, and tension are modulated by lipid composition, temperature, and interactions with cytoskeletal elements. Passive transport processes include simple diffusion of small nonpolar molecules, facilitation by channel proteins and carrier-mediated facilitated diffusion, and osmosis for water across aquaporins and through the bilayer. Active transport consumes metabolic energy to move solutes against electrochemical gradients using primary pumps such as the adenosine triphosphate-dependent sodium-potassium pump and proton pumps, and secondary active transport harnesses existing gradients to drive co-transport. Vesicular transport such as endocytosis and exocytosis accomplishes bulk uptake and secretion; endocytic pathways subdivide into clathrin-mediated, caveolar, and clathrin-independent routes, with receptor-mediated endocytosis providing selectivity. Membrane trafficking is governed by small GTPases including Rabs and Arfs, coat proteins like clathrin and COP complexes, and SNARE proteins that specify membrane fusion events. Cytoplasm, cytosol, and cytoskeleton: scaffold and dynamics The cytoplasm is the aqueous compartment containing soluble metabolic enzymes, ribosomes, inclusions, and the cytoskeleton; the cytosol refers more specifically to the fluid phase. The cytoskeleton consists of three principal filament systems microfilaments composed of actin, microtubules built from alpha- and beta-tubulin heterodimers, and intermediate filaments assembled from a family of cell-type specific proteins that together provide mechanical support, determine cell shape, enable intracellular transport, and power motility. Actin filaments are dynamic polymers that undergo treadmilling regulated by actin-binding proteins and ATP hydrolysis, and they interact with myosin motors to generate contractile force in processes such as cytokinesis and cell migration. Microtubules nucleated from centrosomes or microtubule organizing centers form polarized tracks for kinesin and dynein motor proteins and are essential for mitotic spindle assembly, vesicle transport, and organelle positioning. Intermediate filaments confer tensile strength and maintain structural integrity under stress. The cytoplasm is not a simple well-mixed solution; molecular crowding influences reaction kinetics, phase separation of proteins and RNAs forms membraneless organelles such as stress granules and nucleoli, and local microenvironments regulate biochemical flux. Nucleus, chromatin, and the nucleolus: genomic control center The nucleus in eukaryotic cells houses genomic DNA organized with histone and nonhistone proteins into chromatin, and it is bounded by a double membrane perforated by nuclear pore complexes that regulate bidirectional macromolecular traffic. Chromatin exists in dynamic states of compaction, with euchromatin being transcriptionally active and accessible, and heterochromatin being more condensed and transcriptionally repressed; chromatin structure is governed by nucleosome positioning, histone post-translational modifications such as acetylation and methylation, DNA methylation, and chromatin remodelers that slide or evict nucleosomes. The nucleolus is a specialized subnuclear compartment where ribosomal RNA genes are transcribed by RNA polymerase I, ribosomal RNA is processed, and ribosomal subunits are assembled; nucleolar activity correlates with cellular growth rates. Nuclear pore complexes are large multiprotein assemblies that mediate selective transport through signals such as nuclear localization sequences and nuclear export sequences, mediated by importins and exportins under the control of Ran GTPase. DNA replication is temporally regulated across the S phase of the cell cycle, involving origin licensing, helicase activity, synthesis by DNA polymerases, and error correction by proofreading and repair pathways. Multiple DNA repair mechanisms base excision repair, nucleotide excision repair, mismatch repair, homologous recombination, and non-homologous end joining maintain genome integrity. Telomeres protect chromosome ends and are extended by telomerase in germline and certain stem cells, while somatic cells generally exhibit telomere shortening that contributes to replicative senescence.

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