Discovery of Cell: Robert Hooke (1665) discovered cells while observing thin slices of cork under a microscope. He saw box-like structures and named them 'cells'.
Living Cells: Anton van Leeuwenhoek (1674) first observed living cells in pond water, bacteria, and blood cells.
Cell Theory: Proposed by Schleiden (1838) and Schwann (1839). States: (1) All living organisms are made of cells, (2) Cell is the basic structural and functional unit of life, (3) All cells arise from pre-existing cells (added by Virchow, 1855).
Modern Cell Theory: Includes: (1) Cells contain hereditary information (DNA) passed from cell to cell, (2) All cells have the same basic chemical composition, (3) Energy flow occurs within cells.
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Cell Structure - Plasma Membrane
Plasma Membrane: Outermost covering of the cell (in animal cells). Thin, elastic, living membrane.
Structure: Fluid Mosaic Model by Singer and Nicolson (1972). Composed of lipid bilayer with embedded proteins.
Functions: (1) Protects cell contents, (2) Regulates entry and exit of substances (selectively permeable), (3) Provides shape to cell, (4) Cell recognition and communication.
Selective Permeability: Allows certain substances to pass while blocking others. Small molecules (O₂, CO₂, water) pass easily; large molecules need transport proteins.
Diagram Importance: Structure of plasma membrane showing lipid bilayer, proteins, and carbohydrate chains is crucial for ICSE exams.
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Cell Wall (Plant Cells)
Cell Wall: Rigid, non-living outer covering present only in plant cells, fungi, and bacteria.
Composition: Made of cellulose in plants. Provides rigidity and shape.
Functions: (1) Provides mechanical strength and protection, (2) Maintains cell shape, (3) Prevents excessive water entry (prevents bursting), (4) Allows exchange of materials through plasmodesmata.
Plasmodesmata: Cytoplasmic connections between adjacent plant cells through cell walls, allowing communication and transport.
Difference from Plasma Membrane: Cell wall is rigid and non-living; plasma membrane is flexible and living.
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Cytoplasm and Cell Organelles
Cytoplasm: Jelly-like substance between plasma membrane and nucleus. Contains water, salts, enzymes, and cell organelles.
Cytosol: Liquid portion of cytoplasm where organelles are suspended.
Functions: (1) Site of many metabolic reactions, (2) Provides medium for organelles, (3) Stores chemicals, (4) Gives shape to cell.
Cell Organelles: Specialized structures within cells performing specific functions. Include mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, ribosomes, plastids, vacuoles.
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Nucleus - Control Center
Nucleus: Largest organelle; control center of cell. Contains genetic material (DNA).
Nuclear Membrane: Double-layered membrane with nuclear pores allowing exchange of materials between nucleus and cytoplasm.
Nucleoplasm: Jelly-like substance inside nucleus containing chromatin and nucleolus.
Chromatin: Network of thread-like structures made of DNA and proteins. Condenses to form chromosomes during cell division.
Nucleolus: Dense spherical body inside nucleus. Site of ribosome synthesis (rRNA production).
Functions: (1) Controls all cell activities, (2) Stores hereditary information (genes), (3) Regulates protein synthesis, (4) Controls cell division.
Diagram Importance: Labeled diagram of nucleus showing nuclear membrane, pores, nucleolus, and chromatin is frequently asked.
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Mitochondria - Powerhouse of Cell
Mitochondria: Rod-shaped or oval organelles; site of cellular respiration.
Cristae: Folds of inner membrane increasing surface area for ATP production.
Functions: (1) Cellular respiration - breakdown of glucose to produce ATP (energy currency), (2) Aerobic respiration occurs here.
ATP (Adenosine Triphosphate): Energy currency of cell. Mitochondria produce ATP through oxidative phosphorylation.
Why 'Powerhouse': Mitochondria generate most of the cell's energy (ATP), hence called powerhouse.
Own DNA: Mitochondria have their own DNA and ribosomes, can self-replicate. Evidence of endosymbiotic theory.
Diagram Importance: Cross-section showing outer membrane, inner membrane with cristae, and matrix is essential.
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Endoplasmic Reticulum (ER)
Endoplasmic Reticulum: Network of membrane-bound tubules and sacs extending from nuclear membrane to plasma membrane.
Types: (1) Rough ER (RER) - has ribosomes on surface, (2) Smooth ER (SER) - lacks ribosomes.
Rough ER Functions: (1) Protein synthesis (due to ribosomes), (2) Transport of proteins, (3) Formation of nuclear membrane after cell division.
Smooth ER Functions: (1) Lipid and steroid synthesis, (2) Detoxification of drugs and poisons, (3) Calcium storage in muscle cells.
Structure: Interconnected network providing large surface area for biochemical reactions.
Importance: Acts as cell's transport system, moving materials between organelles.
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Golgi Apparatus - Packaging and Dispatch
Golgi Apparatus: Stack of flattened membrane-bound sacs (cisternae). Discovered by Camillo Golgi.
Structure: Consists of cis face (receiving side) and trans face (shipping side).
Functions: (1) Modification and packaging of proteins from ER, (2) Formation of lysosomes, (3) Secretion of enzymes and hormones, (4) Formation of cell plate during plant cell division.
Process: Proteins from ER → Golgi (modification) → Vesicles (packaging) → Secretion or use in cell.
Importance: Acts as cell's post office - receives, modifies, packages, and dispatches materials.
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Lysosomes - Suicide Bags
Lysosomes: Small spherical vesicles containing digestive enzymes. Discovered by Christian de Duve.
Enzymes: Contain hydrolytic enzymes (lipases, proteases, carbohydrases) that can digest all organic materials.
Functions: (1) Intracellular digestion of food particles, (2) Removal of dead and damaged cells, (3) Digestion of foreign materials (bacteria), (4) Autolysis (self-digestion during starvation).
Why 'Suicide Bags': If lysosome membrane ruptures, enzymes digest cell's own components, leading to cell death.
Autophagy: Process where lysosomes digest damaged organelles to recycle components.
Importance in White Blood Cells: Lysosomes in WBCs digest bacteria and foreign particles, providing immunity.
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Ribosomes - Protein Factories
Ribosomes: Smallest organelles; not membrane-bound. Composed of RNA and proteins.
Structure: Made of two subunits (large and small). Composed of ribosomal RNA (rRNA) and proteins.
Location: (1) Free in cytoplasm, (2) Attached to rough ER, (3) Inside mitochondria and chloroplasts.
Function: Site of protein synthesis. Read mRNA and assemble amino acids into proteins (translation).
Importance: Essential for cell growth and repair as proteins are building blocks of cells.
Polyribosomes: Group of ribosomes reading same mRNA simultaneously, increasing protein production efficiency.
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Plastids (Plant Cells Only)
Plastids: Large, membrane-bound organelles found only in plant cells and some protists.
Diagram Importance: Chloroplast structure showing outer membrane, inner membrane, grana, thylakoids, and stroma is crucial.
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Vacuoles - Storage Organelles
Vacuoles: Membrane-bound sacs filled with cell sap (water, minerals, sugars, pigments).
In Plant Cells: Large central vacuole occupying 50-90% of cell volume. Provides turgidity and rigidity.
In Animal Cells: Small, numerous, temporary vacuoles. Used for storage and transport.
Functions: (1) Storage of water, minerals, sugars, proteins, (2) Maintains turgor pressure in plant cells, (3) Storage of waste products, (4) Provides color to flowers and fruits (anthocyanin pigments).
Tonoplast: Membrane surrounding vacuole, regulating movement of substances in and out.
Turgor Pressure: Pressure exerted by cell sap on cell wall. Keeps plant cells rigid and plant upright.
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Centrosome and Centrioles (Animal Cells)
Centrosome: Organelle near nucleus containing two centrioles. Found in animal cells, absent in plant cells.
Centrioles: Pair of cylindrical structures made of microtubules arranged in 9+0 pattern.
Function: (1) Form spindle fibers during cell division, (2) Help in chromosome separation, (3) Form cilia and flagella.
Importance in Cell Division: Centrioles migrate to opposite poles and form spindle apparatus for chromosome movement.
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Differences: Plant Cell vs Animal Cell
Cell Wall: Present in plant cells; absent in animal cells.
Plastids: Present in plant cells (chloroplasts, chromoplasts, leucoplasts); absent in animal cells.
Vacuoles: Large central vacuole in plant cells; small, numerous vacuoles in animal cells.
Centrosome: Absent in plant cells; present in animal cells.
Shape: Plant cells have fixed shape (due to cell wall); animal cells have irregular shape.
Nutrition: Plant cells are autotrophic (make own food); animal cells are heterotrophic (consume food).
Diagram Importance: Comparative diagrams of plant and animal cells showing these differences are frequently asked in ICSE exams.
Characteristics: (1) No nuclear membrane, (2) DNA lies freely in cytoplasm (nucleoid), (3) No membrane-bound organelles, (4) Small ribosomes (70S), (5) Cell wall present.