Cell – The Structural and Functional Unit of Life

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.

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.

Composition: Lipids (phospholipids, cholesterol), Proteins (integral and peripheral), Carbohydrates (glycoproteins, glycolipids).

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.

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.

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.

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.

Mitochondria: Rod-shaped or oval organelles; site of cellular respiration.

Structure: Double membrane - outer smooth membrane and inner folded membrane forming cristae. Matrix contains enzymes.

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.

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.

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.

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.

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.

Plastids: Large, membrane-bound organelles found only in plant cells and some protists.

Types: (1) Chloroplasts (green), (2) Chromoplasts (colored), (3) Leucoplasts (colorless).

Chloroplasts: Contain chlorophyll; site of photosynthesis. Convert light energy to chemical energy (glucose).

Chloroplast Structure: Double membrane. Internal membrane forms thylakoids stacked into grana. Stroma is the fluid.

Chromoplasts: Contain pigments (carotenoids) giving red, orange, yellow colors to flowers and fruits.

Leucoplasts: Colorless plastids storing food. Types: Amyloplasts (starch), Elaioplasts (oils), Aleuroplasts (proteins).

Diagram Importance: Chloroplast structure showing outer membrane, inner membrane, grana, thylakoids, and stroma is crucial.

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.

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.

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.

Prokaryotic Cells: Primitive cells without true nucleus. Example: Bacteria, blue-green algae.

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.

Eukaryotic Cells: Advanced cells with true nucleus. Example: Plants, animals, fungi, protists.

Characteristics: (1) True nucleus with nuclear membrane, (2) DNA organized into chromosomes, (3) Membrane-bound organelles present, (4) Large ribosomes (80S), (5) Complex cell structure.

Size: Prokaryotic cells (1-10 μm) are smaller than eukaryotic cells (10-100 μm).

Evolution: Eukaryotic cells evolved from prokaryotic cells through endosymbiosis.