Notes Class 11 Chapter 5 Biology Maharashtra Board

Cell Structure and Organization

Introduction

• Cell: The structural and functional unit of life, capable of independent existence and performing all life functions.
• Microscopes: Essential for observing cells.
  • Simple Microscope: Magnifies 50-100 times, suitable for larger cells.
  • Compound Microscope: Magnifies up to 1000 times, used for smaller cells.
  • Light Microscope: Uses a beam of light for visibility.
  • Electron Microscope: Magnifies up to 500,000 times, used to see cell interior details.
• Cell Diversity:
  • Shape: Spherical, rectangular, flattened, polygonal, oval, triangular, conical, columnar, etc.
  • Size: Varies greatly:
    • Smallest: Mycoplasma (0.3 µm).
    • Bacterial cell: 3-5 µm.
    • Largest: Ostrich egg (~15 cm).
    • Longest: Nerve cells.

Historical Contributions

• Robert Hooke (1665): Coined the term “cell” in Micrographia after observing cork cells.
• Leeuwenhoek (late 1600s): Developed the first microscope and observed cells.
• Matthias Schleiden (1838): Concluded that plant tissues are composed of cells.
• Theodore Schwann (1839): Proposed that animal tissues are made of cells and identified the cell wall as unique to plant cells.
• Rudolf Virchow (1855): Stated that new cells arise from pre-existing cells (Omnis cellula-e-cellula).
• Other Scientists:
  • Purkinje and Mohl (1835-37): Discovered protoplasm.
  • Camillo Golgi (1838): Discovered Golgi apparatus.
  • Robert Brown (1881): Discovered the nucleus.
  • Balbiani (1881): Discovered chromosomes.
  • Flemming (1882): Studied mitosis.
  • Porter (1945): Discovered endoplasmic reticulum.
  • C. Benda: Named mitochondria.
  • C. de Duve (1955): Discovered lysosomes.
  • Venkatraman Ramakrishnan (2009): Nobel Prize for explaining ribosome structure.

Cell Theory

• Classical Cell Theory (Schwann and Schleiden):
  • All living organisms are made of cells.
  • Cells are the basic unit of life.
• Modern Cell Theory (modified with advancements):
  • All living organisms are made of cells.
  • Living cells arise from pre-existing cells.
  • Cells are the structural and functional units of life.
  • Cellular activities drive organismal functions.
  • Cells transform energy.
  • Cells contain nucleic acids (DNA and RNA) in the nucleus and cytoplasm.
• Totipotency: The ability of a nucleated cell to differentiate into any cell type and form a new organism. Embryonic animal stem cells are totipotent, with medical applications (e.g., disease treatment).

Types of Cells

A. Prokaryotic Cells

• Characteristics:
  • Simple cellular organization, no well-defined nucleus, or membrane-bound organelles.
  • Chemically complex cell envelope with three layers:
    • Glycocalyx: Slime layer (loose) or capsule (tough).
    • Cell Wall: Made of peptidoglycan (Gram-positive) or murein (Gram-negative), provides mechanical strength.
    • Plasma Membrane: Phospholipid bilayer, aids in transport.
  • Gram Staining: Developed by Hans Christian Gram to differentiate bacteria based on cell wall composition.
• Surface Structures:
  • Flagella/Cilia: Motile bacteria have flagella or cilia driven by a basal body (smallest motor in the world due to rotary movement).
  • Pili: Tubular, aid in intercellular communication.
  • Fimbriae: Help in clinging to surfaces.
• Internal Structures:
  • Mesosomes: Infoldings of the plasma membrane, aid in cell wall formation, respiration, and DNA replication.
  • Chromatophores: In photosynthetic cyanobacteria, carry pigments.
  • Ribosomes: 70S (50S + 30S subunits), for protein synthesis.
  • Nucleoid: Single, circular, coiled DNA (not associated with histones), attached to mesosomes, undergoes theta replication.
  • Plasmids: Small, circular, extrachromosomal DNA (e.g., F-plasmid for reproduction, R-plasmid for antibiotic resistance).
  • Cytoplasm: Contains water, enzymes, amino acids, and inclusion bodies (e.g., cyanophycean starch, glycogen, phosphate, sulphur granules).

B. Eukaryotic Cells

• Characteristics:
  • Complex organization with a defined nucleus (bounded by nuclear membrane) and membrane-bound organelles.
  • Found in protists, plants, animals, and fungi.
  • Components: Plasma membrane, cytoplasm, organelles (mitochondria, ER, ribosomes, Golgi complex, etc.), and nucleus.

Components of Eukaryotic Cells

1. Cell Wall

• Presence: In plant cells, fungi, and some protists (e.g., algae).
• Composition:
  • Algae: Cellulose, galactans, mannans, calcium carbonate.
  • Plants: Hemicelluloses, pectin, lipids, proteins, with cellulose microfibrils for rigidity.
  • Other deposits: Silica (grass stem), cutin (epidermal walls), suberin (endodermal root cells), wax, lignin.
• Structure:
  • Middle Lamella: Thin, pectin-based, formed during cytokinesis, softens in ripe fruits.
  • Primary Wall: In young cells, capable of growth, found in meristematic tissue, mesophyll, pith.
  • Secondary Wall: Formed after primary wall growth stops, with pits (unthickened areas).
• Functions: Provides shape, protects from mechanical injury and infections.
• Plasmodesmata: Cytoplasmic bridges through pores in the cell wall for intercellular communication.

2. Plasma Membrane (Biomembrane)

• Structure:
  • Thin (~75 Å), quasifluid, trilamellate (three-layered) under electron microscope.
  • Composed of phospholipid bilayer (amphipathic: hydrophilic heads, hydrophobic tails), proteins (52% in human RBCs), and carbohydrates (e.g., glycoproteins, glycolipids).
• Fluid Mosaic Model (Singer and Nicolson, 1972):
  • Phospholipid bilayer with embedded proteins (intrinsic/transmembrane or extrinsic/peripheral).
  • Proteins move laterally, contributing to fluidity.
• Functions:
  • Selective Permeability: Regulates molecule transport.
  • Passive Transport: No energy required (e.g., simple diffusion, osmosis).
  • Active Transport: Energy-dependent (e.g., Na+/K+ pump using ATP).
  • Carrier Proteins: Facilitate transport of polar molecules across the non-polar lipid bilayer.

3. Cytoplasm

• Composition: Colloidal, jelly-like cytoplasmic matrix (cytosol) with water, sugars, amino acids, vitamins, enzymes, nucleotides, minerals, and waste products.
• Movement: Exhibits cyclosis (streaming movements).
• Functions:
  • Acts as a source of raw materials and a site for metabolic activities.
  • Facilitates material exchange between organelles.
• Organelles: Includes endoplasmic reticulum, Golgi complex, mitochondria, plastids, nucleus, microbodies, and cytoskeletal elements (microtubules, microfilaments, intermediate filaments).
• Endomembrane System: Includes nuclear membrane, ER, Golgi complex, lysosomes, vesicles, and vacuoles, coordinating specific functions.

4. Endoplasmic Reticulum (ER)

• Structure: Network of membranous tubules and sacs (cisternae), continuous with the nuclear envelope and plasma membrane.
• Types:
  • Smooth ER (SER): No ribosomes; synthesizes lipids (e.g., steroids in adrenal glands), detoxifies drugs (liver), stores calcium (muscle cells).
  • Rough ER (RER): Studded with ribosomes; synthesizes secretory proteins (e.g., insulin in pancreatic cells) and forms cell membranes.
• Functions:
  • Supports intracellular framework, maintains organelle positions.
  • RER produces transport vesicles for protein secretion.

5. Golgi Complex (Golgi Apparatus)

• Structure: Stacks of hollow membranous sacs (cisternae, 0.5-1 µm diameter), with cis face (receiving, near ER) and trans face (maturing, for secretion).
• Cisternal Maturation Model: Cisternae mature from cis to trans, recycling enzymes.
• Functions:
  • Modifies ER secretions (e.g., alters glycoproteins/glycolipids).
  • Produces own secretions (e.g., pectin in plant cells).
  • Packages products into transport vesicles for secretion or targeting specific organelles.

6. Lysosomes

• Structure: Membrane-bound vesicles with hydrolytic enzymes (amylases, proteases, lipases) active in acidic pH, arising from Golgi-associated ER.
• Types:
  • Primary Lysosomes: Inactive enzyme vesicles.
  • Secondary (Hybrid) Lysosomes: Formed by fusion with endocytic vesicles, digest materials (heterophagic vesicles).
  • Residual Bodies: Contain undigested remains.
  • Autophagic Vesicles: Digest damaged organelles or molecules (suicide bags).
• Functions:
  • Intracellular Digestion: Breaks down materials (e.g., in macrophages, amoeba food vacuoles).
  • Extracellular Digestion: Releases enzymes (e.g., hyaluronidase in sperm acrosome for fertilization).
  • Recycling (e.g., liver cells recycle macromolecules weekly).
• Note: Lysosomal enzyme deficiencies cause disorders (e.g., Tay-Sachs disease due to lipase insufficiency).

7. Vacuoles

• Presence: Prominent in plant cells (2-3 per cell, or a single central vacuole occupying 90% volume); smaller and fewer in animal cells.
• Structure: Bound by tonoplast membrane, containing cell sap (hypertonic to cytosol, with ions, pigments, or proteins).
• Functions:
  • Stores ions, nutrients (e.g., proteins in seeds), or protective compounds (unpalatable to herbivores).
  • Maintains turgidity in plant cells.
  • Involved in phagocytosis (food vacuoles) or osmoregulation (contractile vacuoles in Paramecium).
  • Contributes to petal coloration via pigment storage.

8. Microbodies

• Structure: Minute, membrane-bound sacs in plant and animal cells.
• Types:
  • Sphaerosomes: Store fats (e.g., in endosperm of oil seeds), have a half-unit membrane.
  • Peroxisomes: Contain enzymes to produce and break down hydrogen peroxide, detoxify substances (e.g., alcohol in liver cells).
  • Glyoxysomes: Convert fatty acids to sugars in germinating seeds via reverse glycolysis.

9. Mitochondria

• Structure:
  • Double-membraned: Outer membrane (permeable, with porin proteins); inner membrane (selectively permeable, with cristae for increased surface area).
  • Matrix: Contains circular DNA, 70S ribosomes, RNA, lipids, and Krebs cycle enzymes.
  • Oxysomes: Particles on inner membrane with F1 head (ATP synthase) and F0 foot (proton channel) for ATP synthesis.
• Functions: Powerhouse of the cell, producing ATP via aerobic respiration (Krebs cycle, electron transport chain).
• Absence: In prokaryotes and mature RBCs.
• Endosymbiont Theory: Mitochondria evolved from engulfed aerobic prokaryotes.

10. Plastids

• Structure: Double-membraned, containing DNA, RNA, and 70S ribosomes; larger than mitochondria, visible under light microscope.
• Types:
  • Leucoplasts: Colorless, store nutrients (e.g., amyloplasts for starch, elaioplasts for oils, aleuroplasts for proteins).
  • Chromoplasts: Contain carotene/xanthophyll, impart red/yellow/orange colors to flowers/fruits.
  • Chloroplasts: Contain chlorophyll for photosynthesis; found in green plant cells (e.g., mesophyll).
    • Structure: Inner membrane encloses thylakoids (stacked as grana, connected by stromal lamellae), with stroma containing DNA, ribosomes, and photosynthetic enzymes.
• Endosymbiont Theory: Chloroplasts evolved from engulfed photosynthetic prokaryotes.

11. Ribosomes

• Structure: Non-membranous, made of rRNA and proteins; discovered by Pallade (1953).
• Types:
  • Prokaryotic: 70S (50S + 30S subunits).
  • Eukaryotic: 80S (60S + 40S subunits) in cytoplasm; 70S in mitochondria/plastids.
• Location: Attached to RER/nuclear membrane or free in cytoplasm; form polyribosomes for protein synthesis.
• Functions:
  • Bound Ribosomes: Produce proteins for export (e.g., pancreatic enzymes).
  • Free Ribosomes: Synthesize cytoplasmic enzymes (e.g., for sugar breakdown).
• Svedberg Unit (S): Measures ribosome sedimentation rate (density/size).

12. Nucleus

• Structure (visible in interphase):
  • Nuclear Envelope: Double membrane with perinuclear space (10-50 nm), connected to ER, with nucleopores for material exchange.
  • Nuclear Lamina: Protein network maintaining shape.
  • Nucleoplasm (Karyolymph): Contains nucleic acids, proteins, minerals, salts, chromatin, and nucleolus.
  • Nucleolus: Non-membranous, made of rRNA and proteins, site of ribosome biogenesis.
  • Chromatin: DNA, histones, non-histone proteins, RNA; euchromatin (active, extended) vs. heterochromatin (inactive, condensed).
• Functions:
  • Stores genetic information, controls heredity and variation.
  • Site of DNA, RNA, and ribosome synthesis.
  • Regulates protein synthesis and cell division.
  • Constant chromosome number aids phylogenetic studies.

13. Cytoskeleton

• Structure: Network of fibrils throughout cytoplasm.
• Components:
  • Microtubules: Made of tubulin.
  • Microfilaments: Made of actin.
  • Intermediate Filaments: Made of fibrous proteins.
• Functions: Maintains cell shape, enables contraction, mobility, organelle movement, and cell division.

14. Cilia and Flagella

• Structure: Hair-like, membrane-bound protoplasmic outgrowths.
  • Cilia: Small, numerous, act as oars.
  • Flagella: Longer, fewer, in eukaryotes differ from prokaryotic flagella.
  • Components: Basal body (derived from centriole, with nine triplet fibrils), shaft (with sheath and axoneme of 9+2 microtubules).
• Functions: Generate fluid currents for material passage and locomotion.

15. Centrioles and Centrosomes

• Structure:
  • Centrosome: Near nucleus in animal cells, contains two perpendicular centrioles in pericentriolar material.
  • Centriole: Nine triplet microtubules, connected by non-tubulin proteins, with a hub and radial spokes at the proximal end (cartwheel appearance).
• Functions: Forms spindle apparatus during cell division, serves as basal body for cilia/flagella.

Key Differences: Prokaryotic vs. Eukaryotic Cells