← Back 🏠 Home

Biology: Class 11 — Chapter 8: Chapter 8: Cell: The Unit of Life

Introduction to Cell & Cell Theory

All organisms are composed of cells. Some are single-celled (unicellular organisms) capable of independent existence and performing essential life functions, while others are composed of many cells (multicellular organisms).

What is a Cell?
  • The cell is the fundamental structural and functional unit of all living organisms.
  • Antonie Von Leeuwenhoek first saw and described a live cell.
  • Robert Brown later discovered the nucleus.
  • The invention of the microscope and electron microscope revealed structural details of the cell.

Cell Theory
  • In 1838, Matthias Schleiden (German botanist) observed that all plants are composed of different kinds of cells which form plant tissues.
  • In 1839, Theodore Schwann (German zoologist) studied animal cells and reported a thin outer layer now known as the plasma membrane. He also concluded that the presence of a cell wall is a unique character of plant cells.
  • Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and products of cells.
  • Rudolf Virchow (1855) first explained that cells divide and new cells are formed from pre-existing cells (Omnis cellula-e cellula).
  • Modern Cell Theory:
    1. All living organisms are composed of cells and products of cells.
    2. All cells arise from pre-existing cells.

An Overview of Cell
  • Inside each cell is a dense membrane-bound structure called the nucleus, which contains chromosomes and genetic material (DNA).
  • Cells with membrane-bound nuclei are eukaryotic, while those lacking a membrane-bound nucleus are prokaryotic.
  • The cytoplasm is a semi-fluid matrix occupying the cell volume, serving as the main arena for cellular activities.
  • Ribosomes are non-membrane bound organelles found in all cells (cytoplasm, chloroplasts, mitochondria, and on rough ER).
  • Animal cells contain another non-membrane bound organelle called the centrosome, which helps in cell division.
  • Cell Sizes:
    • Smallest cells: Mycoplasmas (0.3 µm in length).
    • Bacteria: 3 to 5 µm.
    • Largest isolated single cell: Egg of an ostrich.
    • Human red blood cells: About 7.0 µm in diameter.
Prokaryotic Cells

Prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma, and PPLO (Pleuro Pneumonia Like Organisms). They are generally smaller and multiply more rapidly than eukaryotic cells.

Cell Envelope and its Modifications
  • The cell envelope consists of a tightly bound three-layered structure: outermost glycocalyx, followed by the cell wall, and the plasma membrane.
  • Based on staining response (Gram stain), bacteria are classified into Gram positive and Gram negative.
  • The glycocalyx can be a loose sheath called the slime layer or a thick and tough layer called the capsule.
  • The cell wall provides structural support to prevent the bacterium from bursting or collapsing.
  • The plasma membrane is selectively permeable and structurally similar to that of eukaryotes.
  • A special membranous structure called the mesosome is formed by extensions of the plasma membrane (vesicles, tubules, lamellae) into the cell. It helps in cell wall formation, DNA replication, respiration, secretion, and increasing surface area.
  • Some prokaryotes (like cyanobacteria) have membranous extensions containing pigments called chromatophores.
  • Bacterial flagella are composed of three parts: filament, hook, and basal body.
  • Surface structures not involved in motility: pili (tubular proteins) and fimbriae (bristle-like fibres for attachment).

Ribosomes and Inclusion Bodies
  • Prokaryotic ribosomes are associated with the plasma membrane, sized about 15 nm by 20 nm, and are of the 70S type (made of 50S and 30S subunits).
  • Several ribosomes may attach to a single mRNA to form a chain called polyribosomes or polysome.
  • Inclusion bodies are non-membrane bound reserve materials stored free in the cytoplasm (e.g., phosphate granules, cyanophycean granules, glycogen granules, and gas vacuoles).
Eukaryotic Cells - Membranes & Walls

Eukaryotes include protists, plants, animals, and fungi. They exhibit extensive compartmentalisation of cytoplasm through membrane-bound organelles.

Cell Membrane
  • Composed mainly of lipids (arranged in a bilayer with polar heads outwards and hydrophobic tails inwards) and proteins.
  • In human erythrocytes, the membrane has approximately 52% protein and 40% lipids.
  • Membrane proteins are classified as peripheral (on the surface) or integral (partially or totally buried).
  • The widely accepted structure is the Fluid Mosaic Model proposed by Singer and Nicolson (1972).
  • The quasi-fluid nature of lipids enables lateral movement of proteins, measured as membrane fluidity.
  • Transport Mechanisms:
    • Passive transport: Movement of molecules along the concentration gradient without energy requirement (e.g., simple diffusion, osmosis).
    • Active transport: Energy-dependent process (utilising ATP) transporting molecules against their concentration gradient (e.g., $Na^+/K^+$ Pump).

Cell Wall
  • A non-living rigid structure forming an outer covering for the plasma membrane of fungi and plants.
  • Algae cell wall: Made of cellulose, galactans, mannans, and minerals like calcium carbonate.
  • Plant cell wall: Consists of cellulose, hemicellulose, pectins, and proteins.
  • The primary wall of a young plant cell is capable of growth, which diminishes as the cell matures, forming the secondary wall on the inner side.
  • The middle lamella is a layer of calcium pectate that glues neighbouring cells together.
  • The cell wall and middle lamellae may be traversed by plasmodesmata which connect the cytoplasm of neighbouring cells.
The Endomembrane System

Organelles whose functions are coordinated are considered together as the endomembrane system. This includes the Endoplasmic Reticulum, Golgi complex, Lysosomes, and Vacuoles.

Endoplasmic Reticulum (ER)
  • A network of tiny tubular structures scattered in the cytoplasm, dividing the intracellular space into luminal (inside ER) and extra-luminal (cytoplasm) compartments.
  • Rough Endoplasmic Reticulum (RER): Bears ribosomes on its surface; actively involved in protein synthesis and secretion.
  • Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; major site for lipid and steroidal hormone synthesis.

Golgi Apparatus
  • First observed by Camillo Golgi (1898).
  • Consists of flat, disc-shaped sacs or cisternae (0.5 µm to 1.0 µm diameter) stacked parallel to each other.
  • Features a distinct convex cis (forming) face and concave trans (maturing) face.
  • Principally performs the packaging of materials to be delivered to intracellular targets or secreted outside the cell.
  • Important site for the formation of glycoproteins and glycolipids.

Lysosomes & Vacuoles
  • Lysosomes: Membrane-bound vesicular structures formed by packaging in the Golgi apparatus. Rich in hydrolytic enzymes (hydrolases - lipases, proteases, carbohydrases) active at acidic pH.
  • Vacuoles: Membrane-bound space in the cytoplasm bound by a single membrane called tonoplast. In plant cells, it can occupy up to 90% of the volume.
  • In plants, the tonoplast facilitates transport of ions against concentration gradients into the vacuole.
  • In Amoeba, the contractile vacuole is important for osmoregulation and excretion.
Other Major Eukaryotic Organelles

Mitochondria
  • Double membrane-bound "power houses" of the cell, responsible for aerobic respiration and ATP generation.
  • Sausage-shaped or cylindrical, with a diameter of 0.2-1.0 µm and length of 1.0-4.1 µm.
  • The inner membrane forms infoldings called cristae towards the matrix to increase surface area.
  • The matrix contains a single circular DNA molecule, a few RNA molecules, 70S ribosomes, and divides by fission.

Plastids
  • Found in all plant cells and euglenoids. Classified based on pigments:
  • Chloroplasts: Contain chlorophyll and carotenoid pigments for photosynthesis. Double-membrane bound; the space inside the inner membrane is the stroma, containing organized flattened sacs called thylakoids stacked into grana.
  • Chromoplasts: Contain fat-soluble carotenoid pigments like carotene and xanthophylls, giving yellow, orange, or red colours.
  • Leucoplasts: Colourless plastids storing nutrients: amyloplasts (starch), elaioplasts (oils/fats), and aleuroplasts (proteins).

Ribosomes & Cytoskeleton
  • Ribosomes: Granular structures composed of RNA and proteins, first observed by George Palade (1953). Eukaryotic ribosomes are 80S (composed of 60S and 40S subunits).
  • Cytoskeleton: An elaborate network of filamentous proteinaceous structures (microtubules, microfilaments, intermediate filaments) providing mechanical support, motility, and maintenance of cell shape.

Cilia, Flagella & Centrosome
  • Cilia and Flagella: Hair-like outgrowths of the cell membrane. Their core (axoneme) shows a 9+2 array of microtubules (nine peripheral doublets and one central pair).
  • Centrosome: Contains two cylindrical structures called centrioles lying perpendicular to each other with a cartwheel organization. They form the basal body of cilia/flagella and spindle fibres.
Nucleus & Chromosomes

Nucleus Structure
  • First described by Robert Brown (1831). Stained material named chromatin by Flemming.
  • The nuclear envelope consists of two parallel membranes with a perinuclear space (10 to 50 nm) and is interrupted by nuclear pores.
  • The nucleoplasm contains the nucleolus (site for active ribosomal RNA synthesis) and chromatin.
  • Chromatin contains DNA, basic proteins called histones, non-histone proteins, and RNA.

Chromosomes
  • Every chromosome has a primary constriction or centromere with disc-shaped structures called kinetochores on its sides.
  • Types of Chromosomes based on Centromere Position:
    • Metacentric: Centromere in the middle, forming two equal arms.
    • Sub-metacentric: Centromere slightly away from the middle, resulting in one shorter and one longer arm.
    • Acrocentric: Centromere situated close to its end, forming one extremely short and one very long arm.
    • Telocentric: Has a terminal centromere.
  • Some chromosomes have non-staining secondary constrictions forming a constant fragment called a satellite.