Cell: The Unit of Life – Comprehensive Study Guide

Introduction to Reductionist Biology

• Definition of Biology: The study of living organisms. While early descriptions focused on the diversity of form and appearance, the cell theory emphasized the underlying unity of all life forms through their cellular organization.

• Reductionist Biology: A physico-chemical approach to understanding biological processes. It involves:

  • Analyzing living tissues for elements and compounds.

  • Investigating processes (digestion, excretion, memory, defense, recognition, etc.) in molecular terms using cell-free systems.

  • Applying concepts and techniques from physics and chemistry to explain both normal and diseased conditions.

• Scope of Unit 3: Includes Chapter 8 (Cell: The Unit of Life), Chapter 9 (Biomolecules), and Chapter 10 (Cell Cycle and Cell Division).

G.N. Ramachandran and Structural Biology

• G.N. Ramachandran ($1922$ – $2001$): An outstanding figure in protein structure and founder of the ‘Madras school’ of conformational analysis of biopolymers.

• Key Contributions:

  • Triple Helical Structure of Collagen: Published in Nature in $1954$.

  • Ramachandran Plot: A tool used to analyze the allowed conformations of proteins; a foundational contribution to structural biology.

• Background: Born October $8$, $1922$, in a town near Cochin, India. Influenced by his father, a mathematics professor.

• Education: Top-ranking student in B.Sc. (Honors) Physics at the University of Madras ($1942$); Ph.D. from Cambridge University ($1949$).

• Influences: Met Linus Pauling at Cambridge; Pauling’s work on the $\alpha$-helix and $\beta$-sheet models directed Ramachandran’s focus toward solving the structure of collagen.

The Fundamental Unit of Life: The Cell

• Definition: The cell is the basic structural and functional unit of all living organisms. All organisms are composed of cells.

• Classification:

  • Unicellular Organisms: Single-celled entities capable of independent existence and performing all essential life functions. Anything less than a complete cell structure cannot ensure independent living.

  • Multicellular Organisms: Composed of many cells working in coordination.

• Early Discoveries:

  • Antonie Von Leeuwenhoek: First to see and describe a live cell.

  • Robert Brown: Discovered the nucleus.

  • Microscopy: The invention of the electron microscope allowed for the revelation of detailed internal structures.

The Cell Theory

• Matthias Schleiden ($1838$): A German botanist who observed that all plants are composed of different kinds of cells forming plant tissues.

• Theodore Schwann ($1839$): A German zoologist who studied animal cells and identified a thin outer layer now called the plasma membrane. He also concluded that the cell wall is a unique characteristic of plant cells.

• Swan and Schleiden's Hypothesis: Proposed that the bodies of animals and plants are composed of cells and their products. However, they could not explain the origin of new cells.

• Rudolf Virchow ($1855$): Formulated the concept of Omnis cellula-e cellula, meaning all cells arise from pre-existing cells through division.

• Modern Cell Theory Principles:

  1. All living organisms are composed of cells and products of cells.

  2. All cells arise from pre-existing cells.

Overview of Cellular Diversity

• Typical Models:

  • Onion Peel Cell: A typical plant cell with a distinct cell wall and a cell membrane.

  • Human Cheek Cell: An animal cell with only an outer delimiting membrane.

• Core Components:

  • Nucleus: A dense, membrane-bound structure containing chromosomes and genetic material (DNA).

  • Cytoplasm: A semi-fluid matrix and the main arena of cellular activities and chemical reactions required for the "living state."

• Broad Categories:

  • Eukaryotic: Cells with membrane-bound nuclei and organelles.

  • Prokaryotic: Cells lacking a membrane-bound nucleus and organelles.

• Commonalities: Ribosomes (non-membrane-bound) are found in both types.

• Cell Size and Shape:

  • Smallest Cell: Mycoplasmas ($0.3\,\mu m$ in length).

  • Bacteria: Typically $3$ to $5\,\mu m$; typical bacteria range from $1$ to $2\,\mu m$.

  • Largest Isolated Single Cell: Ostrich egg.

  • Human RBCs: Approximately $7.0\,\mu m$ in diameter.

  • Longest Cells: Nerve cells.

  • Shapes: Disc-like, polygonal, columnar, cuboid, thread-like, or irregular, often varying with function.

Prokaryotic Cells

• Representatives: Bacteria, blue-green algae (Cyanobacteria), Mycoplasma, and PPLO (Pleuro Pneumonia Like Organisms).

• Characteristics: Generally smaller and multiply faster than eukaryotes. They possess a cell wall (except for Mycoplasma) surrounding the plasma membrane.

• Genetic Structure:

  • Genomic DNA: Single circular chromosome found naked in the cytoplasm (no nuclear membrane).

  • Plasmids: Small circular DNA outside the genomic DNA. They provide unique phenotypes, such as antibiotic resistance, and are used in monitoring bacterial transformation.

• Specialized Structures:

  • Mesosome: A specialized, differentiated form of the cell membrane (infoldings like vesicles, tubules, and lamellae). Functions include cell wall formation, DNA replication/distribution, respiration, secretion, and increasing surface area.

  • Inclusions: Non-membrane-bound reserve materials (e.g., phosphate, cyanophycean, and glycogen granules). Gas vacuoles are found in photosynthetic bacteria.

The Prokaryotic Cell Envelope

• Structure: A tightly bound three-layered system acting as a single protective unit.

  1. Glycocalyx: The outermost layer. Can be a loose slime layer or a thick, tough capsule.

  2. Cell Wall: Determines shape and prevents bursting or collapsing.

  3. Plasma Membrane: Selectively permeable; interacts with the environment.

• Gram Staining: Developed by Gram to classify bacteria based on envelope differences:

  • Gram Positive: Take up the stain.

  • Gram Negative: Do not take up the stain.

• Motility Structures:

  • Flagella: Thin filamentous extensions. Components: Filament (longest), hook, and basal body.

  • Pili: Elongated tubular structures made of special protein.

  • Fimbriae: Small bristle-like fibers for attachment to rocks or host tissues.

Eukaryotic Cell Structure

• Classification: Includes protists, plants, animals, and fungi.

• General Features: Extensive compartmentalization of cytoplasm via membrane-bound organelles, organized nucleus with envelope, and complex locomotory/cytoskeletal structures.

• Plant vs. Animal Cells:

  • Plant Cells: Have cell walls, plastids, and a large central vacuole.

  • Animal Cells: Have centrioles (in the centrosome) for cell division; lack cell walls and plastids.

The Plasma Membrane

• Composition: Composed of lipids (phospholipids and cholesterol), proteins, and carbohydrates.

  • Phospholipid Bilayer: Polar heads face outward (aqueous environment); hydrophobic tails of saturated hydrocarbons face inward.

  • Protein-to-Lipid Ratio: In human erythrocytes (RBCs), it is approximately $52\,\%$ protein and $40\,\%$ lipids.

• Fluid Mosaic Model: Proposed by Singer and Nicolson ($1972$).

  • Quasi-fluid nature: Lipids allow lateral movement of proteins, defining fluidity.

  • Functions: Cell growth, secretion, endocytosis, and cell division.

• Transport Mechanism:

  • Passive Transport: Move along concentration gradient without energy. Includes simple diffusion (neutral solutes) and osmosis (water movement).

  • Facilitated Transport: Polar molecules require carrier proteins.

  • Active Transport: Energy-dependent process using ATP to move molecules against gradient (e.g., $Na^+/K^+$ Pump).

Cell Wall and Endomembrane System

• Cell Wall: A rigid, non-living structure for shape, protection (mechanical damage/infection), and cell-to-cell interaction.

  • Composition: Algae (cellulose, galactans, mannans, calcium carbonate); Plants (cellulose, hemicellulose, pectins, and proteins).

  • Layers: Primary wall (capable of growth) and Secondary wall (inner side). Middle lamella (calcium pectate) glues cells together.

  • Plasmodesmata: Cytoplasmic connections between neighboring cells.

• Endomembrane System: Coordinated organelles including:

  • Endoplasmic Reticulum (ER): Network of tubules.

    • Rough ER (RER): Has ribosomes; involved in protein synthesis/secretion.

    • Smooth ER (SER): No ribosomes; site for lipid and steroidal hormone synthesis.

  • Golgi Apparatus: Discovered by Camillo Golgi ($1898$). Consists of stacked cisternae ($0.5\,\mu m$ to $1.0\,\mu m$).

    • Has a cis (forming) face and a trans (maturing) face.

    • Functions: Packaging and modification of glycoproteins and glycolipids.

  • Lysosomes: Formed by the Golgi; contain hydrolytic enzymes (hydrolases: lipases, proteases, carbohydrases) active at acidic pH.

  • Vacuoles: Membrane-bound (tonoplast) space. In plants, occupies up to $90\,\%$ volume; transports ions against gradients. Includes contractile vacuoles (osmoregulation) and food vacuoles.

Energy-Transducing Organelles

• Mitochondria: The "power houses" of the cell.

  • Structure: Double-membrane bound; outer membrane and inner membrane with infoldings called cristae to increase surface area.

  • Matrix: Contains single circular DNA, RNA, $70\text{S}$ ribosomes, and enzymes for aerobic respiration.

  • Replication: Divide by fission.

• Plastids: Found in plants and euglenoids.

  • Chloroplasts: Contain chlorophyll and carotenoids; site of photosynthesis. Double-membrane bound; internal space is the stroma. Organised sacs are thylakoids, stacked into grana.

  • Chromoplasts: Contain fat-soluble pigments like carotene and xanthophylls (red/yellow/orange color).

  • Leucoplasts: Colorless storage plastids: Amyloplasts (starch), Elaioplasts (oils/fats), and Aleuroplasts (proteins).

Other Eukaryotic Organelles

• Ribosomes: Discovered by George Palade ($1953$). Composed of RNA and proteins. Eukaryotic ($80\text{S}$: $60\text{S} + 40\text{S}$ subunits) and Prokaryotic ($70\text{S}$: $50\text{S} + 30\text{S}$ subunits). S stands for Svedberg’s Unit (sedimentation coefficient).

• Cytoskeleton: Network of microtubules, microfilaments, and intermediate filaments providing mechanical support and motility.

• Cilia and Flagella: Outgrowths of the membrane. Both have an axoneme with a $9+2$ array of microtubules (nine peripheral doublets and two central singles) and emerge from basal bodies.

• Centrosome: Contains two perpendicular centrioles with a "cartwheel" organization (nine peripheral triplets of tubulin). Form the spindle apparatus and basal bodies.

The Nucleus and Chromosomes

• Nucleus: Described by Robert Brown ($1831$).

  • Nuclear Envelope: Double membrane with perinuclear space ($10$ to $50\,nm$) and nuclear pores for RNA/protein transport.

  • Nucleolus: Non-membrane bound site for active rRNA synthesis.

• Chromatin: Termed by Flemming. Contains DNA, histone (basic) proteins, non-histones, and RNA.

• Chromosomes: Visible during division. A single human cell contains approximately $2\,m$ of DNA across $46$ ($23$ pairs) chromosomes.

  • Structure: Contains a centromere (primary constriction) with kinetochores on the sides.

  • Classification:

    1. Metacentric: Middle centromere; equal arms.

    2. Sub-metacentric: Centromere slightly off-center; one short, one long arm.

    3. Acrocentric: Centromere close to end; one extremely short arm.

    4. Telocentric: Terminal centromere.

  • Satellite: A small fragment appearing after a non-staining secondary constriction.

• Microbodies: Tiny membrane-bound vesicles with enzymes found in plants and animals.