Unit 2: Cells - Cell Theory, Prokaryotes, and Eukaryotes

Cell Theory and Foundational Principles

  • The Fundamental Postulates of Cell Theory:

    • All living organisms are composed of one or more cells.

    • The cell is the basic, smallest structural and functional unit of all living organisms.

    • All cells arise solely from pre-existing cells through cellular division.

  • Organismal Scale and Cellularity:

    • Cells are microscopic units of life; consequently, large multicellular organisms contain millions or billions of cells.

    • Unicellular organisms are composed of a single, independent cell that carries out all vital life functions.

    • Common biological examples of single-celled organisms include bacteria and yeast.

Microscopic Tools and Techniques in Cell Biology

  • Limitations of the Naked Eye:

    • The human eye cannot resolve or see most individual cells due to their microscopic dimensions.

  • Hand Lenses:

    • Provide low-power magnification of approximately 10×10\times.

    • Under a hand lens, individual cells generally appear only as minute dots.

  • Light Microscopes:

    • Standard equipment used in school laboratories.

    • Mechanism of Operation: Directs light through a thin specimen of plant or animal tissue and utilizes glass lenses to focus and magnify the specimen image.

    • Magnification Range: Standard light microscopes magnify from 400×400\times to 1500×1500\times.

    • Resolving Capabilities: A high-quality light microscope operating at 1500×1500\times can clearly reveal basic internal structures such as the cell wall, cell surface membrane, cytoplasm, nucleus, nuclear envelope, chloroplasts, large vacuoles, and starch grains.

    • Photomicrograph: Defined as a photograph or digital image captured through a light microscope (e.g., mammalian tracheal cells imaged at 300×300\times, or plant cells imaged at 2000×2000\times).

  • Electron Microscopes:

    • Advanced scientific instruments that utilize a focused beam of electrons rather than visible light.

    • Magnification Range: Magnifies specimens from 40∼000×40\sim000\times to 500∼000×500\sim000\times.

    • Resolving Power: Provides vastly greater resolution and fine ultrastructural detail, making visible submicroscopic organelles, membranes, and structural fibres (such as criss-crossing cellulose microfibrils at 50∼000×50\sim000\times) that are entirely invisible under light microscopy.

  • Biological Scale and Metric Units:

    • 1 millimetre (mm)=1000 micrometres (μm)1\text{ millimetre (mm)} = 1000\text{ micrometres (}\mu\text{m)}.

    • 1 micrometre (μm)=11000 mm=10−3 mm1\text{ micrometre (}\mu\text{m)} = \frac{1}{1000}\text{ mm} = 10^{-3}\text{ mm}.

    • Comparative Cell Sizes in the Human Body:

    • Largest human cell: Nerve cells (neurones), which possess extended axon fibres reaching lengths up to 1 metre1\text{ metre}.

    • Smallest human cell: Sperm cells, measuring approximately 60 μm60\,\mu\text{m} in total length.

Classification of Cellular Forms: Prokaryotic and Eukaryotic Cells

  • Prokaryotic Cells:

    • Etymology: Derived from Greek where pro means "before" and karyon means "nucleus" or "kernel".

    • Key Diagnostic Feature: Absence of a true, membrane-bound nucleus; genetic material is not enclosed within a nuclear envelope.

    • Genetic Material: Contains a primary circular chromosome (circular DNA) situated freely within the cytoplasm, alongside independent, smaller loops of accessory DNA known as plasmids.

    • External and Structural Components:

    • Capsule: An outer protective coating situated outside the cell wall.

    • Cell Wall: Provides structural rigidity, shape, and mechanical protection against osmotic lysis.

    • Cell Membrane (Plasma Membrane): Selectively permeable boundary controlling molecular flux.

    • Pili: Hair-like protein appendages located on the cell surface used for cellular attachment and adhesion.

    • Flagellum: A specialized, whip-like motile appendage utilized for active locomotion.

    • Internal Machinery: Cytoplasm contains ribosomes, which function as the sites of cellular protein synthesis.

  • Eukaryotic Cells:

    • Etymology: Derived from Greek where eu means "true" and karyon means "nucleus".

    • Key Diagnostic Features:

    • Possess a distinct, double membrane-bound nucleus housing the genetic material.

    • DNA is organized into multiple linear chromosomes located inside the nucleus.

    • Contain a wide variety of specialized, membrane-bound cellular compartments termed organelles.

    • Exhibit high architectural and functional complexity compared to prokaryotes, partitioning metabolic tasks across specialized internal environments.

Comprehensive Anatomy of Cellular Organelles and Structures

  • The Nucleus:

    • Command Center: Acts as the executive command and control center of the eukaryotic cell.

    • Membrane Barrier: Enclosed by a double membrane system known as the nuclear envelope.

    • Genetic Storage: Stores deoxyribonucleic acid (DNA), which comprises hereditary genes and chromosomes; contains the master operational instructions for synthesizing cellular proteins and coordinating metabolic processes.

  • The Cell Surface Membrane (Plasma Membrane):

    • Biochemical Composition: Formed from a very thin, flexible bilayer consisting of proteins and lipids (fats).

    • Location: Encloses the outer periphery of animal cells; in plant cells, it is located immediately interior to the rigid cell wall, pressed tightly against it.

    • Permeability Characteristics: Partially permeable (semi-permeable or selectively permeable), permitting the passage of specific water molecules and small solutes while blocking or regulating the transport of other substances.

    • Functional Roles: Isolates the internal biochemical milieu from the external environment, provides physical containment, and maintains cellular homeostasis.

  • The Cytoplasm:

    • Physical State: A clear, jelly-like matrix that fills the interior of the cell and surrounds the organelles.

    • Aqueous Composition: Consists predominantly of water (approximately 70%70\% water in many cell types) acting as a solvent for numerous suspended and dissolved solutes, particularly enzymes and other proteins.

    • Metabolic Platform: Serves as the active site for numerous metabolic reactions (the fundamental chemical processes of life), including enzymatic conversions and cellular synthesis.

  • Mitochondria:

    • Known as the powerhouses or cellular respiration sites of eukaryotic cells.

    • Biochemical sites of aerobic cellular respiration, wherein glucose molecules and other substrates are oxidatively transformed to yield biological energy molecules (ATP).

  • Ribosomes:

    • The universal "protein factories" of both prokaryotic and eukaryotic cells.

    • Function as the enzymatic sites where amino acids are assembled into polypeptides and proteins via translation of genetic codes.

  • Endoplasmic Reticulum (ER):

    • An extensive intracellular membranous network involved in the biosynthesis, folding, and processing of vital structural lipids, membrane lipids, and proteins.

  • Golgi Apparatus (Golgi Body):

    • A specialized endomembrane organelle responsible for receiving, modifying, sorting, storing, packaging, and transporting processed cellular proteins to intra- or extracellular targets.

  • Vacuoles:

    • Fluid-filled spaces enclosed by a selective vacuolar membrane.

    • Vacuoles in Animal Cells: Small, transient, and numerous; typically function in storing ingested food particles, metabolic waste products, or water.

    • Vacuoles in Plant Cells: Very large, permanent, central organelles enclosed by a vacuolar membrane; filled with cell sap (a aqueous solution of dissolved sugars, amino acids, mineral salts, and organic/inorganic substances); functions in maintaining turgor pressure, structural support, and storage.

  • Chloroplasts:

    • Organelles surrounded by their own double membranes, found exclusively in photosynthetic plant cells and absent from animal cells.

    • Contain the green photosynthetic pigment chlorophyll, which traps the radiant energy of sunlight.

    • Convert light energy, carbon dioxide, and water into chemical energy (sugars) through photosynthesis.

    • Often contain internal insoluble starch grains, which represent the stored excess carbohydrates produced by photosynthesis.

  • Plant Cell Wall:

    • Composition: Non-living, rigid external protective layer composed predominantly of insoluble cellulose fibres.

    • Cellulose Chemistry: Cellulose is a complex structural carbohydrate belonging to the chemical class of polysaccharides (long chains of linked sugars).

    • Architecture: Insoluble cellulose fibres criss-cross over one another in a dense network, providing immense structural and tensile strength (also forming the fundamental basis of paper).

    • Permeability: Fully permeable; wide intermolecular spaces between the criss-crossing fibres permit water, dissolved ions, and even very large macromolecules to pass freely.

    • Protective and Mechanical Roles: Supports the plant cell structurally, maintains cell shape, and prevents the cell from lysing (bursting) when it takes up large amounts of water via osmosis and swells.

Comparative Analysis: Plant Cells Versus Animal Cells

  • Cellular Boundaries:

    • Animal Cells: Enclosed solely by an exterior, flexible, partially permeable cell surface membrane; completely devoid of a cellulose cell wall.

    • Plant Cells: Enclosed by both an outer, rigid, fully permeable cellulose cell wall and an inner, partially permeable cell surface membrane positioned directly beneath the wall.

  • Vacuolar Structure:

    • Animal Cells: Possess only small, transient, non-permanent vacuoles (containing water or food vacuoles).

    • Plant Cells: Possess a large, central, permanent vacuole filled with cell sap.

  • Plastids and Chloroplasts:

    • Animal Cells: Chloroplasts and chlorophyll are completely absent; cannot synthesize carbohydrates phototrophically.

    • Plant Cells: Green tissue cells contain numerous chloroplasts equipped with chlorophyll for photosynthesis.

  • Carbohydrate Storage Form:

    • Animal Cells: Never contain starch grains.

    • Plant Cells: Commonly store reserve carbohydrates as insoluble starch grains within the chloroplasts and cytoplasm.

  • Shared Cellular Components:

    • Both plant and animal cells share the fundamental eukaryotic triad: a cell surface membrane, cytoplasm, and a membrane-bound nucleus containing DNA.

    • Both types possess mitochondria for cellular respiration, ribosomes for protein synthesis, an endoplasmic reticulum, and a Golgi apparatus.

Cellular Specialization and Division of Labour

  • The Principle of Division of Labour:

    • Multicellular organisms contain millions of individual cells that do not perform identical tasks.

    • Although nearly all cells perform the fundamental baseline activities of living organisms, cells differentiate structurally to specialize in executing specific physiological functions with maximal efficiency.

  • Specialized Animal Cells:

    • Ciliated Cells:

    • Anatomical Distribution: Form the epithelial lining of the mammalian trachea (windpipe), bronchi, and nasal passages.

    • Structural Adaptations: The apical cell surface possesses hundreds of microscopic, hair-like projections known as cilia that beat in synchronized waves.

    • Physiological Function: Propel mucus containing trapped dust, debris, and airborne bacteria upward away from the lungs toward the throat for expulsion or swallowing.

    • Red Blood Cells (Erythrocytes):

    • Structural Adaptations: Mature cells lack a nucleus, maximizing internal cytoplasmic volume for hemoglobin packaging; contain the iron-rich red protein pigment hemoglobin; exhibit a biconcave disc shape.

    • Physiological Function: Bind and transport molecular oxygen (O2\text{O}_2) from the respiratory exchange surfaces to all metabolizing tissues across the body.

    • Nerve Cells (Neurones):

    • Structural Adaptations: Possess a central nucleated cell body extending into elongated cytoplasmic fibres (axons and dendrites) that can reach lengths up to 1 metre1\text{ metre}.

    • Physiological Function: Rapidly conduct electrical nerve impulses between distant peripheral body regions and the central nervous system (the brain and spinal cord).

    • Sperm Cells (Male Gametes):

    • Structural Adaptations: Composed of three distinct anatomical zones:

      • Head: Houses the haploid nucleus carrying the paternal genetic material.

      • Midpiece (Middle Piece): Packed with high densities of mitochondria to generate copious ATP (energy) required for propulsion.

      • Tail (Flagellum): A long, motile, whip-like flagellum that lashes back and forth to propel the cell forward.

    • Physiological Function: Actively swim toward, locate, and fertilize the female ovum (egg cell) during sexual reproduction; represent the smallest cell in the human body at approximately 60 μm60\,\mu\text{m} in length.

    • Muscle Cells:

    • Structural Adaptations: Contain specialized contractile protein filaments that slide past each other.

    • Physiological Function: Contract and relax to generate mechanical movement, locomotion, and force (e.g., churning food in the digestive tract).

  • Specialized Plant Cells:

    • Root Hair Cells:

    • Structural Adaptations: Outer epidermal cell wall projects outward into an elongated, narrow, hair-like extension that penetrates directly between microscopic soil particles.

    • Physiological Function: Dramatically expands the absorbing surface area of the root epidermis to maximize the absorption rate of water and dissolved mineral salts (ions) from the soil.

    • Xylem Vessel Cells:

    • Structural Adaptations: Long, narrow cells aligned end-to-end in continuous columns. The transverse intervening end-walls completely break down to form hollow, continuous open tubes (vessels). The living protoplasm, including the nucleus and cytoplasm, dies and degenerates. The remaining cell walls are heavily impregnated and thickened with lignin to become woody.

    • Physiological Function: Dual function:

      • Conduction: Transports water and dissolved mineral ions unhindered from roots upward through the stem to leaves.

      • Mechanical Support: Lignified woody walls provide immense structural strength, preventing the stem from collapsing under physical and gravitational stress.

    • Palisade Mesophyll Cells:

    • Structural Adaptations: Elongated columnar cells positioned vertically beneath the upper leaf epidermis; densely packed with numerous chloroplasts.

    • Physiological Function: Primary site for trapping sunlight and manufacturing carbohydrates via photosynthesis.

Hierarchical Organization: From Organelles to Organisms

  • The Structural Hierarchy of Life:

    • The complexity of multicellular life is organized in a stepwise ascending hierarchy:     Organelles→Cells→Tissues→Organs→Organ Systems→Organisms\text{Organelles} \rightarrow \text{Cells} \rightarrow \text{Tissues} \rightarrow \text{Organs} \rightarrow \text{Organ Systems} \rightarrow \text{Organisms}

  • Key Structural Definitions:

    • Organelle: A discrete, specialized subcellular structure contained within a cell that performs a specific metabolic or structural task (e.g., nucleus, mitochondrion, chloroplast, ribosome).

    • Cell: The fundamental structural and functional unit of life; a bounded unit containing organelles and cytoplasm (e.g., liver cell, ciliated tracheal cell, palisade cell, neurone).

    • Tissue: A group of cells with similar structures working harmoniously together to perform a shared, coordinated biological function.

    • Organ: A distinct anatomical structure composed of several different tissues working synchronously together to carry out specific physiological functions.

    • Organ System: A group of interconnected organs with related functions operating cooperatively to carry out major systemic body functions.

    • Organism: A complete, individual living entity composed of integrated organ systems functioning together to sustain life.

  • Tissue-Level Integration:

    • Animal Tissue Examples:

    • Stomach Lining (Epithelial Tissue): Layer of specialized epithelial cells resting on a basement membrane; cells contain secretory vacuoles filled with digestive enzymes and secrete these enzymes into the gastric cavity to chemically digest food.

    • Stomach Muscular Tissue: Formed of muscle cells capable of coordinated contraction and relaxation, churning food and thoroughly mixing it with gastric digestive enzymes.

    • Additional Animal Tissues: Bone tissue, nervous tissue, connective tissue, and blood.

    • Plant Tissue Examples:

    • Onion Epidermis Tissue: Continuous single-cell layer protecting the underlying tissues of an onion bulb.

    • Palisade Tissue: Dense layer of chloroplast-rich palisade cells situated within the leaf lamina specialized for light absorption.

    • Vascular Tissues: Xylem tissue (water transport) and phloem tissue (sugar and nutrient transport).

  • Organ and Organ System Integration:

    • The Stomach as an Organ: Composed of epithelial secretory tissue, connective tissue, and muscular tissue working together to store and churn food.

    • The Digestive System: Comprises a series of connected organs, including the mouth, esophagus, stomach, small intestine, and large intestine, working cooperatively to ingest, digest, absorb, and egest nutrients.

    • The Circulatory System: Consists of the heart (a muscular pumping organ), blood vessels (arteries, veins, capillaries), and blood tissue, responsible for transporting oxygen, carbon dioxide, nutrients, and waste products.

    • The Excretory System: Comprises the kidneys (filtering organs), ureters, bladder, and urethra, specialized for blood filtration and waste removal.

    • Plant Organs: An onion bulb, a plant stem, a root, and a leaf are distinct organs composed of dermal, ground, and vascular tissues acting collectively to sustain plant life.

Questions & Discussion

  • Microscopy Questions and Exercises:

    • Question 2.1: How many times can a good light microscope magnify?

    • Response: A high-quality light microscope used in biological laboratories can magnify up to approximately 1500×1500\times (15001500 times).

    • Question 2.2: If an object was 1 mm1\text{ mm} across, how big would it look if it was magnified 1010 times?

    • Response: The apparent size would be calculated as: 1 mm×10=10 mm1\text{ mm} \times 10 = 10\text{ mm} (or 1 cm1\text{ cm}).

  • Structural Hierarchy and Size Ordering Exercises:

    • Revision Question 1: Arrange these structures in order of size, beginning with the smallest: stomach, tracheal cell, mitochondrion, nucleus, starch grain.

    • Response (from smallest to largest):

      1. Mitochondrion

      2. Starch grain / Nucleus (organelle-scale components; individual starch granules and sub-cellular organelles)

      3. Nucleus

      4. Tracheal cell (whole eukaryotic cell)

      5. Stomach (multicellular organ)

    • Revision Question 2: For each of the following structures, state whether it is classified as an organelle, a cell, a tissue, an organ, an organ system, or an organism:

    • a. Heart: Organ

    • b. Chloroplast: Organelle

    • c. Nucleus: Organelle

    • d. Trachea: Organ (part of the respiratory system)

    • e. Onion epidermis: Tissue

    • f. Onion bulb: Organ

    • g. Onion plant: Organism

    • h. Mitochondrion: Organelle

    • i. Human being: Organism

    • j. Lung: Organ

  • Plant Cell Functional Identification:

    • Revision Question 3: State which specific part of a plant cell performs each listed function:

    • a. Makes food by photosynthesis: Chloroplast (via the pigment chlorophyll).

    • b. Releases energy from food: Mitochondria (via aerobic cellular respiration).

    • c. Controls what goes in and out of the cell: Cell surface membrane (partially permeable plasma membrane).

    • d. Stores information about making proteins: Nucleus (within the DNA of chromosomes).

    • e. Contains cell sap: Vacuole (the large, permanent central vacuole).

    • f. Protects the outside of the cell: Cell wall (cellulose wall).

  • Comparative Terminology Distinctions:

    • Revision Question 4: Distinguish between each pair of terms:

    • a. Chloroplast versus Chlorophyll: A chloroplast is the physical, double membrane-bound cell organelle where photosynthesis occurs; chlorophyll is the green light-absorbing chemical pigment molecule contained inside the chloroplast.

    • b. Cell Wall versus Cell Membrane: The cell wall is a rigid, fully permeable outer protective covering made of criss-crossing cellulose fibres found outside plant cells; the cell membrane (plasma membrane) is a very thin, flexible, partially permeable layer composed of lipids and proteins that surrounds the cytoplasm of all cells and controls molecular transport.

    • c. Organelle versus Organ: An organelle is an internal microscopic structure within a single cell with a specialized cellular function (e.g., nucleus, mitochondrion); an organ is a macroscopic macroscopic or anatomical structure in a multicellular organism made of a collection of different tissues working together to carry out a specific body function (e.g., heart, stomach, leaf).