Chapter 4: Cell Structure and Function

Cellular Level of Organization and Cell Theory

• Historical Origin of Cell Theory:

  • Systematic detailed studies of cell structure and function began in the 1830s, establishing a foundational unifying concept in modern biology.

  • German botanist Matthias Schleiden (1804 to 1881) and German zoologist Theodor Schwann (1810 to 1882) formulated the initial tenets of cell theory between 1838 and 1839.

  • German physician Rudolph Virchow (1821 to 1902) expanded the theory by establishing that cells reproduce from existing biological structures.

• Core Principles of Cell Theory:

  • All living organisms are composed of one or more fundamental units known as cells.

  • Cells represent the basic structural and functional units of organization in all organisms.

  • Cells arise exclusively from preexisting cells because cells are self-reproducing entities.

• Cell Dimensions and Spatial Scale:

  • Most cells range in size from $1\,\text{mm}$ down to $1\,\mu\text{m}$ in diameter.

  • Biological scale comparison of objects across imaging techniques:

  • Visible only with Electron Microscope:

    • Atom: $0.1\,\text{nm}$

    • Amino acid: $1\,\text{nm}$

    • Protein: $5\text{--}10\,\text{nm}$

    • Virus: $10\text{--}100\,\text{nm}$

  • Visible with Electron and Light Microscopes:

    • Chloroplast: $1\,\mu\text{m}$

    • Most bacteria: $1\text{--}10\,\mu\text{m}$

    • Plant and animal cells: $10\text{--}100\,\mu\text{m}$

    • Human egg: $100\,\mu\text{m}$

  • Visible with Light Microscope and Unaided Human Eye:

    • Frog egg: $1\,\text{mm}$

  • Visible with Unaided Human Eye:

    • Ant: $1\,\text{cm}$

    • Mouse: $1\,\text{cm}\text{--}0.1\,\text{m}$

    • Ostrich egg: $0.1\,\text{m}$

    • Rose: $0.1\text{--}1\,\text{m}$

    • Human: $1\,\text{m}$

    • Blue whale: $30\,\text{m}$

• Biophysical Constraint of Surface-Area-to-Volume Ratio:

  • Cells require an adequate plasma membrane surface area to efficiently import nutrients and export metabolic waste products.

  • As a cell increases in volume, its total surface area decreases relative to its internal volume, drastically reducing transport efficiency across the membrane.

  • Smaller cell sizes provide a high surface-area-to-volume ratio, facilitating optimized biochemical exchange.

  • Mathematical demonstration using cube geometry:

  • Scenario 1: One $4\text{-cm}$ cube

    • Total Surface Area: $96\,\text{cm}^2$ ($\text{height} \times \text{width} \times \text{number of sides} \times \text{number of cubes} = 4\,\text{cm} \times 4\,\text{cm} \times 6 \times 1$

    • Total Volume: $64\,\text{cm}^3$ ($\text{height} \times \text{width} \times \text{length} \times \text{number of cubes} = 4\,\text{cm} \times 4\,\text{cm} \times 4\,\text{cm} \times 1$

    • Surface-Area-to-Volume Ratio: $1.5:1$

  • Scenario 2: Eight $2\text{-cm}$ cubes

    • Total Surface Area: $192\,\text{cm}^2$ ($2\,\text{cm} \times 2\,\text{cm} \times 6 \times 8$

    • Total Volume: $64\,\text{cm}^3$ ($2\,\text{cm} \times 2\,\text{cm} \times 2\,\text{cm} \times 8$

    • Surface-Area-to-Volume Ratio: $3:1$

  • Scenario 3: Sixty-four $1\text{-cm}$ cubes

    • Total Surface Area: $384\,\text{cm}^2$ ($1\,\text{cm} \times 1\,\text{cm} \times 6 \times 64$

    • Total Volume: $64\,\text{cm}^3$ ($1\,\text{cm} \times 1\,\text{cm} \times 1\,\text{cm} \times 64$

    • Surface-Area-to-Volume Ratio: $6:1$

Microscopy Techniques and Principles

• Key Optical Concepts in Microscopy:

  • Magnification: The ratio between the visual size of an image and its true physical size. Electron microscopes produce magnifications hundreds of times greater than compound light microscopes.

  • Resolution: The minimum distance separating two distinct objects that permits them to be distinguished as individual entities rather than a single blurred point.

  • Contrast: The degree of shading variation between a specimen feature and its surrounding background. Fluorescently tagged antibodies targeted to specific cellular proteins are used to dramatically enhance contrast.

• Light vs. Electron Optics:

  • Light rays refract and focus as they pass through shaped glass lenses.

  • Charged electron beams cannot pass through glass lenses; they are focused using electromagnetic lenses.

  • Transmitted or scattered electrons strike an electron-sensitive detection screen or photographic plate to build visible image data.

• Characteristics of Major Microscope Types:

  • Compound Light Microscope:

  • Illuminates specimens by passing visible light through glass condenser lenses, objective lenses, and ocular lenses onto the retina.

  • Maximum magnification capacity is approximately $1000\times$.

  • Resolving power limit is $0.2\,\mu\text{m}$, which represents a $500\times$ improvement over the unaided human eye (assuming an eye resolution benchmark of $1.0$).

  • Transmission Electron Microscope (TEM):

  • Directs a beam of electrons through an ultrathin specimen section focused by magnetic lenses.

  • Captures internal cellular fine structure onto a fluorescent screen or photographic plate.

  • Resolving power limit is $0.0002\,\mu\text{m}$, representing a $500,000\times$ improvement over the human eye.

  • Scanning Electron Microscope (SEM):

  • Specimen surface is coated with a microscopic layer of metal.

  • An electron beam scans across the metal surface, causing secondary electrons to be emitted.

  • Secondary electrons are captured and focused by electromagnetic lenses, constructing a detailed three-dimensional surface image on a display screen.

  • Confocal Laser Scanning Microscopy:

  • Uses a narrow laser beam focused on a single thin plane of a transparent biological sample.

  • Enables optical sectioning by adjusting focus up and down through successive depth planes.

  • Computer software compiles the section stack into a 3D electronic image that can be dynamically manipulated and rotated.

  • Video-Enhanced Contrast Microscopy:

  • Connects a specialized TV camera to a microscope to convert light images into digital video signals.

  • Computerized image processing darkens dark pixels and brightens light pixels to produce artificially elevated contrast for viewing live internal cell dynamics.

• Specialized Light Microscopy Optical Techniques:

  • Bright-field: Unmodified light passes directly through specimen; low native contrast limits visualization of small intracellular components.

  • Bright-field (Stained): Chemical dyes color cellular components, increasing contrast by differential stain uptake.

  • Differential Interference Contrast (DIC): Utilizes optical polarization mechanisms to amplify cellular density variations, creating a high-contrast pseudo-3D appearance ideal for living cells, chromosomes, and organelle masses.

  • Phase Contrast: Uses phase shifts in light passing through regions of differing refractive index and density within live cells, translating phase differences into high-contrast bright and dark regions.

  • Dark-field: Oblique light illumination bypasses the objective lens unless diffracted or scattered by cellular structures; cellular organelles shine brightly against a completely dark background.

Structure and Characteristics of Prokaryotic Cells

• Fundamental Domain Features:

  • Lack a true, membrane-bound nucleus.

  • Display significantly smaller overall dimensions ($1.1\text{--}1.5\,\mu\text{m}$ wide by $2.0\text{--}6.0\,\mu\text{m}$ long) and simpler internal structural organization than eukaryotic cells.

  • Taxonomically divided into two distinct domains:

  • Domain Bacteria: Abundant organisms that can cause disease but play essential ecological roles as primary decomposers; widely utilized in industrial manufacturing and drug synthesis.

  • Domain Archaea: Extremophilic organisms adapted to extreme environmental conditions (e.g., high temperature, salinity, or acidity).

  • Biochemical distinction: Despite structural similarities, Domain Bacteria and Domain Archaea possess fundamentally different biochemical and molecular pathways.

• Morphological Classifications:

  • Coccus: Spherical bacterial shape.

  • Bacillus: Rod-shaped bacterial morphology.

  • Spirillum: Rigid spiral shape.

  • Spirochete: Flexible spiral shape.

• Components of the Prokaryotic Cell Envelope:

  • Plasma Membrane: Phospholipid bilayer embedded with integral and peripheral membrane proteins. Internal invaginations called mesosomes expand surface area for metabolic processes.

  • Cell Wall: Rigid external layer that maintains cell shape and structural integrity; strengthened by peptidoglycan in bacteria.

  • Glycocalyx: Outer coating composed of polysaccharides. When dense, highly organized, and tightly bound to the cell wall, it is classified as a capsule.

• Internal Cytoplasm and Genome Architecture:

  • Cytoplasm: Semifluid internal matrix enclosed by the plasma membrane; contains water, dissolved inorganic and organic molecules, ions, and catalytic enzymes.

  • Nucleoid: Unbounded cytoplasmic region housing a single, circular DNA chromosome.

  • Plasmids: Small, autonomous extrachromosomal rings of double-stranded DNA encoding accessory genetic traits (such as antibiotic resistance).

  • Ribosomes: Small cytoplasmic ribonucleoprotein complexes responsible for protein synthesis.

• External Appendages:

  • Flagella: Rotating protein filaments that propel the organism through fluid environments to afford active motility.

  • Fimbriae: Small, hair-like protein bristles sprouting from the cell envelope that mediate surface attachment and adhesion.

  • Conjugation Pili: Tubular, rigid protein structures (singular: pilus) utilized to pass plasmid DNA directly between bacterial cells.

Introduction to Eukaryotic Cells and Endosymbiotic Origin

• Defining Features of Eukaryotes:

  • Enclose genetic material within a membrane-bound nucleus.

  • Contain compartmentalized, membrane-bounded organelles designed to isolate specific chemical reactions and metabolic pathways.

  • Enclosed by a plasma membrane consisting of a phospholipid bilayer with embedded functional proteins that regulates substance traffic.

  • Possess significantly larger cellular volumes than prokaryotic cells.

• Functional Organelle Categories:

  • Endomembrane System: Interconnected cellular structures (including the nuclear envelope, endoplasmic reticulum, Golgi apparatus, and transport vesicles) that communicate via physical continuity or vesicle exchange.

  • Energy-Related Organelles: Mitochondria and chloroplasts; semi-autonomous structures involved in cellular energy conversion.

• Structural Diversity and Specialization:

  • Cellular organelle numbers vary depending on specialized physiological functions:

  • Liver cells contain dense concentrations of smooth endoplasmic reticulum to perform drug detoxification.

  • Nerve cells maintain extensive plasma membrane surfaces to transmit electrical impulses across long distances.

  • Cytoskeletal tracks: Internal protein filaments serve as paths along which vesicles transport molecules.

  • Cell Wall Structures: Surrounds plant cells, fungi, and many protists; plant cell walls are composed of the structural polysaccharide cellulose.

• Evolutionary History and the Endosymbiotic Theory:

  • Fossil evidence establishes that prokaryotic organisms were the earliest cellular life forms on Earth.

  • Biochemical analysis indicates eukaryotic lineages share a closer molecular evolutionary ancestry with Domain Archaea than Domain Bacteria.

  • Evolutionary Steps in Endosymbiosis:

  1. Infolding (invagination) of the primitive prokaryotic plasma membrane around the genome created a double-membrane nucleus, allowing functional division of labor and freeing up resources.

  2. Proliferation of internal invaginated membranes generated the endomembrane system (endoplasmic reticulum and Golgi apparatus), vastly increasing transport surface area.

  3. An ancestral cell engulfed an aerobic heterotrophic bacterium; rather than digesting it, the cell established a mutualistic endosymbiotic relationship, giving rise to mitochondria capable of aerobic respiration.

  4. A lineage of mitochondria-containing cells engulfed a photosynthetic cyanobacterium, giving rise to chloroplasts capable of converting light energy into carbohydrates.

The Nucleus and Ribosomes

• Architecture and Function of the Nucleus:

  • Serves as the primary genetic command center of the eukaryotic cell, typically positioned centrally.

  • Nuclear Envelope: A double-membrane structure separating nucleoplasm from cytoplasm.

  • Nuclear Pores: Octagonal protein complexes spanning both membranes of the nuclear envelope; selectively regulate nuclear transport (permitting proteins into the nucleus and ribosomal subunits out into the cytoplasm).

  • Chromatin: Network of diffuse DNA threads wrapped around proteins located in the fluid nucleoplasm; condenses into distinct chromosomes prior to cellular division.

  • Nucleolus: Dense region inside the nucleus responsible for synthesizing ribosomal RNA (rRNA) and assembling large and small ribosomal subunits.

• Ribosome Structure and Synthesis:

  • Non-membrane-bound complexes consisting of ribosomal RNA (rRNA) and structural proteins arranged into a large subunit and a small subunit.

  • Assembly location: Subunits are synthesized in the nucleolus and exported through nuclear pores to the cytoplasm.

  • Cytoplasmic Distribution:

  • Free Ribosomes: Suspended in cytoplasm singly or in clusters called polyribosomes (strings of ribosomes translating the same mRNA simultaneously); synthesize proteins that function inside the cytoplasm.

  • Bound Ribosomes: Attached to the cytoplasmic face of the rough endoplasmic reticulum; synthesize membrane-bound, organellar, or secreted proteins.

  • Central Dogma of Molecular Biology:     $\text{DNA} \xrightarrow{\text{Transcription}} \text{mRNA} \xrightarrow{\text{Translation}} \text{Protein}$

• Mechanism of Signal Peptide Protein Targeting to the ER:

  1. Gene transcription in the nucleus produces mRNA, which exits into the cytoplasm via nuclear pores. A ribosome attaches to the mRNA and begins translation, generating an initial amino acid sequence called a signal peptide.

  2. A cytoplasmic Signal Recognition Particle (SRP) binds to the signal peptide, temporarily halting polypeptide elongation.

  3. The SRP-ribosome complex binds to an SRP receptor on the ER membrane, opening a translocon channel. SRP detaches, and translation resumes, pushing the growing polypeptide chain into the ER lumen.

  4. A signal peptidase enzyme in the ER lumen cleaves the signal peptide from the elongating polypeptide chain.

  5. Translation finishes, the ribosome and mRNA detach, and the completed polypeptide chain folds into its tertiary structure inside the ER lumen.

The Endomembrane System

• System Architecture:

  • A series of intracellular double and single membranes that compartmentalize the cell, restricting enzymatic reactions to specific regions.

  • Includes the nuclear envelope, rough and smooth endoplasmic reticulum, Golgi apparatus, and various transport vesicles.

• Endoplasmic Reticulum (ER):

  • Extensive network of folded membrane channels and flattened saccules continuous with the outer nuclear envelope membrane.

  • Rough Endoplasmic Reticulum (Rough ER):

  • Coated with ribosomes on its cytoplasmic surface.

  • Receives nascent polypeptides, modifies them (such as adding sugar chains to form glycoproteins), and packages them into transport vesicles directed to the Golgi apparatus.

  • Smooth Endoplasmic Reticulum (Smooth ER):

  • Lacks attached ribosomes.

  • Synthesizes lipids, phospholipids, and steroid hormones (e.g., testosterone production in testicular interstitial cells).

  • Performs drug and toxin detoxification (e.g., highly abundant in liver cells) and stores intracellular calcium ions.

  • Packages lipid products into transport vesicles for delivery to the Golgi apparatus.

• Golgi Apparatus:

  • Named after discoverer Camillo Golgi; consists of a stack of 3 to 20 curved, flattened membranous saccules resembling hollow pancakes.

  • Receives ER-derived transport vesicles at its cis (receiving/inner) face.

  • Modifies protein and lipid carbohydrate chains, adding specific chemical "signal" tags that determine molecular destination.

  • Sorts and packages processed molecules into vesicles discharging from its trans (shipping/outer) face.

  • Molecular destinations include intracellular organelles, return routes to the ER, plasma membrane integration, or extracellular secretion via exocytosis.

• Lysosomes:

  • Membrane-enclosed vesicles formed by the Golgi apparatus (typically absent in plant cells).

  • Contain acidic hydrolytic digestive enzymes designed to break down macromolecules into basic monomeric nutrients.

  • Fuse with endocytic vesicles to digest ingested materials, degrade invading pathogens in phagocytic white blood cells, and destroy worn-out cellular organelles (autophagy).

  • Clinical Pathology: Defects in specific lysosomal hydrolytic enzymes result in lysosomal storage diseases such as Tay-Sachs disease, where undigested lipids accumulate in neural tissues; experimental treatments utilize gene therapy to restore missing functional enzymes.

• Summary of Endomembrane Pathway Dynamics:

  1. Polypeptides synthesized on Rough ER and lipids produced on Smooth ER are encapsulated into transport vesicles.

  2. Transport vesicles shuttle cargo to the cis face of the Golgi apparatus.

  3. The Golgi apparatus modifies, sorts, and packages cargo into specialized vesicles at its trans face.

  4. Secretory vesicles migrate along cytoskeletal tracks to fuse with the plasma membrane, releasing contents externally via exocytosis (e.g., pancreatic digestive enzyme secretion or mammary gland milk production).

  5. Lysosomal vesicles fuse with incoming endocytic vesicles to digest imported materials.

Microbodies and Vacuoles

• Microbodies and Peroxisomes:

  • Microbodies are specialized, single-membrane vesicles loaded with specific metabolic enzymes.

  • Peroxisomes:

  • Vesicles derived from the cytoplasm that are involved in fatty acid oxidation and lipid metabolism.

  • Metabolic reactions in peroxisomes generate toxic hydrogen peroxide ($\text{H}_2\text{O}_2$) as a byproduct.

  • Contain high concentration of the enzyme catalase, which immediately breaks down toxic hydrogen peroxide into harmless water and oxygen:       $2\text{H}_2\text{O}_2 \xrightarrow{\text{catalase}} 2\text{H}_2\text{O} + \text{O}_2$

  • Medical Link: Adrenoleukodystrophy (ALD) is a metabolic disorder caused by a lack of a specific peroxisomal membrane transport protein, causing un-metabolized long-chain fatty acids to accumulate and destroy myelin sheaths, leading to severe neurological damage.

• Vacuoles:

  • Membranous sacs significantly larger than standard transport vesicles, functioning primarily in storage.

  • Specialized Contractile Vacuoles in freshwater protists actively pump excess water out of the cell to prevent osmotic lysis.

  • Plant Central Vacuole:

  • Large fluid-filled central sac occupying up to $90\%$ of total plant cell volume.

  • Stores water, essential nutrients, plant pigments, and metabolic waste products.

  • Exerts turgor pressure against the surrounding cell wall, providing rigid structural support for plant tissues.

  • Stores secondary metabolic toxins that deter plant herbivores.

  • Functions as a lysosome-equivalent in plants by using internal digestive enzymes to break down aged organelles.

Energy-Related Organelles: Chloroplasts and Mitochondria

• Bioenergetic Relationship Between Organelles:

  • Solar energy captured by chloroplasts drives the synthesis of energy-rich carbohydrates from low-energy carbon dioxide and water.

  • Mitochondria break down these carbohydrates during cellular respiration, capturing released energy to form ATP to power cellular work, releasing carbon dioxide and water.

• Chloroplasts:

  • Specialized double-membrane plastids responsible for conducting photosynthesis in plants, algae, and cyanobacteria.

  • Chemical Reaction Equation:     $\text{solar energy} + \text{carbon dioxide} + \text{water} \rightarrow \text{carbohydrate} + \text{oxygen}$     $\text{solar energy} + 6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

  • Structural Features:

  • Outer membrane and inner membrane enclosing an internal space.

  • Stroma: Semifluid matrix containing enzymes that reduce carbon dioxide into carbohydrates.

  • Thylakoids: Disc-like membrane sacs stacked into column units termed grana (singular: granum).

  • Chlorophyll: Green photosynthetic pigment located within thylakoid membranes that absorbs solar energy.

• Mitochondria:

  • Energy-converting organelles present in almost all eukaryotic cells (plants, animals, fungi, and protists).

  • Quantity per cell corresponds directly to metabolic demand (e.g., active human liver cells contain up to $1,000$ individual mitochondria).

  • Contain their own circular DNA chromosome and self-replicating ribosomes.

  • Perform aerobic cellular respiration, consuming oxygen and producing ATP molecules.

  • Structural Anatomy:

  • Bounded by a double membrane.

  • Inner Membrane: Convoluted into deep folds termed cristae, maximizing surface area for electron transport chains and ATP synthase complexes.

  • Matrix: Internal semifluid cavity containing respiratory enzymes that break down carbohydrate derivatives.

The Cytoskeleton and Cellular Movement

• Cytoskeleton Overview:

  • A dynamic network of protein fibers that continuously assembles and disassembles to maintain shape, facilitate motility, and mediate internal transport.

  • Functionally analogous to the skeletal and muscular systems of animals.

• Three Major Types of Cytoskeletal Fibers:

  • Actin Filaments:

  • Ultrathin filaments ($7\,\text{nm}$ diameter) formed by two twisted chains of globular actin monomers.

  • Form a dense web beneath the plasma membrane to maintain cell shape and structurally support intestinal microvilli.

  • Drive cytoplasmic streaming, direct intracellular vesicle trafficking, and power pseudopod movement in amoeboid cells.

  • Form the contractile ring during animal cell cytokinesis.

  • Interact with motor protein myosin: ATP hydrolysis causes myosin heads to bind and pull actin filaments, generating mechanical contraction.

  • Intermediate Filaments:

  • Intermediate diameter ($8\text{--}11\,\text{nm}$) composed of a ropelike assembly of fibrous polypeptide chains.

  • Tissue-specific composition (e.g., keratin filaments provide mechanical strength to epidermal skin cells).

  • Mechanically support the nuclear envelope and reinforce cell-to-cell desmosome junctions.

  • Microtubules:

  • Small hollow cylinders ($25\,\text{nm}$ outer diameter) built from globular $\alpha$ and $\beta$ tubulin heterodimers.

  • Tubulin dimers spontaneously assemble into helical tubes composed of $13$ parallel protofilament dimers around an empty core.

  • Assembly is regulated by the Microtubule-Organizing Center (MTOC), mainly the centrosome.

  • Form the mitotic spindle during cell division to pull chromosomes apart.

  • Serve as tracks for motor proteins kinesin and dynein: ATP hydrolysis enables kinesin receptors to "walk" transport vesicles along microtubule cylinders.

• Centrioles:

  • Short, hollow cylinders composed of $27$ microtubules arranged in $9$ overlapping microtubule triplets ($9 + 0$ pattern).

  • Animal centrosomes contain one pair of centrioles oriented at right angles ($90^\circ$) to each other.

  • Separate during mitosis to organize the mitotic spindle and define the plane of cell division.

  • Give rise to basal bodies that anchor cilia and flagella.

• Cilia and Flagella:

  • Hair-like surface projections that facilitate cell movement or liquid propulsion (distinct from bacterial flagella).

  • Ultrastructure: Covered by plasma membrane, containing an inner cylinder of $18$ microtubules arranged in $9$ outer doublets around $2$ central single microtubules ($9 + 2$ pattern).

  • Anchored to the cell by a basal body at the base ($9$ microtubule triplets with no central tubules, $9 + 0$ pattern).

  • Motility Mechanism: Dynein side arms extend toward adjacent doublets; ATP cleavage causes dynein arms to bind and slide adjacent doublets, causing structural bending.

  • Functional Differences:

  • Cilia: Short, numerous extensions moving in synchronized oar-like strokes (e.g., sweeping debris up the human respiratory tract).

  • Flagella: Long, sparse extensions moving in whip-like propeller motions (e.g., propelling human sperm).