Cell Theory Flashcards

Foundations and Historical Development of Cell Theory

• Definition of the Cell:

  • The cell (derived from the Latin term cella, meaning "small room") is the basic structural, functional, and biological unit of all known living organisms.

  • It represents the smallest unit of life and is universally regarded as the primary building block of life.

  • The scientific study of cells is termed cell biology, cellular biology, or cytology.

• Core Tenets of Cell Theory:

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

  • The cell is the basic structural and functional unit of life.

  • All cells arise exclusively from pre-existing cells through cell division.

• Chronological Timeline of Key Scientific Discoveries:

  • 1665 — Robert Hooke: Utilized an early compound microscope to examine thin slices of bottle cork; observed porous, box-like compartments and coined the biological term "cell."

  • 1674 — Antonie van Leeuwenhoek: Engineered improved, high-magnification single-lens microscopes; examined pond water and bodily fluids, discovering living single-celled microorganisms that he termed "animalcules."

  • 1833 — Robert Brown: Conducted microscopic investigations of plant cells, discovering and naming the internal cell nucleus.

  • 1838 — Matthias Schleiden: Systematically investigated plant tissues under the microscope and concluded that all plants are composed entirely of cells.

  • 1839 — Theodore Schwann: Carried out comparative investigations on animal tissues, concluding that all animals are likewise composed of cells, thereby unifying botanical and zoological sciences under a single cellular paradigm.

  • 1852 — Robert Remak: Analyzed animal embryos and early developmental stages, publishing empirical evidence demonstrating that living cells multiply through binary division.

  • 1855 — Rudolf Virchow: Popularized Remak's findings regarding cellular reproduction across the scientific community, formulating the Latin epigram omnis cellula e cellula ("all cells arise from pre-existing cells").

Cellular Dimensions and the Surface Area-to-Volume Ratio

• Physical Constraints on Cellular Dimensions:

  • Cells are microscopic because their metabolic requirements dictate a high surface area-to-volume ratio ($\text{SA/V}$).

  • The plasma membrane acts as the selective gateway for gas exchange, nutrient uptake, and waste excretion, whereas cellular volume determines the rate at which metabolic reactions occur and waste accumulates.

  • As a spherical or cuboidal cell increases in diameter, its interior volume expands as the cube of its radius ($V \propto r^3$), whereas its surface area increases only as the square of its radius ($\text{SA} \propto r^2$).

• Mathematical Demonstration of Volume Expansion:

  • Small Cell ($r = 1\,\text{unit}$):

  • Surface area: $\text{SA} = 4\pi r^2 = 4\pi(1)^2 \approx 12.57\,\text{unit}^2$

  • Volume: $V = \frac{4}{3}\pi r^3 = \frac{4}{3}\pi(1)^3 \approx 4.189\,\text{unit}^3$

  • Surface area-to-volume ratio: $\frac{\text{SA}}{V} = \frac{12.57}{4.189} \approx 3$

  • Large Cell ($r = 10\,\text{units}$):

  • Surface area: $\text{SA} = 4\pi(10)^2 \approx 1257\,\text{unit}^2$

  • Volume: $V = \frac{4}{3}\pi(10)^3 \approx 4189\,\text{unit}^3$

  • Surface area-to-volume ratio: $\frac{\text{SA}}{V} = \frac{1257}{4189} \approx 0.3$

  • Physiological Implications:

  • A $10\text{-fold}$ increase in cell radius yields a $100\text{-fold}$ increase in membrane surface area but a $1000\text{-fold}$ surge in metabolic volume.

  • When the ratio drops from $3$ to $0.3$, the surface area becomes inadequate to import sufficient nutrients or expel toxic metabolic byproducts rapidly enough, enforcing a strict upper boundary on individual cell size.

Universal Structural Features of All Cells

• Centrally Located Genetic Material:

  • Every biological cell contains deoxyribonucleic acid ($\text{DNA}$), the universal hereditary molecule encoding instructions for protein synthesis and cellular replication.

  • In prokaryotes, genetic material exists predominantly as a single, circular double-stranded $\text{DNA}$ chromosome clustered in an uncompartmentalized cytoplasmic zone known as the nucleoid.

  • In eukaryotes, multiple linear $\text{DNA}$ molecules complexed with histones reside within a double-membrane-bound nucleus.

• The Cytoplasm and Cytosol:

  • The cytoplasm is the entire semifluid matrix occupying the cell interior between the plasma membrane and the genetic core.

  • It contains dissolved amino acids, carbohydrates, lipids, nucleotides, and inorganic ions essential for metabolic functions.

  • Cytosol: The liquid aqueous portion of the cytoplasm in which ions and soluble macromolecules are dissolved.

  • Organelle: Any discrete, organized macromolecular intracellular structure specialized to perform dedicated metabolic functions.

• The Plasma Membrane:

  • Encloses the cytoplasmic contents, establishing a selectively permeable boundary between the intracellular environment and the extracellular surroundings.

  • Consists of an amphipathic phospholipid bilayer measuring approximately $5\text{ to }10\,\text{nm}$ ($5\text{ to }10\times 10^{-9}\,\text{m}$) in thickness.

  • Contains hydrophilic phosphate heads oriented toward aqueous environments and hydrophobic fatty acid tails oriented inward toward one another, with integrated peripheral and integral membrane proteins.

• Ribosomes and the Svedberg Unit:

  • Complex non-membranous ribonucleoprotein assemblies composed of ribosomal RNA ($\text{rRNA}$) and structural proteins that execute translation (protein synthesis).

  • Svedberg unit ($\text{S}$): A unit representing sedimentation velocity during ultracentrifugation, governed by a particle's mass, shape, and surface density. Values are non-additive because sedimentation rates depend on hydrodynamic surface properties rather than simple additive mass.

  • Prokaryotic Ribosomes ($70\text{S}$): Composed of a large $50\text{S}$ subunit and a small $30\text{S}$ subunit.

  • Eukaryotic Cytoplasmic Ribosomes ($80\text{S}$): Composed of a large $60\text{S}$ subunit and a small $40\text{S}$ subunit.

Prokaryotic Cell Organization and Morphology

• General Characteristics of Prokaryotes:

  • Etymology: -pro ("before") and -karyon ("nucleus").

  • Composed of the domains Bacteria and Archaea.

  • Lack membrane-delimited nuclei and endomembrane-bound organelles.

• Intracellular Components:

  • Nucleoid: Non-membrane-enclosed zone holding the primary circular bacterial chromosome.

  • Plasmids: Small, circular, extrachromosomal autonomous $\text{DNA}$ fragments conferring specialized adaptive advantages such as antibiotic resistance or heavy-metal tolerance.

  • Ribosomes ($70\text{S}$): Sites of cytoplasmic translation.

• The Bacterial Cell Wall and Peptidoglycan Architecture:

  • Provides mechanical rigidity, maintains osmotic integrity, and counters turgor pressure.

  • Chemically constituted of peptidoglycan (murein), a polymer of repeating disaccharides ($N\text{-acetylglucosamine}$ and $N\text{-acetylmuramic acid}$) cross-linked by short amino acid oligopeptides.

• Comparative Bacterial Envelope Profiles:

  • Gram-Positive Bacteria:

  • Characterized by a thick, multilayered peptidoglycan cell wall seated directly exterior to the plasma membrane.

  • Retains crystal violet-iodine complexes during alcohol decolorization, appearing deep purple under light microscopy.

  • Gram-Negative Bacteria:

  • Possesses a thin peptidoglycan monolayer localized inside a fluid-filled periplasmic space.

  • Bounded externally by an asymmetric outer lipid bilayer enriched in lipopolysaccharides ($\text{LPS}$), phospholipids, and transmembrane porin channels.

  • Outer membrane lipid components wash out during alcohol washing, releasing initial crystal violet stain; counterstained with safranin to appear pink under light microscopy.

• Gram Staining Laboratory Procedure:

  • Step 1 (Primary Stain): Apply Crystal Violet for $60\,\text{seconds}$.

  • Step 2 (Mordant): Apply Gram's Iodine for $60\,\text{seconds}$ to form insoluble crystal violet-iodine precipitates.

  • Step 3 (Decolorization): Rinse with $95\%$ Ethyl Alcohol for $5\text{ to }10\,\text{seconds}$.

  • Step 4 (Counterstain): Apply Safranin for $45\,\text{seconds}$.

• Accessory Surface and Survival Structures:

  • Glycocalyx: An extracellular gelatinous layer of polysaccharides and glycoproteins.

  • Exists as a well-organized, firmly anchored capsule that inhibits phagocytosis by immune cells, or as a loose, unorganized slime layer promoting surface adherence.

  • Visualized microscopically through negative capsule staining, where background and bacterial rod bodies take up stain while non-ionic capsules remain clear halos.

• Bacterial Flagella and Motility Patterns:

  • Helical appendages driven by rotary motor complexes anchored in the cell envelope.

  • Classified according to flagellar distribution:

    • Monotrichous (polar): A single flagellum positioned at a solitary cellular pole.

    • Lophotrichous: A tuft or cluster of multiple flagella emerging from a single pole.

    • Peritrichous: Multiple flagella distributed randomly across the entire surface.

    • Amphitrichous: Single or tufted flagellar structures emerging from both opposite poles.

• Fimbriae: Numerous hair-like filamentous protein projections that mediate cellular attachment to environmental surfaces and host tissues.

• Pili and Bacterial Conjugation:

  • Elongated, tubular proteinaceous appendages used for motility and genetic transfer.

  • Sex Pilus: Facilitates bacterial conjugation by mechanically bridging a genetic donor ($\text{F}^+$) to a recipient ($\text{F}^-$).

  • The relaxasome nicks double-stranded plasmid $\text{DNA}$, and the transferasome complex transfers a single strand of the $\text{F}$ plasmid through the pilus, followed by complementary strand synthesis catalyzed by $\text{DNA}$ polymerase in both cells.

• Endospores:

  • Highly resistant, dehydrated, dormant cellular structures produced intracellularly under severe nutritional deprivation or environmental stress.

  • Encase genomic $\text{DNA}$ inside specialized dipicolinic acid-rich and keratin-like protein coats, enduring extreme heat, desiccation, ultraviolet radiation, and enzymatic digestion.

The Eukaryotic Endomembrane System and Associated Organelles

• General Architecture of Eukaryotic Cells:

  • Etymology: -eu ("true") and -karyon ("nucleus").

  • Encompasses plants, animals, fungi, and protists.

  • Defined by internal compartmentalization with membrane-bound organelles performing discrete biochemical pathways.

• The Nucleus and Genetic Compartment:

• Acts as the regulatory center of the cell, housing genomic $\text{DNA}$.

• Nuclear Envelope: A double-membrane barrier consisting of two distinct lipid bilayers (inner and outer nuclear membranes) separated by a perinuclear lumen.

• Nuclear Pores and Pore Complexes: Ring-shaped multi-protein complexes that regulate bidirectional macromolecular transport ($\text{RNA}$, ribosomal subunits, transcription factors, and enzymes).

• Nuclear Lamina: A dense protein meshwork of intermediate filaments (lamins) lining the inner surface of the nuclear envelope, maintaining mechanical stability and nuclear shape.

• Chromatin and Chromosomes: Genomic $\text{DNA}$ complexed with basic histone proteins, condensing into distinguishable linear chromosomes during mitosis or meiosis.

• Nucleolus: A dense, non-membrane-bound subnuclear region responsible for transcribing ribosomal RNA ($\text{rRNA}$) and assembling ribosomal subcomplexes.

  • The Endomembrane System Dynamics:

• An integrated network of internal membranes that are physically continuous or functionally coupled through vesicle trafficking.

• Includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and plasma membrane.

• Endoplasmic Reticulum ($\text{ER}$):

  • Accounts for greater than $50\%$ of total membranous surface area in eukaryotic cells, extending continuously from the outer nuclear membrane.

  • Rough Endoplasmic Reticulum ($\text{RER}$): Characterized by outer surfaces studded with ribosomes. Directly translocates nascent polypeptides into the $\text{ER}$ cisternal lumen for folding, post-translational modifications (e.g., formation of glycoproteins), and budding into transport vesicles; functions as a membrane factory for the cell.

  • Smooth Endoplasmic Reticulum ($\text{SER}$): Lacks surface-bound ribosomes. Enriched in enzymes that catalyze lipid and steroid synthesis, carbohydrate metabolism, calcium ion ($\text{Ca}^{2+}$) sequestration and release, and enzymatic detoxification of drugs and poisons.

  • The Golgi Apparatus:

• Consists of stacks of flattened, membrane-bound cisternae.

• Exhibiting structural and functional polarity:

  • Cis Face: The receiving face, positioned proximal to the $\text{ER}$, where incoming transport vesicles fuse and unload proteins and lipids.

  • Trans Face: The shipping face, oriented toward the plasma membrane, where modified macromolecules are sorted into secretable or organelle-bound transport vesicles.

• Chemically alters oligosaccharide side-chains of glycoproteins, synthesizes specific non-cellulose polysaccharides, and tags macromolecules with molecular addresses for targeted sorting.

  • Lysosomes and Hydrolytic Degradation:

• Membrane-delimited spheres containing hydrolytic enzymes operating optimally at acidic $\text{pH}$.

• Catalyze the degradation of proteins, lipids, complex carbohydrates, and nucleic acids.

• Phagocytosis: Degradation of foreign matter or extracellular particles engulfed within a phagocytic food vacuole that fuses with a primary lysosome.

• Autophagy: Intracellular recycling mechanism wherein damaged organelles (such as fragmented mitochondria or peroxisomes) are encapsulated within a double-membrane autophagosome that fuses with a lysosome to recycle organic monomers.

  • Vacuoles:

• Large fluid-filled membrane-enclosed sacs exhibiting varied physiological specializations:

  • Food Vacuoles: Formed by phagocytic invagination to hold engulfed metabolic substrates.

  • Contractile Vacuoles: Osmoregulatory structures in freshwater protists (Amoeba, Paramecium, Euglena) that collect and periodically expel excess water to prevent osmotic lysis.

  • Central Vacuole: Prominent organelle in mature plant cells bounded by a selectively permeable membrane (tonoplast).

• Central vacuoles sequester water, inorganic ions ($\text{K}^+$, $\text{Cl}^-$), hydrolytic enzymes, pigments, and metabolic wastes, generating turgor pressure against the cell wall to provide structural support.

Energy-Transforming Organelles and the Endosymbiotic Theory

• The Endosymbiotic Theory:

  • Evolutionary theory postulating that eukaryotic mitochondria and chloroplasts originated as free-living prokaryotic organisms internalized by primitive ancestral host cells.

  • An ancestral non-photosynthetic eukaryote engulfed an aerobic, non-photosynthetic prokaryote, evolving into an obligate endosymbiont that became the mitochondrion.

  • A lineage of these early mitochondrion-bearing cells later engulfed a photosynthetic prokaryote (an ancestor of cyanobacteria), giving rise to chloroplast-containing photosynthetic eukaryotes.

  • Diagnostic Evidence Supporting Endosymbiosis:

  • Mitochondria and chloroplasts are completely excluded from vesicular endomembrane flow.

  • Bounded by double membrane envelopes.

  • Retain independent, circular, non-histone-bound $\text{DNA}$ molecules.

  • Contain functional internal $\text{70S}$ ribosomes capable of independent translation of organellar proteins.

  • Multiply inside eukaryotic hosts through autonomous binary fission.

• Mitochondria:

  • The sites of cellular respiration, executing aerobic catabolic reactions to generate adenosine triphosphate ($\text{ATP}$) from glucose, fatty acids, and oxygen.

  • Structurally divided into two membranes and two internal compartments:

  • Outer Membrane: Smooth, permeable lipid bilayer.

  • Intermembrane Space: Fluid compartment between inner and outer membranes serving as a reservoir for proton accumulation during oxidative phosphorylation.

  • Inner Membrane: Convoluted into folded projections known as cristae, which vastly amplify surface area available for electron transport chains and $\text{ATP}$ synthase complexes.

  • Mitochondrial Matrix: Fluid interior containing mitochondrial $\text{DNA}$, ribosomes, and metabolic enzymes that drive the citric acid cycle.

• Chloroplasts:

• Specialized photosynthetic plastids found in plant leaves, green tissue organs, and algae.

• Contain photosynthetic pigments (chlorophylls and carotenoids) alongside enzymes driving solar energy transduction.

• Internal Compartments:

  • Thylakoids: Flattened, interconnected membranous sacs containing light-harvesting pigment complexes; stacked to form structures called grana (singular: granum).

  • Stroma: Viscous fluid surrounding the grana, containing circular chloroplast $\text{DNA}$, ribosomes, and enzymes that drive carbon fixation during the Calvin cycle.

  • Peroxisomes:

• Single-membrane-bound metabolic compartments specialized for oxidative breakdown.

• Contain oxidases that transfer hydrogen atoms from various substrates (such as long-chain fatty acids) to molecular oxygen ($\text{O}_2$), forming toxic hydrogen peroxide ($\text{H}_2\text{O}_2$) as an intermediate:     $\text{RH}_2 + \text{O}_2 \rightarrow \text{R} + \text{H}_2\text{O}_2$

• House abundant catalase enzymes that immediately decompose toxic hydrogen peroxide into water and molecular oxygen, shielding the cell from oxidative damage:     $2\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2$

The Cytoskeletal Architecture and Cellular Motility

• Overview of the Cytoskeleton:

  • A dynamic network of protein fibers extending throughout the cytoplasm from the nucleus to the plasma membrane.

  • Functions in mechanical stability, maintaining cell shape, securing organelles, and guiding directional vesicular trafficking via motor protein interactions.

• Comparative Properties of Cytoskeletal Fibers:

• Microtubule Dynamics and Motors:

  • Assemble from polar $\alpha\beta\text{-tubulin}$ dimers.

  • Centrosome and Centrioles: The primary microtubule-organizing center ($\text{MTOC}$) in animal cells situated near the nucleus. Contains a pair of perpendicular centrioles, each displaying an arrangement of nine microtubule triplets in a ring ($9 \times 3$ pattern).

  • Vesicle Transit: Serves as intracellular tracks along which ATP-hydrolyzing motor proteins (e.g., kinesin walking toward positive ends, dynein walking toward negative ends) pull transport vesicles.

  • Cilia and Flagella: Motile appendages featuring a specialized ring of nine doublets and two central single microtubules ($9 + 2$ axoneme structure).

• Flagellar Motion: Undulating snakelike waveforms pushing the cell parallel to the axis of the flagellum (e.g., sperm locomotion).

• Ciliary Motion: Rhythmic alternations between a rigid power stroke and a flexible recovery stroke, propelling fluids across stationary epithelial sheets or propelling organisms through liquid media.

  • Microfilaments (Actin) and Motor Systems:

• Formed by helical polymerization of globular actin monomers into filamentous actin ($\text{F-actin}$).

• Actin Treadmilling: Rapid, simultaneous polymerization of actin subunits at the barbed (plus) end and depolymerization at the pointed (minus) end, generating mechanical force for cellular movement.

• Muscle Contraction: Thousands of parallel actin filaments interdigitate with thick myosin filaments. ATP-dependent cyclic binding and pulling of myosin heads pulls actin filaments toward the sarcomere center, shortening the cell.

• Amoeboid Movement: Cytoplasmic sol-gel conversions drive pseudopodia extension and contraction via localized, reversible actin-myosin assemblies.

• Cytoplasmic Streaming: Circular flow of cytosol around plant central vacuoles driven by actin-myosin interactions, accelerating nutrient and organelle distribution.

  • Intermediate Filament Diversity and Classification:

• Stable, highly tensile fibrous cables that do not exhibit dynamic instability or directional polarity.

• Systematic Human Categorization:

  • Type I: Acidic Keratins ($28$ genes in humans); structural integrity of epithelial cells.

  • Type II: Basic Keratins ($26$ genes in humans); paired with Type I keratins in epithelial sheets.

  • Type III: Homodimerizing/heterodimerizing filaments, including Vimentin (mesenchymal cells), Desmin (muscle structural integrity), and Glial Fibrillary Acidic Protein ($\text{GFAP}$, astrocytes).

  • Type IV: Neurofilaments, Nestin, and Synemin; expressed along neuronal axons and muscular structures.

  • Type V: Lamins; assemble the nuclear lamina underlying the eukaryotic nuclear envelope.

  • Type VI: Lens-specific beaded intermediate filaments, comprising Phakinin ($\text{CP49}$) and Filensin within the ocular lens.

Extracellular Boundaries and Intercellular Junctions

• The Plant Cell Wall:

• Extracellular rigid exoskeleton synthesized outside the plant plasma membrane; found in plants, algae, fungi (chitin-based), and prokaryotes (peptidoglycan-based).

• Constructed from rigid cellulose microfibrils embedded in an amorphous matrix of pectins, hemicelluloses, and structural proteins.

• Structural Layers:

  • Primary Cell Wall: Relatively thin, pliable layer secreted during early growth, enabling cellular elongation.

  • Middle Lamella: Pectin-rich adhesive layer cementing adjacent plant cell walls together.

  • Secondary Cell Wall: Laminated, reinforced matrix containing cellulose and cross-linking lignin deposited between the plasma membrane and primary cell wall after cellular expansion ceases.

  • The Animal Extracellular Matrix ($\text{ECM}$):

• Dense network of secreted glycoproteins and carbohydrate-rich molecules providing tissue scaffolding, cell adhesion, positional communication, and mechanical support in the absence of a cell wall.

• Major Molecular Constituents:

  • Collagen: Highly tensile fibrous glycoprotein forming embedded extracellular networks.

  • Proteoglycan Complexes: Massive aggregates comprising hundreds of glycosaminoglycan-rich proteoglycan monomers attached to long polysaccharide backbones.

  • Fibronectin: Multi-adhesive glycoprotein tethering the extracellular matrix to cell surface receptors.

  • Integrins: Membrane-spanning heterodimeric receptor proteins connecting extracellular fibronectin to intracellular actin microfilaments.

  • Specialized Intercellular Junctions:

• Plasmodesmata (Plant Cells): Trans-wall channels lined by plasma membrane that bridge neighboring plant cells, enabling uninterrupted symplastic flow of water, mineral ions, RNAs, and signaling proteins.

• Tight Junctions (Animal Cells): Occluding junctions where transmembrane proteins tightly stitch adjacent plasma membranes together, preventing paracellular leakage of extracellular fluids across epithelial layers (e.g., intestinal linings).

• Desmosomes / Anchoring Junctions (Animal Cells): Robust mechanical rivets fastening neighboring cells into shear-resistant sheets; anchored in the cytoplasm by intermediate filaments (keratins).

• Gap Junctions / Communicating Junctions (Animal Cells): Ring-shaped hexameric protein assemblies (connexons) creating aqueous cytoplasmic channels between adjacent cells, allowing ions, sugars, and secondary messengers to diffuse directly for coordinated electrical and chemical signaling.

  • Summary Comparison of Cell Structures:

• Structures Unique to Animal Cells: Lysosomes and centrosomes containing pairs of centrioles.

• Structures Unique to Plant Cells: Photosynthetic chloroplasts, rigid cellulose cell walls, large central vacuoles, and plasmodesmata.