Cell Structure and Function Study Guide

Cellular Level of Organization and Cell Theory

• Biologists first undertook detailed structural and functional studies of the cell during the 1830s.
• Cell Theory represents a unifying concept in modern biological science, originating from the foundational work of German scientists Matthias Schleiden and Theodor Schwann between 1838 and 1839.
• German botanist Matthias Schleiden ($1804\text{--}1881$) and German zoologist Theodor Schwann ($1810\text{--}1882$) established that all living organisms are composed of structural units called cells.
• German physician Rudolph Virchow ($1821\text{--}1902$) expanded the theory by establishing that cells arise exclusively from preexisting cells because cells are self-reproducing units.
• Core Tenets of Cell Theory:
  • All organisms are composed of one or more cells.
  • Cells are the fundamental units of structure and function in living organisms.
  • Cells originate only from preexisting cells through self-reproduction.

Cell Size and Surface-Area-to-Volume Relationships

• Cell Sizes and Scale Across Nature:
  • Most cells range in size from $1\,\text{mm}$ down to $1\,\mu\text{m}$ in diameter.
  • Atoms: Approximately $0.1\,\text{nm}$.
  • Amino acids: Approximately $1\,\text{nm}$.
  • Proteins: $5\text{--}10\,\text{nm}$.
  • Viruses: $10\text{--}100\,\text{nm}$.
  • Chloroplasts: Approximately $1\,\mu\text{m}$.
  • Most bacteria: $1\text{--}10\,\mu\text{m}$.
  • Plant and animal cells: $10\text{--}100\,\mu\text{m}$.
  • Human egg cell: Approximately $100\,\mu\text{m}$.
  • Frog egg: Approximately $1\,\text{mm}$.
  • Ant: Approximately $1\,\text{cm}$.
  • Mouse: $1\,\text{cm}$ to $0.1\,\text{m}$.
  • Ostrich egg: Approximately $0.1\,\text{m}$.
  • Rose: $0.1\,\text{m}$ to $1\,\text{m}$.
  • Human: Approximately $1\,\text{m}$.
  • Blue whale: Approximately $30\,\text{m}$.
• Surface-Area-to-Volume Constraints:
  • Cells require an adequately large surface area of plasma membrane relative to their interior volume to efficiently exchange nutrients, gases, and metabolic wastes with the external environment.
  • As cell size increases, volume grows much faster than surface area, causing the surface-area-to-volume ratio to decrease dramatically.
  • Large cells suffer from reduced transport efficiency across the plasma membrane.
  • Small cell size maintains a high surface-area-to-volume ratio, which is highly advantageous for rapid molecule exchange.
• Mathematical Models of Surface-Area-to-Volume Ratios:
  • Single $4\text{-cm}$ Cube:
  • Surface Area Calculation: $\text{height} \times \text{width} \times \text{number of sides} \times \text{number of cubes} = 4\,\text{cm} \times 4\,\text{cm} \times 6 \times 1 = 96\,\text{cm}^2$
  • Volume Calculation: $\text{height} \times \text{width} \times \text{length} \times \text{number of cubes} = 4\,\text{cm} \times 4\,\text{cm} \times 4\,\text{cm} \times 1 = 64\,\text{cm}^3$
  • Surface-Area-to-Volume Ratio: $\frac{96\,\text{cm}^2}{64\,\text{cm}^3} = 1.5:1$
  • Eight $2\text{-cm}$ Cubes (Same Total Volume):
  • Surface Area Calculation: $2\,\text{cm} \times 2\,\text{cm} \times 6 \times 8 = 192\,\text{cm}^2$
  • Volume Calculation: $2\,\text{cm} \times 2\,\text{cm} \times 2\,\text{cm} \times 8 = 64\,\text{cm}^3$
  • Surface-Area-to-Volume Ratio: $\frac{192\,\text{cm}^2}{64\,\text{cm}^3} = 3:1$
  • Sixty-Four $1\text{-cm}$ Cubes (Same Total Volume):
  • Surface Area Calculation: $1\,\text{cm} \times 1\,\text{cm} \times 6 \times 64 = 384\,\text{cm}^2$
  • Volume Calculation: $1\,\text{cm} \times 1\,\text{cm} \times 1\,\text{cm} \times 64 = 64\,\text{cm}^3$
  • Surface-Area-to-Volume Ratio: $\frac{384\,\text{cm}^2}{64\,\text{cm}^3} = 6:1$

Principles and Techniques of Microscopy

• Fundamental Parameters of Microscopy:
  • Magnification: The ratio between the apparent visual size of an image and its actual real physical size.
  • Resolution: The minimum physical distance separating two adjacent points that allows them to be distinguished as separate objects. Poor resolution displays adjacent cellular granules as a single blurred mass, whereas high resolution clearly separates them.
  • Contrast: The degree of difference in shading and intensity between an object and its immediate background. Visualizing specific subcellular structures like individual proteins is enhanced using fluorescently tagged antibodies.
• Illumination Properties:
  • Glass lenses refract (bend) and focus visible light rays.
  • Charged electrons cannot pass through glass or be focused by glass lenses; they require electromagnetic lenses to focus their path onto electron-sensitive screens or photographic plates.
• Compound Light Microscope:
  • Visible light passes through the specimen, focused through glass condenser and objective lenses, projecting an image onto the human retina.
  • Maximum magnification capability: Approximately $1000\times$.
  • Resolving power: Resolves structures separated by $0.2\,\mu\text{m}$, representing a $500\times$ improvement over the human eye (taking human eye resolving power as $1.0$).
  • Visualization Range: Observes chloroplasts ($1\,\mu\text{m}$), bacteria ($1\text{--}10\,\mu\text{m}$), plant and animal cells ($10\text{--}100\,\mu\text{m}$), human eggs ($100\,\mu\text{m}$), and frog eggs ($1\,\text{mm}$).
• Transmission Electron Microscope (TEM):
  • Accelerated electrons pass directly through an ultrathin specimen section and are focused using electromagnetic lenses.
  • Visualized image forms on a fluorescent screen or photographic plate.
  • Resolving power: Resolves structures separated by $0.0002\,\mu\text{m}$, representing a $500{,}000\times$ improvement over human visual resolution.
• Scanning Electron Microscope (SEM):
  • Specimen is coated with a thin layer of metal (such as gold).
  • An electron beam scans across the specimen surface, causing the metal coating to emit secondary electrons.
  • Emitted secondary electrons are collected and focused by magnetic lenses onto a screen, forming a high-resolution, three-dimensional surface topology image.
• Specialized Light Microscopy Modalities (e.g., Human Cheek Cell Analysis):
  • Bright-Field: Light passes directly through the specimen. Low natural contrast makes resolving inner subcellular components difficult without stains.
  • Bright-Field (Stained): Application of selective chemical dyes enhances contrast, as specific organelles absorb distinct stains.
  • Differential Interference Contrast (DIC): Utilizes optical wave-interference methods to accentuate cellular density gradients, rendering living cells, chromosomes, and organelle masses in distinct relief.
  • Phase Contrast: Exploits phase shifts caused by light passing through regions of differing density within living cells, translating phase shifts into contrast intensity variations.
  • Dark-Field: Directs light at an oblique angle so only diffracted and scattered light enters the objective lens, producing bright organelle images against a pitch-black background.
• Advanced Illumination Developments:
  • Confocal Microscopy: A narrow laser beam scans across a transparent specimen focused strictly on a single thin optical focal plane. Systematically focusing up and down produces optical sections that computer software compiles into a rotatable three-dimensional digital reconstruction.
  • Video-Enhanced Contrast Microscopy: A television camera captures microscope optical output, converting light into electronic signals for computer processing. Algorithms darken dark pixels and brighten light pixels, maximizing contrast for downstream image manipulation.
• Model Organism Example: Single-celled flagellated protist Euglena gracilis serves as a standard model across compound light microscopy, TEM, and SEM comparative studies.

Prokaryotic Cell Structure and Function

• General Features of Prokaryotes:
  • Prokaryotes lack a membrane-bound nucleus and membrane-bound internal organelles.
  • Structurally smaller and simpler than eukaryotic cells.
  • Classified into two distinct biological domains:
  • Domain Bacteria: Structurally diverse organisms; function as vital environmental decomposers, disease pathogens, and industrial agents in drug/chemical manufacturing.
  • Domain Archaea: Organisms adapted to extreme environmental habitats (e.g., thermal vents, hypersaline basins).
  • While archaea and bacteria share structural similarities, they possess fundamental biochemical differences.
• Cell Dimensions and Morphology:
  • Typical dimensions: $1.1\text{--}1.5\,\mu\text{m}$ in width and $2.0\text{--}6.0\,\mu\text{m}$ in length.
  • Three primary morphological shapes:
  • Spherical: Coccus.
  • Rod-shaped: Bacillus.
  • Spiral: Rigid forms termed spirillum; flexible forms termed spirochete.
• Structure of the Prokaryotic Cell Envelope:
  • Plasma Membrane: Phospholipid bilayer embedded with peripheral and integral proteins. It regulates molecular exchange and can form invaginated internal pouches known as mesosomes that expand membrane surface area.
  • Cell Wall: Rigid structural layer supporting cell shape and resisting osmotic lysis; strengthened by peptidoglycan.
  • Glycocalyx: Outer polysaccharide coat extending beyond the cell wall. When highly organized and tightly bound, it forms a protective capsule that resists host immune clearance.
• Cytoplasm and Internal Components:
  • Cytoplasm: Semifluid internal matrix surrounded by the plasma membrane; contains water, dissolved inorganic ions, organic molecules, and metabolic enzymes.
  • Nucleoid: Unenclosed nuclear region housing the cell's single, circular double-stranded DNA chromosome (e.g., as seen in Escherichia coli).
  • Plasmids: Small, extrachromosomal circular DNA rings that replicate independently and often carry accessory genes (such as antibiotic resistance).
  • Ribosomes: Granular cytoplasmic ribonucleoprotein structures dedicated to protein synthesis.
• Prokaryotic External Structures:
  • Flagella: Long, rotating helical protein filaments extending from the envelope that propel the bacterium through liquid media.
  • Fimbriae: Abundant, short, bristlelike surface projections enabling bacteria to adhere to physical surfaces and host tissues.
  • Conjugation Pili: Rigid, hollow tubular structures (singular: pilus) used to physically connect bacterial cells and transfer plasmid DNA directly between them.

Introduction to Eukaryotic Cells and Endosymbiotic Theory

• General Characteristics of Eukaryotic Cells:
  • Contain a membrane-bound nucleus housing genomic DNA.
  • Contain specialized membrane-bound organelles that compartmentalize metabolic processes.
  • Possess a plasma membrane composed of a phospholipid bilayer with embedded functional proteins.
  • Significantly larger overall size compared to prokaryotic cells.
  • Cellular compartmentalization isolates chemical reactions, protecting cytoplasm and enhancing metabolic efficiency.
• Structural Diversity and Specialization:
  • Generalized plant and animal cell models represent ideal constructs; actual specialized cells vary in organelle density.
  • Example: Mammalian liver cells possess exceptionally abundant smooth endoplasmic reticulum to perform drug detoxification.
  • Example: Neurons contain extensive specialized plasma membranes to conduct bioelectric impulses.
  • Plants, fungi, and many protists possess cell walls; plant cell walls are composed of structural polysaccharides, primarily cellulose.
• Organelle Functional Classes:
  • Endomembrane System: Interconnected organelles that communicate dynamically via direct physical channels or mobile transport vesicles.
  • Energy-Related Organelles: Mitochondria and chloroplasts; self-sufficient organelles operating independently of endomembrane vesicle flow.
• Endosymbiotic Theory:
  • Microfossils indicate the earliest primitive cells were prokaryotes.
  • Comparative biochemical data demonstrates eukaryotic lineage is more closely related to Domain Archaea than Domain Bacteria.
  • Nuclear Envelope and Endomembrane System Origin: The eukaryotic nucleus evolved when plasma membrane invaginated around genomic DNA, forming a double membrane envelope. Proliferation of invaginated membranes simultaneously gave rise to the endoplasmic reticulum and Golgi apparatus.
  • Mitochondria Origin: A larger anaerobic eukaryotic host cell engulfed an aerobic prokaryote, forming a symbiotic relationship where the host gained aerobic respiration capabilities.
  • Chloroplast Origin: An ancestral eukaryotic lineage containing mitochondria engulfed a photosynthetic cyanobacterium, acquiring light-driven carbohydrate synthesis capabilities.

Nucleus and Ribosomes: Genetic Control and Protein Synthesis

• Structure and Function of the Nucleus:
  • Serves as the primary genetic command center of eukaryotic cells, typically situated near the cell center.
  • Enclosed by the nuclear envelope, a double-membrane complex perforated by intricate protein structures called nuclear pores.
  • Nuclear pores strictly regulate macromolecular transport between the inner nucleoplasm and the outer cytoplasm.
  • Nucleoplasm: Semifluid nuclear matrix containing chromatin.
  • Chromatin: Diffuse network composed of genomic DNA bound to proteins; condenses into visible distinct chromosomes during cell division.
  • Nucleolus: Dense, dark region inside the nucleus composed of ribosomal RNA (rRNA); acts as the assembly site for ribosomal subunits.
• Ribosome Structure and Distribution:
  • Non-membrane-bound complexes composed of structural rRNA and enzymatic proteins.
  • Formed from two distinct subunits (a large subunit and a small subunit) constructed within the nucleolus and exported into the cytoplasm.
  • Cytoplasmic Distribution:
  • Free Ribosomes: Unbound in the cytoplasm (singly or linked along mRNA as polyribosomes); synthesize proteins intended for intracellular cytoplasmic use.
  • Bound Ribosomes: Attached to the outer cytosolic face of the endoplasmic reticulum; synthesize proteins destined for insertion into membranes, organelle sequestration, or cellular secretion.
  • Central Dogma of Molecular Biology: Directs genetic information flow from DNA $\rightarrow$ mRNA $\rightarrow$ Protein.
• Detailed Step-by-Step Mechanism of Signal Peptide Co-Translational Import:
  1. A gene is transcribed into messenger RNA (mRNA) inside the nucleus; mRNA exits into the cytoplasm via a nuclear pore complex. A free ribosome attaches to the mRNA and initiates translation, synthesizing an N-terminal signal peptide.
  2. A cytoplasmic Signal Recognition Particle (SRP) specifically recognizes and binds to the signal peptide, temporarily pausing translation.
  3. The SRP-ribosome complex migrates to the rough ER membrane, where SRP binds to a membrane-bound SRP receptor. A translocon channel opens, SRP dissociates, and translation resumes.
  4. As the growing polypeptide chain is synthesized, it is fed directly into the lumen of the ER through the open translocon channel. An lumenal enzyme (signal peptidase) cleaves off the signal peptide.
  5. Upon translation completion, the ribosomal subunits and mRNA transcript disassemble. The completed polypeptide remains inside the ER lumen, where it folds into its native tertiary protein structure.

The Endomembrane System

• System Architecture:
  • An internal network of distinct membranous compartments restricting specific enzymatic reactions to isolated intracellular domains.
  • Consists of: Nuclear envelope, Rough Endoplasmic Reticulum, Smooth Endoplasmic Reticulum, Golgi apparatus, and diverse vesicle populations.
• Endoplasmic Reticulum (ER):
  • An extensive network of interconnected membranous tubules and flattened sacs called saccules, continuous with the outer layer of the nuclear envelope.
  • Rough ER:
  • Outer cytoplasmic membrane face is densely studded with active ribosomes.
  • Processes, folds, and modifies nascent proteins; attaches oligosaccharide chains to produce functional glycoproteins.
  • Packages modified proteins into membrane-bound transport vesicles directed toward the Golgi apparatus.
  • Smooth ER:
  • Lacks attached ribosomes.
  • Synthesizes diverse lipid species (e.g., steroid hormones such as testosterone produced in testes interstitial cells).
  • Performs specialized synthetic processes, carbohydrate metabolism, calcium storage, and lipid transport.
  • Highly developed in hepatic (liver) tissue to detoxify drugs and metabolic poisons.
  • Packages synthesized lipids into transport vesicles destined for the Golgi apparatus.
• Golgi Apparatus:
  • Discovered by Italian cytologist Camillo Golgi.
  • Composed of a stack of $3\text{--}20$ flattened, curved, hollow membranous saccules, resembling a stack of pancakes.
  • Demonstrates distinct structural and functional polarity:
  • Cis Face (Inner/Receiving Face): Oriented toward the ER; receives incoming transport vesicles carrying proteins and lipids.
  • Trans Face (Outer/Shipping Face): Oriented toward the plasma membrane; dispatches sorted, processed, and packaged products in outgoing vesicles.
  • Adds carbohydrate molecular tags ("zip codes") and signal sequences to modify proteins and lipids.
  • Vesicle Destinations:
  • Retained internally as functional lysosomes.
  • Directed to specific organelle membranes.
  • Returned back to the ER for recycling.
  • Transported as secretory vesicles to the plasma membrane for exocytosis.
• Lysosomes:
  • Specialized membrane-bound digestive vesicles formed by budding from the Golgi apparatus (present in animal cells, generally absent in plant cells).
  • Maintain an internal highly acidic pH containing potent hydrolytic digestive enzymes.
  • Break down complex biological macromolecules (proteins, nucleic acids, lipids, polysaccharides) into monomeric subunits for cellular reuse.
  • Fuse with endocytic/phagocytic incoming vesicles to destroy engulfed bacterial pathogens in human white blood cells.
  • Degrade damaged or aged cell components (autophagy).
  • Clinical Connection: Lysosomal Storage Diseases (e.g., Tay-Sachs disease) result from genetic mutations causing missing or inactive lysosomal enzymes, leading to toxic lipid accumulation in neural tissues. Gene therapy approaches aim to deliver functional enzyme genes to patient cells.
• Integrated Endomembrane Flow (Exocytosis Pathway):
  1. Rough ER synthesizes proteins; Smooth ER synthesizes lipids.
  2. Transport vesicles shuttle these products from ER to the cis face of the Golgi apparatus.
  3. Golgi apparatus modifies, sorts, and packages products into specialized vesicles at its trans face.
  4. Secretory vesicles move along cytoskeletal tracks to fuse with the plasma membrane, releasing products externally via exocytosis (e.g., digestive enzyme secretion by pancreatic acinar cells, milk production in mammary glands).

Microbodies and Vacuoles

• Microbodies and Peroxisomes:
  • Membrane-bounded vesicles loaded with specialized metabolic enzymes.
  • Structural appearance resembles lysosomes, but originate independently of Golgi sorting.
  • Play critical roles in lipid metabolism, oxidizing fatty acids.
  • Enzymatic reactions produce toxic hydrogen peroxide ($\text{H}_2\text{O}_2$) as a metabolic byproduct.
  • Contain high concentrations of the enzyme catalase, which rapidly degrades toxic hydrogen peroxide into harmless water and oxygen gas:     $2\text{H}_2\text{O}_2 \xrightarrow{\text{catalase}} 2\text{H}_2\text{O} + \text{O}_2$
  • Clinical Connection: Adrenoleukodystrophy (ALD) is a severe inherited metabolic disorder caused by a defective peroxisomal membrane transport protein, preventing peroxisomal fatty acid breakdown and causing progressive myelin degradation and severe neurological damage.
• Vacuoles:
  • Membranous sacs significantly larger than standard transport vesicles, functioning primarily in storage and cell maintenance.
  • Unicellular protists utilize specialized contractile vacuoles to pump out excess water and maintain osmotic equilibrium.
  • Plant Cell Central Vacuole:
  • Dominates mature plant cell volume, occupying up to $90\%$ of total cellular volume.
  • Stores water, inorganic ions, nutrients, water-soluble pigments (such as floral anthocyanins), and waste products.
  • Exerts hydrostatic pressure against the cell wall, developing vital cellular turgor pressure that keeps plant tissues upright.
  • Stores toxic secondary metabolites (defensive chemicals) to deter herbivorous animals.
  • Contains hydrolytic enzymes that break down worn-out cellular organelles, fulfilling roles performed by lysosomes in animal cells.

Energy-Related Organelles: Chloroplasts and Mitochondria

• Key Similarities and Bioenergetic Coupling:
  • Chloroplasts and mitochondria are specialized membranous organelles responsible for cellular energy transformations.
  • Both contain their own circular DNA genomes and self-replicating prokaryotic-like ribosomes.
  • Chloroplasts capture solar energy and use carbon dioxide ($\text{CO}_2$) and water ($\text{H}_2\text{O}$) to synthesize energy-rich carbohydrates, releasing oxygen ($\text{O}_2$):     $\text{solar energy} + 6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{photosynthesis}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$
  • Mitochondria oxidize high-energy carbohydrates in the presence of oxygen ($\text{O}_2$) during cellular respiration, producing cellular energy in the form of adenosine triphosphate (ATP), while releasing low-energy $\text{CO}_2$ and $\text{H}_2\text{O}$ back into the environment:     $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \xrightarrow{\text{cellular respiration}} 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$
• Chloroplast Structure and Biochemistry:
  • Specialized photosynthetic plastids present exclusively in plants, algae, and photosynthetic protists/cyanobacteria.
  • Enclosed by a double-membrane envelope (outer and inner membranes).
  • Inner region is filled with a fluid matrix called the stroma.
  • Suspended within the stroma is an intricate internal membrane network of flattened disc-like sacs termed thylakoids.
  • Thylakoids are organized into dense stacks called grana (singular: granum).
  • The interior space enclosed within thylakoids is the thylakoid space.
  • Light-absorbing green photosynthetic pigments (chlorophyll) are embedded strictly within the thylakoid membranes.
  • Enzymes responsible for catalyzing carbohydrate synthesis reactions are located in the fluid stroma.
• Mitochondrion Structure and Bioenergetics:
  • Found in virtually all eukaryotic cells (plants, animals, fungi, protists).
  • Typically smaller than chloroplasts; total number per cell varies according to tissue metabolic demand (e.g., highly active mammalian liver cells contain up to $1{,}000$ mitochondria per cell).
  • Bound by a double membrane system:
  • Smooth Outer Membrane: Encloses the organelle.
  • Highly Folded Inner Membrane: Involuted into deeply convoluted folds called cristae, which greatly increase functional surface area for membrane-bound electron transport chains and ATP synthase complexes.
  • Inner semifluid cavity enclosed by the inner membrane is the matrix.
  • The matrix contains respiratory enzymes that break down carbohydrate fragments and organic molecules, capturing energy to generate ATP.

The Cytoskeleton and Cellular Motility

• Overview of the Cytoskeleton:
  • A dynamic network of interlinked protein fibers extending throughout the cytoplasm.
  • Maintains cell morphology and mechanical integrity.
  • Drives internal transport of vesicles and organelles, facilitates cell motility, and coordinates chromosome segregation during cell division.
  • Dynamic assembly: Polymerizes and depolymerizes rapidly in response to intracellular signaling and environmental cues.
• Actin Filaments:
  • Composition: Extremely thin fibers ($7\,\text{nm}$ diameter) constructed from globular actin monomers arranged like two twisted strands of pearls.
  • Structural Functions: Forms a dense cortical network directly beneath the plasma membrane to support cell shape; provides structural framework for microvilli lining intestinal epithelial cells.
  • Motility Functions: Mediates intracellular traffic, drives cytoplasmic streaming in plant cells (Chara), and supports pseudopod formation in amoeboid organisms.
  • Motor Interactions: Interacts with the motor protein myosin (utilizing ATP hydrolysis) to generate muscle contractions and form the contractile ring (cleavage furrow) during animal cell division.
• Intermediate Filaments:
  • Composition: Intermediate size ($8\text{--}11\,\text{nm}$ diameter); constructed as ropelike assemblies of fibrous polypeptide chains.
  • Properties: Chemically variable across tissue types (e.g., keratin in epithelial cells).
  • Functions: Provides intense mechanical strength to skin cells and animal cell-to-cell junctions (desmosomes); supports the nuclear envelope as the nuclear lamina.
• Microtubules:
  • Composition: Hollow, rigid cylinders ($25\,\text{nm}$ outer diameter) constructed from globular protein heterodimers consisting of $\alpha\text{-tubulin}$ and $\beta\text{-tubulin}$.
  • Assembly Architecture: Spontaneous association of $\alpha\text{-}$ and $\beta\text{-tubulin}$ forms dimers that stack into linear protofilaments; $13$ parallel protofilaments arrange circularly to form a hollow tube.
  • Regulation: Assembly is organized by the cell's Microtubule-Organizing Center (MTOC); the primary MTOC in animal cells is the centrosome.
  • Motor Interactions: Serves as molecular tracks for ATP-driven motor proteins kinesin and dynein.
  • Kinesin attaches to vesicle kinesin receptors and "walks" along microtubules toward the cell periphery.
  • Dynein moves retrogradely toward the cell center.
  • Cell Division: Constructs the dynamic mitotic spindle that attaches to and segregates chromosomes during cell division.
• Centrioles:
  • Structure: Short, hollow cylinder composed of $27$ microtubules organized into $9$ overlapping microtubule triplets ($9 \times 3$ array).
  • Organization: Animal cells contain one pair of centrioles located within the centrosome, positioned strictly at right angles ($90^\circ$) to each other.
  • Function: Replicate prior to cell division and separate to organize mitotic spindle poles; missing in plant and fungal cells; act as basal bodies that template cilia and flagella.
• Cilia and Flagella:
  • Projections from the cell surface enabling fluid movement or cell swimming.
  • Eukaryotic structure is completely distinct from prokaryotic flagella:
  • Enclosed by the cell's outer plasma membrane.
  • Core internal structure (axoneme) consists of a cylinder containing $18$ microtubules arranged into $9$ outer doublets surrounding $2$ central single microtubules ($9 + 2$ arrangement).
  • Anchored to the cell by a basal body situated at the base, which features a $9 \times 3$ triplet microtubule structure identical to a centriole.
  • Mechanism of Movement: Dynein motor side arms attached to outer doublets bind adjacent doublets and walk along them using ATP hydrolysis. Because doublets are structurally anchored, sliding forces are converted into coordinated bending.
  • Functional Differences:
  • Cilia: Short ($2\text{--}10\,\mu\text{m}$), highly abundant, move in synchronized oar-like strokes (e.g., ciliated epithelial cells lining the human upper respiratory tract).
  • Flagella: Long ($10\text{--}200\,\mu\text{m}$), few per cell, move in dynamic propeller-like or corkscrew undulations (e.g., human sperm cells).