Medical School · Year 1 · Foundations · includes a quiz and discussion video

Lecture 2: Cell Structure and Function

Unit 1.1: Foundations of Medicine & Medical Sciences

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the structure and function of major cellular organelles including nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus
2. Explain the structure and properties of the plasma membrane, including the fluid mosaic model
3. Compare and contrast different membrane transport mechanisms: passive diffusion, facilitated diffusion, and active transport
4. Relate cellular ultrastructure to specific tissue and organ functions in the human body
5. Identify the cytoskeletal components and describe their roles in cell shape, movement, and intracellular transport
6. Explain the clinical significance of organelle dysfunction in human disease

Lecture Outline

I. Introduction to the Cell

A. The Cell as the Basic Unit of Life

The cell stands as the fundamental unit of all living organisms—the smallest entity capable of independent life. This principle, known as cell theory, establishes that all living things are composed of cells and that all cells arise from pre-existing cells through division. No spontaneous generation occurs; every cell in your body traces its lineage back through countless generations of cellular division.

The human body contains approximately 37 trillion cells, a number so vast it defies easy comprehension. These cells are not identical copies but rather encompass over 200 distinct cell types, each specialized for particular functions. Neurons transmit electrical signals across vast distances, while red blood cells ferry oxygen from lungs to tissues. Hepatocytes process nutrients and detoxify chemicals, while osteocytes maintain bone structure. This remarkable diversity arises from the same genome through differential gene expression, demonstrating how a single blueprint can yield such varied outcomes.

Understanding cellular structure and function provides the foundation for understanding all of medicine. Diseases ultimately manifest at the cellular level—whether through abnormal cell division (cancer), cell death (myocardial infarction), cellular dysfunction (diabetes), or cellular invasion (infection). The more deeply you understand normal cell biology, the more clearly you will understand how diseases disrupt these processes.

B. Prokaryotic vs. Eukaryotic Cells

Cells fall into two fundamental categories based on their internal organization. Prokaryotic cells, found in bacteria and archaea, lack membrane-bound organelles. Their genetic material floats freely in the cytoplasm rather than being enclosed within a nucleus. Prokaryotes are generally smaller and structurally simpler than eukaryotic cells, though they have proven remarkably successful across Earth's environments.

Eukaryotic cells possess membrane-bound organelles, including a true nucleus that houses and protects genetic material. This compartmentalization allows for specialization—different cellular processes can occur simultaneously in different locations, separated by membranes that regulate the movement of molecules between compartments. Human cells are eukaryotic, and the complexity of eukaryotic cell structure enables the complexity of multicellular organisms.

The evolutionary relationship between prokaryotes and eukaryotes informs our understanding of cellular organelles. Mitochondria, the cell's energy powerhouses, likely originated as free-living bacteria that were engulfed by ancestral eukaryotic cells—an endosymbiotic event that fundamentally shaped cellular evolution. Evidence for this origin includes mitochondria's own circular DNA, bacterial-type ribosomes, and double membrane structure.

<image>Panel A: Prokaryotic cell (bacterium) showing cell wall, plasma membrane, cytoplasm, circular DNA nucleoid region, and scattered ribosomes. Panel B: Prokaryotic flagellum and scale bar indicating 1-2 micrometers size. Panel C: Eukaryotic animal cell displaying nucleus with nuclear envelope, mitochondria, rough and smooth endoplasmic reticulum, and Golgi apparatus. Panel D: Eukaryotic cell lysosomes, peroxisomes, cytoskeleton elements, and scale bar showing 10-30 micrometers size difference.</image>

II. The Plasma Membrane

A. Fluid Mosaic Model

The plasma membrane defines the boundary of the cell, separating the carefully controlled intracellular environment from the variable extracellular space. The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes the membrane's structure and explains its remarkable properties.

The phospholipid bilayer forms the membrane's foundation. Each phospholipid molecule possesses a hydrophilic (water-loving) head containing a phosphate group and hydrophobic (water-fearing) tails composed of fatty acid chains. When placed in aqueous solution, phospholipids spontaneously arrange themselves into bilayers with hydrophilic heads facing the water on both sides and hydrophobic tails hidden in the interior. This arrangement creates a stable barrier that separates aqueous compartments.

The membrane is fluid, not rigid. Phospholipids move laterally within their layer, creating a dynamic structure that can change shape, fuse with other membranes, and accommodate the insertion of proteins. The degree of fluidity depends on temperature and lipid composition—unsaturated fatty acids with kinked tails increase fluidity, while saturated fatty acids pack tightly and decrease fluidity.

The mosaic aspect refers to proteins embedded within or attached to the membrane. These proteins are not uniformly distributed but rather cluster in functional domains, creating a patchwork pattern when viewed from above. Membrane proteins perform the active work of the membrane—transporting molecules, receiving signals, anchoring the cytoskeleton, and catalyzing reactions.

B. Membrane Lipids

Three classes of lipids contribute to membrane structure, each with distinct properties and functions.

Phospholipids constitute the primary structural component, forming the bilayer backbone. Different phospholipids predominate in different membrane leaflets—phosphatidylcholine and sphingomyelin concentrate in the outer leaflet, while phosphatidylserine and phosphatidylethanolamine predominate in the inner leaflet. This asymmetry is actively maintained and serves signaling functions; for example, phosphatidylserine exposure on the outer leaflet signals apoptosis.

Cholesterol, unique to eukaryotic membranes, intercalates between phospholipids to modulate membrane properties. At high temperatures, cholesterol restricts phospholipid movement, reducing fluidity. At low temperatures, cholesterol prevents tight packing, maintaining fluidity. This buffering effect keeps membrane fluidity within functional ranges despite temperature fluctuations. Cholesterol also increases membrane thickness and reduces permeability to small water-soluble molecules.

Glycolipids contain carbohydrate groups attached to lipid molecules. Found exclusively in the outer leaflet, glycolipids contribute to the glycocalyx—the carbohydrate-rich coat covering the cell surface. Blood group antigens (A, B, and O types) are glycolipids, illustrating their role in cell recognition and identity.

C. Membrane Proteins

Membrane proteins perform the membrane's functional work, and they fall into two categories based on their association with the lipid bilayer.

Integral proteins are embedded within the membrane, with portions extending into the hydrophobic core. Transmembrane proteins span the entire membrane, with domains exposed on both surfaces. These proteins typically contain hydrophobic amino acid sequences that anchor them within the lipid bilayer. Channel proteins form aqueous pores allowing specific ions to cross the membrane. Carrier proteins bind substances and undergo conformational changes to transport them across. Receptor proteins bind extracellular signals and transmit information into the cell.

Peripheral proteins associate with the membrane surface without penetrating the hydrophobic core. They attach through interactions with integral proteins or with the polar heads of phospholipids. Many peripheral proteins are enzymes that catalyze reactions at the membrane surface. Others serve structural roles, linking the membrane to the underlying cytoskeleton or to extracellular matrix components.

D. Membrane Carbohydrates

Carbohydrates attached to membrane lipids and proteins form the glycocalyx, a fuzzy coat visible on electron microscopy that extends from the cell surface. All carbohydrate groups face the extracellular space, creating an asymmetry between membrane surfaces.

The glycocalyx serves multiple functions. It protects the cell from mechanical damage and chemical insult. It mediates cell-cell recognition, allowing cells to identify self from non-self and enabling immune surveillance. It facilitates cell adhesion through interactions with carbohydrate-binding proteins on adjacent cells. Changes in glycocalyx composition accompany cancer transformation, contributing to altered cell behavior and metastatic potential.

<image>Panel A: Phospholipid bilayer cross-section showing hydrophilic heads as spheres and hydrophobic tails as wavy lines with cholesterol molecules intercalated. Panel B: Integral membrane proteins including barrel-shaped channel protein with central pore and carrier protein in two conformations. Panel C: Receptor protein with extracellular binding domain and peripheral proteins attached to inner membrane surface. Panel D: Glycocalyx formation with glycoproteins and glycolipids extending branching carbohydrate chains into extracellular space.</image>

III. Membrane Transport Mechanisms

The plasma membrane is selectively permeable—it allows some substances to cross while restricting others. Understanding transport mechanisms is essential for understanding how cells maintain their internal environment, how drugs enter cells, and how transport defects cause disease.

A. Passive Transport (No ATP Required)

Passive transport moves substances down their concentration gradient, from regions of higher concentration to regions of lower concentration. This movement is thermodynamically favorable, requiring no energy input from the cell.

1. Simple Diffusion

Simple diffusion occurs when molecules pass directly through the lipid bilayer without assistance from proteins. Only certain molecules can cross by this route—they must be small enough and sufficiently lipid-soluble to penetrate the hydrophobic core.

Gases diffuse readily across membranes. Oxygen moves from alveoli into blood and from blood into tissues along its partial pressure gradient. Carbon dioxide moves in the opposite direction. Nitrogen and other atmospheric gases also cross easily. This gas permeability is essential for respiratory function.

Small nonpolar molecules, including steroid hormones, also diffuse through membranes. Cortisol, estrogen, and testosterone cross plasma membranes to reach intracellular receptors, explaining why their receptors are located inside cells rather than on the surface.

The rate of simple diffusion depends on several factors: the steepness of the concentration gradient (steeper gradients drive faster diffusion), the membrane's permeability to that substance (determined by lipid solubility and molecular size), and the surface area available for diffusion (larger areas permit more molecules to cross simultaneously).

2. Facilitated Diffusion

Facilitated diffusion requires membrane proteins to assist transport but still moves substances down their concentration gradient without energy expenditure. This mechanism enables hydrophilic and charged molecules to cross the membrane despite their inability to penetrate the lipid bilayer.

Channel proteins form aqueous pores through the membrane. Ion channels allow specific ions—sodium, potassium, calcium, chloride—to flow rapidly across when the channel opens. Channel opening may be regulated by voltage (voltage-gated channels), ligand binding (ligand-gated channels), or mechanical forces (mechanosensitive channels). Aquaporins form channels specific for water, dramatically increasing membrane water permeability in tissues requiring rapid water transport, such as kidney collecting ducts.

Carrier proteins bind their substrates and undergo conformational changes that expose the binding site to the opposite side of the membrane. GLUT transporters facilitate glucose entry into cells—GLUT4 in muscle and fat cells mediates insulin-stimulated glucose uptake, and its dysfunction contributes to insulin resistance in diabetes.

3. Osmosis

Osmosis refers specifically to water movement across a semipermeable membrane. Water moves from regions of lower solute concentration (higher water concentration) to regions of higher solute concentration (lower water concentration), driven by differences in water potential.

Osmotic pressure describes the pressure required to prevent osmotic water flow. Solutions with equal solute concentrations (isotonic) produce no net water movement. Hypotonic solutions have lower solute concentration than the cell interior, causing water to enter and cells to swell. Hypertonic solutions have higher solute concentration, causing water to exit and cells to shrink.

Cells regulate their volume through ion transport mechanisms that adjust intracellular solute concentrations. The clinical importance of osmolarity is evident in intravenous fluid administration—infusing hypotonic solutions too rapidly causes cell swelling and potentially fatal cerebral edema, while hypertonic solutions can cause dangerous cell shrinkage.

<image>Panel A: Simple diffusion with O2 and CO2 molecules passing directly through phospholipid bilayer along concentration gradient from dark to light. Panel B: Facilitated diffusion via GLUT transporter showing glucose hexagons binding and protein conformational change. Panel C: Ion channel facilitated diffusion with ions flowing through aqueous pore down concentration gradient. Panel D: Osmosis through aquaporin channels with water molecules moving from dilute to concentrated solution.</image>

B. Active Transport (ATP Required)

Active transport moves substances against their concentration gradient, from lower to higher concentration. This thermodynamically unfavorable movement requires energy input, typically from ATP hydrolysis.

1. Primary Active Transport

Primary active transport directly couples ATP hydrolysis to substance movement. ATPase pumps bind ATP, hydrolyze it to ADP and phosphate, and use the released energy to drive conformational changes that move substrates across the membrane.

The sodium-potassium ATPase (Na⁺/K⁺-ATPase) is the most important primary active transporter in animal cells. Each cycle hydrolyzes one ATP molecule while exporting three sodium ions and importing two potassium ions. This establishes the sodium and potassium gradients fundamental to cell function—low intracellular sodium and high intracellular potassium. The Na⁺/K⁺-ATPase consumes roughly one-third of cellular ATP, reflecting its importance.

The calcium ATPase (Ca²⁺-ATPase) maintains low cytosolic calcium concentration by pumping calcium ions out of the cell or into the endoplasmic reticulum. Cytosolic calcium concentration is approximately 10,000-fold lower than extracellular concentration, a gradient essential for calcium signaling. When channels open and calcium floods into the cytosol, it triggers cellular responses; calcium must then be pumped out to reset the system.

The hydrogen-potassium ATPase (H⁺/K⁺-ATPase) in gastric parietal cells pumps hydrogen ions into the stomach lumen, generating the acidic environment necessary for digestion and pathogen defense. Proton pump inhibitors—among the most commonly prescribed medications—block this pump to reduce gastric acid secretion.

2. Secondary Active Transport

Secondary active transport harnesses the energy stored in ion gradients (established by primary active transport) to drive transport of other substances. No ATP is directly consumed, but the process depends on ATP-dependent gradients.

Symporters (cotransporters) move two substances in the same direction. The sodium-glucose cotransporter (SGLT) in intestinal and renal epithelial cells uses the sodium gradient to drive glucose uptake. As sodium flows down its concentration gradient into the cell, glucose is carried along, even against its own gradient. SGLT2 inhibitors, a class of diabetes medications, block renal glucose reabsorption by inhibiting this transporter.

Antiporters (exchangers) move two substances in opposite directions. The sodium-hydrogen exchanger uses sodium influx to drive hydrogen ion efflux, contributing to intracellular pH regulation. The sodium-calcium exchanger in cardiac muscle uses sodium entry to drive calcium exit, helping to terminate cardiac contraction.

<image>Panel A: Na+/K+-ATPase stages 1-2 showing three Na+ ions (blue spheres) binding intracellularly, ATP binding, and hydrolysis to ADP + Pi initiating conformational change. Panel B: Stages 3-4 showing completed conformational change opening to extracellular side and Na+ release with decreased Na+ affinity. Panel C: Stages 5-6 showing two K+ ions (purple spheres) binding extracellularly, phosphate release, and return to original conformation releasing K+ inside. Panel D: Summary equation (3 Na+in + 2 K+out + ATP to 3 Na+out + 2 K+in + ADP + Pi) with circular diagram overview.</image>

C. Vesicular Transport

Large molecules and bulk quantities of material cannot cross the membrane through channels or carriers. Instead, they move via vesicular transport—membrane-bound compartments that bud from one membrane and fuse with another.

1. Endocytosis

Endocytosis brings material into the cell by invaginating the plasma membrane to form intracellular vesicles. Three forms exist, distinguished by the nature of the material internalized.

Phagocytosis ("cell eating") engulfs large particles, including bacteria, cell debris, and foreign material. Pseudopods extend around the target, eventually fusing to enclose it within a phagosome. This process is particularly important in immune cells—neutrophils and macrophages use phagocytosis to engulf and destroy pathogens. The phagosome then fuses with lysosomes, which digest the contents.

Pinocytosis ("cell drinking") non-selectively internalizes extracellular fluid and dissolved solutes. Small vesicles continuously form at the plasma membrane, sampling the extracellular environment. This constitutive process occurs in virtually all cells.

Receptor-mediated endocytosis selectively concentrates specific molecules for internalization. Receptors in the plasma membrane bind their ligands, then cluster in clathrin-coated pits that invaginate and pinch off as coated vesicles. Low-density lipoprotein (LDL) cholesterol enters cells through this mechanism—LDL binds receptors, the receptor-LDL complex is internalized, and cholesterol is extracted for cellular use. Familial hypercholesterolemia results from LDL receptor mutations that impair this uptake.

2. Exocytosis

Exocytosis moves material out of the cell by fusing intracellular vesicles with the plasma membrane. The vesicle contents are released into the extracellular space while the vesicle membrane incorporates into the plasma membrane.

Constitutive exocytosis continuously delivers newly synthesized membrane proteins and lipids to the plasma membrane and secretes extracellular matrix components. This ongoing process maintains and renews the cell surface.

Regulated exocytosis releases vesicle contents in response to specific signals. Neurotransmitter release from nerve terminals is perhaps the most dramatic example—calcium influx triggers fusion of synaptic vesicles with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft within milliseconds. Hormone secretion from endocrine cells similarly involves regulated exocytosis.

IV. Cellular Organelles

Eukaryotic cells contain membrane-bound compartments called organelles, each specialized for particular functions. This compartmentalization allows incompatible processes to occur simultaneously in different cellular locations and increases the efficiency of processes by concentrating necessary components.

A. The Nucleus

1. Structure

The nucleus, typically the largest organelle, houses and protects the cell's genetic material. The nuclear envelope consists of two concentric membranes separated by a perinuclear space continuous with the endoplasmic reticulum lumen. Nuclear pore complexes punctuate this envelope, providing regulated passageways between nucleus and cytoplasm.

Within the nucleus, the nucleoplasm contains chromatin—DNA complexed with histone and non-histone proteins. Chromatin exists in two states: euchromatin is loosely packed and actively transcribed, while heterochromatin is densely packed and largely transcriptionally silent. The pattern of euchromatin and heterochromatin varies among cell types, reflecting their different gene expression profiles.

The nucleolus appears as a distinct dense region within the nucleus. It represents the site where ribosomal RNA is transcribed from clustered ribosomal DNA genes and where ribosomal subunits begin assembly. Cells with high protein synthesis demands, such as growing cells and secretory cells, typically have prominent nucleoli.

2. Functions

The nucleus serves as the cell's information center. DNA replication occurs here, copying the genome before cell division. Transcription produces RNA copies of genetic information, which are processed (introns removed, caps and tails added) before export to the cytoplasm. The nucleus thus controls which genes are expressed, determining cell identity and function.

3. Nuclear Pore Complex

Nuclear pore complexes (NPCs) are massive protein assemblies (~120 MDa) that span the nuclear envelope. They regulate all traffic between nucleus and cytoplasm, ensuring that molecules reach their proper destinations.

Small molecules and ions diffuse freely through NPCs. Larger molecules require active transport mediated by signal sequences and transport receptors. Proteins destined for the nucleus carry nuclear localization signals recognized by importins, which ferry cargo through the pore. RNA and ribosomal subunits carry nuclear export signals recognized by exportins. This selective transport maintains the distinct compositions of nuclear and cytoplasmic compartments.

<image>Panel A: Nuclear envelope cross-section showing inner and outer nuclear membranes with perinuclear space and barrel-shaped nuclear pore complexes. Panel B: Enlarged nuclear pore complex inset depicting cytoplasmic filaments, central channel, and nuclear basket structure. Panel C: Nuclear interior showing lighter loosely-packed euchromatin and darker densely-packed heterochromatin near periphery with prominent nucleolus. Panel D: Connection between outer nuclear membrane and rough endoplasmic reticulum with ribosomes and transport direction arrows.</image>

B. Mitochondria

1. Structure

Mitochondria possess a distinctive double membrane architecture. The outer mitochondrial membrane contains porins that make it permeable to molecules up to about 5 kDa, allowing free exchange of metabolites with the cytosol. The inner mitochondrial membrane is highly impermeable, maintaining the electrochemical gradients essential for ATP synthesis. This inner membrane is extensively folded into cristae, dramatically increasing its surface area.

The mitochondrial matrix, enclosed by the inner membrane, contains enzymes of the citric acid cycle, the mitochondrial genome (mtDNA), mitochondrial ribosomes, and other components necessary for mitochondrial function. The intermembrane space, between outer and inner membranes, accumulates protons pumped by the electron transport chain.

2. Functions

Mitochondria are the primary sites of ATP production through oxidative phosphorylation. The citric acid cycle, operating in the matrix, oxidizes acetyl-CoA derived from carbohydrates, fats, and proteins, generating NADH and FADH₂. The electron transport chain, embedded in the inner membrane, oxidizes these electron carriers, using the released energy to pump protons into the intermembrane space. ATP synthase harnesses the resulting proton gradient to synthesize ATP as protons flow back into the matrix.

This process generates approximately 90% of cellular ATP. A single glucose molecule yields roughly 30-32 ATP through complete oxidation, compared to only 2 ATP from glycolysis alone. Cells with high energy demands—cardiac muscle, skeletal muscle, neurons, hepatocytes—contain thousands of mitochondria.

3. Unique Features

Mitochondria retain features of their bacterial ancestors. Mitochondrial DNA is circular, like bacterial chromosomes, and lacks the histones that package nuclear DNA. Mitochondrial ribosomes resemble bacterial ribosomes and are sensitive to antibiotics that target bacteria. Mitochondria replicate semi-autonomously, dividing by fission independently of the cell cycle.

Human mtDNA encodes 37 genes—13 proteins of the electron transport chain, 22 transfer RNAs, and 2 ribosomal RNAs. All other mitochondrial proteins (over 1,000) are encoded by nuclear genes, synthesized in the cytosol, and imported into mitochondria. This division of genetic labor requires coordinated expression of two genomes.

Mitochondrial DNA is maternally inherited. During fertilization, sperm mitochondria are typically destroyed, so offspring receive mitochondria only from the egg. This maternal inheritance pattern is used to trace maternal lineages and has clinical implications for mitochondrial disease inheritance.

4. Clinical Correlations

Mitochondrial diseases result from mutations in mtDNA or in nuclear genes encoding mitochondrial proteins. Because mitochondrial function is essential for energy-demanding tissues, these diseases typically affect brain, heart, muscle, and endocrine organs.

MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes) results from mtDNA mutations affecting the electron transport chain. Patients experience stroke-like episodes, seizures, muscle weakness, and elevated blood lactate. Leigh syndrome causes progressive neurodegeneration beginning in infancy.

Mitochondria also participate in apoptosis (programmed cell death). Mitochondrial outer membrane permeabilization releases cytochrome c into the cytosol, activating caspases that execute the cell death program. This pathway eliminates damaged cells and sculpts tissues during development.

<image>Panel A: Mitochondrion outer membrane with smooth boundary and porin protein channels, inner membrane folded into cristae projecting into matrix. Panel B: Electron transport chain complexes I-IV and ATP synthase embedded in cristae with electron flow and proton pumping arrows. Panel C: Matrix contents including circular mtDNA molecules, mitochondrial ribosomes, and citric acid cycle enzyme complexes. Panel D: Chemiosmotic mechanism inset showing proton gradient in intermembrane space and ATP synthesis through ATP synthase.</image>

C. Endoplasmic Reticulum

1. Rough Endoplasmic Reticulum (RER)

The rough endoplasmic reticulum derives its name from the ribosomes studding its cytoplasmic surface, which give it a "rough" appearance in electron micrographs. These ribosomes are actively translating mRNAs encoding secretory proteins, membrane proteins, and proteins destined for other organelles.

Translation begins on free ribosomes in the cytosol. When a signal sequence emerges from the ribosome, signal recognition particle (SRP) binds it, halts translation, and directs the ribosome-mRNA complex to the RER membrane. Translation resumes with the growing polypeptide threaded into the ER lumen or inserted into the ER membrane.

Within the RER lumen, proteins fold into their proper three-dimensional conformations with assistance from chaperone proteins. Disulfide bonds form, stabilizing structure. N-linked glycosylation adds carbohydrate chains to asparagine residues. Quality control mechanisms recognize misfolded proteins and target them for degradation.

Cells specialized for secretion—plasma cells producing antibodies, pancreatic acinar cells secreting digestive enzymes—possess extensive rough endoplasmic reticulum to support their high rates of protein synthesis.

2. Smooth Endoplasmic Reticulum (SER)

The smooth endoplasmic reticulum lacks ribosomes and performs different functions. It synthesizes lipids, including phospholipids for membrane biogenesis and cholesterol. In steroid-producing cells—adrenal cortex, gonads—the smooth ER is abundant and produces steroid hormones from cholesterol.

The smooth ER also detoxifies drugs and metabolites. Cytochrome P450 enzymes embedded in the SER membrane catalyze oxidation reactions that make lipophilic compounds more water-soluble and easier to excrete. Hepatocytes contain extensive smooth ER for detoxification. Chronic exposure to certain drugs induces smooth ER proliferation, explaining some drug-drug interactions—increased metabolism of one drug when enzyme inducers are present.

Calcium storage represents another smooth ER function. Calcium ATPases pump calcium into the ER lumen, where it is stored bound to calcium-binding proteins. Signal-triggered calcium release from ER stores initiates many cellular responses. Muscle cells possess specialized smooth ER called sarcoplasmic reticulum that releases calcium to trigger contraction and sequesters it to allow relaxation.

<image>Panel A: Rough ER as flattened cisternae studded with ribosomes translating proteins with nascent polypeptides threading into ER lumen. Panel B: Inset showing signal sequence recognition and protein translocation through translocon channel with nuclear envelope connection. Panel C: Smooth ER tubular network depicting lipid synthesis, cytochrome P450 detoxification enzymes, and calcium ion storage in lumen. Panel D: Transition zone between rough and smooth ER with transport vesicles budding toward Golgi apparatus.</image>

D. Golgi Apparatus

1. Structure

The Golgi apparatus appears as a stack of flattened membrane-bound sacs called cisternae, typically 4-8 per stack. The stack has distinct polarity: the cis face receives vesicles from the ER, while the trans face ships vesicles to their final destinations. Medial cisternae lie between, and a trans-Golgi network on the trans side sorts proteins for different routes.

Vesicle traffic flows through the Golgi. COPII-coated vesicles bud from the ER and fuse with the cis-Golgi. As proteins move through the stack—whether by vesicle transport between cisternae or by cisternal maturation—they undergo progressive modifications. COPI-coated vesicles retrieve escaped ER proteins and return them to the ER.

2. Functions

Protein modification continues in the Golgi, particularly glycosylation. The high-mannose oligosaccharides added in the ER are trimmed and elaborated in the Golgi, creating the diverse oligosaccharide structures found on mature glycoproteins. Different Golgi compartments contain different enzymes, creating an assembly line of sequential modifications.

The Golgi also synthesizes complex polysaccharides, including the glycosaminoglycans of extracellular matrix. Proteoglycan core proteins receive their sugar chains in the Golgi, producing molecules like heparan sulfate and chondroitin sulfate essential for tissue structure and signaling.

Protein sorting occurs primarily in the trans-Golgi network. Sorting signals direct proteins to their appropriate destinations: secretory vesicles for proteins destined for export, lysosomes for hydrolytic enzymes, or plasma membrane for membrane proteins. Mannose-6-phosphate residues tag proteins for lysosomal delivery; failure of this tagging causes I-cell disease, in which lysosomal enzymes are secreted rather than delivered to lysosomes.

<image>Panel A: Golgi apparatus stacked cisternae with cis face near ER and trans face near plasma membrane, showing cis-Golgi network and medial cisternae. Panel B: COPII-coated vesicles arriving from ER fusing with cis face and COPI-coated vesicles mediating retrograde transport. Panel C: Trans-Golgi network with vesicles sorting to secretory pathway, lysosomes with mannose-6-phosphate receptors, and plasma membrane. Panel D: Inset showing glycosylation progression from simple high-mannose structures at cis face to complex oligosaccharides at trans face.</image>

E. Lysosomes

1. Structure

Lysosomes are membrane-bound vesicles containing acid hydrolases—enzymes that break down proteins, lipids, carbohydrates, and nucleic acids. These enzymes function optimally at acidic pH (~5), which is maintained by proton pumps in the lysosomal membrane. The lysosomal membrane also contains proteins that transport digestion products to the cytosol and protect the membrane from digestion by its own enzymes.

Over 50 different hydrolytic enzymes have been identified in lysosomes. This enzymatic repertoire enables complete degradation of essentially any biological macromolecule. Lysosomes also contain enzymes that generate reactive oxygen species, contributing to pathogen killing.

2. Functions

Intracellular digestion is the primary lysosomal function. Material delivered to lysosomes—whether from outside the cell (via endocytosis) or from inside (via autophagy)—is broken down to basic components that can be recycled.

Autophagy delivers damaged or excess organelles and cytoplasmic material to lysosomes. An isolation membrane surrounds the target, forming an autophagosome that fuses with lysosomes. This "self-eating" maintains cellular homeostasis by removing damaged components and recycling their building blocks during nutrient deprivation.

Phagocytosis delivers engulfed material to lysosomes for destruction. Professional phagocytes—macrophages and neutrophils—use this pathway to kill pathogens. The phagosome containing engulfed bacteria fuses with lysosomes, creating a phagolysosome where acid, enzymes, and reactive oxygen species combine to destroy the microorganism.

Specialized lysosomal functions operate in certain cell types. Osteoclasts secrete lysosomal contents onto bone surfaces to dissolve bone matrix, enabling bone remodeling. Defects in osteoclast function cause osteopetrosis, in which bone becomes abnormally dense.

3. Clinical Correlations

Lysosomal storage diseases result from inherited deficiencies of specific lysosomal enzymes. Without the missing enzyme, its substrate accumulates in lysosomes, eventually impairing cell function. These diseases typically affect multiple organs and often cause neurological impairment.

Tay-Sachs disease results from hexosaminidase A deficiency. GM2 ganglioside accumulates in neurons, causing progressive neurodegeneration. Affected infants appear normal at birth but develop seizures, blindness, and motor dysfunction, typically dying by age four.

Gaucher disease results from glucocerebrosidase deficiency. Glucocerebroside accumulates in macrophages, causing hepatosplenomegaly and bone disease. It is the most common lysosomal storage disease and can be treated with enzyme replacement therapy.

Pompe disease results from acid maltase (acid alpha-glucosidase) deficiency. Glycogen accumulates in lysosomes, particularly affecting muscle. Infantile-onset disease causes fatal cardiomyopathy; adult-onset disease causes progressive myopathy.

F. Peroxisomes

1. Structure

Peroxisomes are small, single-membrane-bound organelles containing oxidative enzymes. They form from the ER and can divide by fission, similar to mitochondria. Peroxisomal proteins are synthesized on free ribosomes and imported post-translationally via peroxisomal targeting signals.

2. Functions

Peroxisomes oxidize very-long-chain fatty acids (VLCFAs), which are too long to enter mitochondria directly. Beta-oxidation in peroxisomes shortens these fatty acids to lengths that can be further oxidized in mitochondria. Peroxisomes also oxidize branched-chain fatty acids and certain amino acids.

Hydrogen peroxide is generated as a byproduct of peroxisomal oxidations. Catalase, abundant in peroxisomes, converts this toxic hydrogen peroxide to water and oxygen. The peroxisome name derives from this hydrogen peroxide metabolism.

Other peroxisomal functions include bile acid synthesis, plasmalogen synthesis (plasmalogens are important membrane lipids, especially in myelin), and amino acid metabolism.

3. Clinical Correlations

Zellweger syndrome exemplifies peroxisome biogenesis disorders. Mutations affecting peroxisome assembly lead to absence of functional peroxisomes. VLCFAs accumulate, and plasmalogen synthesis fails. Affected infants have severe neurological impairment, hepatomegaly, and characteristic facial features, typically dying within the first year.

X-linked adrenoleukodystrophy results from defective import of VLCFAs into peroxisomes. VLCFA accumulation damages the adrenal gland (causing adrenal insufficiency) and nervous system (causing progressive demyelination). This disease was depicted in the movie "Lorenzo's Oil."

V. The Cytoskeleton

The cytoskeleton is a dynamic network of protein filaments that gives cells their shape, enables movement, organizes intracellular contents, and powers cell division. Three classes of filaments—differing in size, composition, and function—constitute the cytoskeleton.

A. Microfilaments (Actin Filaments)

Microfilaments, the thinnest cytoskeletal elements at 7 nm diameter, are polymers of the protein actin. Actin is among the most abundant proteins in eukaryotic cells, reflecting microfilaments' importance in cellular structure and function.

Actin monomers (G-actin, for globular) polymerize into filaments (F-actin, for filamentous) in an ATP-dependent process. Filaments have polarity, with a faster-growing "plus" end and a slower-growing "minus" end. This polarity enables directional movement of motor proteins along the filament.

Microfilaments concentrate beneath the plasma membrane, forming a cortical network that supports cell shape and membrane stability. They form the core of microvilli, the finger-like projections that increase surface area in absorptive epithelia. In motile cells, actin polymerization drives extension of lamellipodia (broad, flat protrusions) and filopodia (thin, spike-like projections) at the leading edge.

Muscle contraction depends on actin-myosin interactions. In muscle cells, highly organized arrays of actin filaments and myosin motors generate the force for movement. During cytokinesis, an actin-myosin contractile ring pinches the dividing cell in two.

B. Intermediate Filaments

Intermediate filaments, at 10 nm diameter, are intermediate in size between microfilaments and microtubules. Unlike the other cytoskeletal elements, intermediate filaments comprise diverse protein families, with different cell types expressing different intermediate filament proteins.

Keratins form intermediate filaments in epithelial cells, providing mechanical strength to tissues subject to physical stress. The skin contains abundant keratin filaments, and keratin mutations cause skin blistering diseases like epidermolysis bullosa simplex.

Vimentin characterizes mesenchymal cells (fibroblasts, endothelial cells), and its expression in tumors suggests mesenchymal origin. Neurofilaments provide structural support for the long axons of neurons, and their abnormal accumulation occurs in some neurodegenerative diseases. Lamins line the inner nuclear membrane, supporting nuclear structure; lamin mutations cause diseases ranging from premature aging (progeria) to muscular dystrophy.

Intermediate filaments primarily provide mechanical strength and stress resistance. Unlike microfilaments and microtubules, they are not polar and do not support motor protein-based transport. They anchor to desmosomes and hemidesmosomes, transmitting mechanical forces between cells and between cells and extracellular matrix.

C. Microtubules

Microtubules, the largest cytoskeletal elements at 25 nm diameter, are hollow tubes composed of tubulin protein. Each tubulin molecule is a dimer of alpha-tubulin and beta-tubulin; these dimers polymerize head-to-tail into protofilaments, and 13 protofilaments form the microtubule wall.

Microtubules radiate from the centrosome (the microtubule organizing center) near the nucleus, with minus ends anchored at the centrosome and plus ends extending toward the cell periphery. This arrangement creates highways for intracellular transport.

Microtubules undergo dynamic instability—continuous cycles of growth and rapid shrinkage. GTP hydrolysis drives this behavior; GTP-bound tubulin assembles readily, but hydrolysis to GDP destabilizes the polymer. Drugs that alter microtubule dynamics (taxanes stabilize microtubules; vinca alkaloids destabilize them) are important chemotherapy agents, as they disrupt the mitotic spindle required for cell division.

Cilia and flagella are microtubule-based structures that project from cell surfaces. Cilia move fluid across epithelial surfaces (respiratory tract, fallopian tubes) or serve as sensory antennae (kidney tubules, photoreceptors). Flagella propel cells (sperm). The axoneme of cilia and flagella has a characteristic "9+2" arrangement of microtubules, with outer doublets and a central pair.

<image>Panel A: Microfilaments as 7nm helical double-stranded structures with G-actin polymerizing to F-actin and myosin motor walking toward plus end. Panel B: Intermediate filaments as 10nm rope-like structures showing hierarchical assembly from monomers to coiled-coil dimers to tetramers to mature filaments. Panel C: Microtubules as 25nm hollow tubes with alpha-beta tubulin dimers forming 13 protofilaments and kinesin/dynein motors. Panel D: Cell schematic showing cytoskeletal distribution with microfilaments beneath membrane, intermediate filaments throughout, and microtubules radiating from centrosome.</image>

D. Motor Proteins

Motor proteins convert chemical energy (ATP hydrolysis) into mechanical work, enabling directed movement along cytoskeletal filaments.

Myosins move along actin filaments, generally toward the plus end. Different myosin family members serve different functions: myosin II powers muscle contraction and cytokinesis; myosin V transports vesicles and organelles; myosin I operates at the cell cortex.

Kinesins move along microtubules toward the plus end (anterograde transport, toward the cell periphery). Kinesin motors transport vesicles, organelles, and mRNA from the cell body toward the synapse in neurons, and move chromosomes during cell division.

Dyneins move along microtubules toward the minus end (retrograde transport, toward the cell center). Cytoplasmic dynein transports cargo toward the centrosome, including retrieval of material from the cell periphery. Axonemal dynein powers the beating of cilia and flagella.

VI. Cell-to-Cell Communication and Adhesion

Cells in tissues do not exist in isolation. They adhere to neighboring cells and to extracellular matrix, and they communicate through specialized junctions. These connections enable tissues to function as integrated units rather than collections of independent cells.

A. Cell Junctions

Tight junctions (zonula occludens) seal the space between adjacent epithelial cells, creating a barrier that prevents paracellular diffusion. The seal results from claudin and occludin proteins in adjacent membranes interacting to "zip" the cells together. Tight junction permeability varies among tissues—very tight in the blood-brain barrier, leakier in intestinal epithelium (allowing some paracellular nutrient absorption).

Adherens junctions provide adhesion between cells through cadherin proteins. Cadherins in adjacent cells interact in a calcium-dependent manner (hence the name), linking the cells mechanically. Inside the cell, cadherins connect to the actin cytoskeleton through catenin proteins. This connection transmits forces between cells and is important for tissue integrity during development and wound healing.

Desmosomes provide even stronger adhesion, particularly important in tissues subject to mechanical stress like skin and heart. Desmosomal cadherins (desmogleins and desmocollins) connect adjacent cells, while intracellular adapter proteins link to intermediate filaments. This connection distributes mechanical forces across cells rather than concentrating stress at single points. Autoimmune attack on desmosomal proteins causes the blistering disease pemphigus.

Gap junctions allow direct communication between adjacent cells. Connexin proteins form connexons (hemichannels) in each cell membrane; connexons in adjacent cells align to create channels connecting the cytoplasms. Small molecules (up to ~1 kDa)—ions, metabolites, second messengers—pass through, enabling electrical coupling (important in cardiac muscle) and metabolic cooperation.

B. Extracellular Matrix Interactions

Cells also attach to extracellular matrix through integrin receptors. Integrins are heterodimers of alpha and beta subunits, with different combinations recognizing different matrix components (collagens, fibronectin, laminin). The extracellular domain binds matrix, while the intracellular domain connects to the actin cytoskeleton.

Focal adhesions are integrin-rich sites where cells attach firmly to underlying matrix. They serve both mechanical (anchoring) and signaling functions, transmitting information about the extracellular environment into the cell. Focal adhesion signaling influences cell survival, proliferation, and differentiation, explaining why many cell types require matrix attachment to survive—a phenomenon called anchorage dependence that is often lost in cancer cells.

<image>Panel A: Tight junctions (zonula occludens) below apical surface with inset showing claudin and occludin proteins creating intercellular seal. Panel B: Adherens junctions with cadherin proteins linking to actin filaments through catenins depicted in inset. Panel C: Desmosomes as spot-like connections with desmosomal cadherins linking to intermediate filaments through desmoplakin. Panel D: Gap junctions showing connexon channels for small molecule passage and hemidesmosomes at basal surface connecting to basement membrane via integrins.</image>

VII. Clinical Applications

A. Drug Targets and Membrane Transport

Many therapeutic drugs target membrane proteins. Beta-blockers antagonize beta-adrenergic receptors; proton pump inhibitors block H⁺/K⁺-ATPase; calcium channel blockers prevent calcium influx through voltage-gated channels. Understanding transport mechanisms guides drug design and explains drug-drug interactions.

Drug absorption, distribution, and elimination depend on membrane transport. Lipophilic drugs cross membranes by diffusion; hydrophilic drugs require transporters. P-glycoprotein, an ATP-dependent efflux pump, expels many drugs from cells, limiting their intracellular accumulation. Cancer cells may overexpress P-glycoprotein, pumping out chemotherapy agents and causing multidrug resistance.

B. Organelle Dysfunction in Disease

Understanding organelle function illuminates disease mechanisms and suggests therapeutic approaches. Enzyme replacement therapy provides missing lysosomal enzymes. Chaperone therapy helps misfolded proteins achieve proper conformation. Gene therapy offers potential cures for genetic organelle disorders.

Summary

The plasma membrane is a dynamic, selectively permeable barrier organized according to the fluid mosaic model. Phospholipids form the bilayer foundation, cholesterol modulates fluidity, and proteins embedded in or attached to the membrane perform functional tasks including transport, signaling, and adhesion.

Transport mechanisms maintain cellular homeostasis. Passive transport (simple diffusion, facilitated diffusion, osmosis) moves substances down concentration gradients without energy expenditure. Active transport uses ATP to move substances against gradients—directly (primary active transport) or indirectly (secondary active transport). Vesicular transport moves large molecules and bulk quantities by endocytosis and exocytosis.

Each organelle performs specialized functions essential for cell survival. The nucleus houses genetic material and controls gene expression. Mitochondria produce ATP through oxidative phosphorylation. The endoplasmic reticulum synthesizes proteins and lipids. The Golgi apparatus modifies and sorts proteins. Lysosomes digest material. Peroxisomes oxidize fatty acids and detoxify hydrogen peroxide.

The cytoskeleton—microfilaments, intermediate filaments, and microtubules—provides structural support, enables movement, and organizes intracellular transport. Motor proteins convert ATP energy into mechanical work along cytoskeletal tracks.

Cell junctions connect cells into functional tissues. Tight junctions seal epithelia; adherens junctions and desmosomes provide adhesion; gap junctions enable communication. Integrins connect cells to extracellular matrix.

Understanding cell biology is fundamental to understanding disease mechanisms and developing therapies. Organelle dysfunction underlies numerous inherited and acquired diseases, and membrane proteins are targets for many drugs.

Key Terms

This content is subject to the MIT License. © 2024–2026 Hibbert School of Medicine.

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