Cell Shapes and Sizes
About 200 cell types in the human body with varied shapes, sizes, and functions.
Organs and tissues are often named or described by the shapes of their cells:
Squamous — thin, flat, scaly
Cuboidal — squarish-looking
Columnar — taller than wide
Polygonal — irregularly angular shapes, multiple sides
Stellate — star-like shape
Spheroidal to ovoid — round to oval
Discoidal — disc-shaped
Fusiform — thick in the middle, tapered toward the ends
Fibrous — threadlike
A cell’s shape can appear different in different types of sections (longitudinal vs cross section).
Development of Cell Theory
Schwann (1800s) stated that all animals are made of cells.
Pasteur (1859) disproved spontaneous generation.
Modern cell theory posits:
All organisms are composed of cells.
The cell is the simplest structural and functional unit of life.
Cells arise from preexisting cells.
Cells of all species are similar.
Basic Components of a Cell
Improvements in microscopy revealed cell ultrastructure:
Light microscope (LM): visualizes plasma membrane, nucleus, cytoplasm.
Transmission electron microscope (TEM): higher resolution via electron beam; reveals internal detail.
Scanning electron microscope (SEM): 3‑D surface images at high magnification and resolution; limited to surface features.
The Plasma Membrane
Defines cell boundaries; appears as a pair of dark parallel lines under TEM.
Has intracellular (cytoplasmic) face and extracellular face.
Controls passage of materials in and out of the cell.
Membrane Lipids
Approximately 98% of the membrane is lipids, mainly phospholipids.
Phospholipids (~75% of membrane lipids):
Phospholipid bilayer with hydrophilic (water-loving) phosphate heads facing water on each side.
Hydrophobic tails oriented toward the center; keeps water out of the interior region.
Lipids drift laterally, contributing to membrane fluidity.
Cholesterol (~20%):
Helps hold phospholipids in place and stiffens the membrane.
Glycolipids (~5%):
Carbohydrate chains on the extracellular face form part of the glycocalyx (carbohydrate coating on the cell surface).
The Plasma Membrane (continued) – Membrane Proteins
Proteins comprise about 2% of the molecules but ~50% of the membrane weight.
Transmembrane proteins span the membrane; hydrophilic regions contact watery cytoplasm and extracellular fluid; hydrophobic regions pass through lipid bilayer.
Most transmembrane proteins are glycoproteins.
Some proteins drift within the membrane; others are anchored to cytoskeleton.
Peripheral proteins adhere to one face of the membrane; inner-face proteins are often tethered to transmembrane proteins and cytoskeleton.
Functions of membrane proteins include:
Receptors — bind chemical signals to trigger internal changes; may produce second messengers.
Enzymes — catalyze reactions (e.g., digestion, second messenger production).
Channel proteins — allow passage of hydrophilic solutes and water; some are always open (leak channels), others gated (ligand-, voltage-, or mechanically gated).
Carriers — bind solutes and transfer them across the membrane; may exhibit saturation (transport maximum, Tmax).
Pumps — ATP-powered carriers.
Cell-identity markers — glycoproteins used as identification tags.
Cell-adhesion molecules (CAMs) — mechanically link cells to each other and to extracellular material.
Transmembrane Proteins (visual overview)
Transmembrane proteins have hydrophilic (external) and hydrophobic (membrane-embedded) regions and can anchor peripheral proteins to the cytoskeleton.
Functions of Membrane Proteins (overview)
Receptors — bind signals to trigger intracellular changes; may generate second messengers.
Enzymes — catalyze reactions (e.g., digestion, second messengers).
Channel proteins — form pores for solutes or water; some always open (leak channels); others gated (ligand-, voltage-, mechanically gated).
Carriers — bind solutes then change conformation to transport them; can be specific to solutes; may exhibit saturation ( Tmax ).
Pumps — use ATP to move solutes against their gradient.
Cell-identity markers — glycoproteins for cell recognition.
CAMs — anchor cells to each other and to extracellular matrix.
Extensions of the Cell Surface: Microvilli, Cilia, and Flagella
Microvilli — membrane extensions that increase surface area by 15–40x; dense in absorptive cells; brush border appearance.
Cilia — hair-like projections:
Primary (nonmotile) cilium present on nearly all cells; acts as an antenna for sensing conditions; aids balance in inner ear; light detection in retina.
Nonmotile cilia on sensory cells of the nose.
Motile cilia found in the respiratory tract, uterine tubes, brain ventricles, and ducts of testes.
Flagella — tail of sperm; the only functional flagellum in humans.
The Cytosol and the Cytoskeleton
Cytosol — clear, viscous, watery intracellular fluid containing enzymes, proteins, ATP, electrolytes, gases, and wastes.
Cytoskeleton — network of protein filaments/cylinders that provides structural support, determines cell shape, organizes contents, and enables movement.
Cytoskeleton components: microfilaments, intermediate filaments, microtubules.
The Cytoskeleton – Components and Roles
Microfilaments (actin) ~6 nm.
Intermediate filaments ~8–10 nm; provide tensile strength and shape stability; keratin-rich in skin cells.
Microtubules ~25 nm; composed of tubulin; radiate from centrosome; tracks for motor proteins; form axonemes of cilia/flagella; form mitotic spindle.
Key proteins (examples from figures) include kinesin (a motor protein) and dynein arms in cilia/flagella.
Organelles and the Nucleus Context
Organelles are internal structures performing specialized metabolic tasks.
Membranous organelles: nucleus, mitochondria, lysosomes, peroxisomes, endoplasmic reticulum, Golgi complex.
Nonmembranous organelles: ribosomes, centrosomes, centrioles, basal bodies.
The Nucleus
Usually the largest organelle (≈5 μm in diameter).
Most cells have one nucleus; some are anuclear or multinucleated.
Nuclear envelope — double membrane surrounding the nucleus; perforated by nuclear pores (nuclear pore complex).
Nuclear pores regulate molecular traffic; nuclear lamina provides structural support.
Nucleoplasm contains chromatin (DNA + proteins) and one or more nucleoli (sites of ribosome production).
Electron micrographs show nuclear pores, envelope, and lamina.
Endoplasmic Reticulum (ER)
ER is an interconnected network of membranous cisterns.
Rough ER — parallel flattened sacs studded with ribosomes; continuous with outer membrane of the nuclear envelope; synthesizes phospholipids and proteins for membranes; packages proteins into other organelles or for secretion.
Smooth ER — tubular, lacks ribosomes; synthesizes steroids and other lipids; detoxifies alcohol/drugs (liver and kidney); stores calcium in muscle cells.
Rough ER and smooth ER are continuous portions of the same organelle system.
Ribosomes
Granules of protein and RNA; assembled in the nucleolus.
Found attached to rough ER and free in cytosol.
Function: synthesize proteins using the genetic code in mRNA; assemble amino acids into proteins.
Structure: large subunit + small subunit = complete ribosome.
Golgi Complex
Golgi apparatus is a system of stacked membranous cisterns that synthesizes carbohydrates and modifies newly synthesized proteins.
Receives proteins from rough ER; sorts and modifies them (e.g., adds carbohydrate moieties) and packages them into Golgi vesicles.
Some vesicles become lysosomes; some fuse with the plasma membrane to release contents; some become secretory vesicles for later release.
Lysosomes
Lysosomes are enzyme-packed membranes that perform intracellular hydrolytic digestion of proteins, nucleic acids, carbohydrates, lipids, and other substances.
Autophagy — digestion of surplus organelles.
Autolysis — digestion of a cell by itself (cell suicide).
Peroxisomes
Resemble lysosomes but contain different enzymes; contain catalase and oxidation-related radicals.
Function: use molecular oxygen to oxidize organic molecules; produce hydrogen peroxide (H2O2); catalase converts H2O2 to water and O2.
Neutralize free radicals; detoxify drugs and various toxins; beta-oxidation of fatty acids to acetyl groups for mitochondria; abundant in liver and kidney.
Mitochondrion
Double membrane with inner membrane folds called cristae; matrix between cristae contains ribosomes and mtDNA.
Function: powerhouse of the cell; converts energy from nutrients into ATP.
Evolution: derived from engulfed bacteria; mitochondria have their own DNA (mtDNA); maternal inheritance predominates (sperm mitochondria typically degraded in the egg).
Structures: outer membrane, inner membrane, intermembrane space, matrix, cristae.
Centrioles
Centriole: short cylindrical assembly of microtubules in nine triplets.
Two centrioles lie perpendicular to each other within the centrosome.
Important for cell division; form basal bodies of cilia and flagella (basal bodies originate from centriolar organizing center and migrate to the membrane).
Vesicular (Bulk) Transport
Vesicles are membrane-bound sacs that transport large particles, fluid droplets, or many molecules.
Endocytosis brings material into the cell; exocytosis releases material out of the cell.
Endocytosis forms:
Phagocytosis — engulf large particles; “cell eating”; pseudopods surround object; phagosome fuses with lysosome to form phagolysosome for digestion.
Pinocytosis — uptake of droplets of extracellular fluid; vesicles are small and nonspecific; “cell drinking.”
Receptor-mediated endocytosis — selective uptake where particles bind to receptors; membrane pit coated with clathrin forms a coated vesicle that proceeds inside the cell.
Transcytosis — transport material across the cell by capturing it on one side and releasing it on the other (e.g., receptor-mediated endocytosis on one side and exocytosis on the opposite side).
Exocytosis — discharge materials from the cell; examples include insulin release, enzymes for fertilization, milk secretion; also used to replenish plasma membrane lost via endocytosis.
Membrane Transport Review (summary of transport mechanisms)
Movement without carriers:
Filtration
Simple diffusion
Osmosis
Carrier-mediated transport (with carriers):
Facilitated diffusion (down its concentration gradient; no ATP)
Active transport (up concentration gradient; requires ATP)
Vesicular (bulk) transport:
Endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis)
Exocytosis
Key abbreviations and concepts:
Uniport — carrier transports a single solute
Symport (cotransport) — carries two or more solutes in the same direction
Antiport (countertransport) — carries two or more solutes in opposite directions
Primary active transport — directly uses ATP (e.g., Na+/K+ pump)
Secondary active transport — indirectly uses ATP via a primary transport mechanism (e.g., SGLT with Na+ gradient)
Transport maximum (Tmax) — saturation point when all carriers are occupied
The Cytosol and Cytoskeleton (detailed)
Cytosol is a clear, viscous, aqueous-like intracellular fluid.
Cytoskeleton provides structural support and motility; composed of:
Microfilaments (thin filaments) — ~6 nm; actin; form terminal web and contribute to cell shape and traction.
Intermediate filaments — ~8–10 nm; keratin in many cells; resist mechanical stress.
Microtubules — ~25 nm; tubulin polymers; radiate from the centrosome; tracks for motor proteins; form axonemes of cilia/flagella; form mitotic spindle.
Microtubules and other cytoskeletal elements interact with motor proteins (e.g., kinesin) for intracellular transport.
The Nucleus and Associated Structures (recap)
Nuclear envelope encloses the genetic material; perforated by nuclear pores (nuclear pore complex).
Nuclear lamina supports the envelope; nucleoplasm contains chromatin and nucleoli (ribosome production).
Endoplasmic Reticulum and Golgi Relationship (recap)
Rough ER is continuous with the nuclear envelope and studded with ribosomes; synthesizes proteins and phospholipids for membranes; proteins may be secreted or targeted to organelles.
Smooth ER synthesizes lipids/steroids, detoxifies certain compounds, and stores calcium ions (especially in muscle).
Golgi modifies, packages, and sorts proteins received from rough ER; creates vesicles destined for lysosomes, plasma membrane, or secretion.
Osmolarity and Tonicity
Osmolarity: total osmotic concentration of nonpermeating solutes per liter of solution. Typical body fluids have ~300 ext{ mOsm/L}.
Tonicity: effect of surrounding solution on cell volume/pressure; depends on nonpermeating solute concentration.
Terminology:
Hypotonic solution: lowers solute concentration outside the cell relative to intracellular fluid (ICF); water moves into cell; potential swelling and lysis. Example: distilled water.
Hypertonic solution: higher nonpermeating solute concentration outside; water exits cell; cell shrivels (crenation).
Isotonic solution: same osmolarity inside and outside; no net cell volume change (normal saline 0.9% NaCl is isotonic).
Clinical relevance: osmotic imbalances contribute to diarrhea, constipation, edema; IV fluid choices depend on tonicity.
Carrier-Mediated Transport (details)
Carriers are selective for specific solutes; solute binds to carrier at a binding site; transporter undergoes conformational change to release solute on the other side.
Saturation occurs: there is a transport maximum (Tmax) when all carriers are occupied.
Three main mechanisms:
Facilitated diffusion — down the concentration gradient; no ATP used.
Primary active transport — up concentration gradient; ATP used; examples: Na+/K+ pump (uniport for K+ and Na+ exchange), calcium pump (Ca2+ pump).
Secondary active transport — indirect ATP use; relies on the gradient created by primary active transport; example: sodium–glucose transporter (SGLT) moving glucose into the cell as Na+ moves down its gradient.
The Sodium–Potassium Pump (Na+/K+-ATPase)
Important example of primary active transport.
Cycle consumes one ATP per cycle and exchanges three Na+ for two K+ across the membrane.
Maintains Na+ and K+ gradients; keeps intracellular Na+ concentration low relative to ECF and intracellular K+ concentration high.
Functions:
Maintains electrochemical gradient essential for secondary transport.
Regulates cell solute concentration and thus osmosis and cell volume.
Maintains a negatively charged resting membrane potential.
Contributes to heat production.
Classic depiction: 3 Na+ efflux, 2 K+ influx per ATP hydrolyzed.
Transmembrane Transport and Cellular Homeostasis – Practical Connections
Transport systems determine how cells acquire nutrients and eliminate wastes.
Disorders in membrane transport can disrupt cell volume, signaling, and metabolism (e.g., CF involves chloride transport and ionic imbalance affecting mucus consistency).
Understanding tonicity and IV fluid choices is crucial in clinical settings to prevent cellular swelling or shrinkage.
Connections to Foundational Principles
Structure Determines Function: membrane composition (lipids, proteins) sets permeability, signal reception, and interaction with the cytoskeleton.
Homeostasis: membrane transport maintains ion gradients, volume, and metabolic equilibrium.
Evolutionary Perspective: organelles like mitochondria reflect endosymbiotic origins and specialization for energy production; nucleus and ER reflect compartmentalization advantages.
Key Formulas and Numerical References (LaTeX)
Osmolarity reference: ext{Osmolarity} \approx 300\ \,\text{mOsm/L} in body fluids.
Na+/K+ pump stoichiometry per cycle: 3\ \,\mathrm{Na}^+\text{ out},\ 2\ \mathrm{K}^+\text{ in} per ATP hydrolysis: \mathrm{ATP} \rightarrow \mathrm{ADP} + \mathrm{P_i}.
Transport concepts:
Transport maximum: Tmax — the rate level at which all carriers are occupied and transport rate plateaus.
Structural dimensions (from text references):
Typical cell diameter ≈ 5 μm for the nucleus; other organelle sizes are context-dependent within the text figures.
Practical Implications and Real-World Relevance
Cystic fibrosis (CF) highlights the clinical significance of membrane transport defects: defective chloride pumps compromise the saline layer on epithelial surfaces, leading to thick mucus and recurrent infections; management requires understanding ionic transport and mucociliary clearance.
IV fluid therapy relies on tonicity to prevent cellular swelling or shrinkage; isotonic saline is a common baseline, while hypotonic or hypertonic solutions have specific clinical indications.
Drug detoxification and lipid synthesis are linked to ER function (especially Smooth ER) in liver and kidney; detox pathways influence pharmacokinetics of drugs.
Summary of Core Concepts
Cells exhibit diverse shapes; tissue and organ function are related to cell morphology.
Cell theory underpins biology: cells are basic units of life; they derive from preexisting cells and share common features across species.
The plasma membrane is a dynamic, lipid-protein mosaic that governs boundary integrity, signal transduction, transport, and cell interactions.
Membrane lipids (phospholipids, cholesterol, glycolipids) create a fluid yet structured bilayer with a glycocalyx on the extracellular surface.
Membrane proteins provide receptors, enzymes, channels, carriers, and identity/adhesion roles, enabling communication, transport, and cell–cell interactions.
Extensions of the cell surface (microvilli, cilia, flagella) expand surface area, enable sensing, movement, and propulsion.
The cytosol and cytoskeleton organize intracellular architecture, provide mechanical support, and facilitate movement.
Organelles (nucleus, ER, Golgi, lysosomes, peroxisomes, mitochondria, centrioles) perform specialized tasks essential to cell survival, signaling, and energy production.
Vesicular transport (endocytosis, exocytosis, transcytosis) ensures bulk material movement and membrane turnover.
Carrier-mediated transport (facilitated diffusion, primary and secondary active transport) provides selective import/export of solutes, with concepts like specificity, binding, saturation, and transport maximum.
The Na+/K+-ATPase pump is a central example of primary active transport, establishing gradients that enable secondary transport and maintaining cell homeostasis.