Chapter 3 Notes: Cellular Form and Function

Development of the Cell Theory

  • Cytology: study of cells; origins trace to Robert Hooke who coined the term cellulae to describe cork cell walls.
  • Theodor Schwann concluded that all animal tissues are composed of cells.
  • Louis Pasteur showed that cells arise only from preexisting cells, refuting spontaneous generation.
  • Modern cell theory statements:
    • All organisms are composed of cells and cell products.
    • The cell is the simplest structural and functional unit of life.
    • An organism’s structure and functions are due to the activities of its cells.
    • Cells come only from preexisting cells, not from nonliving matter.
    • Cells of all species share fundamental chemical compositions and metabolic mechanisms.

Cell Shapes and Sizes

  • Approximately 200 cell types in the human body.
  • Descriptive shapes:
    • Squamous: thin and flat with nucleus bulge
    • Polygonal: irregular angular with four or more sides
    • Stellate: star-shaped
    • Cuboidal: squarish, about as tall as wide
    • Columnar: taller than wide
    • Spheroid to ovoid: round to oval
    • Discoid: disc-shaped
    • Fusiform: thick in middle, tapered at ends
    • Fibrous: threadlike
  • Note: Some shapes appear in tissue sections; true 3D shapes may differ.
  • Human cell sizes:
    • Most cells:
      extdiameter≈10ext−−15 μmext{diameter} \approx 10 ext{--}15\ \mu\text{m}
    • Egg cell:
      diameter≈100 μm\text{diameter} \approx 100\ \mu\text{m}
    • Nerve cell: can be up to ~1 meter long; longest human cell; slender and often not visible to naked eye.
  • Size limitations and geometry:
    • Surface area increases with the square of the diameter,
      SA∝D2SA \propto D^{2}
    • Volume increases with the cube of the diameter,
      V∝D3V \propto D^{3}
    • Nutrient absorption and waste removal depend on surface area; as a cell grows, volume grows faster than surface area, risking rupture if limits are exceeded.
  • Example illustrating growth effect (Fig. 3.2):
    • Large cell diameter: D=20 mmD=20\,\text{mm}; Small cell diameter: D=10 mmD=10\,\text{mm}.
    • Surface area for large cell: SA=20 mm×20 mm×6=2400 mm2SA = 20\,\text{mm} \times 20\,\text{mm} \times 6 = 2400\ \text{mm}^2
    • Volume for large cell: V=20 mm×20 mm×20 mm=8000 mm3V = 20\,\text{mm} \times 20\,\text{mm} \times 20\,\text{mm} = 8000\ \text{mm}^3
    • Surface area increase factor (if $D$ doubles): SA<em>newSA</em>old=(2DD)2=4\frac{SA<em>{new}}{SA</em>{old}} = \left(\frac{2D}{D}\right)^2 = 4
    • Volume increase factor: V<em>newV</em>old=(2DD)3=8\frac{V<em>{new}}{V</em>{old}} = \left(\frac{2D}{D}\right)^3 = 8

Basic Components of a Cell

  • Light microscopy reveals: plasma membrane, nucleus, cytoplasm (fluid between nucleus and plasma membrane).
  • Electron microscopy reveals ultrastructure: organelles, cytoskeleton, cytosol (ICF).
  • Key components (as seen in Fig. 3.3): nucleus, plasma membrane, Golgi apparatus, mitochondria, ribosomes, etc.

The Plasma Membrane (Structure and Function)

  • Unit membrane: borders the cell and many organelles; appears as two dark parallel lines under electron microscope.
  • Functions of the plasma membrane:
    • Defines cell boundaries and governs interactions with other cells.
    • Regulates passage of materials into and out of the cell.
    • Intracellular face faces cytoplasm; extracellular face faces outward.
  • Composition: phospholipid bilayer with embedded proteins and lipids; diverse regions with different compositions.
  • Extensions and coatings:
    • Glycocalyx: carbohydrate coating formed by membrane glycoproteins and glycolipids; unique to individuals (except identical twins); functions in protection, cell adhesion, immunity, fertilization, defense against cancer, embryonic development, and transplant compatibility.
  • Membrane lipids:
    • Phospholipids make up ~75% of membrane lipids; amphiphilic; hydrophilic heads face water on both sides; hydrophobic tails face the inside; lateral diffusion maintains fluidity.
    • Cholesterol ~20% of membrane lipids; helps stiffen and regulate fluidity.
    • Glycolipids ~5% of membrane lipids; contribute to glycocalyx.
  • Membrane proteins:
    • ~2% of molecules but ~50% of membrane weight; many are glycoproteins.
    • Transmembrane proteins span the membrane; hydrophilic regions contact cytoplasm and extracellular fluid; hydrophobic regions traverse the lipid bilayer.
    • Peripheral proteins adhere to one face of the membrane and are often anchored to the cytoskeleton.
  • Functions of membrane proteins include:
    • Receptors, second-messenger systems, enzymes, ion channels, carriers, cell-identity markers, cell-adhesion molecules (CAMs).
  • Receptors and signaling (overview):
    • Surface receptors bind chemical messengers (hormones, neurotransmitters) and trigger intracellular responses.
    • Second-messenger systems: receptor activation can generate a second messenger in the cytoplasm (e.g., cAMP).
  • The glycocalyx and cell surface interactions:
    • Carbohydrate moieties of glycoproteins/glycolipids form the glycocalyx; important for protection, immunity, recognition, and transplant compatibility.
  • Extensions of membrane to increase surface area (microvilli):
    • 1–2 µm long; brush border can increase absorptive area by 15–40×; actin filaments shorten microvilli to push absorbed contents inward.
  • Microvilli structure: actin microfilaments form and support each microvillus.
  • Cilia and flagella (structure and function):
    • Cilia: hairlike processes; primary cilium (~7–10 µm) is nonmotile and acts as an antenna for sensing conditions; some cells have motile cilia in the respiratory tract, uterine tubes, brain ventricles, and testes.
    • Axoneme structure: 9 + 2 array of microtubules; dynein arms drive bending through ATP energy.
    • Flagella: longer tail (e.g., sperm); uses axoneme identical to cilium but movement is undulatory rather than power/recovery strokes.
  • Cystic fibrosis (CF) – a genetic disease affecting chloride pumps:
    • CF involves failure to install chloride pumps in the plasma membrane, reducing saline layer on cell surface.
    • Thick mucus plugs pancreatic ducts and respiratory tract; leads to poor digestion and nutrient absorption and chronic infections; life expectancy around 30 years.
  • The plasma membrane and transport relevance:
    • Membrane permeability is controlled by number and activity of channels and carriers; some solutes cross via lipid bilayer, others through channels or carriers.
    • Channelopathies: diseases arising from defects in channel proteins.

Membrane Transport: Permeability and Mechanisms

  • Plasma membrane is a barrier and gateway; selectively permeable.

  • Transport types:

    • Passive (no ATP): filtration, diffusion, osmosis; driven by random molecular motion.
    • Active (requires ATP): active transport and vesicular transport.
    • Carrier-mediated: uses membrane proteins to move substances; can be passive or active.

  • Filtration:

    • Driven by hydrostatic pressure; examples include filtration of nutrients into tissues via capillary walls and kidney filtration of wastes while retaining cells and proteins.
    • For example, capillary hydrostatic pressure drives water and small solutes through clefts between endothelial cells.

  • Simple diffusion:

    • Net movement of particles from high to low concentration due to spontaneous motion; movement down the concentration gradient.
    • Substances diffuse either through the lipid bilayer (nonpolar, hydrophobic, lipid-soluble) or through channel proteins for hydrophilic/charged solutes.

  • Diffusion and permeability control:

    • Cells regulate permeability by adjusting the number of channels or gating them on/off.

  • Osmosis:

    • Flow of water across a selectively permeable membrane from side with higher water concentration to side with lower water concentration.
    • Hydration spheres: solute particles attract water, reducing free water available for diffusion.
    • Aquaporins: channel proteins specialized for water transport; cells can increase/decrease osmosis rate by adding/removing aquaporins.
    • Osmotic pressure: hydrostatic pressure required to stop osmosis.
    • Reverse osmosis: applying pressure on one side to drive water against the gradient (e.g., capillary filtration by the heart).

  • Osmolarity and tonicity:

    • One osmole: one mole of dissolved particles; e.g., 1 M NaCl equals 2 osm/L because NaCl dissociates into Na+ and Cl−.
    • Osmolarity: osmoles per liter of solution; physiologic solutions are often expressed in milliosmoles per liter (mOsm/L).
    • Blood plasma: about 300 mOsm/L300\ \text{mOsm/L}.
    • Osmolality: osmoles per kilogram of water; similar to osmolarity in body fluids (small difference).
    • Tonicity: ability of a solution to affect fluid volume and pressure in a cell; depends on solute concentration and membrane permeability.
    • Hypotonic solution: lower nonpermeating solute concentration than intracellular fluid (ICF) → cells swell and may lyse.
    • Hypertonic solution: higher nonpermeating solute concentration → cells crenate (shrink).
    • Isotonic solution: same solute concentration as ICF; no net change in cell volume; e.g., normal saline.

  • Effects on red blood cells (RBCs): hypotonic, isotonic, hypertonic solutions lead to swelling, stability, or crenation respectively.

  • Carrier-mediated transport:

    • Transport proteins carry solutes across the membrane; specificity means a carrier binds only particular ligands; solute binds at a binding site and carrier releases it unchanged on the other side.
    • Saturation: transport rate increases with solute concentration up to a transport maximum (Tm) when all carriers are occupied.
    • Types of carriers:
    • Uniport: carries one solute at a time.
    • Symport (cotransport): carries two or more solutes in same direction.
    • Antiport (countertransport): carries two or more solutes in opposite directions (e.g., Na+–K+ pump).
    • Mechanisms:
    • Facilitated diffusion: down its concentration gradient; does not consume ATP; carrier changes conformation to move solute across.
    • Active transport: up its concentration gradient; consumes ATP; examples include Na+/K+ pump, amino acid uptake, Ca2+ extrusion.

  • Facilitated diffusion mechanism (illustrated in Fig. 3.18): carrier binds solute on extracellular side, carrier changes conformation, solute released intracellularly.

  • Active transport details:

    • Na+/K+ pump exchanges 3 Na+ out for 2 K+ in per ATP hydrolyzed, helping maintain higher K+ inside and lower Na+ inside.
    • Pumps maintain tonicity and are essential because Na+ and K+ leak across the membrane.
    • The Na+/K+ pump uses ATP; about half of daily calories are spent on this pump.

  • Secondary active transport (cotransport):

    • Maintains a steep gradient across the membrane; e.g., SGLT (sodium-glucose cotransporter) uses Na+ gradient to bring glucose into the cell without directly using ATP.
    • Regulation of cell volume: fixed anions attract cations, causing osmosis; Na+/K+ pump activity can adjust ion concentration and osmolarity to counter swelling.

  • Membrane potential and heat production:

    • Ion pumping maintains a membrane potential (inside negative, outside positive).
    • Thyroid hormone increases Na+/K+ pumps, increasing ATP consumption and heat production.

  • Vesicular transport (Fig. 3.21–3.24):

    • Vesicular transport moves large particles, droplets, or many molecules in vesicles.
    • Endocytosis: brings material into the cell; types include:
    • Phagocytosis (cell eating): engulfing large particles; pseudopods form phagosomes; macrophages common examples.
    • Pinocytosis (cell drinking): uptake of ECF droplets via pinocytotic vesicles.
    • Receptor-mediated endocytosis: selective uptake via receptors; involves clathrin-coated vesicles; example: LDL uptake.
    • Exocytosis: discharging material from the cell; vesicle contents released; membrane adds back to the plasma membrane.
    • Receptor-mediated endocytosis vs pinocytosis: specificity vs nonselective uptake.
    • Endocytosis vs exocytosis coordinate to regulate membrane surface area and receptor content.

The Cell Interior: Cytoskeleton and Organelles

  • Cytoskeleton: network of filaments and tubules that give shape, support, organize contents, and facilitate movement.
    • Microfilaments: ~6 nm, actin; form terminal web; involved in cell motility and shape.
    • Intermediate filaments: 8–10 nm; provide tensile strength and support.
    • Microtubules: ~25 nm; tubulin polymers; form tracks for motor proteins; radiate from centrosome; organize cell contents; maintain cell shape; transport intracellular cargo.
  • Cytoskeleton visualization: figures show microfilaments, intermediate filaments, and microtubules; cytoskeletal components support organelles and drive intracellular movements.
  • Organelles (membranous vs nonmembranous):
    • Membranous: nucleus, mitochondria, lysosomes, peroxisomes, endoplasmic reticulum (ER), Golgi complex.
    • Nonmembranous: ribosomes, centrosomes, centrioles, basal bodies.
The Nucleus
  • Nucleus: largest organelle (~5 µm diameter in most cells).
  • Nuclear envelope: two unit membranes; perforated by nuclear pores; regulate molecular traffic; held together by nuclear lamina.
  • Nucleoplasm: contains chromatin (DNA + protein) and nucleoli (ribosome production).
  • Functions: genetic control center; directs protein synthesis; organizes chromatin; regulates the cell life cycle.
Endoplasmic Reticulum (ER)
  • ER: network of interconnected cisternae enclosed by a unit membrane.
  • Rough ER: parallel flattened sacs studded with ribosomes; continuous with outer nuclear envelope; synthesizes phospholipids and proteins for membranes and secreted proteins.
  • Smooth ER: lacks ribosomes; tubular cisternae; synthesizes steroids and lipids; detoxifies alcohol and drugs; manufactures all cell membranes; functionally linked to rough ER.
  • Relationship: rough and smooth ER are different functional parts of the same network.
Ribosomes
  • Ribosomes: small granules of protein and RNA.
  • Locations: nucleoli, cytosol, outer surfaces of rough ER, nuclear envelope.
  • Function: read mRNA and assemble amino acids into proteins (protein synthesis).
Golgi Complex
  • Golgi: system of cisternae that finish protein and glycoprotein processing; sorts, cuts, and adds carbohydrate moieties; packages proteins into Golgi vesicles.
  • Fates of Golgi products: lysosomes, plasma membrane vesicles, secretory vesicles for release.
Lysosomes and Peroxisomes
  • Lysosomes: enzyme-containing vesicles; digest intracellular materials; autophagy (digest worn-out organelles); autolysis (self-duress in certain cells).
  • Peroxisomes: similar to lysosomes but with different enzymes; use molecular oxygen to oxidize organic molecules; produce hydrogen peroxide (H2O2) that is broken down by catalase; detoxify toxins and fatty acids; abundant in liver and kidney.
Mitochondria
  • Mitochondria: ATP synthesis; variable shapes; double membrane with cristae inside; matrix inside cristae contains enzymes, mitochondrial ribosomes, and mtDNA.
  • Mitoch mitochondrial DNA (mtDNA): small circular DNA; replicates independently; inherited almost exclusively from mother; high mutation rate due to limited repair.
  • Evolution: mitochondria likely evolved from bacteria that were engulfed by a primitive cell; two membranes reflect bacterial inner membrane and host-derived outer membrane.
  • Synonyms: powerhouses of the cell; energy from nutrients to ATP.
Centrioles and Centrosome
  • Centriole: short cylindrical assembly of microtubules in nine triplets; two centrioles lie perpendicular within the centrosome.
  • Role in cell division; centriole basal bodies organize cilia/flagella formation.
  • Basal bodies migrate to the plasma membrane and seed axoneme formation in cilia/flagella.
Inclusions
  • Inclusions: two kinds—stored cellular products and foreign bodies.
    • Stored products: glycogen granules, pigments, fat droplets.
    • Foreign bodies: viruses, intracellular bacteria, dust particles; not enclosed by a membrane.
    • Inclusions are not essential for cell survival.
Table 3.1 — Sizes of Biological Structures (in relation to resolution of eye, light microscope, TEM)

  • Visible to the naked eye (resolution 70–100 µm):

    • Human egg, diameter: 100 μm100\ \mu\text{m}

  • Visible with the light microscope (resolution ~200 nm):

    • Most human cells, diameter: 10–15 μm10\text{--}15\ \mu\text{m}
    • Cilia, length: 7–10 μm7\text{--}10\ \mu\text{m}
    • Mitochondria, width × length: 0.2 μm×4 μm0.2\ \mu\text{m} \times 4\ \mu\text{m}
    • Bacteria (Escherichia coli), length: 1–3 μm1\text{--}3\ \mu\text{m}
    • Microvilli, length: 1–2 μm1\text{--}2\ \mu\text{m}
    • Lysosomes, diameter: 0.5 μm0.5\ \mu\text{m} ( = 500\text{ nm})

  • Visible with the Transmission Electron Microscope (resolution ~0.5 nm):

    • Nuclear pores, diameter: 30–100 nm30\text{--}100\ \text{nm}
    • Centriole, diameter × length: 20×50 nm20\times 50\ \text{nm}
    • Polio virus, diameter: 30 nm30\ \text{nm}
    • Ribosomes, diameter: 23 nm23\ \text{nm}
    • Globular proteins, diameter: 5–10 nm5\text{--}10\ \text{nm}
    • Plasma membrane, thickness: 7.5 nm7.5\ \text{nm}
    • DNA molecule, diameter: 2.0 nm2.0\ \text{nm}
    • Plasma membrane channels, diameter: 0.8 nm0.8\ \text{nm}

  • Structural appearance (TEM-based table summary): plasma membrane appears as two dark lines separated by a narrow light space; microvilli show short dense hairlike projections; cilia have an axoneme; mitochondria have cristae and matrix; ER appears as sheets; Golgi as a stack of cisternae; lysosomes and peroxisomes vary in appearance; inclusions are highly variable and often not membrane-bound.

The Cell Surface, Membrane Transport, and Intercellular Interactions (Summary Connections)
  • The cell surface and glycocalyx mediate identity, adhesion, and interactions with the extracellular environment.
  • Transport mechanisms (diffusion, osmosis, filtration, carrier-mediated transport, and vesicular transport) underpin nutrient uptake, waste removal, and cell signaling.
  • Second-messenger systems (e.g., cAMP) translate receptor activation into intracellular responses via kinases and enzymatic cascades.
  • Vesicular transport (endocytosis/exocytosis) regulates membrane turnover and intercellular communication (e.g., receptor-mediated endocytosis via clathrin-coated pits).
  • The cytoskeleton organizes organelle positioning, intracellular transport, vesicle trafficking, and cell movement; microtubules serve as tracks for motor proteins; centrosome and centrioles anchor spindle formation during division and basal bodies anchor cilia/flagella.
  • Organelles collaborate to meet cellular demands: nucleus governs gene expression; ER/ribosomes synthesize proteins; Golgi processes and sorts proteins; lysosomes/peroxisomes manage digestion and detoxification; mitochondria provide ATP with mtDNA inheritance patterns (maternal).
  • Practical/clinical relevance:
    • Channelopathies arise from defects in ion channels.
    • Na+/K+ ATPase activity is vital for cell volume regulation and heat production (e.g., thyroid influence).
    • Cystic fibrosis symptoms stem from defective Cl− transport and saline layer disruption.
    • SGLT-mediated cotransport demonstrates energy-efficient nutrient uptake without direct ATP expenditure.
  • Foundational links: microscopy advances (light vs electron microscopy) transformed understanding of cell structure and organelle localization, and the sizes/resolution table (Table 3.1) contextualizes why certain structures are visible at specific magnifications.

Key Formulas and Numeric References

  • Surface area and volume relationships:
    SA∝D2,V∝D3SA \propto D^{2}, \quad V \propto D^{3}
  • Doubling diameter example:
    • Factor<em>SA=4\text{Factor}<em>{SA} = 4, Factor</em>V=8\text{Factor}</em>{V} = 8 when D→2DD\to 2D.
  • Osmolarity and osmoles:
    • One osmole: 1 osmole1\ \text{osmole} per liter of solution.
    • 1 M NaCl = 2 osm/L2\ \text{osm/L} (because NaCl dissociates into Na⁺ and Cl⁻).
    • Blood plasma: approximately 300 mOsm/L.300\ \text{mOsm/L}.
  • Membrane thickness and structures:
    • Plasma membrane thickness: 7.5 nm7.5\ \text{nm}.
    • Nuclear pores diameter: 30–100 nm30\text{--}100\ \text{nm}.
    • DNA diameter: 2.0 nm2.0\ \text{nm}.
    • Plasma membrane channels diameter: 0.8 nm0.8\ \text{nm}.
  • Second messenger signaling example: ATP→adenylate cyclasecAMP+PPi.\text{ATP} \xrightarrow{\text{adenylate cyclase}} \text{cAMP} + \text{PP}_i.
    • cAMP then activates kinases to phosphorylate other enzymes, altering metabolism and function.
  • Na⁺/K⁺ pump stoichiometry:
    • Each cycle exchanges 3 Na⁺ for 2 K⁺ with ATP hydrolysis.
    • This maintains intracellular K⁺ and extracellular Na⁺, supports membrane potential, and drives secondary transport.
  • 9+2 axoneme description for cilia/flagella: axoneme has 9 peripheral microtubule doublets around a central pair (9+2).
  • Typical cilium length and microvilli length:
    • Cilia length: 7–10 µm; primary cilium is nonmotile, sensory.
    • Microvilli length: ~1–2 µm; brush border increases absorptive surface.

Notes on figures and tables referenced in the transcript

  • Fig. 3.1 and Fig. 3.2 illustrate cell shapes and the growth effects on surface area and volume.
  • Fig. 3.3–3.6 illustrate plasma membrane structure and the unit membrane concept.
  • Fig. 3.7–3.9 illustrate membrane proteins, receptors, and second-messenger signaling.
  • Fig. 3.10–3.12 illustrate microvilli, cilia, and related disorders (e.g., CF).
  • Table 3.1 lists sizes of various structures in relation to resolution of naked eye, light microscope, and TEM.
  • Table 3.4 provides a summary mapping of organelles to their TEM appearance, structure, and function.