SXU_HA_ch.2_pp_new

Cell Theory

  • All living things are composed of cells
  • Cells are the basic units of structure and function in living things
  • New cells are produced from existing cells
  • These statements constitute the cell theory

Cytology and Microscopy History

  • Cytology is the study of cell structure and function
  • Every living organism is made of cells; cells arise only by division of preexisting cells; all cells have the same basic chemical components
  • These statements are the cell theory
  • Microscopy enables cytology: light microscope uses visible light to produce images; easiest and most used but limited in magnification and resolution

Microscopy Technologies

  • Light microscope (LM)
    • Uses visible light to form images
    • Most commonly used
    • Limited magnification and resolution
  • Transmission Electron Microscope (TEM)
    • Uses electrons; higher resolution
    • Specimens sliced ultrathin
  • Scanning Electron Microscope (SEM)
    • Produces 3-D surface images; cannot see through the specimen
    • Specimen coated with vaporized metal (usually gold); secondary electrons are emitted and detected
  • SEM provides surface detail and volume information; LM provides overall morphology

Resolution and Image Quality

  • Resolution: ability to reveal detail; electron microscopes achieve higher resolution than light microscopes by using electron beams instead of light
  • TEM studies ultrathin sections; SEM studies surfaces

Cell Shapes and Sizes (rough catalog of common shapes)

  • There are about ~200 kinds of cells in the body with varying shapes, sizes, and functions
  • Common shapes:
    • Squamous: flat, thin, scaly with a bulged nucleus; line esophagus and form epidermis
    • Cuboidal: box-like; often found in liver cells
    • Columnar: taller than wide; inner lining of small intestines
    • Polygonal: irregular angular shapes with 4+ sides
    • Stellate: star-shaped; nerve cell bodies often stellate
    • Spheroidal/Ovoid: round to oval (e.g., egg cells, white blood cells)
    • Discoid: disc-shaped (e.g., red blood cells)
    • Fusiform: spindle-shaped; elongated with thick middle and tapered ends (e.g., smooth muscle cells)
    • Fibrous: long, slender, threadlike (e.g., skeletal muscle cells, axons)

Epithelia and Surface Orientation

  • Epithelia: cell layers that cover organ surfaces
  • Epithelial cells rest on basal surface attached to basement membrane
  • Apical surface is the upper surface
  • Various factors limit cell size; swelling can rupture a cell like a water balloon

Cellular Terminology and Scale

  • Basal surface, Apical surface, Lateral surface
  • Micrometer (μm) = 10−6extmeter10^{-6} ext{ meter}
    • Naked eye can discern ~100extμm100 ext{ μm}
  • Basic units: Plasma membrane, Cytoplasm (cytoskeleton, organelles, inclusions, cytosol), Nucleoplasm
  • Typical lengths/sizes are often described in μm

Basic Components of a Cell

  • Plasma membrane: boundary of the cell; governs interactions and material passage
  • Cytoplasm: interior content between the plasma membrane and nucleus; also called cytosol
  • Nucleoplasm: material inside the nucleus
  • Cytoskeleton, organelles, inclusions are components of the cytoplasm

Plasma Membrane: Structure and Composition

  • The plasma membrane defines boundaries and regulates material passage; it is a bilayer of lipids with embedded proteins
  • Phospholipid bilayer: hydrophilic heads face ECF and ICF; hydrophobic tails form the interior
  • Fat-soluble substances (steroid hormones, O2, CO2) cross easily; water-soluble molecules require channels or transporters
  • Membrane lipid composition (approximate):
    • Phospholipids: ~75 ext{%}
    • Cholesterol: ~20 ext{%}
    • Glycolipids: ~5 ext{%}
  • Membrane proteins can comprise up to ~10 ext{%} of the membrane
    • Transmembrane proteins span the membrane
    • Peripheral proteins do not traverse the bilayer
    • Functions include receptors, enzymes, channels, transporters, cell-identity markers, and adhesion molecules
  • Glycocalyx: fuzzy, sugary coating on the extracellular face of the plasma membrane; carbohydrate-rich
    • Functions: cell identity, protection, binding to tissues, and mediating interactions

Membrane Proteins and Functions

  • Receptors: bind signaling molecules (hormones, neurotransmitters) to trigger intracellular changes
  • Enzymes: catalyze reactions at the cell surface
  • Channel proteins: form tunnels; allow water and solutes to pass; some channels are always open, others gated
  • Transport (carriers): bind solutes on one side and releases on the other; e.g., glucose, amino acids, Na+, K+, Ca2+
  • Cell-identity markers: enable immune recognition of self vs non-self
  • Cell-adhesion molecules (CAMs): mediate cell-to-cell adhesion

Plasma Membrane Architecture and Visualization

  • Boundary of the cell; lipid composition details above; membrane proteins integrated as described
  • Glycoproteins and glycolipids contribute to the glycocalyx on the extracellular face
  • Membrane structure includes cytoskeletal elements and extracellular matrix connections

Plasma Membrane Transport Overview

  • Filtration: physical pressure forces fluid through a membrane; e.g., movement of water, salts, nutrients from blood to tissue fluid
    • Example: capillary filtration driven by blood pressure; water and small solutes pass through clefts; larger particles retained
  • Simple Diffusion: net movement of particles from high to low concentration; energy-free; key for O2 and steroid hormones
    • Lipid-soluble particles diffuse through the phospholipid bilayer; water-soluble particles may require channels
    • Osmosis is a special case of diffusion for water movement across a selectively permeable membrane
  • Osmosis: net flow of water from side with more water to side with less water across a selectively permeable membrane
    • Mathematical relation not shown here, but involves solute concentrations and membrane permeability
  • Facilitated Diffusion: carrier-mediated transport down its concentration gradient; no energy required
    • Carrier binds solute on the side where it is abundant and releases on the side where it is scarce; may involve conformational change
  • Active Transport: movement of solute against its concentration gradient; energy required (usually ATP)
    • ATP hydrolysis provides energy for conformational change; solute moves from low to high concentration
  • Vesicular Transport: large particles moved via vesicles; energy-requiring
    • Endocytosis (into cell): phagocytosis, pinocytosis, receptor-mediated endocytosis
    • Exocytosis (out of cell): vesicles fuse with plasma membrane to release contents

Glycocalyx, Microvilli, and Surface Extensions

  • Glycocalyx: fuzzy, sugary coat; protective and functional for cell identity and binding
  • Microvilli: plasma membrane extensions that increase surface area for absorption; brush border in dense environments
  • Cilia: hairlike cell surface extensions; a single primary cilium acts as an antenna; motile cilia beat in waves to move substances (e.g., mucus) in respiratory tract, uterine tubes, brain/spinal cord
  • Sperm cells bear a long flagellum (a type of cilium) for motility

Cell Junctions and Connections

  • Tight junctions: encircle an epithelial cell, sealing intercellular space; adjacent plasma membranes linked by cell-adhesion proteins; restrict paracellular transport
  • Desmosomes: patch-like adhesive sites linking cells; not continuous; resist mechanical stress by linking cytoskeletons between cells
  • Gap junctions: connexon channels formed by six transmembrane proteins; allow small solutes to diffuse directly between cytoplasm of adjacent cells
  • Hemidesmosomes: anchor epithelial cells to the basement membrane

Cytoplasm and Cytoskeleton

  • Cytosol: intracellular fluid; fluid component of cytoplasm
  • Cytoskeleton: structural framework; determines cell shape; organizes contents; supports movement
    • Microfilaments (thin): primarily actin; form terminal web; involved in cell movement
    • Intermediate filaments: thicker and stiffer; provide structural integrity and resist stress
    • Microtubules: hollow tubes made of tubulin; form a network that supports organelle position and guides movement; not permanent, assemble/disassemble as needed
  • Motor proteins: e.g., kinesin moves along microtubules carrying cargo

Organelles (Functional Overview)

  • Nucleus
    • Largest organelle; contains chromosomes; genetic control center
    • Nucleolus: site of ribosome production
    • Nuclear envelope with nuclear pores; material inside is nucleoplasm
    • Normally 1 nucleus per cell; red blood cells are anuclear; skeletal muscles