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Chapter 4 Inside the Cell

4.1 Cells Under the Microscope

  • Cells are extremely diverse and each type in the body is specialized for a particular function.
  • Nearly all cells require a microscope to be seen.
  • Light microscope
    • Invented in the seventeenth century.
    • Limited by properties of light.
  • Electron microscope
    • Invented in the 1930s.
    • Overcomes limitations of light by using a beam of electrons.
  • Relative sizes and scale (Figure 4.2)
    • Atoms ≈ 0.1 nm
    • Amino acids ≈ 1 nm
    • Proteins ≈ 10 nm
    • Viruses ≈ 100 nm
    • Chloroplasts ≈ 1 μm
    • Most bacteria ≈ 10 μm
    • Plant and animal cells ≈ 100 μm
    • Human egg ≈ > 100 μm
    • Frog egg ≈ 1 mm
    • Ant ≈ 1 cm
    • Mouse ≈ 0.1 m
    • Man ≈ 1 m
    • Blue whale ≈ 10 m
  • Image/slide context (Figure 4.1–4.2 context): text alternatives available for slide images; examples include epithelial cells, pluripotent stem cells, Euglena, TEM/SEM, and light microscopy visuals.
  • Surface-area-to-volume considerations (Figure 4.3)
    • Why cells are small: need enough surface area for entry and exit of materials.
    • Surface-area-to-volume ratio concept: small cells have more surface area per volume, aiding exchange.
    • Microstructures to increase surface area: microvilli in the small intestine increase surface area for absorption.
  • Surface-area-to-volume relationships example (Figure 4.3):
    • One 4-cm cube
    • Eight 2-cm cubes
    • Sixty-four 1-cm cubes
    • Shared takeaway: smaller units have higher SA:V; larger units have lower SA:V.
    • Mathematical reminder: for a cube of side length s,
    • SA = 6s^2, \ V = s^3, \ SA:V = \frac{SA}{V} = \frac{6s^2}{s^3} = \frac{6}{s}.
  • Practical takeaway: cells optimize surface area to support exchange while maintaining functional volume.

4.2 The Plasma Membrane

  • The plasma membrane marks the boundary between the outside and inside of the cell and regulates passage of materials.
  • Structure: phospholipid bilayer with embedded proteins.
    • Polar heads (hydrophilic) face the watery environments.
    • Nonpolar tails (hydrophobic) face each other.
  • Fluid mosaic model: the membrane is a dynamic, fluid structure with many different proteins embedded.
  • Membrane protein diversity and roles (Figure 4.5 series)
    • Channel proteins: form tunnels for specific molecules to pass.
    • Transport proteins: involved in passage of molecules; sometimes require energy.
    • Cell recognition proteins: enable the body to distinguish self from foreign cells.
    • Receptor proteins: bind signal molecules and trigger a cellular response.
    • Enzymatic proteins: directly participate in metabolic reactions.
    • Junction proteins: form junctions between cells for adhesion and communication.
  • Summary: the plasma membrane regulates material exchange and communication, supported by diverse membrane proteins.

4.3 The Two Main Types of Cells

  • Cell theory:
    • All organisms are composed of cells.
    • All cells arise only from preexisting cells.
    • All cells have a plasma membrane, cytoplasm, and genetic material.
  • Cell types by organization of genetic material:
    • Prokaryotic cells: lack a membrane-bounded nucleus.
    • Eukaryotic cells: have a nucleus that houses DNA.
  • Prokaryotic Cells (Domain Bacteria and Archaea)
    • Generally smaller and simpler in structure than eukaryotic cells.
    • Reproduce quickly and effectively; extremely successful group.
    • Bacteria: some cause disease; others environmental roles; used to manufacture chemicals, foods, drugs, etc.
  • Prokaryotic Structure (Bacterial structure):
    • Cytoplasm surrounded by a plasma membrane and a cell wall.
    • Capsule: protective layer (optional).
    • Plasma membrane: same as in eukaryotes.
    • DNA: single circular, coiled chromosome located in the nucleoid (region, not membrane-enclosed).
    • Ribosomes: site of protein synthesis.
  • Prokaryotic Appendages:
    • Flagella: propulsion.
    • Fimbriae: attachment to surfaces.
    • Conjugation pili: DNA transfer (gene exchange).
  • Prokaryotic Cell Illustration (Figure 4.6) and related descriptions (Pilus, capsule, cell wall, plasma membrane, cytoplasm, nucleoid, ribosomes, plasmid, flagella).
  • Additional notes (extended visuals): tomographic segmentation and 3D reconstructions (e.g., flagellum, nucleoid, ribosome, pilus) illustrate prokaryotic cell architecture at high resolution.

4.4 A Tour of the Eukaryotic Cell

  • Eukaryotic cells (protists, fungi, plants, animals) have:
    • A membrane-bounded nucleus housing DNA.
    • Much larger size than prokaryotes.
    • Compartmentalization with organelles.
    • Four broad categories of organelles:
    • Nucleus and ribosomes
    • Endomembrane system
    • Energy-related organelles
    • Cytoskeleton
  • Nucleus and Ribosomes
    • Nucleus stores genetic information.
    • Chromatin (diffuse DNA + proteins + some RNA).
    • DNA condenses into chromosomes prior to cell division.
    • Genes specify a protein (polypeptide).
    • mRNA relays information to ribosomes for polypeptide synthesis.
    • Nucleolus: site of rRNA synthesis.
    • Nuclear envelope: double membrane with nuclear pores regulating traffic.
  • Ribosomes
    • Carry out protein synthesis in the cytoplasm.
    • Present in both prokaryotes and eukaryotes.
    • Composed of two subunits (large and small), a mix of rRNA and proteins.
    • In eukaryotes: some ribosomes are free in cytoplasm; many are attached to the rough endoplasmic reticulum (RER).
  • Endomembrane System
    • Network of membranes: nuclear envelope, ER, Golgi apparatus, and vesicles.
    • Function: compartmentalization of cellular processes; transport vesicles move materials between components.
  • Endoplasmic Reticulum (ER)
    • Rough ER (RER): studded with ribosomes; modifies proteins in the lumen; forms transport vesicles to the Golgi.
    • Smooth ER (SER): lacks ribosomes; synthesizes lipids (e.g., phospholipids, steroids); detoxifies drugs; activity depends on cell type (example: liver detoxification and testosterone production).
    • ER images and descriptions show the continuity with the outer nuclear membrane.
  • Golgi apparatus
    • Stack of flattened cisternae.
    • Receives vesicles from the ER; modifies molecules within vesicles; sorts and repackages for new destinations (some go to lysosomes or secretory pathways).
    • Size varies with cell type and secretory output (enzyme-secreting cells have larger Golgi; some cells export little protein).
  • Lysosomes
    • Digest molecules or old cell parts via digestive enzymes.
    • Related disease note: Tay-Sachs disease.
  • Vacuoles
    • Membranous sacs larger than vesicles.
    • Function: remove excess water, digestion, storage (e.g., plant pigments, animal adipocytes).
  • Energy-Related Organelles
    • Mitochondria: sites of ATP production; common to plant and animal cells; usually visible under an electron microscope; double membrane; cristae increase surface area; matrix contains enzymes, DNA, and ribosomes; site of cellular respiration (requires oxygen; produces CO2).
    • Chloroplasts (plants and algae): use solar energy to synthesize carbohydrates via photosynthesis; three-membrane system (outer, inner, and thylakoid membranes forming granum and stroma); chloroplasts have their own DNA and ribosomes.
  • The Cytoskeleton and Motor Proteins
    • Cytoskeleton: network of protein filaments and tubules extending from the nucleus to the plasma membrane; unique to eukaryotes; maintains cell shape.
    • Motor proteins:
    • Myosin: interacts with actin; enables amoeboid movement; muscle contraction.
    • Kinesin and dynein: move along microtubules; transport vesicles from the Golgi to final destinations.
  • Microtubules and Intermediate Filaments
    • Microtubules: small hollow cylinders; assembly controlled by the centrosome; act as tracks for organelles and vesicles.
    • Intermediate filaments: ropelike, provide structural support; run from the nuclear envelope to the plasma membrane.
  • Actin Filaments
    • Two chains of monomers twisted into a helix; form a dense web that supports the cell.
  • Centrioles
    • Made of nine triplets of microtubules; two centrioles lie at right angles; present in animal cells; not in plant cells.
  • Cilia and Flagella
    • Eukaryotic structures used for movement of the cell or fluids past the cell.
    • Structural pattern: 9+2 arrangement of microtubules.
    • Cilia are shorter and more numerous; flagella are longer.

4.5 Outside the Eukaryotic Cell

  • Plant cell walls
    • Primary cell walls: cellulose fibrils plus noncellulose substances; wall expands during growth.
    • Secondary cell walls (in some cells): form inside the primary wall; woody plants contain lignin for strength.
    • Plasmodesmata: channels passing through cell walls to connect plant cells for water and small solute exchange.
  • Extracellular Matrix (ECM) in animals
    • Meshwork of fibrous proteins and polysaccharides closely associated with the cell that produced them.
    • Collagen: provides resistance to stretch.
    • Elastin: provides resilience.
    • ECM properties vary by tissue (e.g., more flexible in cartilage, more rigid in bone).
  • Junctions Between Cells (intestine wall; Figure 4.22)
    • Adhesion junctions: cytoplasmic plaques linked by intercellular filaments; sturdy yet flexible.
    • Tight junctions: impermeable barriers; adjacent plasma membranes sealed together.
    • Gap junctions: allow direct communication between neighboring cells via channels.

Notes on visuals and text alternatives

  • Many slides provide text alternatives for images to aid accessibility (e.g., descriptions for Figures 4.1–4.22).
  • The figures illustrate concepts such as organelle structure, cell types, and cell–cell junctions; refer to the figures for spatial context when studying.

Key terms to remember

  • Endomembrane system components: nuclear envelope, ER, Golgi, vesicles, lysosomes, secretory vesicles.
  • Organelles and their primary roles: nucleus (genetic storage), ribosomes (protein synthesis), mitochondria (ATP production), chloroplasts (photosynthesis), lysosomes (digestion), peroxisomes (detoxification context in some slides), vacuoles (storage/digestion), Golgi (modification and sorting), cytoskeleton (shape and transport).
  • Plasma membrane proteins: channels, transporters, receptors, enzymes, cell–cell recognition, junction proteins.
  • Cell junctions: adhesion, tight, gap.
  • Prokaryotic features: capsule, nucleoid, plasmids, flagella, fimbriae, conjugation pili.
  • Key structural relationships: SA:V and limits to cell size; surface area considerations drive microvilli and other adaptations.

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