Cell Membrane, Cytoplasm, and Initial Organelles

Cell (Plasma) Membrane
  • Structural Overview
    • Basic architecture: phospholipid bilayer with polar (hydrophilic) heads on outer surfaces and non-polar (hydrophobic) fatty-acid tails in the interior.
    • Referred to as a “fluid mosaic” due to lipid molecules forming an oily, fluid background where proteins float and move.
    • Selectively permeable (semipermeable): allows some substances to cross while restricting others.
  • Lipid Components
    • Phospholipids: major constituent of the bilayer.
    • Cholesterol molecules: interspersed between phospholipids, rigid structure decreases membrane fluidity, making the bilayer less permeable to water-soluble substances (e.g., extaminoacids,extsugars,extnucleicacids,extcertainionsext{amino acids}, ext{sugars}, ext{nucleic acids}, ext{certain ions}).
  • Membrane Proteins (general)
    • Provide most of the membrane’s specialized functions.
    • Classified by position:
      • Integral proteins: embedded in the lipid interior; may extend from one side.
      • Transmembrane proteins: subclass of integral proteins that span both faces of the bilayer.
      • Peripheral proteins: loosely attached to inner or outer membrane surfaces.
    • Classified by shape: globular, rod-like, or fibrous.
  • Functional Types of Membrane Proteins
    • Receptors: Bind specific ligands (hormones, growth factors) to initiate signal transduction; exhibit “lock-and-key” specificity.
    • Transporters / Carriers: Move ions or molecules across the bilayer, often undergoing conformational change.
    • Ion Channels: Water-filled passages specific for a single ion species (e.g., extCa2+,extNa+,extK+,extCl−ext{Ca}^{2+}, ext{Na}^{+}, ext{K}^{+}, ext{Cl}^{-}). Gated: open/close in response to electrical, chemical, or mechanical stimuli.
      • Clinical note: cystic fibrosis results from misfolded chloride channel proteins; therapeutic drugs can restore extCl−ext{Cl}^{-} flow and reduce thick mucus.
    • Anchoring Proteins: Cytoskeletal link proteins attach the inner face of the membrane to microfilaments/microtubules, stabilizing cell shape.
    • Cell Identity Markers: Many are glycoproteins that mark the cell as “self” to the immune system; critical for organ/tissue recognition (adaptive immunity).
    • Cellular Adhesion Molecules (CAMs): Guide cell–cell interactions; an example is sequential CAM action directing a white blood cell to migrate from the bloodstream to the exact site of injury.
  • Practice Concepts (from text prompts)
    • Selectively permeable membrane: a barrier that allows passage of certain molecules/ions while restricting others based on size, polarity, or charge.
    • Chemical structure: double layer of amphipathic phospholipids with interspersed cholesterol; proteins (integral, transmembrane, peripheral) embedded or attached; extracellular carbohydrate chains on some proteins/lipids form the glycocalyx.
Cytoplasm
  • Definition & Visual Characteristics
    • Gel-like material filling the cell between the plasma membrane and nuclear envelope.
    • Electron microscopy reveals a complex network of membranes and organelles suspended in more fluid cytosol.
  • Major Components
    • Cytosol: aqueous solution containing salts, nutrients, and dissolved enzymes.
    • Cytoskeleton: protein rods and tubules (microfilaments & microtubules) providing structural framework (the “cell skeleton”).
    • Organelles: discrete, specialized sub-compartments performing cellular functions.
Ribosomes
  • Structure & Composition
    • Tiny, spherical particles composed of protein and RNA.
    • Lack surrounding membranes.
  • Function
    • Catalyze peptide-bond formation, linking amino acids during protein synthesis.
    • Provide both structural support and enzymatic activity for translation.
  • Distribution
    • Free ribosomes: scattered throughout cytosol, synthesizing proteins used in the cytoplasm.
    • Bound ribosomes: attached to endoplasmic reticulum, synthesizing proteins destined for membranes, lysosomes, or secretion.
    • Polysomes: clusters of ribosomes translating the same mRNA, enabling rapid mass production of proteins needed in large amounts.
Endoplasmic Reticulum (ER)
  • General Description
    • Extensive network of membrane-bounded, flattened sacs, cylinders, and fluid-filled vesicles.
    • Continuous with the nuclear envelope and communicates with the cell membrane & other organelles.
  • Primary Function
    • Provides a vast tubular system for transporting molecules from one part of the cell to another.