Cell Structure: Plasma Membrane and Organization

Cell Organization

  • Extracellular fluid surrounds cells; cells float in this medium.
  • Plasma membrane separates cytoplasm from extracellular fluid.
  • Cytoplasm = cytosol (fluid) + organelles (structures).
  • Organelles classified as:
    • Nonmembranous (no surrounding membrane): cytoskeleton, microvilli, centrioles, cilia, flagella, ribosomes
    • Membranous: mitochondria, nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes

The Plasma Membrane

  • Outer boundary of the cell; also called plasmalemma.
  • Thickness: 6-10\,\text{nm} (1 nm = 0.001 μm).
  • Key functions arise from selective permeability:
    • Physical isolation of cytoplasm from extracellular fluid
    • Regulation of exchange with the environment (ions, nutrients in; wastes out; secretions out);
    • This involves controlling the entry of essential nutrients and the removal of metabolic waste products.
    • Sensitivity to changes in extracellular fluid; contains receptors;
    • Receptors bind to specific chemical signals (ligands) and trigger responses inside the cell.
    • Cell-to-cell communication, adhesion, and structural support via membrane connections

Components of the Plasma Membrane

  • Phospholipids:
    • Form a bilayer with hydrophilic heads (phosphate groups) facing the aqueous extracellular fluid and cytosol, and hydrophobic tails (fatty acids) hidden from water in the interior.
    • This arrangement creates a barrier to most water-soluble substances, allowing primarily small, lipid-soluble molecules to pass.
  • Proteins:
    • Integral (transmembrane) proteins: span the membrane;
    • Can form channels (pores) for specific ions/water, transporters (carriers) to move substances, receptors to bind chemical signals, enzymes to catalyze reactions, anchoring proteins to link to the cytoskeleton, and cell-identity markers.
    • Some are gated, opening or closing in response to signals.
    • Peripheral proteins: attached to inner or outer surfaces;
    • Roles vary, including enzymatic activity, structural support, and linking cells together.
    • Proteins, overall, enable transport, catalysis, communication, and structure.
  • Carbohydrates:
    • Glycolipids and glycoproteins extend from the outer surface to form the glycocalyx (coat).
    • Glycocalyx functions:
    • Cell recognition: helps distinguish 'self' from 'non-self' cells.
    • Cell adhesion: helps cells bind to each other and the extracellular matrix.
    • Protection: shields the cell from physical and chemical damage.
    • Lubrication: makes cell surfaces slippery, aiding in movement.
    • Also contains receptors and aids in cell recognition.
  • Sterols:
    • Cholesterol is the most common sterol;
    • Interspersed among phospholipids, it stabilizes the membrane and maintains fluidity.
    • It reduces fluidity at warmer temperatures and prevents excessive rigidity at colder temperatures.
  • Membrane composition varies by cell type and over time as components are removed/replaced.

Membrane Permeability

  • Permeability determines barrier effectiveness: more permeable = easier crossing.
  • The plasma membrane is selectively permeable, meaning it allows some substances to pass more easily than others.
    • Easily permeable: Small, nonpolar, lipid-soluble molecules (e.g., O{2} , CO{2}) and small uncharged polar molecules (e.g., water, urea, through aquaporins).
    • Difficult to permeate: Large uncharged polar molecules (e.g., glucose) and ions (e.g., Na^{+} , K^{+}) typically require membrane proteins.
  • Terms:
    • Impermeable: nothing crosses
    • Freely permeable: substances cross easily
  • Transport modes include:
    • Passive transport (no cellular energy required):
    • Diffusion: Movement down a concentration gradient.
    • Osmosis: Diffusion of water.
    • Facilitated Diffusion: Uses membrane proteins to assist movement down a concentration gradient.
    • Active transport (requires cellular energy);
    • Moves substances against their concentration gradient (low to high).
    • Includes specific carrier proteins and Vesicular Transport (endocytosis, exocytosis) for bulk movement.
  • Membrane proteins (channels/transporters) facilitate selective movement of ions and molecules.
  • Fluidity allows lateral movement of proteins within the membrane.