Study Guide: Chapter 5 - Membranes

Study Guide: Chapter 5 - Membranes

I. Membrane Structure

  • Phospholipid Bilayer:
    • Composed of phospholipids arranged in two layers, with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward.
    • Embedded in or attached to this bilayer are various proteins that contribute to membrane functions.
  • Fluid Mosaic Model:
    • Describes the cell membrane's structure, where proteins float in or on the lipid bilayer, resembling a mosaic.
  • Four Components of Memblane:
    1. Phospholipids: Basic structural unit of the membrane.
    2. Integral Proteins: embedded within the membrane and often span its entire width.
    3. Peripheral Proteins: attached to the membrane's outer or inner surface, not embedded.
    4. Cell-Surface Markers: glycoproteins and glycolipids that serve as identification tags for cells.

II. Phospholipids and Fluidity

  • Structure:
    • Comprised of a glycerol backbone, two fatty acid tails (which are nonpolar), and a phosphate group (which is polar).
  • Formation of Bilayer:
    • Phospholipids spontaneously organize into a bilayer due to their amphipathic properties.
  • Factors Influencing Fluidity:
    • Saturated vs. Unsaturated Fatty Acids:
    • Saturated fatty acids pack tightly, reducing fluidity; unsaturated fatty acids (with double bonds) create kinks, increasing fluidity.
    • Presence of Cholesterol:
    • Cholesterol molecules interspersed among phospholipids help stabilize membrane fluidity, especially at varying temperatures.
    • Temperature:
    • Higher temperatures increase fluidity, while lower temperatures decrease it.

III. Membrane Proteins

  • Functions of Membrane Proteins:
    • Transport: facilitate movement of substances across the membrane.
    • Enzymatic Activity: function as enzymes to catalyze reactions.
    • Signaling: involved in reception and transduction of signals.
    • Identity: serve as markers that can indicate cell type or status.
    • Adhesion: help cells stick to each other or to their environment.
    • Anchoring: attach to the cytoskeleton or extracellular matrix for structural support.
  • Transmembrane Domains:
    • Made of hydrophobic α-helices that span the membrane, allowing proteins to exist within the lipid bilayer.
  • Pores:
    • Formed by β-barrel proteins, these create channels that permit polar molecules to pass through the membrane.
  • Anchored Proteins:
    • Proteins that are tethered to the lipid bilayer via covalent bonds with lipid molecules.

IV. Passive Transport

  • Characteristics:
    • Does not require energy (ATP); moves substances along their concentration gradient (from high to low concentration).
  • Types of Passive Transport:
    • Diffusion: simple movement of molecules from an area of higher concentration to one of lower concentration.
    • Facilitated Diffusion: involves the use of channel or carrier proteins to assist in the movement of molecules across the membrane.
  • Selectively Permeable Membrane:
    • Membranes control which substances can enter or leave the cell, allowing for selective transport.

V. Osmosis and Tonicity

  • Osmosis:
    • The process in which water molecules move across a selectively permeable membrane toward a region of higher solute concentration in order to equalize concentrations.
  • Types of Tonicity:
    • Hypertonic: solution with a higher solute concentration compared to another.
    • Hypotonic: solution with a lower solute concentration compared to another.
    • Isotonic: solutions with equal solute concentrations.
  • Aquaporins:
    • Specialized protein channels that facilitate the rapid transport of water across the membrane.

VI. Osmotic Pressure and Balance

  • Osmotic Pressure:
    • Defined as the pressure required to stop the flow of water across a semipermeable membrane due to osmosis.
  • Plant Cells:
    • Maintain turgor pressure, which is the pressure of the cell contents against the cell wall, providing rigidity and structural integrity.
  • Animal Cells:
    • Must maintain isotonic conditions to prevent shrinking or bursting due to changes in osmotic pressure.
  • Extrusion and Isosmotic Regulation:
    • Mechanisms employed by cells to manage osmotic balance and prevent excessive inward or outward water movement.

VII. Active Transport

  • Definition:
    • Active transport requires energy in the form of ATP to move substances against their concentration gradient.
  • Types of Carrier Proteins:
    • Uniporters: transport a single type of molecule.
    • Symporters: transport two molecules in the same direction across the membrane.
    • Antiporters: transport two molecules in opposite directions across the membrane.
  • Example:
    • Sodium-Potassium Pump (Na+/K+ pump): actively transports sodium ions out of the cell and potassium ions into the cell, against their concentration gradients, using ATP.

VIII. Coupled Transport

  • Definition:
    • Refers to the indirect use of ATP, where the energy released from the diffusion of one molecule down its concentration gradient is used to transport another molecule against its concentration gradient.
  • Example:
    • Glucose Symporter: utilizes the concentration gradient of sodium ions to transport glucose into the cell, even against its own concentration gradient.

IX. Bulk Transport

  • Characteristics:
    • Requires energy to move large quantities of materials.
  • Types of Bulk Transport:
    • Endocytosis: process by which cells intake materials.
    • Phagocytosis: engulfing of larger solid particles.
    • Pinocytosis: intake of liquid substances.
    • Receptor-mediated Endocytosis: specific uptake of molecules triggered by their binding to receptors on the cell surface.
    • Exocytosis: process in which materials are expelled from the cell, often in vesicles that fuse with the membrane and release their contents outside.