The Membrane

• Plasma membrane structure

  • Phospholipid bilayer with embedded proteins arranged as a fluid mosaic model
• There are two distinct populations of membrane proteins: Integral proteins and Peripheral proteins
• Types of cell-cell junctions

  • Gap Junctions for communication
  • Desmosomes for holding and anchoring adjacent cells
  • Tight Junctions for preventing ions from passing between cells
• Key players of the plasma membrane

  • Membrane lipids form a flexible lipid bilayer
  • Specialized membrane proteins float through the fluid membrane, resulting in constantly changing patterns
  • Surface sugars form glycocalyx
  • Membrane structures help to hold cells together through cell junctions
• Functions of the plasma membrane

  • Physical barrier
  • Selective permeability
  • Communication
  • Cell recognition
  • Structural support
• Membrane carbohydrates and the glycocalyx

  • Glycocalyx consists of sugars sticking out of the cell surface
  • Functions as specific biological markers for cell-to-cell recognition
  • Allows immune system to recognize "self" vs. "nonself"
• Concept Check – Membrane Permeability: Non-polar small molecules like Ca2+, steroid, DNA, glucose, CO2 & O2 can diffuse through the membrane based on solubility and molecular size
• Plasma membrane is also known as the "cell membrane"
• Membrane protein functions

  1. Integral proteins span the membrane and function as transport proteins, enzymes, or receptors

  2. Peripheral proteins are embedded on just one side of the membrane or loosely attached to integral proteins
• Plasma membrane acts as an active barrier separating intracellular fluid (ICF) from extracellular fluid (ECF)
• Glycocalyx serves as an "ID Card" for cell-to-cell recognition
• Membrane lipids composition

  • 75% phospholipids, 20% cholesterol, 5% glycolipids
• Membrane isn't needed for diffusion to occur
• Passive transport - osmosis

  1. Osmosis is the movement of solvent (water) across a selectively permeable membrane

  2. Water can diffuse through the lipid bilayer or through water channels called aquaporins

  3. Requirements of osmosis include a solute concentration difference and impermeability of the membrane to the solute
• Active versus passive membrane transport

  • Passive transport involves no energy input
  • Three types of passive transport: Simple Diffusion, Facilitated Diffusion, Osmosis
  • All types involve diffusion from areas of high concentration to low concentration
  • Equilibrium is reached when there is no net movement of molecules in one direction only
• Kinetic energy

  Movement
• Passive transport - facilitated diffusion

  1. Carrier-mediated facilitated diffusion involves substances binding to protein carriers

  2. Carriers are transmembrane integral proteins

  3. Each carrier transports specific polar molecules such as sugars and amino acids

  4. Binding of molecule causes carrier to change shape and transport the molecule across the membrane
• Passive transport - facilitated diffusion

  1. Channel-mediated facilitated diffusion involves substances moving through water-filled channels

  2. Channels transport specific molecules down their concentration gradient

  3. Two types: Leakage/Open channels and Gated channels
• Equilibrium


  Is reached when there is no net movement of molecules in one direction only
• Overview of membrane transport

  • Selectively permeable plasma membrane allows some particles to pass while preventing others
  • Substance permeability of the membrane is based on whether the substance can cross it
  • Two properties of particles influence whether they can permeate the plasma membrane: relative solubility in lipid and size of the particle
  • Polarity affects permeability - nonpolar substances pass through
• Passive transport - simple diffusion

  1. Nonpolar lipid-soluble substances diffuse directly through phospholipid bilayer

  2. Examples include oxygen, carbon dioxide, steroid hormones, fatty acids

  3. Small amounts of very small polar substances like water can also pass
• Water moves by osmosis from areas of low solute (high water) concentration to high areas of solute (low water) concentration across a selectively permeable membrane
• Osmotically active


  Capable of exerting an osmotic effect
• Tonicity is the ability of a solution to change the shape or tone of cells by altering the cells’ internal water volume
• Rate of diffusion is measured by the number of diffusing particles per unit of time and depends on various factors
• Hypotonic solution

  Has lower osmolarity than inside the cell, so water flows into the cell, resulting in cell swelling
• Active transport

  Moves solutes against their concentration gradient (from low to high) - requires energy (ATP)
• Speaker: 'Solutes suck'
• Tonicity levels

  • Isotonic
  • Hypertonic
  • Hypotonic
• Exocytosis


  Substances transported out of the cell
• Some carriers transport more than one substance (cotransporter)
• ATP is required to move solutes across a plasma membrane for active transport
• Osmolarity


  Osmolarity is equal to molarity times the number of ions (particles)
• Hypertonic solution

  Has higher osmolarity than inside the cell, so water flows out of the cell, resulting in cell shrinking (crenation)
• Endocytosis


  Substances transport into the cell
• Symporters transport two different substances in the same direction
• Antiporters transport one substance into the cell while transporting a different substance out of the cell
• Isotonic solution


  Has the same osmolarity as inside the cell, so volume remains unchanged
• Osmolarity


  Measures the concentration of the total number of solute particles in a solvent
• Types of active transport

  • Primary active transport
  • Secondary active transport