cell membrane

• Components of biological membranes

  • Glycoprotein
  • Glycolipid
  • Microfilaments of cytoskeleton
  • Fibers of extracellular matrix (ECM)
  • Sterol
  • Peripheral proteins
  • Integral protein
• Fluid mosaic model


  Current model for the structure of the plasma membrane and other cell membranes in which protein molecules "float" in a fluid phospholipid bilayer
• fluid -> phospholipid bilayer

  mosaic -> protein molecules
• Functions of membranes


  • Transport
  • Enzymatic activity
  • Signal transduction
  • Cell-cell recognition
  • Intercellular joining
  • Attachment to the cytoskeleton and extracellular matrix (ECM)
• How is lipid asymmetry created?

  1. All membrane lipids are made in the SER

  2. Enzymes in the SER join fatty acids and glycerol and phosphate and head groups to make phospholipids

  3. The phospholipid is inserted into one of the monolayers

  4. Enzymes called FLIPPASES flip some of these lipids into the other bilayer, so that the whole membrane will grow

  5. They only transfer specific lipids
• Fluidity of membranes


  • Phospholipids in the plasma membrane can move within the bilayer
  • Most of the lipids, and some proteins, drift laterally
  • Rarely, a lipid may flip-flop transversely across the membrane
• Membrane proteins


  • A membrane is a collage of different proteins, often grouped together, embedded in the fluid matrix of the lipid bilayer
  • Proteins determine most of the membrane's specific functions
• Importance of fluidity

  • It allows membrane proteins move rapidly in the plane of bilayer
  • It permits membrane lipids and proteins to diffuse from sites where they are inserted into bilayer after their synthesis
  • It enables membranes to fuse with one another and mix their molecules
  • It ensures that membrane molecules are distributed evenly between daughter cell when a cell divides
• Types of membrane proteins

  • Peripheral proteins - bound to surface of membrane
  • Integral proteins - penetrate hydrophobic core
  • Transmembrane proteins - spans the membrane
• Membrane proteins can move within the plane of the plasma membrane
• Selective permeability of plasma membranes


  A cell must exchange materials with its surroundings, a process controlled by the plasma membrane
• Permeability of the lipid bilayer

  • Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly
  • Hydrophilic molecules including ions and polar molecules do not cross the membrane easily
• Types of transport proteins

  • Channel proteins
  • Carrier proteins
• Channel proteins


  Have a hydrophilic channel that certain molecules or ions can use as a tunnel
• Carrier proteins


  Bind to molecules and change shape to shuttle them across the membrane

  • undergo a subtle change in shape that translocates the solute- binding site across the membrane
• Types of transport

  • Passive
  • Active
• Passive transport


  Diffusion of a substance across a membrane with no energy investment
• Diffusion


  The tendency for molecules to spread out evenly into the available space
• At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other
• Concentration gradient


  The region along which the density of a chemical substance increases or decreases
• No work must be done to move substances down the concentration gradient
• Osmosis


  The diffusion of water across a selectively permeable membrane
• Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides
• Tonicity


  The ability of a surrounding solution to cause a cell to gain or lose water
• Types of tonicity

  • Isotonic
  • Hypertonic
  • Hypotonic
• Facilitated diffusion


  Passive transport aided by proteins
• Active transport


  Uses energy to move solutes against their gradients

  • requires ATP
• Membrane potential


  The voltage difference across a membrane
• Electrochemical gradient


  The combined forces of a chemical force (the ion's concentration gradient) and an electrical force (the effect of the membrane potential on the ion's movement)
• Electrogenic pump


  A transport protein that generates voltage across a membrane

  • help store energy that can be used for cellular work
• Cotransport


  Occurs when active transport of a solute indirectly drives transport of other substances
• Exocytosis


  Transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell
• Endocytosis


  The cell takes in macromolecules by forming vesicles from the plasma membrane
• Types of endocytosis

  • Phagocytosis
  • Pinocytosis
  • Receptor-mediated endocytosis
• Bulk transport across the plasma membrane occurs by exocytosis and endocytosis

  • requires energy
• isotonic solution


  solute concentration is the same as that inside the cell; no net water movement across plasma membrane
• hypertonic solution


  solute concentration is greater than that inside the cell; cell loses water
• hypotonic solution 


  solute concentration is less than that inside the cell; cell gains water
• how do you associate tonicity with traditional food preservation

  High tonicity (salt concentration) draws water out of microbes, dehydrating and inhibiting their growth. This is a key principle in traditional methods like salting fish or brining vegetables.
• a plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid