M3

• Life at the edge

  • The plasma membrane is the boundary that separates the living cell from its surroundings
  • The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others
• Cellular membranes are fluid mosaics of lipids and proteins

  • Phospholipids - the most abundant lipid in the plasma membrane
  • Phospholipids are amphipathic molecules -> hydrophobic and hydrophilic regions
  • A phospholipid bilayer -> a stable boundary between two aqueous compartments
• The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it
• Proteins are not randomly distributed in the membrane
• The fluidity of membranes 

  • Phospholipids in the plasma membrane can move within the bilayer -> hydrophobic interactions
  • Most of the lipids and some proteins drift laterally
  • rarely, a lipid may flip-flop transversely across the membrane
• Membrane proteins and their functions: proteins determine most of the membrane’s specific functions

  • peripheral proteins are bound to the surface of the membrane
  • Integral proteins penetrate the hydrophobic core
  • Integral proteins that span the membrane are called transmembrane proteins
  • The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices
• The role of membrane carbohydrates in cell-cell recognition

  • Cells recognize each other by binding to molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane
  • Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or, more commonly, to proteins (forming glycoproteins) 
  • Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual
• Synthesis and sidedness of membranes

  • Membranes have distinct stride and outside faces
  • The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus
• Membrane structure results in selective permeability

  • A cell must exchange materials with its surroundings, a process controlled by the plasma membrane
  • Plasma membranes are selectively permeable, regulating the cell’s molecular traffic
• The 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
• Transport proteins

  • Allow passage of hydrophilic substances across the membrane
  • Specific to the substance, it moves
  • Channel proteins have hydrophilic channels that certain molecules or ions can use as a tunnel
  • Channel proteins called aquaporins facilitate the passage of water
  • Carrier proteins bind t molecules and change shape to shuttle them across the membrane
• Passive transport is the diffusion of a substance across a membrane with no energy investment

  • Diffusion is the tendency of molecules to spread out evenly into the available space
  • Although each molecules move randomly, diffusion of a population of molecules may be directional
  • At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other
• Passive transport is the diffusion of a substance across a membrane with no energy investment:

  • Substances diffuse down their 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
  • The diffusion of a substance across a biological membrane is passive transport because no energy is expended by the cell to make it happen
• Effects of osmosis on water balance

  • Osmosis: 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
• Water balance of cells without cell walls

  • Tonicity: the ability of a surrounding solution to cause a cell to gain or lose water 
  • Isotonic solution: solute concentration is the same as that inside the cell; no net water movement across the plasma membrane
  • Hypertonic solution: solute concentration is greater than inside the cell; cell loses water
  • Hypertonic solution: solute concentration is less than that inside the cell; cell gains water
• Water balance of cells without cell walls:

  • hypertonic or hypotonic environment creates osmotic problems for organisms
  • Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environments
  • The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump
• facilitated diffusion

  • Transport proteins speed the passive movement of molecules across the membrane
  • Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane
  • Aquaporins facilitate the diffusion of water
  • Ion channels facilitate the diffusion of ions
  • Some ion channels, called gated channels, open or close in response to a stimulus
  • Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane
• Active transport uses energy to move solutes against their gradients

  • Active transport moves substances against their concentration gradients
  • Active transport requires energy, usually in the form of ATP
  • Active transport is performed by specific proteins embedded in the membranes
  • Active transport allows cells to maintain concentration gradients that differ from their surroundings
  • The sodium-potassium pump is one type of active transport system
• How ion pumps maintain membrane potential

  • Membrane potential is the voltage difference across a membrane
  • Voltage is created by differences in the distribution of positive and negative ions across a membrane
• • Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane
  • A chemical force (the ion’s concentration gradient)
  • An electrical force (the effect of the membrane potential on the ion’s movement)
  • Active transports allow cells to maintain concentration gradients that differ from their surroundings
  • The sodium-potassium pump is one type of active transport system
• An electrogenic pump is a transport protein that generates voltage across a membrane 

  • The sodium-potassium pump is the major electrogenic pump of animal cells
  • The main electrogenic pump of plants, fungi, and bacteria is a proton pump
• Cotransport

  • Cotransport occurs when the active transport of a solute indirectly drives the transport of other substances
• Bulk transport across the plasma membrane occurs by exocytosis and endocytosis

  • Small molecules and water enter or leave the cell through the lipid bilayer or via transport proteins
  • Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles
  • Bulk transport requires energy
• In exocytosis, transport vehicles migrate to the membrane, fuse with it, and release their contents outside the cell

  • Many secretory cells use exocytosis to export their products
• In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane

  • There are three types of endocytosis
  • Phagocytosis (cellular eating)
  • Pinocytosis (cellular drinking)
  • receptor-mediated endocytosis