Cell Biology: Cellular Compartments Part I

Cellular Membrane Systems

• Structure
• Bacterial, fungal, plant cells have a cell wall that gives their membrane rigidity
  
• Cell membrane restricts flow of molecules into and out of the cell
  
• Eukaryotes: Membranes separate cell into compartments, organelles
• 2 Layers of phospholipid molecules (hydrophilic and hydrophobic end)
• Lipids (i.e. cholesterol) and proteins are inserted into phospholipids
• Molecules making up the membrane can float sideways, giving the membrane a fluid character allowing shape-change and movement (but attachment to proteins w/in cell can stop movement
• Lipid Composition of the Cell Membrane
  
• Phospholipids spontaneously form bilayers
• Hydrocarbon chains form the hydrophobic core
• Bilayer structure maintained by hydrophobic and van der Waals interactions btwn lipid chains
• Always a cytosolic face and an exoplasmic face
• Phosphatidylcholine, the most abundant phospholipid in the membrane
• Transport
• Membrane Transport Proteins
• Definition
  
• permit passage of nutrients into the cell and metabolic wastes out of it
• Keeps the cytosolic conc of K+ much higher than that of Na+ in the cell (i.e. K+ much higher in the cell, Na+ much higher in the blood)
  
• maintain the proper ionic composition and pH (~7.2) of the cytosol
• are transmembrane proteins containing multi membrane-spanning segments (i.e. alpha helices)
• 3 Types of Membrane Transport Proteins source
• ATP-powered pumps (a.k.a. pumps)
• ATPases that use the energy of ATP hydrolysis to move ions or small molecules across a mem against a chemical conc gradient, electric potential, or both
• Process is called active transport and an ex. of a coupled chemical rxn (i.e. transport of ions 'uphill' against an electrochemical gradient is coupled to the hydrolysis of ATP)
• Process is energetically favorable
• Four classes of ATP-powered transport proteins
• P-class pump: Plasma membrane of plants, fungi, bacteria (H+ pump)
• Possess two identical catalytic alpha subunits that contain an ATP-binding site and one of the alpha subunits is phosphorylated
  
• :Plasma membrane of higher eukaryotes (Na+/K+ pump which maintains the low cytosolic Na+ and high cytosolic K+)
• F-class and V-class ion pumps
• contain several different transmembrane and cytosolic subunits and only transport protons
• F-class power the synthesis of ATP from ADP and Pi by moving protons from the exoplasmic to the cytosolic face of the membrane down the proton electrochemical gradient and are very important in ATP synthesis
• ABC superfamily (ATP-binding cassette)
• specific for a single substrate which could be ions, sugars, a.a.s, phospholipids, peptides, polysaccharides, or proteins
  

• Channel Proteins
• transport water or certain ions and hydrophilic small molecules down their conc or electric potential gradients.
• Protein-assisted transport referred to as facilitated diffusion
• Form a hydrophilic passage way across the membrane through which ions, etc. move through, single file, at a rapid rate
• non-gated channels: open much of the time
• gated channels: what most channels are, open only in response to specific chemical or electric signals
• Transporters (a.k.a. carriers) source
• Maintain ionic gradients across cellular membranes
  
• Three types:
  
• Uniporter transport a single type of molecule down its con gradient (i.e. glucose and amion acids
  
• Antiporters and symporters (a.k.a. cotransporters) couple the movement of one type of ion or molecule against its conc gradient w/ the movement of one or more different ions down its conc gradient
  
• Passive (simple) diffusion
• Doesn't require energy b/c moves down conc gradient
  
• Gases (O2, CO2) & small uncharged polar molecules (urea, EtOH)
• To measure rate, add a small amt radioactive material to one compartment & meas its rate of appearance in other compartment
• Larger the partition coefficient, faster it will diffuse across a mem
  
• Osmosis = water moves semipermeable mem from low solute to high solute conc until both sides equal
• isotonic solution= solute conc of sol equal to the conc inside cell
  
• hypotonic solution = solute conc of solution is LESS than inside the cell
• hypertonic sol = solute conc of solution is MORE than inside the cell
• Features Distinguishing Uniport from Passive Diffusion
• Rate of facilitated iffusion by uniporters is higher than passive diffusion
• B/c the transported molecule never enters the hydrophobic core of the bilayer, the partition coefficient is irrelevant (doesn't have to be large to be fast)
• Transport occurs via a limited number of uniporter molecules rather than throughout the entire bilayer. As a result, there is a max transport rate Vmax that is reached when the conc gradient across the mem is very large and each uniporter is working at its max rate
• Transport is specific; each uniporter transports only a single species of molecule or a single group of closely related molecules
• The GLUT1 Uniporter source

• glucose transporter found in plasma membrane of erythrocytes (no nucleus or other internal organelles making it easy to purify transport proteins)
• GLUT1 expressed by most mammalian cells since most mammalian cells use blood glucose as the major source of cellular energy
• Alternates between two conformations as seen on figure
• Isomeric sugars D-mannose and D-galactose, which differ from D-glucose is the configuration at only one carbon atom, are transporter by GLUT1 @ measurable rates; however, the Km for glucose is much lower than for the other sugars, meaning GLUT1 is still fairly specific for glucose