lec 4

• why is cell transport important?

  understanding how cells exchange materials is key to understanding how the body maintains homeostasis.
• what do membranes do?

  separate material: ICF/ECF
• why transport is important?

  obtaining O2 and nutrients

  getting rid of waste products
• What is Permeability?

  determines what moves in and out of a cell
• Permeable?


  allow molecules to pass through
• selective?


  restrictive
• selective permeability restricts materials based on?

  -size

  -molecular shape

  -electrical charge

  -lipid solubility
• movement between and within body compartments is necessary for?

  communication
• What are non polar molecules?

  easily transported across membrane (O2, CO2, fatty acids)
• What are ions and polar molecules?

  normally not transported
• both forces=?

  electrochemical forcde
• Passive Transport?


  -spontaneous

  -movement down the force; downhill movement
• Active Transport?


  -not spontaneous

  -movement against the force; uphill movement
• Bulk Flow?


  -movement of fluids

  -fluids are gases and liquids

  -pressure gradients

  -e.g. blood through the circulatory system
• Other forms of transport?

  -diffusion

  -protein-mediated transport

  -vesicular transport
• Rate of Transport?


  diffusional equilibrium

  -no net transfer of molecules
• Types of passive transport?

  • simple diffusion
  • facilitated diffusion
  • diffusion through channels
• simple diffusion?

  -no membrane proteins are needed

  -factors affecting the rate

  -magnitude of: 1) driving force 2)membrane surface area 3) membrane permeability
• Factors affecting simple diffusion?


  -distance

  -magnitude of the concentration gradient

  -molecule size, smaller is faster

  -temp, more heat, faster motion

  -electrical forces, opposites attract, like repel

  
  

  Membrane permeability factors?

  -Channel-mediated diffusion and the Interaction with channels

  -Lipid solubility of diffusing substance

  -Size and shape of diffusing particle

  -Temperature (generally not really relevant in human physiology)

  -Thickness of membrane
• Types of Channels?

  1. aquaporins

  2. ion channels

  • leak channels
  • gated channels
  • bidirectional

  factors affecting the rate of transport:

  -Transport rate of each channel• Number of channels in the membrane• Open or closed state
• Diffusion through channels?


  -Passive transport through a channel

  
  

  Characteristics of a channel

  -Transmembrane protein

  -Functions like a passageway or pore

  -Substance specific

  
  

  Percent of “open time” regulated (gating) by:

  •Binding of ligands to the channels

  •Ligand gating

  
  

  Voltage difference across membrane

  •Voltage gating

  
  

  Stretch of membrane

  •Mechanical gating
• Carrier Proteins?


  -Bind to substrates and carry thembetween intra and extracellularcompartments-Carriers are open to only one side ofthe membrane at a time• recall: channels are open toboth at once-Transport molecules too large to fitthrough channel proteins• glucose, amino acids
• Facilitated Diffusion?


  • Passive transport through a carrier

  Characteristics of a carrier:

  -Transmembrane protein

  -Has binding sites for specific particles

  -Binding occurs one side at a time

  -random conformational changes
• Facilitated Diffusion?


  Factors affecting the rate of transport:

  -rate of transport of each carrier

  -number of carriers in the membrane

  -concentration gradient
• Facilitated Diffusion: Glucose Transporter 4?

  GLUT4

  -GLUT4 molecules are synthesized in rough ER

  -Molecules are packaged in vesicles

  -Insulin stimulates GLUT4 insertion into the membrane

  -In diabetes mellitus, lack of insulin action means less GLUT4 is inserted into the membrane

  • Less glucose uptake

  •More glucose in blood
• Two Types of Active Transport?

  1. primary (pump)

  • requires energy
  • ion pumps

  2. secondary (coupled transport)

  • does not require ATP
  • potential energy from another gradient
  • indirectly driven by energy from ATP
• Characteristics of a pump?

  • type of membrane protein
  • functions as a transporter and an enzyme
  • can harness energy
  • has specific binding sites
  • demonstrates saturation
• Primary Active Transport?


  -energy comes from a high-energy compound

  -energy is usually from ATP hydrolysis