BIOC 2300 Lecture Notes - Lecture 13: Faraday Constant, Passive Transport, Cell Membrane
3 May 2018
School
Department
Course
Professor
Membrane Transport
February 3rd, 2016
Membrane Permeability
• Estrogen can diffuse through
• While lipid bilayers are permeable to some non-polar substances, polar molecules require
protein-mediated transport
• Spontaneous passive transport must be down a concentration gradient for transport of X
from compartment A to B
(delta)G = RT ln([X]B/[X]A)
• Active transport requires an energy input
• Transport may be facilitated by channels:
o Non-stoichiometric: fast 106/sec
o Always passive
o Selective
o May be gated (by a ligand or by voltage)
• Transport may be facilitated by transporters:
o Stoichiometric: slower 103/sec
o Passive or active (pumps)
o Specific
o May be regulated
• Hydrophobic – diffuse through (oxygen, CO2, steroid, hormones)
• Small, Uncharged, Polar – only small amount diffuse through (water, urea, glycerol)
• Large, uncharged, polar – very little goes through membrane (glucose, sucrose)
• Ions – cannot diffuse through membrane (H+, Na+, Cl-)
Facilitated Membrane Transport
• Passive – down concentration gradient (fast channels vs. slow transporters)
• Active transport – against concentration gradient (transporter pumps use ATP or coupled
transport)
Ion Transport
• Na+ 150mM outside – 5-15mM inside
• K+ 4 outside – 140 inside
• Ca++ 1-2 outside – 10-4 inside
• Mg++ 1-2 outside – 0.5 inside
• Mammalian cells maintain ion gradient across plasma membrane: the large Na+/K+
gradient is maintained by the Na,K-ATPase pumped (uses 1/3 of energy at rest)
• Although cell overall must be neutral, K+ leak channels allow a small number of K+ to
exit until concentration gradient is balanced by voltage difference or membrane potential
across bilayer; typically about -70 mV
• Thus, the transport of a charged ion is subject to both concentration and voltage gradients
• **see equation with charge and Faraday constant do not memorize for exam
• this membrane potential can be harnessed in electric excitable membranes: e.g. as the
action potential in axonal nerve conduction
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Ions cannot diffuse through membrane (h+, na+, cl-) Facilitated membrane transport: passive down concentration gradient (fast channels vs. slow transporters, active transport against concentration gradient (transporter pumps use atp or coupled transport) Na+ ions dissociate: two extracellular k+ bind, the aspartyl phosphate group is hydrolyzed, protein conformation changes, exposing k+ binding sites to cell interiors k+ ions dissociate. Topological relationship among cell compartments: areas in grey are topologically equivalent within the cytosol, areas in orange are topologically equivalent with the extracellular space, need to cross a membrane to get from one region to the other. Vesicular transport: vesicles bud from donor membrane and fuse with target membrane (er golgi plasma membrane, proteins are targeted depending on embedded signals, most lipids are synthesized in smooth er, thus bulk flow is directed to other membranes. Mechanism of membrane fusion: fusion of lipid bilayers cannot be random: it depends on correct addressing plus physical disruption and reformation of bilayer leaflets.
