Chemistry 451
Lecture #12: Transport Across Membranes
Read: pg. 264-277 AND 671-672 (Glucose transporters)
Optional Reading: pg. 16-18 (section 1-4D) and references of interest (see below)
HW: pg. 277; Problems (6, 7, 10, 11)
Optional HW: pg. 277; Study exercises (8, 10, 12)
Objectives:
1. The free energy of transport of a solute across a nonpermeable membrane is the sum of its chemical potential (
D GA = RTln([A]in/[A]out) and electrical potential (ZAÁ D y ). The resulting electrochemical potential is calculated as: D GA = RTln([A]in/[A]out) + ZAÁ D y .2. Passive transport (down a concentration gradient) can be non-mediated (for lipids and small molecules) or carrier mediated (such as GLUT1,2,3,4,and 7 transporters; and ionophores).
3. Active transport (against a concentration gradient) can be primary (such as Na+/K+ ATPase which is directly coupled to ATP hydrolysis) or secondary (such as GLUT5 glucose transport). Glucose is absrobed from the intestine via a GLUT5 transporter which is driven by the transport of Na+ down its concentration gradient. The Na+ gradient that drives glucose transport is maintained by primary active transport (Na+/K+ ATPase). Therefore GLUT5 glucose transport is called secondary active transport.
4. All carrier mediated (active and passive) transport is saturable, specific, competitive and depends on proper protein structure.
References of interest:
Regulation of ground squirrel Na+K+-ATPase activity by reversible
phosphorylation during hibernation.
Biochem Biophys Res Commun. 1999 Jan 19;254(2):424-9.
Tsukaguchi H, Tokui T, Mackenzie B, Berger UV, Chen XZ, Wang Y, Brubaker RF, Hediger MA.
A family of mammalian Na+-dependent L-ascorbic acid transporters.
Nature. 1999 May 6;399(6731):70-5.