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Chapter 11 – Transmembrane Transport of Ions and
Small Molecules
Chapter 11 – Transmembrane Transport of Ions and
Small Molecules
11.1 Overview of Transmembrane Transport
11.2 Facilitated Transport of Glucose and Water
11.3 ATP-Powered Pumps and the Intracellular Ionic
Environment
11.4 Nongated Ion Channels and the Resting
Membrane Potential
11.5 Cotransport by Symporters and Antiporters
11.6 Transcellular Transport
Transmembrane Transport of Ions and
Small Molecules
11.1 Overview of Transmembrane Transport
• Cellular membranes regulate the traffic of molecules
and ions into and out of cells and their organelles.
• The simple diffusion rate of substance movement across
the phospholipid part of a membrane is proportional to
its concentration gradient and hydrophobicity.
• Most molecules, except O2 and CO2, are moved by
protein channels and some transporters down their
concentrations gradients or by other transporters and
ATP-powered pumps up their concentration gradients.
Transmembrane Transport of Ions and
Small Molecules
11.2 Facilitated Transport of Glucose and Water
• Protein-mediated transport is faster than simple
diffusion, highly specific, and the transport rate is
limited by the number of transporters (Vmax) and
transporter affinity (Km) for the molecule/ion.
• Water moves by osmosis across membranes but
some membranes have aquaporin water channels to
increase rate of water transport.
• Uniport proteins convert between two
conformational states to facilitate diffusion across a
membrane.
Transmembrane Transport of Ions and
Small Molecules
11.3 ATP-Powered Pumps and the Intracellular Ionic
Environment
• Four classes of transmembrane proteins couple
energy released by ATP hydrolysis with energyrequiring transport of substances against their
concentration gradients.
• The combined action of P-class ATP-powered pumps
generates the usual ionic milieu of animal cells.
• ABC superfamily proteins transport a wide array of
substrates, including toxins, drugs, phospholipids,
peptides, and proteins, into or out of the cell.
Transmembrane Transport of Ions and
Small Molecules
11.4 Nongated Ion Channels and the Resting
Membrane Potential
• The animal cell plasma membrane resting potential is
generated by the ATP-powered Na+/K+ pump and
nongated K+ channels.
• The structure and chemical nature of a channel pore
lowers the activation energy for passage of a specific
ion over other ions, which may be even smaller.
• Patch-clamping techniques measure ion movements
through single channels.
Transmembrane Transport of Ions and
Small Molecules
11.5 Cotransport by Symporters and Antiporters
• Two forces constitute an electrochemical gradient
across a membrane – electric potential and ion
concentration gradient.
• Cotransporters use the energy released by ion (H+/
Na+) movement down its electrochemical gradient to
power transport of another molecule or different ion
up its concentration gradient.
Transmembrane Transport of Ions and
Small Molecules
11.6 Transcellular Transport
• Apical and basolateral plasma-membrane regions of
polarized epithelial cells contain different transport
proteins whose activities are coordinated to
accomplish different transport processes:
– Transcellular transport of amino acids and glucose
from the intestinal lumen to the blood
– Stomach acid secretion
– Bone resorption