Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 2Overview of Cellular Physiology in Medical Physiology 45

COATS & VESICLE TRANSPORT

It now appears that all vesicles involved in transport have pro-
tein coats. In humans, 53 coat complex subunits have been
identified. Vesicles that transport proteins from the trans Gol-
gi to lysosomes have assembly protein 1 (AP-1) clathrin
coats, and endocytotic vesicles that transport to endosomes
have AP-2 clathrin coats. Vesicles that transport between the
endoplasmic reticulum and the Golgi have coat proteins I and
II (COPI and COPII). Certain amino acid sequences or at-
tached groups on the transported proteins target the proteins
for particular locations. For example, the amino acid sequence
Asn–Pro–any amino acid–Tyr targets transport from the cell
surface to the endosomes, and mannose-6-phosphate groups
target transfer from the Golgi to mannose-6-phosphate recep-
tors (MPR) on the lysosomes.
Various small G proteins of the Rab family are especially
important in vesicular traffic. They appear to guide and facili-
tate orderly attachments of these vesicles. To illustrate the
complexity of directing vesicular traffic, humans have 60 Rab
proteins and 35 SNARE proteins.

MEMBRANE PERMEABILITY &

MEMBRANE TRANSPORT PROTEINS

An important technique that has permitted major advances in
our knowledge about transport proteins is patch clamping. A
micropipette is placed on the membrane of a cell and forms a
tight seal to the membrane. The patch of membrane under the
pipette tip usually contains only a few transport proteins, al-
lowing for their detailed biophysical study (Figure 2–14). The
cell can be left intact (cell-attached patch clamp). Alterna-
tively, the patch can be pulled loose from the cell, forming an
inside-out patch. A third alternative is to suck out the patch
with the micropipette still attached to the rest of the cell mem-
brane, providing direct access to the interior of the cell (whole
cell recording).
Small, nonpolar molecules (including O 2 and N 2 ) and small
uncharged polar molecules such as CO 2 diffuse across the
lipid membranes of cells. However, the membranes have very
limited permeability to other substances. Instead, they cross
the membranes by endocytosis and exocytosis and by passage
through highly specific transport proteins, transmembrane
proteins that form channels for ions or transport substances
such as glucose, urea, and amino acids. The limited perme-
ability applies even to water, with simple diffusion being sup-
plemented throughout the body with various water channels
(aquaporins). For reference, the sizes of ions and other bio-
logically important substances are summarized in Table 2–2.
Some transport proteins are simple aqueous ion channels,
though many of these have special features that make them
selective for a given substance such as Ca2+ or, in the case of
aquaporins, for water. These membrane-spanning proteins
(or collections of proteins) have tightly regulated pores that
can be gated opened or closed in response to local changes

(Figure 2–15). Some are gated by alterations in membrane

potential (voltage-gated), whereas others are opened or

closed in response to a ligand (ligand-gated). The ligand is

FIGURE 2–14 Patch clamp to investigate transport. In a patch

clamp experiment, a small pipette is carefully maneuvered to seal off a

portion of a cell membrane. The pipette has an electrode bathed in an

appropriate solution that allows for recording of electrical changes

through any pore in the membrane (shown below). The illustrated setup

is termed an “inside-out patch” because of the orientation of the mem-

brane with reference to the electrode. Other configurations include cell

attached, whole cell, and outside-out patches. (Modified from Ackerman

MJ, Clapham DE: Ion channels: Basic science and clinical disease. N Engl J Med

1997;336:1575.)

TABLE 2–2 Size of hydrated ions and other substances

of biologic interest.

Substance Atomic or Molecular Weight Radius (nm)

Cl– 35 0.12

K+ 39 0.12

H 2 O 18 0.12

Ca2+ 40 0.15

Na+ 23 0.18

Urea 60 0.23

Li+ 7 0.24

Glucose 180 0.38

Sucrose 342 0.48

Inulin 5000 0.75

Albumin 69,000 7.50

Data from Moore EW: Physiology of Intestinal Water and Electrolyte Absorption.

American Gastroenterological Association, 1976.

Inside-out patch

Cell

membrane

Pipette

Electrode

Closed

Open

ms

pA