Page 41 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

!
+
+
2 Na -amino acid – symport, or H -peptide 0 sis) in conjunction with the expenditure of
+
symport (! B5). The chemical Na gradient ATP. In cytosis, the uptake and release of mac-
provides the sole driving force for elec- romolecules such as proteins, lipoproteins,
troneutral transport (e.g., Na /H + antiport), polynucleotides, and polysaccharides into and
+
whereas the negative membrane potential out of a cell occurs by specific mechanisms
(! p. 32 ff.) provides an additional driving similar to those involved in intracellular trans-
force for electrogenication–coupled cotrans- port (! p. 12 ff.).
port into the cell. When secondary active into different types, including pinocytosis, re-
Endocytosis (! p. 13) can be broken down
transport (e.g., of glucose) is coupled with the
Fundamentals and Cell Physiology aid of ATPases is necessary, however, if the re- tinuous unspecific uptake of extracellular fluid
ceptor-mediated endocytosis, and phagocyto-
+
influx of not one but two Na ions (e.g., SGLT1
symporter), the driving force is doubled. The
sis. Pinocytosis is characterized by the con-
and molecules dissolved in it through rela-
quired “uphill” concentration ratio is several
tively small vesicles. Receptor-mediated en-
6
decimal powers large, e.g., 10 in the extreme
+
case of H ions across the luminal membrane of
docytosis (! C) involves the selective uptake
parietal cells in the stomach. ATPase-mediated
of specific macromolecules with the aid of re-
transport can also be electrogenic or elec-
ceptors. This usually begins at small depres-
sions (pits) on the plasma membrane surface.
+
+
troneutral, e.g., Na -K -ATPase (3 Na /2 K ; cf.
+
+
+
p. 46) or H /K -ATPase (1 H /1 K ), respectively.
+
+
+
Since the insides of the pits are often densely
Characteristics of active transport:
called clathrin-coated pits. The receptors in-
! It can be saturated, i.e., it has a limited maxi-
volved are integral cell membrane proteins
1 mum capacity (J max). covered with the protein clathrin, they are
such as those for low-density lipoprotein (LPL;
! It is more or less specific, i.e., a carrier
molecule will transport only certain chemi- e.g., in hepatocytes) or intrinsic factor-bound
cally similar substances which inhibit the cobalamin (e.g., in ileal epithelial cells). Thou-
transport of each other (competitive inhibi- sands of the same receptor type or of different
tion). receptors can converge at coated pits (! C),
! Variable quantities of the similar substances yielding a tremendous increase in the efficacy
are transported at a given concentration, i.e., of ligand uptake. The endocytosed vesicles are
each has a different affinity (~1/K M) to the initially coated with clathrin, which is later re-
transport system. leased. The vesicles then transform into early
! Active transport is inhibited when the endosomes, and most of the associated recep-
energy supply to the cell is disrupted. tors circulate back to the cell membrane (! C
All of these characteristics except the last and p. 13). The endocytosed ligand is either
apply to passive carriers, that is, to uniporter- exocytosed on the opposite side of the cell
mediated (facilitated) diffusion (! p. 22). (transcytosis, see below), or is digested by lyso-
The transport rate of saturable transport somes (! C and p. 13). Phagocytosis involves
(J sat) is usually calculated according to Mi- the endocytosis of particulate matter, such as
chaelis–Menten kinetics: microorganisms or cell debris, by phagocytes
C (! p. 94 ff.) in conjunction with lysosomes.
J sat ! J max ! [1.16]
K M + C Small digestion products, such as amino acids,
where C is the concentration of the substrate in sugars and nucleotides, are transported out of
question, J max is its maximum transport rate, the lysosomes into the cytosol, where they can
and K M is the substrate concentration that pro- be used for cellular metabolism or secreted
duces one-half J max (! p. 383). into the extracellular fluid. When certain hor-
Cytosis is a completely different type of ac- mones such as insulin (! p. 282) bind to re-
tive transport involving the formation of mem- ceptors on the surface of target cells, hormone-
brane-bound vesicles with a diameter of receptor complexes can also enter the coated
50–400 nm. Vesicles are either pinched off pits and are endocytosed (internalized) and
from the plasma membrane (exocytosis) or in- digested by lysosomes. This reduces the den-
28 corporated into it by invagination (endocyto- sity of receptors available for hormone bind-
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
All rights reserved. Usage subject to terms and conditions of license.