Page 29 - Color Atlas Physiology

Transport In, Through and Between ! Endoplasmic reticulum (ER): In addition to a
Cells translocator protein (! p. 10), the ER has two
other proteins that transport Ca 2+ (! A). Ca 2+
The lipophilic cell membrane protects the cell can be pumped from the cytosol into the ER by
2+
interior from the extracellular fluid, which has a Ca -ATPase called SERCA (sarcoplasmic en-
a completely different composition (! p. 2). doplasmic reticulum Ca -transporting
2+
This is imperative for the creation and main- ATPase). The resulting Ca 2+ stores can be re-
tenance of a cell’s internal environment by leased into the cytosol via a Ca 2+ channel (ry-
anodine receptor, RyR) in response to a trigger-
means of metabolic energy expenditure. Chan-
Fundamentals and Cell Physiology transmembrane transport of selected sub- tains large pores called porins that render it
nels (pores), carriers, ion pumps (! p. 26ff.)
ing signal (! p. 36).
! Mitochondria: The outer membrane con-
and the process of cytosis (! p. 28) allow
permeable to small molecules (! 5 kDa), and
stances. This includes the import and export of
metabolic substrates and metabolites and the
the inner membrane has high concentrations
of specific carriers and enzymes (! B).
selective transport of ions used to create or
Enzyme complexes of the respiratory chain
modify the cell potential (! p. 32), which plays
–
transfer electrons (e ) from high to low energy
an essential role in excitability of nerve and
levels, thereby pumping H
+
ions from the
muscle cells. In addition, the effects of sub-
stances that readily penetrate the cell mem-
matrix space into the intermembrane space
+
gradient directed into the matrix. This not only
mitigated by selectively transporting certain
other substances. This allows the cell to com-
drives ATP synthetase (ATP production; ! B2),
pensate for undesirable changes in the cell
1 brane in most cases (e.g., water and CO 2) can be (! B1), resulting in the formation of an H ion
but also promotes the inflow of pyruvate and
–
–
volume or pH of the cell interior. anorganic phosphate, P i (symport; ! B2b,c
2+
and p. 28). Ca 2+ ions that regulate Ca -sensi-
Intracellular Transport tive mitochondrial enzymes in muscle tissue
The cell interior is divided into different com- can be pumped into the matrix space with ATP
partments by the organelle membranes. In expenditure (! B2), thereby allowing the mi-
some cases, very broad intracellular spaces tochondria to form a sort of Ca 2+ buffer space
must be crossed during transport. For this pur- for protection against dangerously high con-
pose, a variety of specific intracellular trans- centrations of Ca 2+ in the cytosol. The inside-
+
port mechanisms exist, for example: negative membrane potential (caused by H re-
! Nuclear pores in the nuclear envelope pro- lease) drives the uptake of ADP 3 – in exchange
vide the channels for RNA export out of the nu- for ATP 4 – (potential-driven transport; ! B2a
cleus and protein import into it (! p. 11 C); and p. 22).
! Protein transport from the rough endo-
plasmic reticulum to the Golgi complex Transport between Adjacent Cells
(! p. 13 F); In the body, transport between adjacent cells
! Axonal transport in the nerve fibers, in occurs either via diffusion through the extra-
which distances of up to 1 meter can be cellular space (e.g., paracrine hormone effects)
crossed (! p. 42). These transport processes or through channel-like connecting structures
mainly take place along the filaments of the (connexons) located within a so-called gap
cytoskeleton. Example: while expending ATP, junction or nexus (! C). A connexon is a hemi-
the microtubules set dynein-bound vesicles in channel formed by six connexin molecules
motion in the one direction, and kinesin- (! C2). One connexon docks with another con-
bound vesicles in the other (! p. 13 F). nexon on an adjacent cell, thereby forming a
common channel through which substances
Intracellular Transmembrane Transport with molecular masses of up to around 1 kDa
Main sites: can pass. Since this applies not only for ions
+
! Lysosomes: Uptake of H ions from the cyto- such as Ca , but also for a number of organic
2+
16 sol and release of metabolites such as amino substances such as ATP, these types of cells are
acids into the cytosol (! p. 12);
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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