Page 25 - Color Atlas Physiology

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plasmic reticulum (RER). These ribosomes syn- Hence, the Golgi apparatus represents a
thesize export proteins as well as transmem- central modification, sorting and distribution
brane proteins (! G) for the plasma mem- center for proteins and lipids received from the
brane, endoplasmic reticulum, Golgi appara- endoplasmic reticulum.
tus, lysosomes, etc. The start of protein synthe- Regulation of gene expression takes place
sis (at the amino end) by such ribosomes (still on the level of transcription (! C1a), RNA
unattached) induces a signal sequence to modification (! C1b), mRNA export (! C1c),
which a signal recognition particle (SRP) in the RNA degradation (! C1d), translation (! C1e),
cytosol attaches. As a result, (a) synthesis is
modification and sorting (! F,f), and protein
Fundamentals and Cell Physiology tein synthesis, a translocator protein conveys site of oxidation of carbohydrates and lipids to
degradation (! F,g).
temporarily halted and (b) the ribosome (me-
The mitochondria (! A, B; p. 17 B) are the
diated by the SRP and a SRP receptor) attaches
to a ribosome receptor on the ER membrane.
CO 2 and H 2O and associated O 2 expenditure.
After that, synthesis continues. In export pro-
The Krebs cycle (citric acid cycle), respiratory
chain and related ATP synthesis also occur in
the peptide chain to the cisternal space once
mitochondria. Cells intensely active in meta-
synthesis is completed. Synthesis of membrane
proteins is interrupted several times (depend-
bolic and transport activities are rich in mito-
ing on the number of membrane-spanning
chondria—e.g., hepatocytes, intestinal cells,
domains (! G2) by translocator protein clo-
and renal epithelial cells. Mitochondria are en-
peptide sequence is pushed into the phos-
smooth outer membrane and an inner mem-
brane. The latter is deeply infolded, forming a
pholipid membrane. The smooth endoplasmic
1 sure, and the corresponding (hydrophobic) closed in a double membrane consisting of a
series of projections (cristae); it also has im-
reticulum (SER) contains no ribosomes and is
the production site of lipids (e.g., for lipo- portant transport functions (! p. 17 B). Mito-
proteins, ! p. 254 ff.) and other substances. chondria probably evolved as a result of sym-
The ER membrane containing the synthesized biosis between aerobic bacteria and anaerobic
membrane proteins or export proteins forms cells (symbiosis hypothesis). The mitochondrial
vesicles which are transported to the Golgi ap- DNA (mtDNA) of bacterial origin and the
paratus. double membrane of mitochondria are relicts
The Golgi complex or Golgi apparatus (! F) of their ancient history. Mitochondria also
has sequentially linked functional compart- contain ribosomes which synthesize all pro-
ments for further processing of products from teins encoded by mtDNA.
the endoplasmic reticulum. It consists of a cis- Lysosomes are vesicles (! F) that arise from
Golgi network (entry side facing the ER), the ER (via the Golgi apparatus) and are in-
stacked flattened cisternae (Golgi stacks) and a volved in the intracellular digestion of macro-
trans-Golgi network (sorting and distribution). molecules. These are taken up into the cell
Functions of the Golgi complex: either by endocytosis (e.g., uptake of albumin
! polysaccharide synthesis; into the renal tubules; ! p. 158) or by phagocy-
! protein processing (posttranslational modi- tosis (e.g., uptake of bacteria by macrophages;
fication), e.g., glycosylation of membrane pro- ! p. 94 ff.). They may also originate from the
teins on certain amino acids (in part in the ER) degradation of a cell’s own organelles (auto-
that are later borne as glycocalyces on the ex- phagia, e.g., of mitochondria) delivered inside
ternal cell surface (see below) and γ-carboxy- autophagosomes (! B, F). A portion of the en-
lation of glutamate residues (! p. 102 ); docytosed membrane material recycles (e.g.,
! phosphorylation of sugars of glycoproteins receptor recycling in receptor-mediated en-
(e.g., to mannose-6-phosphate, as described docytosis; ! p. 28). Early and late endosomes
below); are intermediate stages in this vesicular trans-
! “packaging” of proteins meant for export port. Late endosomes and lysosomes contain
into secretory vesicles (secretory granules), the acidic hydrolases (proteases, nucleases, li-
contents of which are exocytosed into the ex- pases, glycosidases, phosphatases, etc., that
12 tracellular space; see p. 246, for example. are active only under acidic conditions). The
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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