Page 27 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

!
+
membrane contains an H -ATPase that creates below. Cholesterol (present in both layers) re-
an acidic (pH 5) interior environment within duces both the fluidity of the membrane and
the lysosomes and assorted transport proteins its permeability to polar substances. Within
that (a) release the products of digestion (e.g., the two-dimensionally fluid phospholipid
amino acids) into the cytoplasm and (b) ensure membrane are proteins that make up 25% (my-
+
charge compensation during H uptake (Cl – elin membrane) to 75% (inner mitochondrial
channels). These enzymes and transport pro- membrane) of the membrane mass, depend-
teins are delivered in primary lysosomes from ing on the membrane type. Many of them span
the Golgi apparatus. Mannose-6-phosphate
the entire lipid bilayer once (! G1) or several
Fundamentals and Cell Physiology docytosis (! p. 28 ), cluster in the membrane teins, hormone receptors, etc. The proteins are
(M6 P) serves as the “label” for this process; it
proteins),
(transmembrane
(! G2)
times
thereby serving as ion channels, carrier pro-
binds to M6 P receptors in the Golgi membrane
which, as in the case of receptor-mediated en-
anchored by their lipophilic amino acid resi-
dues, or attached to already anchored proteins.
with the help of a clathrin framework. In the
Some proteins can move about freely within
acidic environment of the lysosomes, the
enzymes and transport proteins are separated
the membrane, whereas others, like the anion
exchanger of red cells, are anchored to the cy-
from the receptor, and M6 P is dephosphory-
toskeleton. The cell surface is largely covered
lated. The M6 P receptor returns to the Golgi
apparatus (recycling, ! F). The M6 P receptor
by the glycocalyx, which consists of sugar
proteins, which prevents them from returning
the cell membrane (! G1,4) and of the extra-
to the Golgi apparatus.
cellular matrix. The glycocalyx mediates cell–
Peroxisomes are microbodies containing
1 no longer recognizes the dephosphorylated moieties of glycoproteins and glycolipids in
cell interactions (surface recognition, cell
enzymes (imported via a signal sequence) that docking, etc.). For example, components of the
permit the oxidation of certain organic glycocalyx of neutrophils dock onto en-
molecules (R-H 2), such as amino acids and dothelial membrane proteins, called selectins
fatty acids: R-H 2 + O 2 ! R + H 2O 2. The peroxi- (! p. 94).
somes also contain catalase, which transforms The cytoskeleton allows the cell to maintain
2 H 2O 2 into O 2 + H 2O and oxidizes toxins, such and change its shape (during cell division, etc.),
as alcohol and other substances. make selective movements (migration, cilia),
Whereas the membrane of organelles is re- and conduct intracellular transport activities
sponsible for intracellular compartmentaliza- (vesicle, mitosis). It contains actin filaments as
tion, the main job of the cell membrane (! G) well as microtubules and intermediate fila-
is to separate the cell interior from the extra- ments (e.g., vimentin and desmin filaments,
cellular space (! p. 2). The cell membrane is a neurofilaments, keratin filaments) that extend
phospholipid bilayer (! G1) that may be either from the centrosome.
smooth or deeply infolded, like the brush
border or the basal labyrinth (! B). Depending
on the cell type, the cell membrane contains
variable amounts of phospholipids, cholesterol,
and glycolipids (e.g., cerebrosides). The phos-
pholipids mainly consist of phosphatidylcho-
line (! G3), phosphatidylserine, phosphati-
dylethanolamine, and sphingomyelin. The hy-
drophobic components of the membrane face
each other, whereas the hydrophilic com-
ponents face the watery surroundings, that is,
the extracellular fluid or cytosol (! G4). The
lipid composition of the two layers of the
membrane differs greatly. Glycolipids are
14 present only in the external layer, as described
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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