High-Yield Facts
Histology and Cell Biology
CELL MEMBRANESCell membranes consist of a lipid bilayerand associated proteins and car-
bohydrates. In the bilayer, the hydrophilic portions of the lipids are
arranged on the external and cytosolic surfaces, and the hydrophobic tails
are located in the interior. Transmembrane proteinsare anchored to the
core of the bilayer by their hydrophobic regions and can be removed only
by detergents that disrupt the bilayer. Peripheral membrane proteins are
attached to the surface of the membrane by weak electrostatic forces and are
easy to remove by altering the pH or ionic strength of their environment.
CYTOPLASM AND ORGANELLESCytoplasm is a dynamic fluid environment bounded by the cell membrane.
It contains various membrane-bound organelles, nonmembranous struc-
tures (such as lipid droplets, glycogen, and pigment granules), and struc-
tural or cytoskeletal proteins in either a soluble or insoluble form. The
endoplasmic reticulum (ER)is a continuous tubular meshwork that may
be either smooth (SER) or rough (RER)where studded with ribosomes.
RER is involved in protein synthesis while the SER is involved in sterioid
synthesis and detoxification. The discoid stacks (CGN,cis,medial,trans,
and TGNas one moves from the RER-side to the secretory vesicle-side) of
theGolgi apparatusare involved in packaging and routing proteins for
export or delivery to other organelles, including lysosomes and peroxi-
somes.Lysosomesdegrade intracellular and imported debris, and peroxi-
somesoxidize a variety of substrates, through beta-oxidation and are the
sole source of plasmalogens.Targeting sequences include KDEL,which
targets ER proteins from the Golgi to the ER, and mannose 6-phosphate,
which targets proteins to the lysosome. Mannose 6-phosphate receptors
are found in the Golgi and in lysosomes. In the absence of mannose 6-
phosphate on lysosomal enzymes (I-cell disease)they follow the default
pathway and are secreted from the cell. Lysosomal enzymes are specific for
substrate; the absence of specific enzymes results in lysosomal storage
diseasessuch as Tay-Sach’s.Secretory granules leave the TGNto dock
with the plasma membrane. In that process, v-SNAREon the vesicle docks
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