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BIO315H5 Study Guide - Midterm Guide: Desmosome, Phospholipid, Carbohydrate

Course Code
Danton O' Day
Study Guide

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The Human Cell Membrane
All cells are surrounded by a cell
aka plasma membrane,
double layer of lipids in which
proteins are located
1. isolates cell components from
external environment (regulates
flow of materials)
2. communications interface b/w
cell & environment (ex:
intercellular communications &
3. compartmentalizes cellular
functions (i.e. organelles are all
surrounded by membranes)
Fluid Mosaic Model:
concept that the cell membrane
consists of a continuous, fluid,
double layer of phospholipids
proteins may be embedded
(integral) in the bilayer or associated
(peripheral) with either face
carbohydrates link to proteins or
lipids only on extracellular side to
form glycoproteins/lipids
hydrophilic polar head group
hydrophobic non-polar tail
length & # of double bonds
differs in different phospholipids
Bilayer contains lipids involved in cell
communication (aka signal transduction)
outside: more phosphatidylcholine
inside: more phosphatidylserine,
cytoplasmic face is generally
negatively charged
form when lipid amount is low
compared to water content
can form at regions of membrane
bilayered lipid vesicles
form by sonicating lipids in aq.
used in drug delivery & in
biologically active cosmetics
steroid lipid
flat shape--> interdigitates b/w
phospholipids--> stabilizes the
present in animal cell membranes,
absent in bacteria and most plants
Membrane Protein Functions:
membrane transport, cell
adhesion, intercellular comm,
enzymes, channels, adhesion
lipid-spanning domains allow for
membrane passes (1 domain =
single pass)

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may be linked to membrane via a
glycolipid or phospholipid anchor &
interact with other proteins (inside &
many membrane proteins are
covalently linked to sugar residues
(extracellular face only)
if carbohydrate component of
glycoprotein is extensive it can be
seen using electron microscope (ex:
intestinal epithelium)
Proteins are not randomly localized,
they exist in complexes that are localized
to specific domains--> some proteins are
restricted to certain sites in the
ex: in sperm, the cell membrane at
the head contains different
membrane proteins than that at the
midpiece or tail.
Lipid Rafts:
membranes are not made up of a
bilayer containing continuous
proteins domains--> there are
lipid domains (aka rafts) also exist
contain diff concentrations of
lipids, such as cholesterol &
sites where other specific
molecules group for specific
Caveolae = little caves;
special type of lipid raft which
form distinct invaginations of
the cell membrane
stable, cholesterol-rich
membrane domains
containing the structure-
specific protein caveolin on
cytoplasmic side
implicated in uptake of
cholesterol by endocytosis &
accumulation of signal
Cell fusion experiments prove
membrane fluidity:
Junctional Adhesion Complexes
Evolution of higher organisms required
that single cells form multicellular
associations--> began with cell adhesion
molecules (CAMs)
Homotypic cell adhesion = adhesion of
2 identical molecules
Heterotypic cell adhesion = adhesion of
2 diff molecules
These simple associations led to
clustering of cell adhesion molecules to
form complex adhesion structures
highly organized adhesion junctions
consisting of cell adhesion
molecules & proteins (accessory &
adaptor) that allow interactions like
links to intracellular cytoskeleton &
extracellular matrix
Cell Adhesion Mechanisms:
1. Cells adhere to each other via:
Junctional adhesion
mechanisms, JAMs
tight junctions, adherens
junctions, desmosomes, gap

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2. Cells adhere to substratum, basal
lamina, & extracellular matrix via:
focal contacts
integral membrane proteoglycans
different junctions may be grouped
together to form junctional adhesion
complexes which provide strong binding
b/w cells subjected to great stress
Junctional Adhesion Molecules:
aka JAMs
some junctions contain unique
proteins (ex: connexin proteins of
gap junctions)
some junctions contain proteins that
appear elsewhere (ex: cadherins)
Tight Junctions:
aka occluding junctions
prevent movement b/w intercellular
spaces, paracellular movement
restrict flow of membrane
components b/w basal & apical side;
role in cell polarity
tightly aligned rows of tight junction
proteins stitch membranes together,
sealing association b/w adjacent
Tight Junction Proteins:
1. Occludin:
integral membrane protein
specific to tight junctions
knockout (KO) mutants still
form tight junctions; therefore
its only an accessory protein
(not essential for structure)
4 transmembrane domains
a NACo
2. ZO1 (Zona Occludin):
structural tight junction
protein; scaffold protein
KO interferes with tight
junction formation
a NACo
Adherens Junctions:
mainly in epithelial cells
lie just below tight junctions
often form a continuous belt of
cadherin around cells
cadherins bind to ß-catenins in
cytoplasm which anchors the actin
cadherins bind to vinculin which
allow them to associate with actin
strong adhesions found in epithelial
& other cell types that are subjected
to stress or shear (cardiac muscle,
cervix)--> cell-to-cell adhesion
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