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Midterm

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


Department
Biology
Course Code
BIO315H5
Professor
Danton O' Day
Study Guide
Midterm

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The Human Cell Membrane
All cells are surrounded by a cell
membrane
aka plasma membrane,
biomembrane
double layer of lipids in which
proteins are located
functions:
1. isolates cell components from
external environment (regulates
flow of materials)
2. communications interface b/w
cell & environment (ex:
intercellular communications &
adhesion)
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
(cytoplasmic/extracellular)
carbohydrates link to proteins or
lipids only on extracellular side to
form glycoproteins/lipids
Phospholipids:
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,
glycolipids
cytoplasmic face is generally
negatively charged
Micelles:
form when lipid amount is low
compared to water content
can form at regions of membrane
instability
Liposomes:
bilayered lipid vesicles
form by sonicating lipids in aq.
solution
used in drug delivery & in
biologically active cosmetics
Cholesterol:
steroid lipid
flat shape--> interdigitates b/w
phospholipids--> stabilizes the
membrane
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 &
out)
Glycoproteins:
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
membrane
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
discontinuities
lipid domains (aka rafts) also exist
contain diff concentrations of
lipids, such as cholesterol &
sphingolipids
sites where other specific
molecules group for specific
functions
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
transduction
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
junctions

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2. Cells adhere to substratum, basal
lamina, & extracellular matrix via:
hemi-desmosomes
focal contacts
integrins
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
cells
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
cytoskeleton
cadherins bind to vinculin which
allow them to associate with actin
filaments
Desmosomes:
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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