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CHAPTER 7
MEMBRANE STRUCTURE AND FUNCTION
CELL MEMBRANE
• Selectively Permeable –
some substances cross
more easily than others
• Made of:
1. Lipids
2. Proteins
3. Carbs
PHOSPHOLIPIDS
• Amphipathic – has
a hydrophobic
(nonpolar) and
hydrophilic (polar)
side
• Forms a bilayer
FLUID MOSAIC MODEL
• Plasma membrane is fluid
• Proteins are embedded and attached
like a mosaic
DAVSON AND DANIELLI - 1935
• Scientists knew the
membrane has
proteins and lipids
• D & D sandwich
model
• Phospholipid bilayer
between two layers
of protein
SINGER AND NICOLSON
• Used freeze-fracture
to split the
membrane
• Saw that the
proteins are
embedded
• Like chunky peanut
butter!
MEMBRANE FLUIDITY
• Membrane held
together by
hydrophobic
interactions
• Weaker than
covalent bonds
• Phospholipids move
rapidly within
membrane
• About the
consistency of salad
oil
BARRIERS TO FLUIDITY
• Temperature
• Unsaturated can stay liquid longer
• Kinks in lipid tails prevent packing
BARRIERS TO FLUIDITY
• Cholesterol
• Steroid that packs in
between
• high temps restricts
phospholipid
movement
• low temps keeps it from
solidifying
MEMBRANE PROTEIN MOVEMENT
• Some drift
• Some move in
directed manner
• Others are
attached to
cytoskeleton
MEMBRANE PROTEINS
• Integral Proteins
• Within the
hydrophobic core
• Nonpolar amino
acids make up
middle region
• Peripheral Proteins
• Not embedded in
bilayer
• Loosely bound to
surface
Fig. 7-7
Fibers of
extracellular
matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE
TRANSPORT PROTEINS
• Span the
membrane like a
tunnel
• Change shape to
move things in
and out
• Some use ATP
ENZYMATIC PROTEINS
• Active site
exposed to inside
or outside
• Sometimes
several in a
pathway
SIGNAL TRANSDUCTION
• Receptor for a
chemical
messenger
• Ex: hormone
• Usually changes
shape and relays
message
CELL-CELL RECOGNITION
• ID tags that are
recognized by
proteins on
other cells
ECM ATTACHMENT
• Protein
noncovalently
bonded to parts of
ECM
• Microfilaments etc.
• Helps to:
• stabilize ECM
• maintain cell shape
• coordinate changes
INTERCELLULAR JOINING
• Membrane
proteins of
adjacent cells
hook together
• Ex: tight or gap
junctions