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Chapter 7
The Cell Membrane
AP Biology
Overview
 Plasma membrane separates living cell
from nonliving surroundings

thin barrier = 8nm thick
 Controls traffic into & out of the cell
selectively permeable
 allowing some substances to cross
more easily than others

 Made of phospholipids, proteins & other
macromolecules
AP Biology
Phospholipids
 Fatty acid tails
are hydrophobic
 Phosphate
group form a
hydrophilic head
interaction with
H2O is complex
 & very
important!

AP Biology
Phospholipids in water
 At surface of a cell phospholipids are
arranged as a bilayer


AP Biology
hydrophilic heads on the outside in contact with
aqueous solution & hydrophobic tails form core
phospholipid bilayer forms a barrier between
cell & external environment
Fluid Mosaic Model
 In 1972, S.J. Singer & G. Nicolson
proposed that membrane proteins are
inserted into the phospholipid bilayer
AP Biology
A membrane is a collage of different proteins
embedded in the fluid matrix of the lipid bilayer
AP Biology
Membrane Proteins
 Proteins determine most of membrane’s
specific functions
 Plasma membrane & organelle membranes
each have unique collections of proteins
 Membrane proteins:


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peripheral proteins = loosely bound to surface
of membrane
integral proteins = penetrate into lipid bilayer,
often completely spanning the membrane
(a transmembrane protein)
Why are proteins the perfect
molecule to function embedded
in the cell membrane?
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Why proteins?
 Proteins are mixed molecules

hydrophobic amino acids
 stick in the lipid membrane
 anchors the protein in membrane

hydrophilic amino acids
 stick out in the watery
fluid in & around cell
 specialized “receptor”
for specific molecules
AP Biology
2005-
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Integral Proteins
 Within membrane
hydrophobic regions with nonpolar
amino acids
 often coiled into alpha helices

 Outer surfaces of membrane

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hydrophilic regions
with polar amino
acids
2004-200
Inside & outside surfaces are different
 2 layers may differ in
lipid composition
proteins in
membrane have a
direction
 outer surface also
has carbohydrates

AP Biology
2004-200
Membranes provide a variety of cell functions
AP Biology
Fluid membranes
 Permeability

membrane must be fluid, about as fluid
as salad oil
 Changing temperatures
cells alter lipid composition of
membranes
 cold-adapted organisms

 winter wheat increases % unsaturated
phospholipids in autumn
 prevent membranes from solidifying during
winter
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Unsaturated fats
 more fluid membranes

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kinks in unsaturated fatty acid tails
prevent tight packing
Membrane carbohydrates
 Play a key role in cell-cell recognition
ability of a cell to distinguish
neighboring cells from another
 important in organ & tissue
development
 basis for rejection of foreign cells by
immune system

AP Biology
A membrane is a collage of different proteins
embedded in the fluid matrix of the lipid bilayer
AP Biology
2004-20
Any Questions??
AP Biology