AP Bio Chap 7 The Cell Membrane only

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The Cell Membrane
• AP Chapter 7
Overview: Life at the Edge
• The plasma membrane is the boundary that
separates the living cell from its surroundings
• The plasma membrane exhibits selective
permeability, allowing some substances to
cross it more easily than others
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Composition of the cell membrane:
lipids (phospholipids and cholesterol)
and proteins
• Phospholipids are the most abundant lipid in
the plasma membrane
- they are amphipathic molecules, contain
hydrophobic and hydrophilic regions.
• The fluid mosaic model states that a
membrane is a fluid structure with a “mosaic”
of various proteins embedded in it
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Development of the Fluid Mosaic
Model
• In 1935, Hugh Davson and James Danielli proposed a
sandwich model in which the phospholipid bilayer lies
between two layers of globular proteins
• In 1972, J. Singer and G. Nicolson proposed that the
membrane is a mosaic of proteins dispersed within
the bilayer, with only the hydrophilic regions exposed
to water
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Sandwich model of the cell membrane
What is wrong with this model?
Proteins are mostly
hydrophobic.
All phospholipids
are not alike.
Fluid Mosaic Model
Fig. 7-2
This slide shows how the hydrophobic and
hydrophilic regions are set up.
Hydrophilic
head
WATER
Hydrophobic
tail
WATER
Fig. 7-3
Phospholipid
bilayer
Hydrophobic regions
of protein
Hydrophilic
regions of protein
Double phospholipid layer
Different types of phospholipids
Different types of phospholipids making
up the membrane
Fig. 7-4
TECHNIQUE
RESULTS
Extracellular
layer
Knife
Plasma membrane
Proteins
Inside of extracellular layer
Cytoplasmic layer
Inside of cytoplasmic layer
Freeze-fracture studies of the plasma
membrane supported the fluid mosaic
model
The Fluidity of Membranes
• Phospholipids in the plasma membrane can
move within the bilayer
• Most of the lipids, and some proteins, drift
laterally (very rarely transverse flip-flops)
fluidity of cell membrane
fluidity of membrane
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-5
Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)
(a) Movement of phospholipids
Fluid
Biology
Animations
Unsaturated hydrocarbon
tails with kinks
Viscous
Saturated hydrocarbon tails
(b) Membrane fluidity
Cholesterol
(c) Cholesterol within the animal cell membrane
This experiment shows how the proteins
can move about the membrane.
Fig. 7-6
RESULTS
Membrane proteins
Mouse cell
Mixed proteins
after 1 hour
Human cell
Hybrid cell
The fluidity of the membrane depends
on temperature and types of lipids
making up the membrane.
• Membranes rich in unsaturated fatty acids are
more fluid than those rich in saturated fatty
acids
• Membranes must be fluid to work properly;
they are usually about as fluid
as salad oil.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-5b
Fluid
Unsaturated hydrocarbon
tails with kinks
(b) Membrane fluidity
Viscous
Saturated hydrocarbon tails
• Would you expect an
amoeba that lives in a
pond in a cold northern
climate to have a higher
or lower percentage of
saturated fatty acids in
its membranes during
the summer as
compared to the
winter?
• The steroid cholesterol has different effects on
membrane fluidity at different temperatures
• At warm temperatures (such as 37°C),
cholesterol restrains movement of
phospholipids
• At cool temperatures, it maintains fluidity by
preventing tight packing
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-5c
Cholesterol
(c) Cholesterol within the animal cell membrane
Membrane Proteins and Their Functions
• A membrane is a collage of different proteins
embedded in the fluid matrix of the lipid
bilayer
• Proteins determine most of the membrane’s
specific functions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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
• Peripheral proteins are bound to the surface of
the membrane
• Integral proteins penetrate the hydrophobic
core, are called transmembrane proteins
• The hydrophobic regions of an integral protein
consist of one or more stretches of nonpolar
amino acids, often coiled into alpha helices
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-8
N-terminus
C-terminus
 Helix
EXTRACELLULAR
SIDE
CYTOPLASMIC
SIDE
• Six major functions of membrane proteins:
– Transport
– Enzymatic activity
– Signal transduction
– Cell-cell recognition
– Intercellular joining
– Attachment to the cytoskeleton and extracellular
matrix (ECM)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-9
Signaling molecule
Enzymes
ATP
(a) Transport
Receptor
Signal transduction
(b) Enzymatic activity
(c) Signal transduction
(e) Intercellular joining
(f) Attachment to
the cytoskeleton
and extracellular
matrix (ECM)
Glycoprotein
(d) Cell-cell recognition
Intercellular joining
E-selectin is a transmembrane protein expressed by endothelial cells that
binds to an oligosaccharide expressed on the surface of leukocytes
Construct a cell membrane
Try this at home!
constructing a cell membrane
The Role of Membrane Carbohydrates in Cell-Cell Recognition
• Cells recognize each other by binding to surface
molecules, usually carbohydrates
• Membrane carbohydrates may be covalently
bonded to lipids (forming glycolipids) or more
commonly to proteins (forming glycoproteins)
• Carbohydrates on the external side of the
plasma membrane vary among species,
individuals, and even cell types in an individual
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Membranes are bifacial
• Carbohydrates (making glycoproteins and
glycolipids) on outer surface
• Peripheral proteins generally on cytoplasmic
surface
• Proteins have a distinct orientation,
ie…receptor proteins oriented at surface,
enzyme proteins oriented toward cytoplasm
Synthesis and Sidedness of Membranes
• Membranes have distinct inside and outside
faces – determined when the membrane is
built by the ER and Golgi.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-10
ER
1
Transmembrane
glycoproteins
Secretory
protein
Glycolipid
Golgi
2
apparatus
Vesicle
3
4
Secreted
protein
Plasma membrane:
Cytoplasmic face
Extracellular face
Transmembrane
glycoprotein
Membrane glycolipid
Can you guess where you would find
this cell?
This cell?
Notice how thin-walled
they are.
Need a hint?
These cells?

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