Download Chapter 7 Membrane

Membrane Structure and
Function
Chapter 7
Overview: Life at the Edge
• The plasma membrane  selectively
permeable
Cell membranes
• Phospholipids most abundant lipids in plasma
membrane
• amphipathic = hydrophobic and hydrophilic
regions
– Polar head
– Hydrocarbon tails
• Phospholipid
Membrane Models: Scientific Inquiry
Hydrophilic
head
WATER
Hydrophobic
tail
WATER
• 1935Davson and Danielli - bilayer model
• 1972 Singer and Nicolson - fluid mosaic model
– membrane mosaic of proteins dispersed within bilayer
• Current model: mosaicism
TECHNIQUE
RESULTS
Extracellular
layer
Proteins
Knife
Plasma membrane
Inside of extracellular layer
Cytoplasmic layer
Inside of cytoplasmic layer
Proteins embedded in bilayer
Phospholipid
bilayer
Hydrophobic regions
of protein
Hydrophilic
regions of protein
The Fluidity of Membranes
Phospholipids in membrane move laterally within
bilayer
rarely flip flop
• Lipids, proteins, may move laterally
Lateral movement
(∼107 times per second)
(a) Movement of phospholipids
Flip-flop
(∼ once per month)
RESULTS
Membrane proteins
Mouse cell
Mixed proteins
after 1 hour
Human cell
Hybrid cell
Membrane fluidity affected by:
1. Type of phospholipid
Fluid
Unsaturated hydrocarbon
tails with kinks
Viscous
Saturated hydrocarbon tails
2. Temperature
• cool  gel
– Tightly packed tails
• warm  fluid
3. Cholesterol
• Stabilizes membrane fluidity with changing
temperature
FYI
Cholesterol can compose ½ of
the membrane
Bacterial cell membranes do not
contain cholesterol
Plant cells do not contain much
Cholesterol
(c) Cholesterol within the animal cell membrane
Membrane Proteins and Their Functions
• Mosaic of proteins embedded in lipid bilayer
• Proteins determine most of membrane’s
functions
Membrane Proteins
Peripheral proteins
– bound to _____________ of membrane
Integral proteins
– penetrate hydrophobic region
– Transmembrane proteins
Nterminus
C-terminus
1. Receptor proteins for signal transduction
• ex. Insulin receptor
2. Channel proteins for passage of molecules
hydrophilic core
• Hydrophilic core
• Ex. Aquaporins
3. Transport proteins
Ex. Glucose transporter shuttles glucose across
membrane
Signaling molecule
Enzymes
ATP
(a) Transport
Receptor
Signal transduction
(b) Enzymatic activity
(c) Signal transduction
Glycoprotein
(d) Cell-cell recognition
(e) Intercellular joining
(f) Attachment to
the cytoskeleton
and extracellular
matrix (ECM)
4. Cell-cell recognition
• Glycoproteins
– Carbohydrates attached to proteins
mucin
5. Intercellular joining proteins
Ex. gap junctions allow passage
of ions and small molecules
from cell to cell
6. Extracellular matrix proteins
7. Membrane enzymes
Six major functions of membrane proteins:
– Transport
– Enzymatic activity
– Signal transduction
– Cell-cell recognition
– Intercellular joining
– Attachment to the cytoskeleton and extracellular matrix
(ECM)
Selective permeability
• Plasma membrane regulates cell molecular
traffic
Permeability of Lipid Bilayer
• Hydrophobic molecules dissolve in bilayer and pass
through membrane rapidly
– O2, CO2, NO, steroids, nanoparticles
O2
CO2
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Hydrophilic (polar) molecules do not cross easily
– sugar, water, ions
Passive transport = no energy used
molecules move randomly
A. Diffusion
• Molecules diffuse down their concentration
gradient from high to lower concentration until
equilibrium
What molecules can diffuse across the cell membrane?
Answer: O2, CO2, urea
Net diffusion
Net diffusion
(b) Diffusion of two solutes
Net diffusion
Net diffusion
Equilibrium
Equilibrium
• B. Osmosis is diffusion of water across a
selectively permeable membrane
• Water diffuses across membrane from region of
higher water concentration to the region of
lower water concentration until equilbrium
Lower
concentration
of sugar)
Higher
Concentration
of sugar
Same concentration
of sugar
H2O
Selectively
permeable
membrane
Osmosis
Water Balance of Cells Without Walls
• Tonicity =ability of solution to cause cell to gain or
lose water
• Isotonic solution: Solute concentration same as in
cell
– no net water movement across membrane
• Hypertonic solution: Solute concentration greater
than inside cell
– cell loses water
• Hypotonic solution: Solute concentration is less
than inside cell
– cell gains water
Solution type? Isotonic, hypotonic, hypertonic?
Hypotonic solution
H2O
Isotonic solution
H2O
Hypertonic solution
H2O
H2O
(a) Animal
cell
Lysed
Normal
Shriveled
How do cells deal with changing external water concentrations?
• Osmoregulation= control of water balance
• Ex. Paramecium video
– pond water is _____________ to the protista
– contractile vacuole
Filling vacuole
50 µm
(a) A contractile vacuole fills with fluid that enters from
a system of canals radiating throughout the cytoplasm.
Contracting vacuole
(b) When full, the vacuole and canals contract, expelling
fluid from the cell.
Water Balance in plants (cell wall)
• Isotonic solution  no net movement of water
into cell
– flaccid (limp)
• Hypotonic solution  cell (vacuole) swells
– cell wall opposes uptake  turgid (firm)
• Hypertonic  lose water; membrane pulls
away from wall
– plasmolysis (lethal)
Hypotonic solution
H2O
Isotonic solution
H2O
H2O
Hypertonic solution
H2O
Flaccid
Plasmolyzed
(b) Plant
cell
Turgid (normal)
C. Facilitated Diffusion: Passive Transport aided by
proteins
• Channel proteins
• Carrier proteins
What molecules use facilitated diffusion to cross membrane?
Answer: glucose, sodium ions, chloride ions, water
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
(a) A channel protein
Carrier protein
(b) A carrier protein
Solute
Active transport
• Energy (ATP) required to move solutes against
their gradients (from lower to higher conc. !)
Pumps are membrane proteins
FYI:
 Ex. sodium-potassium pump (an enzyme)
 All animals
 Nobel prize 1997 (Jens Skou)
 Uses 1/3 of cells total energy production
 Provides driving force for other cell processes (secondary
transport, volume, gradients)
Examine the figure:
EXTRACELLULAR
FLUID
Na+
Na+ high outside cell
K+ low
Na+ low inside cell
K+ high
According to diffusion?
[Na+] high
[K+] low
Na+
CYTOPLASM
1
Na+
[Na+] low
[K+] high
Cytoplasmic Na+ binds to
the sodium-potassium pump.
1. Cytoplasmic Na+ binds
to pump
2. ADP phosphorylated
to ATP
Na+
Na+
Na+
What is ATP?
P
ADP
ATP
Na+ binding stimulates
phosphorylation by ATP.
2
+
Na
Na+
3. Na+ out of cell
+
Na
Na+
shape change of
pump
Against conc. grad.
+
Na
Na+
PP
Phosphorylation
Phosphorylation causes
causes
the
the protein
protein to
to change
change its
its
++ is expelled to
shape.
Na
shape. Na is expelled to
the
the outside.
outside.
33
4. Extracellular K+
binds to pump
ATP used
P
P
4 K+ binds on the
extracellular side and
triggers release of the
phosphate group.
5+6
K+  inside cell
Pump animation
Step by step
Shape change
Passive transport
Active transport
Review the
difference
passive vs
active transport
ATP
Diffusion
Facilitated diffusion
NO ATP IS USED
WHICH ONE REPRESENTS
THE CHANNEL PROTEIN?
Why do cells need pumps?
a. Maintain membrane potential = voltage
difference across membrane
Inside of cell more electronegative than out
= negative membrane potential
–
ATP
EXTRACELLULAR
FLUID
+
–
+
H+
H+
Proton pump
H+
–
+
H+
H+
–
+
CYTOPLASM
–
H+
+
b. Maintain electrochemical gradients
– chemical =
concentration
gradient
– electrical =
membrane
potential
Bulk transport
• Exocytosis
– To secrete products from cell
– Vesicles fuse with membrane
2. Endocytosis
cell takes in macromolecules by forming vesicles from
membrane
a. Phagocytosis – for large
particle
Vesicle fuses with lysosome
to digest particle
PHAGOCYTOSIS
EXTRACELLULAR
FLUID
1 µm
CYTOPLASM
Pseudopodium
Pseudopodium
of amoeba
“Food” or
other particle
Bacterium
Food
vacuole
Food vacuole
An amoeba engulfing a bacterium
via phagocytosis (TEM)

b. Pinocytosis – for fluids/small molecules
PINOCYTOSIS
0.5 µm
Plasma
membrane
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM)
Vesicle


c. receptor-mediated endocytosis,
ligand binds to receptor  vesicle
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated
vesicle
Coated
pit
Ligand