Membrane Structure & Function cont.
I. Membrane Protein Function
II. Cellular Transport
• Integral proteins
– span lipid bilayer
– called transmembrane proteins
– hydrophobic regions consist of one or more
stretches of nonpolar amino acids
– often coiled into alpha helices
– Visualize and draw membrane with transmembrane
protein containing 2 helices
LE 7-8
EXTRACELLULAR
SIDE
N-terminus
C-terminus
a Helix
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)
LE 7-9a
Signal
Enzymes
Receptor
ATP
Transport
Enzymatic activity
Signal transduction
LE 7-9b
Glycoprotein
Cell-cell recognition
Intercellular joining
Attachment to the
cytoskeleton and extracellular matrix (ECM)
The Role of Membrane Carbohydrates in CellCell Recognition
• Cells recognize each other by binding to surface
molecules, often carbohydrates, on the plasma
membrane
• Carbohydrates covalently bonded to lipids (glycolipids)
or more often to proteins (glycoproteins)
• Much variability of extracellular carbohydrates among
species, individuals, cell types in an individual
• Example of Pneumococcus
Synthesis and Sidedness of Membranes
• Membranes distinct inside and outside faces
• Plasma membrane is added to by vesicles from ER &
Golgi.
• Secreted and integral membrane proteins, lipids and
associated carbohydrates transported to membrane
by these vesicles.
LE 7-10
ER
Transmembrane
glycoproteins
Secretory
protein
Glycolipid
Golgi
apparatus
Vesicle
Plasma membrane:
Cytoplasmic face
Extracellular face
Secreted
protein
Transmembrane
glycoprotein
Plasma membrane:
Transport across cellular membranes
To exchange materials with surroundings in
part to take in nutrients and give off waste
Exchange(or transport) regulated:
selective permeability
Structure Dictates Membrane Permeability
• Hydrophobic (nonpolar) molecules cross
membrane rapidly
• e.g., hydrocarbons, oxygen, CO2 can dissolve in the lipid
bilayer and pass through the membrane rapidly
• Polar molecules cross slowly
• e.g. sugars, charged proteins, water
How do hydrophilic substances cross membranes?
With Help!
• Transport proteins
• Some create hydrophilic channels across membranes for
polar molecules or ions to pass through
Example: Aquaporin
water channel protein
Carrier proteins
binds solutes & change the shape of carrier
help to facilitate passage across membrane
highly specific for transported solutes
Examples: glucose transporter is a carrier protein for
glucose only
Transport Can be Passive or Active
LE 7-11a
Passive Transport: Diffusion
Molecules of dye
Membrane (cross section)
WATER
Net diffusion
Diffusion of one solute
Net diffusion
Equilibrium
• Substances diffuse down their concentration gradient
High to low
• Substances reach dynamic equilibrium
• No work (no added energy) required
LE 7-11b
Net diffusion
Net diffusion
Diffusion of two solutes
Net diffusion
Net diffusion
Equilibrium
Equilibrium
Effects of Osmosis on Water Balance
• Osmosis
– diffusion of water across a selectively permeable membrane
• Diffuses across a membrane from the region of lower
solute (such as an ion) concentration to the region of
higher solute concentration
• The direction of osmosis is determined only by a
difference in total solute concentration
LE 7-12
Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
H2O
Selectively
permeable membrane: sugar molecules cannot pass
through pores, but
water molecules can
Osmosis
Same concentration
of sugar
Water Balance of Cells Without Walls
• Tonicity
– ability of a solution to cause a cell to gain or lose water
• Isotonic solution
solute concentration is equal inside and
outside the cell --> no net water movement
cell remains same size
Hypertonic solution
external solute concentration is
greater than that inside the cell-->
cell loses water
Hypotonic solution
external solute concentration is less than that
inside the cell--> cell gains water
May expand enough to burst!
LE 7-13
Hypotonic solution
Isotonic solution
Hypertonic solution
Animal
cell
H2O
H2O
Turgid (normal)
H2O
H2O
Flaccid
H2O
Shriveled
Normal
Lysed
Plant
cell
H2O
H2O
H2O
Plasmolyzed
Water Balance of Cells with Walls vs No Walls
• Cell walls help maintain water balance
• Plant cell in hypotonic solution swells -->turgid (firm)
• Animal cell?
• Plant cell and its surroundings isotonic--> no net water
movemen; the cell becomes flaccid (limp), and the plant
may wilt
• Animal cell?
• In hypertonic environment, plant cells lose water-->
membrane pulls away from the wall: plasmolysis
• Lethal
• Animal cell?
Passive Transport Aided by Proteins
• Facilitated diffusion
– transport proteins speed movement of molecules
across the plasma membrane
– Channel proteins
– Carrier proteins
LE 7-15a
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
LE 7-15b
Carrier protein
Solute
Active transport
uses energy to move solutes against their gradients
•Requires energy, usually ATP
•Performed by specific membrane proteins
•Example
sodium-potassium pump
LE 7-16
EXTRACELLULAR [Na+] high
FLUID
[K+] low
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
CYTOPLASM
[Na+] low
[K+] high
Na+
Cytoplasmic Na+ bonds to
the sodium-potassium pump
P
ATP
P
ADP
Na+ binding stimulates
phosphorylation by ATP.
Phosphorylation causes
the protein to change its
conformation, expelling Na+
to the outside.
Loss of the phosphate
restores the protein’s
original conformation.
K+ is released and Na+
sites are receptive again;
the cycle repeats.
P
P
Extracellular K+ binds
to the protein, triggering
release of the phosphate
group.
LE 7-17
Passive transport
Active transport
ATP
Diffusion
Facilitated diffusion
• Electrogenic pumps
•
is a transport protein that generates a voltage across a
membrane--> opposite charges across membrane
(membrane potential)
• Example: In animals, Na-K pump
• In plant fungi and bacteria, proton pump
– Requires ATP (active transport)
LE 7-18
–
–
ATP
EXTRACELLULAR
FLUID
+
+
H+
H+
Proton pump
H+
–
+
H+
H+
–
+
CYTOPLASM
–
H+
+
Cotransport
Coupled Transport by a Membrane Protein
When active transport of one solute indirectly drives
transport of another
Example
Plants commonly use the proton gradient generated
by proton pumps to drive transport of nutrients into the
cell
LE 7-19
–
+
H+
ATP
H+
–
+
H+
Proton pump
H+
–
+
H+
–
+
H+
Sucrose-H+
cotransporter
Diffusion
of H+
H+
–
–
+
+
Sucrose
How do large molecules move in and out of cells?
• Small molecules and water enter or leave the cell
through the lipid bilayer or by transport proteins
• Large molecules, such as polysaccharides and
proteins, cross the membrane via vesicles
Exocytosis
• Transport vesicles with cargo migrate to the membrane,
fuse with it, and are release contents
• Example:
– Many secretory cells use exocytosis to export their products
– Pancreatic cells (beta-cells) secrete insulin
LE 7-10
ER
Transmembrane
glycoproteins
Secretory
protein
Glycolipid
Golgi
apparatus
Vesicle
Plasma membrane:
Cytoplasmic face
Extracellular face
Secreted
protein
Transmembrane
glycoprotein
Plasma membrane:
Endocytosis
• Cell takes in macromolecules by forming
vesicles at the plasma membrane
• Reversal of exocytosis, involving different
proteins
• Three types of endocytosis
– Phagocytosis (“cellular eating”): Cell engulfs
particle in a vacuole
– Pinocytosis (“cellular drinking”): Cell creates
vesicle around fluid
– Receptor-mediated endocytosis: Binding of
ligands to receptors triggers vesicle formation
LE 7-20c
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated
vesicle
Coated
pit
Ligand
A coated pit
and a coated
vesicle formed
during
receptormediated
endocytosis
(TEMs).
Coat
protein
Plasma
membrane
0.25 µm