Membrane Structure and Function •The plasma membrane is the ...

Ch 7- Membrane Structure and Function
Goals for today’s lecture:
Learn about the structure of cellular membranes
-Understand osmosis and water balance in cells
-Understand passive vs. facilitated diffusion
-Learn about the traffic of large molecules
-exocytosis and endocytosis
•The plasma membrane is the boundary that
separates the living cell from its nonliving
surroundings
•The plasma membrane exhibits selective
permeability, allowing some substances to cross it
more easily than others
A Fluid Mosaic of Lipids and Proteins
•The membranes of cells are composed of
Hydrophilic
head
Hydrophobic
tail
Phospholipid
bilayer
WATER
WATER
Hydrophilic region
of protein
Hydrophobic region of protein
•This behavior leads to the description of a
membrane as a fluid mosaic
Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)
Movement of phospholipids
Membrane Proteins and Their Functions
•Proteins determine most of the membrane’s
specific functions
Glycoprotein
Peripheral proteins
EXTRACELLULAR
SIDE OF
MEMBRANE
Integral proteins penetrate the
hydrophobic core and often span the membrane
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE

Membrane Proteins and Their Functions
•Six major functions of membrane proteins:
Fibers of
extracellular
matrix
c
Cytoplasm
Enzymatic activity
b
Cell signaling
a
Attachment to
cytoskeleton and
extracellular
matrix
Cytoskeleton
d
e
Intercellular f
joining Cell-cell
Transport
recognition
Cytoplasm
Figure
The Role of Membrane Carbohydrates in Cell-Cell Recognition
Cell-cell recognition
Enzymes
Signal
•Cells recognize each other by binding to surface molecules,
often carbohydrates, on the plasma membrane
ATP
Receptor
Transport Enzymatic activity Signal transduction
Membrane structure results in selective permeability
A cell must exchange materials with its surroundings, a
process controlled by the plasma membrane
Plasma membranes are selectively permeable, regulating
the cell’s molecular traffic
Glycoprotein
Hydrophobic (nonpolar) molecules
Cell-cell recognition Intercellular joining Attachment to the
cytoskeleton and extracellular
matrix (ECM)
Polar molecules
Passive transport is diffusion of a substance across
a membrane with no energy investment
Diffusion
Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one
direction
Diffusion across a membrane is called passive transport.
Passive because the cell does not expend any energy for passive transport to happen.

Effects of Osmosis on Water Balance
Osmosis
Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
Same concentration
of sugar
H 2 O
The direction of osmosis is determined only by a
difference in total solute concentration
Water diffuses across a membrane from the region of
lower solute concentration to the region of higher solute
concentration
Selectively
permeable membrane:
sugar molecules
cannot pass
through pores, but
water molecules can
Osmosis
Tonicity
hypertonic
hypotonic
isotonic
Water Balance in Animal Cells
•Animals and other organisms without rigid cell walls
have osmotic problems in either a hypertonic or
hypotonic environment
osmoregulation

Water Balance in Animal Cells
•To maintain their internal environment, such organisms must
have adaptations for osmoregulation, the control of water
balance
Filling vacuole
The protist Paramecium
Contracting vacuole
Water Balance in Plant Cells
Water balance problems are
somewhat different for plant
cells because of their rigid cell
wall.
Facilitated Diffusion: Passive Transport Aided by Proteins
•In facilitated diffusion, transport proteins speed
movement of molecules across the plasma membrane
EXTRACELLULAR
FLUID
Channel proteins
Channel protein
Solute
CYTOPLASM
Carrier proteins
Carrier protein
Solute

Active Transport: The Pumping of Molecules
Across the Membrane
In contrast to passive transport, active transport requires
that a cell expend energy to move molecules across the
membrane.
In active transport, a specific transport protein
actively pumps a solute across the cell membrane
against the solute’s concentration gradient.
The Sodium-Potassium Pump
EXTRACELLULAR
FLUID
[Na + ] high
[K + ] low
Na +
Na +
Na +
Na +
Na +
Na +
Na +
Na +
CYTOPLASM Na+
[Na + ] low
[K + ] high
Cytoplasmic Na + bonds to
the sodium-potassium pump
P
ADP
ATP
Phosphorylation causes
the protein to change its
conformation, expelling Na +
to the outside.
P
P
P
Loss of the phosphate
restores the protein’s
original conformation.
K + is released and Na +
sites are receptive again;
the cycle repeats.
Maintenance of Membrane Potential by Ion Pumps
•Membrane potential
–
+
EXTRACELLULAR
FLUID
Two combined forces, collectively called the
electrochemical gradient, drive the diffusion of ions across
a membrane:
A chemical force (the ion’s concentration
gradient)
An electrical force (the effect of the membrane
potential on the ion’s movement)
ATP
H +
–
CYTOPLASM
–
Proton pump
+
+
+
H +
H +
H +
H +
H +
An electrogenic pump
The sodium-potassium pump is the major electrogenic pump of animal cells.
The pumping of H+ transfers positive charge from the cytoplasm to the extracellular solution.
By generating voltage across the membranes, electrogenic pumps store energy for cellular work.

Cotransport: Coupled Transport by a Membrane Protein
Cotransport
ATP
H+
Proton pump
•A substance that has been pumped across a membrane can do work as
it moves back through diffusion.
•Plants commonly use the gradient of hydrogen ions
generated by proton pumps to drive active transport
of nutrients into the cell
Sucrose-H+
cotransporter
Sucrose
Diffusion
of H+
Bulk transport across the plasma membrane occurs by exocytosis and
endocytosis
•Small molecules and water enter or leave the cell through the lipid bilayer
or by transport proteins
Exocytosis
During protein production by the cell, secretory proteins exit
the cell in transport vesicles that fuse with the plasma membrane,
and spill the contents outside the cell.
Endocystosis
Three types of endocytosis
Phagocytosis (“cellular eating”)
Pinocytosis (“cellular drinking”)
Plasma
membrane
Vesicle
Pinocytosis
vesicles forming
(arrows) in a cell
lining a small
blood vessel
(TEM).
Receptor
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Coated
vesicle
Receptor-mediated endocytosis
An example is the mechanism human liver
cells use to take up cholesterol particles that
circulate in the blood.
Ligand
Coated
pit
Coat
protein
Plasma
membrane 0.25 µm
A coated pit
and a coated
vesicle formed
during
receptormediated
endocytosis
(TEMs).

Study outline-Chapter 7-Membrane Structure and Function
Understand structure of membranes and the Fluid Mosaic Model
Recognize the following parts of the Fluid Mosaic Model
-phospholipid bilayer
-hydrophobic tail
-hydrophilic head
-membrane proteins
integral proteins
peripheral proteins
Know the functions of membrane proteins (7.10)
-attachment to cytoskeleton
-cell signaling
-enzymatic activity
-transport
-intercellular joining
-cell-cell recognition
Permeability of the lipid bilayer
Understand the permeability of hydrophobic (nonpolar) molecules
vs. polar molecules and how permeability relates to fluidity of the membrane
Passive transport
-diffusion (Fig. 7.13a)
-osmosis (Fig. 7.14)
-facilitated transport
channel proteins
carrier proteins
Tonicity
Understand hypertonic, isotonic, and hypotonic in plant and animal cells (Fig. 7.15)
Active transport
Examples – sodium-potassium pump
proton pump- electrochemical gradient
cotransport
Bulk flow
Exocytosis vs Endocytosis
Three types of endocytosis (Fig. 7.22)
-phagocytosis
-pinocytosis
-receptor-mediated endocytosis