5
Cell Membranes and
Signaling
Chapter 5 Cell Membranes and Signaling
Key Concepts
• 5.1 Biological Membranes Have a
Common Structure and Are Fluid
• 5.2 Some Substances Can Cross the
Membrane by Diffusion
• 5.3 Some Substances Require Energy to
Cross the Membrane
Chapter 5 Cell Membranes and Signaling
• 5.4 Large Molecules Cross the
Membrane via Vesicles
• 5.5 The Membrane Plays a Key Role in a
Cell’s Response to Environmental
Signals
• 5.6 Signal Transduction Allows the Cell
to Respond to Its Environment
Concept 5.1 Biological Membranes Have a Common Structure
and Are Fluid
A membrane’s structure and functions
are determined by its constituents:
lipids, proteins, and carbohydrates.
The general structure of membranes is
known as the fluid mosaic model.
Phospholipids form a bilayer which is like
a “lake” in which a variety of proteins
“float.”
Figure 5.1 Membrane Molecular Structure
Concept 5.1 Biological Membranes Have a Common Structure
and Are Fluid
Lipids form the hydrophobic core of the
membrane.
Most lipid molecules are phospholipids with two
regions:
• Hydrophilic regions—electrically charged
“heads” that associate with water molecules
• Hydrophobic regions—nonpolar fatty acid
“tails” that do not dissolve in water
Concept 5.1 Biological Membranes Have a Common Structure
and Are Fluid
Membranes may differ in lipid composition as
there are many types of phospholipids.
Phospholipids may differ in:
• Fatty acid chain length
• Degree of saturation
• Kinds of polar groups present
Concept 5.1 Biological Membranes Have a Common Structure
and Are Fluid
Two important factors in membrane fluidity:
• Lipid composition—types of fatty acids can
increase or decrease fluidity
• Temperature—membrane fluidity decreases
in colder conditions
Concept 5.1 Biological Membranes Have a Common Structure
and Are Fluid
Biological membranes contain proteins, with
varying ratios of phospholipids.
• Peripheral membrane proteins lack
hydrophobic groups and are not embedded
in the bilayer.
• Integral membrane proteins are partly
embedded in the phospholipid bilayer.
Concept 5.1 Biological Membranes Have a Common Structure
and Are Fluid
Anchored membrane proteins have lipid
components that anchor them in the bilayer.
Proteins are asymmetrically distributed on the
inner and outer membrane surfaces.
A transmembrane protein extends through
the bilayer on both sides, and may have
different functions in its external and
transmembrane domains.
Concept 5.1 Biological Membranes Have a Common Structure
and Are Fluid
Plasma membrane carbohydrates are
located on the outer membrane and can
serve as recognition sites.
• Glycolipid—a carbohydrate bonded to a
lipid
• Glycoprotein—a carbohydrate bonded
to a protein
Concept 5.2 Some Substances Can Cross the Membrane by
Diffusion
Biological membranes allow some
substances, and not others, to pass.
This is known as selective
permeability.
Two processes of transport:
• Passive transport does not require
metabolic energy.
• Active transport requires input of
metabolic energy.
Concept 5.2 Some Substances Can Cross the Membrane by
Diffusion
Passive transport of a substance can
occur through two types of diffusion:
• Simple diffusion through the
phospholipid bilayer
• Facilitated diffusion through channel
proteins or aided by carrier proteins
Concept 5.2 Some Substances Can Cross the Membrane by
Diffusion
Diffusion is the process of random
movement toward equilibrium.
Speed of diffusion depends on three
factors:
• Diameter of the molecules—smaller
molecules diffuse faster
• Temperature of the solution—higher
temperatures lead to faster diffusion
Concept 5.2 Some Substances Can Cross the Membrane by
Diffusion
Simple diffusion takes place through the
phospholipid bilayer.
A molecule that is hydrophobic and
soluble in lipids can pass through the
membrane.
Polar molecules do not pass through—
they are not soluble in the hydrophilic
interior and form bonds instead in the
aqueous environment near the
membrane.
Concept 5.2 Some Substances Can Cross the Membrane by
Diffusion
Osmosis is the diffusion of water across
membranes.
It depends on the concentration of solute
molecules on either side of the
membrane.
Water passes through special membrane
channels.
Concept 5.2 Some Substances Can Cross the Membrane by
Diffusion
When comparing two solutions separated
by a membrane:
• A hypertonic solution has a higher
solute concentration.
• Isotonic solutions have equal solute
concentrations.
• A hypotonic solution has a lower solute
concentration.
Figure 5.3A Osmosis Can Modify the Shapes of Cells
Figure 5.3B Osmosis Can Modify the Shapes of Cells
Figure 5.3C Osmosis Can Modify the Shapes of Cells
Concept 5.2 Some Substances Can Cross the Membrane by
Diffusion
The concentration of solutes in the
environment determines the direction of
osmosis in all animal cells.
In other organisms, cell walls limit the
volume that can be taken up.
Turgor pressure is the internal pressure
against the cell wall—as it builds up, it
prevents more water from entering.
Concept 5.2 Some Substances Can Cross the Membrane by
Diffusion
Diffusion may be aided by channel
proteins.
Channel proteins are integral membrane
proteins that form channels across the
membrane.
Substances can also bind to carrier
proteins to speed up diffusion.
Both are forms of facilitated diffusion.
Concept 5.2 Some Substances Can Cross the Membrane by
Diffusion
Ion channels are a type of channel
protein—most are gated, and can be
opened or closed to ion passage.
A gated channel opens when a stimulus
causes the channel to change shape.
The stimulus may be a ligand, a
chemical signal.
Concept 5.2 Some Substances Can Cross the Membrane by
Diffusion
A ligand-gated channel responds to its
ligand.
A voltage-gated channel opens or closes
in response to a change in the voltage
across the membrane.
Figure 5.4 A Ligand-Gated Channel Protein Opens in Response to a Stimulus
Concept 5.3 Some Substances Require Energy to Cross the
Membrane
Active transport requires the input of
energy to move substances against their
concentration gradients.
Active transport is used to overcome
concentration imbalances that are
maintained by proteins in the
membrane.
Concept 5.3 Some Substances Require Energy to Cross the
Membrane
The energy source for active transport is
often ATP.
Active transport is directional and moves
a substance against its concentration
gradient.
A substance moves in the direction of the
cell’s needs, usually by means of a
specific carrier protein.
Concept 5.3 Some Substances Require Energy to Cross the
Membrane
Two types of active transport:
• Primary active transport involves
hydrolysis of ATP for energy.
• Secondary active transport uses the
energy from an ion concentration
gradient, or an electrical gradient.
Concept 5.3 Some Substances Require Energy to Cross the
Membrane
The sodium–potassium (Na+–K+) pump
is an integral membrane protein that
pumps Na+ out of a cell and K+ in.
One molecule of ATP moves two K+ and
three Na+ ions.
Figure 5.7 Primary Active Transport: The Sodium–Potassium Pump
Concept 5.3 Some Substances Require Energy to Cross the
Membrane
Secondary active transport uses energy
that is “regained,” by letting ions move
across the membrane with their
concentration gradients.
Secondary active transport may begin
with passive diffusion of a few ions, or
may involve a carrier protein that
transports both a substance and ions.
Concept 5.4 Large Molecules Cross the Membrane via Vesicles
Macromolecules are too large or too
charged to pass through biological
membranes and instead pass through
vesicles.
To take up or to secrete macromolecules,
cells must use endocytosis or
exocytosis.
Figure 5.8 Endocytosis and Exocytosis (Part 1)
Figure 5.8 Endocytosis and Exocytosis (Part 2)
Concept 5.4 Large Molecules Cross the Membrane via Vesicles
Three types of endocytosis brings
molecules into the cell: phagocytosis,
pinocytosis, and receptor–mediated
endocytosis.
In all three, the membrane invaginates, or
folds around the molecules and forms a
vesicle.
The vesicle then separates from the
membrane.
Concept 5.4 Large Molecules Cross the Membrane via Vesicles
In phagocytosis (“cellular eating”), part
of the membrane engulfs a large particle
or cell.
A food vacuole (phagosome) forms and
usually fuses with a lysosome, where
contents are digested.
Concept 5.4 Large Molecules Cross the Membrane via Vesicles
In pinocytosis (“cellular drinking”),
vesicles also form.
The vesicles are smaller and bring in
fluids and dissolved substances, as in
the endothelium near blood vessels.
Concept 5.4 Large Molecules Cross the Membrane via Vesicles
Receptor–mediated endocytosis
depends on receptors to bind to
specific molecules (their ligands).
The receptors are integral membrane
proteins located in regions called coated
pits.
The cytoplasmic surface is coated by
another protein (often clathrin).
Concept 5.4 Large Molecules Cross the Membrane via Vesicles
When receptors bind to their ligands, the
coated pit invaginates and forms a
coated vesicle.
The clathrin stabilizes the vesicle as it
carries the macromolecules into the
cytoplasm.
Once inside, the vesicle loses its clathrin
coat and the substance is digested.
Figure 5.9 Receptor-Mediated Endocytosis (Part 1)
Figure 5.9 Receptor-Mediated Endocytosis (Part 2)
Concept 5.4 Large Molecules Cross the Membrane via Vesicles
Exocytosis moves materials out of the
cell in vesicles.
The vesicle membrane fuses with the
plasma membrane and the contents are
released into the cellular environment.
Exocytosis is important in the secretion of
substances made in the cell.
Synopsis of Cellular Transport
Bozeman Biology
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Cells can respond to many signals if they
have a specific receptor for that signal.
A signal transduction pathway is a
sequence of molecular events and
chemical reactions that lead to a cellular
response, following the receptor’s
activation by a signal.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Cells are exposed to many signals and
may have different responses:
• Autocrine signals affect the same cells
that release them.
• Paracrine signals diffuse to and affect
nearby cells.
• Hormones travel to distant cells.
Figure 5.10 Chemical Signaling Concepts
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Only cells with the necessary receptors
can respond to a signal—the target cell
must be able to sense it and respond to
it.
A signal transduction pathway involves a
signal, a receptor, and a response.
Figure 5.11 Signal Transduction Concepts
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
A common mechanism of signal
transduction is allosteric regulation.
This involves an alteration in a protein’s
shape as a result of a molecule binding
to it.
A signal transduction pathway may
produce short or long term responses.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
A signal molecule, or ligand, fits into a
three-dimensional site on the receptor
protein.
Binding of the ligand causes the receptor
to change its three-dimensional shape.
The change in shape initiates a cellular
response.
Figure 5.12 A Signal Binds to Its Receptor
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Ligands are generally not metabolized
further, but their binding may expose an
active site on the receptor.
Binding is reversible and the ligand can
be released, to end stimulation.
An inhibitor, or antagonist, can bind in
place of the normal ligand.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Receptors can be classified by their
location in the cell.
This is determined by whether or not their
ligand can diffuse through the
membrane.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Cytoplasmic receptors have ligands, such
as estrogen, that are small or nonpolar
and can diffuse across the membrane.
Membrane receptors have large or polar
ligands, such as insulin, that cannot
diffuse and must bind to a
transmembrane receptor at an
extracellular site.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Receptors are also classified by their
activity:
• Ion channel receptors
• Protein kinase receptors
• G protein–linked receptors
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Ion channel receptors, or gated ion
channels, change their threedimensional shape when a ligand binds.
The acetylcholine receptor, a ligandgated sodium channel, binds
acetylcholine to open the channel and
allow Na+ to diffuse into the cell.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Protein kinase receptors change their
shape when a ligand binds.
The new shape exposes or activates a
cytoplasmic domain that has catalytic
(protein kinase) activity.(cascade
phosphorylation)
Figure 5.13 A Protein Kinase Receptor
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Protein kinases catalyze the following
reaction:
ATP + protein  ADP + phosphorylated
protein
Each protein kinase has a specific target
protein, whose activity is changed when
it is phosphorylated.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Ligands binding to G protein–linked
receptors expose a site that can bind to
a membrane protein, a G protein.
The G protein is partially inserted in the
lipid bilayer, and partially exposed on
the cytoplasmic surface.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Many G proteins have three subunits and
can bind three molecules:
• The receptor
• GDP and GTP, used for energy transfer
• An effector protein to cause an effect in
the cell
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
The activated G protein–linked receptor
exchanges a GDP nucleotide bound to
the G protein for a higher energy GTP.
The activated G protein activates the
effector protein, leading to signal
amplification.
Figure 5.14 A G Protein–Linked Receptor
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
Signal activation of a specific receptor
leads to a cellular response, which is
mediated by a signal transduction
pathway.
Signaling can initiate a cascade of protein
interactions—the signal can then be
amplified and distributed to cause
different responses.
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
A second messenger is an intermediary
between the receptor and the cascade
of responses.
In the fight-or-flight response, epinephrine
(adrenaline) activates the liver enzyme
glycogen phosphorylase.
The enzyme catalyzes the breakdown of
glycogen to provide quick energy.
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
Researchers found that the cytoplasmic
enzyme could be activated by the
membrane-bound epinephrine in broken
cells, as long as all parts were present.
They discovered that another molecule
delivered the message from the “first
messenger,” epinephrine, to the
enzyme.
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
The second messenger was later
discovered to be cyclic AMP (cAMP).
Second messengers allow the cell to
respond to a single membrane event
with many events inside the cell—they
distribute the signal.
They amplify the signal by activating
more than one enzyme target.
Figure 5.16 The Formation of Cyclic AMP
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
Signal transduction pathways involve
multiple steps—enzymes may be either
activated or inhibited by other enzymes.
In liver cells, a signal cascade begins
when epinephrine stimulates a G
protein–mediated protein kinase
pathway.
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
Epinephrine binds to its receptor and
activates a G protein.
cAMP is produced and activates protein
kinase A—it phosphorylates two other
enzymes, with opposite effects:
• Inhibition
• Activation
Figure 5.17 A Cascade of Reactions Leads to Altered Enzyme Activity (Part 1)
Figure 5.17 A Cascade of Reactions Leads to Altered Enzyme Activity (Part 2)
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
• Inhibition—protein kinase A inactivates
glycogen synthase through
phosphorylation, and prevents glucose
storage.
• Activation—Phosphorylase kinase is
activated when phosphorylated and is
part of a cascade that results in the
liberation of glucose molecules.
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
Signal transduction ends after the cell
responds—enzymes convert each
transducer back to its inactive precursor.
The balance between the regulating
enzymes and the signal enzymes
determines the cell’s response.
Figure 5.18 Signal Transduction Regulatory Mechanisms
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
Cells can alter the balance of enzymes in
two ways:
• Synthesis or breakdown of the enzyme
• Activation or inhibition of the enzymes
by other molecules
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
Cell functions change in response to
environmental signals:
• Opening of ion channels
• Alterations in gene expression
• Alteration of enzyme activities