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Cell Membranes and
Signaling
5
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
Chapter 5 Opening Question
What role does the cell membrane play in
the body’s response to caffeine?
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
A bilayer is formed when the fatty acid “tails”
associate with each other and the polar
“heads” face the aqueous environment.
Bilayer organization helps membranes fuse
during vesicle formation and phagocytosis.
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
Some membrane proteins can move within
the phosopholipid bilayer, while others are
restricted.
Proteins inside the cell can restrict movement
of membrane proteins, as can attachments
to the cytoskeleton.
Figure 5.2 Rapid Diffusion of Membrane Proteins
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 5.1 Biological Membranes Have a Common Structure
and Are Fluid
Membranes are constantly changing by
forming, transforming into other types,
fusing, and breaking down.
Though membranes appear similar, there
are major chemical differences among
the membranes of even a single cell.
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
• The concentration gradient in the
system—the greater the concentration
gradient in a system, the faster a
substance will diffuse
A higher concentration inside the cell
causes the solute to diffuse out, and a
higher concentration outside causes the
solute to diffuse in, for many molecules.
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 5.2 Some Substances Can Cross the Membrane by
Diffusion
Water crosses membranes at a faster
rate than simple diffusion.
It may “hitchhike” with ions such as Na+
as
they pass through channels.
Aquaporins are specific channels that
allow large amounts of water to move
along its concentration gradient.
Figure 5.5 Aquaporins Increase Membrane Permeability to Water (Part 1)
Figure 5.5 Aquaporins Increase Membrane Permeability to Water (Part 2)
Concept 5.2 Some Substances Can Cross the Membrane by
Diffusion
Carrier proteins in the membrane
facilitate diffusion by binding
substances.
Glucose transporters are carrier proteins
in mammalian cells.
Glucose molecules bind to the carrier
protein and cause the protein to change
shape—it releases glucose on the other
side of the membrane.
Figure 5.6 A Carrier Protein Facilitates Diffusion (Part 1)
Figure 5.6 A Carrier Protein Facilitates Diffusion (Part 2)
Concept 5.2 Some Substances Can Cross the Membrane by
Diffusion
Transport by carrier proteins differs from
simple diffusion, though both are driven
by the concentration gradient.
The facilitated diffusion system can
become saturated—when all of the
carrier molecules are bound, the rate of
diffusion reaches its maximum.
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.
Table 5.1 Membrane Transport Mechanisms
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.
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 three-
dimensional shape when a ligand binds.
The acetylcholine receptor, a ligand-
gated 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.
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.
Figure 5.15 The Discovery of a Second Messenger (Part 1)
Figure 5.15 The Discovery of a Second Messenger (Part 2)
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
Answer to Opening Question
Caffeine is a large, polar molecule that
binds to receptors on nerve cells in the
brain.
Its structure is similar to adenosine,
which binds to receptors after activity or
stress and results in drowsiness.
Caffeine binds to the same receptor, but
does not activate it—the result is that
the person remains alert.
Figure 5.19 Caffeine and the Cell Membrane (Part 1)
Figure 5.19 Caffeine and the Cell Membrane (Part 2)

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AP Biology Chapter 5 Cell Membranes and Signalling

  • 2. 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
  • 3. 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
  • 4. Chapter 5 Opening Question What role does the cell membrane play in the body’s response to caffeine?
  • 5. 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.”
  • 6. Figure 5.1 Membrane Molecular Structure
  • 7. 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
  • 8. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid A bilayer is formed when the fatty acid “tails” associate with each other and the polar “heads” face the aqueous environment. Bilayer organization helps membranes fuse during vesicle formation and phagocytosis.
  • 9. 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
  • 10. 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
  • 11. 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.
  • 12. 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.
  • 13. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Some membrane proteins can move within the phosopholipid bilayer, while others are restricted. Proteins inside the cell can restrict movement of membrane proteins, as can attachments to the cytoskeleton.
  • 14. Figure 5.2 Rapid Diffusion of Membrane Proteins
  • 15. 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
  • 16. Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid Membranes are constantly changing by forming, transforming into other types, fusing, and breaking down. Though membranes appear similar, there are major chemical differences among the membranes of even a single cell.
  • 17. 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.
  • 18. 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
  • 19. 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
  • 20. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion • The concentration gradient in the system—the greater the concentration gradient in a system, the faster a substance will diffuse A higher concentration inside the cell causes the solute to diffuse out, and a higher concentration outside causes the solute to diffuse in, for many molecules.
  • 21. 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.
  • 22. 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.
  • 23. 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.
  • 24. Figure 5.3A Osmosis Can Modify the Shapes of Cells
  • 25. Figure 5.3B Osmosis Can Modify the Shapes of Cells
  • 26. Figure 5.3C Osmosis Can Modify the Shapes of Cells
  • 27. 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.
  • 28. 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.
  • 29. 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.
  • 30. 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.
  • 31. Figure 5.4 A Ligand-Gated Channel Protein Opens in Response to a Stimulus
  • 32. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Water crosses membranes at a faster rate than simple diffusion. It may “hitchhike” with ions such as Na+ as they pass through channels. Aquaporins are specific channels that allow large amounts of water to move along its concentration gradient.
  • 33. Figure 5.5 Aquaporins Increase Membrane Permeability to Water (Part 1)
  • 34. Figure 5.5 Aquaporins Increase Membrane Permeability to Water (Part 2)
  • 35. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Carrier proteins in the membrane facilitate diffusion by binding substances. Glucose transporters are carrier proteins in mammalian cells. Glucose molecules bind to the carrier protein and cause the protein to change shape—it releases glucose on the other side of the membrane.
  • 36. Figure 5.6 A Carrier Protein Facilitates Diffusion (Part 1)
  • 37. Figure 5.6 A Carrier Protein Facilitates Diffusion (Part 2)
  • 38. Concept 5.2 Some Substances Can Cross the Membrane by Diffusion Transport by carrier proteins differs from simple diffusion, though both are driven by the concentration gradient. The facilitated diffusion system can become saturated—when all of the carrier molecules are bound, the rate of diffusion reaches its maximum.
  • 39. 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.
  • 40. Table 5.1 Membrane Transport Mechanisms
  • 41. 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.
  • 42. 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.
  • 43. 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.
  • 44. Figure 5.7 Primary Active Transport: The Sodium–Potassium Pump
  • 45. 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.
  • 46. 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.
  • 47. Figure 5.8 Endocytosis and Exocytosis (Part 1)
  • 48. Figure 5.8 Endocytosis and Exocytosis (Part 2)
  • 49. 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.
  • 50. 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.
  • 51. 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.
  • 52. 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).
  • 53. 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.
  • 54. Figure 5.9 Receptor-Mediated Endocytosis (Part 1)
  • 55. Figure 5.9 Receptor-Mediated Endocytosis (Part 2)
  • 56. 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.
  • 57. 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.
  • 58. 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.
  • 59. Figure 5.10 Chemical Signaling Concepts
  • 60. 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.
  • 61. Figure 5.11 Signal Transduction Concepts
  • 62. 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.
  • 63. 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.
  • 64. Figure 5.12 A Signal Binds to Its Receptor
  • 65. 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.
  • 66. 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.
  • 67. 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.
  • 68. 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
  • 69. 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 three- dimensional shape when a ligand binds. The acetylcholine receptor, a ligand- gated sodium channel, binds acetylcholine to open the channel and allow Na+ to diffuse into the cell.
  • 70. 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.
  • 71. Figure 5.13 A Protein Kinase Receptor
  • 72. 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.
  • 73. 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.
  • 74. 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
  • 75. 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.
  • 76. Figure 5.14 A G Protein–Linked Receptor
  • 77. 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.
  • 78. 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.
  • 79. 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.
  • 80. Figure 5.15 The Discovery of a Second Messenger (Part 1)
  • 81. Figure 5.15 The Discovery of a Second Messenger (Part 2)
  • 82. 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.
  • 83. Figure 5.16 The Formation of Cyclic AMP
  • 84. 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.
  • 85. 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
  • 86. Figure 5.17 A Cascade of Reactions Leads to Altered Enzyme Activity (Part 1)
  • 87. Figure 5.17 A Cascade of Reactions Leads to Altered Enzyme Activity (Part 2)
  • 88. 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.
  • 89. 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.
  • 90. Figure 5.18 Signal Transduction Regulatory Mechanisms
  • 91. 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
  • 92. 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
  • 93. Answer to Opening Question Caffeine is a large, polar molecule that binds to receptors on nerve cells in the brain. Its structure is similar to adenosine, which binds to receptors after activity or stress and results in drowsiness. Caffeine binds to the same receptor, but does not activate it—the result is that the person remains alert.
  • 94. Figure 5.19 Caffeine and the Cell Membrane (Part 1)
  • 95. Figure 5.19 Caffeine and the Cell Membrane (Part 2)

Editor's Notes

  1. VIDEO 5.1 Cell Visualization: Membranes, hormones, and receptors
  2. LINK Review the properties of phospholipid bilayers in Concept 2.4
  3. APPLY THE CONCEPT Biological membranes have a common structure and are fluid INTERACTIVE TUTORIAL 5.1 Lipid Bilayer: Temperature Effects on Composition
  4. See Figure 5.1
  5. ANIMATED TUTORIAL 5.1 Passive Transport
  6. ANIMATED TUTORIAL 5.2 Active Transport
  7. LINK Review the discussion of phagocytosis in Concept 4.3
  8. VIDEO 5.2 Pinocytosis and membrane ruffling in a mouse epithelial cell
  9. APPLY THE CONCEPT Some substances require energy to cross the membrane VIDEO 5.3 Cell Visualization: Endocytosis
  10. ANIMATED TUTORIAL 5.3 Endocytosis and Exocytosis VIDEO 5.4 Exocytosis of coccoliths in a marine golden alga, Pleurochrysis
  11. See Figure 5.6
  12. See Figure 5.6
  13. VIDEO 5.5 Cell Visualization: Signals and calcium
  14. VIDEO 5.5 Cell Visualization: Signals and calcium
  15. See Figure 5.4 LINK Nerve cells communicate with muscle cells at neuromuscular junctions, which are described in Concept 36.1
  16. ANIMATED TUTORIAL 5.4 G Protein–Linked Signal Transduction and Cancer
  17. VIDEO 5.6 Chemotaxis of human neutrophils
  18. LINK Review enzyme regulation in Concept 3.4
  19. See Figure 5.17, step 1 See Figure 5.17, steps 2 and 3 ANIMATED TUTORIAL 5.5 Signal Transduction Pathway
  20. See Figure 5.4 See Figure 5.17 VIDEO 5.7 Calcium waves in brain glial cells