Cell Membranes and Signaling

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Cell Membranes and Signaling
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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

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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 Cells
  Response to Environmental Signals
• 5.6 Signal Transduction Allows the Cell to
  Respond to Its Environment

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Chapter 5 Opening Question

• What role does the cell membrane play in the
  bodys response to caffeine?

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Concept 5.1 Biological Membranes Have a Common
Structure and Are Fluid

• A membranes 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.

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Figure 5.1 Membrane Molecular Structure
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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 regionselectrically charged heads
  that associate with water molecules
• Hydrophobic regionsnonpolar fatty acid tails
  that do not dissolve in water

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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.

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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

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Concept 5.1 Biological Membranes Have a Common
Structure and Are Fluid

• Two important factors in membrane fluidity
• Lipid compositiontypes of fatty acids can
  increase or decrease fluidity
• Temperaturemembrane fluidity decreases in colder
  conditions

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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.

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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.

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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.

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Figure 5.2 Rapid Diffusion of Membrane Proteins
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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.
• Glycolipida carbohydrate bonded to a lipid
• Glycoproteina carbohydrate bonded to a protein

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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.

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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.

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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

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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 moleculessmaller molecules
  diffuse faster
• Temperature of the solutionhigher temperatures
  lead to faster diffusion

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Concept 5.2 Some Substances Can Cross the
Membrane by Diffusion

• The concentration gradient in the systemthe
  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.

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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 throughthey are not
  soluble in the hydrophilic interior and form
  bonds instead in the aqueous environment near the
  membrane.

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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.

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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.

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Figure 5.3A Osmosis Can Modify the Shapes of
Cells
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Figure 5.3B Osmosis Can Modify the Shapes of
Cells
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Figure 5.3C Osmosis Can Modify the Shapes of
Cells
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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 wallas it builds up, it prevents more
  water from entering.

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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.

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Concept 5.2 Some Substances Can Cross the
Membrane by Diffusion

• Ion channels are a type of channel proteinmost
  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.

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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.

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Figure 5.4 A Ligand-Gated Channel Protein Opens
in Response to a Stimulus
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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.

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Figure 5.5 Aquaporins Increase Membrane
Permeability to Water (Part 1)
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Figure 5.5 Aquaporins Increase Membrane
Permeability to Water (Part 2)
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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 shapeit releases
  glucose on the other side of the membrane.

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Figure 5.6 A Carrier Protein Facilitates
Diffusion (Part 1)
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Figure 5.6 A Carrier Protein Facilitates
Diffusion (Part 2)
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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
  saturatedwhen all of the carrier molecules are
  bound, the rate of diffusion reaches its maximum.

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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.

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Table 5.1 Membrane Transport Mechanisms
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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 cells
  needs, usually by means of a specific carrier
  protein.

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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.

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Concept 5.3 Some Substances Require Energy to
Cross the Membrane

• The sodiumpotassium (NaK) 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.

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Figure 5.7 Primary Active Transport The
SodiumPotassium Pump
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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.

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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.

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Figure 5.8 Endocytosis and Exocytosis (Part 1)
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Figure 5.8 Endocytosis and Exocytosis (Part 2)
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Concept 5.4 Large Molecules Cross the Membrane
via Vesicles

• Three types of endocytosis brings molecules into
  the cell phagocytosis, pinocytosis, and
  receptormediated endocytosis.
• In all three, the membrane invaginates, or folds
  around the molecules and forms a vesicle.
• The vesicle then separates from the membrane.

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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.

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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.

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Concept 5.4 Large Molecules Cross the Membrane
via Vesicles

• Receptormediated 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).

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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.

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Figure 5.9 Receptor-Mediated Endocytosis (Part 1)
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Figure 5.9 Receptor-Mediated Endocytosis (Part 2)
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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.

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Concept 5.5 The Membrane Plays a Key Role in a
Cells 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 receptors
  activation by a signal.

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Concept 5.5 The Membrane Plays a Key Role in a
Cells 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.

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Figure 5.10 Chemical Signaling Concepts
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Concept 5.5 The Membrane Plays a Key Role in a
Cells Response to Environmental Signals

• Only cells with the necessary receptors can
  respond to a signalthe target cell must be able
  to sense it and respond to it.
• A signal transduction pathway involves a signal,
  a receptor, and a response.

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Figure 5.11 Signal Transduction Concepts
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Concept 5.5 The Membrane Plays a Key Role in a
Cells Response to Environmental Signals

• A common mechanism of signal transduction is
  allosteric regulation.
• This involves an alteration in a proteins shape
  as a result of a molecule binding to it.
• A signal transduction pathway may produce short
  or long term responses.

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Concept 5.5 The Membrane Plays a Key Role in a
Cells 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.

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Figure 5.12 A Signal Binds to Its Receptor
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Concept 5.5 The Membrane Plays a Key Role in a
Cells 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.

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Concept 5.5 The Membrane Plays a Key Role in a
Cells 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.

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Concept 5.5 The Membrane Plays a Key Role in a
Cells 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.

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Concept 5.5 The Membrane Plays a Key Role in a
Cells Response to Environmental Signals

• Receptors are also classified by their activity
• Ion channel receptors
• Protein kinase receptors
• G proteinlinked receptors

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Concept 5.5 The Membrane Plays a Key Role in a
Cells 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.

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Concept 5.5 The Membrane Plays a Key Role in a
Cells 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.

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Figure 5.13 A Protein Kinase Receptor
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Concept 5.5 The Membrane Plays a Key Role in a
Cells 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.

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Concept 5.5 The Membrane Plays a Key Role in a
Cells Response to Environmental Signals

• Ligands binding to G proteinlinked 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.

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Concept 5.5 The Membrane Plays a Key Role in a
Cells 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

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Concept 5.5 The Membrane Plays a Key Role in a
Cells Response to Environmental Signals

• The activated G proteinlinked 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.

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Figure 5.14 A G ProteinLinked Receptor
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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
  interactionsthe signal can then be amplified and
  distributed to cause different responses.

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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.

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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.

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Figure 5.15 The Discovery of a Second Messenger
(Part 1)
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Figure 5.15 The Discovery of a Second Messenger
(Part 2)
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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
  cellthey distribute the signal.
• They amplify the signal by activating more than
  one enzyme target.

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Figure 5.16 The Formation of Cyclic AMP
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Concept 5.6 Signal Transduction Allows the Cell
to Respond to Its Environment

• Signal transduction pathways involve multiple
  stepsenzymes may be either activated or
  inhibited by other enzymes.
• In liver cells, a signal cascade begins when
  epinephrine stimulates a G proteinmediated
  protein kinase pathway.

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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
  Ait phosphorylates two other enzymes, with
  opposite effects
• Inhibition
• Activation

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Figure 5.17 A Cascade of Reactions Leads to
Altered Enzyme Activity (Part 1)
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Figure 5.17 A Cascade of Reactions Leads to
Altered Enzyme Activity (Part 2)
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Concept 5.6 Signal Transduction Allows the Cell
to Respond to Its Environment

• Inhibitionprotein kinase A inactivates glycogen
  synthase through phosphorylation, and prevents
  glucose storage.
• ActivationPhosphorylase kinase is activated when
  phosphorylated and is part of a cascade that
  results in the liberation of glucose molecules.

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Concept 5.6 Signal Transduction Allows the Cell
to Respond to Its Environment

• Signal transduction ends after the cell
  respondsenzymes convert each transducer back to
  its inactive precursor.
• The balance between the regulating enzymes and
  the signal enzymes determines the cells response.

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Figure 5.18 Signal Transduction Regulatory
Mechanisms
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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

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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

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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 itthe result is that the person remains
  alert.

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Figure 5.19 Caffeine and the Cell Membrane (Part
1)
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Figure 5.19 Caffeine and the Cell Membrane (Part
2)