1. Membrane Transport and Cell Signaling5
Membrane Transport and Cell Signaling

2. Overview: Life at the Edge
The plasma membrane separates the living cell from its surroundings
The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others
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3. Figure 5.1
Figure 5.1 How do cell membrane proteins help regulate chemical traffic?
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4. CONCEPT 5.1: Cellular membranes are fluid mosaics of lipids and proteins
Phospholipids are the most abundant lipid in most membranes
Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions
A phospholipid bilayer can exist as a stable boundary between two aqueous compartments
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5. EXTRACELLULAR SIDE OF MEMBRANE
Figure 5.2
Fibers of extra-
cellular matrix (ECM)
Glyco-
protein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Figure 5.2 Current model of an animal cell’s plasma membrane (cutaway view)
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE OF MEMBRANE
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6. Most membrane proteins are also amphipathic and reside in the bilayer with their hydrophilic portions protruding
The fluid mosaic model states that the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids
Groups of certain proteins or certain lipids may associate in long-lasting, specialized patches
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7. Hydrophilic head WATER WATER Hydrophobic tail Figure 5.3
Figure 5.3 Phospholipid bilayer (cross section)
Hydrophobic
tail
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8. The Fluidity of Membranes
Most of the lipids and some proteins in a membrane can shift about laterally
The lateral movement of phospholipids is rapid; proteins move more slowly
Some proteins move in a directed manner; others seem to be anchored in place
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9. Results Membrane proteins Mouse cell Human cell Figure 5.4-1
Figure Inquiry: Do membrane proteins move? (step 1)
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10. Results Membrane proteins Mouse cell Human cell Hybrid cell
Figure 5.4-2
Results
Membrane proteins
Mouse cell
Human cell
Figure Inquiry: Do membrane proteins move? (step 2)
Hybrid cell
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11. Results Membrane proteins Mixed proteins after 1 hour Mouse cell
Figure 5.4-3
Results
Membrane proteins
Mixed proteins
after 1 hour
Mouse cell
Human cell
Figure Inquiry: Do membrane proteins move? (step 3)
Hybrid cell
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12. As temperatures cool, membranes switch from a fluid state to a solid state
The temperature at which a membrane solidifies depends on the types of lipids
A membrane remains fluid to a lower temperature if it is rich in phospholipids with unsaturated hydrocarbon tails
Membranes must be fluid to work properly; they are usually about as fluid as salad oil
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13. The steroid cholesterol has different effects on membrane fluidity at different temperatures
At warm temperatures (such as 37oC), cholesterol restrains movement of phospholipids
At cool temperatures, it maintains fluidity by preventing tight packing
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14. Evolution of Differences in Membrane Lipid Composition
Variations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions
Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary
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15. Unsaturated tails prevent packing. Saturated tails pack together.
Figure 5.5
Fluid
Viscous
Unsaturated tails prevent
packing.
Saturated tails pack
together.
(a) Unsaturated versus saturated hydrocarbon tails
(b) Cholesterol reduces
membrane fluidity at
moderate temperatures,
but at low temperatures
hinders solidification.
Figure 5.5 Factors that affect membrane fluidity
Cholesterol
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16. Membrane Proteins and Their Functions
A membrane is a collage of different proteins, often grouped together, embedded in the fluid matrix of the lipid bilayer
Proteins determine most of the membrane’s specific functions
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17. Peripheral proteins are loosely bound to the surface of the membrane
Integral proteins penetrate the hydrophobic interior of the lipid bilayer
Integral proteins that span the membrane are called transmembrane proteins
The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into  helices
Peripheral proteins are loosely bound to the surface of the membrane
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18. N-terminus EXTRACELLULAR SIDE  helix CYTOPLASMIC SIDE C-terminus
Figure 5.6
N-terminus
EXTRACELLULAR
SIDE
Figure 5.6 The structure of a transmembrane protein
 helix
CYTOPLASMIC
SIDE
C-terminus
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19. Six major functions of membrane proteins
Transport
Enzymatic activity
Attachment to the cytoskeleton and extracellular matrix (ECM)
Cell-cell recognition
Intercellular joining
Signal transduction
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20. (b) Enzymatic activity (c) Attachment to the cytoskeleton and extra-
Figure 5.7a
Enzymes
ATP
Figure 5.7a Some functions of membrane proteins (part 1: transport, enzymatic activity, and attachment to the cytoskeleton and ECM)
(a) Transport
(b) Enzymatic activity
(c) Attachment to the
cytoskeleton and extra-
cellular matrix (ECM)
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21. (d) Cell-cell recognition (e) Intercellular joining
Figure 5.7b
Signaling
molecule
Receptor
Glyco-
protein
Figure 5.7b Some functions of membrane proteins (part 2: cell-cell recognition, intercellular joining, and signal transduction)
(d) Cell-cell recognition
(e) Intercellular joining
(f) Signal transduction
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22. The Role of Membrane Carbohydrates in Cell-Cell Recognition
Cells recognize each other by binding to surface molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane
Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or, more commonly, to proteins (forming glycoproteins)
Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual
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23. Synthesis and Sidedness of Membranes
Membranes have distinct inside and outside faces
The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus
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24. Transmembrane Secretory glycoproteins protein Golgi apparatus Vesicle
Figure 5.8
Transmembrane
glycoproteins
Secretory
protein
Golgi
apparatus
Vesicle ER
ER lumen
Glycolipid
Figure 5.8 Synthesis of membrane components and their orientation in the membrane
Plasma membrane:
Cytoplasmic face
Transmembrane
glycoprotein
Extracellular face
Secreted
protein
Membrane
glycolipid
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25. CONCEPT 5.2: Membrane structure results in selective permeability
A cell must regulate transport of substances across cellular boundaries
Plasma membranes are selectively permeable, regulating the cell’s molecular traffic
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26. The Permeability of the Lipid Bilayer
Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer of the membrane and cross it easily
Polar molecules, such as sugars, do not cross the membrane easily
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27. Transport Proteins
Transport proteins allow passage of hydrophilic substances across the membrane
Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel
Channel proteins called aquaporins facilitate the passage of water
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28. A transport protein is specific for the substance it moves
Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane
A transport protein is specific for the substance it moves
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29. a charged amino acid like lysine a hydrophobic amino acid like valine
Which of the following amino acids would most likely be present in the transmembrane domain of an integral membrane protein?
a charged amino acid like lysine
a hydrophobic amino acid like valine
a polar amino acid like serine
any of the above, with no preference

30. CONCEPT 5.3: Passive transport is diffusion of a substance across a membrane with no energy investment
Diffusion is the tendency for molecules to spread out evenly into the available space
Although each molecule moves randomly, diffusion of a population of molecules may be directional
At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other
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31. Membrane (cross section)
Figure 5.9
Molecules of dye
Membrane (cross section)
WATER
Net diffusion
Net diffusion
Equilibrium
(a) Diffusion of one solute
Figure 5.9 The diffusion of solutes across a synthetic membrane
Net diffusion
Net diffusion
Equilibrium
Net diffusion
Net diffusion
Equilibrium
(b) Diffusion of two solutes
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32. Effects of Osmosis on Water Balance
Osmosis is the diffusion of free water across a selectively permeable membrane
Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides
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33. Lower concentration of solute (sugar) Higher concentration of solute
Figure 5.10a
Lower
concentration
of solute (sugar)
Higher
concentration
of solute
More similar concen-
trations of solute
Sugar
molecule
H2O
Figure 5.10a Osmosis (part 1)
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34. Selectively permeable membrane Water molecules can pass through
Figure 5.10b
Selectively
permeable
membrane
Water molecules
can pass through
pores, but sugar
molecules cannot.
Water molecules
cluster around
sugar molecules.
This side has
fewer solute mol-
ecules, more free
water molecules.
This side has
more solute mol-
ecules, fewer free
water molecules.
Figure 5.10b Osmosis (part 2)
Osmosis
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35. This diagram represents osmosis of water across a semipermeable membrane. The U-tube on the right shows the results of the osmosis. What could you do to level the solutions in the two sides of the U-tube on the right?
Add more water to the left-hand side.
Add more water to the right-hand side.
Add more solute to the left-hand side.
Add more solute to the right-hand side.

36. Water Balance of Cells Without Walls
Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water
Isotonic solution: Solute concentration is the same as inside the cell; no net water movement across the plasma membrane
Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water
Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water
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37. Hypertonic or hypotonic environments create osmotic problems for organisms
Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environments
The single celled eukaryote Paramecium caudatum, which is hypertonic to its pondwater environment, has a contractile vacuole that can pump excess water out of the cell
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38. 50 m Contractile vacuole Figure 5.12
Figure 5.12 The contractile vacuole of Paramecium caudatum
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39. Hypotonic Isotonic Hypertonic H2O H2O H2O H2O Animal cell Lysed Normal
Figure 5.11
Hypotonic
Isotonic
Hypertonic
H2O
H2O
H2O
H2O
Animal cell
Lysed
Normal
Shriveled
Cell wall
H2O
H2O
H2O
H2O
Figure 5.11 The water balance of living cells
Plant cell
Turgid (normal)
Flaccid
Plasmolyzed
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40. Water Balance of Cells with Walls
Cell walls help maintain water balance
A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (very firm)
If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt
In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis
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41. Facilitated Diffusion: Passive Transport Aided by Proteins
In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane
Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane
Channel proteins include
Aquaporins, for facilitated diffusion of water
Ion channels that open or close in response to a stimulus (gated channels)
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42. EXTRA- CELLULAR FLUID A channel protein Channel protein Solute
Figure 5.13
EXTRA- CELLULAR FLUID
A channel
protein
Channel protein
Solute
CYTOPLASM
Figure 5.13 Two types of transport proteins that carry out facilitated diffusion
Carrier protein
Solute
(b) A carrier protein
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43. No net energy input is required
Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane
The shape change may be triggered by binding and release of the transported molecule
No net energy input is required
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44. CONCEPT 5.4: Active transport uses energy to move solutes against their gradients
Facilitated diffusion speeds transport of a solute by providing efficient passage through the membrane but does not alter the direction of transport
Some transport proteins, however, can move solutes against their concentration gradients
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45. The sodium-potassium pump is one type of active transport system
Active transport allows cells to maintain concentration gradients that differ from their surroundings
The sodium-potassium pump is one type of active transport system
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46. Animation: Active Transport
Right click slide / Select play

47. Figure 5.14
EXTRACELLULAR FLUID
[Na] high
[K] low
[Na] low 1
CYTOPLASM
[K] high 2
ADP 6 3
Figure 5.14 The sodium-potassium pump: a specific case of active transport 5 4
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48. Substances diffuse down their concentration gradient, from where it is more concentrated to where it is less concentrated
No work must be done to move substances down the concentration gradient
The diffusion of a substance across a biological membrane is passive transport because no energy is expended by the cell to make it happen
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49. The Need for Energy in Active Transport
Active transport moves substances against their concentration gradients
Active transport requires energy, usually in the form of ATP
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50. Facilitated diffusion
Figure 5.15
Passive transport
Active transport
Figure 5.15 Review: passive and active transport
Diffusion
Facilitated
diffusion
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51. Which of the following best describes the structure of a biological membrane?
two layers of phospholipids with proteins embedded between the two layers
a mixture of covalently linked phospholipids and proteins that determines which solutes can cross the membrane and which cannot
two layers of phospholipids with proteins either crossing the layers or on the surface of the layers
a fluid structure in which phospholipids and proteins move freely between sides of the membrane

52. How Ion Pumps Maintain Membrane Potential
Membrane potential is the voltage across a membrane
Voltage is created by differences in the distribution of positive and negative ions across a membrane
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53. 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)
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54. The sodium-potassium pump is a major electrogenic pump of animal cells
An electrogenic pump is a transport protein that generates voltage across a membrane
The sodium-potassium pump is a major electrogenic pump of animal cells
Another important electrogenic pump of animals, plants, fungi, and bacteria is a proton pump
Electrogenic pumps help store energy that can be used for cellular work
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55. EXTRACELLULAR FLUID Proton pump CYTOPLASM Figure 5.16
Figure 5.16 A proton pump
CYTOPLASM
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56. Cotransport: Coupled Transport by a Membrane Protein
Cotransport occurs when active transport of a solute indirectly drives transport of other solutes
Plant cells use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell
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57. Sucrose-H cotransporter
Figure 5.17
Proton pump
Sucrose-H
cotransporter
Diffusion of H
Figure 5.17 Cotransport: active transport driven by a concentration gradient
Sucrose
Sucrose
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58. CONCEPT 5.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
Small solutes and water enter or leave the cell through the lipid bilayer or by means of transport proteins
Large molecules, such as polysaccharides and proteins, cross the membrane in bulk by means of vesicles
Bulk transport requires energy
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59. Exocytosis
In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents
Many secretory cells use exocytosis to export products
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60. Endocytosis
In endocytosis, the cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane
Endocytosis is a reversal of exocytosis, involving different proteins
There are three types of endocytosis
Phagocytosis (“cellular eating”)
Pinocytosis (“cellular drinking”)
Receptor-mediated endocytosis
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61. An amoeba engulfing a bacterium via phagocytosis (TEM)
Figure 5.18a
Phagocytosis
EXTRACELLULAR
FLUID
Pseudopodium
of amoeba
Solutes
Pseudopodium
Bacterium
1 m
Food vacuole
“Food”
or other
particle
An amoeba engulfing a bacterium
via phagocytosis (TEM)
Figure 5.18a Exploring endocytosis in animal cells (part 1: phagocytosis)
Food
vacuole
CYTOPLASM
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62. 0.25 m Pinocytosis Plasma membrane Coat protein
Figure 5.18b
Pinocytosis
Plasma
membrane
0.25 m
Coat
protein
Pinocytotic vesicles forming
(TEMs)
Coated
pit
Figure 5.18b Exploring endocytosis in animal cells (part 2: pinocytosis)
Coated
vesicle
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63. 0.25 m Receptor-Mediated Endocytosis Coat Plasma protein membrane
Figure 5.18c
Receptor-Mediated
Endocytosis
Plasma
membrane
Coat
protein
Receptor
0.25 m
Figure 5.18c Exploring endocytosis in animal cells (part 3: receptor-mediated endocytosis)
Top: A coated pit Bottom: A coated
vesicle forming during receptor-
mediated endocytosis (TEMs)
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64. receptor-mediated endocytosis phagocytosis facilitated diffusion
Cells (e.g., bacteria) are taken up by other cells (e.g., an immune cell) by which of the following?
pinocytosis
exocytosis
receptor-mediated endocytosis
phagocytosis
facilitated diffusion

65. CONCEPT 5.6: The plasma membrane plays a key role in most cell signaling
In multicellular organisms, cell-to-cell communication allows the cells of the body to coordinate their activities
Communication between cells is also essential for many unicellular organisms
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66. Local and Long-Distance Signaling
Eukaryotic cells may communicate by direct contact
Animal and plant cells have junctions that directly connect the cytoplasm of adjacent cells
These are called gap junctions (animal cells) and plasmodesmata (plant cells)
The free passage of substances in the cytosol from one cell to another is a type of local signaling
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67. In many other cases of local signaling, messenger molecules are secreted by a signaling cell
These messenger molecules, called local regulators, travel only short distances
One class of these, growth factors, stimulates nearby cells to grow and divide
This type of local signaling in animal cells is called paracrine signaling
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68. (a) Paracrine signaling
Figure 5.19a
Local signaling
Target cell
Secreting
cell
Secretory
vesicle
Figure 5.19a Local and long-distance cell signaling by secreted molecules in animals (part 1: local signaling, paracrine)
Local regulator
diffuses through
extracellular fluid.
(a) Paracrine signaling
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69. Another more specialized type of local signaling occurs in the animal nervous system
This synaptic signaling consists of an electrical signal moving along a nerve cell that triggers secretion of neurotransmitter molecules
These diffuse across the space between the nerve cell and its target, triggering a response in the target cell
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70. (b) Synaptic signaling
Figure 5.19b
Local signaling
Electrical signal
along nerve cell
triggers release
of neuro-
transmitter.
Neurotransmitter
diffuses across
synapse.
Figure 5.19b Local and long-distance cell signaling by secreted molecules in animals (part 2: local signaling, synaptic)
Target cell
is stimulated.
(b) Synaptic signaling
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71. Hormones vary widely in size and shape
In long-distance signaling, plants and animals use chemicals called hormones
In hormonal signaling in animals (called endocrine signaling), specialized cells release hormone molecules that travel via the circulatory system
Hormones vary widely in size and shape
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72. Long-distance signaling
Figure 5.19c
Long-distance signaling
Blood
vessel
Endocrine cell
Hormone
travels in
bloodstream.
Target cell
specifically
binds
hormone.
Figure 5.19c Local and long-distance cell signaling by secreted molecules in animals (part 3: long distance signaling, endocrine)
(c) Endocrine (hormonal) signaling
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73. The Three Stages of Cell Signaling: A Preview
Earl W. Sutherland discovered how the hormone epinephrine acts on cells
Sutherland suggested that cells receiving signals undergo three processes
Reception
Transduction
Response
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74. Animation: Signaling Overview
Right click slide / Select play

75. EXTRACELLULAR FLUID CYTOPLASM Plasma membrane Reception Transduction
Figure
EXTRACELLULAR FLUID
CYTOPLASM
Plasma membrane
Reception
Transduction
Response
Receptor
Activation
Relay molecules
Figure Overview of cell signaling (step 3)
Signaling
molecule
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76. Reception, the Binding of a Signaling Molecule to a Receptor Protein
The binding between a signal molecule (ligand) and receptor is highly specific
Ligand binding generally causes a shape change in the receptor
Many receptors are directly activated by this shape change
Most signal receptors are plasma membrane proteins
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77. Receptors in the Plasma Membrane
Most water-soluble signal molecules bind to specific sites on receptor proteins that span the plasma membrane
There are two main types of membrane receptors
G protein-coupled receptors
Ligand-gated ion channels
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78. G proteins bind to the energy-rich molecule GTP (like ATP)
G protein-coupled receptors (GPCRs) are plasma membrane receptors that work with the help of a G protein
G proteins bind to the energy-rich molecule GTP (like ATP)
The G protein acts as an on-off switch: If GTP is bound to the G protein, the G protein is active
Many G proteins are very similar in structure
GPCR pathways are extremely diverse in function
© 2014 Pearson Education, Inc. 78

79. Inactive enzyme Activated Signaling molecule GPCR Plasma membrane
Figure 1
Inactive
enzyme
Activated
GPCR
Signaling molecule
Plasma membrane
Activated
G protein
CYTOPLASM 2
Activated
enzyme
Figure A G protein-coupled receptor (GPCR) in action (step 2)
Cellular response
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80. Ligand-gated ion channels are very important in the nervous system
A ligand-gated ion channel receptor acts as a “gate” for ions when the receptor changes shape
When a signal molecule binds as a ligand to the receptor, the gate allows specific ions, such as Na+ or Ca2+, through a channel in the receptor
Ligand-gated ion channels are very important in the nervous system
The diffusion of ions through open channels may trigger an electric signal
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81. Gate Gate open closed Ions Signaling molecule (ligand) Plasma membrane
Figure 1 2
Gate
open
Gate
closed
Ions
Signaling
molecule
(ligand)
Plasma
membrane
Ligand-gated
ion channel receptor
Cellular
response 3
Gate closed
Figure Ion channel receptor (step 3)
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82. Intracellular Receptors
Intracellular receptor proteins are found in the cytosol or nucleus of target cells
Small or hydrophobic chemical messengers can readily cross the membrane and activate receptors
Examples of hydrophobic messengers are the steroid and thyroid hormones of animals and nitric oxide (NO) in both plants and animals
© 2014 Pearson Education, Inc. 82

83. Testosterone behaves similarly to other steroid hormones
Only cells that contain receptors for testosterone can respond to it
The hormone binds the receptor protein and activates it
The active form of the receptor enters the nucleus, acts as a transcription factor, and activates genes needed for male sex characteristics
© 2014 Pearson Education, Inc. 83

84. EXTRA- CELLULAR FLUID Plasma membrane New protein
Figure 5.23
Hormone
(testosterone)
EXTRA-
CELLULAR FLUID
Plasma
membrane
Receptor
protein
Hormone-
receptor
complex
DNA
Figure 5.23 Steroid hormone interacting with an intracellular receptor
mRNA
New
protein
NUCLEUS
CYTOPLASM
© 2014 Pearson Education, Inc. 84

85. A steroid hormone binds to an intracellular receptor
A steroid hormone binds to an intracellular receptor. When it does, the resulting complex is able to do which of the following?
open channels in the membrane for other substances to enter
open channels in the nuclear envelope for cytoplasmic molecules to enter
mediate the transfer of phosphate groups to/from ATP
act as a transcription factor in the nucleus

86. Transduction by Cascades of Molecular Interactions
Signal transduction usually involves multiple steps
Multistep pathways can amplify a signal: A few molecules can produce a large cellular response
Multistep pathways provide more opportunities for coordination and regulation of the cellular response than simpler systems do
© 2014 Pearson Education, Inc. 86

87. The molecules that relay a signal from receptor to response are mostly proteins
Like falling dominoes, the receptor activates another protein, which activates another, and so on, until the protein producing the response is activated
At each step, the signal is transduced into a different form, usually a shape change in a protein
© 2014 Pearson Education, Inc. 87

88. Protein Phosphorylation and Dephosphorylation
Phosphorylation and dephosphorylation are a widespread cellular mechanism for regulating protein activity
Protein kinases transfer phosphates from ATP to protein, a process called phosphorylation
The addition of phosphate groups often changes the form of a protein from inactive to active
© 2014 Pearson Education, Inc. 88

89. Activated relay molecule Receptor
Figure 5.24
Signaling molecule
Activated relay molecule
Receptor
Inactive
protein
kinase 1
Active
protein
kinase 1
Inactive
protein
kinase 2
Phosphorylation cascade
ADP
Active
protein
kinase 2
Figure 5.24 A phosphorylation cascade
Inactive
protein
ADP
Active
protein
Cellular
response
© 2014 Pearson Education, Inc. 89

90. Protein phosphatases remove the phosphates from proteins, a process called dephosphorylation
Phosphatases provide a mechanism for turning off the signal transduction pathway
They also make protein kinases available for reuse, enabling the cell to respond to the signal again
© 2014 Pearson Education, Inc. 90

91. Small Molecules and Ions as Second Messengers
The extracellular signal molecule (ligand) that binds to the receptor is a pathway’s “first messenger”
Second messengers are small, nonprotein, water-soluble molecules or ions that spread throughout a cell by diffusion
Cyclic AMP and calcium ions are common second messengers
Ca2+
© 2014 Pearson Education, Inc. 91

92. First messenger (signaling molecule such as epinephrine) Adenylyl
Figure 5.25
First messenger
(signaling molecule
such as epinephrine)
Adenylyl
cyclase
G protein
G protein-coupled
receptor
Second
messenger
Figure 5.25 cAMP as a second messenger in a G protein signaling pathway
Protein
kinase A
Cellular responses
© 2014 Pearson Education, Inc. 92

93. Response: Regulation of Transcription or Cytoplasmic Activities
Ultimately, a signal transduction pathway leads to regulation of one or more cellular activities
The response may occur in the cytoplasm or in the nucleus
Many signaling pathways regulate the synthesis of enzymes or other proteins, usually by turning genes on or off in the nucleus
The final activated molecule in the signaling pathway may function as a transcription factor
© 2014 Pearson Education, Inc. 93

94. Growth factor Reception Receptor Transduction CYTOPLASM Inactive
Figure 5.26
Growth factor
Reception
Receptor
Phosphorylation
cascade
Transduction
CYTOPLASM
Inactive
transcription
factor
Active
transcription
factor
Figure 5.26 Nuclear response to a signal: the activation of a specific gene by a growth factor
Response
DNA
Gene
NUCLEUS
mRNA
© 2014 Pearson Education, Inc. 94

95. Which of the following best describes a signal transduction pathway?
binding of a signal molecule to a cell protein
catalysis mediated by an enzyme
sequence of changes in a series of molecules resulting in a response
binding of a ligand on one side of a membrane that results in a change on the other side
the cell’s detection of a chemical or mechanical stimulus