1. © 2016 Pearson Education, Inc.

2. 3.4 Active Membrane Transport
Two major active membrane transport processes
Active transport
Vesicular transport
Both require ATP to move solutes across a plasma membrane for any of these reasons:
Solute is too large for channels, or
Solute is not lipid soluble, or
Solute is not able to move down concentration gradient
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3. Active Transport Requires carrier proteins (solute pumps)
Bind specifically and reversibly with substance being moved
Some carriers transport more than one substance
Antiporters transport one substance into cell while transporting a different substance out of cell
Symporters transport two different substances in the same direction
Moves solutes against their concentration gradient (from low to high)
This requires energy (ATP)
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4. Active Transport (cont.)
Two types of active transport:
Primary active transport
Required energy comes directly from ATP hydrolysis
Secondary active transport
Required energy is obtained indirectly from ionic gradients created by primary active transport
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5. Active Transport (cont.)
Primary active transport
Energy from hydrolysis of ATP causes change in shape of transport protein
Shape change causes solutes (ions) bound to protein to be pumped across membrane
Example of pumps: calcium, hydrogen (proton), Na+-K+ pumps
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6. Active Transport (cont.)
Sodium-potassium pump
Most studied pump
Basically is an enzyme, called Na+-K+ ATPase, that pumps Na+ out of cell and K+ back into cell
Located in all plasma membranes, but especially active in excitable cells (nerves and muscles)
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7. Active Transport (cont.)
Leakage channels located in membranes result in leaking of Na+ into the cell and leaking of K+ out of cell
Both travel down their concentration gradients
Na+-K+ pump works as an antiporter that pumps Na+ out of cell and K+ back into cell against their concentration gradients
Maintains electrochemical gradients, which involve both concentration and electrical charge of ions
Essential for functions of muscle and nerve tissues
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8. Cells that connect body parts, form linings, or transport gases
Figure 3.1 Cell diversity.
Erythrocytes
Fibroblasts
Skeletal
muscle
cell
Smooth
muscle cells
Epithelial cells
Cells that connect body parts, form linings,
or transport gases
Cells that move organs and body parts
Macrophage
Fat cell
Nerve cell
Cell that stores
nutrients
Cell that fights
disease
Cell that gathers information and controls
body functions
Sperm
Cell of reproduction
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9. © 2016 Pearson Education, Inc.
Focus Figure 3.1 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. The action of the Na+-K+ pump is an important example of primary active transport.
Slide 2
Extracellular fluid
Na+
Na+ –K+
pump
ATP K+
ATP-binding site
Cytoplasm
Three cytoplasmic Na+ bind to
pump protein. 1
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10. © 2016 Pearson Education, Inc.
Focus Figure 3.1 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. The action of the Na+-K+ pump is an important example of primary active transport.
Slide 3
Extracellular fluid
Na+
Na+ –K+
pump
ATP K+
Na+ bound
ATP-binding site
Cytoplasm
Three cytoplasmic Na+ bind to
pump protein. 1 P
ADP
Na+ binding promotes hydrolysis
of ATP. The energy released during this
reaction phosphorylates the pump. 2
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11. © 2016 Pearson Education, Inc.
Focus Figure 3.1 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. The action of the Na+-K+ pump is an important example of primary active transport.
Slide 4
Extracellular fluid
Na+
Na+ –K+
pump
ATP K+
Na+ bound
ATP-binding site
Cytoplasm
Three cytoplasmic Na+ bind to
pump protein. 1 P
ADP
Na+ binding promotes hydrolysis
of ATP. The energy released during this
reaction phosphorylates the pump. 2
Na+
released P
Phosphorylation causes the
pump to change shape, expelling
Na+ to the outside. 3
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12. © 2016 Pearson Education, Inc.
Focus Figure 3.1 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. The action of the Na+-K+ pump is an important example of primary active transport.
Slide 5
Extracellular fluid
Na+
Na+ –K+
pump
ATP K+
Na+ bound
ATP-binding site
Cytoplasm
Three cytoplasmic Na+ bind to
pump protein. 1 P
ADP
Na+ binding promotes hydrolysis
of ATP. The energy released during this
reaction phosphorylates the pump. 2
Na+
released P K+
Phosphorylation causes the
pump to change shape, expelling
Na+ to the outside. 3 P 4
Two extracellular K+ bind to pump.
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13. © 2016 Pearson Education, Inc.
Focus Figure 3.1 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. The action of the Na+-K+ pump is an important example of primary active transport.
Slide 6
Extracellular fluid
Na+
Na+ –K+
pump
ATP K+
Na+ bound
ATP-binding site
Cytoplasm
Three cytoplasmic Na+ bind to
pump protein. 1 P
ADP
Na+ binding promotes hydrolysis
of ATP. The energy released during this
reaction phosphorylates the pump. 2
Na+
released
K+ bound P Pi K+
K+ binding triggers release of
the phosphate. The dephosphorylated
pump resumes its original
conformation. 5
Phosphorylation causes the
pump to change shape, expelling
Na+ to the outside. 3 P 4
Two extracellular K+ bind to pump.
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14. © 2016 Pearson Education, Inc.
Focus Figure 3.1 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. The action of the Na+-K+ pump is an important example of primary active transport.
Slide 7
Extracellular fluid
Na+
Na+ –K+
pump
ATP K+
Na+ bound
ATP-binding site
Cytoplasm
Three cytoplasmic Na+ bind to
pump protein. 1
ATP P
K+ released
ADP
Pump protein binds ATP; releases
K+ to the inside, and Na+ sites are ready
to bind Na+ again. The cycle repeats. 6
Na+ binding promotes hydrolysis
of ATP. The energy released during this
reaction phosphorylates the pump. 2
Na+
released
K+ bound P Pi K+
K+ binding triggers release of
the phosphate. The dephosphorylated
pump resumes its original
conformation. 5
Phosphorylation causes the
pump to change shape, expelling
Na+ to the outside. 3 P 4
Two extracellular K+ bind to pump.
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15. Active Transport (cont.)
Secondary active transport
Depends on ion gradient that was created by primary active transport system
Energy stored in gradients is used indirectly to drive transport of other solutes
Low Na+ concentration that is maintained inside cell by Na+-K+ pump strengthens sodium’s drive to want to enter cell
Na+ can drag other molecules with it as it flows into cell through carrier proteins (usually symporters) in membrane
Some sugars, amino acids, and ions are usually transported into cells via secondary active transport
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16. 1 Extracellular fluid Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ K+
Figure 3.10 Secondary active transport is driven by the concentration gradient created by primary active transport.
Slide 2
Extracellular fluid
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+ K+
Na+-K+
pump
ATP
Na+
Cytoplasm 1
Primary active transport
The ATP-driven Na+-K+ pump
stores energy by creating a steep
concentration gradient for Na+
entry into the cell.
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17. 1 2 Extracellular fluid Glucose Na+ Na+ Na+ Na+ Na+-glucose
Figure 3.10 Secondary active transport is driven by the concentration gradient created by primary active transport.
Slide 3
Extracellular fluid
Glucose
Na+
Na+
Na+
Na+
Na+-glucose
symport transporter
releases glucose
into the cytoplasm
Na+-glucose
symport
transporter
loads glucose
from extracellular
fluid
Na+
Na+
Na+
Na+
Na+
Na+ K+
Na+-K+
pump
ATP
Na+
Cytoplasm 1
Primary active transport 2
Secondary active transport
The ATP-driven Na+-K+ pump
stores energy by creating a steep
concentration gradient for Na+
entry into the cell.
As Na+ diffuses back across the membrane
through a membrane cotransporter protein, it
drives glucose against its concentration gradient
into the cell.
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18. Vesicular Transport
Involves transport of large particles, macromolecules, and fluids across membrane in membranous sacs called vesicles
Requires cellular energy (usually ATP)
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19. Vesicular Transport (cont.)
Vesicular transport processes include:
Endocytosis: transport into cell
3 different types of endocytosis: phagocytosis, pinocytosis, receptor-mediated endocytosis
Exocytosis: transport out of cell
Transcytosis: transport into, across, and then out of cell
Vesicular trafficking: transport from one area or organelle in cell to another
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20. Vesicular Transport (cont.)
Endocytosis
Involves formation of protein-coated vesicles
Usually involve receptors; therefore can be a very selective process
Substance being pulled in must be able to bind to its unique receptor
Some pathogens are capable of hijacking receptor for transport into cell
Once vesicle is pulled inside cell, it may:
Fuse with lysosome or
Undergo transcytosis
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21. Figure 3.11 Events of endocytosis mediated by protein-coated pits.
ingests substance. 1
Extracellular fluid
Plasma
membrane
Protein coat
(typically clathrin)
Cytoplasm
Protein-coated
vesicle detaches. 2
Coat proteins are recycled
to plasma membrane. 3
Transport
vesicle
Uncoated
endocytic
vesicle
Endosome
Uncoated vesicle fuses with
a sorting vesicle called an endosome. 4
Transport vesicle
containing membrane
components moves to
the plasma membrane
for recycling. 5
Lysosome
Fused vesicle may (a) fuse
with lysosome for digestion of
its contents, or (b) deliver its
contents to the plasma
membrane on the opposite
side of the cell (transcytosis). 6
(b)
(a)
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22. Figure 3.11 Events of endocytosis mediated by protein-coated pits.
Slide 2
Coated pit
ingests substance. 1
Extracellular fluid
Plasma
membrane
Protein coat
(typically clathrin)
Cytoplasm
(b)
(a)
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23. Figure 3.11 Events of endocytosis mediated by protein-coated pits.
Slide 3
Coated pit
ingests substance. 1
Extracellular fluid
Plasma
membrane
Protein coat
(typically clathrin)
Cytoplasm
Protein-coated
vesicle detaches. 2
(b)
(a)
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24. Figure 3.11 Events of endocytosis mediated by protein-coated pits.
Slide 4
Coated pit
ingests substance. 1
Extracellular fluid
Plasma
membrane
Protein coat
(typically clathrin)
Cytoplasm
Protein-coated
vesicle detaches. 2
Coat proteins are recycled
to plasma membrane. 3
(b)
(a)
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25. Figure 3.11 Events of endocytosis mediated by protein-coated pits.
Slide 5
Coated pit
ingests substance. 1
Extracellular fluid
Plasma
membrane
Protein coat
(typically clathrin)
Cytoplasm
Protein-coated
vesicle detaches. 2
Coat proteins are recycled
to plasma membrane. 3
Uncoated
endocytic
vesicle
Endosome
Uncoated vesicle fuses with
a sorting vesicle called an endosome. 4
(b)
(a)
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26. Figure 3.11 Events of endocytosis mediated by protein-coated pits.
Slide 6
Coated pit
ingests substance. 1
Extracellular fluid
Plasma
membrane
Protein coat
(typically clathrin)
Cytoplasm
Protein-coated
vesicle detaches. 2
Coat proteins are recycled
to plasma membrane. 3
Transport
vesicle
Uncoated
endocytic
vesicle
Endosome
Uncoated vesicle fuses with
a sorting vesicle called an endosome. 4
Transport vesicle
containing membrane
components moves to
the plasma membrane
for recycling. 5
(b)
(a)
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27. Figure 3.11 Events of endocytosis mediated by protein-coated pits.
Slide 7
Coated pit
ingests substance. 1
Extracellular fluid
Plasma
membrane
Protein coat
(typically clathrin)
Cytoplasm
Protein-coated
vesicle detaches. 2
Coat proteins are recycled
to plasma membrane. 3
Transport
vesicle
Uncoated
endocytic
vesicle
Endosome
Uncoated vesicle fuses with
a sorting vesicle called an endosome. 4
Transport vesicle
containing membrane
components moves to
the plasma membrane
for recycling. 5
Lysosome
Fused vesicle may (a) fuse
with lysosome for digestion of
its contents, or (b) deliver its
contents to the plasma
membrane on the opposite
side of the cell (transcytosis). 6
(b)
(a)
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28. Vesicular Transport (cont.)
Phagocytosis: type of endocytosis that is referred to as “cell eating”
Membrane projections called pseudopods form and flow around solid particles that are being engulfed, forming a vesicle which is pulled into cell
Formed vesicle is called a phagosome
Phagocytosis is used by macrophages and certain other white blood cells
Phagocytic cells move by amoeboid motion where cytoplasm flows into temporary extensions that allow cell to creep
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29. Figure 3.12a Comparison of three types of endocytosis.
Phagocytosis
The cell engulfs a large particle by forming
projecting pseudopods (“false feet”) around
it and enclosing it within a membrane sac
called a phagosome. The phagosome is
combined with a lysosome. Undigested
contents remain in the vesicle (now called a
residual body) or are ejected by exocytosis.
Vesicle may or may not be protein-coated
but has receptors capable of binding to
microorganisms or solid particles.
Receptors
Phagosome
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30. Vesicular Transport (cont.)
Pinocytosis: type of endocytosis that is referred to as “cell drinking” or fluid-phase endocytosis
Plasma membrane infolds, bringing extracellular fluid and dissolved solutes inside cell
Fuses with endosome
Used by some cells to “sample” environment
Main way in which nutrient absorption occurs in the small intestine
Membrane components are recycled back to membrane
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31. Figure 3.12b Comparison of three types of endocytosis.
Pinocytosis
The cell “gulps” a drop of extracellular fluid
containing solutes into tiny vesicles. No receptors
are used, so the process is nonspecific. Most
vesicles are protein-coated.
Vesicle
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32. Vesicular Transport (cont.)
Receptor-mediated endocytosis involves endocytosis and transcytosis of specific molecules
Many cells have receptors embedded in clathrin-coated pits, which will be internalized along with the specific molecule bound
Examples: enzymes, low-density lipoproteins (LDL), iron, insulin, and, unfortunately, viruses, diphtheria, and cholera toxins may also be taken into a cell this way
Caveolae have smaller pits and different protein coat from clathrin, but still capture specific molecules (folic acid, tetanus toxin) and use transcytosis
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33. Figure 3.12c Comparison of three types of endocytosis.
Receptor-mediated endocytosis
Extracellular substances bind to specific receptor
proteins, enabling the cell to ingest and concentrate
specific substances (ligands) in protein-coated
vesicles. Ligands may simply be released inside
the cell, or combined with a lysosome to digest
contents. Receptors are recycled to the plasma
membrane in vesicles.
Vesicle
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34. Exocytosis Process where material is ejected from cell
Usually activated by cell-surface signals or changes in membrane voltage
Substance being ejected is enclosed in secretory vesicle
Protein on vesicle called v-SNARE finds and hooks up to target t-SNARE proteins on membrane
Docking process triggers exocytosis
Some substances exocytosed: hormones, neurotransmitters, mucus, cellular wastes
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35. Figure 3.13a Exocytosis.
Extracellular
fluid
Plasma membrane
SNARE (t-SNARE)
The process of
exocytosis
Secretory
vesicle
Vesicle
SNARE
(v-SNARE)
The membrane-
bound vesicle migrates
to the plasma
membrane. 1
Molecule to
be secreted
Cytoplasm
There, proteins at
the vesicle surface
(v-SNAREs) bind with
t-SNAREs (plasma
membrane proteins). 2
Fused
v- and
t-SNAREs
Fusion pore formed
The vesicle and
plasma membrane fuse
and a pore opens up. 3
Vesicle contents
are released to the cell
exterior. 4
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36. Figure 3.13a Exocytosis.
Slide 2
Extracellular
fluid
Plasma membrane
SNARE (t-SNARE)
The process of
exocytosis
Secretory
vesicle 1
Vesicle
SNARE
(v-SNARE)
The membrane-
bound vesicle migrates
to the plasma
membrane.
Molecule to
be secreted
Cytoplasm
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37. Figure 3.13a Exocytosis.
Slide 3
Extracellular
fluid
Plasma membrane
SNARE (t-SNARE)
The process of
exocytosis
Secretory
vesicle 1
Vesicle
SNARE
(v-SNARE)
The membrane-
bound vesicle migrates
to the plasma
membrane.
Molecule to
be secreted
Cytoplasm
There, proteins at
the vesicle surface
(v-SNAREs) bind with
t-SNAREs (plasma
membrane proteins). 2
Fused
v- and
t-SNAREs
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38. Figure 3.13a Exocytosis.
Slide 4
Extracellular
fluid
Plasma membrane
SNARE (t-SNARE)
The process of
exocytosis
Secretory
vesicle
The membrane-
bound vesicle migrates
to the plasma
membrane. 1
Vesicle
SNARE
(v-SNARE)
Molecule to
be secreted
Cytoplasm
There, proteins at
the vesicle surface
(v-SNAREs) bind with
t-SNAREs (plasma
membrane proteins). 2
Fused
v- and
t-SNAREs
Fusion pore formed
The vesicle and
plasma membrane fuse
and a pore opens up. 3
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39. Figure 3.13a Exocytosis.
Slide 5
Extracellular
fluid
Plasma membrane
SNARE (t-SNARE)
The process of
exocytosis
Secretory
vesicle 1
Vesicle
SNARE
(v-SNARE)
The membrane-
bound vesicle migrates
to the plasma
membrane.
Molecule to
be secreted
Cytoplasm
There, proteins at
the vesicle surface
(v-SNAREs) bind with
t-SNAREs (plasma
membrane proteins). 2
Fused
v- and
t-SNAREs
Fusion pore formed
The vesicle and
plasma membrane fuse
and a pore opens up. 3
Vesicle contents
are released to the cell
exterior. 4
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40. Photomicrograph of a secretory vesicle releasing its contents
Figure 3.13b Exocytosis.
Photomicrograph
of a secretory
vesicle releasing
its contents
by exocytosis
(100,000)
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41. 3.5 Membrane Potential Resting membrane potential (RMP)
Electrical potential energy produced by separation of oppositely charged particles across plasma membrane in all cells
Difference in electrical charge between two points is referred to as voltage
Cells that have a charge are said to be polarized
Voltage occurs only at membrane surface
Rest of cell and extracellular fluid are neutral
Membrane voltages range from –50 to –100 mV in different cells (negative sign (–) indicates inside of cell is more negative relative to outside of cell)
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42. K+ is Key Player in RMP
K+ diffuses out of cell through K+ leakage channels down its concentration gradient
Negatively charged proteins cannot leave
As a result cytoplasmic side of cell membrane becomes more negative
K+ is then pulled back by the more negative interior because of its electrical gradient
When drive for K+ to leave cell is balanced by its drive to stay, RMP is established
Most cells have an RMP around –90 mV
Electrochemical gradient of K+ sets RMP
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43. K+ is Key Player in RMP (cont.)
In many cells, Na+ also affects RMP
Na+ is also attracted to inside of cell because of negative charge
If Na+ enters cell, it can bring RMP up to –70 mV
Membrane is more permeable to K+ than Na+, so K+ primary influence on RMP
Cl– does not influence RMP because its concentration and electrical gradients are exactly balanced
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44. 1 2 3 K+ diffuse down their steep concentration gradient (out of the
Figure 3.14 The key role of K+ in generating the resting membrane potential.
K+ diffuse down their steep
concentration gradient (out of the
cell) via leakage channels. Loss of
K+ results in a negative charge on
the inner plasma membrane face. 1
Extracellular fluid
Na+
Na+
Na+
Na+
Na+
Na+ K+
CI
Na+
Na+ K+
CI K+ K+ K+
K+ also move into the cell
because they are attracted to the
negative charge established on
the inner plasma membrane face. 2 + + + + + + + +    
A negative membrane potential
(–90 mV) is established when the
movement of K+ out of the cell
equals K+ movement into the cell.
At this point, the concentration
gradient promoting K+ exit exactly
opposes the electrical gradient for
K+ entry. 3   
Potassium
leakage
channels  K+ K+ K+ K+ K+ K+ K+ K+ K+ A
Na+ A
Protein anion (unable
to follow K+ through the
membrane)
Cytoplasm
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45. 1 K+ diffuse down their steep concentration gradient (out of the
Figure 3.14 The key role of K+ in generating the resting membrane potential.
Slide 2
K+ diffuse down their steep
concentration gradient (out of the
cell) via leakage channels. Loss of
K+ results in a negative charge on
the inner plasma membrane face. 1
Extracellular fluid
Na+
Na+
Na+
Na+
Na+
Na+ K+
CI
Na+
Na+ K+
CI K+ K+ K+ + + + + + + + +       
Potassium
leakage
channels  K+ K+ K+ K+ K+ K+ K+ K+ K+ A
Na+ A
Protein anion (unable
to follow K+ through the
membrane)
Cytoplasm
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46. 1 2 K+ diffuse down their steep concentration gradient (out of the
Figure 3.14 The key role of K+ in generating the resting membrane potential.
Slide 3
K+ diffuse down their steep
concentration gradient (out of the
cell) via leakage channels. Loss of
K+ results in a negative charge on
the inner plasma membrane face. 1
Extracellular fluid
Na+
Na+
Na+
Na+
Na+
Na+ K+
CI
Na+
Na+ K+
CI K+ K+ K+
K+ also move into the cell
because they are attracted to the
negative charge established on
the inner plasma membrane face. 2 + + + + + + + +       
Potassium
leakage
channels  K+ K+ K+ K+ K+ K+ K+ K+ K+ A
Na+ A
Protein anion (unable
to follow K+ through the
membrane)
Cytoplasm
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47. 1 2 3 K+ diffuse down their steep concentration gradient (out of the
Figure 3.14 The key role of K+ in generating the resting membrane potential.
Slide 4
K+ diffuse down their steep
concentration gradient (out of the
cell) via leakage channels. Loss of
K+ results in a negative charge on
the inner plasma membrane face. 1
Extracellular fluid
Na+
Na+
Na+
Na+
Na+
Na+ K+
CI
Na+
Na+ K+
CI K+ K+ K+
K+ also move into the cell
because they are attracted to the
negative charge established on
the inner plasma membrane face. 2 + + + + + + + +    
A negative membrane potential
(–90 mV) is established when the
movement of K+ out of the cell
equals K+ movement into the cell.
At this point, the concentration
gradient promoting K+ exit exactly
opposes the electrical gradient for
K+ entry. 3   
Potassium
leakage
channels  K+ K+ K+ K+ K+ K+ K+ K+ K+ A
Na+ A
Protein anion (unable
to follow K+ through the
membrane)
Cytoplasm
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48. Active Transport Maintains Electrochemical Gradients
RMP is maintained through action of the Na+-K+ pump, which continuously ejects 3Na+ out of cell and brings 2K+ back inside
Steady state is maintained because rate of active pumping of Na+ out of cell equals the rate of Na+ diffusion into cell
Neuron and muscle cells “upset” this steady state RMP by intentionally opening gated Na+ and K+ channels
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49. 3.6 Cell-Environment Interactions
Cells interact with their environment by responding directly to other cells, or indirectly to extracellular chemicals
Interactions always involves glycocalyx
Cell adhesion molecules (CAMs)
Plasma membrane receptors
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50. Role of Cell Adhesion Molecules (CAMs)
Every cell has thousands of sticky glycoprotein CAMs projecting from membrane
Functions:
Anchor cell to extracellular matrix or to each other
Assist in movement of cells past one another
Attract WBCs to injured or infected areas
Stimulate synthesis or degradation of adhesive membrane junctions (example: tight junctions)
Transmit intracellular signals to direct cell migration, proliferation, and specialization
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51. Roles of Plasma Membrane Receptors
Membrane receptor proteins serve as binding sites for several chemical signals
Contact signaling: cells that touch recognize each other by each cell’s unique surface membrane receptors
Used in normal development and immunity
Chemical signaling: interaction between receptors and ligands (chemical messengers) that cause changes in cellular activities
In some cells, binding triggers enzyme activation; in others, it opens chemically gated ion channels
Examples of ligands: neurotransmitters, hormones, and paracrines
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52. Roles of Plasma Membrane Receptors (cont.)
Chemical signaling (cont.):
Same ligand can cause different responses in different cells depending on chemical pathway that the receptor is part of
When ligand binds, receptor protein changes shape and thereby becomes activated
Some activated receptors become enzymes; others act to directly open or close ion gates, causing changes in excitability
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53. Roles of Plasma Membrane Receptors (cont.)
Chemical signaling (cont.):
Activated G protein–linked receptors indirectly cause cellular changes by activating G proteins, which in turn can affect ion channels, activate other enzymes, or cause release of internal second messenger chemicals such as cyclic AMP or calcium
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54. hundreds of reactions.)
Focus Figure 3.2 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell.
Ligand
(1st messenger)
Receptor
G protein
Enzyme
2nd
messenger
Ligand*(1st
messenger) binds to
the receptor. The
receptor changes shape
and activates. 1
The activated receptor binds
to a G protein and activates it.
The G protein changes shape (turns
“on”), causing it to release GDP
and bind GTP (an energy source). 2
Activated G protein
activates (or inactivates)
an effector protein by
causing its shape to
change. 3
Extracellular fluid
Effector protein
(e.g., an enzyme
Ligand
Receptor
Activated effector
enzymes catalyze reactions
that produce 2nd
messengers in the cell.
(Common 2nd messengers
include cyclic AMP and Ca2.) 4
GTP
GTP
Inactive 2nd
messenger
G protein
Second messengers
activate other enzymes or
ion channels. Cyclic AMP
typically activates protein kinase
enzymes. 5
GDP
GTP
Active 2nd
messenger
Activated
Kinase
enzymes
Kinase enzymes activate
other enzymes. Kinase
enzymes transfer phosphate
groups from ATP to specific
proteins and activate a series of
other enzymes that trigger
various metabolic and structural
changes in the cell. 6
Cascade of cellular
Responses
(The amplification
effect is tremendous.
Each enzyme catalyzes
hundreds of reactions.)
* Ligands include
hormones and
neurotransmitters.
Intracellular fluid
© 2016 Pearson Education, Inc.

55. hundreds of reactions.)
Focus Figure 3.2 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell.
Slide 2
Ligand
(1st messenger)
Receptor
G protein
Enzyme
2nd
messenger
Ligand*(1st
messenger) binds to
the receptor. The
receptor changes shape
and activates. 1
Extracellular fluid
Effector protein
(e.g., an enzyme
Ligand
Receptor
GTP
GTP
Inactive 2nd
messenger
G protein
GDP
GTP
Active 2nd
messenger
Activated
Kinase
enzymes
Cascade of cellular
Responses
(The amplification
effect is tremendous.
Each enzyme catalyzes
hundreds of reactions.)
* Ligands include
hormones and
neurotransmitters.
Intracellular fluid
© 2016 Pearson Education, Inc.

56. hundreds of reactions.)
Focus Figure 3.2 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell.
Slide 3
Ligand
(1st messenger)
Receptor
G protein
Enzyme
2nd
messenger
Ligand*(1st
messenger) binds to
the receptor. The
receptor changes shape
and activates. 1
The activated receptor binds
to a G protein and activates it.
The G protein changes shape (turns
“on”), causing it to release GDP
and bind GTP (an energy source). 2
Extracellular fluid
Effector protein
(e.g., an enzyme
Ligand
Receptor
GTP
GTP
Inactive 2nd
messenger
G protein
GDP
GTP
Active 2nd
messenger
Activated
Kinase
enzymes
Cascade of cellular
Responses
(The amplification
effect is tremendous.
Each enzyme catalyzes
hundreds of reactions.)
* Ligands include
hormones and
neurotransmitters.
Intracellular fluid
© 2016 Pearson Education, Inc.

57. hundreds of reactions.)
Focus Figure 3.2 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell.
Slide 4
Ligand
(1st messenger)
Receptor
G protein
Enzyme
2nd
messenger
Ligand*(1st
messenger) binds to
the receptor. The
receptor changes shape
and activates. 1
The activated receptor binds
to a G protein and activates it.
The G protein changes shape (turns
“on”), causing it to release GDP
and bind GTP (an energy source). 2
Activated G protein
activates (or inactivates)
an effector protein by
causing its shape to
change. 3
Extracellular fluid
Effector protein
(e.g., an enzyme
Ligand
Receptor
GTP
GTP
Inactive 2nd
messenger
G protein
GDP
GTP
Active 2nd
messenger
Activated
Kinase
enzymes
Cascade of cellular
Responses
(The amplification
effect is tremendous.
Each enzyme catalyzes
hundreds of reactions.)
* Ligands include
hormones and
neurotransmitters.
Intracellular fluid
© 2016 Pearson Education, Inc.

58. hundreds of reactions.)
Focus Figure 3.2 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell.
Slide 5
Ligand
(1st messenger)
Receptor
G protein
Enzyme
2nd
messenger
Ligand*(1st
messenger) binds to
the receptor. The
receptor changes shape
and activates. 1
The activated receptor binds
to a G protein and activates it.
The G protein changes shape (turns
“on”), causing it to release GDP
and bind GTP (an energy source). 2
Activated G protein
activates (or inactivates)
an effector protein by
causing its shape to
change. 3
Extracellular fluid
Effector protein
(e.g., an enzyme
Ligand
Receptor
Activated effector
enzymes catalyze reactions
that produce 2nd
messengers in the cell.
(Common 2nd messengers
include cyclic AMP and Ca2+.) 4
GTP
GTP
Inactive 2nd
messenger
G protein
GDP
GTP
Active 2nd
messenger
Activated
Kinase
enzymes
Cascade of cellular
Responses
(The amplification
effect is tremendous.
Each enzyme catalyzes
hundreds of reactions.)
* Ligands include
hormones and
neurotransmitters.
Intracellular fluid
© 2016 Pearson Education, Inc.

59. hundreds of reactions.)
Focus Figure 3.2 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell.
Slide 6
Ligand
(1st messenger)
Receptor
G protein
Enzyme
2nd
messenger
Ligand*(1st
messenger) binds to
the receptor. The
receptor changes shape
and activates. 1
The activated receptor binds
to a G protein and activates it.
The G protein changes shape (turns
“on”), causing it to release GDP
and bind GTP (an energy source). 2
Activated G protein
activates (or inactivates)
an effector protein by
causing its shape to
change. 3
Extracellular fluid
Effector protein
(e.g., an enzyme
Ligand
Receptor
Activated effector
enzymes catalyze reactions
that produce 2nd
messengers in the cell.
(Common 2nd messengers
include cyclic AMP and Ca2+.) 4
GTP
GTP
Inactive 2nd
messenger 5
G protein
Second messengers
activate other enzymes or
ion channels. Cyclic AMP
typically activates protein kinase
enzymes.
GDP
GTP
Active 2nd
messenger
Activated
Kinase
enzymes
Cascade of cellular
Responses
(The amplification
effect is tremendous.
Each enzyme catalyzes
hundreds of reactions.)
* Ligands include
hormones and
neurotransmitters.
Intracellular fluid
© 2016 Pearson Education, Inc.

60. hundreds of reactions.)
Focus Figure 3.2 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell.
Slide 7
Ligand
(1st messenger)
Receptor
G protein
Enzyme
2nd
messenger
Ligand*(1st
messenger) binds to
the receptor. The
receptor changes shape
and activates. 1
The activated receptor binds
to a G protein and activates it.
The G protein changes shape (turns
“on”), causing it to release GDP
and bind GTP (an energy source). 2
Activated G protein
activates (or inactivates)
an effector protein by
causing its shape to
change. 3
Extracellular fluid
Effector protein
(e.g., an enzyme
Ligand
Receptor
Activated effector
enzymes catalyze reactions
that produce 2nd
messengers in the cell.
(Common 2nd messengers
include cyclic AMP and Ca2+.) 4
GTP
GTP
Inactive 2nd
messenger 5
G protein
Second messengers
activate other enzymes or
ion channels. Cyclic AMP
typically activates protein kinase
enzymes.
GDP
GTP
Active 2nd
messenger
Activated
Kinase
enzymes
Kinase enzymes activate
other enzymes. Kinase
enzymes transfer phosphate
groups from ATP to specific
proteins and activate a series of
other enzymes that trigger
various metabolic and structural
changes in the cell. 6
Cascade of cellular
Responses
(The amplification
effect is tremendous.
Each enzyme catalyzes
hundreds of reactions.)
* Ligands include
hormones and
neurotransmitters.
Intracellular fluid
© 2016 Pearson Education, Inc.