1. 3
Cells: The Living Units: Part B

2. Membrane Transport: Active Processes
require ATP to move solutes across a living plasma membrane because
Solute too large for channels
Solute not lipid soluble
Solute not able to move down concentration gradient
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3. Requires carrier proteins (solute pumps)
Active Transport
Requires carrier proteins (solute pumps)
Bind specifically and reversibly with substance
Moves solutes against concentration gradient ( low to HIGH)
Requires energy
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4. 4 Two extracellular K+ bind to pump.
Figure Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP.
Slide 1
Extracellular fluid
Na+
Na+–K+ pump K+
Na+ bound
ATP-binding site
Cytoplasm
1 Three cytoplasmic Na+ bind to pump
protein. P
K+ released
6 Pump protein binds ATP; releases K+ to
the inside, and Na+ sites are ready to bind
Na+ again. The cycle repeats.
2 Na+ binding promotes hydrolysis of ATP.
The energy released during this reaction
phosphorylates the pump.
Na+ released
K+ bound P Pi K+
5 K+ binding triggers release of the
phosphate. The dephosphorylated pump
resumes its original conformation.
3 Phosphorylation causes the pump to
change shape, expelling Na+ to the outside. P
4 Two extracellular K+ bind to pump.
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5. Primary active transport
Figure Secondary active transport is driven by the concentration gradient created by primary active transport.
Slide 1
Extracellular fluid
Glucose
Na+-glucose
symport transporter
releases glucose
into the cytoplasm
Na+-glucose
symport
transporter
loads glucose
from extracellular
fluid
Na+-K+
pump
Cytoplasm
Primary active transport
The ATP-driven Na+-K+ pump stores energy by creating a steep concentration gradient for Na+ entry into the cell. 1
Secondary active transport
As Na+ diffuses back across the membrane
through a membrane cotransporter protein, it
drives glucose against its concentration gradient
into the cell. 2
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6. Primary active transport
Figure Secondary active transport is driven by the concentration gradient created by primary active transport.
Slide 2
Extracellular fluid
Na+-K+
pump
Cytoplasm
Primary active transport
The ATP-driven Na+-K+ pump stores energy by creating a steep concentration gradient for Na+ entry into the cell. 1
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7. Primary active transport
Figure Secondary active transport is driven by the concentration gradient created by primary active transport.
Slide 3
Extracellular fluid
Glucose
Na+-glucose
symport transporter
releases glucose
into the cytoplasm
Na+-glucose
symport
transporter
loads glucose
from extracellular
fluid
Na+-K+
pump
Cytoplasm
Primary active transport
The ATP-driven Na+-K+ pump stores energy by creating a steep concentration gradient for Na+ entry into the cell. 1
Secondary active transport
As Na+ diffuses back across the membrane
through a membrane cotransporter protein, it
drives glucose against its concentration gradient
into the cell. 2
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8. Vesicular Transport
Transport of large particles, macromolecules, and fluids across membrane in membranous sacs called vesicles
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9. Vesicular Transport Functions: Exocytosis—transport out of cell
Endocytosis—transport into cell
Phagocytosis, pinocytosis, receptor-mediated endocytosis
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10. Figure 3.12 Events of endocytosis mediated by protein-coated pits.
Slide 1
Coated pit ingests substance. 1
Extracellular fluid
Plasma
membrane
Protein coat
(typically
clathrin)
Cytoplasm
Protein-coated vesicle deta-
ches. 2 3
Coat proteins are recycled to plasma membrane.
Transport
vesicle
Uncoated endocytic
vesicle
Endosome
Uncoated vesicle fuses with a sorting vesicle called an endosome. 4
Transport
vesicle containing 5
membrane compone
-nts moves to the plasma
membrane for recycling.
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
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11. Figure 3.12 Events of endocytosis mediated by protein-coated pits.
Slide 2
Coated pit ingests substance. 1
Extracellular fluid
Plasma
membrane
Protein coat
(typically
clathrin)
Cytoplasm
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12. Figure 3.12 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 deta-
ches. 2
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13. Figure 3.12 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 deta-
ches. 2
Coat proteins are recycled to plasma membrane. 3
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14. Figure 3.12 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 deta-
ches. 2
Coat proteins are recycled to plasma membrane. 3
Uncoated endocytic
vesicle
Endosome
Uncoated vesicle fuses with a sorting vesicle called an endosome. 4
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15. Figure 3.12 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 deta-
ches. 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 5
membrane compone
-nts moves to the plasma
membrane for recycling.
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16. Figure 3.12 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 deta-
ches. 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 5
membrane compone
-nts moves to the plasma
membrane for recycling.
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
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17. Used by macrophages and some white blood cells
Endocytosis
Phagocytosis
Pseudopods engulf solids and bring them into cell's interior
Form vesicle called phagosome
Used by macrophages and some white blood cells
Move by amoeboid motion
Cytoplasm flows into temporary extensions
Allows creeping
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18. Figure 3.13a 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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19. Pinocytosis (fluid-phase endocytosis)
Plasma membrane infolds, bringing extracellular fluid and dissolved solutes inside cell
Fuses with endosome
Most cells utilize to "sample" environment
Nutrient absorption in the small intestine
Membrane components recycled back to membrane
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20. Figure 3.13b 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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21. Figure 3.13c 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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22. Substance enclosed in secretory vesicle Functions
Exocytosis
Substance enclosed in secretory vesicle
Functions
Hormone secretion, neurotransmitter release, mucus secretion, ejection of wastes
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23. Fusion pore formed Cytoplasm
Figure Exocytosis.
Slide 1
The process of exocytosis
Plasma membrane
SNARE (t-SNARE)
Extracellular
fluid
Fusion pore formed
3 The vesicle
and plasma
membrane
fuse and a pore
opens up.
Secretory
vesicle
Vesicle
SNARE
(v-SNARE)
1 The membrane-
bound vesicle
migrates to the
plasma membrane.
Molecule to
be secreted
Cytoplasm
4 Vesicle
contents are
released to the
cell exterior.
2 There, proteins at the vesicle surface (v-SNAREs) bind with t-SNAREs (plasma membrane proteins).
Fused
v- and
t-SNAREs
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24. Photomicrograph of a secretory vesicle releasing its contents
Figure 3.14b Exocytosis.
Photomicrograph
of a secretory
vesicle releasing
its contents
by exocytosis
(100,000x)
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25. Table 3.2 Active Membrane Transport Processes (1 of 2)
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26. Table 3.2 Active Membrane Transport Processes (2 of 2)
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27. Extracellular fluid + + + + + + + +
Figure The key role of K+ in generating the resting membrane potential.
Slide 1
1 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.
Extracellular fluid
2 K+ also move into the cell
because they are attracted to the
negative charge established on the
inner plasma membrane face. + + + + + + + + – – –
3 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. – – – –
Potassium
leakage
channels –
Protein anion (unable to
follow K+ through the
membrane)
Cytoplasm
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28. Extracellular fluid + + + + + + + +
Figure The key role of K+ in generating the resting membrane potential.
Slide 2
1 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.
Extracellular fluid + + + + + + + + – – – – – – –
Potassium
leakage
channels –
Protein anion (unable to
follow K+ through the
membrane)
Cytoplasm
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29. Extracellular fluid + + + + + + + +
Figure The key role of K+ in generating the resting membrane potential.
Slide 3
1 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.
Extracellular fluid
2 K+ also move into the cell
because they are attracted to the
negative charge established on the
inner plasma membrane face. + + + + + + + + – – – – – – –
Potassium
leakage
channels –
Protein anion (unable to
follow K+ through the
membrane)
Cytoplasm
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30. Extracellular fluid + + + + + + + +
Figure The key role of K+ in generating the resting membrane potential.
Slide 4
1 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.
Extracellular fluid
2 K+ also move into the cell
because they are attracted to the
negative charge established on the
inner plasma membrane face. + + + + + + + + – – –
3 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. – – – –
Potassium
leakage
channels –
Protein anion (unable to
follow K+ through the
membrane)
Cytoplasm
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31. Figure G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell.
Slide 1
The sequence described here is like a molecular
relay race. Instead of a baton passed from runner
to runner, the message (a shape change) is
passed from molecule to molecule as it makes its
way across the cell membrane from outside to
inside the cell.
Ligand (1st
messenger)
Receptor
G protein
Enzyme
2nd
messenger
Ligand* (1st messeng-
er) binds to the receptor.
The receptor changes shape
and activates.
The activated receptor
binds to a G protein and acti-
vates it. The G protein changes
shape (turns “on”), causing it to
release GDP and bind GTP (an
energy source).
Activated G protein
activates (or inactivates)
an effector protein by
causing its shape to
change. 1 2 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
Inactive 2nd
messenger
G protein
GDP
Active 2nd
messenger
Second messengers
activate other enzymes or ion
channels. Cyclic AMP typically
activates protein kinase enzymes. 5
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
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