Download Chapter 3B Notes

PowerPoint® Lecture Slides
prepared by
Barbara Heard,
Atlantic Cape Community
College
CHAPTER
3
Cells: The
Living Units:
Part B
© Annie Leibovitz/Contact Press Images
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Membrane Transport: Active Processes
• Two types of active processes
– Active transport
– Vesicular transport
• Both 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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Active Transport
• Requires carrier proteins (solute pumps)
– Bind specifically and reversibly with
substance
• Moves solutes against concentration
gradient
– Requires energy
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Active Transport: Two Types
• Primary active transport
– Required energy directly from ATP hydrolysis
• Secondary active transport
– Required energy indirectly from ionic
gradients created by primary active transport
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Primary Active Transport
• Energy from hydrolysis of ATP causes
shape change in transport protein that
"pumps" solutes (ions) across membrane
• E.g., calcium, hydrogen, Na+-K+ pumps
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Primary Active Transport
• Sodium-potassium pump
– Most well-studied
– Carrier (pump) called Na+-K+ ATPase
– Located in all plasma membranes
– Involved in primary and secondary active
transport of nutrients and ions
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Sodium-Potassium Pump
• Na+ and K+ channels allow slow leakage
down concentration gradients
• Na+-K+ pump works as antiporter
– Pumps against Na+ and K+ gradients to
maintain high intracellular K+ concentration
and high extracellular Na+ concentration
• Maintains electrochemical gradients essential for
functions of muscle and nerve tissues
• Allows all cells to maintain fluid volume
© 2013 Pearson Education, Inc.
Figure 3.10 Primary active transport is the process in which solutes are moved across cell
membranes against electrochemical gradients using energy supplied directly by ATP.
Extracellular fluid
Na+
Na+–K+ pump
K+
Na+ bound
ATP-binding site
Cytoplasm
1 Three cytoplasmic Na+ bind to pump
protein.
P
K+ released
2 Na+ binding promotes hydrolysis of ATP.
The energy released during this reaction
phosphorylates the pump.
6 Pump protein binds ATP; releases K+ to
the inside, and Na+ sites are ready to bind
Na+ again. The cycle repeats.
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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Slide 1
Secondary Active Transport
• Depends on ion gradient created by
primary active transport
• Energy stored in ionic gradients used
indirectly to drive transport of other solutes
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Secondary Active Transport
• Cotransport—always transports more
than one substance at a time
– Symport system: Substances transported in
same direction
– Antiport system: Substances transported in
opposite directions
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Figure 3.11 Secondary active transport is driven by the concentration gradient created by primary active
transport.
Extracellular fluid
Slide 1
Glucose
Na+-K+
pump
Na+-glucose
symport
transporter
loads glucose
from extracellular
fluid
Na+-glucose
symport transporter
releases glucose
into the cytoplasm
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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2 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.
Vesicular Transport
• Transport of large particles,
macromolecules, and fluids across
membrane in membranous sacs called
vesicles
• Requires cellular energy (e.g., ATP)
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Vesicular Transport
• Functions:
– Exocytosis—transport out of cell
– Endocytosis—transport into cell
• Phagocytosis, pinocytosis, receptor-mediated
endocytosis
– 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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Endocytosis and Transcytosis
• Involve formation of protein-coated
vesicles
• Often receptor mediated, therefore very
selective
• Some pathogens also hijack for transport
into cell
• Once vesicle is inside cell it may
– Fuse with lysosome
– Undergo transcytosis
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Figure 3.12 Events of endocytosis mediated by protein-coated pits.
1 Coated pit ingests
substance.
Protein coat
(typically
clathrin)
2 Protein-coated
vesicle detaches.
Extracellular fluid
Plasma
membrane
Cytoplasm
3 Coat proteins are
recycled to plasma
membrane.
Transport
vesicle
Uncoated endocytic
vesicle
Endosome
4 Uncoated vesicle
fuses with a sorting
vesicle called an
endosome.
Lysosome
5 Transport
vesicle containing
membrane compone
-nts moves to the plasma
membrane for recycling.
6 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).
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Slide 1
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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Figure 3.13a Comparison of three types of endocytosis.
Receptors
Phagosome
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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.
Endocytosis
• 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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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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Endocytosis
• Receptor-mediated endocytosis
– Allows specific endocytosis and transcytosis
• Cells use to concentrate materials in limited supply
– Clathrin-coated pits provide main route for
endocytosis and transcytosis
• Uptake of enzymes, low-density lipoproteins, iron,
insulin, and, unfortunately, viruses, diphtheria, and
cholera toxins
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Figure 3.13c Comparison of three types of endocytosis.
Vesicle
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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.
Exocytosis
• Usually activated by cell-surface signal or
change in membrane voltage
• Substance enclosed in secretory vesicle
• v-SNAREs ("v" = vesicle) on vesicle find
t-SNAREs ("t" = target) on membrane and
bind
• Functions
– Hormone secretion, neurotransmitter release,
mucus secretion, ejection of wastes
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Figure 3.14b Exocytosis.
Photomicrograph
of a secretory
vesicle releasing
its contents
by exocytosis
(100,000x)
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Table 3.2 Active Membrane Transport Processes (1 of 2)
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Table 3.2 Active Membrane Transport Processes (2 of 2)
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Active Transport Maintains Electrochemical
Gradients
• Na+-K+ pump continuously ejects 3Na+
from cell and carries 2K+ in
• Steady state maintained because rate of
active transport equal to and depends on
rate of Na+ diffusion into cell
• Neuron and muscle cells "upset" RMP by
opening gated Na+ and K+ channels
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