1. 3
Cells: The Living Units: Part B

2. 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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3. Requires carrier proteins (solute pumps)
Active Transport
Requires carrier proteins (solute pumps)
Bind specifically and reversibly with substance
Moves solutes against concentration gradient
Requires energy
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4. 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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5. 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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6. Requires cellular energy (e.g., ATP)
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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7. 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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8. 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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9. 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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10. 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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11. 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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12. 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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13. 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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14. 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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15. 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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16. 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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17. Table 3.2 Active Membrane Transport Processes (1 of 2)
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18. Table 3.2 Active Membrane Transport Processes (2 of 2)
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