1. (a) A channel protein Channel protein Solute Carrier protein Solute
Figure 7.17
EXTRACELLULAR FLUID
(a) A channel protein
Channel protein
Solute
CYTOPLASM
Figure 7.17 Two types of transport proteins that carry out facilitated diffusion.
Carrier protein
Solute
(b) A carrier protein 1

2. Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane
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3. Active transport uses energy to move solutes against gradients
Facilitated diffusion is still passive because solute moves down its concentration gradient, and transport requires no energy
Some transport proteins, however, can move solutes against concentration gradients
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4. The Need for Energy in Active Transport
Active transport moves substances against their concentration gradients
Active transport requires energy usually in form of ATP
Active transport is performed by specific proteins embedded in membranes
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5. Sodium-potassium pump is one type of active transport system
Active transport allows cells to maintain concentration gradients that differ from their surroundings
Sodium-potassium pump is one type of active transport system
For the Cell Biology Video Na+/K+ATPase Cycle, go to Animation and Video Files.
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6. EXTRACELLULAR FLUID [Na] high [K] low Na Na [Na] low CYTOPLASM
Figure
EXTRACELLULAR FLUID
[Na] high
[K] low
Na
Na
CYTOPLASM
[Na] low
Na 1
[K] high
Figure 7.18 The sodium-potassium pump: a specific case of active transport. 6

7. EXTRACELLULAR FLUID [Na] high [K] low Na Na Na Na Na [Na] low
Figure
EXTRACELLULAR FLUID
[Na] high
[K] low
Na
Na
Na
Na
Na
[Na] low
ATP
CYTOPLASM
Na P 1
[K] high 2
ADP
Figure 7.18 The sodium-potassium pump: a specific case of active transport. 7

8. EXTRACELLULAR FLUID [Na] high Na Na [K] low Na Na Na Na Na
Figure
EXTRACELLULAR FLUID
[Na] high
Na
Na
[K] low
Na
Na
Na
Na
Na
Na
ATP
CYTOPLASM
[Na] low
Na P P 1
[K] high 2
ADP 3
Figure 7.18 The sodium-potassium pump: a specific case of active transport. 8

9. EXTRACELLULAR FLUID [Na] high Na Na [K] low Na Na Na Na Na
Figure
EXTRACELLULAR FLUID
[Na] high
Na
Na
[K] low
Na
Na
Na
Na
Na
Na
CYTOPLASM
[Na] low
ATP
Na P P 1
[K] high 2
ADP 3 K
Figure 7.18 The sodium-potassium pump: a specific case of active transport. K P 4
P i 9

10. EXTRACELLULAR FLUID [Na] high Na Na [K] low Na Na Na Na Na
Figure
EXTRACELLULAR FLUID
[Na] high
Na
Na
[K] low
Na
Na
Na
Na
Na
Na
ATP
CYTOPLASM
[Na] low
Na P P 1
[K] high 2
ADP 3 K
Figure 7.18 The sodium-potassium pump: a specific case of active transport. K K K P 5 4
P i 10

11. EXTRACELLULAR FLUID [Na] high Na Na [K] low Na Na Na Na Na
Figure
EXTRACELLULAR FLUID
[Na] high
Na
Na
[K] low
Na
Na
Na
Na
Na
Na
CYTOPLASM
[Na] low
ATP
Na P P 1
[K] high 2
ADP 3 K
Figure 7.18 The sodium-potassium pump: a specific case of active transport. K K K K P 6 K 5 4
P i 11

12. Facilitated diffusion ATP
Figure 7.19
Passive transport
Active transport
Figure 7.19 Review: passive and active transport.
Diffusion
Facilitated diffusion
ATP 12

13. How Ion Pumps Maintain Membrane Potential
Membrane potential is the voltage difference across a membrane
Voltage is created by differences in the distribution of positive and negative ions across a membrane
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14. 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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15. An electrogenic pump is a transport protein that generates voltage across a membrane
The sodium-potassium pump is the major electrogenic pump of animal cells
The main electrogenic pump of plants, fungi, and bacteria is a proton pump
Electrogenic pumps help store energy that can be used for cellular work
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16. ATP   EXTRACELLULAR FLUID   H Proton pump H H H   H   H
Figure 7.20
ATP  
EXTRACELLULAR FLUID   H
Proton pump H H H   H   H
Figure 7.20 A proton pump.
CYTOPLASM 16

17. Cotransport: Coupled Transport by a Membrane Protein
Cotransport occurs when active transport of a solute indirectly drives transport of other solutes
Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell
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18. Sucrose-H cotransporter
Figure 7.21
ATP H   H
Proton pump H H   H H H   H
Sucrose-H cotransporter
Diffusion of H
Figure 7.21 Cotransport: active transport driven by a concentration gradient.
Sucrose  
Sucrose 18

19. Bulk transport across plasma membrane occurs by exocytosis and endocytosis
Small molecules and water enter or leave the cell through the lipid bilayer or via transport proteins
Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles
Bulk transport requires energy
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20. Exocytosis
In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents
Many secretory cells use exocytosis to export their products
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21. Endocytosis
In endocytosis, the cell takes in macromolecules by forming 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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22. In phagocytosis a cell engulfs a particle in a vacuole
The vacuole fuses with a lysosome to digest the particle
For the Cell Biology Video Phagocytosis in Action, go to Animation and Video Files.
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23. In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles
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24. In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation
A ligand is any molecule that binds specifically to a receptor site of another molecule
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25. Receptor-Mediated Endocytosis
Figure 7.22
Phagocytosis
Pinocytosis
Receptor-Mediated Endocytosis
EXTRACELLULAR FLUID
Solutes
Pseudopodium
Receptor
Plasma membrane
Ligand
Coat proteins
Coated pit
“Food” or other particle
Coated vesicle
Figure 7.22 Exploring: Endocytosis in Animal Cells
Vesicle
Food vacuole
CYTOPLASM 25

26. Pseudopodium of amoeba
Figure 7.22a
Phagocytosis
EXTRACELLULAR FLUID
Solutes
Pseudopodium of amoeba
Pseudopodium
Bacterium
1 m
Food vacuole
“Food” or other particle
An amoeba engulfing a bacterium via phagocytosis (TEM).
Figure 7.22 Exploring: Endocytosis in Animal Cells
Food vacuole
CYTOPLASM 26

27. Pinocytosis 0.5 m Plasma membrane
Figure 7.22b
Pinocytosis
0.5 m
Plasma membrane
Pinocytosis vesicles forming in a cell lining a small blood vessel (TEM).
Figure 7.22 Exploring: Endocytosis in Animal Cells
Vesicle 27

28. Receptor-Mediated Endocytosis
Figure 7.22c
Receptor-Mediated Endocytosis
Receptor
Plasma membrane
Coat proteins
Ligand
Coat proteins
Coated pit
0.25 m
Figure 7.22 Exploring: Endocytosis in Animal Cells
Coated vesicle
Top: A coated pit. Bottom: A coated vesicle forming during receptor-mediated endocytosis (TEMs). 28