1. Concept 7.4: Active transport uses energy to move solutes against their gradients
Facilitated diffusion is still passive because the solute moves down its concentration gradient
Some transport proteins, however, can move solutes against their concentration gradients
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

2. The Need for Energy in Active Transport
Active transport moves substances against their concentration gradient
Active transport requires energy, usually in the form of ATP
Active transport is performed by specific proteins embedded in the membranes
Animation: Active Transport
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3. The sodium-potassium pump is one type of active transport system
Active transport allows cells to maintain concentration gradients that differ from their surroundings
The 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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

4. 1 2 3 6 5 4 EXTRACELLULAR FLUID [Na+] high Na+ [K+] low Na+ Na+ Na+
Fig
EXTRACELLULAR
FLUID
[Na+] high
Na+
[K+] low
Na+
Na+
Na+
Na+
Na+
Na+
Na+
[Na+] low
ATP P
Na+ P
CYTOPLASM
[K+] high
ADP 1 2 3 K+
Figure 7.16, 1–6 The sodium-potassium pump: a specific case of active transport K+ K+ K+ K+ P K+ P 6 5 4

5. Facilitated diffusion
Fig. 7-17
Passive transport
Active transport
ATP
Diffusion
Facilitated diffusion
Figure 7.17 Review: passive and active transport

6. 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
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7. 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)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

8. 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

9. – EXTRACELLULAR FLUID + ATP – + H+ H+ Proton pump H+ – + H+ H+ H+ – +
Fig. 7-18 –
EXTRACELLULAR
FLUID +
ATP – + H+ H+
Proton pump H+ – + H+ H+ H+
Figure 7.18 An electrogenic pump – +
CYTOPLASM H+ – +

10. Cotransport: Coupled Transport by a Membrane Protein
Cotransport occurs when active transport of a solute indirectly drives transport of another solute
Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

11. – + H+ ATP H+ – + H+ H+ – + H+ H+ – + H+ H+ – + – + Diffusion of H+
Fig. 7-19 – + H+
ATP H+ – +
Proton pump H+ H+ – + H+ H+ – + H+
Diffusion
of H+
Sucrose-H+
cotransporter
Figure 7.19 Cotransport: active transport driven by a concentration gradient H+ –
Sucrose + – +
Sucrose

12. Bulk transport requires energy
Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins
Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles
Bulk transport requires energy
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

13. Animation: 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
Animation: Exocytosis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

14. Animation: Exocytosis and Endocytosis Introduction
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
Animation: Exocytosis and Endocytosis Introduction
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

15. Animation: Phagocytosis
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.
Animation: Phagocytosis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

16. Fig. 7-20 Figure 7.20 Endocytosis in animal cells PHAGOCYTOSIS
EXTRACELLULAR
FLUID
CYTOPLASM
1 µm
Pseudopodium
Pseudopodium
of amoeba
“Food”or
other particle
Bacterium
Food
vacuole
Food vacuole
An amoeba engulfing a bacterium
via phagocytosis (TEM)
PINOCYTOSIS
0.5 µm
Plasma
membrane
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM)
Vesicle
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated
vesicle
Figure 7.20 Endocytosis in animal cells
Coated
pit
Ligand
A coated pit
and a coated
vesicle formed
during
receptor-
mediated
endocytosis
(TEMs)
Coat
protein
Plasma
membrane
0.25 µm

17. PHAGOCYTOSIS CYTOPLASM 1 µm EXTRACELLULAR FLUID Pseudopodium
Fig. 7-20a
PHAGOCYTOSIS
EXTRACELLULAR
FLUID
CYTOPLASM
1 µm
Pseudopodium
Pseudopodium
of amoeba
“Food” or
other particle
Bacterium
Food
vacuole
Figure 7.20 Endocytosis in animal cells—phagocytosis
Food vacuole
An amoeba engulfing a bacterium
via phagocytosis (TEM)

18. Animation: Pinocytosis
In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles
Animation: Pinocytosis
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19. PINOCYTOSIS Plasma membrane Vesicle 0.5 µm Fig. 7-20b
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM)
Vesicle
Figure 7.20 Endocytosis in animal cells—pinocytosis

20. Animation: Receptor-Mediated Endocytosis
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
Animation: Receptor-Mediated Endocytosis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

21. Coat protein Receptor Coated vesicle Coated pit Ligand A coated pit
Fig. 7-20c
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated
vesicle
Coated
pit
Ligand
A coated pit
and a coated
vesicle formed
during
receptor-
mediated
endocytosis
(TEMs)
Coat
protein
Figure 7.20 Endocytosis in animal cells—receptor-mediated endocytosis
Plasma
membrane
0.25 µm

22. Facilitated diffusion
Fig. 7-UN1
Passive transport:
Facilitated diffusion
Channel
protein
Carrier
protein

23. Fig. 7-UN2
Active transport:
ATP

24. Environment: 0.01 M sucrose “Cell” 0.01 M glucose 0.01 M fructose
Fig. 7-UN3
Environment:
0.01 M sucrose
0.01 M glucose
0.01 M fructose
“Cell”
0.03 M sucrose
0.02 M glucose

25. Fig. 7-UN4

26. You should now be able to:
Define the following terms: amphipathic molecules, aquaporins, diffusion
Explain how membrane fluidity is influenced by temperature and membrane composition
Distinguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic, hypotonic, and isotonic solutions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

27. Explain how transport proteins facilitate diffusion
Explain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumps
Explain how large molecules are transported across a cell membrane
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings