1. Selectively Permeable Membranes
The plasma membrane is the boundary that separates cells from their surroundings
The plasma membrane exhibits selective permeability, meaning some things can get through more easily than others.
Permeable: Things can get through
Impermeable: Things can’t get through
Selectively permeable: Some kinds of things can get through
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2. Membrane Models
Membranes have been analyzed through experiments and found to be made of proteins and lipids
In the early 1900s, scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer
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3. WATER Hydrophilic head Hydrophobic tail WATER Fig. 7-2
Figure 7.2 Phospholipid bilayer (cross section)
Hydrophobic
tail
WATER

4. In 1935, Hugh Davson and James Danielli proposed a sandwich model – they thought the phospholipids were sandwiched between layers of proteins
Later studies found problems with this, since proteins have hydrophobic and hydrophilic parts
In 1972, J. Singer and G. Nicolson proposed that the proteins are mixed in with the phospholipids, with hydrophobic parts on the inside of the membrane.
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5. Phospholipid bilayer Hydrophobic regions of protein Hydrophilic
Figure 7.3 The fluid mosaic model for membranes
Hydrophobic regions
of protein
Hydrophilic
regions of protein

6. Inside of extracellular layer Knife
Fig. 7-4
TECHNIQUE
RESULTS
Extracellular
layer
Proteins
Inside of extracellular layer
Knife
Figure 7.4 Freeze-fracture
Plasma membrane
Cytoplasmic layer
Inside of cytoplasmic layer

7. The Fluidity of Membranes
Phospholipids in the plasma membrane can move within the bilayer
Most of the lipids, and some proteins, drift laterally (from side to side)
Rarely does a molecule flip-flop transversely (upside down) across the membrane
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8. (a) Movement of phospholipids
Fig. 7-5a
Lateral movement
(107 times per second)
Flip-flop
( once per month)
Figure 7.5a The fluidity of membranes
(a) Movement of phospholipids

9. Fig. 7-6
We know membranes are fluid (liquid) due to experiments that fused mouse cells with human cells.
RESULTS
Membrane proteins
Mixed proteins
after 1 hour
Mouse cell
Figure 7.6 Do membrane proteins move?
Human cell
Hybrid cell

10. As temperatures cool, membranes switch from a fluid (more liquid) state to a more solid state
The temperature at which a membrane solidifies depends on the types of fatty acid tails on the phospholipids
Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids
Membranes must be fluid to work properly; they are usually about as fluid as salad oil
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11. Unsaturated hydrocarbon tails with kinks Saturated hydro- carbon tails
Fig. 7-5b
More Fluid
Less fluid
Unsaturated hydrocarbon
tails with kinks
Saturated hydro-
carbon tails
Figure 7.5b The fluidity of membranes
(b) Membrane fluidity

12. The steroid cholesterol has different effects on membrane fluidity at different temperatures
At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids so the membrane doesn’t get too fluid
At cool temperatures, it maintains fluidity by preventing tight packing
Overall, cholesterol prevents membranes from getting too fluid or too solid.
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13. (c) Cholesterol within the animal cell membrane
Fig. 7-5c
Cholesterol
Figure 7.5c The fluidity of membranes
(c) Cholesterol within the animal cell membrane

14. Membrane Proteins and Their Functions
A membrane is a collage of different proteins embedded in the phospholipid bilayer
Proteins determine most of the membrane’s specific functions
Different organisms and different organelles have different kinds of membrane proteins.
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15. Fig. 7-7 Fibers of extracellular matrix (ECM) Glyco- Carbohydrate
protein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Figure 7.7 The detailed structure of an animal cell’s plasma membrane, in a cutaway view
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE

16. Some proteins are bound to the surface of the membrane
Others penetrate the hydrophobic core
The hydrophobic regions of these proteins consist of one or more stretches of nonpolar amino acids, often coiled into shapes called alpha helices
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17. EXTRACELLULAR SIDE CYTOPLASMIC SIDE  Helix Fig. 7-8
Figure 7.8 The structure of a transmembrane protein
CYTOPLASMIC
SIDE
 Helix

18. Six major functions of membrane proteins:
Transport
Enzymatic activity
Signal transduction
Cell-cell recognition
Intercellular joining
Attachment to the cytoskeleton and extracellular matrix (ECM)
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19. (b) Enzymatic activity (c) Signal transduction
Fig. 7-9
Signaling molecule
Enzymes
Receptor
ATP
Signal transduction
(a) Transport
(b) Enzymatic activity
(c) Signal transduction
Figure 7.9 Some functions of membrane proteins
Glyco-
protein
(d) Cell-cell recognition
(e) Intercellular joining
(f) Attachment to
the cytoskeleton
and extracellular
matrix (ECM)

20. The Permeability of the Lipid Bilayer
Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane quickly
Polar molecules, such as sugars, do not cross the membrane easily
Small molecules move through the membrane faster than large ones
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21. Transport Proteins
Transport proteins allow hydrophilic substances to move through the membrane
Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel
Most cells have specific types of channel proteins called aquaporins, which allow water to pass through
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22. Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane
A transport protein is specific for the substance it moves; each kind of transport protein can only move specific types of solutes.
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23. Channel protein Solute (a) A channel protein Solute Carrier protein
Fig. 7-15
EXTRACELLULAR FLUID
Channel protein
Solute
CYTOPLASM
(a) A channel protein
Figure 7.15 Two types of transport proteins that carry out facilitated diffusion
Solute
Carrier protein
(b) A carrier protein

24. Passive transport is diffusion of a substance across a membrane with no energy investment
Diffusion is the tendency for molecules to spread out evenly into the available space as they move randomly
At equilibrium, as many molecules cross one way as cross in the other direction
hill.com/sites/ /student_view0/chapter2/animation__how_diffusion _works.html
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25. Membrane (cross section)
Fig. 7-11
Molecules of dye
Membrane (cross section)
WATER
Net diffusion
Net diffusion
Equilibrium
(a) Diffusion of one solute
Figure 7.11 The diffusion of solutes across a membrane
Net diffusion
Net diffusion
Equilibrium
Net diffusion
Net diffusion
Equilibrium
(b) Diffusion of two solutes

26. Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another
The diffusion of a substance across a biological membrane is called passive transport because it requires no energy from the cell to make it happen
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27. (b) Diffusion of two solutes
Fig. 7-11b
Net diffusion
Net diffusion
Equilibrium
Figure 7.11b The diffusion of solutes across a membrane
Net diffusion
Net diffusion
Equilibrium
(b) Diffusion of two solutes

28. Effects of Osmosis on Water Balance
Osmosis = diffusion of water
Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration (opposite direction from the solutes)
Lower solute concentration means higher water concentration, so the water is moving down its concentration gradient in osmosis
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29. Higher concentration Lower Same concentration concentration of sugar
Fig. 7-12
Lower
concentration
of solute (sugar)
Higher concentration
of sugar
Same concentration
of sugar
H2O
Selectively
permeable
membrane
Figure 7.12 Osmosis
Osmosis

30. Water Balance of Cells Without Walls
Tonicity is the ability of a solution to cause a cell to gain or lose water
Isotonic solution: Solute concentration is the same on both sides of the membrane; no net water movement across the plasma membrane
Hypertonic solution: Solute concentration is greater than on the other side of the membrane; hypertonic side gains water
Hypotonic solution: Solute concentration is less than that on the other side of the membrane; hypotonic side gains water
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31. cell cell Hypotonic solution Isotonic solution Hypertonic solution H2O
Fig. 7-13
Hypotonic solution
Isotonic solution
Hypertonic solution
H2O
H2O
H2O
H2O
(a) Animal
cell
Lysed
Normal
Shriveled
H2O
H2O
H2O
H2O
Figure 7.13 The water balance of living cells
(b) Plant
cell
Turgid (normal)
Flaccid
Plasmolyzed

32. Hypertonic or hypotonic environments create osmotic problems for organisms
Osmoregulation, the control of water balance, is necessary for life in such environments
The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump
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33. (a) A contractile vacuole fills with fluid that enters from
Fig. 7-14
50 µm
Filling vacuole
(a) A contractile vacuole fills with fluid that enters from
a system of canals radiating throughout the cytoplasm.
Contracting vacuole
Figure 7.14 The contractile vacuole of Paramecium: an evolutionary adaptation for osmoregulation
(b) When full, the vacuole and canals contract, expelling
fluid from the cell.

34. Water Balance of Cells with Walls
Cell walls help maintain water balance
A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm)
If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt
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35. In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis
Video: Plasmolysis
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36. Facilitated Diffusion = Passive Transport Aided by Proteins
In facilitated diffusion, transport proteins help molecules move down their concentration gradient (diffusion through a protein)
Channel proteins provide corridors that allow a specific kind of molecule or ion to cross the membrane
Channel proteins include
Aquaporins, for facilitated diffusion of water
Ion channels that open or close in response to a stimulus (gated channels)
For the Cell Biology Video Water Movement through an Aquaporin, go to Animation and Video Files.
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37. Channel protein Solute (a) A channel protein Solute Carrier protein
Fig. 7-15
EXTRACELLULAR FLUID
Channel protein
Solute
CYTOPLASM
(a) A channel protein
Figure 7.15 Two types of transport proteins that carry out facilitated diffusion
Solute
Carrier protein
(b) A carrier protein

38. Carrier proteins undergo a change in shape that moves the solute across the membrane
Some diseases are caused by malfunctions in specific transport systems, for example the kidney disease cystinuria
Cystinuria is caused by mutations in 2 genes that tell ribosomes how to make transport proteins
People with the disease are missing these transport proteins, which move some amino acids out of the kidneys
These amino acids build up and crystallize in the kidneys, causing painful kidney stones
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39. 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
These types of proteins require added energy (usually in the form of ATP), so they are forms of active transport.
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40. 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.
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41. Cytoplasmic Na+ binds to
Fig
EXTRACELLULAR
FLUID
[Na+] high
[K+] low
Na+
Na+
[Na+] low
Na+
Figure 7.16, 1 The sodium-potassium pump: a specific case of active transport
CYTOPLASM
[K+] high
Cytoplasmic Na+ binds to
the sodium-potassium pump. 1

42. Na+ binding stimulates
Fig
Na+
Na+
Na+
ATP P
Figure 7.16, 2 The sodium-potassium pump: a specific case of active transport
ADP
Na+ binding stimulates
phosphorylation by ATP. 2

43. Phosphorylation causes the protein to change its
Fig
Na+
Na+
Na+
Figure 7.16, 3 The sodium-potassium pump: a specific case of active transport P
Phosphorylation causes
the protein to change its
shape. Na+ is expelled to
the outside. 3

44. extracellular side and triggers release of the phosphate group.
Fig K+ K+ P
Figure 7.16, 4 The sodium-potassium pump: a specific case of active transport P
K+ binds on the
extracellular side and
triggers release of the
phosphate group. 4

45. restores the protein’s original shape.
Fig K+ K+
Figure 7.16, 5 The sodium-potassium pump: a specific case of active transport
Loss of the phosphate
restores the protein’s original
shape. 5

46. K+ is released, and the cycle repeats. 6 K+ K+ Fig. 7-16-6
Figure 7.16, 6 The sodium-potassium pump: a specific case of active transport K+
K+ is released, and the
cycle repeats. 6

47. 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

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

49. 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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50. An electrogenic pump is a transport protein that generates voltage (+ and – charge) across a membrane (contributing to the membrane potential)
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
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51. – 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+ – +

52. 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
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53. – + 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

54. Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
Small molecules and water enter or leave the cell through the membrane or in transport proteins
Large molecules, such as polysaccharides and proteins, cross the membrane in vesicles; this is called bulk transport
Bulk transport requires energy
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55. 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
Exo = out
cyt = cell
Exocytosis moves things out of the cell
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56. Endocytosis
In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane
There are three types of endocytosis:
Phagocytosis (“cellular eating”)
Pinocytosis (“cellular drinking”)
Receptor-mediated endocytosis
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57. In phagocytosis a cell engulfs a particle or cell 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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58. 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)

59. In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles
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60. 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

61. 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
Receptor mediated endocytosis lets the cell be selective about what molecules come into the cell in vesicles.
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62. 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