1. Chapter 7: Membrane Structure and Function

2. Overview: Life at the Edge
Plasma membrane: boundary that separates the living cell from its surroundings
Exhibits selective permeability
allows some substances to cross it more easily than others
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

3. Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins
Phospholipids: most abundant lipid in the plasma membrane
amphipathic molecules: contain hydrophobic and hydrophilic regions
Fluid mosaic model: membrane is a fluid structure with a “mosaic” of various proteins embedded in it
For the Cell Biology Video Structure of the Cell Membrane, go to Animation and Video Files.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

4. History of the Membrane
1935: Hugh Davson & James Danielli proposed sandwich model
phospholipid bilayer lies between two layers of globular proteins
PROBLEM: Placement of membrane proteins which have hydrophilic and hydrophobic regions
1972: J. Singer & G. Nicolson proposed membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water
Freeze-fracture studies of the plasma membrane supported the fluid mosaic model
Freeze-fracture: specialized preparation technique that splits membrane along the middle of the phospholipid bilayer
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

5. 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

6. The Fluidity of Membranes
Phospholipids move within the bilayer
Most lipids, and some proteins, drift laterally
Rarely molecule flip-flop transversely across membrane
Temperatures cool  membranes switch from fluid to solid state
Temperature at which membrane solidifies = types of lipids
Unsaturated fatty acids = more fluid
Saturated fatty acids = more solid
Membranes must be fluid to work properly
usually about as fluid as salad oil

7. (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

8. Unsaturated hydrocarbon tails with kinks Saturated hydro- carbon tails
Fig. 7-5b
Fluid
Viscous
Unsaturated hydrocarbon
tails with kinks
Saturated hydro-
carbon tails
Figure 7.5b The fluidity of membranes
(b) Membrane fluidity

9. Membrane proteins Mixed proteins after 1 hour Mouse cell Human cell
Fig. 7-6
RESULTS
Membrane proteins
Mixed proteins
after 1 hour
Mouse cell
Figure 7.6 Do membrane proteins move?
Human cell
Hybrid cell

10. The Fluidity of Membranes
Steroid cholesterol effects membrane different temperatures
warm temperatures (such as 37°C) cholesterol restrains movement of phospholipids
cool temperatures  maintains fluidity by preventing tight packing

11. Membrane Proteins and Their Functions
Collage of different proteins embedded in the fluid matrix of the lipid bilayer
Proteins  membrane’s specific functions
Peripheral proteins: bound to the surface of the membrane
Integral proteins: penetrate hydrophobic core
Transmembrane proteins: Integral proteins that span the membrane
Hydrophobic regions consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

12. 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

13. Membrane Functions Six major functions of membrane proteins: Transport
Enzymatic activity
Signal transduction
Cell-cell recognition
Intercellular joining
Attachment to the cytoskeleton and extracellular matrix (ECM)

14. (b) Enzymatic activity (c) Signal transduction
Fig. 7-9ac
Signaling molecule
Enzymes
Receptor
Figure 7.9a–c Some functions of membrane proteins
ATP
Signal transduction
(a) Transport
(b) Enzymatic activity
(c) Signal transduction

15. (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to
Fig. 7-9df
Glyco-
protein
Figure 7.9d–f Some functions of membrane proteins
(d) Cell-cell recognition
(e) Intercellular joining
(f) Attachment to
the cytoskeleton
and extracellular
matrix (ECM)

16. The Role of Membrane Carbohydrates in Cell-Cell Recognition
Cells recognize each other by binding to surface molecules, often carbohydrates, on plasma membrane
Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)
Carbohydrates on external side of plasma membrane vary among species, individuals, and even cell types in an individual
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

17. Synthesis and Sidedness of Membranes
Membranes have distinct inside and outside faces
Asymmetrical distribution of proteins, lipids, and carbohydrates in plasma membrane determined when membrane is built by ER and Golgi apparatus

18. Concept 7.2: Membrane structure results in selective permeability
Cell must exchange materials with its surroundings  controlled by the plasma membrane
Plasma membranes = selectively permeable
regulating the cell’s molecular traffic
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

19. Permeability of the Lipid Bilayer
Hydrophobic/nonpolar molecules (like hydrocarbons) can dissolve in lipid bilayer and pass through membrane rapidly
Polar molecules (like sugars) do not cross the membrane easily
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

20. Transport Proteins
Transport proteins: allow passage of hydrophilic substances across the membrane
Channel proteins: have a hydrophilic channel that certain molecules or ions can use as tunnel
Aquaporins: channel proteins that facilitate passage of water
Carrier proteins: bind to molecules and change shape to shuttle them across the membrane
specific for the substance it moves

21. Concept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investment
Diffusion: tendency for molecules to spread out evenly into the available space
Each molecule moves randomly HOWEVER diffusion of molecules may exhibit net movement in one direction
Dynamic equilibrium molecules crossing one direction = other direction

22. Passive Transport Substances diffuse down concentration gradient
the difference in concentration of a substance from one area to another
Passive transport: requires no energy from the cell
The diffusion of a substance across biological membrane
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

23. Effects of Osmosis on Water Balance
Osmosis: diffusion of water across a selectively permeable membrane
Water diffuses across membrane from lower [solute]  higher [solute]
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

24. Water Balance of Cells Without Walls
Tonicity: ability of solution to cause a cell to gain or lose water
Isotonic solution: Solute concentration is the same as that inside the cell
no net water movement across the plasma membrane
Hypertonic solution: Solute concentration is greater than that inside the cell
cell loses water
Hypotonic solution: Solute concentration is less than that inside the cell
cell gains water
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

25. 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

26. Effects of Osmosis on Living Cells
Hypertonic OR hypotonic environments create osmotic problems for organisms
Why? What are some problems?
Osmoregulation: control of water balance
necessary adaptation for life in such environments
Protist Paramecium is hypertonic to its pond water environment has a contractile vacuole that acts as pump

27. Water Balance of Cells with Walls
Cell walls help maintain water balance
Plant cell in…
hypotonic solution swells until wall opposes uptake  cell now turgid (firm)
isotonic  no net movement of water into the cell  cell becomes flaccid (limp) plant may wilt
hypertonic environment  plant cells lose water  membrane pulls away from the wall (usually lethal effect) called plasmolysis

28. Facilitated Diffusion: Passive Transport Aided by Proteins
Facilitated diffusion: transport proteins speed up passive movement of molecules across plasma membrane
Channel proteins: provide corridors that allow specific molecule/ion to cross 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.

29. Facilitated Diffusion: Passive Transport Aided by Proteins
Carrier proteins undergo subtle change in shape that translocates solute-binding site across the membrane
Some diseases caused by malfunctions in specific transport systems
Ex: kidney disease, cystinuria

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

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

32. 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

33. 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

34. 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

35. 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

36. 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

37. 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

38. 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

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

40. How Ion Pumps Maintain Membrane Potential
Membrane potential: voltage difference across a membrane
created by differences in distribution of positive/negative ions
Electrochemical gradient: two combined forces that drive diffusion of ions across membrane:
Chemical force: the ion’s concentration gradient
Electrical force: the effect of the membrane potential on the ion’s movement
Electrogenic pump: transport protein that generates voltage across membrane
Sodium-potassium pump: major electrogenic pump of animal cells
Proton pump: main electrogenic pump of plants, fungi, and bacteria is a

41. – 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+ – +

42. Cotransport: Coupled Transport by a Membrane Protein
Cotransport: active transport of a solute indirectly drives transport of another solute
Plants use gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell

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

44. Exocytosis
Exocytosis: transport vesicles migrate to the membrane, fuse with it, and release their contents
secretory cells use exocytosis to export their products
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

45. Endocytosis
Endocytosis: cell takes in macromolecules by forming vesicles from plasma membrane
reversal of exocytosis, involving different proteins
Three types of endocytosis:
Phagocytosis: cell engulfs a particle in a vacuole then fuses with a lysosome to digest the particle (“cellular eating”)
Pinocytosis: molecules are taken up when extracellular fluid is “gulped” into tiny vesicles (“cellular drinking”)
Receptor-mediated endocytosis: binding of ligands to receptors triggers vesicle formation
Ligand: any molecule that binds specifically to receptor site of another molecule

46. 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)

47. 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

48. 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

49. 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

50. 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

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

52. Fig. 7-UN2
Active transport:
ATP

53. 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

54. Fig. 7-UN4