1. Membrane Structure and Function7
Membrane Structure and Function
Lecture Presentation by Nicole Tunbridge and
Kathleen Fitzpatrick

2. Life at the Edge Plasma Membrane
Boundary separates living cell from surroundings
Selective permeability
Allows some substances to pass through more easily than others

3. Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins
Fluid Mosaic Model: Cell membrane has a fluid structure with a “mosaic” of various proteins embedded in it
Proteins not randomly distributed in the membrane
Phospholipid Bilayer:
Stable boundary between two aqueous compartments
Phospholipids
Most abundant lipid in membrane
Amphipathic molecules, with hydrophobic and hydrophilic regions

4. WATER WATER Hydrophilic head Hydrophobic tail Figure 7.2
Figure 7.2 Phospholipid bilayer (cross section)
Hydrophobic tail

5. Fibers of extra- cellular matrix (ECM)
Figure 7.3
Fibers of extra- cellular matrix (ECM)
Glyco- protein
Glycolipid
Carbohydrate
EXTRACELLULAR SIDE OF MEMBRANE
Cholesterol
Figure 7.3 Updated model of an animal cell’s plasma membrane (cutaway view)
Microfilaments of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC
SIDE OF
MEMBRANE

6. The Fluidity of Plasma Membranes
Phospholipids can move within the bilayer
Most lipids and some proteins drift laterally
Rarely, a lipid may flip-flop transversely
Membranes must be fluid to work properly
Membranes are usually about as fluid as salad oil

7. Mixed proteins after 1 hour
Figure 7.4-3
Membrane proteins
Mixed proteins after 1 hour
Mouse cell
Human cell
Figure Inquiry: Do membrane proteins move? (step 3)
Hybrid cell

8. Temperature and Fluidity
Temperatures   Membranes fluid, solid
Temperature at which a membrane solidifies depends on types of lipids
EX: Unsaturated fatty acids: more fluid than saturated
Steroid: Cholesterol
Different effects at different temperatures
At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids
At cool temperatures, it maintains fluidity by preventing tight packing

9. (a) Unsaturated versus saturated hydrocarbon tails
Figure 7.5
(a) Unsaturated versus saturated hydrocarbon tails
Fluid
Viscous
Unsaturated tails prevent packing.
Saturated tails pack together.
(b) Cholesterol within the animal cell membrane
Cholesterol reduces membrane fluidity at moderate temperatures, but at low temperatures hinders solidification.
Figure 7.5 Factors that affect membrane fluidity
Cholesterol

10. Evolution of Differences in Membrane Lipid Composition
Variations in Lipid Composition Among Species
Possible adaptations to specific environmental conditions
EX: Ability to change the lipid composition
Evolutionary response to temperature changes in organisms’ habitats

11. Membrane Proteins and Their Functions
Membrane is a collage of different proteins
Proteins are often grouped together and embedded in the fluid matrix of the lipid bilayer
Proteins determine most of the membrane’s specific functions
Peripheral Proteins: Bound to the surface
Integral Proteins: Penetrate the 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

12. N-terminus EXTRACELLULAR SIDE α helix CYTOPLASMIC SIDE C-terminus
Figure 7.6
N-terminus
EXTRACELLULAR SIDE
Figure 7.6 The structure of a transmembrane protein
α helix
CYTOPLASMIC SIDE
C-terminus

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

14. Signaling molecule Enzymes ATP Receptor Signal transduction
Figure 7.7
Signaling molecule
Receptor
Enzymes
ATP
Signal transduction
(a) Transport
(b) Enzymatic activity
(c) Signal transduction
Figure 7.7 Some functions of membrane proteins
Glyco- protein
(d) Cell-cell recognition
(e) Intercellular joining
(f) Attachment to the cytoskeleton and extracellular matrix (ECM)

15. Surface Proteins and Receptors
Surface protein CD4 and “co-receptor” CCR5
Needed in order for HIV to bind to the immune cell
HIV cannot enter the cells of resistant individuals that lack CCR5
Thoughts on possible targeted treatment ideas?

16. Receptor (CD4) but no CCR5 Co-receptor (CCR5) Plasma membrane
Figure 7.8
HIV
Receptor
(CD4)
Receptor (CD4) but no CCR5
Figure 7.8 The genetic basis for HIV resistance
Co-receptor (CCR5)
Plasma membrane
(a)
(b)

17. Role of Membrane Carbohydrates in Cell-Cell Recognition
Mr. T-Cell
Cell-Cell Recognition
Bind to molecules on the extracellular surface of the membrane
Often carbohydrates
Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual
Glycolipids:
Carbohydrates covalently bonded to lipids
Glycoproteins:
Carbohydrates covalently bonded to proteins

18. I pity the fool who doesn’t learn about cell signaling!

19. Synthesis and Sidedness of Membranes
Membranes have distinct inside and outside “faces”
Plasma membrane has an asymmetrical distribution of proteins, lipids, and associated carbohydrates
Determined when the membrane is built by the ER and Golgi apparatus

20. Transmembrane glycoproteins
Figure 7.9
Transmembrane glycoproteins
Secretory protein
Golgi apparatus
Vesicle
Attached carbohydrate
Glycolipid
ER lumen
Plasma membrane: Cytoplasmic face
Extracellular face
Figure 7.9 Synthesis of membrane components and their orientation in the membrane
Transmembrane glycoprotein
Secreted protein
Membrane
glycolipid

21. Concept 7.2: Membrane structure results in selective permeability
Cells exchange materials with surroundings
Controlled by the plasma membrane
Plasma Membrane
Selectively permeable
Regulate cell’s molecular “traffic”
The Permeability of Lipid Bilayer
Hydrophobic/Nonpolar Molecules (hydrocarbons)
Can dissolve in the lipid bilayer, pass through rapidly
Hydrophilic/Polar Molecules (ions)
Do not cross easily

22. Transport Proteins
Allow passage of hydrophilic/polar substances across the membrane
Specific for the substance it moves
Channel Proteins
Have a hydrophilic channel that certain ions can use
Aquaporins: Facilitate passage of water
Carrier Proteins
Bind to molecules, change shape, & act as a shuttle

23. Concept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investment
Passive Transport
The movement of a substance across a biological membrane requiring no energy to be expended by the cell
Diffusion
Osmosis

24. Diffusion Across the Membrane
Tendency for molecules to spread out evenly into the available space
Molecules move randomly, but the diffusion may be directional
Dynamic Equilibrium: Equal #’s of molecules cross in both directions
Molecules diffuse down their concentration gradient, from high  low.
Region along which the density of a chemical substance increases or decreases
No work needed to move substances down the concentration gradient

25. Membrane (cross section)
Figure 7.10
Molecules of dye
Membrane (cross section)
WATER
Net diffusion
Net diffusion
Equilibrium
(a) Diffusion of one solute
Figure 7.10 The diffusion of solutes across a synthetic membrane
Net diffusion
Net diffusion
Equilibrium
Net diffusion
Net diffusion
Equilibrium
(b) Diffusion of two solutes

26. Effects of Osmosis on Water Balance
The diffusion of water molecules across a selectively permeable membrane
Moves from the side of the membrane with a lower solute concentration  the region of higher solute concentration
Seeks equal solute concentration on both sides
Molecules never stop moving

27. Lower concentration of solute (sugar) Higher concentration of solute
Figure 7.11
Lower concentration of solute (sugar)
Higher concentration of solute
More similar concentrations of solute
Sugar molecule
H2O
Selectively permeable membrane
Figure 7.11 Osmosis
Osmosis

28. Water Balance of Cells Without Cell Walls
Tonicity: Surrounding solutions cause cells 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

29. Hypotonic Isotonic Hypertonic H2O H2O H2O H2O (a) Animal cell Lysed
Figure 7.12
Hypotonic
Isotonic
Hypertonic
H2O
H2O
H2O
H2O
(a) Animal cell
Lysed
Normal
Shriveled
Cell wall
Plasma membrane
H2O
Plasma membrane
H2O
H2O
H2O
Figure 7.12 The water balance of living cells
(b) Plant cell
Turgid (normal)
Flaccid
Plasmolyzed

30. Osmoregulation Control of solute concentrations and water balance
Necessary adaptation for life in aquatic environments
Hypertonic or hypotonic environments create osmotic problems for organisms
Example: Paramecium, (protist), is hypertonic to its pond water environment
Has a contractile vacuole that acts as a pump

31. 50 μm Contractile vacuole Figure 7.13
Figure 7.13 The contractile vacuole of Paramecium caudatum

32. Water Balance of Cells with Cell Walls
Cell walls help maintain water balance
A plant cell in a hypotonic solution swells until the wall opposes uptake, making the cell turgid (firm)
Isotonic Environment: Plant cell has no net movement of water into the cell  cell becomes flaccid (limp)
Hypertonic Environment: Plant cells lose water
Membrane pulls away from the cell wall
Causes plant to wilt
Plasmolysis: Lethal effect of wilting

33. Facilitated Diffusion: Passive Transport Aided by Proteins
Transport proteins speed the movement of molecules across the plasma membrane
Channel Proteins: Provide corridors for a specific molecule or ion to cross the membrane
Carrier Proteins: Undergo a subtle change in shape and translocates the solute-binding site across the membrane
Aquaporins facilitate the diffusion of water
Ion channels facilitate the diffusion of ions
Gated Channels: Ion channels open or close in response to a stimulus

34. EXTRACELLULAR FLUID (a) A channel protein CYTOPLASM
Figure 7.14
EXTRACELLULAR
FLUID
(a) A channel protein
Channel protein
Solute
CYTOPLASM
Figure 7.14 Two types of transport proteins that carry out facilitated diffusion
Carrier protein
Solute
(b) A carrier protein

35. 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, and the transport requires no energy
Some transport proteins, however, can move solutes against their concentration gradients

36. Active Transport
Moves substances against their concentration gradients
REQUIRES ENERGY – usually ATP
Performed by specific proteins embedded in the membranes
Allows cells to maintain concentration gradients that differ from their surroundings
Example: Sodium-Potassium Pump

37. Figure 7.15
EXTRACELLULAR
FLUID
[Na+] high [K+] low
Na+
Na+
Na+
Na+
Na+
[Na+] low [K+] high P
ATP
Na+
CYTOPLASM
ADP 2 K+ 1 K+ 6
Na+
Na+
Na+ K+
Figure 7.15 The sodium-potassium pump: a specific case of active transport K+ P K+ K+ 3 P P 5 i 4

38. Facilitated diffusion ATP
Figure 7.16
Passive transport
Active transport
Figure 7.16 Review: passive and active transport
Diffusion
Facilitated diffusion
ATP

39. Ion Pumps Maintain Membrane Potential
Membrane Potential: Voltage difference across a membrane
Voltage created by differences in the distribution of + and - ions across a membrane
Electrochemical Gradient: Combined chemical and electrical forces drive the diffusion of ions across a membrane
Chemical Force: Ion’s concentration gradient
Electrical Force: Effect of the membrane potential on the ion’s movement

40. Electrogenic Pumps
Transport protein that generates voltage across a membrane
Example: Sodium-Potassium Pump (animal cells)
Proton Pump: The main electrogenic pump of plants, fungi, and bacteria
Help store energy that can be used for cellular work

41. ATP − + EXTRACELLULAR FLUID − + H+ H+ H+ Proton pump H+ − + H+ − + H+
Figure 7.17
ATP − +
EXTRACELLULAR FLUID − + H+ H+ H+
Proton pump H+ − + H+ − + H+
CYTOPLASM
Figure 7.17 A proton pump

42. Cotransport Coupled transport by a membrane protein
Occurs when active transport of a solute indirectly drives transport of other substances
Commonly Used in Plant Cells
Proton pumps generate a gradient of H+ ions
Use the gradient to drive active transport of nutrients into the cell

43. Sucrose-H+ cotransporter
Figure 7.18 + −
Sucrose
Sucrose
Sucrose-H+ cotransporter
Diffusion of H+ H+ + H+ − H+ + − H+ H+
Figure 7.18 Cotransport: active transport driven by a concentration gradient H+ H+
Proton pump H+
ATP − + H+

44. 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
Via transport proteins
Large Molecules:
Cross the membrane in bulk, via vesicles
Requires energy
EX: Polysaccharides, proteins

45. Exocytosis
Many secretory cells use exocytosis to export their products
Transport vesicles migrate to the membrane
Fuse with membrane
Release their contents outside the cell

46. Endocytosis Reversal of exocytosis, involving different proteins
Plasma membrane folds inward to form vesicles.
Membrane surrounds the macromolecules that the cell takes into it as the vesicles form.
Types of Endocytosis
Phagocytosis (“cellular eating”)
Pinocytosis (“cellular drinking”)
Receptor-mediated endocytosis

47. Receptor-Mediated Endocytosis
Figure 7.19
Receptor-Mediated Endocytosis
Phagocytosis
Pinocytosis
EXTRACELLULAR FLUID
Solutes
Pseudopodium
Receptor
Plasma membrane
Coat protein
“Food” or other particle
Coated pit
Figure 7.19 Exploring endocytosis in animal cells
Coated vesicle
Food vacuole
CYTOPLASM

48. Types of Endocytosis Phagocytosis Pinocytosis
Cell engulfs a particle in a vacuole
Vacuole fuses with a lysosome to digest the particle
Pinocytosis
Molecules dissolved in droplets are taken up when extracellular fluid is “gulped” into tiny vesicles
Receptor-mediated Endocytosis
Binding of ligands to receptors triggers vesicle formation
Ligand: Any molecule that binds specifically to a receptor site of another molecule

49. Pseudopodium of amoeba
Figure 7.19a
Phagocytosis
EXTRACELLULAR FLUID
Solutes
Pseudopodium of amoeba
Pseudopodium
Bacterium
1 μm
Food vacuole
An amoeba engulfing a bacterium via phago- cytosis (TEM)
“Food” or other particle
Figure 7.19a Exploring endocytosis in animal cells (part 1: phagocytosis)
Food vacuole
CYTOPLASM

50. Pinocytotic vesicles forming (TEMs)
Figure 7.19b
Pinocytosis
Plasma membrane
0.25 μm
Coat protein
Pinocytotic vesicles forming (TEMs)
Coated pit
Figure 7.19b Exploring endocytosis in animal cells (part 2: pinocytosis)
Coated vesicle

51. Receptor-Mediated Endocytosis
Figure 7.19c
Receptor-Mediated Endocytosis
Plasma membrane
Coat protein
Receptor
0.25 μm
Figure 7.19c Exploring endocytosis in animal cells (part 3: receptor-mediated endocytosis)
Top: A coated pit.
Bottom: A coated vesicle forming during receptor- mediated endocytosis (TEMs)