1. 5, part 1 Membrane Transport
CAMPBELL BIOLOGY IN FOCUS
Urry • Cain • Wasserman • Minorsky • Reece
5, part 1 Membrane Transport
Kathleen Fitzpatrick and Nicole Tunbridge, Simon Fraser University Modified by James R. Jabbur, Houston Community College
© 2016 Pearson Education, Inc

2. Overview: Life at the Edge (Just don’t go over it)
The plasma membrane is the boundary that separates the living cell from its surroundings
The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily and specifically than others

3. Phospholipids are the most abundant lipid in the plasma membrane
Concept 5.1: Cellular membranes are fluid mosaics of lipids and proteins
Phospholipids are the most abundant lipid in the plasma membrane
Phospholipids are amphipathic*** molecules, containing hydrophobic and hydrophilic regions [what does hydrophobic or hydrophilic mean?]
The fluid mosaic model states that the plasma membrane is a fluid structure with a “mosaic” of various proteins embedded in a lipid bilayer
For the Cell Biology Video Structure of the Cell Membrane, go to Animation and Video Files.

4. The “ampipathic” Lipid Bilayer
WATER
Hydrophilic
head
Figure 7.2 Phospholipid bilayer (cross section)
Hydrophobic
tail
WATER

5. Model: The “Fluid Mosaic”
Phospholipid
bilayer
Figure 7.3 The fluid mosaic model for membranes
Hydrophobic regions
of protein
Hydrophilic
regions of protein
It is “fluid” because lipids and most proteins are free to move, laterally

6. Proof: Freeze-fracture preparation technique
RESULTS
Extracellular
layer
Proteins
Inside of extracellular layer
Knife
Figure 7.4 Freeze-fracture
Plasma membrane
Cytoplasmic layer
Inside of cytoplasmic layer

7. Proof: Mouse-Human Cell Hybridoma
Figure 7.4 Freeze-fracture

8. The Fluidity of Membranes
Membranes must be fluid to work properly
Phospholipids in the plasma membrane can move laterally within the bilayer (fluid movement in 2 dimensions)
Rarely does a molecule “flip-flop” transversely (inside-out) across the membrane
Membranes rich in unsaturated fatty acids and cholesterol are more fluid than those rich in saturated fatty acids

9. Unsaturated hydrocarbon
Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)
(a) Movement of phospholipids
Fluid
(Loose)
Viscous (Dense)
Unsaturated hydrocarbon
tails with kinks
Saturated hydro-
carbon tails
(b) Membrane fluidity
Figure 7.5 The fluidity of membranes
Cholesterol (Intermediate Density)
(c) Cholesterol within the animal cell membrane

10. Cholesterol is important for Membrane Fluidity
The steroid cholesterol has different effects on membrane fluidity at different temperatures
At warm temperatures (such as 37°C), cholesterol restrains the movement of phospholipids; this is useful for animals
At cool temperatures, cholesterol maintains fluidity by preventing tight packing; this is useful for plants (plants are not homeothermic)

11. Membrane Proteins: Structure and Function
Fig. 7-7
Fibers of
extracellular
matrix (ECM)
Glyco-
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
A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer
CYTOPLASMIC SIDE
OF MEMBRANE

12. Structure
Peripheral proteins are loosely bound to the surface of the membrane
Integral proteins penetrate the hydrophobic, interior core of the lipid bilayer
Integral proteins that span the membrane are called transmembrane proteins
The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices

13. How many transmembranous segments does this integral protein have?
EXTRACELLULAR
SIDE
N-terminus
Figure 7.8 The structure of a transmembrane protein
C-terminus
CYTOPLASMIC
SIDE
 Helix
How many transmembranous segments does this integral protein have?

14. Function Proteins determine most of the membrane’s specific functions
Six major functions of membrane proteins:
Transport (in and out; passive and active)
Enzymatic activity
Signal transduction
Cell-to-cell recognition
Intercellular joining
Attachment to the cytoskeleton and extracellular matrix

15. (b) Enzymatic activity (c) Signal transduction
Signaling molecule
Passive
Transport
(Gradient)
Enzymes
Receptor
Active
Transport
(Energy)
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

16. The Role of Membrane Carbohydrates in Cell-Cell Recognition
Cells recognize each other by binding to surface molecules, which are often carbohydrates, on the plasma membrane
Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)
Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual (blood typing is an example between peeps)

17. ABO Blood Typing

18. Synthesis and Sidedness of Membranes
Membranes have distinct inside and outside faces
The asymmetrical distribution of proteins, lipids and associated carbohydrates in the plasma membrane is determined when the membrane is built by the rough Endoplasmic Reticulum and Golgi apparatus [Explain with next slide]

19. ER (secreted protein synthesis) 1
Transmembrane Glycoprotein
(embedded in vesicle membrane)
Secretory Protein
(enclosed inside vesicle)
Glycolipid
Golgi Apparatus (modifies protein) 2
Figure 7.10 Synthesis of membrane components and their orientation on the resulting membrane
Vesicle Relocation 3
Plasma membrane:
Cytoplasmic face 4
Vesicle-Membrane Fusion & Expcytosis
Extracellular face
Transmembrane
glycoprotein
Secreted protein
Membrane glycolipid

20. Concept 5.2: Membrane structure results in selective permeability
A cell must exchange materials with its surroundings in a process that is controlled by components of the cell’s plasma membrane
Plasma membranes are selectively permeable, regulating the cell’s molecular traffic

21. The Permeability of the Lipid Bilayer
Small or Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly
Large or Hydrophilic (polar) molecules, such as sugars, can not dissolve in the lipid bilayer and pass through the membrane – they need help from specialized proteins
Animation: Membrane Selectivity

22. Transport Proteins
Hydrophilic substances pass through the membrane with the help of transport proteins
Channel proteins, have a hydrophilic channel that specific molecules or ions can use as a tunnel
Carrier proteins, bind to specific molecules and change shape to shuttle them across the membrane
Channel proteins called aquaporins facilitate the passage of water

23. Video: Membrane Transport

24. Concept 5.3: Passive transport is diffusion of a substance across a membrane with no energy used
Diffusion is the tendency for molecules to spread out evenly into the available space
Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another (passive means not requiring energy)
At dynamic equilibrium, as many molecules cross one way as cross in the other direction
Animation: Diffusion

25. Membrane (cross section)
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. Effects of Osmosis on Water Balance
Osmosis is the diffusion of water across a selectively permeable membrane
Water diffuses from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides
Osmosis effects cells differently, depending on the presence or absence of a Cell Wall

27. Notice the volume of water shift before and
Lower
concentration
of solute
Higher concentration
of solute
Same concentration
of sugar
Glucose in water
H2O
Notice the volume of water shift before and
after
Selectively
permeable
Membrane
(water only)
Figure 7.12 Osmosis
Osmotic
Process
Osmosis

28. Osmoregulation: Cellular Process to Maintain Water balance within Cells
Tonicity is the ability of a 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

29. cell cell Hypotonic solution Isotonic solution Hypertonic solution 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
Plant cell walls are rigid and push back on water pressure; makes cells stiff

30. Video: Turgid Elodea

31. Facilitated Diffusion: Passive Transport Aided by Integral Membrane Proteins
In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane
Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane. Ion channels that open or close in response to a stimulus are called “gated”
Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane
For the Cell Biology Video Water Movement through an Aquaporin, go to Animation and Video Files.

32. (a) A channel protein (Ion channels are “gated”)
EXTRACELLULAR FLUID
Channel protein
Solute
CYTOPLASM
(a) A channel protein (Ion channels are “gated”)
Figure 7.15 Two types of transport proteins that carry out facilitated diffusion
Solute
Carrier protein
(b) A carrier protein

33. Animation: Active Transport
Concept 5.4: Active transport requires energy to move solutes against their concentration gradients
Active transport moves substances against their concentration gradient, requiring energy, usually in the form of ATP
Active transport is performed by specific proteins embedded in the membranes
Active transport allows cells to maintain concentration gradients that differ from their surroundings
Animation: Active Transport

34. Example: Sodium-potassium pump
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

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

36. Phosphorylation causes the protein to change its
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

37. extracellular side and triggers release of the phosphate group.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

38. restores the protein’s original shape.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

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

40. Review Energy Independent Needs Energy Passive transport
Active transport
ATP
Diffusion
Facilitated diffusion
Figure 7.17 Review: passive and active transport
Energy Independent
Needs Energy

41. How Ion Pumps Maintain Cell Membrane Potential
Membrane potential is the voltage difference across a membrane, created by differences in the distribution of positive and negative ions
Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane. The forces are:
A chemical force, caused by the ion’s concentration gradient
An electrical force, caused by the difference in ion concentrations across the membrane

42. An electrogenic pump is a transport protein that generates voltage across a membrane by expending energy in the process (Primary Active Transport)
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
(next slides)

43. Sodium-Potassium ATPase
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
Sodium-Potassium ATPase 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

44. Proton ATPase EXTRACELLULAR FLUID – + ATP – + H+ H+ Proton pump H+ – +
Figure 7.18 An electrogenic pump H+ H+ – +
CYTOPLASM H+ – +

45. Cotransport: Coupled Transport by a Membrane Protein
Cotransport occurs when active transport of a solute by Primary Transport indirectly drives transport of another solute (Secondary Active Transport)
The Primary Active Transporter generates a concentration gradient used by the Secondary Active Transporter
Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell

46. – + H+ ATP H+ – + H+ H+ – + H+ H+ – + H+ H+ – + – + Diffusion of H+
Proton pump
(1o Mechanism) H+ H+ – + H+ H+ – + H+
Diffusion
of H+
Sucrose-H+
Cotransporter
(2o Mechanism)
Figure 7.19 Cotransport: active transport driven by a concentration gradient H+ –
Sucrose + – +
Sucrose

47. Animation: Exocytosis and Endocytosis Introduction
Concept 5.5: 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
Animation: Exocytosis and Endocytosis Introduction

48. 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

49. 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

50. In phagocytosis a cell engulfs a particle in a vacuole
In phagocytosis a cell engulfs a particle in a vacuole. The vacuole fuses with a lysosome to digest the particle
In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles
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
For the Cell Biology Video Phagocytosis in Action, go to Animation and Video Files.

51. Animation: Phagocytosis
EXTRACELLULAR
FLUID
CYTOPLASM
1 µm
Pseudopodium
Pseudopodium
of amoeba
Animation: Phagocytosis
“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)
Animation: Pinocytosis
Vesicle
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated
vesicle
Figure 7.20 Endocytosis in animal cells
Coated
pit
Ligand
Animation: Receptor-Mediated Endocytosis
A coated pit
and a coated
vesicle formed
during
receptor-
mediated
endocytosis
(TEMs)
Coat
protein
Plasma
membrane
0.25 µm