1. Membrane Transport and Cell Signaling5
Membrane Transport and Cell Signaling

2. Do now: Explain why the membrane is a lipid bilayer
Why is the cell membrane described as a fluid mosaic?
What is meant by the terms “semi-permeable”

3. Hydrophilic head WATER WATER Hydrophobic tail Figure 5.3
Figure 5.3 Phospholipid bilayer (cross section)
Hydrophobic
tail
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4. EXTRACELLULAR SIDE OF MEMBRANE
Figure 5.2 A) B
Carbohydrate F
EXTRACELLULAR
SIDE OF
MEMBRANE
Figure 5.2 Current model of an animal cell’s plasma membrane (cutaway view) C
Microfilaments
of cytoskeleton D
proteins E
CYTOPLASMIC SIDE OF MEMBRANE
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5. EXTRACELLULAR SIDE OF MEMBRANE
Figure 5.2
Fibers of extra-
cellular matrix (ECM)
Glyco-
protein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Figure 5.2 Current model of an animal cell’s plasma membrane (cutaway view)
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE OF MEMBRANE
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6. Figure 5.1
Figure 5.1 How do cell membrane proteins help regulate chemical traffic?
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7. Fluid Mosaic Model Fluid: the phospholipids are capable of drift
Mosaic: the membrane is made up of many structures: phospholipids, proteins, cholesterol, glycoproteins, glycolipids
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8. Overview: Life at the Edge
The plasma membrane separates the living cell from its surroundings
The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others
Video: Membrane and Aquaporin
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9. How did scientists conclude that the C.M is fluid?
Membrane proteins
Mouse cell
Human cell
Figure Inquiry: Do membrane proteins move? (step 1)
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10. Results Membrane proteins Mouse cell Human cell Hybrid cell
Figure 5.4-2
Results
Membrane proteins
Mouse cell
Human cell
Figure Inquiry: Do membrane proteins move? (step 2)
Hybrid cell
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11. Results Membrane proteins Mixed proteins after 1 hour Mouse cell
Figure 5.4-3
Results
Membrane proteins
Mixed proteins
after 1 hour
Mouse cell
Human cell
Figure Inquiry: Do membrane proteins move? (step 3)
Hybrid cell
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12. The Fluidity of Membranes
Most of the lipids and some proteins in a membrane can shift about laterally
The lateral movement of phospholipids is rapid; proteins move more slowly
Some proteins move in a directed manner; others seem to be anchored in place
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13. Unsaturated tails prevent packing. Saturated tails pack together.
Figure 5.5
Fluid
Viscous
Unsaturated tails prevent
packing.
Saturated tails pack
together.
(a) Unsaturated versus saturated hydrocarbon tails
(b) Cholesterol reduces
membrane fluidity at
moderate temperatures,
but at low temperatures
hinders solidification.
Figure 5.5 Factors that affect membrane fluidity
Cholesterol
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14. As temperatures cool, membranes switch from a fluid state to a solid state
The temperature at which a membrane solidifies depends on the types of lipids
A membrane remains fluid to a lower temperature if it is rich in phospholipids with unsaturated hydrocarbon tails
At warm temperatures (such as 37oC), cholesterol restrains movement of phospholipids
At cool temperatures, it maintains fluidity by preventing tight packing
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15. Membrane Proteins and Their Functions
Integral proteins penetrate the hydrophobic interior of the lipid bilayer
Integral proteins that span the membrane are called transmembrane proteins
Peripheral proteins are loosely bound to the surface of the membrane
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16. N-terminus EXTRACELLULAR SIDE  helix CYTOPLASMIC SIDE C-terminus
Figure 5.6
N-terminus
EXTRACELLULAR
SIDE
Figure 5.6 The structure of a transmembrane protein
 helix
CYTOPLASMIC
SIDE
C-terminus
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17. (b) Enzymatic activity (c) Attachment to the cytoskeleton and extra-
Figure 5.7
Enzymes
ATP
(a) Transport
(b) Enzymatic activity
(c) Attachment to the
cytoskeleton and extra-
cellular matrix (ECM)
Signaling
molecule
Receptor
Figure 5.7 Some functions of membrane proteins
Glyco-
protein
(d) Cell-cell recognition
(e) Intercellular joining
(f) Signal transduction
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18. The Role of Membrane Carbohydrates in Cell-Cell Recognition
Cells recognize each other by binding to surface molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane
Glycolipids- carbohydrates to lipids
Ex: Cell signaling
Glycoproteins- carbohydrates to proteins
Ex: antibody recognition
Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual
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19. 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 ER and Golgi apparatus
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20. Transmembrane Secretory glycoproteins protein Golgi apparatus Vesicle
Figure 5.8
Transmembrane
glycoproteins
Secretory
protein
Golgi
apparatus
Vesicle ER
ER lumen
Glycolipid
Figure 5.8 Synthesis of membrane components and their orientation in the membrane
Plasma membrane:
Cytoplasmic face
Transmembrane
glycoprotein
Extracellular face
Secreted
protein
Membrane
glycolipid
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21. CONCEPT 5.2: Membrane structure results in selective permeability
A cell must regulate transport of substances across cellular boundaries
Plasma membranes are selectively permeable, regulating the cell’s molecular traffic
Small, polar ions (K+)
Large polar (sugar)
Small, polar (water)
Nonpolar, small (carbon dioxide)
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22. Substances are able to move through a membrane, but what determines direction?
Diffusion is the tendency for molecules to spread out evenly into the available space
Animation: Diffusion
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23. Diffusion
Substances diffuse down their concentration gradient (high to low)
No work must be done to move substances down the concentration gradient (spontaneous)
Also known as Passive Transport
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24. How do polar molecules diffuse through a membrane?
In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane
How is facilitated diffusion similar to simple diffusion?
How is facilitated diffusion different from simple diffusion?
Video: Aquaporins
Video: Membrane and Aquaporin
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25. Transport Proteins allow passage of hydrophilic substances/polar substances across the membrane
channel proteins
have a hydrophilic channel that certain molecules or ions can use as a tunnel
Channel proteins called aquaporins facilitate the passage of water
carrier proteins
bind to molecules and change shape to shuttle them across the membrane
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26. Effects of Osmosis on Water Balance
Osmosis is the facilitated diffusion of free water across a selectively permeable membrane
Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides
Animation: Membrane Selectivity
Animation: Osmosis
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27. Do now: Is water a polar or nonpolar molecule? Explain.
If water must diffuse what type of transport will it utilize? Explain
In which type of solution would a cell gain mass – hypertonic, hypotonic or isotonic solution? Explain.

28. Lower concentration of solute (sugar) Higher concentration of solute
Figure 5.10
Lower
concentration
of solute (sugar)
Higher
concentration
of solute
More similar concen-
trations of solute
Sugar
molecule
H2O
Selectively
permeable
membrane
Figure 5.10 Osmosis
Osmosis
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29. Hypotonic Isotonic Hypertonic H2O H2O H2O H2O Animal cell Lysed Normal
Figure 5.11
Hypotonic
Isotonic
Hypertonic
H2O
H2O
H2O
H2O
Animal cell
Lysed
Normal
Shriveled
Cell wall
H2O
H2O
H2O
H2O
Figure 5.11 The water balance of living cells
Plant cell
Turgid (normal)
Flaccid
Plasmolyzed
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30. Water Balance of Cells Without Walls
Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water
Isotonic solution: Solute concentration is the same as 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
Video: Turgid Elodea
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31. Hypertonic or hypotonic environments create osmotic problems for organisms
Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environments
Problem: The protist Paramecium caudatum, lives in a hypotonic external environment
Solution: has a contractile vacuole that can pump excess water out of the cell
Video: Chlamydomonas
Video: Paramecium Vacuole
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32. Lab on osmosis: predicting concentrations
You will be given 4 sucrose solutions of varying hypertonic concentrations.
Using a potato, how can you determine which solution is the most concentrated?

33. 50 m Contractile vacuole Figure 5.12
Figure 5.12 The contractile vacuole of Paramecium caudatum
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34. Active Transport
Active transport moves substances against their concentration gradients (L to H)
requires energy, in the form of ATP
Active transport allows cells to maintain concentration gradients that differ from their surroundings
The sodium-potassium pump is one type of active transport system
Animation: Active Transport
Video: Sodium-Potassium Pump
Video: Membrane Transport
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35. Figure 5.14
EXTRACELLULAR FLUID
[Na] high
[K] low
[Na] low 1
CYTOPLASM
[K] high 2
ADP 6 3
Figure 5.14 The sodium-potassium pump: a specific case of active transport 5 4
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36. to the sodium-potassium pump. The affinity for Na
Figure 5.14a
EXTRACELLULAR FLUID
[Na] high
[K] low
[Na] low
CYTOPLASM
[K] high
ADP
Cytoplasmic Na binds
to the sodium-potassium
pump. The affinity for Na
is high when the protein
has this shape. 1
Figure 5.14a The sodium-potassium pump: a specific case of active transport (part 1: Na+ binding)
Na binding stimulates
phosphorylation by ATP. 2
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37. Phosphorylation leads to a change in protein
Figure 5.14b
Phosphorylation leads
to a change in protein
shape, reducing its affinity
for Na, which is released
outside. 3
Figure 5.14b The sodium-potassium pump: a specific case of active transport (part 2: K+ binding)
The new shape has a
high affinity for K, which
binds on the extracellular
side and triggers release
of the phosphate group. 4
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38. group restores the protein’s original shape, which has a
Figure 5.14c
Figure 5.14c The sodium-potassium pump: a specific case of active transport (part 3: K+ release)
Loss of the phosphate
group restores the protein’s
original shape, which has a
lower affinity for K. 5
K is released; affinity
for Na is high again, and
the cycle repeats. 6
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39. __________ transport _______ transport A B Figure 5.15
Figure 5.15 Review: passive and active transport A B
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40. Facilitated diffusion
Figure 5.15
Passive transport
Active transport
Figure 5.15 Review: passive and active transport
Diffusion
Facilitated
diffusion
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41. Bulk Transport
Small solutes and water enter or leave the cell through the lipid bilayer or by means of transport proteins
How can large molecules cross?
Vesicles
Requires energy
Two Types of bulk transport:
Endocytosis
Exocytosis
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42. Exocytosis
In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents
Many secretory cells use exocytosis to export products
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43. Endocytosis
In endocytosis, the cell takes in molecules and particulate matter by forming new 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
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44. Endocytosis Animation: Exocytosis Endocytosis Introduction
Animation: Phagocytosis
Video: Phagocytosis
Animation: Pinocytosis
Animation: Receptor-Mediated Endocytosis
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45. Receptor-Mediated Endocytosis
Figure 5.18
Receptor-Mediated
Endocytosis
Phagocytosis
Pinocytosis
EXTRACELLULAR
FLUID
Solutes
Pseudopodium
Plasma
membrane
Receptor
Coat
protein
“Food”
or other
particle
Coated
pit
Figure 5.18 Exploring endocytosis in animal cells
Coated
vesicle
Food
vacuole
CYTOPLASM
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46. An amoeba engulfing a bacterium via phagocytosis (TEM)
Figure 5.18a
Phagocytosis
EXTRACELLULAR
FLUID
Pseudopodium
of amoeba
Solutes
Pseudopodium
Bacterium
1 m
Food vacuole
“Food”
or other
particle
An amoeba engulfing a bacterium
via phagocytosis (TEM)
Figure 5.18a Exploring endocytosis in animal cells (part 1: phagocytosis)
Food
vacuole
CYTOPLASM
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47. 0.25 m Pinocytosis Plasma membrane Coat protein
Figure 5.18b
Pinocytosis
Plasma
membrane
0.25 m
Coat
protein
Pinocytotic vesicles forming
(TEMs)
Coated
pit
Figure 5.18b Exploring endocytosis in animal cells (part 2: pinocytosis)
Coated
vesicle
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48. 0.25 m Receptor-Mediated Endocytosis Coat Plasma protein membrane
Figure 5.18c
Receptor-Mediated
Endocytosis
Plasma
membrane
Coat
protein
Receptor
0.25 m
Figure 5.18c Exploring endocytosis in animal cells (part 3: receptor-mediated endocytosis)
Top: A coated pit Bottom: A coated
vesicle forming during receptor-
mediated endocytosis (TEMs)
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