1. The Plasma Membrane
Membrane Transport

2. Figure 5.1
Figure 5.1 How do cell membrane proteins help regulate chemical traffic?

3. The Fluid Mosaic Model Phospholipids- the main “fabric”
amphipathic= they have both hydrophilic AND a hydrophobic regions
Proteins- embedded in the phospholipid membrane
Also amphipathic
Determine the function of the membrane
Proteins are not distributed randomly or evenly, but rather according to function

4. How fluid is fluid?
The membrane is held together my hydrophobic interactions-weaker than covalent bonds
Constant lateral movement
Proteins larger than lipids therefore move more slowly

5. Viscosity
A measure of a fluid’s resistance to flow; how “thick” or “sticky” it is
Due to molecular makeup and internal friction
Honey is more viscous than water

6. What determines a membrane’s viscosity?
Hydrocarbon tails on its phospholipids
Saturated- more viscous
Unsaturated- less viscous, more fluid
Temperature
Decrease in temp more viscous; may eventually solidify
Increase in temp less viscous; too fluid, cannot support proteins and their function

7. What determines a membrane’s viscosity?
Cholesterol- helps membranes resist changes in fluidity with changes in temperature
High temps- restricts movement of phospholipids
Low temps- prevents phospholipids from packing together
Evolution
Membrane composition evolves to meet specific environmental needs
Cold

8. 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 8

9. What determines a membrane’s viscosity?
Evolution
Membrane composition evolves to meet specific environmental needs
Cold water fish
Archea that live at 90°C (194°F)
Some alter their composition seasonally

10. Membrane Proteins and Their Functions
The proteins within the phospholipid bilayer determine the function of the membrane.
Different cells  different membrane proteins
Different organelles with a specific cell  different membrane proteins

11. Two Major Types of Proteins
Integral
Peripheral
Can you see the difference?

12. 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

13. Integral Penetrates the membrane
Transmembrane- through to both surfaces
Partially embedded- only exposed on one surface
The embedded portions have hydrophobic amino acids, often in an α helix
Some have hydrophilic channels through them to allow for passage of substances through the membrane

14. 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 14

15. Peripheral Not embedded Bound to either surface
Extracellular matrix (outside)
Cytoskeletal elements (inside)
Provide extra support for the membrane

16. 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

17. 6 Major Functions of Plasma Membrane Proteins
Transport
Enzymatic activity
Attachment to the cytoskeleton and ECM
Cell-cell recognition
Intercellular joining
Signal transduction

18. (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 18

19. Membrane Carbohydrates
Cell-cell recognition
Can be covalent bound to either lipids or proteins on the extracellular side of the membrane
Glycoproteins
Glycolipids
Act as markers to distinguish cells
Ex. ABO blood types

20. Membrane Synthesis
Proteins and lipids- ER
Carbohydrates added –Golgi

21. Selective permeability
2 aspects of “selectivity”
The membrane takes up some small ions and molecules, but not others
Substances that are allowed through, do so at different rates
How does the membrane accomplish this selectivity?

22. Form Follows Function Hydrophilic head WATER WATER Hydrophobic tail
Figure 5.3
Form Follows Function
Hydrophilic
head
WATER
WATER
Figure 5.3 Phospholipid bilayer (cross section)
Hydrophobic
tail

23. Nonpolar substances= hydrophobic
Cross easily
Ex. Hydrocarbons, CO2 ,O2
Ions & polar substances= hydrophilic
Hard to pass
Ex. Glucose, H2O, Na+, Cl-
Ions especially have a hard time as they tend to be surrounded by a “shell” of water molecules

24. Transport Proteins Channel proteins vs. carrier proteins
Channel proteins create a channel through which hydrophilic substances may pass. Ex. Aquaporins
Carrier proteins hold onto substances, change shape and redeposit them on the other side

25. 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 25

26. Directionality of transport
Controlled by
Passive transport
Diffusion
Osmosis
Facilitated diffusion
Active transport
Ion pumps, membrane potential
Cotransport
Bulk transport
Exocytosis
Endocytosis

27. Active transport
Moves substances against their gradient; from an area of low concentration to one of high concentration
Requires energy- supplied by ATP
Allows cells to maintain a different environment inside vs. outside the cell

28. An example is the sodium- potassium pump

29. 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 29

30. 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 30

31. 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 31

32. Ion pumps maintain voltage across membranes
Membrane potential= the voltage across a membrane
Cytoplasmic side relatively negative
Creates electrical potential energy that drives passive transport of cations into the cell and anions out
Electrochemical gradient= chemical (concentration gradient) and electrical forces that drive diffusion across membranes

33. Main electrogenic pumps
Animals-
Sodium-potassium pump
Plants-
Proton pump

34. EXTRACELLULAR FLUID Proton pump CYTOPLASM Figure 5.16
Figure 5.16 A proton pump
CYTOPLASM 34

35. Cotransport
A process by which one protein transports 2 molecules or ions at a time. It uses the diffusion of solute to force the other against it’s gradient.
It does not use ATP directly, but often is coupled with an ion pump that does use ATP

36. Sucrose-H cotransporter
Figure 5.17
Proton pump
Sucrose-H
cotransporter
Diffusion of H
Figure 5.17 Cotransport: active transport driven by a concentration gradient
Sucrose
Sucrose 36

37. Bulk Transport Exocytosis Endocytosis Phagocytosis Pinocytosis
Receptor-mediated endocytosis

38. 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 38