2. Membrane Structure and Function

3. Plasma Membrane
Is the boundary that separates the living cell from its nonliving surroundings
Selectively Permeable (chooses what may cross the membrane)
Fluid mosaic of lipids and proteins
Lipid bilayer
Contains embedded proteins

5. EXTRACELLULAR SIDE OF MEMBRANE
Figure 7.3b
Glycolipid
EXTRACELLULAR SIDE OF MEMBRANE
Figure 7.3b Updated model of an animal cell’s plasma membrane (cutaway view) (part 2)
Peripheral
proteins
Integral
protein
CYTOPLASMIC
SIDE OF
MEMBRANE

6. Phospholipids Are the most abundant lipid in the plasma membrane
Are amphipathic, containing both hydrophilic (head) and hydrophobic regions (tails)
Head composed of phosphate group attached to one carbon of glycerol is hydrophilic
Two fatty acid tails are hydrophobic

7. (a) Structural formula (b) Space-filling model
Phospholipid structure
Consists of a hydrophilic “head” and hydrophobic “tails”
CH2 O P CH C
Phosphate
Glycerol
(a) Structural formula
(b) Space-filling model
Fatty acids
(c) Phospholipid
symbol
Hydrophobic tails
Hydrophilic
head
Hydrophobic
tails –
Hydrophilic head
Choline +
Figure 5.13
N(CH3)3

8. Actually four major phospholipids a. phosphatidylcholine
b. sphingomyelin
c. phosphatidylserine
d. phosphatidylethanolamine
Outside of cell
Inside of cell

9. Hydrophilic
head
Hydrophobic tail
WATER
Phospholipid Bilayer

10. Danielli-Davson model – sandwich model Protein covering every part of the membrane’s outside

11. Hydrophobic region of protein
Singer and Nicolson
In 1972, Singer and Nicolson, Proposed that membrane proteins are dispersed and individually inserted into the phospholipid bilayer of the plasma membrane
Phospholipid
bilayer
Hydrophilic region
of protein
Hydrophobic region of protein

12. Fluid Mosaic Model
A membrane is a fluid structure with a “mosaic” of various proteins embedded in it when viewed from the top
Phospholipids can move laterally a small amount and can “flex” their tails
Membrane proteins also move side to side or laterally making the membrane fluid

13. Freeze-fracture studies of the plasma membrane support the fluid mosaic model of membrane structure
A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers.

14. The Fluidity of Membranes
Phospholipids in the plasma membrane Can move within the bilayer two ways
Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)

15. The Fluidity of Membranes
The type of hydrocarbon tails in phospholipids Affects the fluidity of the plasma membrane
Fluid
Viscous
Unsaturated hydrocarbon
tails with kinks
Saturated hydro-
Carbon tails

16. The Fluidity of Membranes
The steroid cholesterol Has different effects on membrane fluidity at different temperatures
Figure 7.5
Cholesterol

17. 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

19. 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
Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids
Membranes must be fluid to work properly; they are usually about as fluid as salad oil

20. The steroid cholesterol has different effects on membrane fluidity 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

22. Membrane Proteins and Their Functions
A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer
Fibers of ECM
Fibers of
extracellular
matrix (ECM)

23. Types of Membrane Proteins
Integral proteins
Penetrate the hydrophobic core of the lipid bilayer
Are often transmembrane proteins, completely spanning the membrane
EXTRACELLULAR
SIDE

24. Types of Membrane Proteins
Peripheral proteins
Are appendages loosely bound to the surface of the membrane

25. Six Major Functions of Membrane Proteins
Figure 7.9
Transport. (left) A protein that spans the membrane
may provide a hydrophilic channel across the
membrane that is selective for a particular solute.
(right) Other transport proteins shuttle a substance
from one side to the other by changing shape. Some
of these proteins hydrolyze ATP as an energy source
to actively pump substances across the membrane.
Enzymatic activity. A protein built into the membrane
may be an enzyme with its active site exposed to
substances in the adjacent solution. In some cases,
several enzymes in a membrane are organized as
a team that carries out sequential steps of a
metabolic pathway.
Signal transduction. A membrane protein may have
a binding site with a specific shape that fits the shape
of a chemical messenger, such as a hormone. The
external messenger (signal) may cause a
conformational change in the protein (receptor) that
relays the message to the inside of the cell.
(a)
(b)
(c)
ATP
Enzymes
Signal
Receptor

26. Six Major Functions of Membrane Proteins
Cell-cell recognition. Some glyco-proteins serve as
identification tags that are specifically recognized
by other cells.
Intercellular joining. Membrane proteins of adjacent cells
may hook together in various kinds of junctions, such as
gap junctions or tight junctions
Attachment to the cytoskeleton and extracellular matrix
(ECM). Microfilaments or other elements of the
cytoskeleton may be bonded to membrane proteins,
a function that helps maintain cell shape and stabilizes
the location of certain membrane proteins. Proteins that
adhere to the ECM can coordinate extracellular and
intracellular changes
(d)
(e)
(f)
Glyco-
protein

27. https://www. wisc-online
science/ap1101/construction-of-the-cell-membrane

28. The Role of Membrane Carbohydrates in Cell-Cell Recognition
Is a cell’s ability to distinguish one type of neighboring cell from another
Membrane carbohydrates
Interact with the surface molecules of other cells, facilitating cell-cell recognition

29. Conceptual model of cholesterol effects on the traffic of microbial pathogens into or out of the eukaryotic cell.
Conceptual model of cholesterol effects on the traffic of microbial pathogens into or out of the eukaryotic cell. (A) The bacterial pathogen interacts directly with membrane cholesterol, which serves as a “docking site” and stabilizes microbial interaction with membranes. (B) The bacterial pathogen interacts directly with a GPI-anchored protein receptor embedded in lipid rafts. Cholesterol is required to maintain lipid raft integrity. (C) The internalized microorganism resides within a phagosome with a cholesterol-enriched membrane. Cholesterol is required to prevent phagolysosomal fusion. (D) Viral particles (HIV-1) are targeted to lipid rafts. Cholesterol is required to maintain virion integrity and the release of infectious virions.
Goluszko P , Nowicki B Infect. Immun. 2005;73:

30. Conceptual model of cholesterol effects on the traffic of microbial pathogens into or out of the eukaryotic cell.
(A) The bacterial pathogen interacts directly with membrane cholesterol, which serves as a “docking site” and stabilizes microbial interaction with membranes.
(B) The bacterial pathogen interacts directly with a GPI-anchored protein receptor embedded in lipid rafts. Cholesterol is required to maintain lipid raft integrity.
(C) The internalized microorganism resides within a phagosome with a cholesterol-enriched membrane. Cholesterol is required to prevent phagolysosomal fusion.
(D) Viral particles (HIV-1) are targeted to lipid rafts. Cholesterol is required to maintain virion integrity and the release of infectious virions.

31. HIV must bind to the immune cell surface protein CD4 and a “co-receptor” CCR5 in order to infect a cell
HIV cannot enter the cells of resistant individuals that lack CCR5

32. 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)

33. Synthesis and Sidedness of Membranes
Membranes have distinct inside and outside faces
This affects the movement of proteins synthesized in the endomembrane system (Golgi and ER)

34. Synthesis and Sidedness of Membranes
Membrane proteins and lipids are made in the ER and Golgi apparatus ER

35. Membrane Permeability
Membrane structure results in selective permeability
A cell must exchange materials with its surroundings, a process controlled by the plasma membrane

36. Permeability of the Lipid Bilayer
Hydrophobic molecules
Are lipid soluble and can pass through the membrane rapidly
Polar molecules
Do NOT cross the membrane rapidly

37. Transport Proteins Transport proteins
Allow passage of hydrophilic substances across the membrane