1. The Cell Membrane
AP Chapter 7

3. Overview: Life at the Edge
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 than others
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4. Composition of the cell membrane: lipids (phospholipids and cholesterol) and proteins
Phospholipids are the most abundant lipid in the plasma membrane
- they are amphipathic molecules, contain
hydrophobic and hydrophilic regions.
The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it
For the Cell Biology Video Structure of the Cell Membrane, go to Animation and Video Files.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

5. Development of the Fluid Mosaic Model
In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins
In 1972, J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

6. Sandwich model of the cell membrane
Proteins are mostly hydrophobic.
All phospholipids are not alike.
What is wrong with this model?

7. Fluid Mosaic Model

8. This slide shows how the hydrophobic and
Fig. 7-2
This slide shows how the hydrophobic and
hydrophilic regions are set up.
WATER
Hydrophilic
head
Figure 7.2 Phospholipid bilayer (cross section)
Hydrophobic
tail
WATER

9. Phospholipid bilayer Hydrophobic regions of protein Hydrophilic
Fig. 7-3
Phospholipid
bilayer
Figure 7.3 The fluid mosaic model for membranes
Hydrophobic regions
of protein
Hydrophilic
regions of protein

10. Double phospholipid layer

11. Different types of phospholipids

12. Different types of phospholipids making up the membrane

13. Fig. 7-4
TECHNIQUE
RESULTS
Extracellular
layer
Proteins
Inside of extracellular layer
Knife
Figure 7.4 Freeze-fracture
Plasma membrane
Cytoplasmic layer
Inside of cytoplasmic layer
Freeze-fracture studies of the plasma membrane supported the fluid mosaic model

14. The Fluidity of Membranes
Phospholipids in the plasma membrane can move within the bilayer
Most of the lipids, and some proteins, drift laterally (very rarely transverse flip-flops)
fluidity of cell membrane
fluidity of membrane
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15. Biology Animations Fig. 7-5 Lateral movement (~107 times per second)
Flip-flop
(~ once per month)
(a) Movement of phospholipids
Fluid
Viscous
Biology Animations
Unsaturated hydrocarbon
tails with kinks
Saturated hydro-
carbon tails
(b) Membrane fluidity
Figure 7.5 The fluidity of membranes
Cholesterol
(c) Cholesterol within the animal cell membrane

16. This experiment shows how the proteins can move about the membrane.
Fig. 7-6
This experiment shows how the proteins can move about the membrane.
RESULTS
Membrane proteins
Mixed proteins
after 1 hour
Mouse cell
Figure 7.6 Do membrane proteins move?
Human cell
Hybrid cell

17. The fluidity of the membrane depends on temperature and types of lipids making up the membrane.
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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

18. Unsaturated hydrocarbon tails with kinks Saturated hydro- carbon tails
Fig. 7-5b
Fluid
Viscous
Unsaturated hydrocarbon
tails with kinks
Saturated hydro-
carbon tails
Figure 7.5b The fluidity of membranes
(b) Membrane fluidity

19. Would you expect an amoeba that lives in a pond in a cold northern climate to have a higher or lower percentage of saturated fatty acids in its membranes during the summer as compared to the winter?

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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

21. (c) Cholesterol within the animal cell membrane
Fig. 7-5c
Cholesterol
Figure 7.5c The fluidity of membranes
(c) Cholesterol within the animal cell membrane

22. Membrane Proteins and Their Functions
A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer
Proteins determine most of the membrane’s specific functions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

23. Fig. 7-7 Fibers of extracellular matrix (ECM) Glyco- Carbohydrate
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
CYTOPLASMIC SIDE
OF MEMBRANE

25. Peripheral proteins are bound to the surface of the membrane
Integral proteins penetrate the hydrophobic core, 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

26. EXTRACELLULAR N-terminus SIDE C-terminus CYTOPLASMIC SIDE  Helix
Fig. 7-8
EXTRACELLULAR
SIDE
N-terminus
Figure 7.8 The structure of a transmembrane protein
C-terminus
CYTOPLASMIC
SIDE
 Helix

27. Six major functions of membrane proteins:
Transport
Enzymatic activity
Signal transduction
Cell-cell recognition
Intercellular joining
Attachment to the cytoskeleton and extracellular matrix (ECM)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

28. (b) Enzymatic activity (c) Signal transduction
Fig. 7-9
Signaling molecule
Enzymes
Receptor
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 (ECM)

29. Intercellular joining E-selectin is a transmembrane protein expressed by endothelial cells that binds to an oligosaccharide expressed on the surface of leukocytes

30. Construct a cell membrane Try this at home!
constructing a cell membrane

31. The Role of Membrane Carbohydrates in Cell-Cell Recognition
Cells recognize each other by binding to surface molecules, usually carbohydrates
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

32. Membranes are bifacial
Carbohydrates (making glycoproteins and glycolipids) on outer surface
Peripheral proteins generally on cytoplasmic surface
Proteins have a distinct orientation, ie…receptor proteins oriented at surface, enzyme proteins oriented toward cytoplasm

34. Synthesis and Sidedness of Membranes
Membranes have distinct inside and outside faces – determined when the membrane is built by the ER and Golgi.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

35. ER 1 Transmembrane glycoproteins Secretory protein Glycolipid Golgi 2
Fig. 7-10 ER 1
Transmembrane
glycoproteins
Secretory
protein
Glycolipid
Golgi
apparatus 2
Vesicle
Figure 7.10 Synthesis of membrane components and their orientation on the resulting membrane 3
Plasma membrane:
Cytoplasmic face 4
Extracellular face
Transmembrane
glycoprotein
Secreted
protein
Membrane glycolipid

36. Can you guess where you would find this cell?

37. This cell?
Notice how thin-walled
they are.

38. Need a hint?

39. These cells?