1. Membrane Structure and Function
Chapter 7:
Membrane Structure and
Function

2. 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

3. 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
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

4. WATER Hydrophilic head Hydrophobic tail WATER
Figure 7.2 Phospholipid bilayer (cross section)
Hydrophobic
tail
WATER

5. Phospholipid bilayer Hydrophobic regions of protein Hydrophilic
Figure 7.3 The fluid mosaic model for membranes
Hydrophobic regions
of protein
Hydrophilic
regions of protein

6. • Phospholipids in the plasma membrane can move within the bilayer
Lateral movement
(107 times per second)
Flip-flop
( once per month)
(a) Movement of phospholipids
Figure 7.5a The fluidity of membranes
• Phospholipids in the plasma membrane can move within the bilayer

7. 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 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

8. Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids
Viscous
Figure 7.5b The fluidity of membranes
Saturated hydro-
carbon tails
Unsaturated hydrocarbon
tails with kinks
(b) Membrane fluidity

9. Cholesterol affects in membrane
At warm temperatures cholesterol restrains movement of phospholipids
At cool temperatures, it maintains fluidity by preventing tight packing
Cholesterol
Cholesterol within the animal cell membrane
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

10. 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
Animation: Plasma Membrane
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

11. 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

12. Peripheral proteins are bound to the surface of the membrane
Integral proteins penetrate the hydrophobic core (often coiled into alpha helices
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

13. 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

14. (b) Enzymatic activity (c) Signal transduction
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)

15. Carbohydrates in Cell-Cell Recognition
Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane
Membrane carbohydrates may
be covalently bonded to lipids
(forming glycolipids) or more
commonly to proteins (forming
glycoproteins)
Glyco-
protein
Cell to cell recognition
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

16. The Permeability of the Lipid Bilayer
Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly
Polar molecules, such as sugars, do not cross the membrane easily
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

17. Transport Proteins
Transport proteins allow passage of hydrophilic substances across the membrane
Channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel
Aquaporins (channel protein) facilitate the passage of water
Carrier proteins, bind to molecules and change shape to shuttle them across the membrane
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

18. Passive transport
Diffusion is the tendency for molecules to spread out evenly into the available space
Molecules of dye
Membrane (cross section)
WATER
Net diffusion
Net diffusion
Equilibrium
Animation: Diffusion
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

19. Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another
No work must be done to move substances down the concentration gradient
The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

20. Effects of Osmosis on Water Balance
Osmosis is the diffusion of 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

21. Higher concentration Lower Same concentration concentration of sugar
Fig. 7-12
Lower
concentration
of solute (sugar)
Higher concentration
of sugar
Same concentration
of sugar
H2O
Selectively
permeable
membrane
Figure 7.12 Osmosis
Osmosis

22. Water Balance of Cells Without Walls
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 movement)
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

23. cell cell Hypotonic solution Isotonic solution Hypertonic solution H2O
Fig. 7-13
Hypotonic solution
Isotonic solution
Hypertonic solution
H2O
H2O
H2O
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

24. Water Balance of Cells with Walls
A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm)
If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt
In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

25. Facilitated Diffusion: Passive Transport Aided by Proteins (no energy)
In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane (down gradient)
Channel proteins include
Aquaporins, for facilitated diffusion of water
Ion channels that open or close in response to a stimulus (gated channels)
For the Cell Biology Video Water Movement through an Aquaporin, go to Animation and Video Files.
Animation: Facilitated diffusion
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

26. The Need for Energy in Active Transport
Active transport moves substances against their concentration gradient
Active transport requires energy, usually in the form of ATP
Active transport is performed by specific proteins embedded in the membranes
Animation: Active Transport
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

27. 1 2 3 6 5 4 Fig. 7-16-7 EXTRACELLULAR FLUID [Na+] high Na+ [K+] low
[Na+] low
ATP P
Na+ P
CYTOPLASM
[K+] high
ADP 1 2 3 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

28. Facilitated diffusion
Passive transport
Active transport
ATP
Diffusion
Facilitated diffusion
Figure 7.17 Review: passive and active transport

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

30. Exocytosis
In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

31. Animation: Phagocytosis
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: binding of ligands to receptors triggers vesicle formation
Animation: Phagocytosis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

32. Coat protein Receptor Coated vesicle Coated pit Ligand PHAGOCYTOSIS
PINOCYTOSIS
PHAGOCYTOSIS
EXTRACELLULAR
FLUID
CYTOPLASM
Pseudopodium
“Food” or
other particle
Food
vacuole
Vesicle
Plasma
membrane
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Figure 7.20 Endocytosis in animal cells—phagocytosis
Coated
vesicle
Coated
pit
Ligand