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

2. Concept 7.1 Most abundant lipids in membranes are phospholipids.
- phospholipids are amphipathic (head is hydrophilic, tail is hydrophobic )
Phospholipids and proteins are arranged in the “fluid mosaic model”: membrane is fluid w/ proteins embedded in or attached to the bilayer

3. Concept 7.1
Hydrophilic
head
WATER
Hydrophobic
tail

4. Concept 7.1
Phospholipids and proteins are arranged in the “fluid mosaic model”: membrane is fluid w/ proteins embedded in or attached to the bilayer
- disproved the Davson-Danielli “sandwich” model

5. Concept 7.1 Phospholipid bilayer Hydrophobic regions of protein
Hydrophilic
regions of protein

6. Concept 7.1 The membrane is fluid
-membranes are not static sheets of molecules locked in place
-the membrane is held together primarily by hydrophobic interactions

7. Concept 7.1 Lateral movement (107 times per second) Flip-flop
( once per month)
(a) Movement of phospholipids

8. Concept 7.1 (b) Membrane fluidity Fluid Unsaturated hydrocarbon
tails with kinks
Viscous
Saturated hydro-
carbon tails

9. Concept 7.1 Membranes are mosaics of structure and function
- proteins are embedded in the fluid matrix; the lipid bilayer is the main fabric of the membrane, but proteins determine its specific fcn.

10. Concept 7.1 Fibers of extracellular matrix (ECM) Glyco- Carbohydrate
protein
Microfilaments
of cytoskeleton
Cholesterol
Peripheral
proteins
Integral
CYTOPLASMIC SIDE
OF MEMBRANE
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Carbohydrate

11. Concept 7.1 Two major types of membrane proteins:
- Integral proteins: penetrate the hydrophobic core of the bilayer; many are transmembrane proteins
- Peripheral proteins: appendages loosely bound to the surface of the membrane

12. Concept 7.1 EXTRACELLULAR N-terminus C-terminus CYTOPLASMIC SIDE
 Helix
CYTOPLASMIC
SIDE
EXTRACELLULAR

13. Concept 7.1
Membrane carbohydrates are important for cell-cell recognition
- cell-cell recognition is the ability of a cell to distinguish one type of neighboring cell from another
- membrane carbohydrates are usually oligosaccharides (can vary greatly)

14. Concept 7.1 (a) Transport (b) Enzymatic activity
(c) Signal transduction
ATP
Enzymes
Signal transduction
Signaling molecule
Receptor

15. Concept 7.1 Glyco- protein (d) Cell-cell recognition
(e) Intercellular joining
(f) Attachment to
the cytoskeleton
and extracellular
matrix (ECM)

16. Concept 7.2
Hydrophobic molecules can cross the bilayer with ease. However, ions and polar molecules cannot pass through because they are hydrophilic.
- proteins play keys roles in regulating transportation.

17. Concept 7.2
Transport proteins: allow hydrophilic molecules to enter and exit the cell.
The selective permeability of a membrane depends on the specific transport proteins built into the membrane.

18. Concept 7.3 Passive transport involves diffusion across a membrane.
- Diffusion: the tendency for molecules of any substance to spread out into available space
- any substance will move down a [gradient]. [high]  [low]

19. Concept 7.3 Molecules of dye Membrane (cross section) WATER
Net diffusion
(a) Diffusion of one solute
Equilibrium

20. Concept 7.3
(b) Diffusion of two solutes
Net diffusion
Equilibrium

21. Concept 7.3
Passive transport: diffusion of a substance across a biological membrane. (no energy is used)
Osmosis is the passive transport of water
- sln. w/ a higher [solute] = hypertonic
- sln. w/ a lower [solute] = hypotonic
- slns. w/ equal [solute] = isotonic

22. Concept 7.3

23. Concept 7.3
Organisms without cell walls that live in hypertonic or hypotonic environments must have adaptations for osmoregulation, the control of water balance

24. Concept 7.3 Organisms with cell walls
- turgid (very firm) when placed in a hypotonic sln.
- flacid (limp) if the sln. is isotonic
- plasmolysis (shriveled) occurs when put in a hypertonic sln.

25. Concept 7.3 cell cell Hypotonic solution Isotonic solution
Hypertonic solution
H2O
H2O
H2O
H2O
(a) Animal
cell
Lysed
Normal
Shriveled
H2O
H2O
H2O
H2O
(b) Plant
cell
Turgid (normal)
Flaccid
Plasmolyzed

26. Concept 7.3
Facilitated diffusion: passive transport of molecules through transport proteins
- each protein is specific for the solute it transports

27. Concept 7.3 Channel protein (a) A channel protein Solute
EXTRACELLULAR FLUID
Channel protein
(a) A channel protein
Solute
CYTOPLASM
Carrier protein
(b) A carrier protein

28. Concept 7.4
Active transport: movement of molecules across a membrane against the gradient (uses ATP)
- sodium-potassium pump: movement of 3 Na+ for every 2 K+ ions

29. Concept 7.4 2 3 6 5 4 1 EXTRACELLULAR [Na+] high FLUID [K+] low Na+
[Na+] low
[K+] high
Na+
CYTOPLASM
ATP
ADP P 3 K+ 6 5 4 1

30. Concept 7.4 Some ion pumps generate voltage across membranes
- membrane potential: the voltage across a membrane
- electrogenic pump: a transport protein that generates voltage across a membrane (ex. Sodium-potassium pump)

31. Concept 7.4
The main electrogenic pump for plants and fungi is a proton pump which transports H+ ion out of the cell.

32. Concept 7.4
EXTRACELLULAR
FLUID H+
Proton pump + –
ATP
CYTOPLASM

33. Concept 7.4
In cotransport, a ATP powered pump can drive the transport of other solutes.
1) active transport of a substance against a gradient
2) cotransport through a protein w/ 2nd substance

34. Concept 7.4 – + ATP H+ Diffusion of H+ Proton pump Sucrose-H+
cotransporter
Sucrose

35. Concept 7.5
Exocytosis: the secretion of macromolecules by the fusion of vesicles with the plasma membrane
Endocytosis: the cell takes in macromolecules by forming new vessicles
- 3 types: phagocytosis, pinocytosis, and receptor-mediated endocytosis

36. Concept 7.5 PHAGOCYTOSIS CYTOPLASM 1 µm EXTRACELLULAR FLUID
Pseudopodium
“Food” or
other particle
Food
vacuole
Food vacuole
Bacterium
An amoeba engulfing a bacterium
via phagocytosis (TEM)
of amoeba
1 µm