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

2. Question 1
The plasma membrane is the boundary that separates the living cell from its surroundings
8 nm thick (8,000 to equal the thickness of a page.
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. Phospholipids are the most abundant lipid in the plasma membrane
Questions 2 & 3
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. 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

5. Freeze-fracture studies of the plasma membrane supported the fluid mosaic model
Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer
TECHNIQUE
RESULTS
Extracellular
layer
Proteins
Inside of extracellular layer
Knife
Plasma membrane
Cytoplasmic layer
Inside of cytoplasmic layer

6. The Fluidity of Membranes
Phospholipids in the plasma membrane can move within the bilayer
Most of the lipids, and some proteins, drift laterally
Adjacent phospholipids switch positions about 107 times per second
Rarely does a molecule flip-flop transversely across the membrane
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

7. (a) Movement of phospholipids
Fig. 7-5a
Lateral movement
(107 times per second)
Flip-flop
( once per month)
Figure 7.5a The fluidity of membranes
(a) Movement of phospholipids

8. 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
Membranes must be fluid in order to maintain their permeability
Winter wheat
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

9. (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

10. Proteins determine most of the membrane’s specific functions
Question 4
Proteins determine most of the membrane’s specific functions
Peripheral proteins are bound to the surface of the membrane
Integral proteins penetrate the hydrophobic core
Integral proteins that span the membrane are called transmembrane proteins
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. 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

13. (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)

14. Membrane carbs are usually short chains (fewer than 15 subunits)
Question 6
Cell-to-cell recognition is important in the sorting out of cells into tissues and organs in animal embryo.
It is also the basis for the rejection of foreign cells, including those of transplanted organs.
Membrane carbs are usually short chains (fewer than 15 subunits)
Glycolipids/glycoproteins
ABO blood typing is an example of carbohydrate cell recognition.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

15. Polar molecules, such as sugars, do not cross the membrane easily
Question 6
A cell must exchange materials with its surroundings, a process controlled by the plasma membrane
Plasma membranes are selectively permeable, regulating the cell’s molecular traffic
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

16. Channel proteins called aquaporins facilitate the passage of water
Question 7
Transport proteins allow passage of hydrophilic substances across the membrane
Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel
Channel proteins called aquaporins facilitate the passage of water
Each aquaporin molecule allows the entry of up to 3 billion water molecules per second.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

17. A transport protein is specific for the substance it moves
Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane
A transport protein is specific for the substance it moves
For example, carrier proteins allow glucose to enter red blood cells 50,000 times faster than would normally occur.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

18. Animation: Membrane Selectivity
Question 9
Diffusion is the tendency for molecules to spread out evenly into the available space
Diffusion occurs down a concentration gradient
It does not require energy.
Animation: Membrane Selectivity
Animation: Diffusion
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

19. Membrane (cross section)
Fig. 7-11
Molecules of dye
Membrane (cross section)
WATER
Net diffusion
Net diffusion
Equilibrium
(a) Diffusion of one solute
Figure 7.11 The diffusion of solutes across a membrane
Net diffusion
Net diffusion
Equilibrium
Net diffusion
Net diffusion
Equilibrium
(b) Diffusion of two solutes

20. 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

21. 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

22. 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

23. Question 10
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 net water movement across the plasma membrane
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

24. 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

25. Video: Paramecium Vacuole
Hypertonic or hypotonic environments create osmotic problems for organisms
Osmoregulation, the control of water balance, is a necessary adaptation for life in such environments
The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump
Video: Chlamydomonas
Video: Paramecium Vacuole
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

26. (a) A contractile vacuole fills with fluid that enters from
Fig. 7-14
50 µm
Filling vacuole
(a) A contractile vacuole fills with fluid that enters from
a system of canals radiating throughout the cytoplasm.
Contracting vacuole
Figure 7.14 The contractile vacuole of Paramecium: an evolutionary adaptation for osmoregulation
(b) When full, the vacuole and canals contract, expelling
fluid from the cell.

27. Water Balance of Cells with Walls
Cell walls help maintain water balance
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

28. In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis
Video: Plasmolysis
Video: Turgid Elodea
Animation: Osmosis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

29. Facilitated Diffusion: Passive Transport Aided by Proteins
In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane
Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane
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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

30. Channel protein Solute (a) A channel protein Solute Carrier protein
Fig. 7-15
EXTRACELLULAR FLUID
Channel protein
Solute
CYTOPLASM
(a) A channel protein
Figure 7.15 Two types of transport proteins that carry out facilitated diffusion
Solute
Carrier protein
(b) A carrier protein

31. Facilitated diffusion…cont.
Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane
Some diseases are caused by malfunctions in specific transport systems, for example the kidney disease cystinuria
Facilitated diffusion is still passive because the solute moves down its concentration gradient
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

32. The Need for Energy in Active Transport
Active transport moves substances against their concentration gradient
requires energy, usually in the form of ATP
performed by specific proteins embedded in the membranes
Active transport allows cells to maintain concentration gradients that differ from their surroundings
Animation: Active Transport
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

33. Bulk transport requires energy
Concept 7.5: Bulk transport across the plasma membrane 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

34. Animation: Exocytosis
In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents
Many secretory cells use exocytosis to export their products
Animation: Exocytosis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

35. Animation: Exocytosis and Endocytosis Introduction
In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane
Endocytosis is a reversal of exocytosis, involving different proteins
There are three types of endocytosis:
Phagocytosis (“cellular eating”)
Pinocytosis (“cellular drinking”)
Receptor-mediated endocytosis
Animation: Exocytosis and Endocytosis Introduction
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

36. Animation: Phagocytosis
In phagocytosis a cell engulfs a particle in a vacuole
The vacuole fuses with a lysosome to digest the particle
For the Cell Biology Video Phagocytosis in Action, go to Animation and Video Files.
Animation: Phagocytosis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

37. Fig. 7-20 Figure 7.20 Endocytosis in animal cells PHAGOCYTOSIS
EXTRACELLULAR
FLUID
CYTOPLASM
1 µm
Pseudopodium
Pseudopodium
of amoeba
“Food”or
other particle
Bacterium
Food
vacuole
Food vacuole
An amoeba engulfing a bacterium
via phagocytosis (TEM)
PINOCYTOSIS
0.5 µm
Plasma
membrane
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM)
Vesicle
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated
vesicle
Figure 7.20 Endocytosis in animal cells
Coated
pit
Ligand
A coated pit
and a coated
vesicle formed
during
receptor-
mediated
endocytosis
(TEMs)
Coat
protein
Plasma
membrane
0.25 µm

38. Animation: Pinocytosis
In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles
Animation: Pinocytosis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

39. PINOCYTOSIS Plasma membrane Vesicle 0.5 µm Fig. 7-20b
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM)
Vesicle
Figure 7.20 Endocytosis in animal cells—pinocytosis

40. Animation: Receptor-Mediated Endocytosis
In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation
A ligand is any molecule that binds specifically to a receptor site of another molecule
Animation: Receptor-Mediated Endocytosis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

41. Coat protein Receptor Coated vesicle Coated pit Ligand A coated pit
Fig. 7-20c
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated
vesicle
Coated
pit
Ligand
A coated pit
and a coated
vesicle formed
during
receptor-
mediated
endocytosis
(TEMs)
Coat
protein
Figure 7.20 Endocytosis in animal cells—receptor-mediated endocytosis
Plasma
membrane
0.25 µm

42. You should now be able to:
Define the following terms: amphipathic molecules, aquaporins, diffusion
Explain how membrane fluidity is influenced by temperature and membrane composition
Distinguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic, hypotonic, and isotonic solutions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

43. Explain how transport proteins facilitate diffusion
Explain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumps
Explain how large molecules are transported across a cell membrane
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings