1. Cell Membrane Structure and Function
Chapter 5
Cell Membrane Structure and Function

2. Chapter 5 At a Glance
5.1 How Is the Structure of a Membrane Related to Its Function?
5.2 How Do Substances Move Across Membranes?
5.3 How Do Specialized Junctions Allow Cells to Connect and Communicate?

3. 5.1 How Is the Structure of a Membrane Related to Its Function?
Cell membranes isolate cell contents while allowing communication with the environment
Functions of the plasma membrane:
It isolates the cell’s contents from the external environment
It regulates the exchange of essential substances
It allows communication between cells
It creates attachments within and between cells
It regulates biochemical reactions

4. 5.1 How Is the Structure of a Membrane Related to Its Function?
Cell membranes isolate cell contents while allowing communication with its environment (continued)
Phospholipids are responsible for the isolating function of membranes
Proteins are responsible for selectively exchanging substances and communicating with the environment, controlling biochemical reactions, and forming attachments

5. 5.1 How Is the Structure of a Membrane Related to Its Function?
Membranes are “fluid mosaics” in which proteins move within layers of lipids
The “fluid mosaic” model of a membrane was proposed in 1972 by S. J. Singer and G. L. Nicolson
This model indicates that each membrane consists of a mosaic, or “patchwork,” of different proteins that constantly shift and flow within a viscous fluid formed by a double layer of phospholipids

6. The Plasma Membrane extracellular fluid (outside) carbohydrate protein
matrix
glycoprotein
binding
site
receptor
protein
phospholipid
bilayer
attachment
protein
transport
protein
phospholipid
pore
cholesterol
recognition
protein
enzyme
cytoskeleton
Fig. 5-1
cytoplasm (inside)

7. 5.1 How Is the Structure of a Membrane Related to Its Function?
The phospholipid bilayer is the fluid portion of the membrane
Phospholipids are the basis of membrane structure and consist of two very different parts:
A polar, hydrophilic head
Two nonpolar, hydrophobic tails

8. A Phospholipid
tails
(hydrophobic)
head
(hydrophilic)
Fig. 5-2

9. 5.1 How Is the Structure of a Membrane Related to Its Function?
The phospholipid bilayer is the fluid portion of the membrane (continued)
Plasma membranes face both exterior and interior watery environments

10. 5.1 How Is the Structure of a Membrane Related to Its Function?
The phospholipid bilayer is the fluid portion of the membrane (continued)
Hydrophobic and hydrophilic interactions drive phospholipids into bilayers
Hydrogen bonds between water and the hydrophilic heads cause the heads of the outer layer to orient outward toward the watery exterior, while the heads of the inner layer face the watery interior
The nonpolar tails, being hydrophobic, face the inside of the membrane, away from the watery environment around the bilayer

11. Author Animation: Plasma Membrane Structure

12. The Phospholipid Bilayer of the Cell Membrane
extracellular fluid
(watery environment)
phospholipid
hydrophilic
heads
hydrophobic
tails
bilayer
hydrophilic
heads
cytoplasm
(watery environment)
Fig. 5-3

13. 5.1 How Is the Structure of a Membrane Related to Its Function?
The phospholipid bilayer is the fluid portion of the membrane (continued)
The phospholipid bilayer’s flexible, fluid membrane allows for cellular shape changes

14. 5.1 How Is the Structure of a Membrane Related to Its Function?
The phospholipid bilayer is the fluid portion of the membrane (continued)
Individual phospholipid molecules are not bonded to one another
Some of the phospholipids have unsaturated fatty acids, whose double bonds introduce “kinks” into their “tails”
The above features make the membrane fluid

15. Kinks Increase Fluidity
more fluid
less fluid
Fig. 5-4

16. 5.1 How Is the Structure of a Membrane Related to Its Function?
The phospholipid bilayer is the fluid portion of the membrane (continued)
Water-soluble substances such as salts, amino acids, and sugars cannot easily cross phospholipid bilayers
However, the isolation provided by the plasma membrane is not complete, and very small molecules such as water, oxygen, and carbon dioxide as well as larger, lipid-soluble molecules can pass through this selective barrier

17. Author Animation: Cell Membranes

18. 5.1 How Is the Structure of a Membrane Related to Its Function?
The phospholipid bilayer is the fluid portion of the membrane (continued)
Membranes become more fluid at high temperatures (more movement) and less fluid at low temperatures (less movement)
Cell membranes of organisms living in low-temperatures tend to be more unsaturated (more kinks help them maintain fluidity)
Cholesterol stabilizes membranes, affecting fluidity and reducing permeability

19. 5.1 How Is the Structure of a Membrane Related to Its Function?
A variety of proteins form a mosaic within the membrane
Proteins are embedded within, or attached to, the phospholipid bilayer
Many proteins have attached carbohydrates (glycoproteins) on their outer membrane surface

20. 5.1 How Is the Structure of a Membrane Related to Its Function?
A variety of proteins form a mosaic within the membrane (continued)
Categories of membrane proteins
Receptor proteins
Recognition proteins
Enzymatic proteins
Attachment proteins
Transport proteins

21. 5.1 How Is the Structure of a Membrane Related to Its Function?
A variety of proteins form a mosaic within the membrane (continued)
Receptor proteins trigger cellular responses upon binding of specific molecules, such as hormones, sent by other cells
Recognition proteins are glycoproteins that serve as identification tags on the surface of a cell

22. Receptor Protein Activation
(extracellular fluid) 1
A hormone binds
to the receptor
hormone
receptor 2
Hormone binding
activates the receptor,
changing its shape
The activated receptor
stimulates a response
in the cell 3
(cytoplasm)
Fig. 5-5

23. 5.1 How Is the Structure of a Membrane Related to Its Function?
A variety of proteins form a mosaic within the membrane (continued)
Enzymes are proteins that promote chemical reactions that synthesize or break apart biological molecules
Attachment proteins anchor the cell membrane to the inner cytoskeleton, to proteins outside the cell, and to other cells

24. 5.1 How Is the Structure of a Membrane Related to Its Function?
A variety of proteins form a mosaic within the membrane (continued)
There are two types of transport proteins
These proteins regulate the movement of hydrophilic molecules through the plasma membrane
Channel proteins form channels whose central pores allow specific ions or water molecules to pass through the membrane

25. 5.1 How Is the Structure of a Membrane Related to Its Function?
A variety of proteins form a mosaic within the membrane (continued)
There are two types of transport proteins (continued)
Carrier proteins have binding sites that can temporarily attach to specific molecules on one side of the membrane and then move them through the membrane to the other side

26. 5.2 How Do Substances Move Across Membranes?
Molecules in fluids move in response to gradients
Definitions relevant to substance movement are:
A fluid is a substance whose molecules can flow past one another and, therefore, have no defined shape
A solute is a substance that can be dissolved (dispersed as atoms, ions, or molecules) in a solvent
A solvent is a fluid capable of dissolving a solute

27. 5.2 How Do Substances Move Across Membranes?
Definitions relevant to substance movement (continued)
The concentration of a substance defines the amount of solute in a given amount of solvent
A gradient is a physical difference in temperature, pressure, charge, or concentration of a particular substance in a fluid between two adjoining regions of space

28. 5.2 How Do Substances Move Across Membranes?
Molecules in fluids move in response to gradients (continued)
Gradients cause molecules to move from one place to another
Gradients of concentration or pressure cause molecules or ions to move from one region to another in a manner that tends to equalize the difference
Cells use energy and cell membrane proteins to generate concentration gradients of various molecules and ions dissolved in their cytoplasm

29. 5.2 How Do Substances Move Across Membranes?
Molecules in fluids move in response to gradients (continued)
Why gradients cause molecules to move from one place to another:
Molecules and ions in solution are in constant random motion
An increase in temperature increases the rate of this random motion
Random motion produces a net movement from regions of high concentration to regions of low concentration by a process called diffusion

30. Diffusion of a Dye in Water2
Dye molecules
diffuse into the water;
water molecules diffuse
into the dye 3
Both dye molecules
and water molecules are
evenly dispersed 1
A drop of dye is
placed in water
drop of dye
water molecule
Fig. 5-6

31. 5.2 How Do Substances Move Across Membranes?
Movement through membranes occurs by passive transport and energy-driven transport
Concentration gradients of ions and molecules exist across the plasma membranes of all cells
Plasma membranes are selectively permeable because they only allow only certain ions or molecules to permeate

32. 5.2 How Do Substances Move Across Membranes?
Movement through membranes occurs by passive transport and energy-driven transport (continued)
There are two types of movement across the plasma membrane
Passive transport is the diffusion of substances across cell membranes down concentration gradients
Energy-requiring transport is transport that requires the use of cellular energy

33. Table 5-1

34. 5.2 How Do Substances Move Across Membranes?
Passive transport includes simple diffusion, facilitated diffusion, and osmosis

35. 5.2 How Do Substances Move Across Membranes?
Simple diffusion: Substances move down their concentration gradients across a membrane
Molecules that move across membranes by simple diffusion include water, oxygen, carbon dioxide, and lipid-soluble molecules like alcohol and vitamins A, D, and E

36. Author Animation: Passive Diffusion

37. Types of Diffusion Through the Plasma Membrane
(extracellular
fluid)
Cl–
water
glucose O2
phospho-
lipid
bilayer
channel
protein
aquaporin
carrier
protein
(cytoplasm)
(a) Simple diffusion through
the phospholipid bilayer
(b) Facilitated diffusion
through channel proteins
(c) Osmosis through
aquaporins or the
phospholipid bilayer
(d) Facilitated diffusion through
carrier proteins
Fig. 5-7

38. 5.2 How Do Substances Move Across Membranes?
Facilitated diffusion
Water soluble molecules like ions, amino acids, and sugars diffuse down their concentration gradients with the aid of channel and carrier transport proteins

39. Author Animation: Facilitated Diffusion

40. 5.2 How Do Substances Move Across Membranes?
Facilitated diffusion (continued)
Many cells have specialized water channel proteins called aquaporins
Their small size and positive charges (that attract the negative pole of water molecules) make aquaporins selective for water molecules

41. 5.2 How Do Substances Move Across Membranes?
Osmosis is the diffusion of water across selectively permeable membranes
Water diffuses from a region of high water concentration to one of low water concentration across a membrane
Dissolved substances reduce the concentration of free water molecules (and hence the purity of the water) in a solution

42. Author Animation: Diffusion

43. 5.2 How Do Substances Move Across Membranes?
Osmosis is the diffusion of water across selectively permeable membranes (continued)
Dissolved substances displace water molecules, lowering water concentration
Dissolved substances form hydrogen bonds with water molecules, reducing the number that are free to move across a water-permeable membrane

44. 5.2 How Do Substances Move Across Membranes?
Osmosis is the diffusion of water across selectively permeable membranes (continued)
Because of displacement and hydrogen bonding, a higher concentration of dissolved substances means that fewer water molecules are available to move across a membrane
In osmosis, there is a net movement of water molecules from regions where they are more available (because of lower solute concentration) into regions where they are less available (because of higher solute concentration)

45. The Effect of Solute Concentration on Osmosis
No net flow
of water
Water flows out;
the balloon shrinks
Water flows in;
the balloon expands
(a) A balloon in an
isotonic solution
(b) A balloon in a
hypertonic solution
(c) A balloon in a
hypotonic solution
Fig. 5-8

46. 5.2 How Do Substances Move Across Membranes?
Osmosis is the diffusion of water across selectively permeable membranes (continued)
Isotonic solutions have equal concentrations of water and equal concentrations of dissolved substances
No net water movement occurs across the membrane

47. 5.2 How Do Substances Move Across Membranes?
Osmosis is the diffusion of water across selectively permeable membranes (continued)
A hypertonic solution is one with a greater solute concentration
Water moves across a membrane toward the hypertonic solution

48. 5.2 How Do Substances Move Across Membranes?
Osmosis is the diffusion of water across selectively permeable membranes (continued)
A hypotonic solution has a lower solute concentration
Water moves across a membrane away from the hypotonic solution

49. 5.2 How Do Substances Move Across Membranes?
Osmosis across the plasma membrane plays an important role in the lives of cells
The effects of osmosis are illustrated when red blood cells are placed in various solutions
When cells are placed into a hypertonic solution, they shrivel, owing to water loss

50. 5.2 How Do Substances Move Across Membranes?
Osmosis across the plasma membrane plays an important role in the lives of cells (continued)
When cells are placed into a hypotonic solution, they swell, owing to water entry
Cells in isotonic solutions remain unaffected

51. Author Animation: Osmosis

52. The Effects of Osmosis on Red Blood Cells
Fig. 5-9

53. 5.2 How Do Substances Move Across Membranes?
Osmosis explains why fresh water protists have contractile vacuoles
Water leaks in continuously because the cytosol is hypertonic to fresh water
Salts are pumped into the vacuoles, making them hypertonic to the cytosol
Water follows by osmosis and is then expelled by contraction

54. 5.2 How Do Substances Move Across Membranes?
Osmosis explains the flow of water from the cytoplasm of plants into their central vacuole
Water flows into plant cytoplasm because it is more concentrated than the extracellular fluid
Water flows into the vacuole because its contents are more concentrated than the cytoplasm
Water pressure within the vacuole is called turgor pressure

55. Turgor Pressure in Plant Cells
cytoplasm
central vacuole
When water is plentiful, it fills
the central vacuole, pushes
the cytoplasm against the cell
wall, and helps maintain the
cell's shape
Water pressure
supports the
leaves of this
impatiens plant
(a) Turgor pressure provides support
cell wall
plasma membrane
When water is scarce, the
central vacuole shrinks and the
cell wall is unsupported
Deprived of the support
from water, the plant
wilts
Fig. 5-10
(b) Loss of turgor pressure causes the plant to wilt

56. 5.2 How Do Substances Move Across Membranes?
Energy-requiring transport includes active transport, endocytosis, and exocytosis

57. 5.2 How Do Substances Move Across Membranes?
During active transport, membrane proteins use cellular energy to move molecules or ions across plasma membranes against their concentration gradients
Active transport proteins span the entire membrane
They often have a molecule binding site and an ATP binding site
When the high-energy third phosphate of bound ATP is released, some of its stored energy is donated to the protein to move molecules against gradients
Active transport proteins are often referred to as pumps 57

58. Author Animation: Active Transport

59. Active Transport Fig. 5-11 (extracellular fluid) The transport
protein binds both
ATP and Ca2 1
Energy from ATP
changes the shape of the
transport protein and moves
the ion across the membrane 2
The protein
releases the ion and
the remnants of ATP
(ADP and P) and closes 3
ADP
ATP
binding
site
recognition
site
ATP P
ATP
Ca2
(cytoplasm)
Fig. 5-11

60. 5.2 How Do Substances Move Across Membranes?
Cells engulf particles or fluids by endocytosis
The engulfed particles are transported within the cell inside vesicles

61. 5.2 How Do Substances Move Across Membranes?
There are three types of endocytosis
Pinocytosis (“cell drinking”) moves liquids into the cell
Receptor-mediated endocytosis moves specific molecules into the cell
Phagocytosis (“cell eating”) moves large particles into the cell

62. Pinocytosis Fig. 5-12 (extracellular fluid) 1 3 2 vesicle containing
(cytoplasm) 1
A dimple forms in the plasma membrane, which deepens
and surrounds the extracellular fluid The membrane
encloses the extracellular fluid, forming a vesicle. 2 3
(a) Pinocytosis 1
extracellular fluid 2 3
cytoplasm
Fig. 5-12
(b) TEM of pinocytosis

63. Receptor-Mediated Endocytosis1
Receptor proteins for specific molecules
or complexes of molecules are localized at
coated pit sites.
nutrient molecule
(extracellular fluid)
receptor
The receptors bind the molecules and
the membrane dimples inward. 2 1
coated pit 2 3
The coated pit region of the membrane
encloses the receptor-bound molecules. 3 4 4
A vesicle ("coated vesicle") containing
the bound molecules is released into the
cytoplasm.
coated vesicle
(cytoplasm)
(a) Receptor-mediated endocytosis
extracellular particles
bound to receptors
coated vesicle
(extracellular fluid) 1 4 2
(cytoplasm) 3
protein
coating
coated pit
0.1 micrometer
Fig. 5-13
plasma membrane
(b) TEM of receptor-mediated endocytosis

64. Phagocytosis Fig. 5-14 (extracellular fluid) food particle pseudopods2
food
vacuole
(cytoplasm) 3 1
The plasma membrane extends pseudopods toward an
extracellular particle (for example, food) The ends of the
pseudopods fuse, encircling the particle A vesicle called
a food vacuole is formed containing the engulfed particle. 1
(b) An Amoeba engulfs a
Paramecium
(c) A white blood cell ingests
bacteria 2 2 3 3
(a) Phagocytosis
Fig. 5-14

65. 5.2 How Do Substances Move Across Membranes?
Exocytosis moves material out of the cell
Cells use energy to dispose of undigested particles of waste or to secrete substances into the extracellular fluid by exocytosis
Vesicles containing the material to be expelled move to the cell surface, where they fuse with the cell membrane, allowing their contents to diffuse into the outside fluid

66. Exocytosis
Fig. 5-15

67. 5.2 How Do Substances Move Across Membranes?
Exchange of material across membranes influences cell size and shape
As a spherical cell enlarges, its innermost parts get farther away from the plasma membrane
As the cell gets larger, diffusion, which is relatively slow, can take too long to supply important processes deep within the cell
Because its volume increases more rapidly than its surface area, a larger cell has a relatively smaller area of membrane for acquiring nutrients and eliminating waste products than a smaller cell
Nerve and muscle cells and microvilli overcome size restraints by elongating, thus keeping the ratio of surface area to volume relatively high

68. Surface Area and Volume Relationships
distance to
center (r)
1.0
2.0
4.0
surface area
(4r2)
12.6
50.3
201.1
Volume
(4/3r3)
4.2
33.5
268.1
Fig. 5-16
area/volume
3.0
1.5
0.75

69. Desmosomes attach cells together
5.3 How Do Specialized Junctions Allow Cells to Connect and Communicate?
Desmosomes attach cells together
Desmosomes are found where cells need to adhere tightly together under the stresses of movement
Examples include the skin, intestine, and urinary bladder

70. Tight junctions make cell attachments leakproof
5.3 How Do Specialized Junctions Allow Cells to Connect and Communicate?
Tight junctions make cell attachments leakproof
Tight junctions are found where tubes and sacs must hold contents without leaking
Examples include the skin and the urinary bladder

71. Cell Attachment Structures
small intestine
plasma membranes
(edge view)
protein
filaments
in the
cytoplasm
desmosome
microvilli
cells lining
the small
intestine
In desmosomes,
protein filaments
hold cells together
(a) Desmosomes
plasma membranes
(edge view)
urinary
bladder
cells lining
the bladder
In tight junctions,
proteins seal cells
together
Fig. 5-17
(b) Tight junctions

72. 5.3 How Do Specialized Junctions Allow Cells to Connect and Communicate?
Gap junctions and plasmodesmata allow direct communication between cells
Cell-to-cell protein channels allowing for passage of hormones, nutrients, and ions in animal cells are gap junctions
Plant cells have holes in the walls of adjacent cells forming cytoplasmic connections called plasmodesmata

73. Cell Communication Structures
plasma membranes
liver
liver
cells
In gap junctions, channel
proteins connect the
insides of adjacent cells
(a) Gap junctions
plasma
membranes
cell walls
root
In plasmodesmata,
membrane-lined
channels connect
the insides of
adjacent cells
root
cells
Fig. 5-18
(b) Plasmodesmata