Membrane Structure &

Function

BIOL 101: General Biology I

Chapter 7

Rob Swatski Associate Professor of Biology

HACC – York Campus 1

2

3

Plasma Membrane

Fluid mosaic

Lipids & proteins

Selective permeability

Fibers of extracellular matrix (ECM)

Glyco- protein

Microfilaments of cytoskeleton

Cholesterol

Peripheral proteins

Integral protein

CYTOPLASMIC SIDE OF MEMBRANE

Glyco- lipid EXTRA-

CELLULAR SIDE OF MEMBRANE

Carbo- hydrate

4

5

6

7

Phospholipid Bilayer

Amphipathic

Hydrophilic

Hydrophobic

8

Phospholipids

Cell structure & function

Polar phosphate

head

Nonpolar fatty acid tails

9

10

11

Membrane Models

Sandwich model (Davson & Danielli,

1935): 2 outer protein layers with the

phospholipid bilayer on the inside

Fluid mosaic model (Singer & Nicolson,

1972): proteins dispersed within

phospholipid bilayer

12

13

Freeze-Fracture

Specialized preparation technique

Splits membrane along the middle

of the bilayer

Confirmed fluid mosaic model

Knife

Plasma membrane Cytoplasmic layer

Proteins

Extracellular layer

Inside of extracellular layer Inside of cytoplasmic layer

TECHNIQUE

RESULTS

14

15

16

17

Membrane Fluidity

Flip-flop ( once per month)

Lateral movement (107 times per second)

18

Membrane proteins

Mouse cell

Human cell

Hybrid cell

Mixed proteins after 1 hour

19

20

Membrane Fluidity

Essential for proper functioning

Colder temp. viscous

Unsaturated fatty acids = more fluid

Saturated fatty acids = less fluid

21

22

Cholesterol

Membrane steroid that plays important role in

membrane fluidity

Warm temp. restrains movement

Cool temp. maintains fluidity by preventing

tight packing

Cholesterol in plasma membrane

23

24

Membrane Proteins

Membrane = collage of proteins

Embedded in plasma

membrane

Determine most of membrane’s

specific functions

25

26

Membrane Proteins

Peripheral proteins

Integral proteins

Transmembrane proteins

Alpha helices (nonpolar amino

acids)

Fibers of extracellular matrix (ECM)

Glyco- protein

Microfilaments of cytoskeleton

Cholesterol

Peripheral proteins

Integral protein

CYTOPLASMIC SIDE OF MEMBRANE

Glyco- lipid EXTRA-

CELLULAR SIDE OF MEMBRANE

Carbo- hydrate

27

N-terminus

C-terminus Helix

Trans-membrane protein

28

Major Functions of Membrane Proteins

Transport Enzymatic

activity Signal

transduction Cell-to-cell recognition

Intercellular joining

Attachment to

cytoskeleton &

extracellular matrix (ECM)

29

(a) Transport (b) Enzymatic activity (c) Signal transduction

ATP

Enzymes

Signaling molecule

Receptor

30

31

Sucrose porin (Transport protein)

32

Succinate dehydrogenase

(Enzyme)

33 Cytochrome C Oxidase (Enzyme)

34 G protein (Signal transduction)

(d) Cell-to-cell recognition

Glyco- protein

(e) Intercellular joining (f) Attachment to cytoskeleton & ECM

35

36

HIV ligand receptor binding

(Cell-to-cell recognition)

HIV gp120 ligand

CD4 gp receptor

Antibody protein

37 Adherens junction

38 Bacterial ECM on 1 grain of sand

39

Membrane Carbohydrates

Surface sugars

Cell-to-cell recognition

Glycolipids

Glycoproteins

Variation: species, individual, cell types

Cell membrane

WBC membrane

Receptor (CD4)

Co-receptor (CCR5)

HIV

Receptor (CD4) but no CCR5 Plasma

membrane

HIV can infect a cell that has CCR5 on its surface,

as in most people.

HIV cannot infect a cell lacking CCR5 on its surface, as in

resistant individuals.

40

41

Membrane Sidedness

Outside = extracellular face

Inside = cytoplasmic face

Asymmetrical distribution of proteins,

lipids, & sugars determined by ER &

Golgi

Transmembrane glycoproteins

ER

ER lumen

Glycolipid

Plasma membrane:

Cytoplasmic face

Extracellular face

Secretory protein

Golgi apparatus

Vesicle

Transmembrane glycoprotein Secreted

protein

Membrane glycolipid

42

43

Selective Permeability

Regulates the cell’s molecular traffic

Hydrophobic nonpolar molecules:

dissolve & rapidly move across membrane

[hydrocarbons]

Hydrophilic polar molecules: do not easily move across membrane [sugars]

44

Transport Proteins

Allow passage of specific hydrophilic substances across

membrane

Channel proteins (hydrophilic tunnel for

ions)

Aquaporins (water transport)

Carrier proteins (bind to molecules & change shape)

45

K+ Channel Protein

46

47

Diffusion

Tendency for molecules to spread out evenly

into the available space

Each molecule moves randomly

A population of molecules exhibits a

net movement in one direction

Dynamic equilibrium

48 Diffusion

49

Concentration gradient

50

Passive Transport

Substances diffuse down their

concentration gradient

Move from an area of higher to lower concentration

No energy required = Passive Transport

Net diffusion Net diffusion Equilibrium

51

Diffusion of one solute

Net diffusion

Net diffusion

Net diffusion

Net diffusion

Equilibrium

Equilibrium

52

Diffusion of two solutes

53

Osmosis

Diffusion of water across a selectively

permeable membrane

Water moves from an area of lower to

higher solute concentration

Solutes cannot cross membrane

Low solute concentration

H2O

High solute concentration

Selectively permeable membrane

Equal solute concentration

Osmosis

54 Osmosis

Tonicity: the ability of a surrounding

solution to cause a cell to gain or lose water

Isotonic Hypotonic Hypertonic

55

56

Isotonic

57

Hypotonic

58

Hypertonic

Hypotonic solution

Lysis Normal

Isotonic solution

Shriveled

Hypertonic solution

59

Tonicity in Animal Cells

60

Osmo-regulation

Hypotonic & hypertonic

environments create osmotic problems

for organisms

Osmoregulation = the control of solute

concentrations & water balance

Paramecium & contractile vacuoles

61

Contractile Vacuoles

Hypotonic solution

Turgid (normal, firm)

Isotonic solution

Flaccid (limp) Plasmolysis

Hypertonic solution

62

Tonicity in Plant Cells

63

Facilitated Diffusion

Transport proteins help speed up

passive transport

Ion channels

Gated channels

Aquaporins

Carrier proteins

64

Channel Protein

65

Carrier Protein

66

Active Transport

Requires ATP

Moves substances against their

concentration gradient

Ions, amino acids, glucose

Aided by specific membrane proteins

Sodium-potassium pump

ATP

EXTRACELLULAR

FLUID [Na+] high

[K+] low

Na+

Na+

Na+ [Na+] low

[K+] high CYTOPLASM

Cytoplasmic Na+ binds to the sodium-potassium pump.

1

67

Na+ binding stimulates phosphorylation by ATP.

Na+

Na+

Na+

ATP P

ADP

2

68

Phosphorylation causes the protein to change its shape. Na+ is expelled to the outside.

Na+

P

Na+ Na+

3

69

K+ binds on the extracellular side and triggers release of the phosphate group.

P P

4

70

Loss of the phosphate restores the protein’s original shape.

5

71

K+ is released, and the cycle repeats.

72

73

Electro-chemical Gradient

Voltage: created by differences in

distribution of +/- ions

Membrane potential = voltage

difference

Electrochemical gradient =

combination of chemical &

electrical forces

74

Electrogenic Pumps

Transport proteins that

generate voltage across a

membrane

Help store energy that can be used for cellular work

Sodium-potassium pump (major pump in

animal cells)

Proton pump (major pump in

plants, fungi, bacteria)

H+

H+

H+

H+

H+

H+

+

+

+

H+

+

+

H+

–

–

–

–

ATP

–

75

Proton Pump

76

Cotransport

Active transport of a solute indirectly

drives transport of another solute

Also called coupled transport

Plants use H+ gradient from proton pumps to

actively transport nutrients into cells

Proton pump

–

–

–

–

–

–

+

+

+

+

+

+

ATP

H+

H+

H+ H+

H+

H+

H+

H+

Diffusion of H+ Sucrose-H+

cotransporter

Sucrose

Sucrose

LOW

HIGH

LOW HIGH

77

Bulk Transport: Endocytosis & Exocytosis – requires energy

Phagocytosis (“cell eating”)

Pinocytosis (“cell

drinking”)

Receptor-Mediated

Endocytosis

78

Pseudopodium

Solutes

“Food” or other particle

Food vacuole

CYTOPLASM

EXTRACELLULAR FLUID

Pseudopodium of amoeba

Bacterium

Food vacuole

An amoeba engulfing a bacterium via phagocytosis (TEM).

Phagocytosis

1

m

79

80

Phagocytosis

Pinocytosis vesicles forming in a cell lining a small blood

vessel (TEM).

Plasma membrane

Vesicle

0.5

m

Pinocytosis

81

Top: A coated pit. Bottom: A coated vesicle forming during

receptor-mediated endocytosis (TEMs).

Receptor

0.2

5

m

Receptor-Mediated Endocytosis

Ligand

Coat proteins

Coated pit

Coated vesicle

Coat proteins

Plasma membrane

82

83

Exocytosis

Secretory vesicles

Enzymes, hormones, NT’s,

wastes