Cell Membrane Function

• The purpose of cells membranes is to?

Large and small substances move across cell membranes in fundamentally different ways.

• Small molecules-

A. Passive transport (simple and facilitated diffusion)

B. Active transport

• Large molecules(endo and exocytosis)

A membrane’s molecular organization results in selective permeability

• Membrane permeability is influenced by size, chemical composition and charge/polarity of the molecule trying to cross the membrane

a. Membranes are more permeable to small molecules than larger ones

b. Membranes are more permeable to hydrophobic molecules

c. Membranes are most permeable to uncharged/nonpolar molecules

Simple Diffusion

• Defined-the spontaneous net movement of a substance from an area of its higher concentration to an area of its lower concentration until an equilibrium is achieved

• Diffusion occurs because of the second law of thermodynamics

LE 7-11a

Molecules of dye Membrane (cross section)

WATER

Net diffusion Net diffusion Equilibrium

Diffusion of one solute

LE 7-11b

Net diffusion Net diffusion Equilibrium

Diffusion of two solutes

Net diffusion Net diffusion Equilibrium

Osmosis

• Osmosis is a special case of diffusion

• It involves the diffusion of water across a differentially permeable membrane

• Cell and tissues can gain or lose water by osmosis depending on the type of environment they exist in

Effect of solute on cell solutions

• The solute concentration of the environment determines whether a cell gains or loses water

• The addition of solute lowers the concentration of water (makes it less than 100%)

• Three terms describe the tendency of one solution to gain or lose water to another solution

a. Hypertonic (salty) solutions tend to gain water from hypotonic solutions (less salty)

b. Isotonic solutions gain and lose water to one another at the same rate.

LE 7-12Lowerconcentrationof solute (sugar)

Higherconcentrationof sugar

Same concentrationof sugar

Selectivelypermeable mem-brane: sugar mole-cules cannot passthrough pores, butwater molecules can

H2O

Osmosis

LE 7-UN140

“Cell”

0.03 M sucrose

0.02 M glucose

0.01 M sucrose

0.01 M glucose

0.01 M fructose

Environment

Cell survival depends on balancing water uptake and loss

• Plant and animal responses to being placed in

• A. hypertonic solution

• B. hypotonic solutions

• C. Isotonic solutions

LE 7-13

Animalcell

Lysed

H2O H2O H2O

Normal

Hypotonic solution Isotonic solution Hypertonic solution

H2O

Shriveled

H2OH2OH2OH2OPlantcell

Turgid (normal) Flaccid Plasmolyzed

LE 7-14

Filling vacuole50 µm

50 µmContracting vacuole

Traffic across membranes

• A membrane’s molecular organization results in selective permeability

• Passive transport is diffusion across a membrane• Osmosis is the passive transport of water• Cell survival depends on balancing water uptake and loss• The solute concentration of the environment determines

whether a cell gains or loses water• Specific proteins facilitate the passive transport of selected

solutes (facilitated diffusion)• Active transport is the pumping of solutes against their gradients• Some ion pumps generate voltage across membranes• In cotransport, a membrane protein couples the transport of one

solute to another• Exocytosis and endocytosis transport large molecules

How do small molecules move across cell membranes?

• Passive Transport is diffusion across a membrane

A. Simple diffusion-membrane is permeable; highlow concentration; no energy required

B. Facilitated diffusion-diffusion-membrane is impermeable (carrier molecule required) highlow concentration; no energy required

LE 7-17a

Diffusion Facilitated diffusion

Passive transport

Facilitated Diffusion

• Specific proteins facilitate the passive transport of selected solutes (facilitated diffusion)

• Hydrophilic channels

• Rotating carriers (conformational changes)

LE 7-15a

EXTRACELLULARFLUID

Channel protein Solute

CYTOPLASM

LE 7-15b

Carrier protein Solute

Active Transport

• Active transport is the pumping of solutes against their gradients

A. Membrane is impermeable (carrier required); movement from low concentration to high concentration; energy required

B. Sodium/potassium pump (neurons)

C. Plants often actively transport nutrients from soil into the root cell (advantage of doing this?)

LE 7-17b

ATP

Active transport

Solution A (.2M glucose) is separated from solution B (.4 M glucose) by a membrane which is impermeable to glucose .

Which solution is hypertonic?

1 2 3 4

21%

9%12%

58%1. A

2. B

3. Both A and B

4. Neither A nor B

Solution A (.2M glucose) is separated from solution B (.4 M glucose) by a membrane which is impermeable to glucose .

Which solution will have a net gain of water?

1 2 3 4

47%

11%

3%

39%

1. A

2. B

3. Both A and B

4. Neither A nor B

In the Na+/K+ pump, the ATPase enzyme is activated by

1 2 3 4 5

16%

6%

29%29%

19%

1. Release of K+

2. Binding of K+

3. Binding of Na+

4. Release of Na+

5. phosphorylation

Figure 8.15 The sodium-potassium pump: a specific case of active transport

LE 7-16

Cytoplasmic Na+ bonds tothe sodium-potassium pump

CYTOPLASMNa+

[Na+] low[K+] high

Na+

Na+

EXTRACELLULARFLUID

[Na+] high[K+] low

Na+

Na+

Na+

ATP

ADP

P

Na+ binding stimulatesphosphorylation by ATP.

Na+

Na+

Na+

K+

Phosphorylation causesthe protein to change itsconformation, expelling Na+

to the outside.

P

Extracellular K+ bindsto the protein, triggeringrelease of the phosphategroup.

PP

Loss of the phosphaterestores the protein’soriginal conformation.

K+ is released and Na+

sites are receptive again;the cycle repeats.

K+

K+

K+

K+

K+

LE 7-18

H+

ATP

CYTOPLASM

EXTRACELLULARFLUID

Proton pump

H+

H+

H+

H+

H+

+

+

+

+

+

–

–

–

–

–

Co-transport

• In co-transport, a membrane protein couples the transport of one solute to another

• In plants, transport of sucrose into cells is coupled to the active transport of H+ ions out of the cell

LE 7-19

H+

ATP

Proton pump

Sucrose-H+

cotransporter

Diffusionof H+

Sucrose

H+

H+

H+

H+

H+

H+

+

+

+

+

+

+

–

–

–

–

–

–

Movement of large molecules/cells into and out of cells

• Exocytosis and endocytosis transport large molecules into and out of cells

• Exocytosis-out

• Endocytosis-in

a. Pinocytosis

b. Phagosytosis

c. Receptor-mediated endocytosis

LE 7-20b

Plasmamembrane Pinocytosis

vesicles forming(arrows) in a celllining a smallblood vessel(TEM).

0.5 µm

Vesicle

PINOCYTOSIS

LE 7-20a

CYTOPLASM

Pseudopodium

“Food” orother particle

EXTRACELLULARFLUID

Bacterium

Food vacuole

An amoeba engulfing a bacterium viaphagocytosis (TEM)

Pseudopodiumof amoeba

1 µm

Food vacuole

PHAGOCYTOSIS

LE 7-20c

Receptor

RECEPTOR-MEDIATED ENDOCYTOSIS

Ligand

Coatedpit

Coatedvesicle

Coat protein

Coat protein

Plasmamembrane

0.25 µm

A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs).

Familial Hypercholesterolemia

• Symptoms/consequences

• Causes

• Genetics

Membrane Structure and Function

• Membrane structure• Membrane models have evolved to fit

new data (science as a process)• A membrane is a fluid mosaic of lipids,

proteins and carbohydrates• There is a lot of experimental evidence

that favors the fluid mosaic model of membrane structure.

History of Membrane Models

• Overton (1875) –Membranes contain lipids (like dissolve like)

• Langmuir(1917)-Membranes have amphipathic lipids (phospholipids)

• Gorter and Grendel(1925)-Phospholipid bilayer

• Davson and Danielli (1935)-Phospholipids and proteins (sandwich)

Figure 8.1 Artificial membranes (cross sections)

LE 7-2

Hydrophilichead

Hydrophobictail

WATER

WATER

Figure 8.2 Two generations of membrane models

History of Membrane Models (continued)

• Robertson (1950)-Electron micrographs showing “trilaminate” structure

• Problems with current models

• Singer and Nicholson (1975)-Fluid mosaic model

Figure 8.19 The three types of endocytosis in animal cells

Fluid Mosaic Model

• Consistent with all observations of membrane properties to date

Figure 7-01

LE 7-5

Lateral movement(~107 times per second)

Flip-flop(~ once per month)

Viscous

Movement of phospholipids

Fluid

Unsaturated hydrocarbontails with kinks

Membrane fluidity

Saturated hydro-carbon tails

Cholesterol

Cholesterol within the animal cell membrane

In sucrose co-transport in plants, the active transport of sucrose into plant cells is couple to

1 2 3 4

25% 25%25%25%1. Facilitated diffusion

2. ATP hydrolysis

3. A proton pump

4. 1 and 3

This model of membrane structure consisted of a phospholipid bilayer sandwiched between 2 layers of

protein:

1 2 3 4 5

20% 20% 20%20%20%1. Gorter and Grendle

2. Davson and Danielli

3. Singer and Nicholson

4. Overton

5. Robertson

An increased synthesis of phospholipids containing unsaturated fatty acids may be an adaptation by plants to:

1 2 3 4 5

20% 20% 20%20%20%1. Predators

2. Decreasing sunlight

3. Hypertonic environments

4. Cooling temperatures

5. Warming temperatures

LE 7-4

Knife

Cytoplasmic layerExtracellular layer

Cytoplasmic layer

Plasmamembrane

Extracellular layer

Proteins

LE 7-6

Membrane proteins

Mixedproteinsafter1 hourHybrid cell

Human cell

Mouse cell

Figure 8.9 Some functions of membrane proteins

LE 7-8

EXTRACELLULARSIDEN-terminus

C-terminusCYTOPLASMICSIDE

Helix