9/21/2011

1

1

Chapter 4 Membrane

Structure and

Function

4.1 Plasma Membrane

Structure and Function • Plasma membrane separates interior of a

cell from outside environment

• Is not an impermeable wall!

• Maintains homeostasis (nearly constant

conditions) within the cell

• Molecules can enter and exit in regulated

manner

– Depends on size, charge etc.

4.1 Plasma Membrane

Structure and Function • Structure?

• Mostly lipids and proteins

– Phospholipid bilayer (two layers of phospholipid

molecules)

• Hydrophilic (water-loving) polar heads

• Hydrophobic (water-fearing) nonpolar tails

– Cholesterol in plasma membrane of animal

cells provides slight stiffness and strength

– Various kinds of proteins

4.1 Plasma Membrane

Structure and Function

• Membrane proteins may be

– Peripheral proteins – associated with

only 1 side of membrane

• Provide support and stabilize

– Integral proteins – span the membrane

• Can protrude from 1 or both sides

• Can move laterally

4.1 Plasma Membrane

Structure and Function • Membrane is fluid in nature (light oil)

• Stabilized by:

– Hydrophilic and hydrophobic forces

– Peripheral proteins – Inside and outside surface

– Cytoskeleton from inside

Outside

Inside

plasma membrane

glycolipid

glycoprotein

integral protein

cholesterol

peripheral protein

filaments of cytoskeleton

hydrophobic tails

hydrophilic heads

phospholipid bilayer

carbohydrate

chain

Fluid Mosaic model of plasma

membrane

Mosaic of proteins in

phospholipids bilayer

Membrane is asymmetric

9/21/2011

2

• Membrane proteins have critical functions

• Repertoire of proteins varies in membranes

• They define functions of various cellular

membranes

• Diseases occur if function fails!

Function of the proteins

• Few categories of membrane proteins

– Channel proteins

– Carrier proteins

– Cell recognition proteins

– Receptor proteins

– Enzymatic proteins

Function of the proteins

Function of the proteins

Channel Protein

Allows a particular molecule or ion to

cross the plasma membrane freely .

Specific for a particular molecule or ion.

Cystic fibrosis, an inherited disorder, is

caused by a faulty chloride (Cl–) channel;

a thick mucus collects in airways and in

pancreatic and liver ducts.

a.

Carrier Protein

Selectively interacts with a specific

molecule or ion so that it can cross the

plasma membrane.

Different form channel protein in that

proteins shape changes while carrying

molecules.

The family of GLUT carriers transfers

glucose in and out of the various cell

types of the body . Different carriers

respond differently to blood levels of

glucose.

b.

Function of the proteins

c.

Cell Recognition Protein

The MHC (major histocompatibility

complex) glycoproteins are different

for each person, so organ

transplants are difficult to achieve.

Cells with foreign MHC

glycoproteins are attacked by white

blood cells responsible for

immunity.

Function of the proteins

d.

Receptor Protein

Shaped in such a way that a specific

molecule can bind to it.

Some types of dwarfism result not

because the body does not produce

enough growth hormone, but because the

plasma membrane growth hormone

receptors are faulty and cannot interact

with growth hormone.

Function of the proteins

9/21/2011

3

e.

Enzymatic Protein

Catalyzes a specific reaction. The

membrane protein, adenylate

cyclase, is involved in ATP

metabolism.

Cholera bacteria release a toxin

that interferes with the proper

functioning of adenylate cyclase,

which eventually leads to severe

diarrhea.

Function of the proteins 4.2 Permeability of the Plasma

Membrane • Permeability?

– Measure of ability to transmit fluid

• Differentially permeable

– Most molecules can not passage freely

• Factors that determine how a substance may be transported across a plasma membrane:

– Size

– Nature of molecule – polarity, charge

4.2 Permeability of the Plasma

Membrane • Concentration gradient

– More of a substance on one side of the

membrane

– Going “down” a concentration gradient

• From an area of higher to lower concentration

– Going “up” a concentration gradient

• From an area of lower to higher concentration

• Requires input of energy

4.2 Permeability of the Plasma

Membrane • Some molecules freely cross membrane

– Water, small, noncharged molecules

– Water may also use special channels called

aquaporins to cross membrane

• Other molecules need aid of:

– Channel proteins

– Carrier proteins

– Vesicles

• Endocytosis or exocytosis

macromolecule

H2O

protein

+

+ -

- charged molecules

and ions

phospholipid

molecule

noncharged

molecules

4.2 Permeability of the Plasma

Membrane

• Diffusion

–Movement of molecules from an

area of higher to lower

concentration

• Down a concentration gradient

–Solution contains a solute (solid)

and a solvent (liquid)

9/21/2011

4

• Once the solute and solvent are evenly distributed, their

molecules continue to move about, but there is no net

movement of either one in any direction

water molecules

(solvent)

dye molecules

(solute)

a. Crystal of dye is placed

in water

b. Diffusion of water and

dye molecules

c. Equal distribution of

molecules results

Diffusion is spontaneous

No chemical energy is required

• Gases can diffuse

through a

membrane

• Oxygen enters

blood capillaries

from air sacs

because its

concentration is

high in air sacs

• Carbon dioxide

exits capillaries

because of higher

concentration in

capillaries than air

sacs

capillary alveolus

bronchiole

oxygen

O2

O2 O2

O2

O2

O2

O2 O2

O2

O2

O2

O2

Gas Exchange in Lungs 4.2 Permeability of the Plasma

Membrane

• Osmosis

– Diffusion of water across a differentially

permeable membrane

– Membrane is not permeable to solute

– Water moves across membrane from

the side of dilute solution to the side of

more concentrated solution

• Membrane is not permeable to solute

• Water can move from dilute solution (beaker) to

concentrated solution (thistle tube)

– Level of water in the beaker is reduced

– Level of water in the thistle tube is increased

a.

10%

5%

< 10%

> 5%

solute water

b.

c.

beaker

less water (higher

percentage of solute)

more water (lower

percentage of solute)

more water (lower

percentage of solute)

less water (higher

percentage of solute)

differentially

permeable

membrane

thistle

tube

9/21/2011

5

4.2 Permeability of the Plasma

Membrane • Put a cell in a solution

• Solution can be: – Isotonic: the solute concentration in solution is

same as inside of a cell

– Hypotonic: a solution has a lower solute concentration than the inside of a cell

– Hypertonic: a solution has a higher solute concentration than the inside of a cell

• Isotonic

– No net gain or loss of water

– 0.9% NaCl

• Hypotonic

– Cell gains water

– Cytolysis or (hemolysis in case of blood cells)

• Hypertonic

– Cell loses water

– Crenation

nucleus

6.6 µm 6.6 µm 6.6 µm

Animal cells

plasma

membrane

In an isotonic solution, there is no net

movement of water .

In a hypotonic solution, water enters the cell,

which may burst (lysis).

In a hypertonic solution, water leaves the

cell, which shrivels (crenation).

• Isotonic

– No net gain or loss of water

• Hypotonic

– Cell gains water

– Turgor pressure keeps plant erect – cell wall

• Hypertonic

– Cell loses water

– Plasmolysis

chloroplast

nucleus

25 µm 40 µm 25 µm

In an isotonic solution, there is no

net movement of water.

In a hypotonic solution, the central vacuole

fills with water, turgor pressure develops, and

chloroplasts are seen next to the cell wall.

In a hypertonic solution, the central vacuole loses

water, the cytoplasm shrinks (plasmolysis), and

chloroplasts are seen in the center of the cell.

central

vacuole

cell

wall plasma

membrane

Plant cells 4.2 Permeability of the Plasma

Membrane

• Diffusion is spontaneous from high to low

concentration

• Osmosis is diffusion of water across a

membrane

• Above two account for transport of very

few molecules across membrane

• Membrane proteins aid transport in many

cases

4.2 Permeability of the Plasma

Membrane • Channel proteins

– Few molecules passage through specific

channel proteins

• Transport by Carrier Proteins

– Carrier proteins are specific

• Combine with a molecule or ion to be transported

across the membrane

– Carrier proteins are required for:

• Facilitated Transport

• Active Transport

• Facilitated transport

– Small molecules that are not lipid-soluble

– Molecules follow the concentration gradient

– Energy is not required

Inside

plasma

membrane carrier

protein

solute

Outside

9/21/2011

6

4.2 Permeability of the Plasma

Membrane • Active Transport

– Molecules move against the concentration

gradient

• Entering or leaving cell

• Examples

– Glucose accumulates in the cells lining digestive tract

– Iodine accumulates in cells of thyroid glands

– Molecules combine with carrier proteins

• Often called pumps

– Energy is required

Sodium-Potassium Pump

K+

Inside

carrier

protein

Outside K+

K+

K+

1. Carrier has a shape that allows

it to take up 3 Na+.

K+

P

ADP ATP

K+ K+

K+

2. ATP is split, and phosphate

group attaches to carrier.

K+

K+ K+

K+

P

3. Change in shape results and

causes carrier to release 3 Na+

outside the cell.

K+

K+

K+

K+

P

4. Carrier has a shape that

allows it to take up 2 K+.

9/21/2011

7

K+

K+

K+

K+

P

5. Phosphate group is released

from carrier.

K+

K+

K+ K+

6. Change in shape results and

causes carrier to release 2 K+

inside the cell.

K+

K+

K+

K+

K+

K+ K+

K+

K +

K+

K+

K+

K+

K+

K+

K+

K+ K+

P

P

P

P

Inside

6. Change in shape results and

causes carrier to release 2 K+

inside the cell.

carrier

protein Outside K+

K+

K+

ADP ATP

K+ K+

K+

3. Change in shape results and

causes carrier to release 3 Na+

outside the cell.

2. ATP is split, and phosphate

group attaches to carrier.

4. Carrier has a shape that

allows it to take up 2 K+.

5. Phosphate group is released

from carrier.

1. Carrier has a shape that allows

it to take up 3 Na+.

Na/K pump actively

transports Na+ and

K+ against their

concentration

gradient

Energy is spent in

form of ATP

4.2 Permeability of the Plasma

Membrane

• Vesicle Formation

– Membrane-assisted transport

– Transport of macromolecules

– Requires energy

– Keeps the macromolecule contained

• Exocytosis – exit out of cell

• Endocytosis – enter into cell

4.2 Permeability of the Plasma

Membrane • Exocytosis

– Vesicle fuses with plasma membrane as

secretion occurs

– Membrane of vesicle becomes part of plasma

membrane

– Cells of particular organs are specialized to

produce and export molecules

• Pancreatic cells release insulin when blood sugar

rises

9/21/2011

8

plasma membrane

Inside

Outside

secretory

vesicle

Vesicle Formation

• Endocytosis

– Cells take in substances by vesicle formation

• Phagocytosis: Large, particulate matter

• Pinocytosis: Liquids and small particles dissolved

in liquid

• Receptor Mediated Endocytosis: A type of

pinocytosis that involves a coated pit

paramecium

solute

solute

a. Phagocytosis

b. Pinocytosis

vacuole

coated vesicle

plasma membrane

coated pit

c. Receptor-mediated endocytosis

399.9 µm

vesicle

vacuole

forming

pseudopod

of amoeba

0.5 µm

vesicles

forming

coated

vesicle

coated

pit

receptor

protein

Outside Inside of cell Endocytosis

Passage of Molecules Into and

Out of the Cell