inquiry into life twelfth...

1

Chapter 4 Membrane Structure

and Function

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4.1 Plasma Membrane

Structure and Function

• Regulates the entrance and exit of

molecules into and out of the cell

• Phospholipid bilayer with embedded

proteins

– Hydrophilic (water-loving) polar heads

– Hydrophobic (water-fearing) nonpolar tails

– Cholesterol (animal cells)

4.1 Plasma Membrane

Structure and Function • Membrane proteins may be

– Peripheral proteins – associated with only 1

side of membrane

– Integral proteins – span the membrane

• Can protrude from 1 or both sides

• Can move laterally

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Outside

Inside

plasma membrane

glycolipid

glycoprotein

integral protein

cholesterol

peripheral protein

filaments of cytoskeleton

hydrophobic

tails

hydrophilic

heads phospholipid

bilayer

carbohydrate

chain

4.1 Plasma Membrane

Structure and Function

• 5 Membrane Protein Functions

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Channel Protein

Allows a particular

molecule or ion to

cross the plasma

membrane freely .

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.

Carrier Protein

Selectively interacts

with a specific

molecule or ion so

that it can cross the

plasma membrane.

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. a.

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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.

d. 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.

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.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Channel Protein

Allows a particular

molecule or ion to

cross the plasma

membrane freely .

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.

Carrier Protein

Selectively interacts

with a specific

molecule or ion so

that it can cross the

plasma membrane.

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. 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.

d. 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.

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.

a.

4.2 Permeability of the Plasma

Membrane

• Differentially permeable

• 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 aquaporins to cross

membrane

• Other molecules cannot – use…

– Channel proteins

– Carrier proteins

– Vesicles

• Endocytosis or exocytosis

macromolecule

H2O

protein

+

+ -

- charged molecules

and ions

phospholipid

molecule

noncharged

molecules

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Passage of Molecules Into and

Out of the Cell

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)

• 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

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• Gases can

diffuse

through a

membrane

• Oxygen and

carbon

dioxide enter

and exit this

way

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capillary alveolus

bronchiole

oxygen

O2

O2 O2

O2

O2

O2

O2 O2

O2

O2

O2

O2

4.2 Permeability of the Plasma

Membrane • Several factors influence the rate of

diffusion

– Temperature

• As temperature increases, the rate of diffusion

increases

– Pressure

– Electrical currents

– Molecular size

4.2 Permeability of the Plasma

Membrane • Osmosis

– Diffusion of water across a differentially

permeable membrane

– Diffusion always occurs from higher to lower

concentration

– Osmotic pressure is the pressure that

develops in a system due to osmosis

• The greater the possible osmotic pressure, the

more likely it is that water will diffuse in that

direction

• Membrane is not permeable to solute

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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

4.2 Permeability of the Plasma

Membrane • Osmosis

– Isotonic: the solute concentration is equal inside and outside 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 – hemolysis

• Hypertonic

– Cell loses water

– Crenation

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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).

© David M. Phillips/Photo Researchers, Inc.

• Isotonic

– No net gain or loss of water

• Hypotonic

– Cell gains water

– Turgor pressure keeps plant erect – cell wall

• Hypertonic

– Cell loses water

– Plasmolysis

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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

(bottom left, center): © Dwight Kuhn; (bottom right): © Ed Reschke/Peter Arnold

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Plant cells

chloroplast

nucleus

nucleus

6.6 µm 6.6 µm 6.6 µm

25 µm 40 µm 25 µm

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).

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

Animal cells

(all top): © David M. Phillips/Photo Researchers, Inc.; (bottom left, center): © Dwight Kuhn; (bottom right): © Ed Reschke/Peter Arnold

4.2 Permeability of the Plasma

Membrane

• 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

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Inside

plasma

membrane carrier

protein

solute

Outside

4.2 Permeability of the Plasma

Membrane • Active Transport

– Molecules combine with carrier proteins

• Often called pumps

– Molecules move against the concentration

gradient

• Entering or leaving cell

– Energy is required

Sodium-Potassium Pump

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K+

Inside

carrier

protein

Outside K+

K+

K+

1. Carrier has a shape that allows

it to take up 3 Na+.

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K+

P

ADP ATP

K+ K+

K+

2. ATP is split, and phosphate

group attaches to carrier.

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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+.

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

K+

K+

K+

K+

P

5. Phosphate group is released

from carrier.

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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+.

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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

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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

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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

a(right): © Eric Grave/Phototake; b(right): © Don W. Fawcett/Photo Researchers, Inc.; c(both): Courtesy Mark Bretscher