Membrane Structure and Function selective

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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

PowerPoint® Lecture Presentations for

Biology

Eighth Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp

Chapter 7

Membrane Structure and

Function

• Essential knowledge 2.B.1: Cell membranes

are selectively permeable due to their

structure.

• a. Cell membranes separate the internal environment of the cell from the external

environment.

• b. Selective permeability is a direct consequence of membrane structure, as described

by the fluid mosaic model

• c. Cell walls provide a structural boundary, as well as a permeability barrier for some

substances to the internal environments

• Essential knowledge 4.C.1: Variation in

molecular units provides cells with a wider

range of functions.

• a. Variations within molecular classes provide

cells and organisms with a wider range of

functions.

– Different types of phospholipids in cell

membranes

Overview: Life at the Edge

• The plasma membrane is the boundary that

separates the living cell from its surroundings

• The plasma membrane exhibits selective

permeability, allowing some substances to

cross it more easily than others


Fig. 7-7

Fibers of extracellular matrix (ECM)

Glyco- protein

Microfilaments of cytoskeleton

Cholesterol

Peripheral proteins

Integral protein

CYTOPLASMIC SIDE OF MEMBRANE

Glycolipid

EXTRACELLULAR SIDE OF MEMBRANE

Carbohydrate

Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins

• The main macromolecules in membranes are lipids and

proteins, but carbohydrates are also important.

• Phospholipids

–Are the most abundant lipid in the plasma membrane

–Are amphipathic, containing both hydrophobic and

hydrophilic regions

• The fluid mosaic model of membrane structure

• States that a membrane is a fluid structure with a “mosaic”

of various proteins embedded in it


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Membrane Models: Scientific Inquiry

• Membranes have been chemically analyzed

• And found to be composed of proteins and

lipids

• The arrangement of phospholipids and proteins

in biological membranes is described by the

fluid mosaic model


Fig. 7-2

Hydrophilic head

WATER

Hydrophobic tail

WATER

Scientists studying the plasma

membrane reasoned that it must

be a phospholipid bilayer

Fig. 7-3

Phospholipid

bilayer

Hydrophobic regions of protein

Hydrophilic regions of protein

(a) Movement of phospholipids

Lateral movement

(107 times per second)

Flip-flop

( once per month)

The Fluidity of Membranes-

-Most everything is in motion

Phospholipids in the plasma membrane

–Membrane molecules are held in place by relatively weak

hydrophobic interactions.

–Can move within the bilayer

Fig. 7-5

Lateral movement

(~107 times per second)

Flip-flop

(~ once per month)

(a) Movement of phospholipids

(b) Membrane fluidity

Fluid Viscous

Unsaturated hydrocarbon tails with kinks

Saturated hydrocarbon tails

(c) Cholesterol within the animal cell membrane

Cholesterol

Fig. 7-6

RESULTS

Membrane proteins

Mouse cell Human cell

Hybrid cell

Mixed proteins after 1 hour

Shows that surface proteins move

3

(b) Membrane fluidity

Fluid

Unsaturated hydrocarbon tails with kinks

Viscous

Saturated hydrocarbon tails

Fluidity

•Membrane fluidity is influenced by temperature.

•As temperatures cool, membranes switch from a fluid state to a solid state

as the phospholipids pack more closely.

•The type of hydrocarbon tails in phospholipids affects the fluidity of the

plasma membrane

Fig. 7-5c

Cholesterol

(c) Cholesterol within the animal cell membrane

The steroid cholesterol

–Has different effects on membrane fluidity at different temperatures

–At warm temperatures (such as 37°C), cholesterol restrains the

movement of phospholipids and reduces fluidity.

–At cool temperatures, it maintains fluidity by preventing tight packing.

• As temperatures cool, membranes switch from a fluid state to a solid state

• The temperature at which a membrane solidifies depends on the types of lipids

• The type of hydrocarbon tails in phospholipids affects the fluidity of the plasma

membrane

• Membranes rich in unsaturated fatty acids are more fluid that those rich in

saturated fatty acids

• Cells can alter the lipid composition of membranes to compensate for changes

in fluidity caused by changing temperatures.

• For example, cold-adapted organisms such as winter wheat increase the

percentage of unsaturated phospholipids in their membranes in the autumn.

• This prevents membranes from solidifying during winter.


Fluidity

Membrane Proteins and Their Functions

• A membrane is a collage of different proteins

embedded in the fluid matrix of the lipid bilayer

• Proteins determine most of the membrane’s

specific functions


• Peripheral proteins are bound to the surface

of the membrane

• Integral proteins penetrate the hydrophobic

core

• Integral proteins that span the membrane are

called transmembrane proteins


• There are two major populations of

membrane proteins.

1. Integral proteins

• Penetrate the hydrophobic core of

the lipid bilayer

• Are often transmembrane proteins,

completely spanning the membrane

Proteins

4

Fig. 7-8

N-terminus

C-terminus

Helix CYTOPLASMIC SIDE

EXTRACELLULAR SIDE

How is this protein arranged?

Proteins

• Peripheral proteinsare not embedded in the lipid bilayer at all. instead,

they are loosely bound to the surface of the protein, often connected to

integral proteins.

• Are appendages loosely bound to the surface of the membrane

• Six major functions of membrane proteins:

– Transport

– Enzymatic activity

– Signal transduction

– Cell-cell recognition

– Intercellular joining

– Attachment to the cytoskeleton and

extracellular matrix (ECM)


Proteins

Fig. 7-9ac

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

ATP

Enzymes

Signal transduction

Signaling molecule

Receptor

Fig. 7-9df

(d) Cell-cell recognition

Glyco-

protein

(e) Intercellular joining (f) Attachment to

the cytoskeleton

and extracellular

matrix (ECM)

The Role of Membrane Carbohydrates in Cell-Cell Recognition (more on this later)

• Cells recognize each other by binding to surface

molecules, often carbohydrates, on the plasma

membrane

• Membrane carbohydrates may be covalently

bonded to lipids (forming glycolipids) or more

commonly to proteins (forming glycoproteins)

• Carbohydrates on the external side of the plasma

membrane vary among species, individuals, and

even cell types in an individual

• (unity and diversity)


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Concept 7.2: Membrane structure results in selective permeability (What)

• A cell must exchange materials with its

surroundings, a process controlled by the

plasma membrane

• Plasma membranes are selectively permeable,

regulating the cell’s molecular traffic because

of their composition

• A steady traffic of small molecules and ions

moves across the plasma membrane in both

directions.

• Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Permeability of the Lipid Bilayer What types of substance can pass through the membrane?

• Hydrophobic molecules are lipid soluble and

can pass through the membrane rapidly

• –Small molecules like oxygen and carbon

dioxide

• –Polar molecules-Do not cross the membrane

rapidly


What cannot pass through?

• The hydrophobic core of the membrane

impedes the direct passage of ions and polar

molecules, which cross the membrane with

difficulty.

• •This includes small molecules such as glucose

and other sugars.

• •An ion, whether a charged atom or molecule,

and its surrounding shell of water also has

difficulty penetrating the hydrophobic core.

Getting other “stuff “ across Transport Proteins

• Things that cannot pass thru: ions, water,

other large or polar molecules

• Transport proteins allow passage of hydrophilic

substances across the membrane

– Channel proteins, have a hydrophilic channel that

certain molecules or ions can use as a tunnel

– Carrier proteins, bind to molecules and change

shape to shuttle them across the membrane

• A transport protein is specific for the

substance it moves.


Aquaporins: Special transport protein

For many years, scientists assumed that water

leaked through the cell membrane, and some

water does.

The very rapid movement of water through

some cells was not explained by this theory

Water molecules traverse through the pore of

the channel in single file. The presence of

water channels increases membrane

permeability to water.

Aquaporin

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Transport

• Two Types

– Passive Transport-

• No energy required

• Moves substances with the gradient (swimming

with the current)

– Active Transport-

• Energy required

• Moves substances against the gradient (swimming

with the current)

Concept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investment (How)

There are three types of passive transport

Diffusion is the tendency for molecules to

spread out evenly into the available space

Osmosis- The diffusion of water

Facilitated diffusion-Diffusion of a substance

aided by a protein


Concentration gradient

• In the absence of other forces, a substance will

diffuse from where it is more concentrated to

where it is less concentrated, down its

concentration gradient.

• •No work must be done to move substances

down the concentration gradient.

• •Each substance diffuses down its

ownconcentration gradient, independent of the

concentration gradients of other substances.

Fig. 7-11a

Molecules of dye Membrane (cross section)

WATER

Net diffusion

Net diffusion

(a) Diffusion of one solute

Equilibrium

Diffusion

–Is the tendency for molecules of any substance to

spread out evenly into the available space

Fig. 7-11 Molecules of dye Membrane (cross section)

WATER

Net diffusion Net diffusion Equilibrium

(a) Diffusion of one solute

Net diffusion

Net diffusion

Net diffusion

Net diffusion

Equilibrium

Equilibrium

(b) Diffusion of two solutes

Effects of Osmosis on Water Balance

• Osmosis is the diffusion of water across a selectively

permeable membrane

• Is the movement of water across a

semipermeablemembrane down it’s concentration gradient

• –It is affected by the concentration gradient of dissolved

substances

• –The direction of osmosis is determined only by a

difference in totalsolute concentration.

• –The kindsof solutes in the solutions do not matter.

• –Look at water only to determine which direction it will

move


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Lower

concentration of solute (sugar)

Fig. 7-12

H2O

Higher

concentration of sugar

Selectively permeable

membrane

Same concentration

of sugar

Osmosis

Water Balance of Cells Without Walls

• Tonicity is the ability of a solution to cause a

cell to gain or lose water

• It has a great impact on cells without cell walls

• Isotonic solution: Solute concentration is the same as that inside the

cell; no net water movement across the plasma membrane

• Hypertonic solution: Solute concentration is greater than that inside

the cell; cell loses water

• Hypotonic solution: Solute concentration is less than that inside the

cell; cell gains water


Types of Solutions and water movement

• If a solution is isotonic

–The concentration of

solutes is the same

as it is inside the cell

–There will be no net

movement of water

–Water will move

equally both

directions


a solution is hypertonic

• –The concentration of

solutes is greater than

it is inside the cell

• –Water concentration

is greater inside the

cell

• –Water will move from

high to low

concentration

• –The cell will lose

water


• If a solution is hypotonic

–The concentration of solutes is

less than it is inside the cell

–The concentration of water is

greater outside the cell

–Water will move from high

concentration to low

concentration

–The cell will gain water

Hypotonic solution

(a) Animal

cell

(b) Plant

cell

H2O

Lysed

H2O

Turgid (normal)

H2O

H2O

H2O

H2O

Normal

Isotonic solution

Flaccid

H2O

H2O

Shriveled

Plasmolyzed

Hypertonic solution

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• Hypertonic or hypotonic environments create

osmotic problems for organisms

• Osmoregulation, the control of water balance,

is a necessary adaptation for life in such

environments

• The protist Paramecium, which is hypertonic to

its pond water environment, has a contractile

vacuole that acts as a pump

• http://www.youtube.com/watch?v=GAmOrIRslg

8 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-14

Filling vacuole 50 µm

(a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm.

Contracting vacuole

(b) When full, the vacuole and canals contract, expelling

fluid from the cell.

Water Balance of Cells with Walls

Cell walls

• –Help maintain water balance

If a plant cell is turgid

–It is in a hypotonic environment

–It is very firm, a healthy state in most plants

–This is a normal environment for plants

–Turgid cells contribute to the mechanical support of the plant.

If a plant cell is flaccid

–It is in an isotonic or hypertonic environment

–The cell wall provides no advantages when a plant cell is immersed in a hypertonic

solution. As the plant cell loses water, its volume shrinks. Eventually, the plasma

membrane pulls away from the wall. This plasmolysisis usually lethal.


What types of solutions are these cells in?

Facilitated Diffusion: Passive Transport Aided by Proteins

• In facilitated diffusion, transport proteins

speed the passive movement of molecules

across the plasma membrane

• Channel proteins provide corridors that allow a

specific molecule or ion to cross the membrane

• Channel proteins include

– Aquaporins, for facilitated diffusion of water

– Ion channels that open or close in response

to a stimulus (gated channels)


EXTRACELLULAR FLUID

Channel protein

(a) A channel protein

Solute CYTOPLASM

Solute Carrier protein

(b) A carrier protein

http://www.youtube.com/watch?v=GAmOrIRslg8

http://www.youtube.com/watch?v=GAmOrIRslg8

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• Carrier proteins undergo a subtle change in

shape that translocates the solute-binding site

across the membrane

• Some diseases are caused by malfunctions in

specific transport systems, for example the

kidney disease cystinuria


Concept 7.4: Active transport uses energy to move solutes against their gradients

• Active transport

• –Moves substances against their concentration

gradient

• –Requires energy, usually in the form of ATP


The Need for Energy in Active Transport

• Active transport moves substances against

their concentration gradient

• Active transport requires energy, usually in the

form of ATP

• Active transport is performed by specific

proteins embedded in the membranes


• Active transport allows cells to maintain

concentration gradients that differ from their

surroundings

• The sodium-potassium pump is one type of

active transport system

• http://www.youtube.com/watch?v=SByeTZKAR

1Q


2

EXTRACELLULAR

FLUID [Na+] high [K+] low

[Na+] low

[K+] high

Na+

Na+

Na+

Na+

Na+

Na+

CYTOPLASM ATP

ADP P

Na+ Na+

Na+

P

3

6 5 4

P P

1

Fig. 7-16-7

Fig. 7-17 Passive transport

Diffusion Facilitated diffusion

Active transport

ATP

http://www.youtube.com/watch?v=SByeTZKAR1Q

http://www.youtube.com/watch?v=SByeTZKAR1Q

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How Ion Pumps Maintain Membrane Potential

• Membrane potential is the voltage difference across a membrane

• Voltage is created by differences in the distribution of positive and negative

ions

• All cells maintain a voltage across their plasma membranes.

• •Voltage is electrical potential energy due to the separation of opposite

charges.

• •The cytoplasm of a cell is negative in charge compared to the extracellular

fluid because of an unequal distribution of cations and anions on opposite

sides of the membrane.

• •The voltage across a membrane is called a membrane potential,and ranges

from −50 to −200 millivolts (mV). The inside of the cell is negative

compared to the outside.

• •The membrane potential acts like a battery.


• Two combined forces, collectively called the

electrochemical gradient, drive the diffusion

of ions across a membrane:

– A chemical force (the ion’s concentration

gradient)

– An electrical force (the effect of the membrane

potential on the ion’s movement)


Electrogenic pumps

• An electrogenic pump is a transport protein that

generates the voltage across a membrane

EXTRACELLULAR

FLUID

H+

H+

H+

H+

Proton

pump

+

+

+

H+

H+

+

+

H+

–

–

–

–

ATP

CYTOPLASM

–

Cotransport: Coupled Transport by a Membrane Protein

• Cotransport

• –A single ATP-powered pump that transports

one solute can indirectly drive the active

transport of several other solutes in a

mechanism called cotransport.

• –Occurs when active transport of a specific

solute indirectly drives the active transport of

another solute

Fig. 7-19

Proton pump

–

–

–

–

–

–

+

+

+

+

+

+

ATP

H+

H+

H+ H+

H+

H+

H+

H+

Diffusion

of H+ Sucrose-H+

cotransporter

Sucrose

Sucrose

Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis

• Large proteins cross the membrane by different

mechanisms

• Large molecules, such as polysaccharides and

proteins, cross the membrane in bulk via

vesicles

• Bulk transport requires energy

• http://www.youtube.com/watch?v=FJmnxbYBlr

4


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Fig. 7-20 PHAGOCYTOSIS

EXTRACELLULAR

FLUID

CYTOPLASM

Pseudopodium

“Food”or other particle

Food vacuole

PINOCYTOSIS

1 µm

Pseudopodium

of amoeba

Bacterium

Food vacuole

An amoeba engulfing a bacterium

via phagocytosis (TEM)

Plasma membrane

Vesicle

0.5 µm

Pinocytosis vesicles forming (arrows) in a cell lining a small

blood vessel (TEM)

RECEPTOR-MEDIATED ENDOCYTOSIS

Receptor

Coat protein

Coated vesicle

Coated pit

Ligand

Coat protein

Plasma membrane

A coated pit

and a coated vesicle formed during receptor- mediated endocytosis (TEMs)

0.25 µm

Exocytosis

• In exocytosis, transport vesicles migrate to the

membrane, fuse with it, and release their

contents

• Many secretory cells use exocytosis to export

their products


Endocytosis

• In endocytosis, the cell takes in macromolecules

by forming vesicles from the plasma membrane

• Endocytosis is a reversal of exocytosis, involving

different proteins

• There are three types of endocytosis:

– Phagocytosis (“cellular eating”)

– Pinocytosis (“cellular drinking”)

– Receptor-mediated endocytosis


• In phagocytosis a cell engulfs a particle in a

vacuole

• The vacuole fuses with a lysosome to digest

the particle

• In pinocytosis, molecules are taken up when

extracellular fluid is “gulped” into tiny vesicles


Fig. 7-20a

PHAGOCYTOSIS

CYTOPLASM EXTRACELLULAR

FLUID Pseudopodium

“Food” or

other particle

Food vacuole Food vacuole

Bacterium

An amoeba engulfing a bacterium

via phagocytosis (TEM)

Pseudopodium

of amoeba

1 µm

Fig. 7-20b

PINOCYTOSIS

Plasma membrane

Vesicle

0.5 µm

Pinocytosis vesicles

forming (arrows) in

a cell lining a small

blood vessel (TEM)

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• In receptor-mediated endocytosis, binding of

ligands to receptors triggers vesicle formation

• A ligand is any molecule that binds specifically

to a receptor site of another molecule


Fig. 7-20c RECEPTOR-MEDIATED ENDOCYTOSIS

Receptor

Coat protein

Coated pit

Ligand

Coat protein

Plasma membrane

0.25 µm

Coated vesicle

A coated pit and a coated vesicle formed during receptor- mediated endocytosis (TEMs)

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Membrane Structure and Function selective

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