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CAMPBELL

BIOLOGYReece • Urry • Cain • Wasserman • Minorsky • Jackson

© 2014 Pearson Education, Inc.

TENTH

EDITION

CAMPBELL

BIOLOGYReece • Urry • Cain • Wasserman • Minorsky • Jackson


TENTH

EDITION

7Membrane

Structure and

Function

Lecture Presentation by

Dr Burns

NVC

Outline

I. Biological membranes

II. Plasma membrane

A. Structure and function

B. Proteins

C. Transport

Fig. 4.6

Biological membranes

Membranes all have a similar structure

Plasma membrane surrounds the cell – this

membrane controls what enters and exits

the cell - exhibits selective permeability

There are many membrane bound

organelles within the cell. The membranes

that surround these organelles have the

same basic structure

Functions of the Plasma Membrane

1. Regulates passage of materials

2. Participates in biochemical reactions

3. Receives information about environment – signal transduction

4. Communicates with other cells

membranes are fluid mosaics of lipids and proteins

Phospholipids are the most abundant lipid in the

plasma membrane

Phospholipids are amphipathic molecules,

containing hydrophobic and hydrophilic regions

The fluid mosaic model states that a membrane is a

fluid structure with a “mosaic” of various proteins

embedded in it


2


• In 1935, Hugh Davson and James Danielli proposed

a sandwich model in which the phospholipid bilayer

lies between two layers of globular proteins

Later studies found problems with this model,

particularly the placement of membrane proteins,

which have hydrophilic and hydrophobic regions

In 1972, S. J. Singer and G. Nicolson proposed that

the membrane is a mosaic of proteins dispersed

within the bilayer, with only the hydrophilic regions

exposed to water

Plasma Membrane

Bilayer

Mixture of phospholipids, sterols (cholesterol), and

proteins

Phospholipids arrange to have the polar head on

the outside and the non-polar, lipid portion on the

inside

Proteins arranged on surfaces, some form

channels

Figure 7.2

Hydrophilic

head

Hydrophobic

tail

WATER

WATER

Figure 7.3

Phospholipid

bilayer

Hydrophobic regions

of protein

Hydrophilic

regions of protein

The Fluidity of Membranes

Phospholipids in the plasma membrane can move

within the bilayer

Most of the lipids, and some proteins, drift laterally

Rarely does a molecule flip-flop transversely

across the membrane


Figure 7.7

Membrane proteins

Mouse cellHuman cell

Hybrid cell

Mixed proteins

after 1 hour

RESULTS

3

Figure 7.6

Lateral movement occurs

107 times per second.

Flip-flopping across the membrane

is rare ( once per month).

Figure 7.5

Glyco-

proteinCarbohydrate

Glycolipid

Microfilaments

of cytoskeleton

EXTRACELLULAR

SIDE OF

MEMBRANE

CYTOPLASMIC SIDE

OF MEMBRANE

Integral

protein

Peripheral

proteins

Cholesterol

Fibers of extra-

cellular matrix (ECM)

Membrane Fluidity

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

Membranes rich in unsaturated fatty acids are

more fluid than those rich in saturated fatty

acids

Membranes must be fluid to work properly; they

are usually about as fluid as salad oil


The steroid cholesterol has different effects on

membrane fluidity at different temperatures

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

restrains movement of phospholipids

At cool temperatures, it maintains fluidity by

preventing tight packing


Membrane Fluidity – Cholesterol’s Role

Figure 7.8

Fluid

Unsaturated hydrocarbon

tails

Viscous

Saturated hydrocarbon tails

(a) Unsaturated versus saturated hydrocarbon tails

(b) Cholesterol within the animal

cell membrane

Cholesterol

Plasma Membrane – Lipid Rafts

Cholesterol aids in maintaining the fluidity of

the membrane and making “lipid rafts”

4

Figure 7.5

Glyco-

proteinCarbohydrate

Glycolipid

Microfilaments

of cytoskeleton

EXTRACELLULAR

SIDE OF

MEMBRANE

CYTOPLASMIC SIDE

OF MEMBRANE

Integral

protein

Peripheral

proteins

Cholesterol

Fibers of extra-

cellular matrix (ECM)

Five main components in membrane

1. Phospholipid bilayer – controls what passes

through membrane

2. Cholesterol – maintains proper fluidity of

membrane

3. Proteins – transport, support, communication,

recognition, enzymatic, junctions

4. Glycoproteins - form binding sites and function

in cell recognition

5. Glycolipids - form binding sites and function in

cell recognition

Phospholipids

Amphipathic - have both hydrophilic and

hydrophobic regions

Phosphate head (hydrophilic)

Fatty acid tail (hydrophobic)

Glycoproteins and Glycolipids

Short chains of carbohydrates (usually

less than 15 sugars) attached to proteins

or lipids

Glycoproteins

Glycolipids

Cell to Cell Interactions

Indentity Markers:

Glycolipids – Identify tissue types

Glycoproteins – Identify cells as “our cells”.

MHC proteins – major histocompatibility

complex proteins

5

Six major functions of membrane proteins

Transport

Enzymatic activity

Signal transduction

Cell-cell recognition

Intercellular joining

Attachment to the cytoskeleton and extracellular

matrix (ECM)


Membrane Proteins - FunctionsFigure 7.10

Enzymes

Signaling molecule

Receptor

Signal transduction

Glyco-

protein

ATP

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

(d) Cell-cell recognition (e) Intercellular joining (f) Attachment to

the cytoskeleton

and extracellular

matrix (ECM)

Figure 7.10a

Enzymes

Signaling molecule

Receptor

Signal transductionATP

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

Figure 7.10b

Glyco-

protein

(d) Cell-cell recognition (e) Intercellular joining (f) Attachment to

the cytoskeleton

and extracellular

matrix (ECM)

Synthesis and Sidedness of Membranes

Membranes have distinct inside and outside

faces

The asymmetrical distribution of proteins, lipids,

and associated carbohydrates in the plasma

membrane is determined when the membrane is

built by the ER and Golgi apparatus


Membrane Proteins - Production

Proteins that will be associated with the inner

surface of the membrane are manufactured

by free floating ribosomes

Proteins that will be associated with the outer

surface of the membrane are manufactured

by ribosomes on the rough endoplasmic

reticulum.

6

The proteins destined to be on the outer surface

of the plasma membrane have a sugar chain is

added to these proteins

transmembrane integral

outer surface peripherial proteins

These proteins are glycoproteins.

The sugar chain is added in the lumen of the

RER

Figure 7.12

Transmembraneglycoproteins

ER

ER lumen

Glycolipid

Plasma membrane:

Cytoplasmic face

Extracellular face

Secretoryprotein

Golgiapparatus

Vesicle

Transmembraneglycoprotein Secreted

protein

Membraneglycolipid

Membrane Proteins

Peripheral membrane proteins

Not embedded in the cell membrane, either

found inner or the outer surface of the

membrane

Integral membrane proteins

Embedded in the cell membrane.

Some completely cross the membrane =

transmembrane proteins

34

Membrane Proteins - Peripheral

Peripheral membrane proteins

anchored to a phospholipid in one layer of the

membrane

possess nonpolar regions that are inserted in

the lipid bilayer

are free to move throughout one layer of the

bilayer

35 36

Membrane Proteins - Intergral

Integral membrane proteins

May span the lipid bilayer (transmembrane

proteins)

nonpolar regions of the protein are embedded

in the interior of the bilayer

polar regions of the protein protrude from both

sides of the bilayer

7

Integral Proteins

Integral proteins penetrate the hydrophobic

core

Integral proteins that span the membrane are

called transmembrane proteins

The hydrophobic regions of an integral protein

consist of one or more stretches of nonpolar

amino acids, often coiled into alpha helices


Figure 7.9

N-terminus

helix

C-terminus

EXTRACELLULAR

SIDE

CYTOPLASMIC

SIDE

39 40

Membrane Proteins - integral

Extensive polar regions within a

transmembrane protein can create a pore

through the membrane.

b–pleated sheets in the protein secondary

structure form a cylinder called a b-barrel

b -barrel interior is polar and allows water and

small polar molecules to pass through the

membrane

41

Types of membrane proteins

1. Spectrins – give shape to the cell, form a

scaffold.

2. Clatherins – line coated pits to facilitate

receptor mediated endocytosis

3. Adhesion proteins – adhere cells to each other

or to surface

Integrins and cadherins – proteins that anchor

the cell to a substrate or cell to cell, they also

attach to microfilaments inside the cell, also

can play a role in signal transduction

8

Types of membrane proteins

4. Cell Surface Receptors – transmit messages

from outside the cell to inside the cell = signal

transduction

5. Cell Surface identity markers – proteins and

glycoproteins characteristic for that cell type

6. Junction proteins – form channels between

cells (Gap junctions)

7. Transport proteins – Carrier and transport

proteins. Transport of items across the cell

membrane

Aquaporins – Transport water, important in

kidneys and in certain bacteria

Ion channels

8. Enzymes – catalyze reactions

45

Proteins that anchor the cell to a substrate, they also attach

to microfilaments inside the cell

1. Spectrins

2. Receptors

3. Integrins

4. Enzymes

Junct

ion

Rec

epto

rs

Inte

grins

33% 33%33%

Passage Through the Cell Membrane

The cell membrane is selectively permeable

or semi-permeable.

This means some molecules can pass but

not all molecules.

The Permeability of the Lipid Bilayer

Hydrophobic (nonpolar) molecules, such as

hydrocarbons, can dissolve in the lipid bilayer and

pass through the membrane rapidly

Polar molecules, such as sugars, do not cross the

membrane easily


9

What can freely pass through the membrane

Hydrophobic compounds (non-polar)

Gases – oxygen, carbon dioxide

Very small uncharged polar compounds

(Water, glycerol) – but they will pass slowly

What can not freely pass through the membrane

Ions

Most hydrophillic compounds

Charged compounds

Transport Proteins

Transport proteins allow passage of hydrophilic

substances across the membrane

Some transport proteins, called channel proteins,

have a hydrophilic channel that certain molecules or

ions can use as a tunnel

Channel proteins called aquaporins facilitate the

passage of water


Other transport proteins, called carrier proteins,

bind to molecules and change shape to shuttle

them across the membrane

A transport protein is specific for the substance it

moves


Passive transport is diffusion of a substance across a membrane with no energy investment

Diffusion is the tendency for molecules to spread out

evenly into the available space

Although each molecule moves randomly, diffusion of

a population of molecules may be directional

At dynamic equilibrium, as many molecules cross the

membrane in one direction as in the other


10


Animation: Membrane Selectivity

Right-click slide / select “Play”


Animation: Diffusion


Figure 7.13a

Molecules of dyeMembrane (cross section)

WATER

(a) Diffusion of one solute

Net diffusion Net diffusion Equilibrium

Figure 7.13b

(b) Diffusion of two solutes

Net diffusion Net diffusion

Net diffusion Net diffusion

Equilibrium

Equilibrium

Movement through the membrane

If you add molecules to water they will disperse

until it is equally distributed in the water =

diffusion

Equilibrium is reached when the molecules are

equally distributed

Substances diffuse down their concentration

gradient, the region along which the density of a

chemical substance increases or decreases

No work must be done to move substances down

the concentration gradient

The diffusion of a substance across a biological

membrane is passive transport because no energy

is expended by the cell to make it happen


11

Movement through the membrane

Concentration gradient

Molecules will go from higher concentration

to lower concentration.

62

Movement through the membraneOsmosis

The cell membrane is semipermeable

The cytoplasm is mainly water (solvent) and

molecules and ions are dissolved in it

(solutes)

Water will travel across the membrane to try

to restore the balance of solutes = Osmosis

Effects of Osmosis on Water Balance

Osmosis is the diffusion of water across a

selectively permeable membrane

Water diffuses across a membrane from the

region of lower solute concentration to the

region of higher solute concentration until the

solute concentration is equal on both sides


Animation: OsmosisFigure 7.14 Lower

concentrationof solute (sugar)

Higher concentrationof solute

Sugarmolecule

H2O

Same concentrationof solute

Selectivelypermeablemembrane

Osmosis

12

Water Balance of Cells Without Walls

Tonicity is the ability of a surrounding solution

to cause a cell to gain or lose water

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


Figure 7.15

Hypotonicsolution

Osmosis

Isotonicsolution

Hypertonicsolution

(a) Animal cell

(b) Plant cell

H2O H2O H2O H2O

H2O H2O H2O H2OCell wall

Lysed Normal Shriveled

Turgid (normal) Flaccid Plasmolyzed

Hypertonic or hypotonic environments create

osmotic problems for organisms

Osmoregulation, the control of solute

concentrations and 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


Figure 7.16

Contractile vacuole50 m

Water Balance of Cells with Walls

Cell walls help maintain water balance

A plant cell in a hypotonic solution swells until the

wall opposes uptake; the cell is now turgid (firm)

If a plant cell and its surroundings are isotonic,

there is no net movement of water into the cell; the

cell becomes flaccid (limp), and the plant may wilt


In a hypertonic environment, plant cells lose

water; eventually, the membrane pulls away from

the wall, a usually lethal effect called plasmolysis


13

73

If you put a cell in very salty water, would water enter or

leave the cell?

1. Enter

2. Leave

Ente

r

Lea

ve

50%50%

Can Calcium (Ca2+) pass freely through the membrane?

1. Yes

2. No

Yes N

o

50%50%

Can glucose pass freely through the membrane?

1. Yes

2. No

Yes N

o

50%50%

BioFlix: Membrane Transport Transporting molecules across the membrane

Passive transport – molecules travel across

using a concentration gradient

1. Simple diffusion

2. Facilitated diffusion

Active Transport – requires energy, usually

in the form of ATP, to move molecules

against a concentration gradient

14

Passive Transport: 1. Simple diffusion

Simple diffusion: molecules that can freely passthrough the membrane are controlled by concentration gradient

They will travel from a high concentration to a low concentration

Gases like oxygen and CO2

Very small molecules that are not charged (H2O, glycerol)

hydrophobic (non-polar) molecules (steroids)

Passive Transport: Facilitated diffusion

Passive Transport: 2. Facilitated diffusion

Facilitated diffusion: Aided by a transport protein, still controlled by concentration gradient, does not require energy

Molecules use this type of transport if they can’t pass freely through the membrane, but travels from high concentration to low concentration

Hydrophilic molecules like glucose and polar amino acids

Ions or charged compounds

Passive Transport: Facilitated diffusion

Figure 7.17

EXTRACELLULARFLUID

CYTOPLASM

Channel proteinSolute

SoluteCarrier protein

(a) A channelprotein

(b) A carrier protein

Passive Transport

15

Active Transport

Sometimes our cells want to move a molecule

across the membrane but against the

concentration gradient. The cell wants more of

the solute on one side of the membrane.

The cell will use energy to maintain the higher

concentration. The energy is usually in the

form of ATP

Example: Used to transport ions

Types of Active Transport Proteins

1. Uniporters – transport a single type of

molecule

2. Symporter – transport two types of

molecules in the same direction

3. Antiporter – transport two molecules in the

opposite direction

Active Transport - Example

Sodium-potassium pump

Energy is used to transport sodium ions to the

outside of a cell in exchange for potassium ion

which enter the cell.

Transport proteins are used

Two potassium ions enter in exchange for three

sodium ions that leave the cell

Used in nerve cells = neurons

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 across a membrane



Animation: Active Transport


Figure 7.18-1

EXTRACELLULAR

FLUID[Na] high

[K] low

[Na] low

[K] highCYTOPLASM

Na

Na

Na

1

16

Figure 7.18-2

EXTRACELLULAR

FLUID[Na] high

[K] low

[Na] low

[K] highCYTOPLASM

Na

Na

Na

1 2

Na

Na

Na

PATP

ADP

Figure 7.18-3

EXTRACELLULAR

FLUID[Na] high

[K] low

[Na] low

[K] highCYTOPLASM

Na

Na

Na

1 2 3

Na

Na

Na

Na

Na

Na

PP

ATP

ADP

Figure 7.18-4

EXTRACELLULAR

FLUID[Na] high

[K] low

[Na] low

[K] highCYTOPLASM

Na

Na

Na

1 2 3

4

Na

Na

Na

Na

Na

Na

K

K

PP

PP i

ATP

ADP

Figure 7.18-5

EXTRACELLULAR

FLUID[Na] high

[K] low

[Na] low

[K] highCYTOPLASM

Na

Na

Na

1 2 3

45

Na

Na

Na

Na

Na

Na

K

K

K

K

PP

PP i

ATP

ADP

Figure 7.18-6

EXTRACELLULAR

FLUID[Na] high

[K] low

[Na] low

[K] highCYTOPLASM

Na

Na

Na

1 2 3

456

Na

Na

Na

Na

Na

Na

K

K

K

K

K

K

PP

PP i

ATP

ADP

Figure 7.19Passive transport Active transport

Diffusion Facilitated diffusion ATP

17

If a transport protein is used to move glucose using

a concentration gradient this is called:

1. Simple diffusion

2. Facilitated diffusion

3. Active transport

Sim

ple d

iffusi

on

Fac

ilita

ted

diffus

ion

Act

ive

tran

spor

t

33% 33%33%

Bulk Transport

When the cell needs to transport larger things they can use vesicles to transport things in and out of the cell.

Exocytosis: moving things out of the cell

Endocytosis: moving things into the cell

Used to transport macromolecules including: whole cells (bacteria), cholesterol, and proteins


Animation: Exocytosis


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


Endocytosis

Phagocytosis – when cells transport large particles and cells (bacteria) into the cell using vesicles

Pinocytosis – when cells transport fluid into the cell using vesicles

Receptor-mediated endocytosis – molecules attach to a receptor, and are then transported into the cell. The molecules that specifically bind to a receptor are called ligands


Animation: Exocytosis and Endocytosis Introduction


18

In phagocytosis a cell engulfs a particle in a

vacuole

The vacuole fuses with a lysosome to digest

the particle

© 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc.

Animation: Phagocytosis


In pinocytosis, molecules are taken up when

extracellular fluid is “gulped” into tiny vesicles


Animation: Pinocytosis


receptor-mediated endocytosis

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

© 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc.

Animation: Receptor-Mediated Endocytosis


19

Figure 7.22

Solutes

Pseudopodium

“Food” or

other particle

Food

vacuole

CYTOPLASM

Plasma

membrane

Vesicle

Receptor

Ligand

Coat proteins

Coated

pit

Coated

vesicle

EXTRACELLULAR

FLUID

Phagocytosis Pinocytosis Receptor-Mediated Endocytosis

Figure 7.22a

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

Figure 7.22b

Pinocytosis vesicles forming

in a cell lining a small blood

vessel (TEM).

Plasma

membrane

Vesicle

0.5

m

Pinocytosis

Figure 7.22c

Top: A coated pit. Bottom: Acoated vesicle forming duringreceptor-mediated endocytosis(TEMs).

Receptor

0.2

5

m

Receptor-Mediated Endocytosis

Ligand

Coat proteins

Coated

pit

Coated

vesicle

Coat

proteins

Plasma

membrane

113

Receptor-mediated endocytosis

HIV (Human Immunodeficiency Virus) entering a cell. HIV infects a number of cells in the

body, though the main target is the lymphocyte cell, which is a type of T-cell. Once the HIV

virus has infected the host cell it then replicates itself with thousands of copies. Image 1 of 3

in series. TEM X120,000.

Credit: © Dr. Hans Gelderblom/Visuals Unlimited

229674

20

HIV (Human Immunodeficiency Virus) being surrounded by a cell. HIV infects a number of

cells in the body, though the main target is the lymphocyte cell, which is a type of T-cell. Once

the HIV virus has infected the host cell it then replicates itself with thousands of copies.

Image 2 of 3 in series. TEM X120,000.


229675HIV (Human Immunodeficiency Virus) in a cell. HIV infects a number of cells in the body,

though the main target is the lymphocyte cell, which is a type of T-cell. Once the HIV virus

has infected the host cell it then replicates itself with thousands of copies. Image 3 of 3 in

series. TEM X120,000.


229676

when cells transport fluid into the cell using

vesicles this is called:

1. Phagocytosis

2. Pinocytosis

3. Exocytosis

Phag

ocyt

osis

Pin

ocyto

sis

Exo

cyto

sis

33% 33%33%

Important Concepts

Know the vocabulary from this lecture

What are the functions of the plasma membrane

Identify the main components in membrane and

their functions. Be able to draw a membrane.

Be able to identify what can pass freely through a

membrane and what can’t pass freely

What happens to cells in isotonic, hypertonic, and

hypertonic situations

Important Concepts

Be able to describe how things are transported

across membranes – know all the mechanisms

What are the types of membrane proteins, what

are the secondary structures of intergral

membrane proteins

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