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Membrane Structure & Function Chapter 7

You should be able to:

1. Define the following terms: amphipathic

molecules, aquaporins, diffusion

2. Explain how membrane fluidity is influenced by

temperature and membrane composition

3. Distinguish between the following pairs or sets of

terms: peripheral and integral membrane

proteins; channel and carrier proteins; osmosis,

facilitated diffusion, and active transport;

hypertonic, hypotonic, and isotonic solutions

4. Explain how transport proteins facilitate diffusion

5. Explain how an electrogenic pump creates

voltage across a membrane, and name two

electrogenic pumps

6. Explain how large molecules are transported

across a cell membrane

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Overview: Life at the Edge

o The plasma membrane is the boundary that separates the living cell from its surroundings

o Membranes are made of Proteins and Lipids

o The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others

Cellular membranes are fluid

mosaics of lipids and proteins o Phospholipids are the most abundant lipid in the

plasma membrane

o Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions

o The fluid mosaic model states that a membrane

is a fluid structure with a “mosaic” of various

proteins embedded in it

o Fluid-Substance that flows (vs. locked in place

like a solid)

o Mosaic: Art made of smaller units

Fig. 7-2

Hydrophilic head

WATER

Hydrophobic tail

WATER

A Phospholipid Bilayer

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Phospholipid

bilayer

Hydrophobic regions of protein

Hydrophilic regions of protein

TECHNIQUE

Extracellular layer

Knife Proteins Inside of extracellular

layer

RESULTS

Inside of cytoplasmic layer

Cytoplasmic layer

Plasma membrane

Freeze Fracture

The Fluidity of Membranes

(a) Movement of phospholipids

Lateral movement

(107 times per second)

Flip-flop

( once per month)

o Phospholipids in the plasma membrane can move within

the bilayer

o Most of the lipids, and some proteins, drift laterally

o (Like a person weaving through a crowd)

o Rarely does a molecule flip-flop transversely across the

membrane

Fig. 7-6

RESULTS

Membrane proteins

Mouse cell Human cell

Hybrid cell

Mixed proteins after 1 hour

oScientists at John Hopkins University labeled the PM proteins of

a mouse cell and a human cell with 2 different markers and fused

the cells

oThe mixing of the mouse and human membrane proteins

indicates that at least some membrane proteins move sideways

within the plane of the PM

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o As temperatures cool, membranes switch

from a fluid state to a more solid state

o Membranes must be fluid to work properly;

they are usually about as fluid as salad oil

o The temperature at which a membrane

solidifies depends on the types of lipids

o Membranes rich in unsaturated fatty acids

are more fluid that those rich in saturated

fatty acids

Fig. 7-5b

(b) Membrane fluidity

Fluid

Unsaturated hydrocarbon tails with kinks

Viscous

Saturated hydro- carbon tails

Unsaturated hydrocarbon tails of phospholipids have kinks

that keep the molecules from packing together, enhancing

membrane fluidity.

oCholesterol

o(c) Cholesterol within the animal cell membrane

o The steroid cholesterol has different effects on

membrane fluidity at different temperatures

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

restrains movement of phospholipids

o At cool temperatures, it maintains

fluidity by preventing tight

packing

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Membrane Proteins and Their

Functions

o A membrane is a collage of different

proteins embedded in the fluid matrix of the

lipid bilayer

o Proteins determine most of the membrane’s

specific functions

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

o Peripheral proteins are bound to the surface

of the membrane

o Integral proteins penetrate into the hydrophobic core

o Integral proteins that span the membrane are called transmembrane proteins

o Integrins (Chapter 6) connect extracellular matrix to the cytoskeleton--External forces can change cellular activities

o Membrane proteins- from Rough ER ribosomes

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Fig. 7-8

N-terminus

C-terminus

Helix

CYTOPLASMIC SIDE

EXTRACELLULAR SIDE

The hydrophobic

regions of an

integral protein

consist of one or

more stretches of

nonpolar amino

acids, often coiled

into alpha helices

o Six major functions of membrane proteins

o Transport

o Enzymatic activity

o Signal transduction

o Cell-cell recognition

o Intercellular joining

o Attachment to the cytoskeleton and

extracellular matrix (ECM)

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

ATP

Enzymes

Signal transduction

Signaling molecule

Receptor

Six major functions of membrane proteins:

Left: A protein that spans the memb

may provide a hydrophilic channel

across the memb that is selective for

a particular solute. Right: Other

transport proteins shuttle a substance

from 1 side to the other by changing

shape

A protein built into the memb

many be an enzyme with its

active site exposed to

substances in the adjacent

solution. In some cases.

Several enzymes in a memb

are organized as a team that

carries out sequential steps

of a metabolic pathway

A memb protein (receptor) may

have a binding site with a specific

shape that fits the shape of a

chemical messenger, such as a

hormone. The external

messenger (signaling molecule)

may cause the protein to change

shape, allowing it to relay the

message to the inside of the cell,

usually by binding to a

cytoplasmic protein

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(d) Cell-cell recognition

Glyco-

protein

(e) Intercellular joining (f) Attachment to

the cytoskeleton

and extracellular

matrix (ECM)

Some glycoproteins serve as

identification tags that are

specifically recognized by

memb proteins of other cells.

This type of cell-cell binding is

usually short-lived

Memb proteins of adjacent

cells may hook together in

various kinds of junctions,

such as gap junctions or tight

junctions. This type of binding

is more long lasting. Microfilaments or other

elements of the cytoskeleton

may be noncovalently bound

to the memb proteins, a

function that helps maintain

cell shape and stabilizes the

location of certain membrane

proteins

The Role of Membrane Carbohydrates

in Cell-Cell Recognition

o Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane

o Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)

o Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual

Figure 7.11

Receptor (CD4)

Co-receptor (CCR5)

HIV

Receptor (CD4) but no CCR5 Plasma

membrane

HIV can infect a cell that has CCR5 on its surface, as in most people.

HIV cannot infect a cell lacking CCR5 on its surface, as in resistant individuals.

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Synthesis and Sidedness of

Membranes o Membranes have distinct inside and outside

faces

o 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

1. In the ER: syn of memb proteins and lipids.

Carbs may be added to proteins here

2. In the Golgi: Glycoproteins modified and

lipids acquire carbs – glycolipids

3. Transmemb proteins, glycolipids, and

secretory proteins are transported by

vesicles to the PM

4. Vesicles fuse with the memb, releasing

secretory proteins

Membrane structure results in

selective permeability

o A cell must exchange materials with its

surroundings, a process controlled by the

plasma membrane

o Plasma membranes are selectively

permeable, regulating the cell’s molecular

traffic

The Permeability of the Lipid

Bilayer

o Hydrophobic (nonpolar) molecules, such as

hydrocarbons, can dissolve in the lipid

bilayer and pass through the membrane

rapidly

o Polar molecules, such as sugars, do not

cross the membrane easily

Animation: Membrane Selectivity

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Transport Proteins

o Transport proteins allow passage of

hydrophilic substances across the

membrane

o Some transport proteins, called channel

proteins, have a hydrophilic channel that

certain molecules or ions can use as a

tunnel

o varying degrees of specificity

o Channel proteins called aquaporins

facilitate the passage of water

o Other transport proteins, called carrier

proteins, bind to molecules and change

shape to shuttle them across the membrane

o A transport protein is specific for the

substance it moves

Passive transport is diffusion of a substance

across a membrane with no energy

investment

o Diffusion is the tendency for molecules to spread out evenly into the available space

o Ex. A spilled liquid releasing a smell (molecules into the air). Eventually, the molecules will spread evenly throughout the room, and everybody will smell it.

o Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one direction

o Ex. From the place of the spill to the rest of the room

o At dynamic equilibrium, as many molecules cross one way as cross in the other direction

Animation: Diffusion

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o Substances diffuse down their concentration

gradient, the difference in concentration of a

substance from one area to another

oHigher concentration → Lower concentration

o No work (no energy used) must be done to move

substances down the concentration gradient

o The diffusion of a substance across a biological

membrane is passive transport because it

requires no energy from the cell to make it happen

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

o Osmosis is the diffusion of water across a selectively permeable membrane (here, only water can pass)

o Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration

o Because water molecules H-bond to solute

molecules, they become “part” of the

solute-water unit.

o Here, only think about the free (unbound) water molecules. They will be lower on the side with more solute molecules (higher concentration). So free water will move from the side with more to the side with less.

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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

o Tonicity is the ability of a solution to cause a cell

to gain or lose water

o Isotonic solution: Solute concentration is the

same as that inside the cell; no net water

movement across the plasma membrane

o Hypertonic solution: Solute concentration is

greater than that inside the cell; cell loses water

o Hypotonic solution: Solute concentration is less

than that inside the cell; cell gains water

Fig. 7-13

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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o Hypertonic or hypotonic environments

create osmotic problems for organisms o Your blood is kept isotonic with your cells to prevent water loss or

gain

o Osmoregulation, the control of water

balance, is a necessary adaptation for life in

such environments

o The protist Paramecium, which is hypertonic

to its pond water environment, has a

contractile vacuole that acts as a pump

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

o Cell walls help maintain water balance

o A plant cell in a hypotonic solution swells until the wall

opposes uptake; the cell is now turgid (firm)

o 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

o In a hypertonic environment, plant cells lose water;

eventually, the membrane pulls away from the wall, a

usually lethal effect called plasmolysis

Video: Plasmolysis

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Facilitated Diffusion: Passive

Transport Aided by Proteins

o In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane

oCan’t go through plasma membrane at all

OR

oCan’t go through Fast Enough for cell’s needs

oChannel proteins provide corridors that allow a specific molecule or ion to cross the membrane (selective “tube”)

oChannel proteins include:

oAquaporins, for facilitated diffusion of water

oIon channels that open or close in response to a stimulus (gated channels)

oCarrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane

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Active transport uses energy to

move solutes against their gradients

o Facilitated diffusion is still passive, because the solute moves down its concentration gradient

o Some transport proteins can move solutes against their concentration gradients

o Active transport moves substances against their concentration gradient

o Active transport requires energy, usually in the form of ATP

o Like using a pump to move water uphill

o Active transport is performed by specific proteins embedded in the membranes

o Active transport allows cells to maintain

concentration gradients that differ from their

surroundings

o The sodium-potassium pump is one type

of active transport system

Animation: Active Transport

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

Cytoplasmic Na binds to

Na/K pump. High affinity

for Na

Binding of Na stimulates

phosphorylation of the

protein by ATP

Phos causes prot to change

shape, Na affinity decreases,

Na expelled outside

New shape has high affinity

for K, phos released

Loss of phos restores protein

to original shape and has

lower affinity for K

K released, pump has high

affinity for Na again

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Passive transport – Substances diffuse

spontaneously down their concentration gradient, crossing a membrance with no expenditure of energy by the cell. The rate of diffusion can be greatky increased by transport proteins in the membrane

Diffusion

Facilitated diffusion

Active transport – Some transport

proteins act as pumps, moving substances across a membrane against their concentration (or electrochemical) gradients. Energy for this work is usually supplied by ATP

ATP

Hydrophobic

molecs & (at a

slow rate)

very sm

uncharged

polar molecs

can diffuse

through the

lipid bilayer

Many hydrophilic

substances diffuse

through membs with

the assistance of

transport proteins,

either channel or

carrier proteins

How Ion Pumps Maintain Membrane Potential

o Membrane potential is the voltage difference across a membrane

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

o Areas (around a barrier) do not have a “+” or “-” charge (except for electrons and protons). It has a difference. If one side of a barrier (membrane or battery terminal) has more + charges than the other, then the side with more is “+”, and the side with less is negative, “-”, compared to the side with more.

o Cells are usually Negative compared to outside (fewer positive ions) (-50-200 mV)

o 2 combined forces, collectively called the

electrochemical gradient, drive the diffusion of

ions across a membrane:

o A chemical force (the ion’s concentration

gradient)

o An electrical force (the effect of the membrane

potential on the ion’s movement)

o If the components are opposite, ion flow is

slowed

o If the components (chemical and electrical) are

together, flow is very fast and easy!

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o An electrogenic pump is a transport protein

that generates voltage across a membrane

o The sodium-potassium pump is the major

electrogenic pump of animal cells

o The main electrogenic pump of plants, fungi,

and bacteria is a proton pump

o Electrogenic pumps help store energy that can

be used for cellular work

The main electrogenic pump of plants, fungi, and

bacteria is a Proton (H+) Pump

Cotransport: Coupled Transport by a

Membrane Protein

o Cotransport occurs when active transport of a

solute indirectly drives transport of another

solute

o Plants commonly use the gradient of hydrogen

ions generated by proton pumps to drive active

transport of nutrients into the cell

o You do this with Na+

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Fig. 7-19

Proton pump

–

–

–

–

–

–

+

+

+

+

+

+

ATP

H+

H+

H+ H+

H+

H+

H+

H+

Diffusion

of H+ Sucrose-H+

cotransporter

Sucrose

Sucrose

Bulk transport across the plasma membrane

occurs by exocytosis and endocytosis

o Small molecules and water enter or leave

the cell through the lipid bilayer or by

transport proteins

o Large molecules, such as polysaccharides

and proteins, cross the membrane in bulk

via vesicles

o Bulk transport requires energy

Animation: Exocytosis and Endocytosis Introduction

Exocytosis

o In exocytosis, transport vesicles migrate to

the membrane, fuse with it, and release their

contents

o Many secretory cells use exocytosis to

export their products

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Endocytosis

o In endocytosis, the cell takes in

macromolecules by forming vesicles from the

plasma membrane

o Endocytosis is a reversal of exocytosis,

involving different proteins

o There are three types of endocytosis:

o Phagocytosis (“cellular eating”)

o Pinocytosis (“cellular drinking”)

oReceptor-mediated endocytosis

o In phagocytosis a cell engulfs a particle in

a vacuole

o The vacuole fuses with a lysosome to digest

the particle

Animation: Phagocytosis

oIn pinocytosis, molecules

are taken up when

extracellular fluid is “gulped”

into tiny vesicles

oIn receptor-mediated

endocytosis, binding of

ligands to receptors triggers

vesicle formation

oA ligand is any molecule that

binds specifically to a receptor

site of another molecule

Animation: Pinocytosis

Animation: Receptor-Mediated Endocytosis

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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: A coated vesicle forming during receptor-mediated endocytosis (TEMs).

Receptor

0.2

5

m

Receptor-Mediated Endocytosis

Ligand

Coat proteins

Coated

pit

Coated

vesicle

Coat

proteins

Plasma

membrane