Biological membranes

Life at the Edge

The plasma membrane

Is the boundary that separates the living cell from its nonliving

surroundings

About 8 nm thick

Controls traffic into and out of the cell

The plasma membrane exhibits selective permeability

It allows some substances to cross it more easily than others

Figure 7.1

Transport Across Membranes:

Overcoming the Permeability Barrier

•Overcoming the permeability barrier of cell membranes is

crucial to proper functioning of the cell.

•Specific molecules and ions need to be selectively moved

into and out of the cell or organelle .

•Membranes are selectively permeable.

Definitions

•Solution – mixture of dissolved molecules in a liquid

•Solute – the substance that is dissolved

•Solvent – the liquid

Ion Concentrations

•The maintenance of solutes on both sides of the membrane is critical to the cell

–Helps to keep the cell from rupturing

•Concentration of ions on either side varies widely

–Na+ and Cl- are higher outside the cell

–K+ is higher inside the cell

–Must balance the number of positive and negative charges, both inside and outside cell

•Ions and hydrophilic

molecules cannot easily

pass trough the

hydrophobic membrane

•Small and hydrophobic

molecules can

•Must know the list to the

left

Cells and Transport Processes

Cells and cellular compartments - accumulate a variety of

substances

concentrations -very different from those of the surroundings

substances that move across membranes - dissolved gases,

ions, and small organic molecules; solutes

Transport is central to cell function

A central aspect of cell function - selective transport

movement of ions or small organic molecules (metabolites)

Cellular membranes are fluid mosaics of lipids and

proteins

Phospholipids

Are the most abundant lipid in the plasma membrane

Are amphipathic, containing both hydrophobic and hydrophilic regions

For those who forgot…

HYDROPHOBIC SUBSTANCE cannot be dissolved in water

because they do not have affinity to water. Example is oil

HYDROPHILIC SUBSTANCE can be dissolved in water because

they have affinity to it.

How are phospholipids and proteins arranged in the

membranes of the cell?

• The fluid mosaic model of membrane structure

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

of various proteins embedded in it

– Or attached to a double layer of phospholipids

Membrane Models: Scientific Inquiry

Membranes have been chemically analyzed

And found to be composed of proteins and lipids

Scientists studying the plasma membrane

Reasoned that it must be a phospholipid bilayer

This bilayer of molecules exists as stable boundary between two

aqueous compartments

Hydrophilic

head Hydrophobic

tail

WATER

WATER

The Davson-Danielli sandwich model of membrane structure

Stated that the membrane was made up of a phospholipid bilayer

sandwiched between two protein layers

Was supported by electron microscope pictures of membranes

However, there were 2 problems

1. generalization that all membranes of the cell are identical was

challenged

Plasma membrane is 7/8 nm thick and has three layered structure, and inner

mitochondrial membrane is 6 nm thick and looks like a row of beads

2. placement of the proteins since membrane proteins are not very

soluble in water

Membrane proteins have hydrophobic and hydrophilic regions.

If placed on the surface, hydrophobic parts would be in an aqueous environment…

In 1972, Singer and Nicolson

Proposed that membrane proteins are dispersed and individually

inserted into the phospholipid bilayer

Only their hydrophilic regions protrude far enough from the bilayer to

be exposed to water

According to this, the membrane is a mosaic of protein molecules

bobbing in a fluid bilayer of phospholipids

Phospholipid

bilayer

Hydrophilic region

of protein

Hydrophobic region of protein

The Fluidity of Membranes

Membranes are not static sheets of molecules !

Held together by hydrophobic interactions which are weaker than

covalent bonds

Most of the lipids and some of the proteins can drift about laterally

That is in the plane of the membrane

Movement is rapid

However, proteins are larger than lipids and they move slower

The Fluidity of Membranes

Membrane remains fluid as temperature decreases

Phospholipids settle into closely packed arrangement and the membrane

solidifies

The solidification temperature depends on the types of lipids it is made of

The membrane remains fluid at lower temperatures if it is rich in

phospholipids with unsaturated hydrocarbon tails

Those hydrocarbons have kinks in the tails where the double bonds are

located so they cannot pack closely as saturated hydrocarbons

The type of hydrocarbon tails in phospholipids

Affects the fluidity of the plasma membrane

Fluid Viscous

Unsaturated hydrocarbon

tails with kinks Saturated hydro-

Carbon tails

(b) Membrane fluidity

The Fluidity of Membranes

Phospholipids in the plasma membrane

Can move within the bilayer

Lateral movement

(~107 times per second) Flip-flop

(~ once per month)

(a) Movement of phospholipids

Lateral movement

Within the same membrane surface

Fast process

Flip-flop

Or transverse diffusion

From one membrane surface to another

Slow process

The Fluidity of Membranes

The membranes must be fluid to work properly

Fluid as salad oil

When solid it changes its permeability and enzymatic proteins in

the membrane become inactive

Solutes Cross Membranes

Simple Diffusion, Facilitated Diffusion, and Active

Transport

•Three quite different mechanisms are involved in moving

solutes across membranes

•A few molecules cross membranes by simple diffusion, the

direct unaided movement dictated by differences in

concentration of the solute on the two sides of the membrane

•However, most solutes cannot cross the membrane this way

The Role of Membrane Carbohydrates in

Cell-Cell Recognition

Cell-cell recognition

Is a cell’s ability to distinguish one type of neighboring cell from

another

Important for organisms functioning

Basis for the rejection of foreign cells by immune system

The way cells recognize other cells is by binding to surface

molecules

Usually carbohydrates

Membrane carbohydrates Interact with the surface molecules of other cells, facilitating cell-cell recognition

Usually short

Some are covalently bonded to lipids forming molecules called glycolipids

Most of them are bonded to proteins forming glycoproteins

Synthesis and Sidedness of

Membranes

Membranes have distinct inside and outside faces

This affects the movement of proteins synthesized in the

endomembrane system

Membrane proteins and lipids

ER

Transmembrane

glycoproteins

Secretory

protein

Glycolipid

Golgi

apparatus

Vesicle

Transmembrane

glycoprotein

Membrane glycolipid

Plasma membrane:

Cytoplasmic face

Extracellular face

Secreted

protein

4

1

2

3

•Synthesis of membrane proteins and

lipids in the ER. Carbohydrates are

added to the proteins making them

glycoproteins

•Inside Golgi they undergo

carbohydrate modifications becoming

glycolipids

•Proteins are transported in vesicles to

the plasma membrane

•The vesicles fuse with the membrane

releasing secretory proteins form the

cell

Membrane structure results in selective permeability

A cell must exchange materials with its surroundings, a process

controlled by the plasma membrane

A steady traffic of small molecules and ions moves across the

membrane in both directions

Sugars, amino acids and other nutrients enter the cell while waste

products leave the cell

The cell takes in oxygen for cellular respiration and expels CO2

It also regulates concentration of inorganic ions

The Permeability of the Lipid Bilayer

Hydrophobic molecules

Are lipid soluble and can pass through the membrane rapidly

Examples are oxygen, hydrocarbons and CO2

Polar molecules

Do not cross the membrane rapidly

Examples are glucose and other sugars, water

Charged atom or molecule and its surrounding shell of water penetrate

the membrane even more difficult

Transport Proteins

Transport proteins

Allow passage of hydrophilic substances across the membrane

Some of them act as channel proteins where they have hydrophilic

channel that certain molecules use as a tunnel

Others act as carrier proteins which hold onto their passengers and

change shape in a way that shuttles them across the membrane

In both cases the transport protein is specific for the substance it

translocates

Active transport

In other cases, transport proteins move solutes against the

concentration gradient; this is called active transport.

Active transport requires energy such as that released by the

hydrolysis of ATP or by the simultaneous transport of

another solute down an energy gradient.

Concentration gradient or

Electrochemical Potential

The movement of a molecule that has no net charge is

determined by its concentration gradient

Simple or facilitated diffusion involve exergonic movement

“down” the concentration gradient (negative ΔG)

Active transport involves endergonic movement “up” the

concentration gradient (positive ΔG)

The electrochemical potential

The movement of an ion is determined by its

electrochemical potential

the combined effect of its concentration gradient and the

charge gradient across the membrane

The active transport of ions across a membrane creates a

charge gradient or membrane potential (Vm)

Active transport of ions

Most cells have an excess of negatively charged solutes inside

the cell

This charge difference favors the inward movement of cations

such as Na+ and outward movement of anions such as Cl–

In all organisms, active transport of ions across the plasma

membrane results in asymmetric distribution of ions inside

and outside the cell

Functions of active transport

Active transport couples endergonic transport to an

exergonic process, usually ATP hydrolysis

•Active transport performs three important cellular functions

-Uptake of essential nutrients

-Removal of wastes

-Maintenance of nonequilibrium concentrations of certain ions

Direct active transport

accumulation of solute molecules on one side of the

membrane is coupled directly to an exergonic chemical reaction

This is usually hydrolysis of ATP

•Transport proteins driven by ATP hydrolysis are called

transport ATPases or ATPase pumps

Indirect active transport

Indirect active transport depends on the simultaneous

transport of two solutes.

•Favorable movement of one solute down its gradient - drives the

unfavorable movement of the other up its gradient.

•symport or an antiport, depending on whether the two

molecules are transported in the same or different directions.

Direct Active Transport Depends on Four Types of Transport

ATPases

Four types of transport ATPases have been identified

-P-type

-V-type

-F-type

-ABC-type

•They differ in structure, mechanism, location, and roles

P-type ATPases

members of a large family

reversibly phosphorylated by ATP on a specific aspartic acid

residue

8-10 transmembrane segments in a single polypeptide

crosses the membrane multiple times

5 subfamilies (P1-P5)

V-type ATPases

pump protons into organelles

vacuoles, vesicles, lysosomes, endosomes, and the Golgi

complex

two multisubunit components:

integral component embedded in the membrane

peripheral component that juts out from the membrane

surface

F-type ATPases

found in bacteria, mitochondria and chloroplasts

They transport protons and have two components:

–a transmembrane pore (Fo) and

–a peripheral membrane component (F1) that contains the ATP

binding site.

•Both are multisubunit components

ABC-type ATPases

(ATP binding cassette) transporters

cassette describes the catalytic domain that binds ATP as part

of the transport process

comprise a very large family of transport proteins found in

all organisms

Medical significance of ABC-type

ATPases

some of them pump antibiotics or drugs out of cells,

rendering the cell resistant to the drug

Some human tumors are resistant to drugs that normally

inhibit growth of tumors

resistant cells have high concentrations of an ABC transporter

called MDR (multidrug resistance) transport protein

MDR transport protein

pumps hydrophobic drugs out of cells

reducing the cytoplasmic concentration and hence their

effectiveness

transports a wide range of chemically dissimilar drugs

Indirect Active Transport Is Driven by

Ion Gradients

not powered by ATP hydrolysis

inward transport of molecules up their electrochemical

gradients - coupled to and driven by simultaneous inward

movement of Na+ (animals) or protons (plant, fungi,

bacteria) down their gradients

Summary

Structure of plasma membrane

Transport across the membrane

Fluid mosaic model

Fluidity

Crossing membranes

Active transport

Transport ATPases

Simple Diffusion

Unassisted Movement Down the Gradient

movement of a solute from high to lower concentration

only possible for gases, nonpolar molecules, or small polar

molecules

Oxygen and the function of

erythrocytes

Oxygen gas transfers the lipid bilayer readily by simple

diffusion

Erythrocytes take up oxygen in the lungs, where oxygen

concentration is high

release it in the body tissues, where oxygen concentration is

low

Diffusion Always Moves Solutes Toward

Equilibrium

tends to create a random solution in which the concentration

is the same everywhere

Solutes will move toward regions of lower

concentration until the concentrations are equal

Thus diffusion is always movement toward equilibrium !!!!

Osmosis

Diffusion of Water Across a Selectively Permeable

Membrane

Water molecules are polar and so are not affected by the

membrane potential

Water concentration is not appreciably different on opposite

sides of a membrane

Osmosis

If two solutions are separated by a selectively permeable

membrane, permeable to the water but not the solutes, the water will

move toward the region of higher solute concentration.

Osmosis

For most cells, water tends to move inward

Water Balance of Cells Without Walls

Tonicity

Is the ability of a solution to cause a cell to gain or lose water

Has a great impact on cells without walls

Depends in part on its concentration of solutes that cannot cross the

membrane relative to that in the cell itself.

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

Isotonic solution

H2O H2O

Normal

Animal cell. An

animal cell fares best

in an isotonic environ-

ment unless it has

special adaptations to

offset the osmotic

uptake or loss of

water.

If a solution is hypertonic

The concentration of solutes is greater than it is inside the cell

The cell will lose water

Hypertonic solution

H2O

Shriveled

If a solution is hypotonic

The concentration of solutes is less than it is inside the cell

The cell will gain water

Hypotonic solution

H2O

Lysed

A cell without rigid walls can tolerate neither excessive uptake or

excessive loss of water

This is automatically solved if a cell lives in isotonic surrounding

Animals and other organisms without rigid cell walls living in

hypertonic or hypotonic environment must have special adaptations

for osmoregulation

Control of water balance

Water Balance of Cells with Walls

Cell walls

Help maintain water balance

Example: plant cell

This cell swells as water enters by osmosis

The elastic wall will expand only so much before it exters back

pressure on the cell that opposes further water uptake

At this point, the cell is turgid

If a plant cell is turgid

It is in a hypotonic environment

It is very firm, a healthy state in most plants

Plant cell. Plant cells

are turgid (firm) and

generally healthiest in

a hypotonic environ-

ment, where the

uptake of water is

eventually balanced

by the elastic wall

pushing back on the

cell.

H2O

Turgid (normal)

If a plant cell is flaccid

It is in an isotonic or hypertonic environment

There is not tendency for water to enter

H2O H2O

Flaccid

A wall is of no advantage if the cell is immersed in hypertonic

environment

Plant cell will lose water to its surroundings and shrink

Plasma membrane pulls away from the wall

Plasmolysis

Causing plant to wilt and can be lethal

Plasmolyzed

H2O

Solute Size

lipid bilayers are more permeable to small molecules

without a transporter even these small molecules move more

slowly than in the absence of a membrane

Solute Polarity

Lipid bilayers are more permeable to nonpolar substances than to polar ones

Nonpolar substances dissolve readily into the hydrophobic region of the bilayer

Large nonpolar molecules such as estrogen and testosterone cross membranes easily, despite their large size

Polarity of a solute can be measured by the ratio of its solubility in an organic solvent to its solubility in water

This is called the partition coefficient

In general, the more nonpolar (hydrophobic) a substance is, the higher the partition coefficient is.

Solute Charge and relevance to cell function

The relative impermeability of polar substances, especially ions, is due to their association with water molecules

The molecules of water form a shell of hydration around polar substances

In order for these substances to move into a membrane, the water molecules must be removed, which requires energy

Every cell must maintain an electrochemical potential across its plasma membrane in order to function.

In most cases this potential is a gradient of either sodium ions (animal cells) or protons (other cells).

Membranes must still be able to allow ions to cross the bilayer in a controlled manner.

Facilitated Diffusion

Protein-Mediated Movement Down the Gradient

In facilitated diffusion

Transport proteins speed the movement of molecules across the

plasma membrane

Most transport proteins are very specific

They transport only particular substances but not others

Transport proteins

•Transport proteins assist most solute across membranes.

•These integral membrane proteins recognize the substances

to be transported with great specificity.

•Some move solutes to regions of lower concentration; this

facilitated diffusion (or passive transport) uses no energy.

Figure 7.7

Glycoprotein

Carbohydrate

Microfilaments

of cytoskeleton Cholesterol Peripheral

protein Integral

protein

CYTOPLASMIC SIDE

OF MEMBRANE

EXTRACELLULAR

SIDE OF

MEMBRANE

Glycolipid

Membrane Proteins and Their Functions A membrane

Is a collage of different proteins embedded in the fluid matrix of the

lipid bilayer

Fibers of

extracellular

matrix (ECM)

Membrane Proteins and Their

Functions

Example red blood cells

More than 50 types of proteins have been found in the plasma

membrane of RBC

Phospholipids form the main fabric of the membrane

Proteins determine most of the membrane functions

Different types of cells contain different sets of membrane proteins

Integral proteins

Penetrate the hydrophobic core of the lipid bilayer

Are often transmembrane proteins, completely spanning the

membrane

Usually α helical proteins

EXTRACELLULAR

SIDE N-terminus

C-terminus

a Helix CYTOPLASMIC

SIDE

Peripheral proteins

Are appendages loosely bound to the surface of the membrane

Not embedded in the lipid bilayer

An overview of six major functions of membrane proteins

Figure 7.9

Transport. (left) A protein that spans the membrane

may provide a hydrophilic channel across the

membrane that is selective for a particular solute.

(right) Other transport proteins shuttle a substance

from one side to the other by changing shape. Some

of these proteins hydrolyze ATP as an energy ssource

to actively pump substances across the membrane.

Enzymatic activity. A protein built into the membrane

may be an enzyme with its active site exposed to

substances in the adjacent solution. In some cases,

several enzymes in a membrane are organized as

a team that carries out sequential steps of a

metabolic pathway.

Signal transduction. A membrane protein may have

a binding site with a specific shape that fits the shape

of a chemical messenger, such as a hormone. The

external messenger (signal) may cause a

conformational change in the protein (receptor) that

relays the message to the inside of the cell.

(a)

(b)

(c)

ATP

Enzymes

Signal

Receptor

Cell-cell recognition. Some glyco-proteins serve as

identification tags that are specifically recognized

by other cells.

Intercellular joining. Membrane proteins of adjacent cells

may hook together in various kinds of junctions, such as

gap junctions or tight junctions (see Figure 6.31).

Attachment to the cytoskeleton and extracellular matrix

(ECM). Microfilaments or other elements of the

cytoskeleton may be bonded to membrane proteins,

a function that helps maintain cell shape and stabilizes

the location of certain membrane proteins. Proteins that

adhere to the ECM can coordinate extracellular and

intracellular changes (see Figure 6.29).

(d)

(e)

(f)

Glyco-

protein

Figure 7.9

Channel proteins

Facilitate Diffusion by Forming Hydrophilic Transmembrane Channels

allow specific solutes to cross the membrane directly

There are three types of channels:

ion channels, porins, and aquaporins

EXTRACELLULAR

FLUID

Channel protein Solute

CYTOPLASM

A channel protein (purple) has a channel through which

water molecules or a specific solute can pass.

(a)

Ion Channels

Allow Rapid Passage of Specific Ions

Ion channels - lined with hydrophilic atoms

remarkably selective

most allow passage of just one ion

separate proteins needed to transport Na+, K+, Ca2+, and

Cl–, etc.

Selectivity - based on both binding sites involving amino acid

side chains, and a size filter

Gated channels

Most ion channels are gated, meaning that they open and close in

response to some stimulus

-Voltage-gated channels open and close in response to

changes in membrane potential

-Ligand-gated channels are triggered by the binding of

certain substances to the channel protein

-Mechano-sensitive channels respond to mechanical forces

acting on the membrane

Porins

Transmembrane Proteins That Allow Rapid Passage of Various Solutes

Pores on outer membranes of bacteria, mitochondria and chloroplasts are larger and less specific than ion channels

The pores are formed by multipass transmembrane proteins called porins

The transmembrane segments of porins cross the membrane as β barrels

Aquaporins

Transmembrane Channels That Allow Rapid Passage

of Water

through membranes of erythrocytes and kidney cells in

animals

root cells and vacuolar membranes in plants.

discovered only in 1992

Carrier proteins

Undergo a subtle change in shape that translocates the solute-binding

site across the membrane

Carrier protein Solute

A carrier protein alternates between two conformations, moving a

solute across the membrane as the shape of the protein changes.

The protein can transport the solute in either direction, with the net

movement being down the concentration gradient of the solute.

(b)

Carrier Proteins Are Analogous to Enzymes in Their Specificity and Kinetics

Carrier proteins are analogous to enzymes

-Facilitated diffusion involves binding a substrate, on a specific solute binding site

-The carrier protein and solute form an intermediate

-After conformational change, the “product” is released (the transported solute)

-Carrier proteins are regulated by external factors

Competitive inhibition of carrier

proteins

Competitive inhibition of carrier proteins -in the presence of

molecules or ions that are structurally related to the correct

substrate

Example: transport of glucose by glucose carrier proteins

inhibited by the other monosaccharides that the carrier

accepts (mannose and galactose)

Carrier Proteins Transport Either One or

Two Solutes

When a carrier protein transports a single solute across the

membrane - uniport

A carrier protein that transports a single solute uniporter

When two solutes are transported simultaneously, and their

transport is coupled - coupled transport

Coupled transport

If the two solutes are moved across a membrane in the same

direction - symport (or cotransport)

If the solutes are moved in opposite directions-antiport (or

countertransport)

Transporters that mediate these processes are symporters

and antiporters

Endocytosis/Exocytosis

For substances the cell needs to take in (endo = in)

or expel (exo = out) that are too large for passive or

active transport

Exocytosis

Large molecules that are manufactured in the cell are

released through the cell membrane.

Endocytosis

Two types: phagocytosis (“cellular eating” for solids)

and pinocytosis (“cellular drinking” for fluids)

Receptor mediated Endocytosis – ligands bind to

specific receptors on cell surface

Summary 2

Simple diffusion

Osmosis

Facilitated diffusion

Channel and carrier proteins

Coupled transport

Endocytosis

Exocytosis