Membranes: Structure, Function and...

Membranes:

Structure,

Function and

Chemistry

Chapter 7

Becker’s The World of Cell

The Functions of Membranes

Cells have a variety of membranes that

define the boundaries of the cell and its

internal compartments. All biological

membranes have the same general

structure: a fluid phospholipid bilayer

containing a mosaic of embedded

proteins.

While the lipid component of membranes

provides a permeability barrier, specific

proteins in the membrane regulate

transport of materials into and out of cells

and organelles.

Membranes serve as sites for specific

proteins and thus can have specific

functions. They can detect and transduce

external signals, mediate contact and

adhesion between neighboring cells, or

participate in cell-to-cell communication.

They also help to produce external

structures such as the cell wall or

extracellular matrix.

Models of Membrane

Structure:

An

Experimental

Perspective

Our current understanding of membrane

structure represents the culmination of

more than a century of studies, beginning

with the recognition that lipids are an

important membrane component.

■

Once proteins were recognized as

important components, Davson and

Danielli proposed their “sandwich”

model—a lipid bilayer surrounded on both

sides by layers of proteins. As membranes

and membrane proteins were examined

in more detail, however, this model was

eventually discredited.

In place of the sandwich model, Singer

and Nicolson’s fluid mosaic model

emerged and is now the universally

accepted description of membrane

structure. According to this model,

proteins with varying affinities for the

hydrophobic membrane interior float in

and on a fluid lipid bilayer.

■ We now know that lipids and proteins

are not distributed randomly in the

membrane but are often found in

microdomains known as lipid rafts that are

involved in cell signaling and other

interactions.

Membrane Lipids: The “Fluid”

Part of the Model

Prominent lipids in most membranes

include numerous types of phospholipids

and glycolipids. The proportion of each

lipid type can vary considerably

depending on the particular membrane

or monolayer.

In eukaryotic cells, sterols are also

important membrane components,

including cholesterol in animal cells and

phytosterols in plant cells. Sterols are not

found in the membranes of most

prokaryotes, but some bacterial species

contain similar compounds called

hopanoids.

Proper fluidity of a membrane is critical to

its function. Cells often can vary the

fluidity of membranes by changing the

length and degree of saturation of the

fatty acid chains of the membrane lipids

or by the addition of cholesterol or other

sterols.

Long-chain fatty acids pack together well

and decrease fluidity. Unsaturated fatty

acids contain cis double bonds that

interfere with packing and increase

fluidity.

Most membrane phospholipids and

proteins are free to move within the plane of the membrane unless they are specifically anchored to structures on the inner or outer membrane surface. Transverse diffusion, or “flip-flop,” between monolayers is not generally possible, except for phospholipids when catalyzed by enzymes called phospholipid translocators, or flippases.

As a result, most membranes are

characterized by an asymmetric

distribution of lipids between the two

monolayers and an asymmetric

orientation of proteins within the

membranes so that the two sides of the

membrane are structurally and

functionally dissimilar.

Membrane Proteins: The “Mosaic” Part of the Model

Proteins are major components of

all cellular membranes. Membrane

proteins are classified as integral,

peripheral, or lipid anchored, based

on how they are associated with

the lipid bilayer.

Proteins are major components of

all cellular membranes. Membrane

proteins are classified as integral,

peripheral, or lipid anchored, based

on how they are associated with

the lipid bilayer.

Integral membrane proteins have one or

more short segments of predominantly

hydrophobic amino acids that anchor the

protein to the membrane. Most of these

transmembrane segments are a-helical

sequences of about 20–30 predominantly

hydrophobic amino acids.

Peripheral membrane proteins are

hydrophilic and remain on the membrane

surface. They are typically attached to

the polar head groups of phospholipids

by ionic and hydrogen bonding.

Lipid-anchored proteins are also

hydrophilic in nature but are

covalently linked to the membrane

by any of several lipid anchors that

are embedded in the lipid bilayer.

Membrane proteins function as enzymes, electron

carriers, transport molecules, and receptor sites for

chemical signals such as neurotransmitters and

hormones. Membrane proteins also stabilize and shape

the membrane and mediate intercellular

communication and cell-cell adhesion.

Many proteins in the plasma

membrane are glycoproteins, with

carbohydrate side chains that

protrude from the membrane on

the external side, where they play

important roles as recognition

markers on the cell surface.

Thanks to current advances in SDS-PAGE, molecular

biology X-ray crystallography, affinity labeling, and the

use of specific antibodies, we are learning much about

the structure and function of membrane proteins that

were difficult to study in the recent past.