Cell Membranes

Cell Membranes

"Cell Membranes are phospholipid 

bilayers with embedded proteins"

The Plasma membrane made of phospholipid sheets  only 5 to 

10  nm thick encases all living cells. Biologists have long known the 

molecular components of membranes—lipids, proteins, and other 

molecules—but for many years the organization of these 

membrane components remained elusive.

Note: when we say plasma membrane,it means the outermost membranous boundary in animal cells but it is present below the cell wall in plant cells. The plasma membrane surrounds the cell.

It separates the internal

metabolic events from the environment and allows them to proceed in organized,

controlled ways. The plasma membrane also has specific receptors for external molecules that alter the cell’s function.

But when we talk about cell membranes it means the total membranous system of the cell. 

Fluid Mosaic Model of Membrane Strucure 

The lipid layer that forms the foundation of a cell’s membranes is 

actually a bilayer formed of two phospholipid sheets (figure above) . 

For many years biologists thought that the protein components of 

the plasma membrane covered the inner and outer surfaces of the 

phospholipid bilayer like a coat of paint. An early model portrayed 

the plasma membrane as a sandwich, with a phospholipid bilayer 

between two layers of globular protein.

             In 1972, S. Jonathan Singer and Garth J. Nicolson revised 

the model in a simple but profound way: They proposed that the 

globular proteins are inserted into the lipid bilayer, with their 

nonpolar segments in contact with the nonpolar interior of the 

bilayer and their polar portions protruding out from the membrane 

surface. In this model, called the fluid mosaic model, a mosaic 

of proteins floats in or on the fluid lipid bilayer like boats on a 

pond (again see above figure).

Phospholipid Molecule Structure 

         Phospholipids 

are composed of glycerol 

(pink) linked to two fatty

acids and a phosphate 

group. The phosphate group 

(yellow) can have additional 

molecules attached, such as 

the positively charged 

choline (green) shown. 

Phosphatidylcholine is a 

common component of 

membranes; it is shown in 

(a) with its chemical formula,

(b) as a space-filling model, 

and (c) as the icon.

                      Integral proteins protrude 

through the plasma membrane, with nonpolar 

regions that tether them to the membrane’s 

hydrophobic interior. Carbohydrate chains are 

often bound to the extracellular portion of these 

proteins, forming glycoproteins. Peripheral 

membrane proteins are associated with the 

surface of the membrane. Membrane 

phospholipids can be modified by the addition 

of carbohydrates to form glycolipids. Inside the 

cell, actin filaments and intermediate filaments 

interact with membrane proteins. Outside the 

cell, many animal cells have an elaborate 

extracellular matrix composed primarily of 

glycoproteins.

   Composition of Cell Membranes 

A eukaryotic cell contains many membranes. Although they 

are not all identical, they share the same fundamental architecture.  Cell membranes are assembled from four components.

1. Phospholipid bilayer. Every cell membrane is 

composed of phospholipids in a bilayer. The other 

components of the membrane are embedded within 

the bilayer, which provides a flexible matrix and, 

at the same time, imposes a barrier to permeability. 

Animal cell membranes also contain a significant 

amount of cholesterol, a steroid with a polar hydroxyl 

group (—OH). Plant cells have other sterols, but little 

or no cholesterol.

2.Transmembrane proteins. A major component of every 

membrane is a collection of proteins that float in the
lipid bilayer. These proteins have a variety of functions,
including transport and communication across the
membrane. Many integral membrane proteins are not
fixed in position. They can move about, just as the
phospholipid molecules do. Some membranes are
crowded with proteins, but in others the proteins
are more sparsely distributed.
 3.Interior protein network. Membranes are structurally
supported by intracellular proteins that reinforce the
membrane’s shape. For example, a red blood cell has  a characteristic biconcave shape because a scaffold made
of a protein called spectrin links proteins embedded in
the plasma membrane bilayer with actin filaments in
the cell’s cytoskeleton. Membranes use networks of
other proteins to control the lateral movements of some
key proteins within the bilayer, anchoring them to
specific sites.
4. Cell-surface markers. Membrane sections are
assembled in the endoplasmic reticulum, transferred to
the Golgi apparatus, and then transported to the plasma
membrane. During passage, the ER adds chains of sugar
molecules to the membrane proteins and lipids, converting
them into glycoproteins and glycolipids. Different cell
types exhibit different varieties of glycoproteins and
glycolipids on their surfaces, which act as cell
identity markers.

Originally it was believed that because of its fluidity, the plasma
membrane was uniform, with lipids and proteins free to diffuse
rapidly in the plane of the membrane. However, in the last decade
evidence has accumulated suggesting that the plasma membrane is
not at all homogeneous and contains microdomains with distinct
lipid and protein composition. One type of microdomain, the lipid
raft, is heavily enriched with cholesterol, which fills space between
the phospholipids, packing them more tightly together than the
surrounding membrane.
                                             Although the distribution of different membrane lipids on
the two sides of the bilayer is symmetrical in the ER where membranes are synthesized, this distribution is asymmetrical in the
plasma membrane, Golgi apparatus, and endosomes. This shift is
accomplished by enzymes that transport lipids across the bilayer
from one face to the other.

Next Topic
1.Spontaneous Formation of Lipid Bilayer
2.Membrane Proteins and their Functions 

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