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Cell Membranes
Introduction
Cell membranes are very important structures to cells because they function as a barrier
between the components of the cell and the outside environment
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The cell membrane is
what defines the cell and keeps its components separate from outside cells or organisms
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The structure of the lipid bilayer prevents the free passage of
most molecules into and out of the cell
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We will then go on to
discuss the role of membrane-bound proteins, and finally, will discuss membrane transport
structures
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 Carrier protein - A protein responsible for mediating the active transport of molecules from
one side of the lipid bilayer to the other
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 Channel protein - A protein responsible for mediating the passive transport of molecules
from one side of the lipid bilayer to the other
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 Diffusion - The transport process in which molecules naturally travel from an area of higher
concentration to an area of lower concentration
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Functions in protecting the cell from damage
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 Hydrophobic - A nonpolar molecule that does not readily associate with water through
hydrogen bonds
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 Ionophore - A class of membrane transport proteins
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 Lipid bilayer - A thin double layer of phospholipid molecules
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Structure is a result of hydrophobic and hydrophilic forces
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 Multi-pass protein - Transmembrane proteins that cross the lipid bilayer more than one
time
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 Passive transport - Transport mediated by channel proteins
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 Peripheral protein - A membrane protein that can be easily removed from the lipid bilayer
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 Transmembrane protein - A membrane protein that spans the lipid bilayer having portions
in contact with both the inside and outside of the cell
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The Lipid Bilayer
Lipid Bilayer Structure
The lipid bilayer is a universal component of all cell membranes
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The structure is
called a "lipid bilayer" because it is composed of two layers of fat cells organized in two sheets
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Lipids and Phospholipids
The structure of the lipid bilayer explains its function as a barrier
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There are two important regions of a lipid that provide the structure of the lipid
bilayer
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Figure %: Basic Lipid Structure
The hydrophilic region is attracted to aqueous water conditions while the
hydrophobic region is repelled from such conditions
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The most abundant class of lipid molecule found in cell membranes is the phospholipid
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It also sports two
nonpolar fatty acid chain groups as its tail
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It has a kink in one of the
chains because of its double-bond structure
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This organization is spontaneous, meaning it is a natural

process and does not require energy
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Figure %: Lipid Bilayer
Properties of the Lipid Bilayer
As we have already mentioned, the most important property of the lipid bilayer is that it is a highly
impermeable structure
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Only water and gases can easily pass through the bilayer
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Another important property of the lipid bilayer is its fluidity
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The bilayer's fluidity allows
these structures mobility within the lipid bilayer
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Fluidity is dependent on both the specific structure of the fatty acid chains
and temperature (fluidity increases at lower temperatures)
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Problems
Problem : Identify the lipid bilayer in the following diagram of a cell
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Problem : What is the main function of the lipid bilayer?

The lipid bilayer acts as a barrier between the inside and outside of the cell
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Problem : Why is the structure called a lipid bilayer?
It is called a lipid bilayer because it is composed of two layers of fat, or lipid, molecules
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A phospholipid molecule contains two distinct regions
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As a result of its both polar and
nonpolar regions, it is classified as a(n) __________ molecule
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Hydrophobic
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Problem : Are the two layers of the lipid bilayer identical in composition?
No
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Membrane Proteins
In addition to the lipid bilayer, the cell membrane also contains a number of proteins
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In this section we will
discuss the different classes of proteins found there
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As we discussed in the previous section, membrane proteins are free to move within the
lipid bilayer as a result of its fluidity
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Membrane proteins perform various
functions, and this diversity is reflected in the significantly different types of proteins associated
with the lipid bilayer
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Integral Proteins
Integral proteins are embedded within the lipid bilayer
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Integral proteins
float rather freely within the bilayer, much like oceans in the sea
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The stretch of the integral protein within the
hydrophobic interior of the bilayer is also hydrophobic, made up of non-polar amino acids
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Figure %: Membrane Proteins
When a protein crosses the lipid bilayer it adopts an alpha-helical configuration
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The former are referred to as
single-pass proteins and the later as multi-pass proteins
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Peripheral Proteins
Peripheral proteins are attached to the exterior of the lipid bilayer
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Peripheral proteins
are less mobile within the lipid bilayer
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The Cell Surface
The protein and lipid cell membrane is covered with a layer of carbohydrate chains on its outer
surface
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The exact composition and distribution of
these chains is very diverse
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Glycocalyx are only found on the surface of the cells of higher organism's
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Membrane proteins perform a number of functions within cells, as a result, different proteins
are necessary in different regions of cells depending on the function of the cell and the
interactions it may take part in
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Since
peripheral proteins are easily dissociated from the lipid bilayer, one could treat a cell with a mild
detergent that does not disrupt the cell membrane and then see if the specific protein remains
associated with the lipid bilayer or is removed
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It is the same structure that DNA adopts naturally
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They are relatively
mobile in the cell membrane
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What name is given to
these structures and what is their function in the cell?
The cell surface is covered with a cell coat or glycocalyx which consists of carbohydrate chains
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Structures Responsible for Membrane Transport
We have discussed how the lipid bilayer acts as an efficient barrier by only allowing a very small
number of non-polar molecules to freely enter or exit a cell
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As
a result, cell membranes require specific structures that allow for the transport of certain
molecules
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Some such means, like diffusion and osmosis, are natural processes that require no
expenditure of energy from the cell and are called passive transport
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In addition to these two forms of
transport, there exist other forms of transport such as endocytosis and exocytosis, which will be
discuss later and do not require the same set of membrane proteins for their function
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Osmosis is a similar process, but refers
specifically to water molecules
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As seen in , neither diffusion nor osmosis require the
expenditure of energy
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As seen in , such transport requires
energy
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shows the two main classes of transport proteins: carrier proteins and
channel proteins
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Carrier proteins create an opening in the lipid bilayer by
undergoing a conformational change upon the binding of the molecule
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When open, these pores allow specific molecules to
pass through
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These are small,
hydrophobic proteins that increase bilayer permeability for specific ions
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They allow for the proper
distribution of ions and molecules in multicellular organisms
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Problems
Problem : Why is it necessary for cell membranes to have proteins that help transport
molecules?
The cell membrane is composed of a lipid bilayer that is highly impermeable to most molecules
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Problem : What is the name of the natural process by which molecules flow from an area of
higher concentration to one of lower concentration?
Diffusion
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The conformational change opens a
hole through which the molecule can enter or exit a cell
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Channel proteins allow molecules to
steadily diffuse across the membrane through diffusion
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Problem : Name two specific functions of membrane transport
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