What are membranes?
Membranes cover the surface of every cell, and also surround
most organelles within cells. They have a number of
functions, such as:
Protection: keeping all cellular components inside the cell
and allowing a cell to change shape.
Transport (like a gate): Selectively allow substances to
move in and out of the cell.
isolating organelles from the rest of the cytoplasm, allowing
cellular processes to occur separately.
Recognition: a site for biochemical reactions.
Communication: cell- cell interactions.
Membrane Models: Scientific Inquiry
• In 1915, membranes isolated from red blood cells were
chemically analyzed and found to be composed of lipids and
• In 1924, using an electron microscope, two Dutch
physicians, E. Gorter and F. Grendel found that the cell
membrane was composed primarily of phospholipids. They
deduced, based on the properties of phospholipids, that the
cell membrane was in fact a bilayer.
• By the 1930s experimental evidence showed that proteins
were also part of the cell membrane.
• In 1935, James Danielli and Hugh Davson proposed
the sandwich model: a phospholipid bilayer between two
layers of protein with pores for molecules to travel through.
• In 1972, S. J. Singer and Garth Nicholson presented
the fluid mosaic model of the cell membrane, which
displayed the cell membrane as an integration of proteins
and other molecules into the phospholipid bilayer.
• The head is charged and so polar; the tails are not charged
and so are non-polar. Thus the two ends of the
phospholipid molecule have different properties in water.
The phosphate head is hydrophilic and so the head will
orient itself so that it is as close as possible to water
molecules. The fatty acid tails are hydrophobic and so
will tend to orient themselves away from water.
• Amphipathic - having both:
Fluid mosaic model of cell membranes
1.The Fluidity of Membranes
Membranes are not static sheets of molecules locked rigidly in
place. Membranes are fluid and are rather viscous – like
olive oil. The greater concentration of unsaturated fatty acids ,
the more fluid is the bilayer. Cholesterol also allows the cell
membrane to stay fluid over a wider range of temperatures.
The molecules of the cell membrane are always in motion,
most of the lipids and some of the proteins can shift about
laterally. so the phospholipids are able to drift across the
membrane, changing places with their neighbor (lateral
movement). Rarely, for a molecule to flip- flop
transversely across the membrane, switching from one
phospholipid layer to the other; to do so, the hydrophilic part
of the molecule must cross the hydrophobic interior of the
2. The Mosaic Quality of the membrane
Somewhat like a tile mosaic, a membrane is a collage of
different proteins, often clustered together in groups, embedded
in the fluid matrix of the lipid bilayer.
Proteins, both in and on the membrane, form a mosaic,
floating in amongst the phospholipids.
Various proteins are associated with the cell
• Integral Proteins penetrate the hydrophobic interior of the
lipid bilayer. The majority are transmembrane proteins,
which span the width of membrane; other integral proteins
extend only partway into the hydrophobic interior and create
channels through which charged molecules or large
molecules can pass through.
• Peripheral proteins are not embedded in the lipid bilayer
at all; they are appendages loosely bound to the surface of the
membraneand are primarily used in cell to cell signaling with
surface carbohydrate chains or linking with the cytoskeleton
The Role of Membrane Carbohydrates in Cell-Cell
Cell-cell recognition, a cell s ability to distinguish one type of
neighboring cell from another, is crucial to the functioning of
Membrane carbohydrates are usually short, branched chains
of fewer than 15 sugar units. Some are covalently bonded to
lipids, forming molecules called glycolipids. (Re-call that
glyco refers to the presence of carbohydrate.) However, most
are covalently bonded to proteins, which are thereby
Cells recognize other cells by binding to molecules, often
containing carbohydrates, on the extracellular surface of the
• Selectively (Differentially) Permeable: membrane
regulates what passes into and out of the cell. The cell
membrane controls which substances pass into and
out of the cell. Carrier proteins in or on the membrane are
specific, only allowing a small group of very similar
molecules through. For this reason, the cell membrane is said
to be selectively permeable.
• A steady traffic of small molecules and ions moves across the
plasma membrane in both directions. Consider the chemical
exchanges between a muscle cell and the extracellular fluid
that bathes it. Sugars, amino acids, and other nutrients enter
the cell, and metabolic waste products leave it.
Membrane structure results in selective
Non-polar molecules, such as hydrocarbons, carbon
dioxide, and oxygen, are hydrophobic and can therefore
dissolve in the lipid bilayer of the membrane and cross it
easily, without the aid of membrane proteins.
Polar molecules such as glucose and other sugars pass only
slowly through a lipid bilayer, and even water, an extremely
small polar molecule, does not cross very rapidly.
A charged atom or molecule and its surrounding shell of
water, find the hydrophobic interior of the membrane even
more difficult to penetrate. Furthermore, the lipid bilayer is
only one aspect of the gate-keeper system responsible for the
selective permeability of a cell. Proteins built into the
membrane play key roles in regulating transport.
Cell membranes are permeable to specific ions and a variety of
polar molecules. These hydrophilic substances can avoid
contact with the lipid bilayer by passing through transport
proteins that span the membrane.
Some transport proteins, called channel proteins, function
by having a hydrophilic channel that certain molecules or
atomic ions use as a tunnel through the membrane. For
example, the passage of water molecules through the
membrane in certain cells is greatly facilitated by channel
proteins known as aquaporins. Without aquaporins, only a
tiny fraction of these water molecules would pass through the
same area of the cell membrane in a second, so the channel
protein brings about a tremendous increase in rate.
Other transport proteins, called carrier proteins, hold onto
their passengers and change shape in a way that shuttles
them across the membrane.
A transport protein is specific for the substance it
translocates (moves), allowing only a certain substance (or a
small group of related substances) to cross the membrane.
For example, a specific carrier protein in the plasma
membrane of red blood cells transports glucose across the
membrane 50,000 times faster than glucose can pass through
on its own. This glucose transporter is so selective that it even
rejects fructose, a structural isomer of glucose.
The diffusion of a substance across a biological membrane is
called passive transport because the cell does not have to
expend energy to make it happen
Molecules have a type of energy called thermal energy
(heat), due to their constant motion. One result of this motion
is diffusion, the movement of molecules of any substance so
that they spread out evenly into the available space.
A substance will diffuse from where it is more concentrated to
where it is less concentrated. Put another way, any substance
will diffuse down its concentration gradient, the region
along which the density of a chemical substance increases or
decreases (in this case, decreases).
Diffusion is a spontaneous passive process, which means
it does not require any energy input.
It can occur across a living or non-living membrane and
can occur in a liquid or gas medium. Due to the fact that
diffusion occurs across a concentration gradient it can result
in the movement of substances into or out of the cell.
Examples of substances moved by diffusion include carbon
dioxide, oxygen, water and other small molecules that are able
to dissolve within the lipid bilayer.
Passive Transport Aided by Proteins
Facilitated diffusion is a special form of diffusion
which allows rapid exchange of specific substances.
Facilitated diffusion can only occur across living,
biological membranes which contain the carrier
proteins. A substance is transported via a carrier
protein from a region of high concentration to a region
of low concentration until it is randomly distributed.
Particles are taken up by carrier proteins which
change their shape as a result. The change in shape
causes the particles to be released on the other side of
• Water diffuses across a selectively permeable membrane from
the region of lower solute concentration (higher free
water concentration) to that of higher solute
concentration (lower free water concentration) through
semi-permeable membrane, whether artificial or cellular, is
• Osmosis is a passive process and does not require any input
• Cell membranes allow molecules of water to pass through, but
they do not allow molecules of most dissolved substances, e.g.
salt and sugar, to pass through. As water enters the cell via
osmosis, it creates a pressure known as osmotic pressure.
• Tonicity is the difference in water concentration of two
solutions separated by a semi-permeable
membrane. Knowing the tonicity of solutions will tell you
which direction water will diffuse.
• Refers to the concentration of solutes
• Is a relative term, comparing two different solutions:
•Hypertonic solutions will have a higher concentration of
solute (glucose, salt, etc) than the cell. Mainly water will move
across the cell membrane in order to even out
the concentration of solutes in both the cell and the environment
around the cell. The cell will shrink as water leaves the cell to
decrease the higher concentration of solute in the environment.
•Hypotonic environments will have a lower concentration of
solute than the cell. Water will move from the environment into
the cell in order to balance the concentration of solute. When
water diffuses into the cell it will swell. Sometimes the cell may
lyse or burst due to the excess water uptake.
•Isotonic environments have the same concentration of
solutes as the cell. Water will diffuse both in and out of the cell,
but no net effect will be seen.
Active transport is the movement of substances against a
concentration gradient, from a region of low concentration to
high concentration using an input of energy.
In biological systems, the form in which this energy occurs is
adenosine triphosphate (ATP). The process transports
substances through a membrane protein. The movement of
substances is selective via the carrier proteins and can occur
into or out of the cell.
As in other types of cellular work, ATP supplies the energy for
most active transport. One way ATP can power active transport
is by transferring its terminal phosphate group the ion s
movement). This combination of forces acting on an ion is
called the electrochemical gradient.
The cytoplasmic side of the membrane is negative in charge
relative to the extracellular side because of an unequal
distribution of anions and cations on the two sides. The
voltage across a membrane, called a membrane potential.
The membrane potential acts like a battery, an energy
source that affects the traffic of all charged substances across
the membrane. Because the inside of the cell is negative com-
pared with the outside, the membrane potential favors the
passive transport of cations into the cell and anions out of the
Thus, two forces drive the diffusion of ions across a
membrane: a chemical force (the ions concentration
gradient) and an electrical force (the effect of the
Some membrane proteins that actively transport ions con-
tribute to the membrane potential.
An example is the sodium-potassium pump. The pump
does not translocate Na* and K* one for one, but pumps three
sodium ions out of the cell for every two potassium ions it
pumps into the cell. With each crank of the pump, there is a
net transfer of one positive charge from the cytoplasm to the
extracellular fluid, a process that stores energy as voltage. The
sodium-potassium pump appears to be the major electrogenic
pump of animal cells.
The main electrogenic pump of plants, fungi, and bacteria is a
proton pump, which actively transports protons (hydrogen
ions, H*) out of the cell. The pumping of H* transfers positive
charge from the cytoplasm to the extracellular solution.
The sodium-potassium pump
Bulk transport across the plasma membrane
occurs by exocytosis and endocytosis
Exocytosis---Cellular secretion the cell secretes certain
biological molecules by the fusion of vesicles with the plasma
An Example: A transport vesicle that has budded from the
Golgi apparatus moves along microtubules of the
cytoskeleton to the plasma membrane. When the vesicle
membrane and plasma membrane come into contact, speci c
proteins rearrange the lipid molecules of the two bilayers so
that the two membranes fuse. The contents of the vesicle
then spill to the outside of the cell, and the vesicle membrane
becomes part of the plasma membrane.
– Phagocytosis— “Cell eating”
– Pinocytosis– “Cell drinking”
The cell takes in biological molecules and particulate matter
by forming new vesicles from the plasma membranein a
process called endocytosis.
Although the proteins involved in the processes are different,
the events of endocytosis look like the reverse of exocytosis. A
small area of the plasma membrane sinks inward to form a
pocket. As the pocket deepens, it pinches in, forming a vesicle
containing material that had been outside the cell.