Plasma Membrane
Life Science
1st Semester

• The plasma membrane is the boundary that
separates the living cell from its surroundings

Predominant constituent: phospholipids
Membrane Structure
Dispersed protein components

Phospholipids- building blocks
A balloon
with three
strings
Amphipathic molecule

Hydrocarbon ring structure of cholesterol plays a distinct
role in determining membrane fluidity
Membrane sterols
Sterols=Steroid ring + Alcohol group

Cholesterol molecules insert into
the bilayer with their polar hydroxyl
groups close to the hydrophilic head
groups of the phospholipids.
 The rigid hydrocarbon rings of
cholesterol therefore interact with the
regions of the fatty acid chains that
are adjacent to the phospholipid
head groups.
This interaction decreases the
mobility of the outer portions of the
fatty acid chains, making this part of
the membrane more rigid.
Insertion of cholesterol interferes with interactions between
fatty acid chains, thereby maintaining membrane fluidity at
lower temperatures.

• The hydrophobic regions of an integral protein consist of
one or more stretches of nonpolar amino acids, often coiled
into alpha helices
• Peripheral proteins
are bound to the
surface of the
membrane
• Integral proteins
penetrate the
hydrophobic core
• Integral proteins that
span the membrane
are called
transmembrane
proteins

Membrane organization and properties described by:
Sandwitch model proposed by Danielli‐Davson
Unit membranes model of Robertson
Fluid Mosaic Model by Singer and Nicolson 1972

*
● FLUID- because individual phospholipids and
proteins can move side-to-side within the
layer, like it’s a liquid.
● MOSAIC- because of the pattern produced by
the scattered protein molecules when the
membrane is viewed from above
FLUID MOSAIC MODEL

*
Functions of Plasma
Membrane
● Protective barrier
● Regulate transport in & out of cell
( a selective barrier to the passage of molecules
selectively permeable)
● Allow cell recognition
● Provide anchoring sites for filaments
of cytoskeleton

*
Functions of Plasma
Membrane
● Provide a binding site for enzymes
● Interlocking surfaces bind cells
together (junctions)
●Contains the cytoplasm (fluid in cell)

Properties of the plasma
membrane
1.Dynamic
2.Fluid
3.Asymmetric
4.Semipermeable

1. Dynamic
Lateral movement occurs
107 times per second.
Flip-flopping across the membrane
is rare ( once per month).
These studies have also shown that individual lipid molecules
rotate very rapidly about their long axis and that their
hydrocarbon chains are flexible.

The fluidity of a lipid bilayer depends on -composition and
temperature.
A synthetic bilayer made from a single type of phospholipid
changes from a liquid state to a two-dimensional rigid
crystalline (or gel) state at a characteristic freezing point. This
change of state is called a phase transition.
2. Fluid
Membranes must be fluid to work properly;
they are usually about as fluid as salad oil

The temperature at which it occurs is lower (that is, the membrane
becomes more difficult to freeze) if the hydrocarbon chains are short or
have double bonds.
A shorter chain length reduces the tendency of the hydrocarbon tails to
interact with one another, and cis-double bonds produce kinks in the
hydrocarbon chains that make them more difficult to pack together, so that
the membrane remains fluid at lower temperatures.
Bacteria, yeasts, and other organisms whose temperature fluctuates with
that of their environment adjust the fatty acid composition of their
membrane lipids to maintain a relatively constant fluidity. As the
temperature falls, for instance, fatty acids with more cis-double bonds are
synthesized, so the decrease in bilayer fluidity that would otherwise result
from the drop in temperature is avoided.
2. Fluid
Role of phospholipids

Cholesterol tends to make lipid bilayers less fluid, at
the high concentrations found in most eucaryotic
plasma membranes
It also prevents the hydrocarbon chains from coming
together and crystallizing.
In this way, it inhibits possible phase transitions
2. Fluid
Role of Cholesterol

3. Asymmetry
The Plasma Membrane Contains Lipid Rafts That Are
Enriched in Sphingolipids, Cholesterol, and Some
Membrane Proteins
For some lipid molecules, such as the sphingolipids which tend to
have long and saturated fatty hydrocarbon chains, the attractive
forces can be just strong enough to hold the adjacent molecules
together transiently in small microdomains. Such microdomains,
or lipid rafts, can be thought of as transient phase separations in
the fluid lipid bilayer where sphingolipids become concentrated.

3. Asymmetry
Lipid asymmetry is functionally important. Many cytosolic
proteins bind to specific lipid head groups found in the cytosolic
monolayer of the lipid bilayer.
The enzyme protein kinase C (PKC), for example, is activated in
response to various extracellular signals.
It binds to the cytosolic face of the plasma membrane, where
phosphatidylserine is concentrated, and requires this negatively
charged phospholipid for its activity.

*
Solubility
● Materials that
are soluble in
lipids can pass
through the
cell membrane
easily

Small molecules and larger hydrophobic
molecules move through easily.
e.g. O2, CO2, H2O
4.Semipermeable Membrane

Synthesis and Sidedness of Membranes
• Membranes have distinct inside and outside faces
• The asymmetrical distribution of proteins, lipids,
and associated carbohydrates in the plasma
membrane is determined when the membrane is
built by the ER and Golgi apparatus

New phospholipid molecules are synthesized in the ER by membrane-bound enzymes which use
substrates (fatty acids) available only on one side of the bilayer.
Flipases transfer specific phospholipid molecules selectively so that different types become
concentrated in the two halves. One sided insertion and selective flippases create an
asymmetrical membrane
Plasma Membrane Biosynthesis

The Permeability of the Lipid Bilayer
-Transport across membrane
• Hydrophobic (nonpolar) molecules, such as
hydrocarbons, can dissolve in the lipid bilayer and
pass through the membrane rapidly
• Polar molecules, such as sugars, do not cross the
membrane easily

Transport Proteins
• Transport proteins allow passage of hydrophilic
substances across the membrane
• Some transport proteins, called channel proteins,
have a hydrophilic channel that certain molecules
or ions can use as a tunnel
• Channel proteins called aquaporins facilitate the
passage of water

• Other transport proteins, called carrier proteins,
bind to molecules and change shape to shuttle
them across the membrane
• A transport protein is specific for the substance it
moves
• Some diseases are caused by malfunctions in specific
transport systems, for example the kidney disease
cystinuria

Transport
Can be active (energy
requiring) or passive
Three general
classes of
transport
systems.
Transporters
differ in the
number of
solutes
(substrates)
transported and
the direction in
which each
solute moves.

Passive transport is diffusion of a
substance across a membrane with no
energy investment
• Diffusion is the tendency for molecules to spread out
evenly into the available space
• Although each molecule moves randomly, diffusion of a
population of molecules may be directional
• At dynamic equilibrium, as many molecules cross the
membrane in one direction as in the other

*
Diffusion through a
Membrane
●
Cell membrane
Solute moves DOWN concentration gradient (HIGH to LOW)

• Substances diffuse down their concentration
gradient, the region along which the density of a
chemical substance increases or decreases
• No work must be done to move substances down
the concentration gradient
• The diffusion of a substance across a biological
membrane is passive transport because no energy is
expended by the cell to make it happen

Effects of Osmosis on Water Balance
• Osmosis is the diffusion of water across a selectively
permeable membrane
• Water diffuses across a membrane from the region
of lower solute concentration to the region of higher
solute concentration until the solute concentration
is equal on both sides

Figure 7.14
Lower
concentration
of solute (sugar)
Higher
concentration
of solute
Sugar
molecule
H2O
Same concentration
of solute
Selectively
permeable
membrane
Osmosis

Water Balance of Cells Without Walls
• Tonicity is the ability of a surrounding solution to
cause a cell to gain or lose water
• Isotonic solution: Solute concentration is the same
as that inside the cell; no net water movement
across the plasma membrane
• Hypertonic solution: Solute concentration is
greater than that inside the cell; cell loses water
• Hypotonic solution: Solute concentration is less
than that inside the cell; cell gains water

Figure 7.15
Hypotonic
solution
Osmosis
Isotonic
solution
Hypertonic
solution
(a) Animal cell
(b) Plant cell
H2O H2O H2O H2O
H2O H2O H2O H2O
Cell wall
Lysed Normal Shriveled
Turgid (normal) Flaccid Plasmolyzed

• Hypertonic or hypotonic environments create osmotic
problems for organisms
• Osmoregulation, the control of solute concentrations
and water balance, is a necessary adaptation for life in
such environments
• The protist Paramecium, which is hypertonic to its
pond water environment, has a contractile vacuole
that acts as a pump

Figure 7.16
Contractile vacuole
50 m

Water Balance of Cells with Walls
• Cell walls help maintain water balance
• A plant cell in a hypotonic solution swells until the
wall opposes uptake; the cell is now turgid (firm)
• If a plant cell and its surroundings are isotonic,
there is no net movement of water into the cell;
the cell becomes flaccid (limp), and the plant may
wilt

• In a hypertonic environment, plant cells lose water;
eventually, the membrane pulls away from the wall, a
usually lethal effect called plasmolysis

Facilitated Diffusion: Passive Transport
Aided by Proteins
• In facilitated diffusion, transport proteins speed the
passive movement of molecules across the plasma
membrane
• Channel proteins provide corridors that allow a
specific molecule or ion to cross the membrane
• Channel proteins include
– Aquaporins, for facilitated diffusion of water
– Ion channels that open or close in response to a
stimulus (gated channels)

Active transport uses energy to move
solutes against their gradients
• Facilitated diffusion is still passive because the solute
moves down its concentration gradient, and the
transport requires no energy
• Some transport proteins, however, can move solutes
against their concentration gradients

The Need for Energy in Active
Transport
• Active transport moves substances against their
concentration gradients
• Active transport requires energy, usually in the form
of ATP
• Active transport is performed by specific proteins
embedded in the membranes

• Active transport allows cells to maintain concentration
gradients that differ from their surroundings
• The sodium-potassium pump is one type of active
transport system

Figure 7.18-1
EXTRACELLULAR
FLUID
[Na] high
[K] low
[Na] low
[K] high
CYTOPLASM
Na
Na
Na
1

Figure 7.18-2
EXTRACELLULAR
FLUID
[Na] high
[K] low
[Na] low
[K] high
CYTOPLASM
Na
Na
Na
1 2
Na
Na
Na
P
ATP
ADP

Figure 7.18-3
EXTRACELLULAR
FLUID
[Na] high
[K] low
[Na] low
[K] high
CYTOPLASM
Na
Na
Na
1 2 3
Na
Na
Na
Na
Na
Na
P
P
ATP
ADP

Figure 7.18-4
EXTRACELLULAR
FLUID
[Na] high
[K] low
[Na] low
[K] high
CYTOPLASM
Na
Na
Na
1 2 3
4
Na
Na
Na
Na
Na
Na
K
K
P
P
P
P i
ATP
ADP

Figure 7.18-5
EXTRACELLULAR
FLUID
[Na] high
[K] low
[Na] low
[K] high
CYTOPLASM
Na
Na
Na
1 2 3
4
5
Na
Na
Na
Na
Na
Na
K
K
K
K
P
P
P
P i
ATP
ADP

Figure 7.18-6
EXTRACELLULAR
FLUID
[Na] high
[K] low
[Na] low
[K] high
CYTOPLASM
Na
Na
Na
1 2 3
4
5
6
Na
Na
Na
Na
Na
Na
K
K
K
K
K
K
P
P
P
P i
ATP
ADP

Figure 7.19
Passive transport Active transport
Diffusion Facilitated diffusion ATP

Cotransport: Coupled Transport by a
Membrane Protein
• Cotransport occurs when active transport of a
solute indirectly drives transport of other solutes
• Plants commonly use the gradient of hydrogen
ions generated by proton pumps to drive active
transport of nutrients into the cell

Figure 7.21
ATP
H
H
H
H
H
H
H
H
Proton pump
Sucrose-H
cotransporter
Sucrose
Sucrose
Diffusion of H







