Plasma Membrane Note ppt

Plasma membrane and the functions
of each of its components

STRUCTURE OF PLASMA
MEMBRANE

(Based on Fluid Mosaic Model by
Singer and Nicolson)

STRUCTURE

•A membrane Phospholipid
that encloses a bilayer
cell

•Consists of
phospholipid
bilayer

Plasma membrane

• A membrane that encloses a cell
• Control traffic into and out of the cell it surrounds
• Plasma membrane is selective permeability
• Allow some substances to cross it easily than other

substance

Phospholi
pid
bilayer

• Plasma membranes
are typically about
7nm thick

• consist
mainly of
phospholipids and
proteins, arranged as
a fluid mosaic

Phospholipid bilayer

PLASMA
MEMBRANE

§ Phospholipids are the most abundant lipids in the
plasma membranes

§ A phospholipid is an amphipathic molecule,
containing both hydrophobic and hydrophilic
regions

Hydrophilic (head) WATER
Hydrophobic (tail) WATER

Figure 7.2

WATER • The phospholipid
tails are non-polar,
Hydrophilic hydrophobic
(head) (repelled for water)
which face inwards
Hydrophobic
(tail) • The phospholipid
heads are polar,
WATER hydrophilic (attract
water) which point
outwards

• The phospholipid tails are non-polar, hydrophobic (repelled for water) which
face inwards

• The phospholipid heads are polar, hydrophilic (attract water) which point
outwards

Membrane proteins and their functions

• There are various proteins
attached to or embedded in
phospholipid bilayer

• There are two types of
protein

• 1. integral protein
• 2. peripheral protein

Membrane proteins and their functions

INTEGRAL PROTEIN

Some integral proteins
embedded partially in the
phospholipids bilayer

Major integral protein extend
completely through the
phospholipids bilayer called
transmembrane protein

Membrane proteins and their functions

PERIPHERAL PROTEIN

They are not embedded in the
phospholipid bilayer at all

The are appendages loosely
bound to the surface of the
membrane.
(often to exposed parts of
integral protein)

Plasma Membrane: Fluid Mosaic
Model

§ Proposed by S.J Singer & G.L Nicolson (1972)
§ It is called ‘Fuid Mosaic Model’ because:

Phospholipid can move laterally or
Fluid flip-flop

Protein are free to move laterally in
phospholipid bilayer

Mosaic The various proteins are attached to

or embedded in the phospholipids

bilayer

Phospholipid bilayer

PLASMA
MEMBRANE

COMPONENTS AND ITS
FUNCTIONS

Functions of each of plasma membrane
components

• Protein

• Carbohydrate

• Phospholipid

• Cholesterol

Functions of proteins at plasma
membrane

• Transport
• Enzymatic activity
• Signal transduction
• Cell-cell recognition
• Intercellular joining
• Attachment to the cytoskeleton and

extracellular matrix

1. Transport Protein

• A protein that spans the
membrane may provide a
channel across the membrane
that is selective for a particular
solute

• Some transport proteins
hydrolyze ATP as an energy
source to solute across the
membrane

1. Transport

There are many
functions of protein in
membrane:

1. Transport ATP

a. Channel protein Carrier protein
b. Carrier protein

Channel protein

FACILITATED DIFFUSION: PROTEIN TRANSPORT
(E.G WATER CHANNEL PROTEINS – AQUAPORINS)

Channel proteins - Channel protein

• Protein that provided FACILITATED DIFFUSION: PROTEIN
hydrophilic channel TRANSPORT (E.G WATER CHANNEL
(provide corridors)
PROTEINS – AQUAPORINS)
• Allow water ATP
molecules or small
ions to flow very
quickly from one side
of the membrane to
the other

Channel proteins
function as:

• Aquaporins (water
channel protein)

• Gated channels, a
stimulus (electrical
or chemical) causes
them to open or
close

Carrier protein ATP

• Carrier protein -protein
shuttle a substance from
one side to the other by
changing shape

• Some of these proteins
hydrolyse ATP as an
energy source to actively
pump substances across
the membrane (e.g. in
sodium potasium pump)

Carrier protein

Transport proteins

Channel protein Carrier protein

2. Enzymatic activity ATP
Enzymes
• The protein may be an
enzyme with its active site Signal
exposed to substances in the
adjacent solution

• Enzymes catalyze reactions
that take place within or
along the membrane surface

3. Signal transduction (Receptor protein molecules
for hormones/neurotransmitters)

§ The protein has binding site Enzymes
with a specific shape that
fits the shape of a chemical
messenger, such as a
hormone

• The external messenger Signal
(signal) may cause a Receptor
conformational change in
the protein (receptor) that
relays the message to the
inside of the cell

4. Intercellular joining

• Cell adhesion proteins
attach membrane of
adjacent cell

• Membrane proteins of
adjacent cells may hook
together in various kinds
of junctions, such as gap
junctions or tight junctions

5. Cell-cell recognition

• Some glycoproteins serve
as identification tags that
are specifically recognized
by membrane proteins of
other cells

• A cell’s ability to
distinguish one type of
neighboring cell from
another

5. Cell-cell recognition

• Basis for rejection of
foreign cells by the
immune system

• The way cells recognize
other cells is by binding
to the surface molecules,
often carbohydrates on
the plasma membrane

Role of membrane carbohydrate in cell-cell recognition

• Carbohydrate chains are
found only on the outer
surface of the plasma
membrane

• Carbohydrate chain attach to
protein is called glycoprotein

• Carbohydrate chain attach to
phospholipid is called
glycolipid

Function of glycolipids and
glycoproteins

• Some glycoproteins serve as identification tags
that are specifically recognized by other
cells

• E.g.: The four human blood types (A, B, AB and
O) reflects variation in the carbohydrates on
the surface of the red blood cells.

• As receptors for chemical signals or external
stimulus

• E.g. Hormones

Functions of Phospholipids

• The
phospholipid
bilayer
provides the
basic structure
of membranes

• It restricts Hydrophilic head WATER
WATER
entry and exit Hydrophobic tail
of polar
molecules and
ions

Functions of Phospholipids

a) Movement of phospholipids
• Phospholipids can move within the

phospholipids bilayer either by laterally or flip-
flop (rare)

Functions of Phospholipids

b) Membrane fluidity
• The type of hydrocarbon tails in phospholipids

affects the fluidity of the plasma membrane

Functions of Phospholipids

b) Membrane fluidity

• The membrane remains fluid to a lower temperature if it is rich in
phospholipids with unsaturated hydrocarbon tails

• Because of kinks in the tails where double bonds are present
• Unsaturated hydrocarbons do not pack together as closely as saturated

hydrocarbons
• This makes the membrane more fluid

• That’s why cold blooded animal (body
temperature that depends on surrounding)
has abundant of unsaturated fatty acids

• Unsaturated fat melt at lower temperature
compare to saturated fat.

Function of Cholesterol

The steroid cholesterol has different effects on
membrane fluidity at different temperatures

Function of Cholesterol

• At 37⁰C of human (relatively warm temperature), cholesterol
makes the membrane less fluid by restraining the movement
of phospholipids

• At cool temperature, cholesterol also hinders close packaging
of phosholipids thus maintain the membrane fluidity. Prevent
phospholipid become solidify

Functions of cholesterol:

a. Regulates membrane fluidity -It prevents
phospholipids molecules from getting too close
when the temperature decreases or getting too far
apart from each other when the temperature
increases

b. Give mechanical stability, increases
flexibility / stability of membrane

Cholesterol
Cholesterol within the animal cell membrane

c. Regulates the movement of hydrophobic
molecules across the plasma membrane

Cholesterol
Cholesterol within the animal cell membrane

a. Cholesterol can be thought as a “temperature

buffer” for the membrane fluidity, resisting changes
in membrane fluidity that can be caused by changes
in temperature
b. Give mechanical stability

• Separate the contents of
cells from their external
environments

• As a selective barrier that
allows sufficient passage
of oxygen, nutrients and
wastes products

• Also act as a receptor sites Glyco-
for protein

– recognizing hormones,
– neurotransmitters and
– other chemicals from the

external environment

• To compartmentalise the
organelles within a cell to
allow specialised
metabolic process to
occur

• Gives physical protection

*The specific functions of a membrane depend on the

kinds of phospholipids and proteins present

6. Attachment to the cytoskeleton and extracellular matrix
(ECM)

• Microfilaments or other elements
of the cytoskeleton may be
bonded to membrane proteins, a
function that helps maintain cell
shape and stabilizes the location
of certain membrane proteins

• Proteins that adhere to the ECM
can coordinate extracellular and
intracellular changes