BIOMEMBRANES
(Lecture # 01)
Introduction, structure and functions

DEFINITION
Biological membranes are thin, flexible surfaces separating cells and cell
compartments from their environments

ORGANELLES CONTAINING MEMBERANE
Lysosomes Golgi bodies Mitochondria Nucleus

FLUID MOSAIC MODEL

COMPONENTS
Components of membrane are:
Proteins
lipids
Carbohydrates

1.PROTEINS
Two basic types of membrane proteins:
Integral (Intrinsic) proteins
Peripheral (extrinsic) proteins
INTEGRAL (INTRINSIC) PROTEINS
They are embedded in hydrophobic core of lipid
bilayer
For example : Transmembrane proteins are
exposed on both sides of the membrane
FUNCTION:
Anchorage
Transport

• Membranes are not merely passive barriers. They include
an array of proteins specialized for promoting or catalyzing
various cellular processes.

THREE GENERAL CLASSES OF TRANSPORT SYSTEMS

PERIPHERAL (EXTRINSIC)
PROTEINS
Peripheral membrane
proteins are proteins that
adhere outside the biological
membrane
These molecules attach to
integral membrane proteins,
or penetrate the peripheral
regions of the lipid bilayer
FUNCTION:
Cell-to-cell interaction
Cell signaling

MANY FUNCTIONS OF MEMBRANE PROTEINS
Transporter Enzyme action Cell surface receptors
Cell surface markers Cell adhesion Attachment to cytoskeleton
Outside
Plasma
membrane
Inside

2. LIPIDS
Lipids in membranes are Amphipathic
Orientation of Amphipathic compounds (Lipids) in aqueous
solution is to prevent Hydrophobic region coming into contact with
water molecules
Major lipids in biological membranes are:
Phospholipids
Glycolipids
Cholesterol

PHOSPHOLIPIDS
Phospholipid is made up of:
Two Non-polar (Hydrophobic) fatty
acid chains: Tails
One polar (Hydrophilic) group: Head

Phospholipid
molecules are
orientated to form:
micelle
lipid bilayer
liposome

GLYCOLIPIDS
Glycolipids are lipids that form part of the
plasma membrane
They have a short carbohydrate chain
covalently attached and this is exposed on
the outer surface of the cell
Function:
Communicative or cellular recognition

CHOLESTEROL
It is a steroid that is amphipathic
in nature
Function:
Provide stability
Maintain the fluidity
Bilayer stronger
Bilayer flexible

3. CARBOHYDRATES
Carbohydrates covalently linked to proteins
(glycoproteins) or lipids (glycolipids) are also a part of cell
membranes
FUNCTION:
cell-cell recognition
Basis for rejection of foreign cells by immune system
GLYCOCALYX
Entire surface of cell membrane contain carbohydrate
coat called glycocalyx
Function:
Provide negative charge to the membrane
Provide attachment between the cells

Function of cell membrane
The cell membrane is selectively
permeable to ions and organic molecules
and controls the movement of substances
in and out of cells
The basic function of the cell
membrane is to protect the cell from its
surroundings
Fluidity. It permits the shape changes
that accompany cell growth and
movement (such as amoeboid movement).

At the cell surface:
• Transporters move specific organic solutes and inorganic ions
across the membrane;
• Receptors sense extracellular signals and trigger molecular
changes in the cell;
• Adhesion molecules hold neighboring cells together.

Major Components of Plasma Membranes in Various Organisms
(components % by weight)
Serial
No.
Organism
name
Protein Phospholipid Sterol Sterol type Other lipids
1 Human
Myelin sheath
30 30 19 Cholesterol Galactolipids,
plasmalogens
2 Mouse liver 45 27 25 Cholesterol ---
3 Maize leaf 47 26 7 Sitosterol Galactolipids
4 Yeast 52 7 4 Ergosterol Triacylglycerol,
steryl esters
5 E. coli 75 25 0 --- ---