The cell membrane is a thin, flexible bilayer composed of phospholipids, cholesterol, proteins, and carbohydrates. It separates the interior of the cell from the external environment. According to the fluid mosaic model, the phospholipid bilayer forms the basic structure of the membrane, with proteins and carbohydrates embedded within it or attached to it. The phospholipids are arranged with their hydrophobic tails facing inward and hydrophilic heads facing outward, forming a semi-permeable barrier that regulates what passes in and out of the cell.

1. CELL
MEMBRANE

2.  Cell membrane also called as plasma membrane is a thin,
pliable, elastic structure of 7.5 – 10 nm diameter
 It separates the contents of the cell from the external
environment & controls exchange of materials between the
ECF & the ICF.
 It is composed almost entirely of proteins & lipids.
INTRODUCTION

3.  The basic structure of cell membrane is a lipid bilayer, which is a
thin, double layered film of membranes.
 In 1972, Singer and Nicolson put forward the fluid mosaic model.
According to this model,
1) Phospholipid bilayer is a basic continuous structure forming
the cell membrane. The phospholipids are present in fluid form.
2) The protein molecules are present as a discontinuous
mosaic of globular proteins which float about in the phospholipid
bilayer thus forming the fluid mosaic pattern.
STRUCTURE AND COMPOSITION

4. STUCTURE OF CELL MEMBRANE

5.  Each lipid molecule in the lipid bilayer primarily consists of
phospholipids (phosphatidyl choline, phosphatidyl
ethonalamine,sphingomyelin), cholesterol & glycolipids.
 The lipid molecule is clothes pin shaped & consists of a head end &
a tail end. The head end contains phosphate moiety of phospholipid
or hydroxyl radicle of cholesterol. It is positively charged and is
soluble in water i.e., polar or hydrophilic.
 The tail end consists of two chains of fatty acids/steroid radicle of
cholesterol. It is insoluble in water i.e., non polar or hydrophobic.
ARRANGEMENT OF LIPID BILAYER

6.  These lipid molecules are arranged in such a way that their
hydrophobic ends are directed towards the centre of the
CM whereas their hydrophilic ends are directed outwards
on either side of the CM. In this way, the head ends face
the aqueous phase ( ECF on the outside, ICF on the inner
side)

7.  The lipid bilayer is a semi permeable membrane. It is
impermeable to water soluble substances like glucose,
urea, ions and is permeable to lipid soluble substances like
oxygen, carbon dioxide and alcohol.
FUNCTIONAL SIGNIFICANCE OF THE LIPID BILAYER

8.  Protein molecules float about in the lipid bilayer.
 Proteins containing lipids – Lipoproteins, Proteins
containing carbohydrates – Glycoproteins.
 The proteins in the CM are described as:
1. Peripheral proteins
2. Integral proteins
ARRANGEMENT OF PROTEINS

9. PERIPHERAL PROTEINS
 These are present peripheral to the lipid bilayer and also attached
to the integral proteins.
 They function mainly as enzymes or as controllers of transport of
substances through the CM pores.
INTEGRAL PROTEINS
 These are proteins that extend into the CM . Some penetrate only
part of the way into the membrane while others penetrate all the
way through.
PROTEINS IN THE CM

10.  Some of the integral protein molecules serve as channels for water
soluble substances. These are called channel proteins.
 The protein molecules that help in transport of substances by active &
passive transport are called carrier proteins.
 Some proteins function as receptors that bind neurotansmitters &
hormones initiating physiological changes inside the cell. These are called
receptor proteins.
 Certain proteins in the CM act as pumps & form active transport of the cell
e.g., Na+ K+ ATPase
 Some of the proteins are cell adhesion molecules that anchor cells to their
neighbors or to basal lamina.
FUNCTIONS OF PROTEINS IN THE CM

11.  The carbohydrates are attaches either to the proteins -
Glycoproteins or to the lipids - Glycolipids.
 Throughout the surface , the carbohydrate molecules form a thin
loose covering called Glycocalyx.
ARRANGEMENT OF CARBOHYDRATES

12.  Being negatively charged, the carbohydrate molecule do not allow
negatively charged particles to move out of the cell.
 The Glycocalyx helps in tight fixation of the cells with one another.
 Some of the carbohydrate molecule serve as receptor sites for
binding hormones.
 Some carbohydrate moieties enter into immune reactions.
FUNCTIONS OF CARBOHYDRATES IN THE CM