 The cell membrane, or plasma
membrane, is a biological membrane that
separates the interior of all cells from
the outside environment.
 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.
 It consists of the phospholipids bilayer with
embedded proteins.
 Cell membranes are involved in a variety of cellular
processes such as cell adhesion, ion conductivity and cell
signalling.
 It serves as the attachment surface for several extracellular
structures, including the cell wall, glycocalyx, and
intracellular cytoskeleton.

 According to the fluid mosaic model of S . J. Singer and G.L.
Nicolson, biological membranes can be considered as a two-
dimensional liquid in which lipid and protein molecules diffuse
more or less easily.

 Like all other cellular membranes, the plasma
membrane consists of both lipids and proteins.
 The fundamental structure of the membrane is
the phospholipids bilayer, which forms a stable barrier between
two aqueous compartments.
 In the case of the plasma membrane, these compartments
are the inside and the outside of the cell.
 Proteins embedded within the phospholipids bilayer carry out
the specific functions of the plasma membrane, including
selective transport of molecules and cell-cell recognition.

 Lipids
Phospholipids bilayer
Cholesterol
 Proteins
Transport proteins
Receptor proteins
Recognition proteins
 Carbohydrates
Oligosaccharides on glycoprotein

Lipids
 Phospholipids bilayer
1. Forms boundary to isolate cell contents from
environment.
2. Restricts passage of hydrophilic substances
across the membrane.
HYDROPHILIC END
HYDROPHOBIC END

 Cholesterol
1. Increases bilayer strength, flexibility.
2. Reduces membrane fluidity.
3. Reduces permeability to water-soluble substances.
Proteins
 Transport proteins
1. Regulate movement of water soluble substances
Channel proteins have pores that allow passage of ions and
small water-soluble molecules
Carrier proteins bind to molecules and change shape for
delivery across membrane

 Receptor proteins
1. Docking site for molecules outside the cell
2. Trigger internal cellular response
 Recognition proteins
1. Identification tags
Oligosaccharides aid in cell-cell recognition
2. Cell-surface attachment sites
 Carbohydrates
1. Attach to the external surface of integral proteins.
2. Hold adjoining cells together
3. Act as sites where viruses or chemical messengers
such as hormones can attach.

Movement of substances across the cell
membrane
Passive transport
 Substances move from [high][low].
 No energy input required.
 Simple Diffusion, Facilitated Diffusion.
Active transport
 Substances move from [low][high].
 Requires energy input.
 Protein carriers, Endocytosis, Exocytosis.

Example: Oxygen or water diffusing
into a cell and carbon dioxide
diffusing out.
Examples: Glucose or amino acids moving
from blood into a cell.
An nerve electrical impulse results from
opening protein channels for ions that
move by facilitated diffusion.

It is an Active process for movement of large molecules and organisms.
 Substance is taken in by vesicle formed from cell membrane.
 Phagocytosis : solid substance in vesicle.
 Pinocytosis : liquid droplets in vesicle.
 Receptor-Mediated Endocytosis: incoming substance binds to
receptor.

 Substance is expelled after being enclosed in a vesicle
within the cell
 Used to move large molecules out of the cell