Plasma Membrane Notes for NEET Cell Structure, Download PDF

By Noushin Chaudhary|Updated : November 16th, 2018

Protoplasm is the living components of a cell. The protoplasm is a jelly like translucent, viscous and colorless semi solid-living substance. The protoplasm of a cell is made up of 3 complex compounds.

  • Plasma membrane
  • Cytoplasm
  • Nucleus

Plasma Membrane Notes

byjusexamprepPlasma membrane or cell membrane is found just beneath the cell wall. It is a soft living membrane surrounding the whole protoplasm. It act as a selective membrane and controls the movement of substances across the boundary. They are also called amphipathic which means they contain both hydrophilic and hydrophobic. Also, Microvilli, finger-like out-growths are present in the cell membrane of some epithelial cells. They increase the absorption surface of the cell.

Chemical Composition of the Membrane

  • LIPIDS (20-79% lipids) 
    • The lipid molecules are arranged in 2 layers. Hydrophilic (water loving) towards the outside and hydrophobic tails (water repelling) facing the inner side.
    • The Lipids found on the outer and inner surface of the membrane are different
    • Sterols, especially cholesterol is present in between these phospholipids to make it more rigid and provide stability to animal membranes.

  • PROTEINS (20-70%protein)

    • The proteins found in the plasma membrane are globular and are of 2 types: Intrinsic and Extrinsic Proteins.
    • The amount and types of extrinsic proteins are more abundant on the inner surface than on the outer surface.
  • CARBOHYDRATES (1-5%carbohydrates)
    • It consists of hexose, hexosamine, fructose and sialic acid, glycoproteins, and glycolipids.
    • Oligosaccharides are attached to the external surface of lipids and proteins and are absent on inner.
  • 20%water

Various Models of Plasma Membrane


Proposed by 

Features of the Model

Overton Model

Charles Ernest Overton (1895)

Suggested that membrane consists of thin lipid single layer

Sandwich model or Trilamellar model


Danielli and Davson (1935)

  • Suggested that membrane consists of  bimolecular layer of phospholipids (35 Å thick) sandwiched between two layers of proteins (each 20 Å thick).
  • Thus, the total thickness of plasma membrane, as per this model, should be

20 Å + 35 Å + 20 Å = 75 Å.

Unit membrane concept


J.D. Robertson (1959)

  • Suggested that all biological membranes shared the same basic structure: These are about 75 Å thick.
  • When viewed with electron microscope, they have a characteristic trilamellar appearance (phospholipid bilayer sandwiched between two electron dense layer of Proteins)
  • The three layers are a result of the same arrangement of proteins and lipids as proposed by Danielli and Davson

Fluid mosaic model


Singer and Nicolson (1972)

Discussed below


It was proposed by Singer and Nicolson (1972) and is the most accepted model.

  • According to their model, cell membranes are composed of a lipid bilayer with globular proteins embedded in the bilayer. 
  • Lipid bilayer mainly has 2 layers of phospholipids, the nonpolar tails point towards the inner side and the polar heads are on the surface.
  • The layer also consists of cholesterol in animal cells.
  • The layer allows proteins to move around within the bilayer as it is fluid.
  • This arrangement ensures resistance to water.


The proteins found in the plasma membrane are globular and are two types



Intrinsic or integral proteins or transmembrane proteins


 Extrinsic or peripheral proteins

  • These are the globular proteins that run through both layers of the lipid bilayer and float freely within the bilayer.
  • Intrinsic proteins have hydrophobic regions that anchor them in the hydrophobic interior of the lipid bilayer and hydrophilic regions that protrude from the bilayer
  • Those proteins that are associated with the surface of the membrane are called peripheral membrane proteins or extrinsic proteins.
  • The interior surface of the plasma membrane is structurally supported by a network of proteins called spectrin  (intracellular side of the plasma membrane in eukaryotic cells) and clathrins (formation of vesicles)

Membrane transport system

Two types of the transport system are seen in the plasma membrane. They are passive and active transport systems.

Passive transport:  It occurs by 2 means, diffusion and osmosis. No energy is spent in this type of transport system.

  • Diffusion- Diffusion is the net movement of particles (atoms, ions or molecules) from a region of higher concentration to regions of lower concentration. It continues until the concentration of substances is uniform throughout. It is further of two types:
    • Simple Diffusion: whereby molecules pass through a membrane without any intermediary such as an integral membrane protein.
    • Facilitated Diffusion: whereby molecules or ions pass across a cell's membrane via specific transmembrane integral proteins. It can be 


Channel Proteins Mediated TransportCarrier Proteins Mediated Transport
Channel proteins span the membrane and form pores, allowing free diffusion of molecules.Carrier proteins can change their shape to move a target molecule from one side of the membrane to the other by binding with them non - covalently.
It does not undergo any conformational changes.It undergoes conformational changes
It is selective in nature Carrier proteins are typically Selective for one or a few substances. 
It allows the movement of polar and charged compounds in order to avoid the hydrophobic core of the plasma membrane, which would otherwise slow or block their entry into the cell.The carrier proteins involved in facilitated diffusion simply provide hydrophilic molecules with a way to move down an existing concentration gradient (rather than acting as pumps).
  • Osmosis- It is the diffusion of water across a semipermeable membrane that occurs under the influence of an osmotically active solution.

Active transport: It requires energy and occurs against a concentration gradient i.e. from lower concentration to higher concentration. Special proteins within the cell membrane act as specific protein ‘carriers’. The energy for active transport comes from ATP generated by respiration (in mitochondria).  It occurs by two means

  • Na+-K+ exchange pump- These transport pumps require energy i.e. ATP. The pump works across n number of animal membranes.
  • Bulk transport- It is the movement of macromolecules such as proteins or polysaccharides into and out of the cell. Bulk transport occurs by exocytosis and endocytosis. Both of these require ATP.
    • Endocytosis is the process wherein the materials move into the cell rather is engulfed by the cell. The process occurs by an infolding or extension of plasma the membrane to form a vacuole or a vesicle (small vacuole).
    • Exocytosis is the process wherein the materials are exported out of the cell via secretory vesicles by reverse pinocytosis. Exocytosis is also known as reverse endocytosis.

 Functions of the Plasma Membrane

  • Transportation of different materials inside and outside the cell.
  • Absorption of different substances (mainly nutrients) from outside the cell.
  • Protection of the cell body and giving the cell a definite shape.
  • Cell membrane contains antigens which determine blood grouping.
  • Cell membrane helps in cell recognition and cell adhesion.



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