Which Components form the Structure of the Plasma Membrane

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Which Components form the Structure of the Plasma Membrane

Ultra Structure of Cells

Which Components form the Structure of the Plasma Membrane 1
Which Components form the Structure of the Plasma Membrane 2
Plasma Membrane :

Introduction :
Cell surface in all the cells is enclosed by a living membrane which is called cell membrane by
C. Nageli and C. Kramer (1855).
Historical Account :
J.Q. Plower (1931) coined the term Plasmalemma for cell membrane.

Ultrastructure :                

  • Plasma membrane forms outer covering of each cell.
  • It is present in both plant and animal cells.
  • Plasma membrane is a living, thin, delicate elastic, selectively permeable membrane.
  • It separates contents of a cell from the surrounding medium.

Fluid Mosaic Model :

  • In 1972, Singer Nicolson proposed this model. According to this, cell membrane consists-two layers of phospholipid molecules, phospholipid & protein molecules are arranged as a mosaic.
  • Phospholipid molecules have their polar heads  directed outward non polar tail pointing inward.
  • The proteins are of two types ­:
  • Peripheral and integral. Peripheral proteins are located superficially while integral proteins are embeded in the phospholipid matrix. The protein monolayer have elasticity & mechanical support to the lipid matrix.
    Which Components form the Structure of the Plasma Membrane 3

Functions of Plasma Membrane :

  • The main function of plasma membrane is to regulate the movement of molecules inside and outside the cell.
  • It allow the movement of gaseous substance from high concentration to low concentration which known as diffusion.
  • Water also obeys the low of diffusion . The movement of water molecule through a selectively permeable membrane is called osmosis.
  • The flexibility of cell membrane also enables the cell to engulf in food, which is also known as endocytosis. For example – in Amoeba

Cell wall :
In plants, one another rigid is called ‘Cell wall’. It is made up of cellulose which provide structural strength to plant.

Function of cell wall :

  • It maintains the shape of cell.
  • It protect the cells from mechanical injury & prevents their desication.
  • It provide mechanical support against gravity. It is due to rigid cell walls that the aerial part of plant are able to keep erect & expose their leaves to sunlight.  
  • Cell walls permit the cells to with stand very dilute external media without bursting.

Nucleus : It is the most important part of cell which control all the activities of cell.

Structure of Nucleus  : 

  • The nucleus has a double layered covering called nuclear membrane.
  • The nuclear membrane has pores inside the nucleus to its outside, that is, to the cytoplasm.
  • The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.
  • Chromosomes are composed of DNA and protein.
  • Functional segments of DNA are called genes.
  • In a cell which is not dividing, this DNA is present as part of chromatin material.

Function of nucleus :

  • It play a important role in cellular reproduction.
  • DNA contain the information necessary for constructing & organizing cells.
  • Nucleoid – In some organisms nuclear region of cell may be poorly defined due to the absence of nuclear membrane. Such an undefined nuclear region called nucleoid.

Note :

  • Prokaryotic cell – Cell which do not have well defined nuclear region . called prokaryotic cells. Pro – Primitive   Karyon –  nucleus
  • Eukaryotic cells – Cells which have well defined nuclear region, called eukaryotic cells.
  • Along with nucleus membrane, prokaryotic cells lack most of cell organells.

Cytoplasm :
The fluid & semifluid matrix of  a cell between the nucleus & the plasma membrane, containing various organelles is called cytoplasm.

Cell organelles :
Small membrane bound structures, which perform a lot of chemical activities to support the function & structure of a cell, called cell organelles –

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