Membranous Structures of the Cell

Membranous Structures of the Cell

Most organelles of the cell are covered by membranes composed  primarily of lipids and proteins. The lipids of the membranes provide a barrier that  impedes the movement of water and water-soluble  substances from one cell compartment to another  because water is not soluble in lipids. Protein  molecules in the membrane often do penetrate

all the  way through  the membrane, thus providing specialized  pathways, often organized into actual pores, for  passage of specific substances through the membrane, other membrane proteins are enzymes that catalyze   a  multitude   of different chemical reactions.

Fig:  Reconstruction of a typical cell, showing the internal organelles in the cytoplasm  and in the nucleus.

The cell membrane (also called the plasma membrane),which envelops the cell, is a thin, pliable, elastic structure only 7.5 to 10 nanometers thick. It is composed almost entirely of proteins and lipids. The approximate composition is proteins 55%;  phospholipids 25%; cholesterol  13%;  other lipids  4%;  and carbohydrates  3%.

Lipid Barrier of the Cell Membrane Impedes Water Penetration:

 The  basic structure of  cell membrane    is a lipid  bilayer, which is a thin, double-layered  film of lipids—each layer only one molecule thick—that is continuous over the entire cell surface. Interspersed in this lipid film are large globular protein molecules. The basic lipid bilayer is composed of phospholipid molecules. One end of each phospholipid molecule  is  soluble in water; that is, it is hydrophilic. The other end  is soluble only in fats; that is, it is hydrophobic.  The lipid layer in the

middle of the membrane is  impermeable to the usual water-soluble substances, such as ions, glucose, and urea.

 Conversely, fat-soluble  substances, such as oxygen, carbon dioxide, and  alcohol, can penetrate this portion of the membrane  with ease.