written by: Rishi Prakash•edited by: Paul Arnold•updated: 6/27/2011
Cells are highly organised and specialized structures. Their valuable organelles are protected by cell membranes. Find out more about these vital cell structures, and the role they play in transporting molecules.
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Plant And Animal Cell Membrane
Cell membranes are amongst the most important components of all plant and animal cells. The cell membrane is a specialized structure that contains proteins and lipid components that allows it to perform unique tasks. The cell membrane performs various functions including protection, the regulation of the transport of solutes in and out of the cell, and it provides a site for the binding of specific proteins and enzymes.
The detailed cell structure was revealed only after the advent of the electron microscope in 1950’s. However, previous chemical studies of human red blood cells revealed that cell membranes are composed of a lipid bilayer with a polar head facing out, and the hydrophobic tails facing towards the inner side. The lipid bilayer is mainly composed of phosphoglycerides.
Biochemical studies revealed that the cell membrane also consists of proteins and carbohydrates. The ratio of proteins and lipids vary considerably depending on the cell types and functions. Membrane proteins are also classified as integral and peripheral. Integral proteins are buried in the cell membrane, while peripheral proteins are situated on the surface.
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Fluid Mosaic Model
Singer and Nicolson in 1972 proposed an improved model of the cell membrane, the fluid mosaic model, which is the most widely accepted. According to this model, the quasi-fluid nature of lipid allows lateral movement of proteins within the lipid bilayer. The fluid nature of the cell membrane is important in that it allows the cell to grow, and form intracellular junctions. In addition, it also plays vital roles in secretion, cell division, and cell structure formation.
One of the vital functions of the cell membrane is the transport of molecules across it. Many molecules can move across the membrane without any energy requirement, which is called passive transport. Neutral molecules can readily cross the cell membrane by simple diffusion along the concentration gradient. However, there are other molecules that might require energy such as ATP in order to cross the semipermeable membrane. The energy dependent transport of molecules is called active transport.
In addition, the transport of molecules is facilitated by exocytosis and endocytosis respectively. In the example of exocytosis, molecules are excreted outside the cell membrane through special vesicles. Endocytosis is where the membrane engulfs the molecule using a special mechanism.
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Ncbi. “Structure of the plasma membrane," retrieved on 26 February 25, 2009 from fluid mosaic model.
Library. “Cell membrane" retrieved on 25 February 25, 2009 from cell membrane.