Each and every cell in a living organism has a membrane that surrounds the contents of the cell known as ‘plasma membrane’. The plasma membrane is a selectively permeable membrane that allows only certain substances to enter the cell and certain substances to exit the cell. The plasma membrane is about a sixty-millionth of an inch in thickness, and surrounds the protoplasm of the cell. It provides shape and structure to the cell. The plasma membrane is composed of double layer of hydrophobic lipids.
Hence substances that are hydrophilic in nature (that is have an attraction to water) are not allowed into the cell. The plasma membrane will not permit molecules greater than a particular size to actually enter into the cell. In certain instances, some substances that would otherwise not pass through the plasma membrane (such as certain proteins) are permitted across through a process known as ‘facilitated diffusion’ (very useful for larger molecules and hydrophilic molecules to enter the cell). For the process of facilitated diffusion expenditure of imagery is required.
In actually sense, substances would diffuse across the plasma membrane by a process of passive diffusion in which energy is not required. Molecules would simply move across an area of high concentration to an area of lower concentration by simple diffusion. As living things require several substances for performing several function including metabolism, generation of energy, growth, development, etc, a plasma membrane is required so that substances that help in functioning can move in and out of the cell. The plasma membrane also helps in communicating from one cell to another.
IT would be very difficult for a living organism to survive without a plasma membrane as foreign substances would enter and destroy the cell. The cell would have no integrity.
References:
Beauty Magic Online (2008). Beauty therapy, retrieved on May 21, 2008, from BMO Web site: http://www. beautymagonline. com/pages/Dictionary. htm Pearson (2008). Chapter 5: End of Chapter Quiz, Retrieved on May 21, 2008, from USU Web site: http://www. biology. usu. edu/courses/biol1010-woertendyke/End%20of%20chapter%20questions/Ch05. doc