August 18, 2007

Cell Membranes: Smart Wrappers

Cell MembranesThe cells in our bodies are covered by a coat called the cell membrane, which is also known as plasma membrane. This membrane is bi-layered, and is composed of phospholipids, among other things such as proteins. The interior of the cells contain water and the cells themselves bathe in an ocean of saline water (a legacy from our marine ancestors). Hence, the bilayer is fashioned in such a way that the water loving (hydrophilic/polar) portions of phospholipid molecules are in contact with water lying external and internal to it. The structure of phospholipids resemble matchsticks; the tips representing hydrophilic phosphate parts while the shafts represent lipid portions, hydrocarbon chains. Thus, it can be very clearly comprehended that the tips of these matchsticks will be oriented exteriorly and inwardly (as is seen in the picture), while the hydrocarbon ends (non polar/hydrophobic), lie buried deep within the membranes' middle portion.

This membrane is in no way static, as the picture might convey. It is highly dynamic and this gives rise to membrane fluidity, which is contributed largely by cholesterol. Imagine a pond with numerous soccer balls floating on the water. You throw a heavy object, say a stone on them. The balls will make way for the stone to go through them before it sinks. Moments later the balls will again fill the gap, the stone created, thus realigning themselves. The cell membrane possesses this property, fluidity. This is essential for the cell to change shape (RBCs change shape while negotiating small capillaries), endocytosis (a process by which cells engulf external particles), receptor conformational changes and many others.

The protein molecules that are studded/embedded within the cell membrane serve various functions. They act as receptors (molecules that interact with various ligands), structural proteins, pumps, carrier proteins (helps in ferrying molecules by facilitated diffusion), ion channels (conduits for ions), enzymes and adhesion molecules (helps cells stick/adhere to themselves and to the underlying basal lamina or basement membrane), to name a few.

Cell membranes also help maintain the polarity of the cell, both by electrical (ionic) insulation and Na+/K+ ATPase pump activity. Membranes that cover intracellular bodies (organelles), also resemble cell membranes and they serve vital functions too. For example, mitochondria generate harmful free radicals, which could wreck havoc had it not been covered by this biological membrane. This intelligent mosaic carpet thus continue to amaze us.
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