Another elegant process that has evolved as part of the endocrine system is that of RECEPTORS. Here we have glucose entering the cell through the action of insulin. Glucose receptors, which are actually protein molecules incorporated into cell membranes, are like locks crafted for a very specific key. These keys, also proteins such as hormones (in this case insulin) and neurotransmitters, unlock the "door" into the cell and activate a second messenger which then "does its stuff"--creating energy from glucose and oxygen for other cellular processes.
It's simple, really, but incredibly versatile, as the thousands of processes that use similar systems demonstrate. Be kind to your receptors this weekend--they are very sensitive to and can be damaged by poisons such as alcohol, nicotine, other drugs, and real physiologic stress.
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