The cell’s INPUT devices are the protein receptors which extend from both of the cell membrane’s surfaces. Receptors facing inwards “read” the status of the cytoplasm’s environmental conditions. These receptors receive information concerning cytoplasmic pH, salt balance, membrane potential, the availability of metabolites and energy molecules and other parameters related to the cell’s physiology. Protein receptors displayed on the outer surface of the membrane provide the cell with awareness of the external environment. Cells use information derived from external receptors to “navigate” through their world. Internal membrane receptors are concerned with visceral needs, externally deployed receptors primarily regulate somatic behaviors. Consequently, information of the external environmental profoundly influences the cell’s cytoskeleton and behavior.

To PROCESS the environmental information (i.e., convert signals into biological responses), “activated” receptors couple with complementary effector proteins. The activity of membrane effector proteins, which include ion channels, enzymes and components of the cytoskeleton, is controlled by receptor proteins. 6 The OUPTUT behavior is mediated by activated effector proteins. Effector proteins primarily serve as “switches” or “second messengers” that turn on or off more complex protein pathways deployed within the cell. Effector proteins regulate cytoplasmic pathways, which include motility, digestion, excretion, and respiration among others.

The MEMORY system of the cell, the genes, are also controlled by the membrane. Sometimes cells receive environmental signals necessitating specific responses, however, the cell may not have the necessary proteins in the cytoplasm to enact the required behavior. In this case, activated receptor-effector protein complexes are able to target the regulatory proteins that mask specific genes. These membrane “messengers,” known as transcription factors, alter the binding of regulatory proteins causing them to detach from the DNA, exposing specific genes that need to be read. 3,4 This is how “environmental signals” control gene expression. As the cell experiences new environments, it is capable of dynamically adjusting its genetic readout to accommodate any environmental exigencies. Consequently, the structural and behavioral expression of the cell is a reflection of the organism’s environment.

The primal role of “environment” in controlling gene expression is revealed in recent studies of newly discovered stem cells. Stem cells, akin to multipotential embryonic cells, proliferate forming large colonies of undifferentiated cells. The developmental destiny of stem cell progeny can be experimentally “controlled” by regulating their environment. Environmental signals activate stem cell transcription factors, which in turn select specific gene programs controlling the differentiation of these cells.7,8 Genes are coded “programs” that enable the organism as an individual, and the species as a whole, to survive. Gene programs can be subdivided into two functional groups. One group, representing “growth” mechanisms, is expressly designed to provide for the physical construction and physiologic maintenance of the body. However, an organism possessing only “growth” mechanisms would most likely be called “food,” and would soon become extinct. Environmental threats are managed by the second group of genes which code for “protection” programs. These genes provide for physical mechanisms and behaviors that are deployed in life-threatening situations.

9 Survival = Growth Programs + Protection Programs Protection behaviors do not provide growth, and visa-versa. Both growth and protection behaviors require an energy expenditure on the part of the organism. An individual’s ability to grow and reproduce is ultimately based upon the amount of energy available to support those processes. However, their ability to protect themselves is also dependent upon the same energy source. Organisms engaging in protection behaviors utilize energy from their reserves, leaving less energy for growth processes. Under extreme environmental stress, protection demands may deplete the energy budget to the extent that the organism dies from an inability to sustain normal metabolic functions. In simple economics, survival is inversely related to the need for protection. More protection equates to less growth.