Rates of Change
Prospects for matrimony notwithstanding, it is axiomatic that time is of the essence, even more in biology than in law and real estate. When journalists refer to the ticking of the biological clock, they usually mean that the choice between career and family is difficult for young women, because fertility declines after the age of thirty more rapidly in women than in men. However, one of the most amazing features of human biology is that important, even crucial, processes occur within vastly different time frames. Our senses must enable us to respond within milliseconds to changes that may threaten us, the secretion of insulin may change within minutes of taking a meal, and the expression of our genes may be reprogrammed over a few hours or less. Other cycles involved in response to sunlight and darkness take place over a day, while reproductive cycles tend to require several days to a month, and reproduction itself may require a significant part of a year. One of the most important aspects of cellular physiology, the cell cycle, requires about a day to complete in mammals, but only 20 minutes or so in microbes. If a Salmonella typhimurium ever challenges you to a race to complete cell division, decline the invitation, for mammals can’t win!
KeywordsOutput Rate Input Rate Elimination Rate Constant Fractional Change Temporal Assistant
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