Characteristics of Life

Why do we consider a growing child to be alive, but not a growing crystal? Is abortion the taking of a human life? If so, what about a contraceptive foam that kills only sperm? As a patient is dying, at what point does it become ethical to disconnect life-support equipment and remove organs for donation? If these organs are alive, as they must be to serve someone else, then why isn't the donor considered alive? Such questions have no easy answers, but they demand a concept of what life is—a concept that may differ with one's biological, medical, or legal perspective.

From a biological viewpoint, life is not a single property. It is a collection of properties that help to distinguish living from nonliving things:

• Organization. Living things exhibit a far higher level of organization than the nonliving world around them. They expend a great deal of energy to maintain order,

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Physiology: The Unity of Anatomy and Physiology Companies, 2003 Form and Function, Third Edition

Chapter 1 Major Themes of Anatomy and Physiology 15

Hand Anatomy And Physiology

Figure 1.10 Variation in Human Anatomy. The left-hand figure in each case depicts the most common anatomy. (a) Variations in stomach shape correlated with body physique. (b) Variations in the position of the appendix. (c) Variations in the bile passages of the liver and gallbladder.

Figure 1.10 Variation in Human Anatomy. The left-hand figure in each case depicts the most common anatomy. (a) Variations in stomach shape correlated with body physique. (b) Variations in the position of the appendix. (c) Variations in the bile passages of the liver and gallbladder.

Saladin: Anatomy & I 1. Major Themes of I Text I I © The McGraw-Hill

Physiology: The Unity of Anatomy and Physiology Companies, 2003 Form and Function, Third Edition

16 Part One Organization of the Body and a breakdown in this order is accompanied by disease and often death.

  • Cellular composition. Living matter is always compartmentalized into one or more cells.
  • Metabolism and excretion. Living things take in molecules from the environment and chemically change them into molecules that form their own structures, control their physiology, or provide them with energy. Metabolism14 is the sum of all this internal chemical change. It consists of two classes of reactions: anabolism,15 in which relatively complex molecules are synthesized from simpler ones (for example, protein synthesis), and catabolism,16 in which relatively complex molecules are broken down into simpler ones (for example, protein digestion). Metabolism inevitably produces chemical wastes, some of which are toxic if they accumulate. Metabolism therefore requires excretion, the separation of wastes from the tissues and their elimination from the body. There is a constant turnover of molecules in the body; few of the molecules now in your body have been there for more than a year. It is food for thought that although you sense a continuity of personality and experience from your childhood to the present, nearly all of your body has been replaced within the past year.
  • Responsiveness and movement. The ability of organisms to sense and react to stimuli (changes in their environment) is called responsiveness, irritability, or excitability. It occurs at all levels from the single cell to the entire body, and it characterizes all living things from bacteria to you. Responsiveness is especially obvious in animals because of nerve and muscle cells that exhibit high sensitivity to environmental stimuli, rapid transmission of information, and quick reactions. Most living organisms are capable of self-propelled movement from place to place, and all organisms and cells are at least capable of moving substances internally, such as moving food along the digestive tract or moving molecules and organelles from place to place within a cell.
  • Homeostasis. While the environment around an organism changes, the organism maintains relatively stable internal conditions. This ability to maintain internal stability, called homeostasis, is explored in more depth shortly.
  • Development. Development is any change in form or function over the lifetime of the organism. In most organisms, it involves two major processes:
  • 1) differentiation, the transformation of cells with no

14metabol = change + ism = process

15ana = up

16cata = down specialized function into cells that are committed to a particular task, and (2) growth, an increase in size. Some nonliving things grow, but not in the way your body does. If you let a saturated sugar solution evaporate, crystals will grow from it, but not through a change in the composition of the sugar. They merely add more sugar molecules from the solution to the crystal surface. The growth of the body, by contrast, occurs through chemical change (metabolism); for the most part, your body is not composed of the molecules you ate but of molecules made by chemically altering your food.

  • Reproduction. All living organisms can produce copies of themselves, thus passing their genes on to new, younger containers—their offspring.
  • Evolution. All living species exhibit genetic change from generation to generation and therefore evolve. This occurs because mutations (changes in DNA structure) are inevitable and because environmental selection pressures endow some individuals with greater reproductive success than others. Unlike the other characteristics of life, evolution is a characteristic seen only in the population as a whole. No single individual evolves over the course of its life.

Clinical and legal criteria of life differ from these biological criteria. A person who has shown no brain waves for 24 hours, and has no reflexes, respiration, or heartbeat other than what is provided by artificial life support, can be declared legally dead. At such time, however, most of the body is still biologically alive and its organs may be useful for transplant.

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