wiki: 生命没有公认定义。无论是左还是右派,请不要自定义,胎儿=婴儿=奴隶。。。
生命现象
另外,新陈代谢和自我复制的能力有时被视为判断生命的根本条件,称之为
生命现象。
病毒在有寄主可
寄生的时候,会表现出
生命现象;但在没有寄主可寄生的时候,不会表现生命现象,所以病毒是介于生命与非生命之间的一种奇妙的有机物。
定义
生命没有公认
定义,不同的科学家曾提出过各种定义。
传统定义
科学家经常认为只有生物体会展现以下全部现象:
- 恒定性:能够调节体内环境以维持身体处于一个相对恒定的状态,例如恒温动物能发汗来降低过热的体温,也能靠发抖来产生额外的热量以保持体温。
- 组织性:由一个或以上的生物基本单位──细胞所组成。
- 新陈代谢:能够转换非生物为细胞成分(组成代谢)以及分解有机物(分解代谢)来获取和转化能量。生物体需要能量来维持体内平衡及产生其他生命现象。
- 生长:使组成代谢的速率高于分解代谢的速率来让细胞体积增大,并在细胞分裂后使细胞成长。一个生长中的有机体增加其细胞的数量和体积,而不止是将得到的物质积存起来。某些物种的个体可以长得很巨大,例如蓝鲸。
- 适应:对环境变化作出反应的能力,与生物当前的身体构造、生活习性及遗传有关。这种能力对生存是很重要的。生物可以通过进化适应环境。
- 感应:反应可以以很多方式进行,从单细胞变形虫被触碰时的收缩到高等生物在不同情况下的复杂反射。最常见的反应是运动,例如植物的叶片转向太阳以及动物追捕其猎物。
- 繁殖:能够产生新的个体。包括只需一个亲本的无性生殖和需要至少两个亲本的有性生殖。
大部分科学家称这样的现象为生命的表现方式。通常必须具备全部七个特征才能被视作生命。
Definitions
The definition of life has long been a challenge for scientists and philosophers.
[17][18][19] This is partially because life is a process, not a substance.
[20][21][22] This is complicated by a lack of knowledge of the characteristics of living entities, if any, that may have developed outside of Earth.
[23][24] Philosophical definitions of life have also been put forward, with similar difficulties on how to distinguish living things from the non-living.
[25] Legal definitions of life have also been described and debated, though these generally focus on the decision to declare a human dead, and the legal ramifications of this decision.
[26] As many as 123 definitions of life have been compiled.
[27] One definition seems to be favored by
NASA: "a self-sustaining chemical system capable of Darwinian evolution".
[28][29][30][31] More simply, life is "matter that can reproduce itself and evolve as survival dictates".
[32][33][34]
Biology
See also:
Organism
Since there is no unequivocal definition of life, most current definitions in biology are descriptive. Life is considered a characteristic of something that preserves, furthers or reinforces its existence in the given environment. This characteristic exhibits all or most of the following traits:
[19][35][36][37][38][39][40]
- Homeostasis: regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature
- Organization: being structurally composed of one or more cells – the basic units of life
- Metabolism: transformation of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.
- Growth: maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter.
- Adaptation: the ability to change over time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity, diet, and external factors.
- Response to stimuli: a response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (phototropism), and chemotaxis.
- Reproduction: the ability to produce new individual organisms, either asexually from a single parent organism or sexually from two parent organisms.
These complex processes, called
physiological functions, have underlying physical and chemical bases, as well as
signaling and control mechanisms that are essential to maintaining life.