Hisory of live

what are the key characteristics of living things?

The earth formed as a hot mass of molten rock about 4.5 billion years ago. As the earth cooled, much of the water vapor present in its atmosphere condensed into liquid water that accumulated on the surface in chemically rich oceans. One scenario for the origin of life is that it originated in this dilute, hot, smelly soup of ammonia, formaldehyde, formic acid, cyanide, methane, hydrogen sulfide, and organic hydrocarbons. Whether at the oceans' edge, in hydro-thermal deep-sea vents, or elsewhere, the general consensus among researchers is that life arose spontaneously from these early waters. While the way in which this happened remains a puzzle, we cannot escape a certain curiosity about the earliest steps that eventually led to the origin of all living things on earth, including our-selves. How did organisms evolve from the complex molecules that swirled in the early oceans?

what is life?

Before we can address the question "What is life?", we must first consider what qualifies something as "living." This is not a simple concept because of the loose manner in which the term "alive" is used. Imagine a situation in which two astronauts encounter a large, amorphous blob on the surface of a planet. How would they determine whether it is alive?


01.Movement.
 Animals have evolved mechanisms that allow them to move about in their environment. While some animals, like this giraffe, move on land, others move through water or air.
  One of the first things the astronauts might do is observe the blob to see if it moves. Most animals move about,  but movement from one place to another in itself is not diagnostic of life. Most plants and even some animals do not move about, while numerous nonliving objects, such as clouds, do move. The criterion of movement is thus neither necessary (possessed by all life) nor sufficient (possessed only by life).

02. Sensitivity.
 The astronauts might prod the blob to see if it responds. All living things respond to stimuli. Plants grow toward light, and animals re-treat from fire. Not all stimuli produce responses, how-ever. Imagine kicking a redwood tree or singing to a hibernating bear. This criterion, although superior to the first, is still inadequate to define life.

03. Death.
 The astronauts might attempt to kill the blob. All living things die, while inanimate objects do not. Death is not easily distinguished from disorder, how-ever; a car that breaks down has not died because it was never alive. Death is simply the loss of life; SQ this is a circular definition at best. Unless one can detect life, death is a meaningless concept, and hence a very inadequate criterion for defining life
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04. Complexity.
 Finally, the astronauts might cut up the blob, to see if it is complexly organized. All living things are complex. Even the simplest bacteria contain

05.Sensitivity. 
 This father lion is responding to a stimulus: He has just been bitten on the rump by his cub. As far as we know, all organisms respond to stimuli, although not always to the same ones or in the same way. Had the cub bitten a tree instead of its father, the response would not have been as dramatic.a bewildering array of molecules, organized into many complex structures. However, a computer is also complex, but not alive. Complexity is a necessary criterion of life, but it is not sufficient in itself to identify living things because many complex things are not alive.



To determine whether the blob is alive, the astronauts would have to learn more about it. Probably the best thing they could do would be to examine it more carefully and determine whether it resembles the organisms we are familiar with, and if so, how.

 

what are the fundamental properties of life?

As we discussed in chapter 1, all known organisms share certain general properties. To a large degree, these proper-ties define what we mean by life. The following fundamental properties are shared by all organisms on earth.

• Cellular organization. All organisms consist of one or more cells—complex, organized assemblages of molecules enclosed within membranes.

•  Sensitivity. All organisms respond to stimuli—though not always to the same stimuli in the same ways.  

• Growth.  All living things assimilate energy and use it to maintain order and grow, a process called metabolism. Plants, algae, and some bacteria use sunlight to create covalent carbon-carbon bonds from CO2 and H2O through photosynthesis. This transfer of the energy in covalent bonds is essential to all life on earth. 

•  Development. Both unicellular and multi cellular organisms undergo systematic, gene-directed changes as they grow and mature.

  

•  Reproduction. All living things reproduce, passing on individuals from one generation to the next.  

• Regulation. All organisms have regulatory mechanisms that coordinate internal processes. 

•  Homeostasis. All living things maintain relatively constant internal conditions, different from their environment.

The key role of heredity 

Are these properties adequate to define life? Is a membrane-enclosed entity that grows and reproduces alive? Not necessarily. Soap bubbles and protein micro spheres spontaneously form hollow bubbles that enclose a small volume of air or water. These spheres can enclose energy-processing molecules, and they may also grow and subdivide. Despite these features, they are certainly not alive. Therefore, the criteria just listed, although necessary for life, are not sufficient to define life. One ingredient is missing—a mechanism for the preservation of improvement.



Heredity. All organisms on earth possess a genetic system that is based on the replication of a long, complex molecule called DNA. This mechanism allows for adaptation and evolution over time, and is a distinguishing characteristic of living organisms.

 To understand the role of heredity in our definition of life, let us return for a moment to protein micro spheres. When we examine an individual micro sphere, we see it at that precise moment in time but learn nothing of its predecessors. It is likewise impossible to guess what future droplets will be like. The droplets are the passive prisoners of a changing environment, and in this sense they are not alive. The essence of being alive is the ability to encompass change and to reproduce the results of change permanently. Heredity, therefore, provides the basis for the great division between the living and the nonliving. Viruses, while possessing nucleic acids, do not possess a functional genetic system, as they cannot use or reproduce their genes outside of a living cell. A genetic system is the sufficient condition of life. Some changes are preserved because they increase the chances of survival in a hostile world, while others are lost. Not only did life evolve, but evolution is the very essence of life.