Before we will address this question, we must first consider what qualifies something as “living.” What's life? This can be a difficult question to answer, largely because life itself isn't a simple concept. If you are trying to put in writing a definition of “life,” you will find that it's not a straightforward task, due to the loose manner in which the term is employed.

     Imagine a situation within which two astronauts encounter a giant, amorphous blob on the surface of a planet. How would they determine whether it's alive?

Movement: one amongst the primary things the astronauts might do is observe the blob to work out if it moves. Most animals move about (figure 1), but movement from one place to a different in itself isn't diagnostic of life.


FIGURE: 1 (one)
Movement: animals have developed mechanisms that allow them to move about in their environment. While several animals, like this giraffe, move on land, others move through water (H2O) or air.



       Most plants and even some animals don't move about, while numerous nonliving objects, like clouds, do move. The criterion of motility is thus neither necessary. (possessed by all life) nor decent (possessed only by life).


Sensitivity:  The astronauts might prod the blob to see if it acts. most living things response stimuli (figure 2).

FIGURE: 2 (two),
Sensitivity:  This father lion is reacting to a stimulus: he has just been bitten on the rump by his cub. As far as we see, all organisms react to stimuli, although not always to the equal ones or in the same like way. Had the cub bitten a tree or else of its father, the response would not have been as striking.


     Plants grow up toward the light, and animals retreat from fire. Not all stimuli produce responses, however. Guess kicking a redwood tree or singing to a hibernating bear. This criterion, although superior to the primary, remains inadequate to define life.


Death: The astronauts might try to kill the blob. All living things die, while inanimate objects don't. Death isn't easily distinguished from disorder, however; a car that breaks down has not died because it had been never alive. Death is just the loss of life, so this can be a circular definition at the best. Unless one can detect life, death may be a meaningless concept, and hence a really inadequate criterion for outlining life.


Complexity:  Finally, the astronauts might hack the blob, to determine if it's complexly organized. All living things are complex. Even the only microorganisms contain a bewildering array of molecules, organized into many complex structures. Nonetheless, a computer is also complex, but not alive. Complexity may be a essential criterion of life, but it's not enough in itself to point living things because many complex things aren't alive..

     To regulate whether the blob is alive , the astronauts would must learn more about it. Probably the most effective thing they could do would be to look at it more carefully and determine whether it resembles the organisms we are familiar with, and if so, how. 


Fundamental Properties of Life:

     As we discussed in chapter 1, all known organisms share certain general properties. To an outsized degree, these properties define what we mean by life. the subsequent fundamental properties are shared by all organisms on earth. 

 Cellular organization: All organisms contain one or more cells—complex, organized assemblages of molecules enclosed within membranes (figure 3).

FIGURE: 3 (three),
Cellular organization (150×):These Paramecia, which are complex, one-celled organisms called protists, have just ingested some yeast cells. The yeasts, stained red in this pic, are enclosed within membrane-bounded sacs called digestive vacuoles. A substance of other organelles are also apparent.






   Sensitivity: All organisms reply to stimuli— though not always to the identical stimuli within the same ways.

  Growth: All living things absorb energy and use it to grow, a process called metabolism. Plants, algae, and some bacteria use sunlight to make covalent carbon-carbon bonds from CO2 and H2O through photosynthesis. This transportation of the energy in covalent bonds is essential to all or any life on earth.

  Development: Multicellular organisms undergo systematic gene-directed occurrences as they grow up and fully grown.


  Reproduction: All living things reproduce, passing on traits from one generation to the subsequent. Although some organisms live for a really while, no organism lives forever, as far as we all know. Because all organisms die, continuing life is hopeless without reproduction.


  Regulation: All organisms have regulatory mechanisms that coordinate interior activities.


  Homeostasis: All living things maintain relatively constant interior states, antithetical from their environment.

 

The Key Role of Heredity 

     Are these properties capable of defining life? May be a membrane-enclosed entity that grows and reproduces alive? Not necessarily. Soap bubbles and proteinoid microspheres spontaneously form hollow bubbles that enclose alittle volume of water. These spheres can enclose energy-processing molecules, and that they can also grow and subdivide. Despite these features, they are certainly not alive. Therefore, the factors just listed, although necessary always, aren't sufficient to define life. 1 (One) element is absent —a mechanism for the preservation of betterment.

  Heredity: All organisms on earth possess a genetic. system that's supported the replication of a          protracted, complex Molecule called DNA. This mechanism allows for. adaptation and evolution over    time, also distinguishing characteristics of living things.


    To realize the role of heredity in our definition of life, allow us to return for a flash to proteinoid microspheres. When we examine a personal microsphere, we see it at that precise present moment in time but learn nothing of its predecessors. It is likewise impossible to guess what future droplets are going to be like. The droplets are the passive prisoners of a changing environment, and it's during this sense that they are not alive. The essence of survival is that the ability to encompass change and to breed the results of change permanently. Heredity, therefore, provides the premise for the great division between the living and also the nonliving. Change doesn't become evolution unless it's passed on to a new generation. A genetic system is that the sufficient condition. of life. Some changes are preserved because they increase the probabilities of survival in an exceedingly hostile world, while others are lost. Not only did life evolve—evolution is that the very essence of life