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Scientific Meaning about What is Life..

What is Life?

Definitions

“Life is an succession of energy-producing electro-chemical processes by a naturally occurring, simple or complex organism composed of a combination of molecules, each consisting of systematically arranged carbon, hydrogen and oxygen atoms, and a few other elements, forming cells, which consume 'food' and produce 'waste', both consisting of solid, aqueous, and gaseous matter; the process is called metabolism; the organism is capable of living within the environment without dependency on any other organism; energy use is manifest by growth with size limits for most; self-healing; possibly movement; self-replication with each offspring slightly different; irritability; capable of modifying their living environment, both beneficially and detrimentally; with eventual termination of energy production, or death. Exceptions are egg, sperm, spore, seeds and virus, which do not consume food or produce waste; the first four are replication structures, and the fifth has premature life-terminating capabilities.” C. Gordon Winder (a “concensus” definition after a 1993 meeting at Brandeis University)

There cannot be life without energy because there cannot be life order without it. Life treads a thin line between order and chaos, related to its use of energy. Energy can be used to build strong chemical bonds between atoms. But life is based mainly on the plasticity of weak bonds that can be continuously broken and re-formed. Life exists in a world of weak bonds. In a world of strong bonds life is impossible because the bonds are essentially inbreakable. That is the realm of rocks and crystals, not of life.” (Renato Dulbecco. 1987. The Design of Life. Yale University Press, New Haven: 449)

We regard as alive any population of entities which has the properties of muliplication, heredity and variation.” (J. Maynard Smith, 1975)

“Life is a partial, continuous, progresive, multiform and conditionally interactive, self-realization of the potentialities of atomic electron states.” (J. D. Bernal, 1967)

“Life is the repetetive production of ordered heterogeneity.” (R. D. Hotchkiss, 1956)

“Life is a potentially self-perpetuating open system of linked organic reactions, catalysed by stepwise and almost isothermally by complex and specific organic catalysts [enzymes] which are themselves produced by the system.” (J. Perret 1952)

“The broadest and most complete definition of life will be ‘the continuous adjustment of internal relations to external relations.’ ” (H. Spencer 1884)

Properties of living systems

We define life more by generating a list of common properties that living systems have in common with each other. Although, non-living systems may exhibit one or more of the items in this list, they do not possess all of them...

· Self-organizing. Living systems utilize energy from light and chemical bonds to produce organized structures. They exhibit autopoesis: A system that continuously renews itself in a regulated manner whilst maintaining integrety of structure over time (from Jantsch, 1980).

· Carbon based. All living systems we know of require and are built on carbon-based chemistry.

· Replication. All organisms can replicate themselves in some form or another. One consequence of this observation is that organisms grow.

· Cells. Life as we know it manifests itself in the form of fundamental structures called, cells.

 

Homeostasis

Organisms use chemical bond energy to manipulate and maintain the chemical and physical medium within cells and within whole organisms. In this way, organisms maintain control over their internal environment which is considerably buffered from the vagaries of their immediate external environment. This mechanism of control to maintain an optimal set of internal conditions for life is called homeostasis.

Living vs non-living chemistry

Living systems utilize only a small set of the elements that exist. The principal elements are, C H N O P S, followed by lesser amounts of, Ca, K, Na and B, Mg, Cl, Mn, Fe, Co, Ni, Cu, Zn. Some, but not all organisms contain the elements, Li, F, Si, V, As, Br, Mo, I, Ba.

chirality. Living systems utilize one one of two possible forms (enantiomers) of those compounds that possess symmetric centers, ie. have mirror images. E.g. only L amino acids are used (with a few exceptions) and only D sugars are used.

Carbon chemistry: Why carbon? There are a list of properties of carbon that seem to make it suitable for biochemistry, of course this is hindsight, but these properties include:

· Valence. Carbon is very flexible in its bonding based on available electrons. Thus it readily forms single, double and triple covalent bonds

· C-C, carbon to carbon bonds allows for the construction of long chains on compounds

· Macroscopic Properties and phase. Small carbon-based molecules can exist as gasses and liquids, medium-sized compounds exist as gels and colloids, and larger polymers may be solids. There is an enormous macroscopic flexibility in this range of properties.

· Abundance. Carbon is abundant in the universe (N=12) is an artifact of He nucleogenesis in evolving stars.

 

The Cell Theory

The smallest unit of life, that is a structure that can perpetuate itself and that all agree is “alive,” is called a cell. We can characterize the components that compose a cell to generate a definition. These include:

· a surrounding (enclosing) bilipid membrane that separates the internal contents from an external environment. The cell membrane is also called a plasmalemma.

· genes in the form of DNA

 

Entropy, order and disordered systems

Life is far from chemical equilibrium with respect to its immediate environment.

Entropy as rigorously defined by the second law of thermodynamics is beyond this course, but we can get a secondary feel for what entropy (S) means. Entropy is a measure of thermal disorder (randomness) in a system. When systems are defined in terms of the energy flow between them, we see that energy is always conserved. In perfectly reversible reactions (exchanges of energy) there is no change in entropy, but for any, so-called irreversible reactions, there is an increase in entropy. So entropy can either stay the same or increase during reactions where there is any transfer of energy. An equilibrium state is one in which a system and its surroundings is at maximal entropy.

Life exhibits an increase in complexity and in order both over its immediate surroundings and in evolutionary time.

In this case the notion of order or and ordered state implies one in which the components of the system are not distributed at random in 3D space. Since organisms are highly organized, that is they are composed of complex and clearly non-random structures, they are systems which are lower in entropy (S) than their surroundings.

Biological order (=structure) requires work to maintain itself. The energy to produce that work comes from chemical bond energy which is ultimately maintained by electromagnetic radiation in the Sun-Earth system. " The sun as a high-temperature source and outer space as a low-temperature sink provide two effectively infinite isothermal resevoirs for the continual performance of work." (Morowitz, 1970: 169)