Sunday, November 20, 2011

Part 2: The Criteria of Life (When do we say a thing is living?)

Dr Abe V Rotor
Living with Nature - School on Air

Binding of virus with host cell, initial step

Adenovirus, Obesity is suspected to be caused by this type of virus (Ad-36), lower photo. The virus is not considered by biologists as a living organism because it cannot meet all the criteria of life.

Hierarchical Organization: Organelles to Organ- System

Let us imagine a ladder with four rungs. Cells make up the bottommost rung, tissues occupy the second, while the third is made up of organs, and finally the topmost rung comprises of the organ-systems. Since many cells make up a tissue, and tissues make up an organ, and several organs make an organ-system. The ladder in our mind has a broad base tapering at the top.

At the top are the "higher" plants such as the mango and "higher" animals such a Philippine eagle - or man, himself. The more complex an organism is, the more developed are its organs and organ-systems. The circulatory system in man, for example, consists of a pumping organ - the heart, and a network of blood vessels with specialized functions, locations, size and structure, through which blood flows. Whereas the circulatory system of an insect consists merely of a simple pumping organ and blood that openly circulates in the body cavity.

But what about the unicellular organisms - the protists and monerans? Do the paramecium and euglena for example, also have organs?

We do not call their body parts organs, but rather organelles, since these parts are confined in their single-cell body structure. They are "organs" of the cell.

To illustrate, the amoeba ingests food with its pseudopodia (false feet), and is stored and consequently digested in food vacuoles. Here the pseudopodia are organelles for locomotion and ingestion, while the food vacuoles are the organelles that function like a digestive system.

Community to Ecosystem

Another hierarchical organization in the biological world maybe similar imagined as another ladder, but in this case, the lowest rung consist of the organism, followed on the next rung by the population of that organism, which together with the population of other organisms form a community. Communities form ecosystems, and ecosystems make up the biosphere which constitutes all parts of the earth where life exists.

In the physical world we also realize the existence of a hierarchy of atoms of an element, which join to produce molecules of a compound which resulted in the combination of a compatible element. These molecules combine to produce macromolecule, which brings us back to our study of carbohydrates, proteins lipids and nucleic acids.

We wonder if the course of evolution, looking at the hierarchical organization of organisms, a cell might have "deviated" and "emerged" from its own organization, hence became an independent unicellular organism, a protist or a moneran. On the other hand, could it be that a group of independent cells united and "emerged" into an organ, which became the precursor of a prototype multicellular organism? Evolutionists believe in both. This is true to certain facultative bacteria to demonstrate the first case, and to the Poriferans - the sponges- for the second.

And yet we know that certain protozoans have developed affinity to their host, and vice versa, that together they are almost integral as one. This is the case of the termite and its protozoan-symbionts living in its digestive system. Could we consider another case whereby certain cells or tissues "deviate" from their structure and function, if and when the conditions of their existence and well-being are threatened? If such a deviation is dictated by a primitive evolutionary archetype, then we are looking into a new field of research about cancer.

Metabolism: Acquisition and Expenditure of energy

All organisms, with no exception, needs energy. How they acquire it distinguishes four groups of organisms, namely:

1. Photosynthesizers - Plants, algae and Eubacteria or BGAs, formerly known as blue- green algae.

2. Chemosynthesizers - Sulfur bacteria

3. Herbivores and carnivores - predators, foragers, parasites, and most animals, including man.

4. Saprophytes - carcass feeders and decomposers, mainly fungi and archaeobacteria, and many protists.

The distinction of each kind is not well defined. We know that there are organisms which obtain their energy by not only one means. The Euglena for example, obtains energy in three ways, namely, by photosynthesis (it has chloroplasts), by feeding on microscopic algae (herbivore), and by ingesting dead organic matter: (saprophyte). Man is both herbivore and carnivore, or in short, omnivore. There are bacteria that are parasitic; they cause diseases, at the same time decomposers.

Thermodynamics in Biology

Organisms expend energy to carry out various life processes. We call this respiration or catabolism (destructive metabolism) as distinguished from anabolism (constructive metabolism, or energy acquisition). All organisms are governed by the law of thermodynamics, which principally explains the transformation of energy from one form to another, as it is passed on from one organism to another through the food chain. Solar energy is transformed into chemical energy (sugar), and the organism that feeds on this sugar releases it into mechanical energy, heat energy, or a kind of electrical energy which makes nerve cells and brain cells function. In all organisms, energy is stored, transformed, and converted through a complex bio- chemical process involving the compound adenosine tri-phosphate or ATP which releases the energy, while its counterpart, adenosine di-phosphate (ADP) absorbs energy, both compounds working in a synchronized cycle.


When does reproduction start? Let us look at it this way. On the part of unicellular organisms, a simple fission or budding is itself reproduction. In higher organisms, reproduction means the birth of one or more individuals. The union of gametes or fertilization precedes this. In plants, reproduction may be by cutting, grafting and other vegetative means. And in a very rare case like in certain insects, an immature individual may produce young even without reaching maturity, which is called paedogenesis, a rare phenomenon in the living world which demonstrates the possibility of "virgin" birth.

On the cellular level, cells divide as the organism grows develops, and reproduces. For somatic or body cells, cell division is called mitosis, while in the reproductive organs or gonads, cell division which results in the production of male or female gametes is called meiosis.

Growth and Development

When we talk of growth and development, which are two biological processes intertwined, we have in mind the illustration of a life cycle. That illustration maybe a circle divided into parts, each part representing a phase.

It may be a square to emphasize the four-stage development of say, a butterfly or a triangle, which is represented by organisms undergoing simple metamorphosis. Arthropods like shrimps and spiders have only three stages in their life cycle - egg, young, and adult.

In plants, algae and other protists, we encounter the term, alternation of generation. It is worth to mention that algae and the lower plants, for example, are dimorphic - sporophyte and gametophyte - so that their life cycle is made up of two parts. A living thing, no matter how simple, changes throughout its life span, growing in size, duplicating its parts as this is necessary in duplication.

Heredity and Genetics

All living organisms, modem and primitive, carry a genetic code, which is passed on from one generation to the next, the DNA. The DNA is the plantilla of organisms so that their offspring will carry the characteristics of the parents. The vehicle of this genetic code is the gene. The genes on the other hand, are built in larger structures called chromosomes. The science that deals with the study of heredity is genetics. Today's genetics encompasses such revolutionary techniques as genetic engineering and cloning.

Response and Adaptation

All living organisms respond to the conditions of the environment in which they live in two ways. First, is day-to-day response as the organism undergoes through its life cycle, as influenced by weather, biological rhythms, and the like. The second is a long-term response called adaptation. Harsh changes of the environment may force organisms to adopt to such conditions, otherwise they perish. The survivors carry the acquired characters and pass them on their offspring. Over time these characters may be accumulated, ultimately setting the conditions of evolution in which case new species may arise, a process called speciation. This is the basis of the Darwinian principle of evolution summarized in a very simple phrase, “survival of the fittest".

Re-creating Life through DNA

While we look at these criteria to guide us in differentiating living and nonliving things, we cannot help picture in our mind "Jurassic Park" which showed the possibility of re- creating a long dead if not extinct organisms from its DNA imprints. If this will be true in the future, then life would assume a new dimension, and therefore a new definition. We would think of the day when organisms or tissues "sleeping" in cryonics laboratories would be brought to life, in a similar way the DNA of a dinosaur was reconstructed to life.

But whatever breakthroughs about life that science and technology shall bring, we do recognize that there is but one and only source - an Omnipotent Being – who is the Author of life, the Creator of all living things, and the Maker of the whole universe. ~

NOTE: This lesson provides a framework for further studies in biology and related studies. On the other hand, it can be simplified for basic instruction.

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