Environment and Biology : Part 1 - The Criteria of Life (When do we say a thing is living?)

Dr Abe V Rotor

The Coral Reef is sanctuary of "living fossils" Painting in acrylic by AVR

All of us may have witnessed in one or many occasions the transformation of the landscape when the first monsoon rains finally put an end to a long, hot summer.

The hills and the mountains begin to turn green; the fields come to life with frogs croaking, mudfish or dalag emerging from their encrusted abode, and herons hovering over the paddies. It is a moment of rejoice, a new beginning of life.

Let us analyze the scenario in the context of biology.

As the landscape turns from brown to green let us examine the germinating seeds, the young plants and the new shoots of shrubs and trees. What brought them suddenly to life? What broke the dormancy of buds of stems and tubers and made them sprung to life? What loosened the soil and made the sleeping seeds germinate'? Where did the mushrooms come from? And mosses carpeting the rocks? This scenario depicts one most appropriate to call “a celebration to life."

Here we see that the monsoon brought in the life giving substance that no other planet within our knowledge possesses - water. With water, the nutrient in the soil are released and made available to plants and animals. Water feeds the brooks and rivers and on its way down to the sea, wakes up the aestivating organisms, releasing them from their prison of mud crust.

Animals respond to the favorable change in season, and now they have food to eat. They emerge from their lair and soon move to new places, mate and multiply. Among the first organisms to dominate the fields are insects that will soon become food of birds, and fishes. Food chains are restored and new ones are built which in turn will form food webs and food pyramids.

This is a microcosm of a web of life simultaneously taking place in many parts of the world at a particular time. It is the essence of changing seasons that influences the living world. The elements of the physical world determine the kind of life that exists in a place, and how its diverse forms interact among themselves and with their immediate environment.

Biology, the Science of Living Things

We are studying biology, from the Greek word bios meaning "life", and logus, meaning study or "science of”. The scenarios we presented could well be traced to early man visioning his environment, and not only to students and scientists. But more than mere curiosity to know what was happening, our ancestors were more interested on plants and animals that relate to his survival. We can then say that biology to early man was an applied form as he searched for food and shelter, sought for cure from plants, and later, as he domesticated plants and animals he found useful to him and the settlement he built.

We have been mentioning the term life. What really is life? When do we say a thing is alive? It is possible that there are non- living things that exhibit certain properties attributed to living things. For example, there are rocks and stones that "grow," but their increase in size is by accretion and not by any biological means. Even those that exhibit life-like functions like viruses, according to scientists, are not living things even if they possess fragments of genetic materials that enable them to stay inside living cells.

The best way to determine whether a thing is alive or not is to follow these criteria which are fundamental properties shared by all living things. They are the following:

1. Molecular organization
2. Hierarchical organization
3. Metabolism
4. Reproduction
5. Development
6. Heredity

Let us take up these criteria one by one. Living things, without exception, are made up of large, complex organic molecules which are of four kinds, namely, carbohydrates, protein, lipids and nucleic acids.

Molecular Organization Based on
Four Basic Organic Compounds

Let us imagine the scenario after the first heavy rain and relate how these compounds became products of the newly germinated plants. The first product of photosynthesis is simple sugars which are then assembled into carbohydrates, calorie-rich food passed on through the food chain. Thus it is called "go" food.

Proteins, on the other hand, are built from amino acids produced through a series of chemical reactions in the cells. They are the building blocks of the cells and tissues, and equally important is their role as enzymes. We eat protein-rich food often referred to as "grow" food.

Lipids on the other hand, are components of fat and oil which are actually reserve food, and chief material that makes up the cell membranes. If we look under a special microscope the inside of a nucleus of every living cell, be it plant or animal - or protist for that matter - we find chromosomes which contain the genes. The genes in turn contain nucleic acids that constitute the code in transmitting hereditary characters from one generation to the next.

These four types of molecules are found all living things and are basically the same in both plants and animals, except in certain molecules which differ in form and configuration such as the case of enzymes. It is no wonder then to see plants and animals, and unicellular organisms, sharing with one another these compounds through the food chain and food web in which they are a part. We recall the second scenario after the first heavy rain when plants and animals have established themselves ecologically. The key to renewal of life is in the universality of these four organic substances in the living world. (Continued)