Biology Lecture: Criteria of Life: When do we say a thing is living?

Dr Abe V Rotor 

Paaralang Bayan sa Himpapawid with Ms Melly C Tenorio 
738 DZRB AM Band, 8 to 9 evening class, Monday to Friday

All of us 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.

Century-old acacia harbors countless organisms from its roots to its branches. 

As the landscape turns from brown to green let us examine the germinating seeds, the young plants and the raw shoots of shrubs and trees. What brought them suddenly to life? What broke the dormancy of buds of stems and tubers and made them spring 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 appropraite to call "a celebration of life."

Here we see that the monsoon brought in the life giving substance that no other planet within our knowledge posses - water. With water, the nutrients in the soil are released and made available to plants and animals. Water feed 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 field 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 the 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 influence 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.

We study 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 is 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 living? 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 tghings even if they possess fragments of genetic materials that enable them to stay inside living cells.

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 are simple sugars which are then assembled into carbohydrates, calorie-rich food passed on through the food chain. Thus it is called "go" food. Protein, on the other hand, are built from amino acids produced through a series of chemical reaction 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 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. They key to renewal of life is in the universality of these four organic substances in the living world.

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.

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

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