Needs of living things

What do you need to live comfortably? The list probably includes food, a home, clothes, and water. But, the basic necessities of every living thing includes:

Light and Carbon Dioxide:

All life forms need energy to survive. Living things use energy to grow, to defend themselves, and to move around – this energy is provided by the Sun – the primary source of energy. Plants use sunlight and carbon dioxide from the air to create their own food by photosynthesis. Many animals then eat the plants, taking this energy into their own bodies. Other animals then eat these plant eaters, passing the Sun's energy from one organism to another. The food organisms take in provides them with energy, and also provides them with the resources, and raw materials they need to build up their bodies, grow, and repair damage.

Water:

Living things need water to survive. But why is water so important? All life forms on Earth are comprised almost entirely of water. Your own body is about 60-80% water.Water in your blood helps transport food, and chemicals to your cells. It helps remove waste products from your body. Water is used to cool you down, to warm you up, and to carry out the chemical reactions that allow you to move and grow. Another important use of water, is to keep your body clean. Plants use water to grow, to transport food, and to carry out chemical reactions. In addition, plants use water as part of photosynthesis, to create their own food.

Oxygen:

Without food, your body would die in a matter of weeks. Without water, you would day in days. How long do you think you would live without oxygen? Most life forms use oxygen as the main ingredient in many of the chemical reactions needed for life. Organisms get oxygen from their environment in a variety of ways. Many land animals breath oxygen directly from the air, while ocean bearing animals often use the oxygen dissolved in the water to survive.

Minerals:

The Earth’s soil contains minerals, which are essential for health and growth. Plants take in minerals through their roots. Animals get minerals by eating plants and/or other animals.

Warmth:

If it gets too hot or cold, the chemical changes which are necessary for life will stop. In many parts of the Earth, temperatures lie between 25 deg Celsius and 30 deg Celsius. Most living things are adapted to live at these temperatures.

The biosphere:

Biosphere is all those parts of the earth’s surface where living things are found. All of the needs we have mentioned so far, energy, food, water, and oxygen are obtained by organisms in their environment, or the space around them.
Living things are found almost everywhere, from about 9000 meters up mountains to at least 5000 meters under the sea.

Classification of living things

Biologists believe that there may be over two million (2,000,000) different kinds of organisms. Already over 1.5 million (1,500,000) different kinds have been identified and new ones are still being discovered. One biologist estimates that for each kind of organism now alive, another 400 kinds once lived but have since become extinct. Therefore, as many as one billion (1,000,000,000) different kinds of living things may have existed on the earth at one time or another.

How can we keep track of such a bewildering number of organisms? How can we even name the organisms now alive when no known language has two million words in it? Biologists have answers to these questions.

What is classification?

Whenever we work with a large number and variety of things, we usually sort them into groups. Each group contains those things that are similar to one another. We may then separate each of those groups into smaller groups that are even more alike.
The grouping of similar things for a specific purpose is called classification. Although it may be instinctive for human to classify things, there are also practical reasons for doing this. For example, a supermarket manager classifies the foods in his/her store by storing all the cereals together, all the meats together, all the cookies together, and so on. Stamp collectors classify their stamps. They place all the Canadian stamps in one page and all the American stamps in another. The words in a dictionary are classified by alphabetical listings. Clearly, we classify things to make it easier to keep track of what we have, and to find particular items.

Early Biological Classification

Biologists have long recognized the need to classify living things. In fact, humans have been classifying living things for thousands of years. The earliest humans probably classified organisms as plants and animals. They may have further classified plants as edible or poisonous, and the animals as harmful or harmless. However, it was 300 BC before the first serious attempt was made to classify all the organisms known. This attempt was made by the Greek philosopher and scientist, Aristotle and his students.

Aristotle’s Classification System

Since only about 1000 kinds of organisms were known at that time, a very simple classification scheme could be used. Aristotle and his students first classified the organisms as plant or animal. They then classified the animals according to where they lived. This resulted in three groupings: air animals, water animals, and land animals. They classified the plants according to the structure of stems. Those with soft stems were called herbs; those with a single woody stem were called trees; and those with many small woody stems were called shrubs.

Aristotle’s classification system survived for almost two thousand years. However, by the beginning of the 18th century, over 10,000 kinds of organisms were known and Aristotle’s system was unable to classify them all. Many of newly discovered organisms would not fit into any category of Aristotle’s simple system. A new system was obviously needed.

Modern Biological Classification

Taxonomy

Taxonomy is the science that deals with the classification of organisms.

The Contribution of Carolus Linnaeus

Carolus Linnaeus, a Swedish botanist, developed a simple classification system that forms the basis of our modern method of classification. At the start of the 18th century about 10,000 kinds of organisms were known. By the end of that century over 70,000 kinds were known. Linnaeus tried to develop a classification system for this large number of organisms. By 1753 his system was well developed and modern taxonomy began.

The Basis for Linnaeus Classification 

Biologists use the word diversity to mean differences, or the number of kinds of living things.
There seem to be so many kinds of living things and they seem to be so different from one another. Yet, if we study them closely, we can see many likenesses. For example, at first glance lions, horses, humans, and mice seem to have little in common. A closer look however, shows that all have hair, a distinct head, four limbs, two ears, and warm blood. That is, they have similar structural features.
Linnaeus decided to use structural features as the basis for his classification system. Therefore, he grouped organisms according to their structural similarities. These organisms with very similar structural features were considered to be the same species. Thus all modern-day humans belong to one species, all house cats belong to one species, and all sugar maple trees belong to one species.

Modern Basis for Classification

Homologous Structure

Carolus Linnaeus used structural features as the basis for his classification system. He grouped organisms according to their structural similarities. Today, taxonomists still use structural similarities as a basis for classification. They look for homologous structures just as Linneaus did.
Homologous structures are structures that show the same basic pattern, the same general relationship to other parts, and the same pattern of development.
However, they need not have the same function. For example, the human arm, the whale flipper, and the bat’s wing, all these appendages are homologous structures that show the same basic pattern. Also, all three appendages are found in the same part of the body. Finally, the bones in all three appendages develop in similar ways. Although their functions are different, they are homologous structures.

Similar Biochemistry

Biochemistry is the study of the chemical compounds formed by living things. Many biologists believe that closely related organisms form similar chemical compounds in their body. They use this belief to help classify organisms. For example, the horseshoe crab was, at one time, classified as a close relative of the true crab. However, chemical analysis showed that its blood was more like spider’s blood than crab’s blood. Thus, the horseshoe crab is now classified as a close relative of spiders.

Genetic Similarity

Most biologists agree that genetic similarity is the best evidence that organisms are closely related. Every organism makes a special compound called DNA that bears hereditary characters. Thus it seems reasonable to assume that the greater the similarity of DNA among organisms, the more closely they may be related.

Selecting a Classification System

Some biologists feel that two kingdom, Plantae and Animalia, are enough to classify all living things. Others prefer three kingdoms; still others use four, and some use five kingdoms.

The Main Classification Groups (Taxa)

There are seven main taxa or classification groups. This system of classification can be compared to a tree. Many leaves (species) are on a tiny twig (genus). Several tiny twigis (genera) are on a larger twig (family). Several larger twigs (families) are on a little branch (order). Some little branches (orders) are on a larger branch (class). Some larger branches (classes) are on a main limb of the tree (phylum). The few main limbs (phyla) make up the whole tree (kingdom).

1. Species: Species (plural also species) is a group of individuals that are alike in many ways and interbreed under natural conditions to produce fertile offspring (children).

2. Genus: Genus (plural genera) is a group of species that are closely similar in structure and evolutionary origin.

3. Family: Family is a group of similar kinds of genera. That is, similar genera are grouped to form a taxon called Family.

4. Order: Similar families are grouped to form a taxon called order.

5. Class: Similar orders are grouped to form a taxon called class.

6. Phylum or Division: Similar classes are grouped to form a taxon called phylum or division. Zoologists favor phylum and botanists favor division.

7. Kingdom: All the phyla or divisions that contain animals are grouped in the kingdom Animalia,
and all the phyla or divisions that contain plants are grouped in the kingdom Plantae.

Phylogenetic System of Classification

Carl R Woese introduced a completely new approach in classification of organisms in 1977 that is based on evolutionary (phylogenetic) relationship among organisms. He meticulously analysed ribosomal ribonucleic acid (rRNA), genetic molecules that coordinate part of protein production. Because rRNA shows only slight variation from one generation to the next, it is an excellent tool for revealing the evolutionary, or family, relationships among organisms. Woese spent nearly a decade analysing the rRNA of various types of bacteria and arranging them into a microbial evolutionary tree. Use this concept all organisms are now classified into three kingdoms.
 They are
(1) Archaea,
(2) Bacteria, and
(3) Eukarya.

Binomial Nomenclature

Once Linnaeus had decided on a basis for classifying organisms, he then developed a system for naming them. His system is quite simple. He gave each species a name that consists of two words. This system is called binomial nomenclature. He used Latin words for these names because all scientists wrote in Latin in time of Linnaeus. Thus, the human is Homo sapiens, and the domestic (house) cat is Felis domesticus. The first word of each name is called the genus and the second word is called the species. The genus begins with a capital letter and the species does not. The genus and species are either printed in italics or underlined.

The Genus Concept
A genus (plural genera) groups species that are similar. For example, maple trees belong to the genus Acer. Thus sugar maple (Acer saccharum), silver maple (Acer saccharinum), and red maple (Acer rubrum) belong to the same genus Acer. Their leaves are similar and other features are similar but not identical. Every genus has characteristics that make it stand out clearly from other living things.

The Species Concept
Linnaeus grouped as a species those organisms that he felt were very similar in structural features. In simple terms, a single species is a distinct kind of organism, with a characteristic shape, size, behaviour, and habitat that remains constant from year to year.
A species (plural also species) is defined as a group of individuals that are alike in many ways and interbreed under natural conditions to produce fertile offspring (children).
Potato (Solanum tuberosum) and the eggplant (Solanum melongena) belong to the same genus because they are similar in many ways. However, they belong to two different species because they are not identical and they have reproductive barrier, that is, they cannot mate (cross or breed) with one another to produce fertile offspring. The members within a species can mate or cross. Thus all varieties of potatoes are in the species because they can interbreed to produce fertile offspring.

Why Use Scientific Names?

One reason for using Latin scientific name instead of common names is that common names can be confusing or misleading. For example the common name for a domestic cow is “la vache” in French, ‘die Kuh” in German, “la vaca” in Spanish, and “garoo” in Bengali. However, in all languages the scientific name is the same, and there is no confusion if we call the cow Bos taurus.