Biology is a science that deals with the living world. Its methods try are similar to those employed in all natural sciences. It is a fascinating study that ranges from microscopic cellular molecules to the multicellular macro forms of life, encompassing the earth’s surface and its living organisms. 

For example, a branch of biology called virology studies viruses, which exhibit some of the characteristics of living entities but lack others. It turns out that although viruses can attack living organisms, cause diseases, and even reproduce, they do not meet the criteria that biologists use to define life.

Consequently, virologists are not biologists, strictly speaking. Similarly, some biologists study the early molecular evolution that gave rise to life; since the events that preceded life are not biological events, these scientists are also excluded from biology in the strict sense of the term.

The article examines the characteristics of living things which distinguish them from non-living things, plant and animal cells, cell division and the exchange of materials within a cell. Science defines the difference between living and non-living things using seven basic characteristics.

 

What are the Living things?

Living things have life, though some might not show its evident signs. For instance, a tree would probably not react the same way a human would. It would not react when we hit it, and it might not be able to walk around. 

Though the signs of life displayed by them are not very observable, it does not make them non-living.

Living things exist and are alive and are made of microscopic structures called cells. They grow and exhibit movement or locomotion. They experience metabolism, which includes anabolic and catabolic reactions.

Living things are capable of producing a new life which is of their own kind through the process of reproduction. Living things have a particular life span and are not immortal.

Cellular Respiration enables living organisms to acquire energy which is used by cells to perform their functions. They digest food for energy and also excrete waste from the body.

 

Read: Characteristics of Living Things and Non-living Things

Seven Characteristics of living things

There are seven activities which make organisms different from non-living things.

These are the seven characteristics of living organisms:

1. Nutrition

2. Respiration

3. Movement

4 Excretions

5. Growth

6. Reproduction

7. Sensitivity

1. Nutrition: Living things take in materials from their surroundings that they use for growth or to provide energy. Nutrition is the process by which organisms obtain energy and raw materials from nutrients such as proteins, carbohydrates and fats.

2. Respiration:Respiration is the release of energy from food substances in all living cells. Living things break down food within their cells to release energy for carrying out the following processes.

3. Movement:All living things move. It is very obvious that a leopard moves but what about the thorn tree it sits in? Plants to move in various different ways. The movement may be so slow that it is very difficult to see.

4. Excretion:All living things excrete. As a result of the many chemical reactions occurring in cells, they have to get rid of waste products which might poison the cells. Excretion is defined as the removal of toxic materials, the waste products of metabolism and substances in excess from the body of an organism.

5. Growth:Growth is seen in all living things. It involves using food to produce new cells. The permanent increase in cell number and size is called growth.

6. Reproduction:All living organisms have the ability to produce offspring.

7. Sensitivity:All living things are able to sense and respond to stimuli around them such as light, temperature, water, gravity and chemical substances.

Many other things carry out one or more of the above processes, only living organisms possess all of these characteristics.

 

Levels of Organization of Living Things

Living things are highly organized and structured, following a hierarchy that can be examined on a scale from small to large. The atom is the smallest and most fundamental unit of matter.

It consists of a nucleus surrounded by electrons. Atoms form molecules. A molecule is a chemical structure consisting of at least two atoms held together by one or more chemical bonds.

Many molecules that are biologically important are macromolecules, large molecules that are typically formed by polymerization (a polymer is a large molecule that is made by combining smaller units called monomers, which are simpler than macromolecules). 

An example of a macromolecule is deoxyribonucleic acid (DNA), which contains the instructions for the structure and functioning of all living organisms.

Some cells contain aggregates of macromolecules surrounded by membranes; these are called organelles. Organelles are small structures that exist within cells.

Examples of organelles include mitochondria and chloroplasts, which carry out indispensable functions: mitochondria produce energy to power the cell, while chloroplasts enable green plants to utilize the energy in sunlight to make sugars.

All living things are made of cells; the cell itself is the smallest fundamental unit of structure and function in living organisms. (This requirement is why viruses are not considered living: they are not made of cells.

To make new viruses, they have to invade and hijack the reproductive mechanism of a living cell; only then can they obtain the materials they need to reproduce.) Some organisms consist of a single cell and others are multicellular.

Cells are classified as prokaryotic or eukaryotic. Prokaryotes are single-celled or colonial organisms that do not have membrane-bound nuclei or organelles; in contrast, the cells of eukaryotes do have membrane-bound organelles and a membrane-bound nucleus.

In larger organisms, cells combine to make tissues, which are groups of similar cells carrying out similar or related functions. 

Organs are collections of tissues grouped together performing a common function. Organs are present not only in animals but also in plants.

An organ system is a higher level of organization that consists of functionally related organs. Mammals have many organ systems. For instance, the circulatory system transports blood through the body and to and from the lungs; it includes organs such as the heart and blood vessels. 

Organisms are individual living entities. For example, each tree in a forest is an organism. Single-celled prokaryotes and single-celled eukaryotes are also considered organisms and are typically referred to as microorganisms.

All the individuals of a species living within a specific area are collectively called a population. For example, a forest may include many pine trees. All of these pine trees represent the population of pine trees in this forest.

Different populations may live in the same specific area. For example, the forest with the pine trees includes populations of flowering plants and also insects and microbial populations. 

A community is the sum of populations inhabiting a particular area. For instance, all of the trees, flowers, insects, and other populations in a forest form the forest’s community.

The forest itself is an ecosystem.

An ecosystem consists of all the living things in a particular area together with the abiotic, non-living parts of that environment such as nitrogen in the soil or rain water. At the highest level of organization.

The biosphere is the collection of all ecosystems, and it represents the zones of life on earth. It includes land, water, and even the atmosphere to a certain extent.

 

Read: 10 Major Problems of Agricultural Planning in Developing

Classification of living organisms

If you have ever been to a library, you will know how much easier it is to find a book on a particular subject if the books are arranged in subject groups. When the librarian has a new book to add to the library, he or she will group it with books on a similar topic, according to a classification system.

 

The use of the hierarchical classification system

Classification helps us to impose order and a general plan on the diversity of living things. Scientists have always tried to organize and classify the objects, including living organisms, around them. Classification can be defined as grouping organisms according to their structural similarities.

This means that organisms that share similar features are placed in one group. These groups are arranged from the largest group of organisms to the smallest group of organisms. The groups, from largest to smallest, are arranged as follows: kingdom, phylum (plural phyla), class, order, family, genus (plural genera) and species.

The species is the smallest group of organisms. As you go through the classification hierarchy, you will see that scientists have used broader features to put organisms into kingdoms, which are the largest groups of organisms.

When you move down towards the species, which are the smallest groups of organisms, features are becoming specific. In other words, two organisms that belong to the same species share more features than those in the same kingdom but in different species. A species can be defined as a group of organisms with similar features, and these organisms are capable of breeding and produce fertile offspring.

You are probably aware of the fact that horses and donkeys belong to the same kingdom, phylum, class, order, family as well as genus but they are from different species. Therefore, if a donkey and the horse happen to breed, they produce an offspring called a mule.

The mule is infertile, meaning that it cannot reproduce offspring because it is a product of organisms of different species. Classification hierarchy has many uses.

First, it helps scientists to sort organisms in order. Second, it helps them to identify new organisms by finding out which group they fit. Third, it is easier to study organisms when they are sorted in groups.

 

The hierarchical classification system

There are various sizes of groups into which living organisms are put. The largest group is the kingdom. There are five kingdoms: prokaryotes (which includes bacteria), protoctista, fungi, plants and animals. Each kingdom is further divided into smaller groups called phyla, based on a few features that are shared by some organisms.

For example, the arthropod phylum contains all the animals without a backbone that also have jointed legs and a hard covering over their body, such as insects, crustaceans and spiders. A phylum is then subdivided into classes, orders, families, genera, and finally species.

In this system of classification the various groups are called taxa (singular: taxon).

This chart shows the hierarchical system of classification.

Kingdom

Phylum

Class

Order

Family

Genus

Species

 

Types of Classification System

Classification can be based on two different systems:

1.   Natural

2.   Artificial.

 

1. Natural classification

The hierarchical classification system described above is based on a natural classification system that uses common features shared by organisms. Natural classification is based on two ideas:

• Homologous structures

• evolutionary relationships 

Homologous structures Homologous structures are features of organisms that are similar in structure but may look very different from each other and may be used for different purposes.

They have the same number and arrangement of bones and this means that they probably evolved from a single type of structure that was present in a common ancestor millions of years ago.

A fly’s wing is not homologous with a bat’s wing. It may look similar and do the same job but it develops from a completely different origin.

The fly’s wing has no bones and is not covered by feathers. A bat’s wing and a fly’s wing are termed analogous. A bat and a fly would not be grouped together!

 

Evolutionary relationships

If you look at photographs of people who share a common ancestor, such as a grandparent or great grandparent, you often see startling similarities in appearance. The people in the photos are obviously related to each other and have inherited some features from their grandparents.

In a natural classification system, biologists group together organisms which are structurally similar and share common ancestors.

Natural classification produces a branching set of relationships. This shows how the plants are divided into major subgroups such as mosses, ferns, conifers and flowering plants. Each of these subgroups can be divided further.

In this diagram only the two main groups of flowering plants have been shown. Where organisms are divisions of the same subgroup, such as the monocotyledons and cotyledons, they are more closely related and may share more similar features than with the mosses and ferns.

In the animal kingdom, humans, Homo sapiens, are found, as is the cockroach Periplaneta americanus. Humans and cockroaches share a common ancestor, but that was more than 500 million years ago! You can see many structural differences between humans and cockroaches and so there is no natural relationship. Because of this we classify Homo sapiens and Periplaneta americanus into very different groups!

 

Artificial classification

With artificial classification you can use any grouping you like. You could put all the animals that fly in the same group. This group would then include birds, bats and many insects. 

You could put all animals that live in water and have streamlined, fish-like bodies in the same group. This group would then include fish and whales.

Artificial classification systems are also used as the basis for dichotomous keys that biologists use to identify organisms.

 

Read: The Concept of Planning and Agricultural Development Planning

Binomial system of naming species

Carl Linnaeus, a Swedish botanist who lived from 1707 to 1778, introduced the hierarchical classification system that we have discussed so far. In addition to that, he gave each and every species a scientific name in Latin.

The binomial system of naming species means giving organisms two names in Latin (scientific names). The term binomial literally means two names – ‘bi’ means two and ‘nomial’ means name. Linnaeus derived scientific names from the genus and the species to which organisms belong. 

When writing a scientific name, the genus name is written first and starts with a capital letter, and the species name is written second and starts with a small letter.

The scientific name ought to be printed in italics when typed and underlined separately when handwritten. The tiger belongs to the genus called Panthera and the species called tigris, therefore its scientific name will be typed as Panthera tigris, or handwritten as Panthera tigris. Scientific names are universal because, for instance, every biologist will understand that Felis catus means ‘house cat’ without resorting to the dictionary, no matter what language they speak. Can you think of the scientific names for some more organisms?

Read: Characteristics of Living Things and Non-living Things