Biology is the study of living things. It is a branch of science and like other sciences, it is a way of understanding nature. Biologists deal with the living part of nature and with the non-living things which affect the living things in any way. they strive to understand, explain, integrate and describe the natural world of living things. The literal meaning of biology is the study of life.

It is very difficult to define life. There are certain aspects of life that lie beyond the scope of the science of biology like the answers to the questions: what is the meaning of life? Why should there be life? These are the questions not usually taken up by Biologists and are left to philosophers and theologians. Biologists mainly deal with matters relating to how life works.

Life, for biologists, is a set of characteristics that distinguish living organisms from non-living objects (including dead organisms). Living organisms are highly organized, complex entities; are composed of one or more cells; contain the genetic program of their characteristics;

  •  Acquire and use energy.
  • Can carry out and control numerous chemical reactions.
  • Grow in size.
  • Maintain a fairly constant internal environment.
  • Produce offspring similar to themselves; respond to changes in their environment.

Any object processing all these characteristics simultaneously can be declared as a living thing and in an object for biological studies.

The science of biology is a very wide based study. It includes every aspect of a living thing. Volumes and volumes of information are available under this major head. It is but natural to divide the science into quite a number of branches for our convenience of comprehending and studying biology.

You are surely familiar, at this stage, with Ecology, Embryology, Physiology, Morphology (external Morphology or Anatomy), Paleontology, Histology, Evolution, Genetics, Zoo-geography etc. These are branches of biology which deal with environmental relations, development, functions, structure, form and internal gross structure, fossil, tissues, ancestral history, heredity and distribution of animals in nature, respectively. In addition to these branches, there are a number of other branches of biology such as Molecular Biology, Microbiology, Marine Biology, Environmental Biology, Freshwater Biology, Parasitology, Human Biology, Social Biology, Biotechnology, etc.

Molecular Biology:

Molecular biology is a branch of biology which deals with the structure of organisms, the cells and their organelles at the molecular level.

Environmental Biology:

Environmental Biology is the study of organisms in relation to their environment. This includes
the interaction between the organism and their inorganic and organic environment, especially as it
relates to human activities.


This is the study of microorganisms which include Bacteria, Viruses, Protozoa and microscopic
algae and fungi.

Freshwater Biology:

This branch of biology deals with the organisms living in freshwater bodies i.e., rivers, lakes etc and physical and chemical parameters of these water bodies.


Marine Biology:

This is the study of life in seas and oceans. This includes the study of the marine life and the physical
and chemical characteristics of the sea acting as factors for marine life.


This is the branch of biology which deals with the study of parasites. The structure, mode of transmission, life histories and host-parasite relationships are studied in parasitology.

Human Biology

It deals with the study of man. This includes form and structure, function, histology, anatomy, morphology, evolution, genetics, cell biology and ecological studies etc. of human beings.

Social Biology

This is the branch of biology which deals with the study of social behaviour and communal life of human beings.


It deals with the use of living organisms, systems or processes in manufacturing and service industries.


Hundreds of chemical reactions are involved in maintaining the life of even the simplest organism. In view of this, it is something of a surprise to find that of the 92 naturally occurring chemical elements, only 16 are commonly used in forming the chemical compounds from which living organisms
are made. These 16 elements and a few others which occur in a particular organism are called elements.
In the human body, only six bio-elements account for 99%of the total mass.

The fact that the same 16 chemical elements occur in all organisms, and the fact that their properties differ from those in the non-living world, shows that bio-elements have special properties which make them particularly appropriate as the basis for life.

The biological organization is not simple. It has a high degree of complexity because of which the living organisms are able to carry out a number of processes (some very complicated) which distinguish them from the non-living things. A living thing has built-in regulatory mechanisms which interact with the environment to sustain its structural and functional integrity.

A living thing is, therefore, composed of highly structured living substance or protoplasm. In order to understand the various phenomena of life, biologists for their convenience, study the biological organization at different levels starting from the very basic level of subatomic and atomic particles to the organism itself and beyond which the study of community, population and the entire world are included. The biological organization can be divided into the following levels,

Levels Of Organization

Atomic & Subatomic Levels:

All living and nonliving matter is formed of simple units called atoms and subatomic particles such as protons, electrons & neutrons.

Molecular Level:

In organisms elements usually, do not occur in isolated forms. The atoms of different elements combine with each other through ionic or covalent bonding to produce compounds. This stable form is called a molecule. Hydrogen, carbon, oxygen, nitrogen, phosphorous and Ca are the most common atoms found in biological molecules. The different types of bonding arrangement permit biological molecules to be constructed in great variety and complexity. These may be micro-molecules with low molecular weight like C02, H20 etc. or macro-molecules with high molecular weights e.g. starch, proteins etc.

Biological world has two types of molecules: organic and inorganic. An organic molecule is any molecule containing both carbon and hydrogen. Inorganic molecules do not include carbon and hydrogen together in a molecule.

An organism is usually formed by an enormous number of micro and macromolecules of hundreds of diferent types. Some most important and abundant organic molecules in organisms are glucose, amino acids, fatty acids, glycerol, nucleotide-like ATP, ADP, AMP etc.

Organelles & Cell:

A different and enormous number of macromolecules and macromolecules arrange themselves in a particular way to form cells and their organelles. In the case of simple organisms like bacteria and most protists, the entire organism consists of a single cell. In most fungi, plants and animals, the organism may consist of up to trillions of cells.

Numerous sub-cellular structures like mitochondria, Golgi-complex, endoplasmic reticulum, ribosomes etc have been studied for their structure and function. It has become clear that the functions of the cells are accomplished by these specialised structures comparable to the organs of the body. These structures are called organelles.

The arrangement of the organelles speaks of the division of labour within the cell. The prokaryotes have only a limited number and type of organelles in their cytoplasm. Eukaryotes are rich in number and kinds of membranous organelles. A cell membrane is however present in all cells whether prokaryotic or eukaryotic.

Tissue Level:

In multicellular animals and plants, groups of similar cells are organized into loose sheets or bundles performing similar functions; these are called tissues. Each tissue has a particular function in the life of the organism e.g. muscle tissue, glandular tissue, xylem tissue, phloem tissue etc. They are specialized for contraction (movement), secretion, conducting water and for translocation of sugar, proteins etc.

Organ & System:

Different tissues having related functions, assemble together in a structure to carry out its function with great efficiency. Such structures are called organs and they are specialized to perform particular functions. For example, stomach which is an organ has a function of food digestion (protein part), has a secretory epithelium which secretes the gastric juice, and muscular tissue (smooth) for contracting the walls of the stomach and mixing the food with the enzyme thoroughly and moving the food to the posterior end. The formation of organs also has a selective value because this leads to an efficient accomplishment of their functions both qualitatively and quantitatively. In animals,

organ formation is far more complex and denied. Organs are part of organ systems where total functions involved in one process or phenomenon are carried out. The organ level of organization is much less definite in plants than it is in animals. At the most, we might distinguish roots, stems, leaves and reproductive structures. Clear cut functions, the distinguishing features, can be assigned to each of these structures. Roots are involved in anchoring the plant, storage of food and procuring water and minerals. The shoot supports the entire plant while the leaves are primary organs for food manufacture. Flowers or other reproductive structures are involved in producing the next generation (reproduction). The complexity of the organ systems of animals is associated with a far greater range of functions
and activities that are found in plants.

Individual (Whole Organism)

Various organs in plants and various organ systems in animals are assembled together to form an individual – the whole organism. The whole organism has its individuality as far as its characteristics are concerned. It is diferent from other members of the same species in certain respects. The various functions, processes, activities of an organism are coordinated. In an animal, all the systems work in coordination with each other. For instance, if a man is engaged in continuous and hard exercise, not only his muscles are working but there is an increase in the rate of respiration and heartbeat to supply the muscles with increased oxygen and food which they need for continuous exercise. In animals, the coordination is achieved by means of the nervous system and endocrine system, whereas in plants only long term regulation of activities is brought about by hormones.

Organism works as a whole and it interacts and responds to the environmental changes as a whole.


A population is a group of living organisms of the same species located in the same place at the same time. Examples are the number of rats in a field of rice, the number of students in your biology class, or the human population in a city.

The population is a higher level of biological organization than organism (whole) because here a group of organisms of the same species is involved. This level of organization has its own attributes which come into being by living together of a group of organisms of the same species.

Some of these attributes are gene frequency, gene low, age distribution, population density, population pressure etc. All these are new parameters which have appeared due to the population of an organism. You will study them in detail in population ecology,


Populations of different species (plants and animals) living in the same habitat form a community. Communities are dynamic collections of organisms, in which one population may increase and others may decrease due to fluctuation in abiotic factors. Some communities are complex and well interrelated, other communities may be simple. In a simple community, any change can have drastic and long-lasting effects.

The foregoing account makes it clear that an organism can be studied at different levels of the organization. It can be studied at subatomic, atomic, molecular, macromolecular, organelle, cell, tissue, organ and organ system level. We can also look at it as an individual, as a part of the population of similar individuals, as a part of a community that includes other populations and a part of the community of an ecosystem which includes abiotic factors as well as living organisms, Fig. 1
The organisms, interaction can take many shapes. It may be predation, parasitism, commensalism, mutualism and competition.

Living World in Space:

Living world of today is enormous in size. It has been reproducing and evolving since the time of its origin on this planet. Today almost all parts of the world abound in living organisms. The distribution of organisms in space can be studied through biomes.

A biome is a large regional community primarily determined by climate. It has been found that the major type of plant determines the other kind of plants and animals. These biomes have, therefore, been named after the type of major plants or major feature of the ecosystem. The major biomes of the world you will study in the chapter of ecology.


The time of origin of life on this planet, various organisms were evolved and dominated this planet during various periods of geological time chart. This has been found by the evidence obtained from the discovery and study of fossils which allows biologists to place organisms in a time sequence. As geological time passes and new layers of sediments are laid down, the older organisms should be in the deeper layer, provided the sequence of the layers has not been disturbed.

It is possible to date/age rocks by comparing the amounts of certain radioactive isotopes they contain. The older sediment layers have less of these specific radioactive isotopes than the younger layers. A comparison of the layers gives an indication of the relative age of the fossils found in the rocks. The fossils found in the same layer must have been alive during the same geological period.

You can have an idea about the temporal distribution of various forms of life both plants and
animals in the various geological periods ( fig.1.3)

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The science of biology has been helping mankind in many ways in increasing food production; in combating diseases and in protecting and conserving the environment. Biological advances in the field of food and health have resulted in a high standard of living.

Plant production has been tremendously increased by improving existing varieties and developing new high-yield and disease – resistant varieties of plants and animals used as food.

Plant and animal breeders have developed, through selective breeding, using the principles of genetics, new better varieties of wheat, rice, corn, chicken, cow and sheep. Poultry breeders have developed broilers for getting quick and cheap white meat. Genes for disease resistance and other desirable characters are introduced into the plant, using the techniques of genetic engineering. Such transgenic plants (plants having foreign DNA incorporated into their cells) can be propagated by cloning (production of genetically identical copies of organisms/cells by asexual reproduction)
using special techniques, tissue culture techniques.

Plant pathogenic fungi and insect pests of crops which weaken the plants. So, Use of these chemicals poses toxicity problems for human beings as well as environmental pollution. There are chances of insects becoming resistant to the effect of these chemicals. Biological control (control by some living organisms) eliminates all such hazards.  That compete with or even eat them up. An aphid that attacks the walnut tree is being controlled biologically by a wasp that parasites this aphid. Learn More Here


The industrialization has helped mankind to raise the standard of living. It has destroyed our environment. These effluents frequently contain the sizeable amount of certain very toxic even carcinogenic materials. Heavy metals like lead from automobiles, chromium from tanneries, are playing havoc to human health. Environmental pollution has reached an alarming level in some countries.

Biology has helped mankind in attracting attention to this problem and the biologists are striving to end the solution to set this environment right wherever it has deteriorated: Biologists have already asked for the treatment of industrial effluents to be made obligatory. Similarly, Several ways of bio-remediation (removal or degradation of environmental pollutants or toxic materials by living organisms) are also under investigation. For example, algae reduce the pollution of heavy metals in the environment. Biologists are also working out the list of endangered species of plants and animals.

They have stressed the needs for their protection. Environmental pollution is a national problem in Pakistan.  The Pollution is spreading in rivers and canals with the mixing of city sewage and industrial wastes. The life in the fresh water of Pakistan is towards decline.  We need to take protective measures as early as possible. In cities, the silencer from cars is adding lead to the environment. There is a need for lead-free petrol to reduce pollution.