The Levels of Organization

In order to understand the various phenomena of life, biologists study biological organization at different levels, which are as follows:

1. Subatomic and Atomic level

All types of matter are made up of elements and each element contains a single kind of atoms (‘a’: not, ‘tom’: cut). The atoms are actually made up of many subatomic particles.

The most stable subatomic particles are electrons, protons and neutrons. Out of the 92 kinds of elements that occur in nature, 16 are called bioelements. These take part in making the body mass of a living organism.


Protons and neutrons are located inside nucleus of atom while electrons orbit in energy levels (electron shells) around the nucleus. The number of electrons in the outermost shell determines the manner in which atoms react with each other.

Out of these bioelements;

Only six (O, C, H, N, Ca, & P) make 99% of the total mass.

Other ten (K, S, Cl, Na, Mg, Fe, Cu, Mn, Zn, & I) collectively make 01% of the total mass.

Percentage composition (by mass) of bioelements in the protoplasm of living organisms
Percentage composition (by mass) of bioelements in the protoplasm of living organisms

2. Molecular level

In organisms, bioelements usually do not occur in isolated forms rather they combine through ionic or covalent bonding The stable particle formed by such bonding is called as molecule or biomolecule.


A molecule is the smallest part of a compound that retains the properties of that compound.

An organism is formed by enormous number of biomolecules of hundreds of different types. These molecules are the building material and are themselves constructed in great variety and complexity due to specific bonding arrangements. Biomolecules are classified as micromolecules and macromolecules. Micromolecules are with low molecular weight e.g. glucose, water etc) and macromolecules are with high molecular weights e.g. starch, proteins, lipids etc.

3. Organelle and Cell level

Biomolecules assemble in a particular way and form organelles. The organelles are actually sub-cellular structures in the cells.

Each type of organelle is specialized to perform a specific function For example; mitochondria are specialized for cellular respiration and ribosomes are specialized for protein synthesis/ In this way, functions of the cell are accomplished by these specialized structures. It is an example of the division of labour within the cell.

In the case of prokaryotes and most protists, the entire organism consists of a single cell. In the case of most fungi, all animals and all plants, the organism consists of up to trillions of cells.

4. Tissue level

In multicellular organisms, similar cells (performing similar functions) are organized into groups, called tissues. We can define a tissue as a group of similar cells specialized for the performance of a common function. Each cell in a tissue carries on its own life processes (like cellular respiration, protein synthesis), but it also carries on some special processes related to the function of the tissue.

There are different types of plant tissues eg epidermal tissue, ground tissue, etc.. Animal tissues are also of different types eg. nervous tissue, muscular tissues etc.

5. Organ and Organ system level

In higher multicellular organisms more than one type of tissue having related functions are organized together and make a unit, called organ. Different tissues of an organ perform their specific functions and these functions collectively become the function/s of that organ. For example stomach is an organ specialized for the digestion of proteins and for storing food. Two major types of tissue are present in its structure Epithelial (glandular) tissue secretes gastric juice for the digestion of proteins. Muscular tissue performs contractions of stomach walls for grinding of food and moving food to posterior end) So two tissues perform their specific functions, which collectively become the function of stomach.

The next level of organization in multicellular organisms is the organ system level. Different organs performing related functions are organized together in the form of an organ system. In an organ system, each organ carries out its specific function and the functions of all organs appear as the function of the organ system For example, digestive system is an organ system that carries out the process of digestion. Major organs in its framework are oral cavity, stomach, small intestine, large intestine, liver, and pancreas. All these organs help in the process of digestion.

The organ system level is less complex in plants (e.g. root system) as compared to animals. This is due to a greater range of functions and activities in animals than in plants.

6. Individual level

Different organs and organ systems are organized together to form an individual or organism. In organism, the functions, processes and activities of various organs and organ systems are coordinated. For example, when a man is engaged in continuous and hard exercise, not only his muscles are working but also there is an increase in the rate of respiration and heart beat. This accelerated rate of respiration and heart beat supplies more oxygen and food to the muscles which they need for continuous work.

7. Population level

Biologists extend their studies to the population level where they study interactions among members of the same species living in the same habitat. A population is defined as a group of organisms of the same species located at the same place, in the same time. For example, human population in Pakistan in 2010 comprises of 173.5 million individuals (according to the Ministry of Population Welfare, Government of Pakistan).

A species is defined as a group of organisms capable of interbreeding and producing fertile offspring.

Habitat means the area of the environment in which  organism lives.

8. Community level

A community is an assemblage of different populations, interacting with one another within the same environment. A forest may be considered as a community. It includes different plant, microorganisms, fungi and animal species.

Communities are collections of organisms, in which one population may increase and others may decrease. Some communities are complex e.g. a forest community, a pond community etc. Other communities may be simple e.g. a fallen log with various populations under it. In a simple community number and size of populations is limited. So any change in biotic or abiotic factors may have drastic and long lasting effects.

9. Biosphere level

The part of the Earth inhabited by organisms’ communities is known as biosphere. It constitutes all ecosystems (areas where living organisms interact with the nonliving components of the environment) and is also called the zone of life on Earth.

Cellular Organizations

All the organisms have been divided into five major groups i.e prokaryotes, protists, fungi, plants and animals. All organisms are made of cells. There are two basic types of cells. The organisms in first group are made of prokaryotic cells while all other groups have eukaryotic cells.

levels of organization
levels of organization

Cells organize in three ways to make the bodies of organisms. Cells make unicellular, colonials and multicellular organizations and the organisms formed through these organizations are unicellular organisms, colonial organisms and multicellular organisms.)

In unicellular organisms, only one cell makes the life of an organism) All the life activities are carried out by the only cell. Amoeba, Paramecium, and Euglena are common examples.

Amoeba, Paramecium and Euglena
Amoeba, Paramecium and Euglena

In colonial type of cellular organization, (many unicellular organisms live together but do not have any division of labour among them). Each unicellular organism in a colony lives its own life and does not depend on other cells for its vital requirements Volvox is a green alga found in water that shows colonial organization. Hundreds of Volvox cells make a colony.

Volvox colony
Volvox colony

In multicellular organization, cells are organized in the form of tissues, organs and organ systems. Frog and mustard are the familiar examples of multicellular organization.

Mustard plant

Mustard plant (scientific name: Brassica campestris) is sown in winter and it produces seeds at the end of winter. The plant body is used as vegetable and its seeds are used for extracting oil. The organs of the body can be divided into two groups on the basis of their functions. Root, stem, branches and leaves are the vegetative organs, which do not take part in the sexual reproduction of the plant. Flowers are the reproductive parts of the plant because they take part in sexual reproduction and produce fruits and seeds.

Mustard plant
Mustard plant


Frog (scientific name: Rana tigrina) shows the multicellular organization. The body is made of organ systems and each organ system consists of related organs. All the organs are made of specific tissues (epithelial, glandular, muscular, nervous etc). Some organs and organ systems of frog have been described in the practical activity given next.

Anatomy of dissected frog
Anatomy of dissected Frog

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