Have you heard the words cell, tissue, organ, etc. in biology? These names gradually proceed towards complexity and sophistication. Is there any systematic graduation from subcellular to cellular to multicellular levels? Cells are the simplest among these, and organisms that may be made up of trillions of different types of cells, are the most complicated. What is such an arrangement called? This arrangement is known as levels of organisation.

Levels of Organisation

Biological systems range from simplest unicellular organisms to most complex multicellular giant organisms. Any entity to be called a level of organisation must fulfil two conditions-

1. The parts that form a higher-level whole must interact with each other.
2. They must be homologous with organisms in a-living state.

The levels of organisation can thus be arranged in a hierarchical fashion which is based on complexity seen in living organisms. On a broader scale, the levels of organisation can be categorised into biological and ecological groups. Biological levels of organisation are seen in living organisms whereas ecological levels are those used in ecological systems. Let’s look at each of these levels.

Acellular or Pre-cellular Level

1. One can start at the acellular level, which has atoms, molecules and biological macromolecules.
2. At this stage, these entities exist and carry out reactions, but those are not considered living.
3. They make the very core of all biological systems, but still, life is not present in them.
4. \(C, H, O, N, P,\) and \(S\) are the six elements that are biologically most important. Besides these, many other elements are needed in this most basic level, like \(Fe, Cu, Ca, Mg, Mn, Se, Ni, K, Zn, Cl,\) etc.
   Examples: Oxygen atom, Oxygen molecule, Phosphate group, Proteins, RNA, DNA

Subcellular Level

1. At this stage, the biological complexes or macromolecules assemble to form cell organelles.
2. Each cell organelle has a different function, structure and characteristics. Their location inside the cell is also different.
3. Cell organelles have no independent existence.
   Examples: Nucleus, Ribosomes, Mitochondria, Chloroplast, Vacuoles, Golgi complex

Cellular Level

1. The cell organelles together make a cell that has life in it.
2. Every single cell is the complete unit and carries out all the vital functions such as respiration, digestion, excretion etc. thus, it forms the basic unit of life.
3. It is capable of independent reproduction. Hence many authors prefer to start the levels of organisation from the cellular level.
4. A cell can be visualized as an ensemble of many cell organelles that work together.
5. A cell can be prokaryotic or eukaryotic based on the presence or absence of the membrane-bound nucleus.
6. Many life forms are unicellular, i.e., made up of a single cell, like Paramoecium, Euglena, Diatoms, etc.
7. Some organisms like sponges exist at the cellular level of organisation despite their multicellularity. In these organisms, cells are very loosely organized.
8. Many colonial organisms are unicellular and are at the cellular level of organisation.
   Examples: Bacteria (prokaryotic), Amoeba, Euglena, Paramoecium (eukaryotic, solitary), Volvox (eukaryotic, colonial)

Tissue Level

1. It is believed that during evolution, the unicellular organisms living in colonies learnt to share the work.
2. Groups of unicellular organisms in the colony started carrying out specific functions. Thus, the system slowly evolved to a stage where the distribution of work became necessary for survival.  At this stage, multicellularity evolved.
3. In multicellular organisms, a group of cells specialized for a particular function is called a tissue. Thus came into existence the tissue level of organisation.
   Examples: Epithelial tissue, connective tissues, muscle tissue and nervous tissue in animals and epidermal tissue, ground tissue and vascular tissue in plants.

Organ Level

1. Tissues are a specialized group of cells performing one function.
2. But as the complexity level increases, many types of tissues are assembled to form an organ.
3. An organ can be defined as a structure made up of different tissues that work together to perform physiological functions.
4. Thus, an organ performs a more complex function than tissue. Organs can be hollow or solid.
   Example: Heart, Lung, Kidney, Stomach, Ovary, Flower, Bud, Leaf etc

Organ-System Level

1. The organ system level of organisation is where many organs work together to perform a specific physiological process.
2. There are eleven organ systems in the human body, each of which has a specific function.
3. While most organ systems perform a single physiological process, some processes require more than one organ system.
   Example: Digestive system contains different organs such as the oesophagus, stomach, intestines and glands and all together perform the process of digestion. Blood pressure is controlled by three organ systems- the circulatory system, the renal system and the nervous system.

Organism Level

1. Many organ systems work together in an organism.
2. An organism is a more complex level of organisation that shows properties or attributes different from those of its component systems.
3. An individual organism is the basic unit of ecological hierarchy.
4. In ecology, an organism is a living being that has the independent ability to work and reproduce.
   Examples: plant, animal, insect, bacteria etc

Population Level

1. This is the next level of organisation in ecological schemes.
2. A population is a group of organisms of single species that live together in the same habitat and interbreed.
3. The population has its own attributes different from those of individual organisms.
4. Sex ratio, age structure, growth rate, death rate, birth rate are few examples of population attributes.
   Examples: Honey bees in a comb, Ants in the anthill, People living in a village

Community Level

A more appropriate word in biology would be a biotic community.

1. A biotic community is a group of populations of different kinds of organisms sharing the same habitat.
2. Populations of many species co-exist, and they make the biotic community.
3. Each community has its own structure, and it depends on the factors such as roles played by each population, size of each population, type of habitat occupied by the community, diversity of species and interactions between the species of different populations etc.
4. Members of the community interact with each other and with the environment also.
5. Communities can be major and minor depending on the size and dependence.
6. Communities can also be natural and man-made.
   Examples: a tropical green forest is an example of a major community; a patch of lichen growing on a rock is a minor community. A Mixed crop farm is a man-made community.

Ecosystem Level

1. An ecosystem can be defined as a community of organisms interacting with the environment where constant exchange of materials takes place.
2. It is the functional unit of nature with complex interactions between biotic and abiotic factors.
3. Each ecosystem has two components: biotic and abiotic.
4. Every organism in an ecosystem is dependent on other species and elements in nature such that damage to one part affects the entire ecosystem.
5. Each ecosystem has four main functions as energy flow through the food chain, nutrient cycling, ecosystem development and ecological succession.
6. Ecosystems can be natural or man-made.
   Examples: Ponds, lakes, meadows, grasslands, crop fields, aquarium, garden

Biome Level

a. The terrestrial part of the biosphere is divided into many biomes based on characteristic features such as general soil type, climate, vegetation and animal life.
b. Important climatic factors are temperature and precipitation.
Examples: Tundra biome, Taiga grasslands. Temperate forests, Desert, Steppe grasslands.

Biosphere Level

a. The biosphere is the part of the earth where life exists.
b. It comprises of atmosphere, lithosphere and hydrosphere
c. The biosphere is absent at extreme poles, deepest parts of oceans and highest peaks of mountains.

Summary

The organisational hierarchy in biological systems exists from atoms to the biosphere. Each of these levels can also be further subdivided. For example, an atom has subatomic particles as protons, neutrons and electrons. The levels of organisation described above are widely used and accepted.  Each level in this hierarchy represents the organisational complexity of the unit. Such an organisational structure is used in many fields of science like evolution, medicine, pharmacology and ecology.

FAQs

Q.1. What is the significance of levels of organisation in biological systems?
Ans: Levels of the organisation tell us the increased complexity in biological systems. These levels are used in the science fields like medicine, pharmacology, ecology. Theories of evolution are also based on these levels.

Q.2. What are the \(7\) levels of organisation in living organisms?
Ans: The \(7\) levels of organisation in living organisms, from simplest to most complex, are biological complexes, cell organelles, cells, tissues, organs, organ systems and organisms.

Q.3. What do the levels of the organisation say?
Ans: Levels of the organisation is the hierarchy of complex structures present in biological systems. Each level in the hierarchy represents an increased complexity in the structure and organisation.

Q.4. What is the highest level of organisation?
Ans: In biological organisms, an organism itself is the highest level, whereas, in ecological levels of organisation, the biosphere is the highest level.

Q.5. What are the ecological levels of the organisation?
Ans: The ecological organisation starts with the most simple unit as an organism or individual, and the highest level is the biosphere. Ecological levels of the organisation are an organism, population, community, ecosystem, biome and biosphere.

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