Levels of Biological Organization (FSC-Biology) Part 1

Hundreds of chemical reactions are involved in maintaining 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 bioelements. In the human body only six bio-elements account for 99% of the total mass.

Biology the six commonest bioelements account for 99% of body mass.


Oxygen 65%

Carbon 18%

Hydrogen 10%

Nitrogen 3%

Calcium 2%

Phosphorus 1%

Other bioelements include (about 1%) – potassium (0.35%), sulphur (0.25%), chlorine 0.15%), sodium (0.15%), magnesium (0.05%), iron (0.004%), copper (trace), manganese (trace), zinc (trace),

Fig.  1.1 percentage composition of bioelements by mass of a human being.

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 bioelements have special properties which make them particularly appropriate as basis for life.

Biological organization is not simple. It has 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 sub atomic and atomic particles to the organism itself and beyond which the study of community, population and entire world are included.

Biological organization can be divided into the following levels.


That part of earth inhabited by living organisms; includes both the living and the nonliving



A community together with its nonliving surroundings



Two or more populations of different species living and interacting in the same area



Members of one species in habiting the same area



Very similar, potentially interbreeding organisms


Multicellular organism

An individual living thing composed of many cells


Organ system

Two or more organs working together in the execution of s specific body function



A structure normally composed of several tissue types that from a functional unit



a group of similar cells that perform a specific function



The unit of life



A structure within a cell that performs a specific function


Micro-molecules and macro molecules


A combination of atoms



The smallest particle of an element that retains the properties of that element


Subatomic particle

Particles that make up an atom



Fig 1.2 levels of organization

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