The hierarchy of complex biological structures and systems that define life is called level of biological organization. These levels are as follows.

  1. Subatomic and Atomic level: An Atom is the smallest entity of matter that can enter into a chemical reaction. An element is the substance that can not be further simplified into other substances by physical and chemical means. Atoms of same elements are alike. Atoms further consists of subatomic particles protons, neutrons and electrons.

BIOELEMENTS: The elements that make the body mass of living organisms are called bioelements.

NUMBER OF BIOELEMENTS: Out of 92 elements, 16 are called bioelements. 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.

Role of bioelements: Mainly carbon, hydrogen and oxygen make most structures such as, sugars, starches and other energy molecules while nitrogen, phosphorus and sulphur make proteins.
Other elements are needed in less quantity. E.g. Iron is essential for haemoglobin (red blood cells), calcium for teeth and bones. Sodium and potassium are important for sending messages along nerves. Zn work as a coenzyme. Magnessium help in photosynthesis, etc.
2) M0lecular level: Atoms combined to form molecules.

Biomolecules: molecules that are present in living organisms are called biomolecules. Biomolecules are of two types.

  1. Micromolecules: molecules with low molecular weight are called micromolecules. e.g. glucose, water etc.
  2. Macro molecules: Molecules with high molecular weights are called macromolecules. e.g. starch, proteins, lipids etc.

Criterion for classifying biological molecules: Biological molecules are classified on the basis of molecular size. The small sized molecules are placed in micromolecules while large sized molecules are placed in macromolecules. In short words the micro molecules combined to form macro molecules.

Difference between molecule and biomolecule:

              Molecule             Biomolecule
A molecule is the smallest unit of substance usually non living. Biomolecule is the smallest particle of a living substance.
Molecules may be organic or inorganic. Biomolecules are organic.
Molecule can consist of any element found in periodic table. Biomolecules consists of carbon, hydrogen, oxygen, nitrogen, sulphur, calcium and phosphorous mainly.
Molecules combined to form compounds. These may be ionic, covalent, dative, etc. Biomolecules may be categorized as monomers or polymers. These are organic in nature having covalent linkage.


  • Organelle level and Cell level:

Organelle level: The building blocks of a cell are called organelle. Biomolecules (Macro molecules) combined to form organelles. Organelles are basically sub cellular structures. Each and every organelle perform a specific function. For example, mitochondria is involved in respiration, chloroplast is responsible for photosynthesis, etc.

Cell level: Cell is the basic structural and functional unit of life. Organelles combined to form a cell. Some organisms consists of only one cell like bacteria, protists, etc. while other consists of many cells. E.g;Human.

4) Tissue level: Tissue a collections of similar cells working together to perform a specific job. E.g, epithelial tissues, connective tissues, etc.

5) Organ and Organ system level:

Organ: Two or more types of tissues work together to complete a specific task is called organ. E.g, heart, stomach, brain, etc.

Organ system (system) : several organs working in union to achieve a common goal is called organ system. E.g, digestive system, cardiovascular system, respiratory system, etc.

Protons and neutrons are located inside nucleus of atom while electrons orbit in energy levels (electrons shells) around the nucleus. The number of electrons in the outermost shell determines the manner in which atoms react with each other.
A molecule is the smallest part of a compound that retains the properties of that compound.


6) Organism level or Individual level :  several organ systems functioning together forms an organism. E.g, human, dog, tree, frog, mustard,etc.
7) Population level: A group of organisms of the same species located at the same place, in the same time and sharing the same resources is called population. Individual organisms of the same species working together to achieve a common goal. Examples: population of human, population of mustard, population of frogs, etc.

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

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.

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

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.

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