Levels of Organization in an Organism

Order of Hierarchy 

Cells combine to form tissues. tissues combine to form organs. organs combine to form organ systems and organ systems combine to form organisms. The order of hierarchy shows the level of organism formation, in a structured and organized way. 

Cell – The cell is the fundamental structural and functional unit of all living beings. Our body is made up of hundreds and thousands of cells. It is the smallest part of the body that can exist independently and perform essential functions of life. Each cell has its own life. Old and weak cells in our body continually die and are replaced by new cells.  

Tissues – The cells combine to form tissues. Tissues work together to perform specific tasks. Prokaryotes are one-celled; hence it does not possess tissue.  

Organ – Tissues work together to form an organ. There are different organs present in different organs. 

Organ system – For a body to perform body functions properly all the organs should work together.  

Organisms – A living thing that has an organized structure and can reproduce, grow, adapt, and maintain homeostasis. An organism is classified in many ways. Based on the number of cells, organisms are classified into unicellular and multicellular. Unicellular organisms in which organisms are made up of only one cell, while in a multicellular organism are made up of more than one cell, plants and animals are multicellular. On the basis of the presence of a well-defined nucleus, they are classified as prokaryotes and eukaryotes. Eukaryotes have a well-defined nucleus and prokaryotes have no well-defined nucleus. 

Levels of Organization  

The levels of organization in organisms help us to understand components easier to understand. 

Example: Club cell – Epithelial tissue – Lung (Organ) – Respiratory System (Organ system) 

There are four levels of organization in an organism: 

1. Cells – The cells may be prokaryotic or eukaryotic. The prokaryotic cell means primitive cells in which there is no well-defined nucleus, a single length of only DNA (deoxyribonucleic acid) and no other cell organelles except small ribosomes are present. In a eukaryotic cell, there is a well-defined nucleus with a nuclear membrane, chromosomes containing DNA wound around certain proteins and larger ribosomes with several organelle-like mitochondria, Golgi apparatus, endoplasmic reticulum etc. are present. Organisms such as bacteria, blue-green algae etc. are examples of prokaryotes and animals and plants are eukaryotes. The plant cells and animal cells also have differences in their structure. A plant cell has a definite cell wall made up of cellulose and it usually contains plastids. While an animal cell has no cell wall and do not contain plastids. 

  Examples: Red blood cells carry oxygen to body tissues.  

Tissues – Tissues are different in different organisms. Epithelial tissue, Connective tissue, Muscular tissue, and Nervous tissue are animal tissues. The epithelial tissues cover the surfaces of organs. It also performs the function of secretion, excretion, and absorption. The connective tissues give shape to organs and hold them in place. The muscular tissue helps in the movement of body parts. The nervous tissue is responsible for the control and coordination of various activities in the body. While in plants there are four tissue systems such as epidermis, ground tissue, vascular tissue, and the meristematic tissue. The epidermis forms the outer layer of stems, roots and leaves and it protects plant tissues and prevents loss of water. The ground tissues and vascular tissues are present in the stems, roots and leaves and the ground tissues make up bulk of plant mass while in vascular tissues the xylem transports water and phloem transports sugar. The xylem is made up of dead cells and is present in the center of the vascular bundle. The phloem is mainly made up of living cells and is present on the outer side of the vascular bundle. The meristematic tissues are present in the tip of shoots and roots, buds and as a ring around the stem in woody plants. 

Organ – In animals there are many organs. In humans’ muscles, heart, lungs, skin, hair, nails etc. are organs. But in other organisms there are other organs also like scales in fish and feathers in birds etc. The organs of plants can be divided into two. They are vegetative which includes roots, stems, leaves and reproductive which include flower, seed, and fruit.

Organ system – The organ system present in a human body are musculoskeletal system, digestive system, respiratory system, urinary system, male and female reproductive system, endocrine system, circulatory system, nervous system, and integumentary system. Similarly, the organ systems present in plants are shoot system and root system. The major organ in respiratory organ system is lungs, it takes in oxygen and removes carbon dioxide. The shoot system consists of stem, leaves and reproductive parts of plants such as flowers and fruits. The root system supports the plants from underground and helps absorb water and minerals.  

Organ System and Organs Involved 

• Cardiovascular system: In cardiovascular system the pumping and channeling blood to and from the body and lungs with heart, blood, and blood vessels. 
• Excretory system: In excretory system, organs such as kidneys, ureters, bladder, and urethra are involved in excretion of urine and fluid and electrolyte balance. 

• Digestive system: In which digestion and processing food with digestive organs such as salivary glands, esophagus, stomach, liver, gallbladder, pancreas, intestines, colon, rectum, and anus. 

Examples of Cell to Organ System