Biology · Structural Organisation in Animals · NEET
The order from smallest to largest is: Cell to Tissue to Organ to Organ System (and then the whole Organism). A cell is the basic unit. Many similar cells make a tissue. Different tissues join to make an organ. Many organs together form an organ system. NEET often asks this exact sequence, so learn the direction: small to big.
A tissue is a group of SIMILAR cells doing ONE common function, for example muscle tissue. An organ is made of DIFFERENT tissues working together for a bigger function, for example the stomach has epithelial, muscular, connective and neural tissue. So a tissue is one cell type; an organ mixes several tissue types.
Yes. The cell is the first and most basic level of organisation because it is the smallest living unit of the body. In NCERT the hierarchy for a multicellular animal starts at the cell, then tissue, then organ, then organ system. A single-celled organism does everything inside one cell, so it has no tissues or organs.
An organ system is a group of organs that work together to carry out one major body function. For example, the digestive system includes the mouth, stomach, small intestine and liver, all working to digest food. The circulatory system includes the heart and blood vessels. Each organ has its own role, but together they complete one job.
As animals became larger and more complex, one cell could not do every job. So cells specialised and grouped into tissues, tissues into organs, and organs into systems. This is called division of labour: each level handles a specific task, making the whole body work more efficiently.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Cell, tissue, organ and organ system. A cell is the basic unit, similar cells form a tissue, different tissues form an organ, and organs together form an organ system.
The cell is the basic (lowest) level of organisation because it is the smallest living unit of the animal body.
Yes. The complete organism is the highest level, formed when all organ systems work together as one living body.
No. In unicellular animals a single cell performs all functions, so there are no tissues, organs or organ systems.
Division of labour means different cells, tissues and organs each do a specific job, so the whole body works in an organised and efficient way.
