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In biology,a cell is the basic unit of life, a group of similar cells form a tissue, and multiple tissues working together create an organ, essentially building the structure of a living organisms,meaning cells are the building blocks, tissues perform specific functions by grouping similar cells together, and organs are complex structures made up of different tissue types to carry out specialized tasks within the body. 

Examples

• Cells: A red blood cell is a cell in the circulatory system. 

• Tissues: Cardiac muscle in the heart is a tissue. 

• Organs: The heart is an organ made up of multiple tissues. 

• Organ systems: The circulatory system is an organ system that includes the heart, blood vessels, and other organs.

Using the circulatory system as an example, a cell in this system is a red blood cell, the heart's cardiac muscle is a tissue, an organ is the heart itself, and the organ system is the circulatory system. An organism is made up of four levels of organization: cells, tissues, organs, and organ systems.

The order of cell, tissue, organ, organ system, and organism increasis the complexity in the human body. 

Order of organization 

1. Molecules: Combine to form cells

2. Cells: Combine to form tissues

3. Tissues: Combine to form organs

4. Organs: Combine to form organ systems

5. Organ systems: Combine to form organisms

   
   

Explanation

Each level of organization is built from the lower level. For example, muscle cells combine to form muscle tissue, and different tissues combine to form organs like the kidney. Organs can then work together to form systems like the urinary system. 

Examples —A muscle fiber is a cell,Cardiac mucles is a tissue,and the heart is an organ.

Some living things are made from a single cell, and are called unicellular