The level of organization for a human is as follows:
organelles < cells < tissues < organs < organ systems < organism
- Organelles are specialized systems or units found inside a cell. Some examples include the ribosome which produces proteins, and mitochondria which provides cell energy.
- A cell is the basic unit of life. It consists of organelles. A cell is the smallest unit of life that is classified as a living thing.
- A group of cells constitute a tissue. It is a group of similar types of cells - from the same origin - that carry out a special/specific function. Examples of tissues are muscles and connective tissues (e.g. blood).
- A group of tissues will constitute an organ. It is a group of tissues that make up a structure that serves a function. An example is the heart - it pumps blood supply throughout the body. A heart consists of nervous tissues, muscle tissue, and blood tissue.
- When a group of organs function, usually in succession, towards a specific goal, you have an organ system. For example, the respiratory system - which consists of the lungs, trachea, nasal cavity, diagphram, among others - is responsible for the gas exchange (oxygen-carbon dioxide) in mammals.
- Lastly, an organism is any living system whose basic unit is a cell, and consists of tissues, organs, and organ systems to function and survive. [Here, we consider multicellular organisms. Otherwise, the level of organization will end at a cellular level - unicellular means single cell.]