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Levels of Organization
The parts of a multi-cellular organism can be divided
into five categories: cells, tissues, organs, systems,
organism. When we are organizing these parts, you
can consider them as levels or parts of a whole.
Organisms are made of multiple systems; each
system is composed of different organs; each organ
can be divided into different tissues; each tissues is
made up of various kinds of cells.
Level 1 - Cells
Cells are the basic structural unit of living things. Cells
can live independently - as a single-cellular organism,
or they can be part of an intricate networking of
specialized cells. Some examples of cell types in
humans are:
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blood cells
nerve cells
bone cells
Level 2 - Tissues
Groups of similar cells that work closely together are called tissues. These cells are similar in structure
and function. Together, they perform a certain activity. Humans have four basic tissues:
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connective tissue
epithelial tissue
muscle tissue
nerve tissue
Level 3 - Organs
Organs are groups of tissues working together to perform a small range of actions. The activities of
organs range from pumping blood through the body (the heart's job) to protecting the body from outside
elements (the skin's job). Examples of human organs are:
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stomach
lungs
liver
Heart
Kidney
Brain
Small intestine
Lungs
Large intestine
Level 4 - Systems
When multiple organs are interconnected, they have the ability to aid each other in regulating the broader
functions of the body. The human body has eleven organ systems:
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circulatory
digestive
endocrine
excretory (urinary)
immune (lymphatic)
integumentary (skin)
muscular
nervous
reproductive
respiratory
skeletal
Level 5 - Organism
Organism is the name of the independent living thing. It can carry out the basic functions of life.
Organisms can be made of a single cell or of multiple cells. The definition of "living thing" is still debated
in scientific circles (a virus, for example, is considered by some to be living, but others believe it lacks
some of the requirements to fit that description). In general, a living thing must be able to take in matter,
release energy from food, release waste, grow, and reproduce.