Animal Cells, Tissues, Organs and Organ Systems

A cutaway view of a typical animal cell. Wikimedia Commons

The building blocks of all matter, atoms and molecules, form the substrate for the increasingly complex chemicals and structures that make up living organisms. For example, simple molecules like sugars and acids combine to form more complex macromolecules, such as lipids and proteins, which in turn are the building blocks for the membranes and organelles that make up living cells. In order of increasing complexity, here are the basic structural elements that, taken together, make up any given animal:

  • atoms
  • simple molecules
  • macromolecules
  • membranes
  • organelles
  • cells
  • tissues
  • organs
  • organ systems
  • animal

The cell, toward the middle of this list, is the basic unit of life. It is within the cell that the chemical reactions necessary for metabolism and reproduction take place. There are two basic kinds of cells, prokaryotic cells (single-celled structures that do not contain a nucleus) and eukaryotic cells (cells that contain a membranous nucleus and organelles that perform specialized functions). Animals are composed exclusively of eukaryotic cells, though the bacteria that populate their intestinal tracts (and other parts of their bodies) are prokaryotic.

Eukaryotic cells have the following basic components:

  • A plasma membrane that forms the outermost boundary layer of the cell, separating the cell's internal processes from the external environment.
  • Cytoplasm, which consists of the semifluid substance called cytosol as well as various organelles.
  • A well-demarcated nucleus, which contains the animal's chromosomes inside a nuclear membrane.

During the development of an animal, eukaryotic cells differentiate so they can perform specific functions. Groups of cells with similar specializations, and which perform a common function, are referred to as tissues. Organs (examples of which include lungs, kidneys, hearts and spleens) are groups of several tissues that function together. Organ systems are groups of organs that work together to perform a specific function; examples include skeletal, muscular, nervous, digestive, respiratory, reproductive, endocrine, circulatory, and urinary systems. (For more on this subject, see The 12 Animal Organ Systems.)