The buildling blocks of matter (elements and molecules) form the basis for increasingly complex substances that make up living organisms. Simple molecules combine to form more complex macromolecules which in turn are the building blocks for the membranes and organelles that make up living cells. In order of increasing complexity, the list below shows the different structural elements that together make up an animal:
- atoms
- simple molecules
- macromolecules
- membranes
- organelles
- cells
- tissues
- organs
- organ systems
- animal
The cell is the basic unit of life. It is within the cell that the chemical reactions necessary for maintenance and reproduction of life take place. There are two types of cells, prokaryotic cells (single-celled organisms that do not contain a nucleus) and eukaryotic cells (cells that contain a membraned nucleus and organelles that perform specialized functions).
Eukaryotic cells have the following basic components:
- A plasma membrane which forms the outermost boundary layer of the cell, separating the cells internal processes from the external environment.
- Cytoplasm which consists of the semifluid substance cytosol and organelles.
- A nucleus which contains the chromosomes inside a nuclear membrane.
During the development of an animal, cells differentiate so they can perform specific functions. Groups of cells with similar specializations that form a common function are referred to as a tissue. Organs are groups of several tissues that function together. Examples of organs include kidneys, spleen, liver, heart, lungs. Organ systems is a group of organs that work together to perform a function. Examples of organ systems include skeletal, muscular, nervous, digestive, respiratory, reproductive, endocrine, circulatory, and urinary systems.
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