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:
- simple molecules
- organ systems
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.