A cell is the basic unit of life, being the smallest unit of life that can carry on all life processes, including maintenance, growth, replication, and self-repair.
The cell membrane is the barrier that maintains the essential balance between the individual distinctness of the cell and communal interactions. As such, it functions to retain key components of the cell and to keep out toxic or unwanted substances, while selectively controlling the flow of nutrients and biochemical signals into the cell.
According to Unification Thought, a cell represents an individual truth body, being composed of united entities engaged in giving and receiving. For the eukaryote cell, the individual body is composed of the unity of the cell nucleus and the cytoplasm, all encased by the cell membrane. The cell is but one level of individual truth bodies, with nature having such levels, from smallest to largest, of elementary particle, atom, molecule, DNA, chromosome, cell nucleus, cell, human body, person, family, nation, Earth, solar system, galaxy, universe, and great macrocosm. Each of these become an entity based on a harmonious relationship of elements on the lower level, and in turn help to constitute the higher level, giving an receiving of elements drawn from the lower levels.
Every entity, that is every individual truth body, exhibits a purpose for the whole and purpose for the individual. The purpose for the whole means that by which the individual contributes to the preservation and development of the larger entity, and the individual purpose refers to the need to support an individual's own multiplication, development, self-preservation, and self-strengthening. These purposes for the individual and the whole are not independent but interdependent. For example, in the cells of a multicellular organism, each cell provides a useful function for the body as a whole. A cell's function may be to convert sugar to ADP energy, attack foreign invaders, or produce hormones. A cell in the epithelial tissue of the stomach may secrete the enzyme pepsin to help with digestion. The cell's purpose of providing pepsin to the body is harmonized with the body's purpose of maintenance, development, and reproduction. The body, on the other hand, supports the individual cell and its individual purpose by providing food, oxygen, and other necessary materials, and by transporting away the toxic waste materials. Each cell actually depends on the other cells in the body to perform their functions and thus keep the body in proper functioning order.
In this great circle of life, the cell membrane plays a key role in maintaining the integrity of the cell and regulating its relationship with the universe beyond.