Cells are the basic unit of life. Humans are multi-celled complex organisms that require each part to work together. Every organ and system of the body depends on each cell to perform three major functions. These functions are vital to the life of the cell and the life of the organism.
Transport of Molecules
Cells need nutrients in order to survive. Since each cell is surrounded by a membrane that acts as a barrier to most molecules, it must transport the molecules it needs. There are two types of transport: passive transport and active transport.
Passive transport, according to faculty at Estrella Mountain Community College, requires no energy from the cell. Small molecules such as oxygen, carbon dioxide and water are able to enter the cells through the process of diffusion. Diffusion is the flow of a molecule from the area of higher concentration to the area of lower concentration. For instance, carbon dioxide is produced in the cell as a by-product of normal cell functions. Since the concentration is high inside the cell, the carbon dioxide diffuses out the cell membrane.
Active transport requires the cell to expend energy in order to transport larger molecules into and out of the cell. There are two main types of active transport; exocytosis moves molecules out of the cell while endocytosis moves molecules into the cell.
Conversion of Energy
In order to perform its normal functions, a cell needs energy. Animal cells produce energy through a process called cellular respiration. The ultimate source of energy for living cells is adenosine triphosphate, or ATP. ATP is a specific type of nucleotide, which is the building block of deoxyribose nucleic acid, or DNA. Since animals do not eat ATP, cells must convert the nutrients from foods, such as proteins, fats and sugars, into usable ATP.
Food is broken down to the basic units, amino acids and glucose, in the digestive system. The nutrients are then absorbed into the bloodstream through the intestinal wall. The basic units are transported into the cells where they undergo several processes including glycolysis and aerobic (meaning it requires oxygen) respiration. The end result is the conversion of the basic units into ATP to be used as energy.
In order for cells, and ultimately the organism, to survive the cell must reproduce to continually replenish those that die. Cellular reproduction, according to ThinkQuest.com, is necessary to allow a species to increase the population and to introduce genetic variation within the species.
There are two types of cellular reproduction. Most cells reproduce through the process of mitosis in which one cell splits to form two identical daughter cells. Meiosis is the process that the sex cells, the eggs and sperm, use to reproduce. During meiosis one diploid cell---a cell that contains two sets of chromosomes---splits to form four haploid cells which are cells that contain only one set of chromosomes.