Living things are composed of cells, and the concept of the cell is the fundamental to understanding the phenomenon of growth. Minimally, a cell is made up of cytoplasm (the filling of the cell), DNA (genetic material of the cell) and a cell membrane that encloses the cell. Cells are the smallest units that satisfy all conditions necessary to be called living--they use energy, reproduce without aid of a host organism, pass on their genetic material, respond to their environment and are able to regulate their physical condition to adapt to the environment. Some structures, like viruses, share some but not all characteristics of cells; therefore they are not alive.
Growth of complex organisms occurs because of cell division. A cell begins its life relatively small. It grows larger and accumulates energy. It uses this energy to replicate it's own DNA so that there is DNA for two cells. Having replicated its DNA, the cell grows larger again and stores energy in preparation for division. Once it has sufficient energy, the cell divides and becomes two new cells. The process repeats.
Cells Become Specialized
Some organisms, like algae, are made of only a single cell, but these are not organisms that grow in the traditional sense, as a human being grows from embryo stage into a baby ready for birth. Complex living things, like plants and animals, need different types of cells to perform all the functions necessary for survival. All these cells originate from a single cell, but some cells become differentiated. The DNA of the cells remains the same but is expressed differently due to modified gene expression--some genes are turned on, while others are turned off.
Living things vary greatly between each other in their structures and life cycles. Depending upon the species, an individual may mature in a matter of days or many years. But in all living things, growth is regulated by hormones, and often must occur during certain stages of maturity or not at all. In humans for instance, full height must be obtained before the end of puberty. Also, each type of cell has a finite number of times it may divide--around 50 for most cells.
Each type of cell has its own life expectancy--nerve cells can live more than 100 years, blood cells only about 120 days. Some types of cells, like skin cells, blood cells and liver cells, are replaced by like cells when they die. Other cells, like brain cells, do not replace themselves when damage occurs.
For all cells, the effects of the environment, disease and trauma inhibit cell replacement; and so we experience the phenomenon of aging. However, our understanding of cells continues to grow. We now know that certain portions of DNA called telomeres limit how many times a cell may divide, giving a cell generational age as well as individual age. It is thought that telomeres define the upper limit of human lifespan at about 120 years.