This chapter opens with the very difficult assessment of life. A careful definition
is necessary to know how to search for the existence of life in the universe, especially on other
planets. Until the end of the twentieth century it was hardly proven from observations that
planets outside our Solar System exist. However, with the technological advancement we
know more than 1000 extrasolar planets, with the number increasing steeply because of the
new satellite missions (see chapter on extrasolar planets). The main problem however is, that
till now we only know about the existence of life on Earth. Could life evolve elsewhere under
completely different conditions than on Earth? Based on our knowledge on terrestrial life, we
will also give some basic information on how living cells function. We give an overview on how
life works on Earth by discussing photosynthesis, respiration, organic molecules, molecules
that are essential for life. The complex organic compounds such as carbohydrates, proteins,
amino acids are discussed and how they work together in a cell. Cell division is also reviewed
and it is demonstrated why this mechanism has evolved from a simple fusion process to such
a complex process like meiosis. The main message of this chapter is that life is complex,
based on complex molecules, reproduces itself and needs energy.
Keywords: Life; definition of life; prokaryotes; eukaryotes; cells; organic molecules; photosynthesis;
respiration; cell division; mitosis; meiosis
