<span>Early pre-cambrian period the atmosphere consisted only of nitrogen and carbon dioxide without oxygen.
<span>A
Prokaryotic cell does not contain a nucleus. It only contains one
chromosome and is a single-celled organism. It was the only form of life
on earth for millions of years. Examples of a Prokaryotic cell are the
different types of bacteria present today.
</span><span>Autotrophs
obtain energy by the process of photosynthesis. Any living organism
need energy to survive and autotrophs are no different. Two types of
autotrophs are present and they are Photoautotrophs and Chemoautotrophs.
The Photoautotrophs rely on the energy produced during the process of
photosynthesis and the Chemoautotrophs get their energy from most of the
inorganic substances. Phtoautotrophs actually get their energy from the
sunlight and then convert it into reusable form. In case of
Chemoautotrophs ammonia is an important source of energy for them.</span></span>
One easy way to measure the "size" of a ball is how far it is from side to side. Your tennis ball example is a good one: a typical tennis ball is about 2.5 inches form side to side.
