Answer:
Electrons Can Donate - 0
Electrons Can Receive- 2
Valence - 2
Explanation:
The electronic configuration of oxygen is 
. Thus in the p-orbital there is deficiency of two electrons. Hence, it can receive/share electron but it cannot donate electrons. This makes oxygen as a good electron acceptor. Since there is a deficiency of two electrons in the outermost shell of oxygen, the valence of oxygen in thus -2
There are many answers to your question but one way is the evaporation cycle.
The main difference between prokaryotes and eukaryotes is the presence of nucleus in eukaryotes and its absence in prokaryotes. Also, prokaryotes are unicellular, so one cell is a whole organism. On the other hand, eukaryotes are mainly multicellular, and thus more complex. More complex DNA is inside the nucleus in the eukaryotes which enables them to conduct a wider range of functions. Prokaryotes, as a single cell, have simple functions present.
All stages respiration occur in every single cell.
