Which organelle do you think is the most important and why

 All organelles are important. However, to answer your question...the most important organelle in my opinion is the nucleus. The nucleus is like the floor general or the point guard on a basketball team. It contains most of the cell's DNA, directs and controls cell function through gene expression, and mediates DNA replication during cell cycles. 

Note: Not all cells have nuclei. An example of this are the red blood cells, that lack nuclei as well as mitochondria.

Each calcium ion has a charge of +2, which can also be written as ca2+. if those ions move out of the neuron and no other ions are moving, the neuron would lose the positive charge associated with those ions and become more negatively charged.

Solar thermal energy and solar electric energy