There are multiple reasons for the resting membrane potential of a cell to be at -70 mV. The most important is that 1) the cell pumps Na+, creating an ion concentration gradient between the intercellular fluid and the extracellular fluid (aka depolarization), and 2) the cell membrane is much more permeable to K+ than to Na+, meaning it allows a lot more potassium to enter the cell than sodium (known as hyperpolarization).
Comments
Numbers
Instead of listing the reasons numerically, it would serve you better to write them out as separate sentences to give your paragraph more body and allow the reader to read it better.
Abbreviations
The paragraph makes sense to one who has a knowledgable science background, yet not to an audiance who does not know what a Na+ or K+ ion is. I suggest stating the full name of these ions and introducing the abbrevian in parenthasis during the first use in order to use the abbreviations throughout the rest of the paragraph. This will ensure that your reader comprehends your writing.