Biological membranes are said to be selectively permeable. what does this term mean, and how does this affect the way the molecules are able to move through cellular membranes?

When lactate builds up in a runners muscles it causes a burning sensation what causes this to occur

Which best describes the nucleus of an atom a. it is where all of the particle s of the atom are located b. it is the negatively charged part of the atom c. it is where the electrons and the protons are located d. it is the part of the atom with the greatest mass

Which is a function performed by stem cells in the skin a. replacing lost skin cells b. making cells for the intestines c. growing new organs d. differentiating into brain cells plz apex