The plasma membranes of some plant cells use transport proteins to move protons out of the cell against their concentration gradient (from areas of low proton concentration to areas of high proton concentration). this is an example of:

The greenhouse effect a. something that has occurred for millions of years b. an unnatural phenomenon caused by humans that maintains earths temperature range. c. the results of the differences in the angle of the suns rays d. an unnatural phenomenon that causes heat energy to be radiated back into the atmosphere