Explain how active loading of sucrose into sieve elements accounts for the following observations:

a. The phloem sap has a relatively high pH of about pH 8.

b. The inside of sieve element/companion cell units is negatively charged relative to the outside. (There is a difference in electrical potential across the cell surface membrane, with a potential of about –150 mV on the inside.)

c. ATP is present in relatively large amounts inside sieve tubes.