How is sucrose loaded from mesophyll cells into the phloem sieve-tube elements?

1. Photosynthates such as sucrose are made in the mesophyll cells of photosynthesising leaves. 2. They travel through cytoplasmic channels called plasmodesmata to reach the phloem sieve-tube elements. 3. Loading into the phloem moves sucrose against its concentration gradient, and that requires ATP. 4. The transport uses the electrochemical potential of the proton gradient, coupled to sucrose uptake by a carrier protein. 5. Passive movement down a gradient would not concentrate sugar in the sieve tube, so it could not build the loading pressure. 6. Bulk flow is what happens after loading, moving sap along the tube rather than loading it in the first place. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 30 "Plant Form and Physiology", section 30.5 Transport of Water and Solutes in Plants_