Through which primary process does sucrose enter the phloem tissue in plants?
Active transport with energy from ATP
Phloem loading is the process by which sucrose produced in mesophyll cells of leaves is transferred into the sieve tubes of the phloem for long-distance translocation to sinks.
Since sucrose moves against its concentration gradient from source cells to the phloem, it requires an energy-dependent mechanism. Companion cells use ATP to drive proton pumps, and the resulting proton gradient powers sucrose–H⁺ symporters that push sucrose into the sieve tubes.
Osmosis moves water, simple and facilitated diffusion follow concentration gradients, so none of these can push sucrose uphill. Hence, the correct answer is Active transport with energy from ATP.
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