For sucrose to be moved from a leaf (source) cell to a root (sink) cell, sucrose is first actively transported into the phloem from the source cell. What happens next?

water diffuses into the phloem from the xylem and sucrose moves to the sink

Explanation:Translocation is the movement of ready produced food substances from the site of production to places in the plant where food is needed. such as the roots.the place where food is produced is called the source, the the place where food is used up is called the sink.the leaves are the source in a plant.photsoynthates, which is the plant manufactired food, may move up and down the stem ,majorly to areas of storage and growth.

Surose is the major intermediate product of photosynthesis.it is the form in which sugar is transported.it is conducted by the phloem vessels.this takes place in the long sieve elements that forms the sieve tubes

The high percentage of sucrose in the phloem sap causes water to pass into it from the xylem. This then causes the sap to move from source to sink.

At the sink, sucrose diffuses out of the phloem.it is either stored up or used for growth and repairs