In phloem transport, what is the role of a source?

• A. Transports sugars to leaves
• B. Produces or stores sugars
• C. Consumes sugars
• D. Transports water

Answer: (B)

In phloem transport, the source is the place where sugars are produced or stored and then loaded into the phloem for distribution. Leaves typically act as sources because photosynthesis creates sugars (mainly sucrose). These sugars may also be stored elsewhere as starch and mobilized when needed, becoming available for transport. Loading the sugars into the phloem raises the solute concentration in the sieve tubes and draws water into the tubes by osmosis, creating pressure that pushes the sap toward tissues that need or store sugar, called sinks. So the role of a source is to produce or store sugars and supply them to the rest of the plant via the phloem. The other options don’t fit this role: phloem’s job isn’t to transport water (that’s the xylem’s main function), nor to have leaves as destinations for sugar transport, nor to consume sugars at the source.