Transport of Sugar in Plants

Photosynthates, such as sucrose, are produced in the mesophyll cells of photosynthesizing leaves. From there they are translocated through the phloem to where they are used or stored. Mesophyll cells are connected by cytoplasmic channels called plasmodesmata. Photosynthates move through these channels to reach phloem sieve-tube elements (STEs) in the vascular bundles. From the mesophyll cells, the photosynthates are loaded into the phloem STEs. Phloem STEs have reduced cytoplasmic contents, and are connected by a sieve plate with pores that allow for pressure driven bulk flow, or translocation, of phloem sap. Companion cells are associated with STEs. They assist with metabolic activities and produce energy for the STEs. Once in the phloem, the photosynthates are translocated to the closest sink. Phloem sap is an aqueous solution that contains up to 30 percent sugar, minerals, amino acids, and plant growth regulators. Unloading at the sink end of the phloem tube occurs by either diffusion or active transport of sucrose molecules from an area of high concentration to one of low concentration. Water diffuses from the phloem by osmosis and is then transpired or recycled via the xylem back into the phloem sap.



*LICENSES AND ATTRIBUTIONS

CC licensed content, Shared previously, Transport of Water and Solutes in Plants, Authored by: OpenStax College. License: CC BY: Attribution. License Terms: Download for free at https://cnx.org/contents/GFy_h8cu@9.36:5aq8b3HZ@5/Transport-of-Water-and-Solutes*