8.4 Source to Sink

Phloem Structure

The products of photosynthesis, such as sucrose, are produced in the mesophyll cells of photosynthesizing leaves, which are considered a type of source cell. From there they are translocated through the phloem to where they are used or stored. The phloem is made up of two different types of vascular cells: companion cells and sieve-tube elements. Phloem sieve-tube elements 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 sieve tube elements. They assist with metabolic activities and produce energy for the sieve tube elements. Sucrose can move into from mesophyll cells into the sieve-tube elements through cytoplasmic channels called plasmodesmata. In addition, the sucrose is actively transported against its concentration gradient (a process requiring ATP) into the phloem companion cells using the electrochemical potential of the proton gradient. This is coupled to the uptake of sucrose with a carrier protein called the sucrose-H⁺ symporter.

Movement of Water and Sucrose through the Phloem

Once in the phloem, sucrose is 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. The high percentage of sugar decreases the solute potential_, which decreases the total water potential and causes water to move by osmosis from the adjacent xylem into the phloem tubes, thereby increasing pressure. This increase in total water potential causes the bulk flow of phloem sap from source to sink (Figure 30.4.1). Sucrose concentration in the sink cells is lower than in the phloem sieve tube elements because the sink sucrose has been metabolized to fuel new growth, converted to starch for storage or other polymers, such as cellulose, for structural integrity. 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.

Figure Descriptions

Figure 8.4.1. The image illustrates the process of sucrose transport in plant cells. It features a vertical arrangement of cells, divided into two main sections: xylem on the left and phloem on the right. The xylem section is depicted as a green, tube-like structure with an arrow indicating the upward transportation of water. The phloem is shown as an elongated column composed of stacked, green, oval-shaped cells. Adjacent to the phloem are two labeled structures: the source cell (leaf) at the top, and the sink cell (root) at the bottom. The source cell is depicted releasing small red dots, representing sucrose, which move into the companion cell and then into the phloem. The phloem has an arrow labeled “Translocation of sucrose” indicating downward movement. At the bottom, the water reenters the xylem from the phloem, highlighted by another arrow. Text labels identify the different parts and processes illustrated. [Return to Figure 8.4.1]

Licenses and Attributions

“8.4 Source to Sink” is adapted from “30.5 Transport of Water and Solutes in Plants” by Mary Ann Clark, Matthew Douglas, and Jung Choi for OpenStax Biology 2e under CC-BY 4.0. “8.4 Source to Sink” is licensed under CC-BY-NC 4.0.