Studyledge

Transport in Plants: Question 5

Syllabus 8.4

Multiple choice Extended 1 mark

In an actively growing tomato plant, mature leaves lower down the main stem are photosynthesising strongly and loading sucrose into the phloem. Some of this sucrose travels upward through the phloem to a cluster of small, actively dividing leaves near the shoot tip, while at the same time other sucrose travels downward through the phloem to actively dividing cells in the root tips.

Which statement correctly explains why sucrose moves in two different directions within the phloem of this same plant?

Choose an answer to check it, then compare with the worked solution below.
Show worked solution Hide worked solution

Worked solution

Step 1: Recall how xylem and phloem differ in direction

Xylem transports water and dissolved mineral ions in one direction only: upward, from the roots to the stem and leaves. Phloem, however, transports sucrose (and amino acids) from a source, where it is produced or released, to a sink, where it is used or stored, and a sink can be positioned either above or below the source in the plant.

Step 2: Apply this to the tomato plant described

The mature lower leaves are producing sucrose by photosynthesis, so they act as a source. The young leaves near the shoot tip are growing and using sucrose to build new cells, so they act as a sink positioned above the source, and sucrose moves upward through the phloem vessels supplying them. The root tips are also actively dividing and using sucrose for growth, so they too act as a sink, but one positioned below the source, and sucrose moves downward through the phloem vessels supplying them.

Step 3: Why the other options are wrong

  • B wrongly assumes phloem transport is fixed and upward only, like xylem, and wrongly reassigns downward-moving sucrose to the xylem, but xylem never carries sucrose.
  • C makes the opposite fixed-direction error, again wrongly invoking the xylem to explain sucrose reaching the shoot tip.
  • D invents a day/night reversal that plays no part in the real explanation, which is simply the relative position of sources and sinks.

Final answer

Sucrose in phloem moves toward whichever sink is using or storing it, and because a sink can lie above or below the source, the direction of flow differs between different phloem vessels in the same plant at the same time, option A.