Most of the work you do on transport in plants concerns the movement of water and minerals from the roots to the leaves of the plants in xylem vessels. (see previous post on xylem transport) You should understand what transpiration is, and how the properties of water allow a transpiration pull provide the energy to move large volumes of water up the xylem in the plant.
But what about the second plant transport tissue phloem? How does it differ from xylem in both structure and function?
Well structurally the tissues are very different. Xylem vessels are large, dead, empty thick-walled cells with cell walls strengthened with lignin. The transport cells in phloem are called sieve-tubes. Phloem sieve tubes are living cells with thin cell walls.
In xylem vessels the end walls break down completely but in phloem sieve tubes, the end walls are filled with many holes forming a structure called a sieve plate. Each phloem sieve tube has a smaller cell called a companion cell alongside and both these two cells must be alive for phloem transport to occur.
What is transported in phloem sieve tubes?
Plants transport the products of photosynthesis (food molecules) up and down the stem in phloem. The main carbohydrate transported is sucrose but phloem cells also contain lots of amino acids and a few other sugars.
The mechanism by which these sugars are moved around the plant is less well understood than for water movement in xylem. Translocation is an active process and requires energy from respiration in the cells. It is possible that a bulk flow exists as shown in the diagram below, but this mechanism cannot be the whole story….. Can you think why? Post a comment if you want to explain some of the difficulties with this theory…..