Phloem

In the syllabus and the exam

Phloem is examined together with xylem as the plant's second major transport tissue, but candidates must clearly separate the two, since phloem carries manufactured food, mainly sucrose, rather than water and minerals. A key fact tested is that phloem tissue remains alive at maturity, consisting of sieve tube elements connected end to end and supported by companion cells that provide the energy needed for active loading and unloading of sugars, which distinguishes phloem from the dead xylem vessels. Another commonly examined feature is that phloem transport, called translocation, is bidirectional rather than one-directional: sugars may move upward or downward depending on whether the nearest source (usually a leaf) is above or below the tissue receiving the food. Structured questions often ask candidates to complete a comparison table contrasting phloem with xylem across cell structure, contents transported, and direction of flow, and marks are commonly lost when candidates state that phloem transport is always downward or that phloem cells are dead like xylem vessels. Confusing translocation, the movement of food in phloem, with transpiration, the loss of water vapour, is another frequent mistake in this section. Candidates should also be able to state that phloem sieve tube elements have perforated end walls, called sieve plates, that allow cytoplasm to connect between adjacent cells, and that these cells lack a nucleus at maturity, relying instead on their neighbouring companion cell to remain functional. This is also why girdling a fruit tree branch just above its base can, within weeks, cause fruit closer to the tip to grow unusually large, since sugar is trapped there instead of being transported back towards the roots.

Example

Explaining food transport, a candidate writes: 'Phloem consists of living sieve tube elements with companion cells, which actively transport dissolved sugars made in the leaves to growing regions and storage organs, in whichever direction the plant currently needs them.'

Related terms

XylemTranslocationMeristem

Source:SRC-DSKP-EN