Phloem tissue structure and function

Phloem tissue transports the food made by photosynthesis, mainly sucrose and amino acids, from the leaves to other parts of the plant. This process is called translocation.

Phloem is made of living cells, which sets it apart from xylem, whose water-carrying vessels are dead and hollow. In the leaf, stem and root, phloem and xylem run side by side in the vascular bundles, so a labelling question often asks you to tell the two tissues apart and to give the direction each one carries substances.

Parts and functions

How structure suits function

The mature sieve tube element loses its nucleus and most of its organelles, and it depends on the companion cell beside it.

• Sieve tube with few organelles, losing the nucleus and most organelles leaves an open channel, so the sugar solution flows with little obstruction.
• Sieve plates, the perforations join the elements end to end into a continuous tube while still supporting the column of sap.
• Companion cell packed with mitochondria, it holds a high density of mitochondria that release the ATP needed to load sucrose actively into the sieve tube.
• Companion cell with a nucleus, because the sieve tube has no nucleus of its own, the companion cell controls its activity through connections between the two cells.
• Living tissue, because the cells are alive, phloem can load and unload sugar actively and move it either up or down the plant, unlike the one-way flow in xylem.

Related processes

Phloem carries out translocation, the movement of sucrose and amino acids from a source to a sink. A source is a part making or releasing sugar, such as a photosynthesising leaf or a store being used up; a sink is a part using or storing sugar, such as a growing shoot, a root or a developing fruit.

Because a part can be a source at one time and a sink at another, phloem moves substances in both directions.

This links phloem to photosynthesis, which supplies the sucrose, and contrasts it with the xylem, which carries water and mineral salts upward only, driven by transpiration pull. In the leaf, the products of photosynthesis are loaded into the phloem for transport, while water arrives through the xylem.

Common labelling errors

Worked question

Question (in the style of Paper 2 Section A): Diagram P shows part of phloem tissue. Cell V is a long tube with perforated end walls and no nucleus.

Cell W lies beside it, has a nucleus and is dense with mitochondria. (a) Name cell V, cell W and the perforated end wall. [3 marks] (b) Explain why cell V depends on cell W. [2 marks] (c) A complete ring of bark, including the phloem, is removed from a tree trunk.

Explain why sugars build up in the bark above the ring. [2 marks]

Model answer: (a) V is the sieve tube element; W is the companion cell; the end wall is the sieve plate. (b) The sieve tube element has no nucleus and few organelles, so it relies on the companion cell to control its activity and to supply ATP for loading sucrose.

(c) Removing the phloem breaks the pathway for translocation, so sucrose made in the leaves cannot be carried down past the ring and accumulates above it.

Marking note: in part (c) the marks are for the phloem/translocation pathway is cut and sugars cannot move down past the ring.

Source:SRC-DSKP-EN

Frequently asked questions