Plant organ system: Phloem is composed of tubes of elongated cells. Cell sap can move from one phloem cell to the next through pores in the end walls.

The idea

Phloem is a transport tissue in plants. It is made of tubes formed from elongated cells joined end to end, running through stems, roots and leaves. The end walls between neighbouring phloem cells are not solid; they contain pores, so they are perforated. These pores allow cell sap, a sugar-rich fluid, to move from one phloem cell to the next along the tube. This movement transports dissolved sugars and other solutes around the plant, for example from a leaf where photosynthesis makes sugars to a growing bud or storage organ. In the required practical context, you may observe phloem in stained transverse sections of a stem, where it appears as small cells near the outside. You do not need to name every cell type or explain the transport mechanism in detail.

What earns the marks

• Phloem is a plant transport tissue arranged as tubes running through the plant.
• The tubes are composed of elongated cells joined end to end.
• The end walls between adjacent phloem cells contain pores.
• Cell sap moves from one phloem cell to the next through these pores.
• Cell sap is a fluid containing dissolved sugars and other solutes.
• Phloem transport moves substances such as sugars around the plant, for example from a photosynthesising leaf to a growing region or storage organ.
• In a stained stem section, phloem can be identified as small cells, often near the outside of the stem.

Common mistakes and how to put them right

• Confusing phloem with xylem.

  Put it right: Link phloem to sugar transport and xylem to water transport.

• Describing phloem as a single hollow pipe.

  Put it right: State that the tube is made of cells and that the end walls are perforated.

Exam tips

• Use the phrase 'elongated cells joined end to end' when describing the tube, and 'pores in the end walls' when describing the route of cell sap.
• If asked to identify phloem in a diagram or micrograph, look for small cells near the outside of a stem section and link them to sugar transport.
• Keep answers to the level required: describe the tube, the elongated cells, the pores and the movement of cell sap; do not attempt a detailed mechanism of translocation.