Summary
Vascular tissue made of living cells that transports sugars from sources to sinks.
In depth
Sieve tube elements with companion cells, sieve plates and no nucleus. Sap contains sucrose, amino acids and hormones at 10 to 25 percent solute. Transport is two-way in the plant and uses energy at loading.
Sugar loading uses companion cell transporters and proton gradients. Sieve plates allow sap flow, plugged by P-protein and callose when damaged. Phloem also carries signals such as florigen (FT protein).
Common mistake: phloem is not a one-way pipe; flow direction depends on source and sink locations.
Phloem transports sugars, amino acids, hormones and RNAs. Sieve tube elements are living but lack nucleus and most organelles, and rely on companion cells connected by plasmodesmata. P-protein and callose seal damaged tubes to stop sap loss. Rates are about 0.5 to 1 m per hour. Aphids insert stylets into phloem, and researchers use their mouthparts to collect pure sap, showing sugar concentrations and pressure.
Ringing (girdling) a tree, removing a ring of bark with phloem, kills it by starving the roots, an experiment done by Malpighi. Radioactive carbon-14 tracing showed that sugars from a labelled leaf move to growing parts. Viruses spread through phloem.
Remember: xylem moves up mostly, but phloem moves in both directions, source to sink.
Frequently asked questions
What is Phloem?
Vascular tissue made of living cells that transports sugars from sources to sinks.
Which area of biology covers Phloem, and how advanced is it?
Phloem is one of the 56 topics in Plant Biology. It is rated Beginner, so it suits learners who are starting out in biology. Related topics in this area include Translocation, Xylem, Auxin and Phototropism.
Can you give an example of Phloem?
Aphid stylets sample phloem sap to study contents.
What should I learn before Phloem?
Phloem is a Beginner topic, so no earlier topic is required. Other Beginner topics in Plant Biology that pair well with it include Plant Kingdom, Photosynthesis, Chlorophyll.
What topics are related to Phloem?
The closest topics to Phloem are Translocation, Xylem, Auxin and Phototropism, Plant Tissues. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Phloem?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Phloem. To practise the whole of Plant Biology, use the Plant Biology quiz.
How is Phloem different from Translocation?
Phloem: Vascular tissue made of living cells that transports sugars from sources to sinks. Translocation: The pressure-flow movement of sucrose through phloem from sources to sinks. Read both side by side and note what each one explains.
What is a common mistake about Phloem?
Phloem is not a one-way pipe; flow direction depends on source and sink locations.
How should I study Phloem step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Translocation and explain the difference aloud. Finally, take the Plant Biology quiz and come back to this page for any question you miss.