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Key concept
Phloem tissue transports dissolved sugars (mainly sucrose) from sources such as leaves to sinks such as roots, fruits and growing tips. This process is called translocation. Phloem sieve tubes are living cells with sieve plates and companion cells.
Translocation is a Supplement-only topic, but it appears frequently in Paper 4. The key distinction from xylem transport is that phloem carries organic solutes in both directions and uses living cells, whereas xylem moves water upward through dead cells.
Structure of phloem tissue
Sieve tube elements are elongated living cells arranged end to end. Their end walls have pores forming sieve plates, through which cytoplasm and dissolved sucrose can pass. Mature sieve tube elements have little or no nucleus and reduced organelles, leaving more space for sap flow.
Companion cells sit alongside sieve tubes and are metabolically active, with a large nucleus, many mitochondria and ribosomes. They provide the energy and proteins that sieve tubes need to function. Each companion cell is connected to its sieve tube element by numerous plasmodesmata.
Translocation: sources and sinks
Translocation is the movement of sucrose and amino acids through phloem from a source (where substances are produced or released) to a sink (where they are used or stored). A photosynthesising leaf is a source; a growing root tip, developing fruit or storage organ is a sink.
Unlike the one-way upward flow in xylem, phloem can transport in both directions at different times. A storage organ like a potato tuber is a sink during growing season but becomes a source when the plant sprouts from it.
Comparing xylem and phloem
Feature
Xylem
Phloem
Main substance
Water and mineral ions
Sucrose and amino acids
Direction
Upward (roots to leaves)
Both directions (source to sink)
Cell status
Dead at maturity
Living
End walls
Absent (broken down)
Present as sieve plates with pores
Requires energy?
No (driven by transpiration pull)
Yes (companion cells supply energy)
Try a focused question (3 marks)
Explain why companion cells are important for the function of sieve tube elements.
Show the answer and marking guidance
Sieve tube elements have no nucleus or few organelles so cannot carry out their own metabolic processes.
Companion cells have many mitochondria and release energy from respiration.
This energy supports the loading of sucrose into sieve tubes for translocation.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying phloem carries water: that is the role of xylem.
Describing phloem cells as dead like xylem.
Writing glucose instead of sucrose as the transported sugar.