Translocation is the transport of dissolved SUGARS (mainly SUCROSE) through the PHLOEM from where they are produced to where they are needed or stored.
Source → Sink:
SOURCE — the place where sugars are made or released (mainly the LEAVES, where photosynthesis takes place).
SINK — any place where sugars are used or stored:
Growing shoot tips — sugars needed for cell division and growth.
Roots — sugars needed for respiration and converted to starch for storage.
Fruits and seeds — sugars needed for development.
Flowers — sugars needed for reproduction.
Unlike the transpiration stream, translocation can move sugars in BOTH DIRECTIONS in the phloem — up to growing tips AND down to roots and storage organs.
How Translocation Works
Sucrose is ACTIVELY LOADED into phloem sieve tubes at the source (leaves) using energy (ATP) from companion cells.
This creates a high concentration of sucrose in the phloem at the source end.
Water enters the phloem by osmosis (moving from xylem, where it's more dilute) → increases pressure at the source end.
This pressure drives the flow of sugar solution THROUGH the phloem towards the sink.
At the sink, sucrose is actively UNLOADED from the phloem and used or converted to starch for storage.
This reduces the concentration at the sink end, maintaining the pressure difference and keeping the flow going.
Transpiration vs Translocation — Key Differences
Students often confuse these two transport processes. Here is a clear comparison:
TRANSPIRATION:
Substance moved: WATER (and dissolved minerals)
Vessel: XYLEM
Direction: UPWARDS ONLY (roots → leaves)
Cells: DEAD cells
Driving force: evaporation from leaves creating transpiration pull
Energy: PASSIVE (no ATP needed)
TRANSLOCATION:
Substance moved: SUGARS (sucrose)
Vessel: PHLOEM
Direction: BOTH DIRECTIONS (source → sink)
Cells: LIVING cells
Driving force: active loading of sucrose creates pressure
Energy: ACTIVE (ATP required to load/unload sucrose)
⚠️ Common Mistake
Translocation = SUGARS in PHLOEM. Transpiration = WATER in XYLEM. These are the two most commonly confused terms in plant biology. Translocation moves in BOTH directions — transpiration only goes UPWARDS. Phloem cells are LIVING — xylem cells are DEAD.
📌 Key Note
Translocation: sucrose in phloem, source (leaves) to sink (roots, fruits, tips), both directions, living cells, uses ATP. NOT the same as transpiration (water in xylem, upwards only, dead cells, passive).
🎯 Matching Activity — Transpiration or Translocation?
Sort each statement into transpiration or translocation. — drag the symbols on the right to match the component names on the left.
Transpiration
Drop here
Translocation
Drop here
Transpiration
Drop here
Translocation
Drop here
Transpiration
Drop here
Translocation
Drop here
Involves dead, hollow, lignified cells
Water moves up through xylem from roots to leaves — pulled by evaporation
Can move substances both upwards and downwards in the plant
Requires ATP energy — companion cells supply energy to sieve tubes
Sucrose moves through phloem from leaves to growing roots and fruits
Rate increases in hot, bright, dry and windy conditions
🎯 Test Yourself
Question 1 of 3
1. What is the main substance transported in phloem?
2. In which direction does translocation move?
3. What is a 'sink' in the context of translocation?
⭐ How Well Do You Understand This Topic?
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