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The key difference
Transpiration is the loss of water vapour from leaves through stomata, pulling water up through xylem (passive). Translocation is the movement of dissolved sugars through phloem from source to sink (active, requires energy).
Plants move substances using two distinct transport systems. Transpiration moves water upward through xylem, while translocation moves sugars in any direction through phloem. These two processes are frequently confused: this guide clarifies every testable difference.
Feature
Transpiration
Translocation
Substance transported
Water (and dissolved mineral ions)
Dissolved sugars (sucrose) and amino acids
Tissue involved
Xylem vessels
Phloem sieve tubes
Direction
Upward only: roots to leaves
Both up and down: from source to sink
Driving force
Evaporation from leaves creates a transpiration pull (passive)
Active loading of sugars into phloem requires energy from respiration
Energy required?
No: passive process driven by evaporation
Yes: active process
Cell status of transport tissue
Dead xylem vessels (no cytoplasm, no end walls)
Living sieve tube elements with companion cells
Where water/sugar exits
Water evaporates from spongy mesophyll surfaces in the leaf and diffuses out through stomata
Sugars are unloaded at sinks: growing points, storage organs, fruits
The Transpiration Stream
The transpiration stream is the continuous movement of water from soil → roots → stem → leaves → atmosphere. The driving force is evaporation from leaf surfaces:
Water evaporates from the surfaces of spongy mesophyll cells into air spaces inside the leaf
Water vapour diffuses out through open stomata
This creates a pull (tension) that draws water up through the xylem: the transpiration pull
Water is absorbed from the soil into root hair cells by osmosis to replace what was lost
The entire process is passive: no energy is expended by the plant to move water upward.
Source and Sink in Translocation
In translocation, sugars move from a source (where sugars are produced or released) to a sink (where sugars are used or stored):
Source examples: photosynthesising leaves (produce sugars), storage organs releasing stored sugars in spring
This means translocation can go upward (leaves → growing shoot tip) or downward (leaves → roots for storage). The direction depends on where sugars are needed, not on gravity.
Try a comparison question (3 marks)
Explain why translocation requires energy but transpiration does not. [3]
Show the answer and marking guidance
Translocation involves active loading of sucrose into the phloem sieve tubes against a concentration gradient
This requires energy from respiration (ATP), which is why companion cells have many mitochondria
Transpiration is driven passively by evaporation of water from leaf surfaces: the transpiration pull moves water without the plant expending energy
Original Cambridge-style practice written for this site.
Common comparison mistakes
When answering a compare question, write matched pairs rather than two separate descriptions.
Confusing transpiration with translocation: transpiration = water in xylem, translocation = sugars in phloem
Saying translocation only goes downward: it moves in both directions, from source to sink
Claiming transpiration requires energy: it is entirely passive, driven by evaporation
Forgetting that phloem cells are alive (and must be for active transport) while xylem cells are dead