Translocation

In the syllabus and the exam

Translocation is tested as the phloem-based counterpart to the xylem-based transpiration stream, and exam questions frequently require candidates to keep the two processes, and the tissues and substances involved in each, clearly separated. Candidates should state that translocation moves organic substances, chiefly sucrose produced during photosynthesis, from a source, typically a mature photosynthesising leaf, to a sink, such as a growing shoot, a developing fruit, or a storage root, and that the direction of flow can change depending on the plant's needs at a given time. Because sucrose must be actively loaded into and unloaded from phloem sieve tubes, candidates are expected to know that this process requires energy from respiration, provided by the companion cells attached to each sieve tube element. A frequently tested comparison table contrasts translocation with the transpiration stream across tissue (phloem versus xylem), substance moved (food versus water and minerals), direction (variable versus always upward), and energy requirement (active versus passive). A common error is stating that translocation only moves substances downward, or confusing it with transpiration simply because both terms begin with the same syllable and involve plant transport. Candidates should also be able to explain, using a ringing experiment where a strip of bark and phloem is removed from around a stem, why sugars accumulate above the ring while the tissue below it eventually starves, since this classic evidence is sometimes presented as an unfamiliar structured question. This is also why fruits and storage roots are described as strong sinks during certain growth stages, since they can draw sugar away from other growing parts of the plant, a competitive relationship that plant growers sometimes manage deliberately through pruning.

Example

Distinguishing the two transport processes, a candidate writes: 'Translocation is the movement of manufactured food, mainly sucrose, through the phloem from a source such as a leaf to a sink such as a root or fruit, unlike the transpiration stream, which moves water through the xylem.'

Related terms

PhloemTranspirationMeristem

Source:SRC-DSKP-EN