Translocation — the four-part definition
Translocation is the movement of sucrose and amino acids through the phloem, from a source (an organ that loads solute in, e.g. a photosynthesising leaf) to a sink (an organ that unloads it for use or storage, e.g. a growing tip, fruit or storage organ). All four elements score. Glucose from photosynthesis is converted to sucrose in the leaf before loading — sucrose is a stable non-reducing disaccharide built for long-distance transport.
Source and sink — organs, not processes
Source = an organ that loads sucrose and amino acids into the phloem: photosynthesising mature leaves in the growing season, and storage roots, tubers or bulbs in spring. Sink = an organ that unloads them: growing shoot and root tips (sucrose for respiration, amino acids for protein synthesis), developing fruits and seeds, and storage organs in summer and autumn. Flow always runs source to sink. Name the organ, never the process.
Reversal of direction (Extended)
Reversal — the same organ switches role with season or stage. In summer and autumn the leaves are the source and the storage root, tuber or bulb the sink, so sucrose moves down and starch accumulates. In spring that starch is hydrolysed back to sucrose, the storage organ becomes the source and the growing shoot tips the sink, so flow runs upward until new leaves open. A seed or fruit is likewise a strong sink; after germination its food store acts as the source.
Drawn from real examiner reports.
Sucrose alone is half the answer
The 0610 definition names two organic solutes: sucrose and amino acids. On a 2-mark definition, sucrose alone forfeits the second mark. Amino acids matter for a reason worth stating — sinks such as shoot tips and developing seeds need them to build proteins, not just sucrose to respire.
Flagged s23 P23 Q16 · w22 P43 Q4c
Sucrose, not glucose, in phloem
Glucose is the product of photosynthesis, but the leaf converts it to sucrose before loading it into the phloem. Writing "glucose is translocated" scores zero for the substance mark. Glucose is the working molecule inside cells — respiration, starch and cellulose synthesis — while sucrose is the form moved between organs.
Flagged s23 P23 Q16 · w22 P43 Q4c · w22 P42 Q3b ii
Sucrose does not travel in xylem
Xylem carries water and mineral ions only; sucrose and amino acids move in the phloem. "Sucrose is carried up the xylem from the leaves" scores zero. Hold the split in mind: xylem = inorganic solutes in dead, hollow, lignified vessels; phloem = organic solutes in living cells.
Flagged w22 P43 Q4c · w22 P42 Q3b ii
Phloem flow is not upward only
Xylem transport is unidirectional — root to leaf, pulled by transpiration. Phloem transport runs wherever the sink happens to be, upward or downward, and can reverse with the season. Describing phloem as "upwards only", or as "the opposite of xylem, so always downwards", loses the direction mark.
Xylem and phloem swapped in sections
Cross-section labelling is confused at almost every sitting. In a leaf vascular bundle the xylem is on the upper (adaxial) side and the phloem on the lower (abaxial) side. In a root, xylem forms the central star and phloem sits in the arms between its points — not the other way round.
Flagged s23 P11/13 Q17 · w22 P12/13 Q7 · s22 P12/22 Q15/Q17
Source and sink the wrong way round
A leaf is the source because it makes sucrose and loads it into the phloem; a growing root tip or developing fruit is the sink because it removes sucrose for respiration, growth or storage. "The root is the source because it is at the bottom" is wrong, and circular answers such as "the source is where it starts" earn nothing.
Flagged s23 P23 Q16 · w22 P43 Q4c
Forgetting to name translocation
When a question asks for the name of the process moving sucrose from leaf to root tip, the answer is one word: translocation. Candidates routinely describe the movement correctly and never name it, losing the recall mark. Osmosis, diffusion and active transport are all wrong answers here.
Flagged s23 P23 Q16 · w22 P43 Q4c
Build the definition in four parts
Answer "define translocation" as substance + vessel + from + to: sucrose and amino acids [1], in the phloem, from sources such as leaves to sinks such as growing regions or storage organs [1]. Naming an example of each end secures the second mark.
Compare means both sides, every time
A compare question needs contrasting pairs, not a description of one tissue. Use three: substance, direction, cell type — stating xylem and phloem for each. An answer that is all about xylem cannot score the phloem half of any pair.
Structure a reversal answer
For Extended reversal questions work in a fixed order: name the source organ, name the sink organ, state the direction of flow, then the mechanism — starch hydrolysed to sucrose, loaded into phloem, carried to the growing region. "The direction reverses" alone scores nothing.
Use the precise noun
Mark schemes want "mineral ions", not "minerals"; "sucrose", not "sugar" or "glucose"; "phloem", not "bark". On a 1-mark name-the-process question, write translocation and stop — extra guesses can cost the mark.
Translocation is the movement of sucrose and amino acids through the phloem from sources to sinks.
The Cambridge 0610 mark scheme requires all four elements of this definition:
| Element | Detail |
|---|---|
| Substances | Sucrose AND amino acids (both required) |
| Vessel | Phloem (NOT xylem) |
| Source | Organ that produces/loads solute into phloem (e.g. photosynthesising leaf) |
| Sink | Organ that removes/uses solute from phloem (e.g. growing root tip, developing fruit) |
Why sucrose and not glucose? Photosynthesis produces glucose in the leaf. This is converted to sucrose (a non-reducing disaccharide) before entering the phloem. Sucrose is chemically stable for long-distance transport; glucose remains the molecule used inside cells for respiration and biosynthesis.
What substances are transported in (a) the xylem and (b) the phloem?
(a) Define translocation. [2]
(b) Name the tissue through which translocation occurs. [1]
