The sugar a leaf makes is no use to a root tip or a ripening fruit unless it gets there. Translocation is the movement of sucrose and amino acids through the phloem, from the parts of the plant that release them to the parts that use or store them. The same organ can switch roles with the seasons. All of this sub-topic is Extended content.
Extended. Translocation is the movement of sucrose and amino acids in the phloem from sources to sinks. A source is a part of the plant that releases sucrose or amino acids, such as a photosynthesising leaf. A sink is a part that uses or stores them, such as a growing tip, a root, a developing fruit or a storage organ. Because sources and sinks can be anywhere, phloem carries its contents both up and down the plant, and an organ such as a tuber can be a sink at one time and a source at another.
Extended. In the sim, green glows mark sources and orange glows mark sinks; the pink particles are sucrose moving in the phloem. Start with summer, then switch to spring and watch which way the sucrose travels.
| Source | Sink | |
|---|---|---|
| Definition | releases sucrose or amino acids into the phloem | uses or stores sucrose or amino acids taken from the phloem |
| Examples | mature leaves photosynthesising; a storage organ in spring | growing shoot and root tips, young leaves, roots, developing fruits and seeds, storage organs in summer |
| What happens to the sucrose | made in photosynthesis (or from stored starch) and loaded into the phloem | used in respiration, used for growth, or converted to starch and stored |
Translocation happens in the phloem and carries sucrose and amino acids. Do not say glucose is translocated (it is converted to sucrose first), and do not confuse it with transpiration, which is water loss through the xylem and leaves.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the summer view of the translocation sim, which parts glow green as sources?
In the spring view of the sim, which part has become the source?
In the summer view, in which direction does sucrose move in the phloem of the stem?
What happens to the sucrose that reaches the tuber in summer?
Extended. Whether a part of the plant is a source or a sink depends on whether it is releasing sucrose or taking it in at that moment, and that changes with the season and the age of the organ.
A storage organ is not "always a sink" and a leaf is not "always a source". Exam questions often describe a season or a growth stage on purpose: decide whether the organ is releasing or receiving sucrose at that time.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Extended: which statement describes translocation?
Extended: which part of a plant is a sink?
Extended: a potato tuber is a sink in summer and a source in spring. The best explanation is:
Extended: a ring of bark containing the phloem is removed from a tree trunk. Which part will soon be short of sucrose?
This skill is now recorded on your profile. Keep the chain going.