(Extended) Translocation: sucrose and amino acids
Translocation is the movement of sucrose and amino acids through phloem tissue, from sources to sinks. Sucrose comes from photosynthesis; the amino acids are built from the products of photosynthesis plus nitrate ions absorbed by the roots. Phloem forms a continuous network of tubes throughout the plant, so translocation has no fixed direction -- unlike water in xylem, which always travels up. It follows wherever the sources and sinks are, changing through the year.
(Extended) Source: releases into the phloem
A source is any part of the plant that releases (loads) sucrose and/or amino acids into the phloem. The commonest source is a mature photosynthesising leaf, which makes more sucrose than its own cells can use, so the surplus is loaded into the phloem. A storage organ -- a tuber, a bulb, or a germinating seed's cotyledon or endosperm -- is also a source whenever it converts stored starch back into sucrose and releases it, for example to fuel new spring growth.
(Extended) Sink: uses or stores from phloem
A sink is any part of the plant that uses or stores sucrose and/or amino acids removed (unloaded) from the phloem. Growing root and shoot tips respire the sucrose for energy and build the amino acids into new proteins. A developing fruit or seed uses both to lay down food reserves. A storage organ filling with food converts the arriving sucrose into insoluble starch, so it can be stored without affecting the plant's water potential.
Drawn from real examiner reports.
Phloem ≠ xylem contents
Xylem carries water and mineral ions, one way (roots to leaves), through dead cells, and gives support. Phloem is living tissue carrying sucrose and amino acids from source to sink. "Water", "mineral ions", "nutrients" and "glucose" are all wrong for phloem -- the first two belong to xylem, the last two are too vague or the wrong molecule.
Flagged Jun 2022 P32 Q10d, P42 Q7d · Jun 2023 P43 Q4c
Source and sink are jobs, not organs
A mature photosynthesising leaf is usually a source, but a young growing leaf is a sink. A potato tuber is a sink while it fills with starch and a source when it sprouts. Never recall a fixed list -- re-read the scenario and ask whether that part is releasing sucrose and amino acids into the phloem, or removing them from it.
Phloem arrows are not always upward
On a whole-plant diagram, xylem arrows always point up, so candidates copy that pattern onto phloem. Phloem flow runs source to sink in whichever direction that puts it: down from leaves to a filling tuber, but up from a sprouting tuber to new shoots. Draw the arrows only after identifying the source and the sink.
Definitions that fit either term
Asked to define source and sink, candidates write answers that could apply to both, such as "a part of the plant involved in transport". The mark is for the direction: a source releases sucrose and amino acids into the phloem; a sink uses or stores them after they are removed from the phloem. Name the phloem in both.
Storing food alone earns no mark
In source/sink explain questions, "the tuber stores food" and "the tuber releases food" are not credited on their own. The examiner wants the substances linked to the phloem: sucrose and amino acids unloaded from the phloem and stored as starch (sink), or starch converted back to sucrose and loaded into the phloem (source).
Sucrose, not glucose
Photosynthesis makes glucose inside the leaf cell, but the plant converts it to sucrose (glucose joined to fructose) before loading it into the phloem. Sucrose is what travels. Answering "glucose", "sugar", "food" or "nutrients" loses the mark -- and naming sucrose alone is still incomplete, because amino acids travel with it.
Flagged Jun 2022 P42 Q7d
Use the three-part scaffold
Answer any translocation question in the same order: name both substances (sucrose and amino acids), identify the source in this scenario and say why it is releasing them, then identify the sink and say why it is using or storing them.
Identify, then explain why
Candidates name the source and sink correctly, then stop, losing the explanation marks. Each identification needs its reason attached -- "the mature leaf is the source because photosynthesis makes more sucrose than the leaf's own cells need".
Read which quantity is asked for
In translocation calculations, subtract before you divide. A percentage translocated uses the mass that actually left the leaf, not the mass produced or the mass respired; a rate uses that same translocated mass over the time. Always write the unit, such as g/hour or mg/hour.
Split a two-phase answer in two
For a storage organ across a whole year, run the scaffold twice -- once for its sink phase while it fills with food, once for its source phase while it sprouts -- rather than blending both into one paragraph. A question tracking three stages needs three separate answers.
Cambridge 0654 spec reference: Section B8 "Transport in plants", sub-topic B8.4 "Translocation" (Extended). Covers the movement of sucrose and amino acids through phloem tissue, from sources to sinks, and how sources and sinks are defined by function rather than by a fixed organ list.
Translocation -- the movement of sucrose and amino acids through phloem tissue, from sources to sinks.
Source -- a part of the plant that releases sucrose and/or amino acids into the phloem (loads the phloem). Example: a mature, photosynthesising leaf, producing more sucrose than it currently needs.
Sink -- a part of the plant that uses or stores sucrose and/or amino acids removed from the phloem (unloads the phloem). Example: a growing root tip, using sucrose for respiration and amino acids for building new proteins.
Substances translocated: sucrose (not glucose) and amino acids -- always state both together. Water and mineral ions are transported by xylem, a separate tissue, not by translocation.
Define "translocation".
(a) Define "source", in the context of translocation. (2 marks)
(b) Define "sink", in the context of translocation. (2 marks)
(c) Name the TWO substances that are translocated through the phloem. (1 mark)
