Transport in Plants
Senior 2 · Biology
Transport in Plants
Lesson 5 of 5
18 min
Let's begin
Dig up an Irish potato plant in Musanze at harvest time and you find a cluster of tubers packed with starch. None of that food was made underground. It was made in the leaves by photosynthesis and carried down through the phloem. Sweet potatoes, cassava roots, banana bunches and sugar cane stems are all filled the same way.
Translocation is the movement of dissolved food, mainly sucrose (with amino acids), through the phloem from where it is made or released to where it is used or stored.
Worked example
During the growing season, a potato plant's leaves are the source and its developing tubers are a sink. The next season, a stored tuber is planted. It sprouts before it has any leaves, so the tuber breaks down its starch to sucrose and sends it up to the new shoots. Conclusion: the same organ was a sink one season and a source the next. What makes something a source or a sink is what it is doing at that time.
Flow in the xylem is always upwards. Flow in the phloem goes from each source to the nearest sink, so it can go both ways: upper leaves usually feed the growing shoot tip above them, lower leaves feed the roots, and leaves in the middle may send food in both directions.
| Water transport (xylem) | Translocation (phloem) | |
|---|---|---|
| Substances | water, mineral ions | sucrose, amino acids |
| Direction | up only | source to sink, up or down |
| Energy from the plant | not needed (evaporation drives it) | needed to load sucrose |
Tip
Source = where sugar comes from (made or released). Sink = where sugar goes into (used or stored).
In ringing questions, always say which tissue was removed (phloem, in the bark) and which was left (xylem, in the wood), then explain the swelling above the ring.
Define translocation.
Solution: The movement of dissolved organic food, mainly sucrose and amino acids, in the phloem from sources to sinks.
Classify each as a source or a sink in a bean plant that is flowering: (a) a mature leaf in sunlight; (b) a developing pod; (c) a growing root tip.
Solution: (a) source; (b) sink; (c) sink.
After bark is ringed on a branch, the fruits above the ring grow larger than usual. Explain why.
Solution: Sugar made by the leaves above the ring cannot pass down through the phloem, so more of it goes to the fruits above the ring.
Why is energy needed for translocation but not for water movement in the xylem?
Solution: Sucrose is actively loaded into the sieve tubes by the companion cells, which uses energy from respiration. Water in the xylem is pulled up by evaporation from the leaves, powered by the sun, not by the plant's own energy.
Remember
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