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58 Translocation
Key Idea: Phloem transports the organic products of photosynthesis (sugars) through the plant by translocation. In vascular plants, the products of photosynthesis move as phloem sap. Apart from water, phloem sap contains mainly sucrose (up to 30%). It may also contain minerals, hormones, and amino acids in transit around the plant. Movement of sap in the phloem is from a source (an organ where sugar is made or mobilised) to a sink (an organ where sugar is stored
or used). The sap moves through the phloem sieve-tube members, which are arranged end-to-end and perforated with sieve plates. Loading sucrose into the phloem at a source (leaf) involves energy expenditure. We know this because it is slowed or stopped by high temperatures or respiratory inhibitors. In some plants, unloading the sucrose at the sinks also requires energy, although in others unloading into the cells of the sink organ (e.g. root) occurs by diffusion alone.
Phloem transport
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Phloem transport
Phloem sap moves from source to sink at rates as great as 100 m/h, which is too fast to be accounted for by cytoplasmic streaming. The most acceptable model for phloem movement is the mass flow hypothesis (also know as the pressure flow hypothesis). Phloem sap moves by bulk flow, which creates a pressure (hence the term “pressure-flow”). The key elements in this model are outlined below and right. For simplicity, the cells that lie between the source (and sink) cells and the phloem sieve-tube have been omitted.
Source cell (leaf cell)
Sucrose
Water
Water
Loading sugar into the phloem increases the solute concentration inside the sieve-tube cells. This causes the sieve-tubes to take up water by osmosis.
Measuring phloem flow
USDA
Aphids can act as natural phloem probes to measure phloem flow. The sucking mouthparts (stylet) of the insect penetrates the phloem sievetube cell. While the aphid feeds, it can be severed from its stylet, which remains in place and continues to exude sap. Using different aphids, the rate of flow of this sap can be measured at different locations on the plant.
Sieve tube (phloem)
Xylem vessel
Sink cell (root cell)
Sucrose
Transpiration stream
Xylem recycles the water from sink to source.
Cell wall
ure flow
The pressure gradient in the sieve tube is reinforced by the active unloading of sugar and consequent loss of water by osmosis at the sink (e.g. root cell).
Sieve plate
Press
The water uptake creates a hydrostatic pressure that forces the sap to move along the tube, just as pressure pushes water through a hose.
Water
Source: Modified after Campbell Biology 1993
1. (a) From what you know about osmosis, explain why water follows the sugar as it moves through the phloem:
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(b) What is meant by 'source to sink' flow in phloem transport?
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2. Why does a plant need to move food around, particularly from the leaves to other regions?
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