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Transportation and Storage of Nutrients

Transportation and Storage of Nutrients - into sieve tubes...

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Transportation and Storage of Nutrients Plants make sugar by photosynthesis, usually in their leaves. Some of this sugar is directly used for the metabolism of the plant, some for the synthesis of proteins and lipids, some stored as starch. Other parts of the plant also need energy but are not photosynthetic, such as the roots. Food must therefore be transported in from a source, an action accomplished by the phloem tissue. Phloem, Sugar, and Translocation Phloem consists of several types of cells: sieve tube cells (aka sieve elements ), companion cells , and the vascular parenchyma . Sieve cells are tubular cells with endwalls known as sieve plates . Most lose their nuclei but remain alive, leaving an empty cell with a functioning plasma membrane. Companion cells load sugar into the sieve element (sieve elements are connected
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Unformatted text preview: into sieve tubes ). Fluids can move up or down within the phloem, and are translocated from one place to another. Sources are places where sugars are being produced. Sinks are places where sugar is being consumed or stored. Food moves through the phloem by a Pressure-Flow Mechanism. Sugar moves (by an energy-requiring step) from a source (usually leaves) to a sink (usually roots) by osmotic pressure. Translocation of sugar into a sieve element causes water to enter that cell, increasing the pressure of the sugar/water mix (phloem sap). The pressure causes the sap to flow toward an area of lower pressure, the sink. In the sink, the sugar is removed from the phloem by another energy-requiring step and usually converted into starch or metabolized....
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