vacuole.htm

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Plant Cell Vacuoles

Plant cell vacuoles are bounded by a membrane, the tonoplast. Assume this membrane is similar to the plasma membrane with integral transport mechanisms. As such, it can contribute to some important functions. The vacuole can also determine in a strong way how cells appear following sectioning and staining.

Plant cells use vacuoles to their advantage to sustain rapid cell enlargement during growth. Young cells typically possess many small vacuoles and ample cytoplasm. As cells mature, the small vacuoles merge to form a single, large vacuole, the contents of which are predominately water. Rapidly enlarging cells need only fill vacuoles with water rather than attempt to fill expanding cell volumes with metabolically costly cytoplasm. Cells subsequently enlarge rapidly when water is available. The symplast with its important organelles, enzymes; and substrates is pushed to the periphery of the cell nearer to transport mechanisms of the plasma membrane, the net result is a more efficient, localized metabolism. In addition, light gathering chloroplasts are pushed to the periphery of the cell and by streaming cytoplasm, all chloroplasts can be cycled to sides where light levels are optimal.

Vacuoles are also used as storage sites for wastes, ions, and metabolic products. Wastes within vacuoles of leaf cells may discourage herbivores. Plant cells commonly appear empty after staining as their vacuoles do not retain stains. In roots and stems, the interiors of many empty cells contain starch granules, a storage molecule for glucose.

Now, run your mouse over the nucleus in the young cell to view empty-looking parenchyma cells.

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Parenchyma Cells

As you can see, these cells appear as mere shells. Parenchyma cells only possess primary cell walls. They also can contain very large vacuoles. Under these conditions, stains have very little solid structure to penetrate. Notice how the nuclei sectioned here are displaced to the side, another sign vacuoles are present.

Now, move your mouse over the round nucleus.

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As you can see here, these parenchyma cell types contain granular structures where the vacuoles are located. These dark staining bodies are starch granules. Glucose molecules, products of photosynthesis, are bonded together to form large polysacharides. Cellulose, the cell-wall polymer is another glucose-based polysacharide very similar in structure to starch. Parenchyma cells like these are found in many roots, functioning to store energy for the plant.

Now, roll your mouse over a starch granule in the upper right corner.

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In this view of a plant leaf you will notice that some cells possess nuclei and chloroplasts that are displaced to the periphery of the cells. In this photosynthetic tissue vacuoles might help cells move chloroplasts to the peripheral areas of cells for optimal light reception.

Remember that vacuoles are important in the elongation of cells during growth. They are also sites for storage of wastes and metabolites. Try to remember that the empty vacuolar spaces in stained and sectioned plant cells serve some very important functions in the living cells.

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