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Plant cells are typical eukaryotic cells, very similar to animal cells in structure and function. However, there are some important differences. As we learned in a previous topic, plant cells possess cell walls. Although cell walls can be restrictive, they are advantageous. Remember, plant cells remain in contact with one another across cell walls via plasmodesmata. In theory then, the entire cytoplasmic volume of a plant is continuous. Botanists refer to this as the "living" symplast. Extracellular water, minerals, and chemical compounds found around cells, in spaces of cell walls, and within intercellular spaces make up the "non-living" apoplast. Transport between cells in the "non-living" fluids is termed apoplastic whereas transport across cytoplasmic membranes through the "living" cytoplasm of cells is called symplastic.
Now, roll over the nucleus in the upper right cell to see plasmodesmata.
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You can see some plasmodesmata here between these sclerenchyma type cells. Cells to the top are fibers and the larger, irregular cell to the bottom is a sclereid. Look carefully and you can see part of the symplast in this large sclereid. We will learn more about these cell types later.
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Cytoplasmic Membranes and Transport
Within the supportive cell walls, cytoplasmic, vacuolar, mitochondrial and other membranes of plant cells are very active, using transport mechanisms to create osmotic and electrochemical gradients of considerable value to the whole plant. Active transport mechanisms enable plant cells to adjust solute concentrations across membranes. The osmotic forces that result promote movements of water. Plants use these forces for support and transport.
Consider, for example, what happens to an herbaceous plant or freshly picked flower kept out of water. Both droop, indicating water has a key role in structural support. This support is primarily a product of internal turgor pressure within plant cells. Turgor pressures develop as membrane transport mechanisms increase concentrations of osmotically active solutes within cells. Water drawn to these solutes causes expansion of the cytoplasmic membranes (i.e. like tiny balloons). Cell walls protect the cytoplasmic membranes from bursting as the continued influx of water creates a substantial internal pressure. In plant stems, cells "pressurized" in this manner provide strong but elastic support during shoot elongation. In seeds and roots, these same forces help fuel the expansion of cells that enables embryonic tissues to break through the hardest soils.
Solutes moved across membranes also contribute to transport. In early spring, deciduous trees emerge from dormancy to mobilize energy by splitting smaller sucrose molecules from starch. Membranes of root cells use active transport to concentrate the sucrose in phloem, the sugar transporting tissue. The osmotically active sugar molecules strongly attract water and this in turn creates a driving force for sugar movement up the stem. Although the sugar is intended as fuel for production of new cells in apical meristems of buds, we collect some as maple syrup.
Now, roll over the upper part of the membrane in the diagram!