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When roots take up nutrients and water from the soil, it is distributed across diverse tissues within the whole plant. The products of photosynthesis, likewise, must travel throughout the plant to cells with storage functions or energy-requiring processes, crossing cell walls, membranes, and the cytoplasm of many cells along the way.

Plants can move solutes using three general pathways. Transport via the apoplastic pathway occurs through the extracellular space including cell walls, while transport via the symplastic pathway occurs through plasmodesmata, pores that directly connect the cytoplasm of neighboring cells. 

A third pathway, the transmembrane pathway, moves substances in and out of cells through the plasma membrane. Repeatedly moving substances across the plasma membrane is sufficiently fast over 2 or 3 cells, but much slower over longer distances. 

Transport across membranes in plant cells bears some general similarities to transport in animal cells. The selectively-permeable plasma membrane allows some substances, such as carbon dioxide and oxygen gas, to diffuse through passively, moving along their concentration gradients from areas of high concentration to areas of l