The apoplast and symplast pathways are two different pathways by which water can move through the root of a plant.

Water in the apoplast pathway moves from cell to cell via spaces in the cellulose cell walls until it reaches the endodermis. At the endodermis, there is the ‘apoplast block’ – the cellulose cell walls of the cells of the endodermis have a substance called suberin which is impermeable and prevents the movement of water. The suberin makes up what is called the Casparian strip. At this point, all the water has to move into the vacuolar and symplast pathways. The function of the apoplast block is to prevent harmful substances from entering the xylem.

The symplast pathway is where water moves from cell to cell in the cytoplasm via the plasma membranes and plasmodesmata. Water moves along the root by osmosis down a water potential gradient (as water moves into one cell, this cell then has a higher water potential than the adjacent cell, so water moves from cell to cell by osmosis). Water moves in this way along in the cytoplasm from the root hair cell to the endodermis.

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