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The Pathway of Water through a Plant OCR Unit 1 Module 2 Root Hair Cell 1 1. Minerals ions either move down their diffusion gradient into the root hair cell or are actively transported across the membrane into the root hair cell using energy from ATP. 2. This lowers the water potential in the root hair cell so water will flow into it from the soil. 3. Water including dissolved mineral ions will move through the cortex cells down the diffusion gradient until it reaches the xylem. The pathway taken through the cortex cells could be the APOPLASTIC, the SYMPLASTIC or the VACUOLAR pathway. 3 2 Pathways A) B) C) The apoplast pathway. The cell walls are made of cellulose strands. Water can travel in between these strands and through spaces between cells. The water does not pass through any membranes so the mineral ions are carried with the water. The symplast pathway. Water enters the cytoplasm through the plasma membrane of the root hair cell. It can then travel from cell to cell through plasmodesmata. These are gaps in the cell wall that allow cytoplasm from adjacent cells to connect. The vacuolar pathway. Similar to the symplast pathway but the water can enter the vacuoles as well. Casparian Strip • The endodermis is a layer of cells surrounding the xylem in the root. It is sometimes called the starch sheath as it contains granules of starch (a sign that energy is being used). • The endodermal cells also have a Casparian Strip. This is a band of waterproof suberin in their cell wall. This blocks the apoplast pathway between the cortex and the xylem. Functions • The strip ensures water and dissolved ions (e.g. nitrate ions) have to pass into the cytoplasm through cell membranes. • There are transporter proteins in the cell membranes that actively transport nitrate ions from the cortex into the xylem. •This lowers the water potential in the xylem and so water enters in by osmosis. • In this way the plant has control over substances entering the xylem. • Once in the xylem the water can move by root pressure, capillary action or by the transpiration pull. • Water molecules are attracted to each other by forces of COHESION. These forces hold the water molecules together in one continuous column. • As water is lost from the top of the column, the whole column is pulled up as one chain – the transpiration stream. • The pull can create tension in the column of water. The xylem vessels need to be reinforced with lignin to stop them collapsing under the tension. The COHESION- TENSION theory relies on an unbroken column of water. If one xylem vessel gets damaged the column can be maintained through another vessel via the pits. • Most of the water exits the plant through stomata (tiny pores in the epidermis) • Water evaporates from the cells immediately above the guard cells and diffuses out of the leaf. • This lowers the water potential in those cells, and so water enters them by osmosis from surrounding cells and so on, until eventually water leaves the xylem and enters the innermost leaf cells.