34.11:
Water and Mineral Acquisition
Water accounts for over 80% of the mass of most plants. It is essential for photosynthesis, metabolism, and the transport of nutrients and other molecules. Inorganic nutrients are likewise crucial for plant growth and reproduction. For instance, nitrogen is a building block for key biomolecules, such as amino acids, while potassium is critical for the opening and closing of stomata. How do plants reliably absorb nutrients and large amounts of water?
Most plants acquire water and minerals from the soil in which they are rooted. Plant roots release hydrogen ions and carbon dioxide. Carbon dioxide reacts with water, forming a bicarbonate anion, and additional hydrogen ions. The hydrogen ions bind to negatively charged soil particles, which releases positively charged ions, like potassium, into the soil solution. This process is called cation exchange and makes nutrients available to the plant.
The root architecture and branching patterns are essential for efficient water and nutrient uptake. For instance, the outer layer of cells that is in contact with the soil forms tiny root hairs. These specialized cells have a large surface area that absorbs water through both active and passive processes.
For instance, water and dissolved nutrients may passively diffuse into the apoplast, which comprises all cell walls and spaces between cells. The space within the cell membrane is called the symplast. Water enters the symplast via osmosis across the cell membrane, while nutrients are taken up via active transport.
Once water and minerals enter the symplast, they move freely towards the center of the root through cell-to-cell connections. Water and nutrients in the apoplast, however, are blocked from entering the stele by Casparian strips. Casparian strips are layers of water-impermeable suberin in the cells of the endodermis, which surrounds the stele. These structures block the passage of potentially undesirable or toxic elements.
In order to enter the center of the root, all solutes need to cross a plasma membrane. Once they enter the symplast water and nutrients enter the stele, which distributes them throughout the plant to fulfill their essential roles.
34.11:
Water and Mineral Acquisition
Specialized tissues in plant roots have evolved to capture water, minerals, and some ions from the soil. Roots exhibit a variety of branching patterns that facilitate this process. The outermost root cells have specialized structures called root hairs that increase the root surface, thus increasing soil contact. Water can passively cross into roots, as the concentration of water in the soil is higher than that of the root tissue. Minerals, in contrast, are actively transported into root cells.
Soil has a negative charge, so positive ions tend to remain attached to soil particles. To circumvent this, roots pump carbon dioxide into the soil, which spontaneously breaks down, releasing positively charged protons (H+) into the soil. These protons displace soil-associated positively charged ions that are available to be pumped into the root tissue, a process called cation exchange. Negatively charged anions exploit the tendency of H+ ions to diffuse down their concentration gradient and back into root cells using co-transport: ions like Cl- are cotransported into the root tissue in association with H+ ions.
Molecules can travel into the core of the root tissue, called the stele, by two routes. Apoplastic transport is the movement of molecules in the spaces created between the continuous cell walls of neighboring cells and their corresponding membranes. In contrast, symplastic transport is the movement of molecules through the cytoplasm of plant cells, which utilizes cellular junctions called plasmodesmata, which allow the free cytoplasmic passage of molecules between adjacent cells. In order to enter the stele, molecules must move into the symplast, as Casparian strips located in the root's endodermis prevent passage of solutes in the apoplast from entering the stele. Therefore, in order to enter into the symplast, solutes must pass through a cell's semipermeable membrane, protecting cells from toxic or unwanted molecules from the soil.
Suggested Reading
Barberon, Marie, and Niko Geldner. "Radial Transport of Nutrients: The Plant Root as a Polarized Epithelium." Plant Physiology 166, no. 2 (October 1, 2014): 528–37. [Source]
Kim YX, Ranathunge K, Lee S, Lee Y, Lee D, Sung J. "Composite Transport Model and Water and Solute Transport across Plant Roots: An Update." Front Plant Sci. 2018 Feb 16;9:193. [Source]