UPTAKE AND TRANSLOCATION OF MINERAL NUTRIENTS:
· There are 113 or so different elements in this planet, in which 14 or 15 are absolutely required for the life processes without which plants exhibit diseased symptoms and ultimately die.
· Such elements which are absolutely required for the normal growth and development of the plant body are called essential nutrients. Among them,, 8 elements are required in sufficient quantities are known asmacronutrients like carbon, nitrogen, phosphorous, potassium, calcium, sulphur, magnesium, iron while others required in small quantities are referred as micronutrients, such arezinc, boron, copper, molybdenum, cobalt. Non-essential elements are sodium, aluminum, silicon,, chlorine, gallium etc. in the atmosphere. However, their remaining nutrient requirement
· Plants obtain their carbon and most of their oxygen from CO2 in the atmosphere. However, their remaining nutritional requirement is obtained from minerals and water for hydrogen in the soil.
Uptake of mineral ions:
Unlike water, all minerals cannot be passively absorbed by the roots due to two reasons:
a) The concentration of minerals in the soil is usually very low.
b) They are present as charged particles which cannot move across cell membrane.
These minerals move into the roots by using energy in the form of ATP. Specific proteins are present in the membrane of root hair cells which actively pump ions from the cells to the cytoplasm of the epidermal cells that act as control points for the type and quantity of the solutes that reach the xylem. Ions are absorbed from the soil.
Movement of mineral ions:
After the uptake, minerals are transported to all other parts by transpiration pull. Minerals are also moved from the older parts of the plant to the newer parts of plant. For example, when the old leaves are about to fall, the minerals are transported to the other parts of plants. Some mobilized minerals are phosphorous, sulphur, nitrogen, potassium.
Absorption of ions:
Ions are absorbed from the soil by both passive and active transport. Specific proteins in trans membrane of root hair cells actively pump ions from the soil into the cytoplasm of the epidermal cells. Like all cells, the epidermal cells have many transport proteins embedded in their plasma membrane, they let some solutes cross the membrane. Transport proteins of endodermal cells are control points, where a plant adjusts the quantity and type of solutes that reach the xylem.
Translocation of mineral ions:
· After the ions have reached xylem through active or passive uptake or a combination of two, their further transport up to the stem to all parts of the plant is through the transpirational stream.
· The chef sinks for the mineral elements are the growing regions of the plant, such as the apical and lateral meristems, young leaves developing flowers, fruits and seeds and the storage organs.
· Unloading of mineral ions occurs at the fine vein endings through diffusion and active uptake by these cells.
· Mineral ions are frequently remobilized particularly from older senescing parts older dying leaves export much of their mineral content to younger leaves. Similarly, before leaf fall in deciduous plants; minerals are removed to other parts. Elements more readily mobilized are phosphorous, sulphur, nitrogen and potassium. Some elements that are structural compounds like calcium are not remobilized.
· An analysis of the xylem exudates shows that though one of the nitrogen travels as inorganic ion, much of it is carried in the organic form as amino acid and related compounds. Similarly small amounts of phosphorous and sulphur are carried as organic compounds.
Plants need water to grow and maintaining the turgidity of cell. There are three types of transport occur in plants: Diffusion, facilitated diffusion and active transport.
Water is the most abundant constituent of all physiologically active plant cells. Several physiological processes of plants like osmosis, imbibition, plasmolysis are accomplished by water.
Transpiration explains how water moves up the plant against gravity in tubes via a xylem. Plants pump the water up from the soil to deliver nutrients to their leaves. It is influenced by several factors.
Opening and closing of stomata is controlled by concentration of solutes in guard cells. Some factors like light, temperature, CO2, O2,hormones regulate the opening and closing of stomata.
Diffusion is the movement of particles from the region of higher concentration to the region of lower concentration. It is affected by temperature, pressure, density of medium, distance,number of particles per unit volume.
Food is synthesized through the process of photosynthesis. It can be transported to different parts of the plant, this function is performed by phloem.
Water transport is important for the uptake of mineral nutrients from the soil. There are three pathways which facilitate the movement of water in plants like apoplast, symplast, trans-membrane pathways.
Mass flow hypothesis states that the mass flow of solute take place from the source(mesophyll cells) to the sink (where photosynthesis needed). This theory was proposed by Ernst Munch.