Plants get their carbon and most of their oxygen from the atmosphere’s CO2, their hydrogen from water, and the rest from minerals they pick up individually from the soil. Minerals reside in the soil as ions that cannot cross cell membranes. Ion concentrations are 100 times higher in the root interior than in the soil. As a result, no minerals can be absorbed passively.
The passage of ions from the soil to the interior of the root goes against the concentration gradient and necessitates active transport. The membrane of root hairs contains specific ion pumps. They transport mineral ions from the earth to the cytoplasm of root hair epidermal cells. ATP is the source of energy. Respiratory inhibitors such as cyanide, which inhibit ATP generation, are commonly used.
Even without ATP, the minimal amount that enters the root must be accomplished through a passive method. ATPases are present on the plasma membranes of root epidermal cells for active transport.
They create an electrochemical proton gradient to provide energy for ion transport. Ions are examined again and transferred inwardly by transport proteins found on endodermal cells. Endodermis permits ions to move within but not outward.
Uptake of mineral ions
- Unlike water and other minerals, most minerals are actively absorbed by the roots. It is due to the following factors:
- Minerals are found in soil as charged ions that cannot cross cell membranes.
- Mineral concentrations in soil are often lower than those in roots. As a result, roots cannot take all minerals passively.
- For the reasons stated above, the majority of minerals enter the root via active absorption into the cytoplasm of epidermal cells. This transfer of ions from the soil to the core of the root demands energy and works against a concentration gradient.
- Ion active uptake is responsible for the water potential gradient in roots, which aids in water uptake via osmosis.
- Passive transport occurs via mass flow or bulk flow systems and diffusion, whereas active transport happens via particular proteins in root hair cell membranes. These proteins actively pump ions from the soil into the cytoplasms of root hair epidermal cells. Energy is provided through ATP.
- Every cell, including endodermal cells, has several transport proteins in its plasma membrane; certain solutes pass the membrane but others are restricted. The xylem is responsible for upward movement, while the phloem is responsible for bidirectional flow.
- Endodermal cell transport proteins act as control points for plants, allowing them to regulate the amount and kind of solutes that reach the xylem based on their fluctuating needs. The layer of suberin covers the root endodermis, and they let the ions move through one direction only.
Translocation Routes
After absorption, dissolved minerals can be translocated in a variety of directions:
- Organic Produce Downward Translocation: From the leaves to the root and other sections of the plant.
- Organic Produce Upward Translocation: Roots to leaves or other apical regions.
- Mineral ion uptake occurs via active transport across the xylem.
- Mineral Upward Movement: The movement of ions from the sap to the leaves.
- Lateral mineral translocation occurs in woody stems in a tangential orientation.
- Radial translocation is the movement of organic solutes from the pith to the cortex via medullary rays.
Translocation of mineral ions
- The inorganic solute substances are transported in the transpiration stream by the ascending sap of water in the xylem vessels. In contrast, phloem is the channel for organic solute downward translocation. Translocation always occurs from the supply end (source), i.e., the region with the highest concentration of soluble form, to the consumption end, which has the lowest concentration of soluble form (sink).
- After the ions reach the xylem vessels by active or passive absorption, or a mix of the two, they are transported upwards to the stem and other regions of the plant via the transpiration stream, allowing minerals to flow from their conducting tissue to the area of their sink.
- Diffusion happens at the tips of fine veins and allows minerals to escape. Active uptake occurs when cells take them up. Mineral ions are often remobilized throughout the plant, especially from older senescing portions to younger growing parts.
- Falling or dying leaves export a large portion of their minerals to younger leaves and other sections. Phosphorus, nitrogen, and potassium are the elements that are most easily mobilised. Only elements contained in structural components, such as calcium, are not remobilised. Young developing leaves and other sinkers have access to mineral remobilisation.
- Translocation of organic solutes is critical in higher plants because during seed germination, the insoluble reserve food material of a seed is converted into a soluble form that is supplied to the plant’s growing regions, such as the apical and developing flowers, lateral meristems, young leaves, fruits and seeds, and storage organs.
- Only the green sections of higher plants can produce food. As a result, the position of photosynthesis differs from the place of nutrient storage. As a result, nutrients must be translocated throughout the plant and delivered to other non-green regions for consumption and storage.
Conclusion
Though it is commonly assumed that xylem delivers inorganic nutrients and phloem transports organic nutrients, this is not entirely correct. Nitrogen flows in xylem sap as inorganic ions as well as organic forms of amino acids and associated substances. Small amounts of P and S are carried as organic molecules in the xylem.
Materials are also exchanged through xylem and phloem. As a result, mineral elements travel through the xylem in both inorganic and organic forms. They get to their sink, which is young leaves, developing flowers, fruits and seeds, apical and lateral meristems, and individual storage cells. Diffusion allows minerals to be emptied at fine vein ends. Active uptake occurs when they are taken up by cells.
Minerals from previous senescing sections are being remobilized. Nickel plays a significant part in this activity. Many minerals, such as nitrogen, sulphur, phosphorus, and potassium, are released by the senescing leaves. Calcium, for example, is not remobilized since it is included in structural components. Minerals that have been remobilized become accessible to developing leaves and other sinks.