Introduction
Mineral uptake is when minerals enter the cellular material, typically following the same pathway as water. The uptake of minerals occurs both at roots and leaves. The majority of mineral nutrients required by plants are essential for their growth and maintenance. There are some Essential minerals without which the plants cannot complete their life cycle. Plants get their carbon and most of their oxygen from carbon dioxide present in the atmosphere. Their remaining nutritional requirements are fulfilled from water and minerals present in the soil.
Body
Uptake of mineral ions
Unlike water, all minerals are not passively absorbed by the roots.
Two factors account for this:
- Minerals which are present in the soil are present as charged particles or ions which cannot move across cell membranes and
- Usually, the concentration of minerals in the soil is lower than in the roots.
Therefore, most minerals must enter the roots by active absorption into the cytoplasm of epidermal cells.
The uptake of ions through active absorption is partially responsible for the water potential gradient in roots and, therefore, for water uptake by osmosis. There are also some ions that move into the epidermal cells through passive transport.
There are some specific proteins located in the membranes of root hair cells that actively pump ions from the soil into the cytoplasms of the epidermal cells.
Like every other cell, the endodermal cells contain many transport proteins embedded in their plasma membrane, so they let some solutes cross the membrane, but not others.
Note:
➤ Transport proteins of endodermal cells are control points, where the plant can adjust the quantity and the types of solutes that can reach the xylem.
➤ The endodermis of the root, due to the layer of suberin, can transport ions in a single direction only actively.
Translocation of Mineral ions
➨When the ions have reached xylem through active or passive absorption or by combining these two, their further transport upward to all the parts of the plant is through transpiration stream.
➨ The top sink for the nutrient elements is the growing parts of plants like young leaves, apical and lateral meristems, developing flowers, fruits, seeds, etc.
➨Unloading of ions occurs at the fine vein endings through diffusion and active uptake by these cells.
➨Mineral ions are remobilised a lot, specifically from older parts. Old dying leaves give much of their mineral content to younger leaves.
➨ Elements most readily mobilized are potassium, nitrogen, sulfur, and phosphorus. Calcium cannot remobilize as it is a structural component of a plant.
➨ A small number of materials do exchange between xylem and phloem. Hence we cannot make a difference and say that xylem transports only the inorganic nutrients while the phloem transports only organic nutrients, which was believed traditionally.
Phloem Transport: Flow from Source to Sink
➨ Sucrose is transported by the phloem from source to sink. Generally, the source is understood to be that part of the plant which prepares food, i.e. leaves, and sinks to that part that needs or stores the food.
Note: The source and sink may be reversed depending on the plant’s need or the season.
➨Though the source and sinks’ relationship is variable, the direction of movement of materials in the phloem can be both upwards or downwards, i.e., bi-directional. Still, the movement in the xylem is always upwards, i.e., uni-directional.
➨ Phloem sap mainly contains water and sucrose, but other materials like sugars, hormones, and Amino acids are also translocated through the phloem.
The Pressure Flow or Mass Flow Hypothesis
➨The mechanism which is understood for the translocation of sugars from source to sink is known as the Pressure Flow Hypothesis.
➨ The sucrose moves into the companion cells and then directly into the living phloem sieve tube cells by active transport, producing hypertonic conditions in the phloem.
➨ Water from the adjacent xylem moves into the phloem sap by osmosis which helps in building pressure in the phloem sap, which will move to the areas of lower pressure.
➨ Active transport is again required to move the sucrose out of the phloem sap and into the cells, which will use sugar to convert it into energy, starch, or cellulose.
➨ As soon as the sugars are removed, the osmotic pressure decreases, and water moves out of the phloem.
Conclusion
The pressure-flow hypothesis is a mechanism used for the translocation of food from source to sink. The process of photosynthesis in plants led to food production in leaves in the form of glucose. It is then converted to sucrose which is moved to the companion cells and then the live phloem sieve tube cells by active transport. The water moves in the phloem from the xylem due to the hypertonic condition created in the phloem by the process of osmosis. Phloem sap moves to the areas of lower pressure due to the build-up of osmotic pressure. At the sink, the atmospheric is reduced. In order to move sucrose out of the sap active transport is needed which will use sugar and it gets converted to energy, starch, or cellulose. When sucrose moves out of the sap led to decrease in the osmotic pressure and water moves out from the phloem.
Minerals may enter roots by Active absorption, which uses energy and by passive absorption which does not use any energy. Transport of mineral nutrients in plants may be multidirectional or unidirectional. Substances that are to be transported are plant growth regulators, organic nutrients, mineral nutrients, and water. For small distances, substances move by diffusion, and for longer distances, substances move through with the help of the vascular system in plants. Transport of nutrients may be affected by internal factors and external factors as well.