Table of Contents
Introduction
Plants obtain carbon and oxygen from the atmosphere. The remaining mineral nutrients are absorbed from the soil through the plant roots.
Mineral nutrients are the chemical elements required as essential nutrients by all plant species to perform all the necessary functions.
Altogether, plants require thirteen different types of minerals. Among calcium, magnesium, nitrogen, phosphorus, potassium and sulphur are six essential mineral nutrients which are required in large quantities and are often called macronutrients. These macronutrients are involved in different functions including:
- Rapid growth
- Provides strength to plants
- Essential for photosynthesis
- Improving the quality of fruits
- Reduction of diseases in plants
- Helps in chlorophyll formation
- Improves root growth and seed production
- For building proteins and other biomolecules
Uptake of Mineral Nutrition
Mineral uptake is the natural process in which all the essential minerals enter the plant’s cellular material, typically following the same pathway as water. The uptake of mineral nutrients occurs at both the roots and the leaves.
The process of uptake or transport of mineral nutrients is carried out by the plant cells in two different ways:
Passive Absorption
It is the absorption of minerals without the direct expenditure of metabolic energy.
Active Absorption
It is the absorption of minerals with the direct expenditure of metabolic energy. During the active absorption of minerals, ions from the outer space of the cell move into the inner space and it generally occurs against the concentration gradient. Hence it requires metabolic energy and this energy is obtained from the cell’s metabolism either directly or indirectly.
All the minerals cannot be transported passively in the roots as the concentration is low in the soil and moreover they are present as a charged particle, which cannot cross the cell membrane. These are actively transported in the roots using energy stored as ATP. Minerals are transported by specific proteins present in the membrane of the root hairs. Transport proteins are embedded in the plasma membranes of endodermal cells and control the type and amount of solutes reaching the xylem.
Movement of Mineral Ions
From roots, minerals are then transported to all the other parts by transpiration pull. Minerals are required in more quantity by all the growing parts. Minerals from the older parts also get transported to the new parts. For example, minerals from old leaves are transported to other parts when they are about to fall. Elements that are mobilized are nitrogen, phosphorus, potassium, sulphur, etc.
Transport in Phloem
Transport of food occurs by phloem from the leaves to the parts of the plant where it is needed or stored. The leaves and the parts of plants where the food is needed are called “source” and “sink” respectively. The source and sink may change with seasons. The roots might become the source in early spring when the buds act as a sink. The direction of the movement of food in phloem is bidirectional which means it could be upwards or downwards. The phloem sap mainly consists of water and sucrose.
Pressure Flow or Mass Flow Hypothesis
The mechanism used for the translocation of food (sugars) from source to sink is called the pressure flow hypothesis. Food production happens in the leaves through the process of photosynthesis. This food is mainly glucose. It is then converted to sucrose which is moved to the companion cells and the live phloem sieve tube cells by active transport. A hypertonic condition is created in the phloem because of which water moves in the phloem from the xylem by the process of osmosis. Due to the buildup of osmotic pressure, phloem sap moves to areas of lower pressure. Osmotic pressure is reduced at the sink. Active transport is needed to move sucrose out of the sap and into the cells which will use sugar and it gets converted to energy, starch or cellulose. When sucrose moves out of the sap, osmotic pressure decreases and water moves out of the phloem.
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