Plants obtain carbon and oxygen from the atmosphere. The remaining mineral nutrients are absorbed from the soil.
Uptake of Mineral Ions
All the minerals cannot be passively absorbed by the roots as the concentration of minerals in the soil is usually low and they are present as charged particles. So, they cannot move across cell membranes. These move into the roots by using energy in the form of ATP. Specific proteins are present in the membrane of the root hair cells which actively pump ions from the cell to the cytoplasm of the epidermal cells. Transport proteins are embedded in the plasma membranes of endodermal cells that act as control points for the type and quantity of solutes that reach the xylem.
Movement of Mineral Ions
After the uptake, minerals are transported to all other parts by transpiration pull. Minerals are required the most at all the growing points. These are also moved from the older parts to the new parts. For example, when the old leaves are about to fall, the minerals are transported to the other parts. Elements that are mobilized are phosphorus, sulphur, nitrogen, and potassium.
Transport in Phloem
Transport of food occurs in 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 movement of food in phloem is bi-directional viz. 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 translocation of food (sugars) from source to sink is called pressure flow hypothesis. Food is prepared in the leaves by 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.
This was a small discussion on the entire topic. To learn more about mineral transport, visit Byju’s.