Transport in Plants - Notes | Class 11 | Part 5: Uptake and Transport of Mineral Nutrients, Phloem Transport
Cell Cycle and Cell Division - Mitosis
Uptake and Transport of Mineral Nutrients
Uptake of Mineral Ions
Most minerals are actively absorbed by the roots because:
Minerals occur in the soil as charged particles (ions) that cannot move across cell membranes.
The concentration of minerals in the soil is lower than that in the root.
Active uptake of ions contributes to the water potential gradient in roots, facilitating water uptake by osmosis.
Some ions are absorbed passively.
Specific membrane proteins in root hair cells actively pump ions from the soil into the epidermal cells.
The endodermal cell membrane also contains transport proteins that selectively allow certain solutes to cross, acting as control points where the plant adjusts the quantity and types of solutes reaching the xylem.
The suberin in the root endodermis ensures active transport of ions occurs in one direction only.
Translocation of Mineral Ions
Ions that reach the xylem are transported to all parts of the plant through the transpiration stream.
The chief sinks for mineral elements include:
Growing regions such as apical and lateral meristems.
Young leaves.
Developing flowers, fruits, and seeds.
Storage organs.
Unloading of mineral ions at fine vein endings occurs through diffusion and active uptake by cells.
Mineral ions are remobilized, particularly from older, senescing parts (e.g., older dying leaves) to younger leaves.
Elements most readily mobilized are phosphorus, sulfur, nitrogen, and potassium. Structural components like calcium are not remobilized.
Nitrogen is primarily transported in organic forms, such as amino acids and related compounds, though some travels as inorganic ions. Phosphorus and sulfur are also carried as organic compounds.
There is an exchange of materials between xylem and phloem, so xylem does not exclusively transport inorganic nutrients, nor does phloem transport only organic materials.
Phloem Transport: Flow from Source to Sink
Phloem transport is the long-distance movement of organic substances (primarily sucrose) from a source (region of synthesis, e.g., leaves) to a sink (region of storage or utilization).
The source and sink may reverse depending on the season or plant needs. For example, in early spring, sugar stored in roots moves to tree buds for growth, making roots the source and buds the sink.
Movement in the phloem can be upward or downward (bi-directional), unlike xylem, which is always upward (unidirectional). Thus, phloem sap can transport food in any direction.
Phloem sap mainly consists of water and sucrose, but also includes other sugars, hormones, and amino acids.
The Pressure Flow (Mass Flow) Hypothesis
This hypothesis explains the mechanism of sugar translocation in the phloem.
Glucose produced at the source (via photosynthesis) is converted to sucrose.
Sucrose is actively transported (loading) into companion cells and then into living phloem sieve tubes, creating a hypertonic condition in the phloem (lowering water potential).
Sieve tube cells form long columns with holes in sieve plates. Cytoplasmic strands pass through these holes, forming continuous filaments.
Water from adjacent xylem moves into the phloem by osmosis. As osmotic/hydrostatic pressure builds, phloem sap moves to areas of lower osmotic pressure (sink).
Diagrammatic presentation of mechanism of translocation
Sucrose from phloem sap is actively transported into sink cells, which convert it into energy, starch, or cellulose.
As sugars are removed, osmotic pressure decreases (water potential increases), and water moves out of the phloem.
Identification of the Tissue that Transports Food (Girdling)
Carefully remove a ring of bark (including the phloem layer) from a tree trunk.
After a few weeks, the portion above the ring swells due to the blocked downward movement of food.
This demonstrates that phloem is responsible for food translocation and that transport occurs in one direction, toward the roots.
