Uptake and Translocation of Mineral Nutrients     

Definition:

Mineral uptake is the process in which 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 important 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. Whereas, their remaining nutritional requirements are fulfilled from water and minerals present in the soil.

A brief outline of the topic:

Uptake and translocation of mineral nutrients by plants is an important aspect of the life cycle of plants. Basically, the roots of plants take minerals from the soil and pass them further towards other parts of plants through vascular tissues present in plants like Xylem and 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.

A brief note:

Uptake of mineral ions:

Unlike water, all minerals are not passively absorbed by the roots.

There are two factors that account for this:

1. Minerals which are present in the soil are present as Charged particles or ions which cannot move across cell membranes and
2. The concentration of minerals in the soil is lower as compared to that in the roots.

• Therefore, the majority of the 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 the uptake of water by osmosis. There are also some ions that move into the epidermal cells through passive transport.
• Certain proteins found in the membranes of root hair cells actively pump ions from the soil into the cytoplasms of epidermal cells.
• Like every other cell, the endodermal cells contain many transport proteins which are embedded in their plasma membrane; so they let some solutes to cross the membrane, but not to others.

Note:

➤ Transport proteins of endodermal cells are control points, where the plant is able to adjust the quantity and the types of solutes that can reach the xylem.

➤ The endodermis of the root, due to the layer of suberin, has the capability to actively transport ions in a single direction only.

Translocation of Mineral ions:

• When ions reach the xylem via active or passive absorption or a combination of the two, they are transported upward to all parts of the plant via the transpiration stream.
• The chief sink for the nutrient elements is the growing parts of plant-like, young leaves, apical and lateral meristems, developing flowers, fruits, seeds, etc.
• Unloading of ions occurs at the very fine vein endings through diffusion and active uptake by these cells.
• Mineral ions are remobilized a lot specifically from older parts. Old dying leaves give much of their mineral content to younger leaves.
• Elements that are 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 were believed traditionally.

Phloem Transport: Flow from Source to Sink

Basically, 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 sink relationship are variable so the direction of movement of materials in the phloem can be both upwards or downwards, i.e., Bi-directional. But the movement in the xylem is always upwards i.e., Uni-directional.

Phloem sap primarily contains water and sucrose, but it also transports other materials such as sugars, hormones, and amino acids.

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 which produces 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.
• The osmotic pressure drops and water moves out of the phloem as soon as the sugars are removed.

Also read: Important Topic of Biology: Mineral Toxicity

FAQs

What is Active Transport?

It is the transport of molecules from the area of low concentration to high concentration. It does take place outside the cell and moves from inside. It requires energy in the form of ATP.

What is Passive Transport?

It is the transport of molecules from the area of high concentration to low concentration. It is the major form of transport in the cell. It does not require any kind of energy.

What is Translocation?

It is basically the transport of mineral nutrients from the source towards the sink.

What are Organic nutrients?

Organic nutrients are nutrients that contain carbon in their structural makeup. For example, carbohydrates, Lipids, Proteins, and Vitamins.

What are Inorganic nutrients?

The nutrients that do not contain any carbon are referred to as inorganic nutrients. For example, Iron, Selenium, Zinc, etc.