Uptake and Translocation of Mineral Nutrients: Mechanism of Nutrient Uptake, Xylem Loading, Phloem Unloading

Uptake and Translocation of Mineral Nutrients: Did you know some plant species can accumulate nearly 60 types of elements? Some can accumulate selenium; some can gold. In fact, plants growing near nuclear test sites are seen to take up radioactive strontium. The elements which plants accumulate are not necessarily used by plants. Fascinating! Isn’t it? It is curious to think how a plant can take up minerals present in soil and utilise them or store them. What’s even more curious is how the plants distribute and translocate these minerals evenly. Read further to learn in detail about the uptake and translocation of mineral nutrients.

Learn About Phloem Transport Here

Essential Mineral Elements

Few elements have been found to be absolutely essential for the growth and metabolism of plants, and they are called essential elements. In general, plants require 13 different types of minerals. These elements can be divided into two based on their quantitative requirements.

1. Macronutrients: These are required in large quantities. These include carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, potassium, calcium and magnesium.
2. Micronutrients: These are required in trace amounts. These include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.

Uptake of Mineral Nutrient

Plants can absorb minerals via aerial parts like leaves, stomata etc., as well as roots. The majority of nutrients are absorbed from the soil through the root system. Plant roots have three different regions: root hair, zone of elongation and meristematic zones. Each zone has a different rate of nutrient absorption. Based on studies, the meristematic zone absorbs more nutrients than others. The presence of fungal hyphae called Mycorrhiza can enhance the absorption ability of the root system.

Fig: Essential Elements Required for Plants

Mechanism of Nutrient Uptake

The mechanism of uptake of mineral nutrients in plant cells can be of two types: active and passive.

Passive Uptake of Nutrients

Passive uptake of nutrients is simple absorption along concentration gradients without any energy expense. Passive uptake of nutrients may be by simple or facilitated diffusion. Transport can occur through channels for a few smaller ions or water.

Fig: Passive Absorption Vs. Active Absorption

Active Uptake of Nutrients

The amount of some minerals is relatively lower in soil than inside the root. Active uptake of minerals is required to transfer minerals from outside the cell to inside the cell. The energy for active uptake can be derived directly by absorption of light or indirectly by ATP for the process of nutrient uptake. Molecules or ions present in soil move across a membrane against a concentration gradient with the help of protein molecules present in the membrane called carrier proteins. Carrier protein binds to the mineral and transfers it inside the cell at the expense of energy.

Xylem Loading

Xylem loading is the process of transfer of ions from the root system to conducting xylem cells. After entering root cells, ions dissolved in water move radially to enter the xylem through both apoplastic and symplastic pathways.

The apoplastic pathway involves the transportation of dissolved ions through intercellular spaces. The apoplast is an additional pathway that consists of the cell wall and intracellular space. It is a permeable route where mineral movement occurs by passive diffusion. No involvement of the metabolic path causes rapid water movement.

The symplastic pathway involves the transportation of dissolved ions via cell-to-cell connection through the plasmodesmata. Symplastic movement occurs through protoplasts. It is a selectively permeable route where mineral movement occurs by osmosis. The involvement of metabolic pathways causes the slower movement of water and minerals.

Minerals along with water reach the cortex via these pathways. The endodermis is impervious to water due to the presence of the Casparian strip. The Casparian strip is made up of suberin, after and minerals which reach there via apoplastic or symplastic route now cross the casparian strip and endodermis via plasmodesmata. Once inside the endodermis, water and minerals can again travel by either the apoplastic or Symplastic pathway.

Ions exit the symplast to enter the trachea by crossing a plasma membrane.

Translocation of Mineral Nutrients

1. Translocation is defined as the bulk transfer of water and dissolved minerals from one part of the plant to another through the xylem and phloem.
2. Mineral salts are translocated through the xylem along with the ascending stream of water. A small amount of exchange of materials does take place between the xylem and phloem.
3. Mineral ions like phosphorus, sulphur, nitrogen and potassium are stored in older, senescing parts like old leaves. These are frequently remobilised to the younger leaves. Some elements like calcium that are structural components are not remobilised.
4. Most nitrogen, phosphorus and sulphur are carried in the organic form as amino acids and related compounds. Some amounts of nitrogen are translocated as inorganic ions as well.

Fig: Translocation of Minerals

Translocation in Phloem

The photosynthetically produced sugar is converted into sucrose in the green part of plants. Now the sugar is loaded into the phloem and translocated to the rest of the plant. The sucrose is transferred from the source to the sink, where the source is the green part of the plant, and the sink is considered a part that stores sucrose. The direction of movement in phloem is bi-directional, i.e., both upward and downward. Translocation in phloem is slower than xylem transport. Fluid within phloem tissue moves both upward and downward.

Pressure-Flow Hypothesis

This is the most accepted hypothesis of translocation in the phloem. This hypothesis states that dissolved sugar moves in phloem due to a pressure gradient.

1. Phloem loading: It is an active process, i.e., it uses the energy of ATP. Transportation occurs to the sieve tubes by the veins of a leaf. The pressure difference exists between source and phloem, which cause phloem loading and also between phloem and sink, where phloem is unloaded. The source is the area of higher turgor pressure, whereas sink is the area of lower turgor pressure.
2. Translocation: Sugar is actively loaded into the sieve-tube element at the source at the expense of ATP. It causes water movement into the sieve-tube element, increasing pressure in the phloem. The increased pressure allows the movement of fluid through the sieve tube.
3. Phloem unloading: Once the fluid reaches the sink, sugar is actively or passively unloaded, causing water movement from the sieve-tube element to the xylem. It is a passive process that occurs along the concentration gradient of sucrose. Phloem unloading requires energy for respiration and biosynthetic reactions.

Summary

Few elements are absolutely essential for the growth and metabolism of plants, and they are called essential minerals. The plant can absorb minerals via aerial parts like leaves, stomata etc., as well as roots. The mechanism of uptake of mineral nutrients in plant cells can be of two types: active and passive. Passive uptake of nutrients is simple absorption along concentration gradients without any energy expense. Active uptake of minerals is required to transfer minerals from outside the cell to inside the cell utilising energy. The process of transfer of ions from the root system to conducting xylem cells is called xylem loading. Translocation is defined as the bulk transfer of water and dissolved minerals from one part of the plant to another through the xylem and phloem. Mineral salts are translocated through the xylem along with the ascending stream of water. The sucrose is transferred from the source to the sink. This is the most accepted hypothesis of translocation in the phloem. This hypothesis states that dissolved sugar moves in phloem due to a pressure gradient. The pressure difference exists between source and phloem, which cause phloem loading and also between phloem and sink, where phloem is unloaded.

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Study About Means of Transport in Plants

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