Transport of Food and Minerals in Plants (By:- Ajay Raj)
Important Terms
(1) Translocation:- a change in location
(2) Transpiration:- The process by which water evaporates from the leaves and draws more water from the roots is called ‘transpiration’.
(3) Transportation:- Transportation is the process that involves the movement of water and necessary nutrients to all parts of the plant for its survival.
Short Notes:-
(1) Xylem transports plant water and minerals.
(2) Root hair is the hair found in the roots of plants.
(3) These root hairs absorb water and minerals from the soil because they are in direct contact with the water film between the soil particles.
(4) Through the diffusion process, water enters the root hairs.
(5) The xylem is made up of small vessels that connect the roots and leaves.
(6) Xylem transports water, minerals, and nutrients from the soil to all the plant parts.
(7) There are two types of "transport" tissues in plants- xylem and phloem.
(8) Water and solutes are transported by the xylem from the roots to the leaves, and food is transported from the leaves to the rest of the plant by the phloem.
Transportation
Transportation is a vital/important process in plants. Trees transport all the nutrients and water it needs for survival from its roots to the tips/top of the leaves.
In the case of transportation in plants, the biggest constraint is water as it ends up being a limiting factor in growth. To overcome this problem, trees and other plants have the perfect system for the absorption and translocation of water.
Plants contain a vast network of conduits which consist of xylem and phloem. This is more like the circulatory system that transports blood throughout the human body. Similar to the circulatory system in humans, the xylem and phloem tissues extend throughout the plant. These conducting tissues originate from the roots and move up through the trunks of trees. Later they branch off into the branches and then branching even further into every leaf, like spider webs.
Transportation occurs in three levels in the case of plants:
(a) Transportation of substance from one cell to another.
(b) Long-Distance transport of sap within phloem and xylem.
(c) The release and uptake of solute and water by individual cells.
Let us have a detailed look at the process of absorption and transportation in plants.
Water Absorption In Plants
The water is absorbed in two ways by the plants:
(1) Active Absorption
(2) Passive Absorption
(1) Active Absorption
In the case of active absorption, water moves through symplast and it is absorbed according to the Diffusion Pressure Deficit changes. The rate of absorption is slow. It comprises osmotic and non-osmotic forces.
Humidity and temperature can have an impact. The force required for the absorption of water is mainly generated in the root cells itself. There would be a decrease in the rate of water absorption if the metabolic inhibitors are applied.
(2) Passive Absorption
The rate of absorption is fast. It occurs in rapidly transpiring plants. The movement of plants is through apoplast and it is absorbed due to transpiration pull and it is created due to the stress created in xylem sap. The rate of absorption significantly depends upon the rate of transpiration. The force required for the absorption of water is mainly generated in the mesophyll cells.
Transportation in Plants
The water and minerals are transported in plants by two types of conducting tissues:
(1) Xylem
(2) Phloem
(1) Xylem
Xylem is a long, non-living tube running from the roots to the leaves through the stem. The water is absorbed by the root hair and undergoes cell to cell movement by osmosis until it reaches the xylem. This water is then transported through the xylem vessels to the leaves and is evaporated by the process of transpiration.
The xylem is also composed of elongated cells like the phloem. However, xylem is especially accountable for transporting water to all plant parts from the roots. Since they serve such an important function, a single tree would have a lot of xylem tissues.
(2) Phloem
The phloem is responsible for translocation of nutrients and sugar like carbohydrates, produced by the leaves to areas of the plant that are metabolically active. It is made up of living cells. The cells walls of these cells form small holes at the ends of the cells known as sieve plates.
The plant body is generally divided into roots, stem, and leaves. The roots are in the soil, which is the major source of nutrients in plants. Water and other nutrients enter the plant through the roots. The leaves are the food production centres. Using the sunlight and Carbon dioxide they synthesize food through photosynthesis in the chloroplasts.
Now, food from the leaves has to reach the other parts and the water, along with other nutrients has to reach leaves and other parts. All of this takes place through the vascular tissues of the plants. This is basically the transportation in plants.
You have already learned in earlier classes about the specialized cells and tissues in plants, which are the xylem and phloem. Together, they constitute the vascular structure in plants.
Vascular Structure
When talking about transportation in plants we must discuss Xylem and Phloem. Xylem and Phloem tissues are present throughout the plant. They begin at the root and then move up to the stem, branches, and leaves.
The xylem tissue transports water and minerals from the roots to the leaves whereas the phloem tissue transports food from the leaves to the other parts of the plant. Xylem tissue has tracheids and vessel elements. Phloem tissue has companion cells and sieve tubes.
When transpiration in plants occurs, water gets evaporated from the leaves. This results in more water being pulled from the root. This phenomenon explains how water moves up in the plants, against gravity, without the use of any pump! The flow of water in the xylem tissues is unidirectional. It moves up the stem from the roots. It occupies the centre of the vascular bundle.
The phloem, on the other hand, is responsible for the translocation of the nutrients like carbohydrates and amino acids from the leaves to other areas of the plants. Here, the flow is bidirectional. It moves up and down the stem. Phloem occupies the edge of the vascular bundle, as seen in the following figure. Food movement in the phloem occurs due to the pressure flow mechanism. The differences in the osmotic pressure help in the movement of food from the area of high concentration to areas of low concentration.