All cells and tissues must get the required energy in order to maintain life through various life processes. The energy is obtained from food, oxygen, water, etc. Therefore, it is essential that the organism have some kind of a transport mechanism to provide essential substances to every cell of the body. Complex multicellular organisms have well-defined transport systems to supply the substances to the cells. A similar transport system is present in every plant to ensure its survival. In higher plants, xylem and phloem are highly organised vascular tissues that are responsible for the transport of substances in plants.

What is Transport?

Transport is one of the essential life processes in which the substances synthesised in one part of the organisms are carried to the other parts. In unicellular organisms, the transport of substances occurs through diffusion. Complex organisms like plants and humans require a well-defined transport system for carrying substances.

Significance of Transport of Substances in Plants

Transportation is a vital process that is significantly required in plants because:

1. Transport of water through the xylem provides water to the leaves to carry out photosynthesis.
2. A continuous transport of water and minerals helps to maintain the turgidity of cells and thereby maintains the cell shape.
3. A continuous movement of water from the roots to the leaves prevents the wilting of leaves due to transpiration.
4. The translocation of food is essential to provide energy for other physiological activities in plants.

Transport Tissues

Xylem: Xylem is a complex tissue that conducts water and minerals absorbed by the roots to the aerial parts of the plant. It comprises tracheids, vessels, parenchyma, and xylem fibres.

1. Tracheids are long, thin spindle-shaped cells with pits in their cell wall.
2. Water flows from one tracheid to another through pits.
3. In non-flowering plants, tracheids are the only conducting cells.
4. In flowering plants, tracheids and vessels both conduct water from roots to stem and leaves.

Phloem: Phloem is the tissue that transports food to different plant parts. It consists of sieve tubes, companion cells, phloem parenchyma, and phloem fibres.

1. Sieve tubes are elongated, multicellular tubular structures that form a continuous channel for the conduction of food. Several sieve tubes join end to end to form such channels.
2. The end wall (septa) between the sieve tube element has pores. These are bulged out regions called sieve plates.
3. The food is transported through a sieve tube element to the different plant parts.

Fig: Vascular Tissues in Plants

Transport of Water & Minerals in Plants

Roots are the non-green, highly branched underground part of the plant that absorb the water and minerals from the soil and transport them to the aerial part of the plant through the xylem. The entire mechanism of transport of water can be broadly studied in the following steps:

1. Absorption of Water & Minerals
2. Pathway of Water & Minerals
3. Mechanism of Transport of Water & Minerals to Stem & Leaves

Absorption of Water & Minerals

The branching pattern of roots provides a large surface area for the absorption of water and minerals from the soil. Root hairs are the extension of the outermost layer (epidermis) of the root that remains in direct contact with the soil water. The following characteristics enable root hairs to absorb water efficiently:

1. Root hairs have a thin and permeable cell wall that allows the movement of water and dissolved substances freely into the cell and out of the cell through the process of diffusion.
2. Further, the thin and semipermeable cell membrane of the root hairs allows only selective minerals and salts to move across along with the water. This phenomenon is called osmosis.
3. The concentration of cell sap inside the root hairs is higher than the surrounding water. This enables them to draw water inside from the soil by endosmosis.

Absorption of water by the roots takes place by the following two processes:

1. Passive Absorption: It is the movement of substances through the cell membrane that does not require energy. It is always in the region of their higher concentration to the region of their lower concentration. Diffusion is an example of passive transport.
   Since the water concentration in the root hairs is lower than that of their surroundings, therefore the water moved into the cells through passive transport.
2. Active Absorption: It is the movement of substances through the cell membrane with an expenditure of energy. It is always in the region of their lower concentration to the region of their higher concentration.
   Mineral elements like potassium, zinc, nitrates, sulphates do not pass through the cell membrane because of their higher concentration inside the root cells. Hence these molecules are forcibly carried inside the cell against the concentration gradient.

Fig: Absorption of Water by Roots

Pathway of Water & Minerals Within the Root Cells

Root tissue system: Roots are complex and highly organised plant parts. The transverse section of the root shows three distinct regions, namely the epidermis, ground tissue system, and vascular tissue system.

1. The epidermis is the outermost single-layered tissue that is made up of compactly arranged cells. The epidermis bears root hairs.
2. Ground tissue system comprises endodermis, cortex, hypodermis, pericycle, and pith.
3. The vascular tissue system is formed by the xylem and phloem that are always organised in groups called vascular bundles.

Pathway of Water and Minerals

1. The root hairs absorb the water and minerals from the soil. The water and minerals absorbed by root hairs pass from cell to cell through the epidermis, endodermis, root cortex, hypodermis, pericycle and reach the root xylem.
2. The Xylem vessels of the root are connected to the xylem vessels of the stem.
3. The xylem vessels of the stem reach the leaves through branched xylem vessels that enter from the petiole into each and every part of the leaf.
4. The water can flow in two possible ways to the root cortex region and then to the xylem vessels. Apoplast pathway (water flows between the adjacent cells through the intercellular spaces). Symplast pathway (water flows through the interconnected cytoplasmic strand called plasmodesmata).

Fig: Pathway of Water and Minerals

Mechanism of Transport of Water and Minerals to Stem and Leaves

The water, along with the dissolved substances like minerals, salts, sugars, amino acids in the cell, is called cell sap. The upward movement of cell sap from the roots to the stem and leaves against the pull of gravity is called the Ascent of Sap. There are the following theories (driving forces) related to the ascent of sap:

1. Transpiration Pull Theory: It is proposed by Dixon and Jolly. Transpiration is the driving force behind the ascent of sap in the plant. Plants lose a large amount of absorbed water through the process of transpiration. Only about 1% of the total water is utilised by plants, and 99% of water is evaporated through stomata and leaf surfaces. Pressure is then created on the root cells to pull more water for the continuous transport of water to the aerial parts of the plant.
The lower concentration of water vapour in the atmosphere as compared to the substomatal cavity and intercellular spaces creates negative water pressure that causes water to move upwards from the roots through the xylem. This theory is based on the following features:
I. Cohesion: Mutual attraction between the water molecules is called cohesion. The cohesion between the water molecules leads to the formation of an unbroken column of water in the root xylem vessels.
II. Adhesion: Mutual attraction between the water molecules and the wall of xylem vessels is called adhesion. The xylem vessels are placed one above another to reach the stem xylem. Due to the adhesive property of water with the wall of xylem vessels, the water moves upward to the stem and leaves.
III. Surface Tension: The loss of water from the xylem creates a negative pressure called surface tension, which is transmitted down to the roots, thereby lifting the water column of the plant.

Fig: Transpiration Pull

2. Root Pressure Theory: The pressure developed in the tracheary element of the xylem is called root pressure. Root pressure is built up due to the cell to cell osmosis in the root tissues. The turgid cell (due to the endosmosis) creates pressure on the adjacent cell, and the water moves into the cell.
The pressure developed is about 1-2 atm. However, a pressure of about 20 atm is needed for raising water in tall plants. However, root pressure is not universal, and it is not found in all the seasons. Water can rise up even in the absence of root pressure.

3. Capillarity Theory: The thin tubular xylem vessels act as a capillary system and pull up the water and dissolved minerals through the capillary action due to the strong adhesive and cohesive force. This force is, however, not sufficient to pull up water beyond a short distance. Moreover, the capillary force cannot operate in plants that have tracheids instead of vessels.

Translocation of Food in Plants

Synthesis of food takes place in the green leaves of the plants by utilising carbon dioxide and water in the presence of sunlight. The process of synthesising food is called photosynthesis. The process by which the synthesised food along with the amino acids, growth hormones, and other essential substances reaches the storage organs and parts where they are utilised is called translocation. This process is carried out by a complex conducting tissue called phloem.

1. The sugar (food) synthesised in mesophyll cells of leaves is shifted into the sieve tubes of the phloem by utilising the energy from ATP (Adenosine Triphosphate).
2. The high concentration of the sugar in the sieve tubes exerts osmotic pressure that causes the water to move into the cell, and the cell becomes turgid.
3. The high turgor pressure in the cell causes the outflow of the sugar to the adjacent cell.
4. In this way, the sugar concentration of the nearby cell increases and leads to endosmosis. As a result, the turgor pressure rises, and the sugar content moves to another cell in the series.
5. This process is repeatedly performed by each cell one after another, and the food is translocated to different parts of the plant. The translocation of food is bidirectional that can be from leaves to root and from roots to leaves depending on the gradient formed due to turgor pressure.

Fig: Diagrammatic Presentation of the Mechanism of Translocation

Summary

The term “transport” means to carry things from one place to another. Transport in plants refers to the movement of food, water, and minerals to all the plant parts. Plants are highly organised complex organisms that have a well-defined vascular tissue system for the transport of substances from one part to another. The upward movement of water and minerals from the roots to the leaves through the xylem is called the ascent of sap.

Transpiration of water contributes to the continuous flow of water from the roots to the leaves. The phloem transport the food prepared in the leaves to all the parts of the plant. The turgor pressure of the cell due to endosmosis contributes to the translocation of food.

Frequently Asked Questions (FAQs) on Transport of Substances in Plants

Q.1. How does the transport of substances occur in plants?
Ans: Transport of substances in plants occurs by vascular tissues. Xylem transports water from roots to leaves, and phloem transports food from leaves to other parts of the plant.

Q.2. What process in plants is known as transpiration?
Ans: The loss of water from the aerial parts of the plants is called transpiration.

Q.3. What is the role of transpiration in plants?
Ans: Transpiration plays an important role in the transport of water from the roots to the leaves through the cohesion of water molecules. It also causes a cooling effect and therefore maintains the temperature of the plant.

Q.4. Which substance is transported by phloem?
Ans: Phloem transport food (sugar) synthesised in the leaves to all the parts of the plant.

Q.5. What is the pathway of the water through a plant?
Ans: Water is absorbed by the root hairs and then transported to the epidermis, cortex, endodermis, pericycle, and xylem vessels in a sequential manner. The xylem vessels of roots transport the water to the xylem vessels of the stem and then reach the leaves of the plant.

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