TRANSPORTATION

CLASS X   |    SCIENCE    |    TRANSPORTATION

      Notes prepared by Subhankar Karmakar

---

 

TRANSPORTATION

As the cells of an organism needs food, oxygen and water, some arrangement is required inside an organism which can carry the essential substances to all its parts, so that they reach each and every cell of its body. In biology, transport is a life process in which a substance is absorbed in one part of the body of an organism is carried to other parts of its body. Special tissues and organs are needed for the transport of substances in plants and animals because these tissues and organs can pick up the essential substances like food, oxygen and water at one end and carry them to all other parts.

TRANSPORT IN PLANTS

·         THE FUNCTIONS OF TRANSPORT SYSTEM IN PLANTS:

·       Transport system in plants does not required to carry oxygen to every parts of the plant as all the cells of a plant get oxygen for respiration and carbon dioxide for photosynthesis directly from the air through stomata.

Book For a Trial Class:

·       The transport system in plants carries water and mineral to the every cell of the plants.

·       Transport system in plants is also responsible for transporting foods prepared in the leaves to the various parts of the plant like stem, roots etc.

• TYPES OF TRANSPORT SYSTEM IN PLANTS:

The plants have two transport system:

• XYLEM which carries water and minerals, and
• PHLOEM which carries the food materials which the plant makes (Phloem also carries the hormones made by the plants in their root and shoot tips)

The transport of materials in a plant can be divided into two parts: 

• Transport of water and minerals in the plant, and
• Transport of food and other substances (like hormones) in the plant. 

TRANSPORTATION OF WATER AND MINERALS 

• 1. Plants require water for making food by photosynthesis and mineral salts for various  purposes like protein making etc. 
• 2. Water and mineral salts are absorbed from the soil by the roots of the plants and transported to the other parts of the plants like stem, leaves and flowers. 
• 3. Water and minerals dissolved in it move from the roots to the leaves through the two types of xylem tissues called (i) xylem vessels and (ii) tracheids. Both of them are non-living conducting tissues which have thick walls. 

XYLEM VESSELS: 

• 1. The xylem vessel is a non-living, long tube which runs like a drain pipe through the plant. 
• 2. A xylem vessel is made of many hollow, dead cells joined end to end and the end walls are broken down to form a long tube.
• 3. Xylem vessels run from the roots of the plant right up through the stem and reach the leaves and branched into every leaf of the plant. 
• 4. Xylem vessels do not contain the cytoplasm or nuclei.
• 5. The walls of xylem vessels are made of cellulose and lignin. Lignin is a very hard and strong substance which provides strength to the stem and help to keep the plant upright. 
• 6. Wood is made almost entirely of lignified xylem vessels.
• 7. Xylem vessels have pits (thin area of cell wall) in their thick cell walls. Pits are not open pores. They are the thin area of the cell wall where no lignin has been deposited.
• 8. In flowering plants, either xylem vessels or both xylem vessels and tracheids transport water. 

TRACHEIDS: 

• 1. Tracheids are long, thin, spindle shaped cells with pits in their thick cell walls. 
• 2. Tracheids are dead cells with lignified walls but they do not have open ends, so they do not form vessels. Water flows from one tracheid to another through pits as they do not have open ends. 
• 3. Although all the plants have tracheids, they are the only water conducting tissue in non-flowering plants. 

EPIDERMIS: The outer layer of the cells in the root is called epidermis. It is only one layer thick. 

ENDODERMIS: The layer of cells around the vascular tissues (xylem and phloem) in the root is called endodermis.

ROOT CORTEX: The part of root between the epidermis and endodermis is called root cortex. 

ROOT XYLEM: Xylem tissue present in the roots is called root xylem. Root hair are at its outer edge but the root xylem vessels are at the centre of the root. 

In between root hair and root xylem, there are epidermis, root cortex and endodermis. 

MECHANISM OF WATER AND MINERAL TRANSPORT IN A PLANT: 

• 1. The water containing minerals called xylem sap is carried by xylem vessels to all the parts of the plant. 
• 2. The roots of a plant have hair called root hairs. The function of root hairs is to absorb water and minerals from the soil. The root hairs are directly in contact with the film of water in-between the soil particles. Water is absorbed by the root hair through the process of diffusion.
• 3. The water and dissolved minerals absorbed by the root hair from the soil pass from cell to cell by osmosis through the epidermis, root cortex, endodermis and reach the root xylem. 
• 4. The xylem vessels of the root is connected to the xylem vessels of the stem of the plant. Water along with dissolved minerals enters from the root xylem vessels to the stem xylem vessels. The xylem vessels of the stem branch into the leaves of the plants. So, the water and minerals carried by the xylem vessels in the stem reach the leaves through the branched xylem vessels which enter from the patiole (stalk of the leaf) into each and every part of the leaf. Only 1% to 2% of the water absorbed by the plant is used up by the plant in photosynthesis and other metabolic activities. The rest of water is lost as water vapour to the air through transportation. 

MECHANISM OF WATER SUCKED UP BY THE XYLEM VESSELS:

The mechanism of water uptake in plants through xylem vessels is mainly driven by two processes: transpiration and cohesion-tension.

1. Transpiration: Transpiration is the process by which water vapor is lost from the aerial parts of the plant, primarily through small pores called stomata in the leaves. As water evaporates from the stomata, it creates a negative pressure or tension in the leaf's cell walls and intercellular spaces. This negative pressure extends throughout the plant's water-conducting tissues, including the xylem vessels.
2. Cohesion-tension theory: The cohesion-tension theory explains how water is pulled up through the xylem vessels from the roots to the leaves. It relies on the cohesion of water molecules and the adhesive properties of water and the xylem vessel walls. Here's how it works:

a. Cohesion: Water molecules have a strong attraction to each other due to hydrogen bonding. This cohesion allows water molecules to stick together and form a continuous, unbroken column of water in the xylem vessels.

b. Adhesion: Water molecules also have an affinity for the inner walls of the xylem vessels, creating an adhesive force between water and the xylem cell walls.

c. Capillary action: The combination of cohesion and adhesion creates capillary action, where water is drawn up through the narrow xylem vessels against gravity. As water evaporates from the leaves during transpiration, it creates a negative pressure or tension at the top of the xylem vessels.

d. Continuous water column: The cohesive forces between water molecules ensure that as water molecules are lost from the leaves, they pull on the adjacent water molecules below them, maintaining a continuous column of water throughout the xylem from the roots to the leaves.

e. Root pressure (optional): In some plants, root pressure may contribute to the initial movement of water into the xylem. Root pressure results from osmotic processes in the root cells, but it is generally not the primary force responsible for long-distance water transport in most plants.

By combining transpiration, which creates negative pressure at the top of the xylem, and cohesion-adhesion forces, which allow water to form a continuous column and be pulled upward, the xylem vessels efficiently transport water and minerals from the roots to the rest of the plant. This process is crucial for maintaining the plant's structure, cooling the leaves, and facilitating nutrient transport.