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World's largest tree World's largest tree: Coast Redwood (Sequoia sempervirens) The tree is 115.72 m (379.7 feet) tall! How can the water reach the height of 115 meters? What kind of pumps these trees have? What enables a vascular plant to conduct water, minerals, and organic nutrients over such long distances? The mechanisms responsible for internal transport are the subject of this chapter.

Learning Objectives

After completing the topic, the student will be able to:

  • Investigate how roots absorb water and minerals from the soil and systematize the pathway of lateral transport of minerals and water through roots.
  • Understand the fundamental differences between diffusion and osmosis in plants.
  • Define and discuss the terms–Turgidity, Plasmolysis, Flaccidity in plants.
  • Define the term “ascent of sap” and analyze the role of different forces that contribute to the upward movement of sap.
  • Understand the mechanism of transpiration in plants and list the factors affecting transpiration.
  • Examine how cohesion and adhesion facilitate long-distance transport in plants.
  • Define the term “translocation and discover how food made by leaves transported to different parts of the plant.
Mechanism of transport Transport mechanism in plants Plants have two different types of 'transport' tissue. Xylem transports water and solutes from the roots to the leaves, phloem transports food from the leaves to the rest of the plant.
Transport in Plants

The body of every organism (plant or animal) is made up of cells. In order that the organism may be able to maintain its life and survive, all its cells must be supplied with essential substances like food, oxygen, water, etc.

In these organisms, the sites of absorption and synthesis are very specific and are separated by a greater distance from the other parts of the body. The actual movement of materials into the individual cells is by diffusion, osmosis or active transport.

In biology, transport is a life process in which a substance absorbed (or made) in one part of the body of an organism is carried to other parts of its body. The process of transportation is dependent on the existence of concentration gradient. Concentration gradient is the difference in the concentrations of a particular substance.

Diffusion is the movement of particles from a high concentration to a low concentration until they are spread out evenly. Diffusion is a major method by which transport of materials occurs in single–celled (unicellular) organisms like Chlamydomonas and simple multicellular plants like Spirogyra

Water conducting cells of xylem Water conducting cells of xylem The xylem transports water and soluble mineral nutrients from the roots throughout the plant.
Mechanism of plant transport - Movement of water, nutrients

The mechanism of transport in multicellular higher plants is much more elaborate. Due to the branching shape of a plant, all the cells of a plant can get oxygen for respiration and carbon dioxide for photosynthesis directly from the air by diffusion. So, the only substances, which are to be supplied to a plant through a transport system are water and minerals (which they can't get from the air).

Another function of transport system of plants is to transport food prepared in the leaves to the various parts of the plants like stems, roots, etc. The process of diffusion, which is slow, cannot meet the requirements of transport to all parts in a short period of time. Hence some special tubes called xylem and phloem, which comprise the conducting tissues or vascular tissues pick up the essential substances like food, water, etc., at one end of their body and carry them to all other parts.

Xylem tissues carry water and minerals whereas phloem tissues carry the food prepared by the plants (Phloem also carries the hormones made by the plants in their root and shoot tips). Xylem tissue consists of tracheids, vessels and xylem parenchyma. The xylem vessels and tracheids together form long tubes that have a narrow diameter. Thus they function as capillaries (narrow tubes) to transport water and minerals. Xylem parenchyma consists of living parenchyma cells associated with xylem. These cells serve for the storage of food (sugars and starch), and also help in the lateral conduction of water and minerals. Phloem transports organic substances like sucrose across the plant body. Sieve tubes and companion cells are mainly involved in the transport of the materials.