ICSE Class 8 Biology Chapter 02 Transport in Plants Absorption Conduction and Rise of SAP

Transport In Plants - Absorption, Conduction And Rise Of Sap

Syllabus

Transport, absorption, conduction, rise of cell sap in plants.

Demonstrating conduction in plants (E).

Transpiration in plants.

Transport In Plants

Plants need water and mineral for their survival. Do you know, where these substances come from? These substances are absorbed by the plants from the soil through their roots. They are then transported upwards to various parts of the plant, like stem, leaves, etc.

Similarly, the leaves prepare the food from water and carbon dioxide during photosynthesis, and this food is transported downwards to all parts of the plant including roots.

Besides absorbing water and minerals from the soil, roots also fix the plant firmly in the ground.

Transportation in plants is the process in which a substance absorbed or synthesized in one part of the plant is moved to the other parts of the plant.

Transportation in plants is carried out by a conducting system consisting of two tissues - xylem and phloem.

Transport Of Water And Minerals

You have already learnt about the conducting tissues of plants i.e. xylem and phloem. The xylem has two types of components, the tracheids and vessels. It is through these components that water and minerals absorbed by the roots move up to the stem and leaves. The tracheids and vessels are dead and highly thickwalled. These are long thin, spindle-shaped cells with pits in their walls. Water and minerals flow from one tracheid to another through these pits.

Similarly, vessels are in the form of perforated long cells. These cells have tubular passages which are joined with each other end to end forming continuous channels. Water and minerals flow through these channels by the process of osmosis and diffusion.

Fig 2.1 The root (underground) and the shoot (above ground) are the conducting systems of a plant

Teacher's Note

Just like how water flows through pipes in your house to reach every room, plants use special tubes called xylem to transport water from roots to leaves.

Water Absorption By The Roots

We are familiar with the absorption of water by blotting paper or by cotton dipped in water. In both cases, water is absorbed by the property of capillarity (attraction of water molecules towards narrow spaces) and surface tension. But the absorption of water by the roots is quite different. To understand it, let us first consider the structure of the roots.

The root system of plants consist of the main root, which gives out lateral (branch) roots. The lateral roots bear a large number of fine outgrowths called root-hairs (Fig. 2.2).

Fig 2.2 Root system of a plant

Root-hair: Given below is a magnified view of a root-hair (Fig. 2.3). It is a long protuberance of a cell. This cell contains a fluid (sap) which is more concentrated than the surrounding soil water. The root-hair is covered by a very thin cell membrane that allows water molecules to pass through, but prevents the other larger molecules. Such a membrane is called semi-permeable membrane.

Fig. 2.3 Root-hair (Highly magnified)

Speciality Of Root-hairs

The root-hairs are suited for absorbing water from the soil in three ways:

1. The root-hairs present a large surface area. The more the surface area, the greater is the absorption.

2. Each root-hair has a semi-permeable membrane.

3. It contains a solution (cell sap) of a higher concentration than the surrounding soil water.

Let us try another experiment to understand how the root-hairs absorb water.

Activity 2

Raisins Swell In Water And Grapes Shrink In Thick Syrup

Put some raisins in a bowl containing water, and some soft-skinned grapes in another bowl containing thick syrup of sugar. After 10-12 hours, you will notice that the raisins have swelled up, whereas the grapes have shrunken. The raisins have absorbed water but the grapes have lost it. In both cases, the gain or loss of water is through the peel acting as a kind of semi-permeable membrane.

This experiment shows that the peel over the raisins and that over the grapes acts like a semi-permeable membrane. This membrane has very minute pores which allow water molecules to pass through, but prevent the larger ones of sugar.

Fig. 2.4 Diagrammatic representation of the nature of a semi-permeable membrane

The semi-permeable membrane lies between water on one side and a strong sugar solution on the other side. While the sugar solution has a higher concentration of sugar molecules, there are no sugar molecules in water. In other words, we can say that there are more water molecules on the left side of the membrane than in the sugar solution on the right side.

After some time, you will notice that the strong sugar solution has become diluted. It is because, the water molecules from the other side have moved into the sugar solution. Such movement of water molecules is known as osmosis.

Three Types Of Movements Of Molecules

1. Diffusion: Soil water moves into the root-hairs by the simple process of diffusion (movement of molecules - gas, liquid or solid) i.e. from higher concentration to lower concentration. For example, while adding sugar in milk, sugar molecules diffuse into the milk evenly, occupying intermolecular spaces of milk.

2. Osmosis: It is the movement of water molecules from the region of higher concentration through a semi-permeable membrane to the region of lower concentration.

3. Active transport: The root-hairs absorb both water and minerals from the soil. Water diffuses into the root-hairs directly, because its concentration is higher in the soil than inside the root-hairs. But in the case of minerals, it is just the opposite. They have more concentration inside the root-hairs than outside in the soil. If it was simple diffusion, the minerals from the root-hairs would have passed out into the soil. But here, the requirement is opposite. The minerals have to be absorbed from a level of lower concentration to a level of higher concentration. It could not be possible by simple diffusion, so there is an active transport in the opposite direction which needs energy.

The water is absorbed by the root-hairs from the surrounding soil by osmosis. It, then, diffuses into deeper parts of the root by cell-to-cell osmosis and reaches the central portion (Fig. 2.5). Here it enters certain vertical tubes (xylem), which continues upward into the stem and the leaves. This upward movement of sap that contains water and minerals is called ascent of sap.

Fig 2.5 Movement of water in plant

Teacher's Note

When you eat a pickle or salted vegetable, it loses water because of a higher salt concentration outside, similar to how grapes shrink in syrup.

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