Molecular genetics, an important area within Biology Topics, focuses on the structure and function of genes.
What is Transport System in Plants?
We have already learnt that the plants take up water (and dissolved minerals) from the soil through their roots and transport it to their leaves for the preparation of food. The leaves of plants prepare food by using water brought in from soil and carbon dioxide taken from air during photosynthesis. And this food has to be transported from the leaves to all other parts of the plant (which cannot make food themselves). This means that: Plants need a transport system
- to carry water (and dissolved minerals) absorbed by the roots up to the leaves, and
- to carry the food made in the leaves to all the parts of the plant (including roots).
In order to carry water and dissolved minerals, as well as prepared food, a plant has a transport system inside it. The transport system in plants consists of long tubes (or vessels) called xylem and phloem. In other words, xylem and phloem transport substances in plants. The xylem and phloem carry the various substances in solution form’ up and down the plant and constitute the transport system of plant. The xylem and phloem vessels are present in all the parts of a plant such as roots, stem, branches and -leaves. The xylem and phloem vessels form the vascular tissues in plants. The part played by xylem and phloem tissues in the transport of substances in plants is described below.
(i) The vascular tissue called ‘xylem’ carries water (and minerals dissolved in it) from the roots upwards through the stem and branches up to the leaves (see Figure). The xylem tissue forms a continuous network of vessels (or channels) that connect the roots to the leaves through the stem and branches, and thus transports water (and dissolved minerals) to the leaves of the entire plant. The water rises from the roots upwards in a plant as follows : In a plant, water evaporates continuously from the leaves through the pores called ‘stomata’. This process is called transpiration (see Figure).
The continuous evaporation of water from the leaves (or transpiration) produces a ‘suction force’ which pulls the water from roots upwards through the stem and branches up to the leaves (The suction force produced in xylem vessels by transpiration in plants is similar to the one which we produce while drinking Coca-Cola or Pepsi from a bottle with a straw). Thus, transpiration generates a ‘suction force’ which pulls up water absorbed by the roots from the soil to reach the leaves (through the stem and branches).
(ii) The vascular tissue called ‘phloem’ carries the food from the leaves to all other parts of the plant (including roots) (see Figure). The phloem tissue forms a network of vessels (or channels) that connect the leaves to all the parts of the plant including roots, and thus transports food to the entire plant. Please note that the food which is made in the leaves during photosynthesis consists of glucose (sugar) and it is transported within the plant in the form of solution (called ‘glucose solution’). The force needed to push the dissolved food (glucose solution) in phloem vessels is generated by the living phloem cells present in the phloem tissue of the plant.
The process of transpiration (evaporation of water from leaves) serves a very useful function in the plants because it generates a ‘suction force in xylem which can pull water from the roots up to great height in the tall trees. Another function of transpiration is that it cools the plant in hot weather.
This happens due to cooling caused by evaporation. Please note that the rate of transpiration increases in moving air (or wind). This is because moving air carries away water vapour from leaves as fast as it comes out of stomata. And when the rate of transpiration increases, then the rate of absorption of water through the roots also increases. Thus, water absorption through roots can be increased by keeping a potted plant under the fan.
Stomata are the tiny pores found on the surface of leaves of plants. The opening and closing of the stomata controls the passage of gases and water vapour into and out of the leaf. For example :
- Carbon dioxide gas ‘enters in and oxygen gas goes out’ through stomata during photosynthesis.
- Water vapour ‘passes out’ through stomata during transpiration.
How Water Reaches From Soil to Xylem Vessels in Roots
We have just studied that water (and minerals dissolved in it) are carried from the roots of a plant to the leaves by the xylem vessels. Now, the xylem vessel of a root is in the centre of the root and it is surrounded by a large number of other cells of the root (see Figure).
This means that the xylem vessel of a root is not in direct contact with the water present in the soil particles. So, an important question now arises : How does water from the soil reach the xylem vessel which is in the centre of the root of a plant ? The answer to this question is that: For very short distances, water can move from cell to cell. So, the water present in soil and absorbed by root hair first moves from cell to cell in the outer part of a root till it reaches the xylem vessel in the centre of the root. This point will become more clear from the following discussion.
(i) Movement of Water From Soil to Root Hair. The roots have large number of root hair. The root hair are in contact with the water present between the soil particles (see Figure). Water from the soil enters the root through root hair. The large number of fine root hair increase the surface area of the root in contact with the soil water due to which more water can be absorbed by the root at a rapid rate (To keep the diagram simple, we have shown only two root hair in Figure).
(ii) Movement of Water From Root Hair to Xylem Vessel in Root. The root hair are on the outer surface of the root, xylem vessel is in the centre of the root, and there are many cells of the root in-between the root hair and xylem vessel (see Figure). Water absorbed by the root hair from the soil moves from cell to cell in the root and ultimately enters the xylem vessel which is located in the centre of the root (see Figure). Once the water reaches the xylem vessel .of root, it moves upwards and goes into interconnected xylem vessels of stem, branches and leaves of the plant.
From the above discussion we conclude that water (containing dissolved minerals) from the soil enters a plant through its root hair. From root hair, water moves from cell to cell in the root till it reaches xylem vessel in the centre of the root. From the xylem vessel in root, water moves upward to the xylem vessels in stem, branches and leaves.
Activity 3
The fact that for very short distances, water can move from cell to cell (without xylem vessels) can be demonstrated by performing an activity as follows. Take a large raw potato and cut it into two halves. Now, take one cut potato piece and peel off its outer skin. Cut its round end slightly to make its base flat. Make a deep cavity in the centre of potato piece. Fill half of the cavity of potato piece with sugar solution and mark the level of sugar solution in it by inserting a pin through the potato wall [as shown in Figure (a)], Place this potato piece (with sugar solution in it) in a petri-dish containing small amount of water. The level of water in the petri-dish should be below the level of pin inserted in the potato [see Figure (a)], Allow the apparatus to stand for a few hours.
After a few hours, we will see that the level of sugar solution in the potato piece rises above the level of pin mark [see Figure (b)]. This means that there is an increase in the level of sugar solution inside the potato cavity. The level of sugar solution can rise only if some water from the petri-dish passes through the wall of potato and goes inside it. Now, the wall of potato is made of tiny cells joined together. Since the water from petri dish passes through the wall of potato made of cells, it shows that water can move from cell to cell in the potato wall and reach inside the cavity.