Transport system in plants
Transport in biology means carrying substance absorbed or made in the body of an organism to all other parts of its body.
In plants, it is only water and minerals that need to be transported to its other parts. Another thing that needs to be transported to other parts of the plants is the food prepared in leaves. This is because a plant has a branching shape so it gets carbon dioxide for photosynthesis and oxygen for respiration from air directly through diffusion.
The two types of conducting tissues that perform the function of transport system in plants are:
Transport of water and minerals
Plants need water to make food through the process of photosynthesis and minerals for making proteins. Thus, a plant absorbs water and minerals from soil through roots and transport it other parts like stem, leaves, flowers etc. It is through two kinds of elements of xylem tissue called, xylem vessels and tracheid that water and minerals move from roots of a plant to its leaves.
Xylem vessel is a long tube made up of dead cells joined end to end. It is a non-living tube which runs from roots of the plants and runs through the stem and reaches every leaf. The end walls of the cells are broken so that an open tube is formed.
Xylem vessels do not have cytoplasm or nuclei and the walls of the vessels are made of cellulose or lignin. Other than transporting water and minerals, xylem vessel also provides strength to the stem and keeps it upright. This is because lignin is very hard and strong. Wood is made of lignified xylem vessels. Xylem vessels have pits in their cell walls where lignin is not deposited. Either xylem vessel or both xylem vessel and tracheid transport water in flowering plants.
In non-flowering plants tracheids are the only water conducting tissues. Tracheids are dead cells with lignified walls with no open ends. They are long, thin and spindle shaped cells. They have pits in them and it is through pits only that water flows from one tracheid to another. All the plants have tracheid in them.
Before understanding the mechanism of transport of water and minerals in a plant it is necessary to know the meaning of some important terms:
Epidermis: The outer layer of the cells in the root of a plant is called epidermis. The thickness of epidermis is equal to one cell.
Endodermis: It is the layer of cells around the vascular tissue (xylem and phloem) in the root of a plant. Endodermis is the innermost layer of cortex.
Root cortex: It is the part of root between the epidermis and endodermis.
Root xylem: It is the xylem tissue present in the roots. It is present at the centre of the root.
Epidermis, root cortex and endodermis are present between the root hair and root xylem. So, the water which is absorbed by the root hairs from soil first passes through epidermis, root cortex and endodermis and then finally reaches to root xylem.
Also, minerals are present in soil. Plants take these minerals from soil in inorganic form such as nitrates and phosphates. Minerals from soil gets dissolve in water to form an aqueous solution. So, when water is transported from roots to leaves, minerals dissolved in water is also transported.
Mechanism of transport of water and minerals in a plant
Root hair absorbs the water containing dissolved minerals from the soil. Root hair is directly in contact with the film of water present in-between the soil particles. Water containing minerals gets into the root hair and passes from cell to cell through the process of osmosis and reaches epidermis, root cortex, endodermis and root xylem.
Xylem vessels of the root are connected with the xylem vessel of the stem of a plant. So the water enters from root xylem vessel to the stem xylem vessel and further reaches into the leaves of the plant from petiole. The plant uses only one or two per cent of the water in photosynthesis. The remaining water is lost in air as water vapour.
Water is sucked up by the xylem vessel
The pressure at the top of the plant (in the leaves) is low whereas pressure at the bottom of the plant is high. It is due to transpiration that the pressure is low at the top of a plant. And it is because of low pressure at the top of the plant that water flows up the xylem vessel into the leaves of a plant.
The continuous evaporation of water from the leaves of a plant is called transpiration. The leaves of a plant have tiny pores called stomata. It is through them that the water evaporates into the air. This reduces the pressure at the top of xylem vessels and thus water flows up into them.
Transport of food and other substances
The food which is prepared by the process of photosynthesis in the leaves of a plant has to be transported to other parts like stem, roots, branches etc. Therefore this food is transported to other parts of the plant through a kind of tubes called phloem. The transport of food from leaves to other parts of a plant is called translocation. The food made by the leaves is in the form of simple sugar.
Phloem is present in all the parts of a plant.
Phloem contains Sieve Tubes
Phloem is a long tube made of many living cells joined end to end. The living cells of phloem are called sieve tubes. The end walls of cells in the phloem have sieve plates which have tiny holes in them. It is through these holes that the food passes along the phloem tubes. Sieve tubes contain cytoplasm in them but have no nucleus. Each sieve tube cell has a companion cell which has a nucleus and many other organelles. The cell wall of sieve tubes contains cellulose but no lignin.
The food is made by the mesophyll cells of a leaf and from there it enters into the sieve tubes of the phloem. These phloem tubes are interconnected and once the food reaches the phloem tube of a leaf, it is then transported to all other parts of a plant.
The transport of food is necessary because every part of a plant needs food for:
Other substance like hormones made in the tips of roots and shoots are also transported through phloem tubes.
Mechanism of transport of food in a plant
It is by using the energy from ATP that the food made in the leaves of a plant is loaded in sieve tubes of phloem tissue. Then by the process of osmosis water enters into sieve tubes that contain sugar. This raises pressure in phloem tissue. This high pressure produced in phloem tissue moves the food to all other parts of a plant having less pressure. In this way food is transported to all parts of a plant through phloem tissue.