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Biology Form 5

Text of 1.7

1.7 The Transport Substances in Plants 1. Plants have a transport system for carrying water and dissolved solutes to different parts of the plant. The vascular tissue in stem, root and leaf 2. Transport in plants is provided by the vascular tissues. 3. There are two types of vascular tissues: a) xylem - transports water and dissolved mineral salts absorbed in the roots, up the stem and to the leaves. - in woody plants, the xylem tissues also provide mechanical support to the plant. b) phloem. - not only transports organic substances downwards from the leaves to the storage organs, but also upwards from the storage organs such as the roots to the growing regions such as the buds. 4. Vascular tissues are found in the roots, stems and leaves.


The stem 1. The stem has an epidermis layer that helps maintain the shape of the stem. 2. In young plants, the epidermis cells may secrete a waterproof cuticle, and in older plants the epidermis may be absent, replaced by bark. 3. Just inside the epidermis is the cortex layer. 4. The cortex layer is made up of parenchyma cells which provide support and flexibility to the stem. 5. The inner parts of the stem consist of the vascular tissues and the pith which is the central region of a stem. 6. The pith is used for food storage in young plants. 7. It may be absent in older plants.

8. In dicotyledonous plants, the vascular tissues of the stem are grouped together to form vascular bundles. 9. The vascular bundles are arranged in a ring around the pith, the central region. 10. In each bundle, the xylem is found towards the inside of the stem, while the phloem is found towards the outside. 11. A tissue called the cambium is found between them. 12. The cambium cells can divide resulting in an increase in the radius of the stem. The root


1. The outermost layer is the epidermis. 2. The epidermis of the root does not have waxy cuticles. 3. In one region of the root, specialised epidermal cells grow outwards to form root hairs. 4. Root hairs increase the surface area for water absorption. 5. A single plant may have more than 10 million root hairs. 6. The region next to the epidermis is called the cortex. 7. The cortex is made up of parenchyma cells which may store starch grains. 8. Located immediately after the cortex is a single layer of cells called the endodermis. 9. Inside the endodermis is the pericycle. 10. The pericycle consists of scierenchyma tissue which provides mechanical support for the root. 11. In the roots, the vascular tissues are located in the vascular cylinder. 12. The vascular cylinder consists of the vascular tissues and the pericycle. 13. The vascular tissues of roots are continuous with the vascular tissues of stems. 14. The xylem radiates from the centre of the vascular cylinder, forming a star shape, while the phloem fills the area between the xylem.

15. In the monocotyledonous root, the vascular cylinder has a central core called the pith. 16. The pith contains parenchyma cells. 17. The vascular tissues form a ring around the pith, with the xylem tissues alternating with the phloem tissues. 3

The leaf

1. The leaf consists of a broad portion called the blade. 2. The blade is connected to the stem by a stalk called the petiole. 3. Inside the petiole are vascular tissues of xylem and phloem that are continuous with those in the stem, root and blade. 4. The leaf blade contains leaf veins. 5. Vascular tissues are found in the leaf veins. 6. The xylem forms the upper part of a vascular bundle in the leaf, while the phloem forms the lower part of the vascular bundle. The structure of xylem in relation to transport 1. Xylem contains four types of cells. 2. These are the xylem vessels, tracheids, parenchyma and fibres (a type of sclerenchyma). 3. Xylem vessels and tracheids are water-conducting cells. 4. They are elongated cells arranged end to end. 5. During development, the walls of xylem vessels and tracheids are thickened with lignin deposits, making them woody and impermeable. 6. Mature xylem vessels and tracheids are hollow and dead. 7. The walls of xylem vessels and tracheids are perforated by a series of holes called pits, which allow water and mineral salts to pass sideways between the cells. 8. Tracheids differ from xylem vessels in terms of their shape 9. Tracheids are longer and have a smaller diameter compared to xylem vessels. 10. Tracheids are pointed at the ends.


11. The end walls break down in pits that allow water to pass from cell to cell. 12. The end walls of xylem vessels are open so that the cells join end to end to form a continuous hollow tube. 13. This arrangement allows water to flow upwards continuously. 14. The cell walls are thickened with lignin. 15. The lignin makes the xylem vessels strong, so that they do not collapse under the tension created by the upward pull of water during transpiration. 16. The function of the parenchyma is to store food substances while the function of the fibres is to provide support.


The structure of phloem in relation 1. Phloem is composed of four types of cells. 2. These are the sieve tube, companion cell, parenchyma and fibres. 3. Organic substances are transported along the sieve tubes. 4. The sieve tube is a cylindrical column of long cells arranged end to end. 5. The sieve tube is a living cell. 6. When mature, it has no nucleus and its cytoplasm is pushed to the sides of the cell. 7. The end walls of each cell are perforated by pores to form sieve plates. 8. Each sieve tube cell is kept alive and supported in their function by one or more companion cells. 9. A companion cell is a normal cell with a nucleus and a large number of mitochondria, indicating that it is very active metabolically. 10. The function of the parenchyma is to store food substances while the function of the fibres is to provide support. 11. Bark ringing can be carried out on a plant to see the effect of removing phloem tissue from a plant.


1.8 Transport of organic substances in plants 1. The phloem contains a concentrated solution of dissolved organic solutes such as sugars (mainly sucrose), amino acids, and other metabolites. 2. The transport of dissolved organic solutes in the phloem is called translocation. The importance of translocation in plants 1. Translocation is important because a plant's survival depends on the transport of organic substances. 2. Organic substances are translocated downwards from the leaves to the storage organs such as the roots. 3. Later, they are translocated upwards from storage organs to the growing regions such as buds. 4. Translocation is important as it enables organic substances such as sucrose to be stored or converted to other sugars once it reaches its destination. Transport of water in plants Transpiration and its importance 1. Like animals, plants also lose water. 2. Most of the water is lost through a process called transpiration. 3. It is replaced by the absorption of water from the soil in the roots. 4. Transpiration is the loss of water vapour from a living plant due to evaporation. 5. A large tree can absorb water at a rate of 1 dm3 min-1. 6. However, only 1% of this water is used by its cells for photosynthesis and for turgidity. 7

7. The remaining 99% evaporates from the leaves and is lost to the atmosphere through transpiration. 8. In a typical plant, about 99% of transpiration takes place through the stomata. 9. Transpiration also takes place through the lenticels. 10. Transpiration helps in the absorption and transport of water and mineral ions from the roots to different parts of the shoots. 11. The continuous stream of flowing water from the roots to the leaves is called the transpiration stream. 12. Water is needed not only for photosynthesis but also to prevent wilting of the plant. 13. On a hot and sunny day, transpiration produces a cooling effect in the plant. The process of transpiration 1. As soon as water is absorbed by the roots from the soil, the water is transported through the xylem vessels to the mesophyll cells of the leaves. 2. The surfaces of the mesophyll cells are covered by a thin layer of water. 3. Heat from the Sun causes water on the external surface of the mesophyll cells to evaporate, thus saturating the air spaces in the mesophyll with water vapour. 4. Outside the stomata, the air in the atmosphere is drier. 5. This means the concentration of water vapour in the atmosphere is lower than the concentration of water vapour in the air spaces. 6. Hence, water vapour in the air spaces evaporates and the water vapour diffuses from the plant cells through the stoma. 7. The movement of air carries water vapour away from the stoma. 8. The loss of water from a mesophyll cell makes the cell hypertonic as compared to an adjacent cell. 9. Water from the adjacent cell diffuses into mesophyll cells by osmosis. 10. In the same way, water continues to diffuse into adjacent cells from neighbouring cells. 11. Eventually, water is drawn from the xylem vessels in the veins. 12. A pulling force is thus created for pulling water up the xylem vessels due to the evaporation of water from the mesophyll cells. 13. This pull is called the transpirational pull.


The external conditions that affect the rate of transpiration 1. Factors that affects the rate of transpiration; a) Light intensity b) Temperature c) Relative humidity d) Air movement 2. The rate of transpiration is increased by an increase in temperature, light intensity, wind speed and a decrease in humidity. 3. Light stimulates the opening of stomata. 4. This results in the upward movement of the transpiration stream. 5. Stomata close with darkness and transpiration stops. 6. Evaporation of water from stomata increases with the increase in temperature and wind speed. 9

7. This produces a faster rate of transpiration. 8. High humidity surrounding leaves reduces evaporat