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.

1

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 olderplants 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

2

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 ofsclerenchyma). 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 shape9. Tracheids are longer and have a smaller diameter compared to xylem vessels. 10. Tracheids are pointed at the ends.

4

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 acontinuous 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.

5

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.

6

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 plantsTranspiration 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 neighbouringcells. 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.

8

The external conditions that affect the rate of transpiration

1. Factors that affects the rate of transpiration;a) Light intensityb) Temperaturec) Relative humidityd) Air movement 2. The rate of transpiration is increased by an increase in temperature, lightintensity, 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 evaporation of water from the stomata. 9. This causes transpiration to slow down.

The movement of water from the soil to the leavesMovement of water through the roots

1. The root hairs are surrounded by soil particles. 2. Soil particles are usually covered by a thin film of water. 3. The root hairs provide a large surface area for absorption of water and minerals. 4. The cytoplasm of root hair cells are hypertonic compared to the surrounding soil water. 5. This means that root cells have a higher concentration of solutes than the water in the surrounding soil.6. Hence, water enters a root cell via osmosis. 7. When this occurs, the cell becomes hypotonic compared to adjacent cells. 8. Water then diffuses into the adjacent cell. 9. In this way, water moves inwards to the cortex. 10. In the cortex, water flows through the cytoplasm, vacuole, and cell walls of the parenchyma cells in the cortex until it reaches the endodermis.

11. Once it reaches the endodermis, the water moves through the cytoplasm and vacuole of the endodermal cells instead of the cell walls. 12. This is because the endodermal cell has a special feature called Casparian strip which line its sides. 13. Since the Casparian strip is impermeable to water, water movement through the cell

10

walls is blocked. 14. Thus, water moves through the cytoplasm and the vacuole of the endodermal cells to the xylem vessels.

15. The gradient of water concentration that exists across the cortex creates a pushing force that results in the inflow of water into the xylem. 16. At the same time, ions from the soil are actively secreted into the xylem and this causes osmotic pressure to increase.17. Consequently, water flows continuously into the xylem.18. This generates pressure known as root pressure. 19. Root pressure results in the upward push of water and mineral ions into the xylem of the stem. 20. Root pressure can be demonstrated by cutting a stem at soil level. 21. After some time, water can be seen exuding from the cut surface.22. In small plants, root pressure can push water all the way up the stem and out of special pores called hydathodes at the edges of leaves. 23. This process is called guttation.24. Guttation occurs on cool humid mornings when the air is too saturated for the water drops to evaporate from the leaves.25. While root pressure may account for some of the upward movement of water in plants, it is insufficient to overcome the force of gravity which is needed to push water upwards to the height of most trees. 26. In addition, root pressure moves water too slowly to account for the rapid transport of minerals from roots to leaves. 27. Hence, the driving force for water transport must come from some other source other than root pressure.

11

Movement of water through the stem

1. The continuous upward movement of water through the xylem vessels in the stems can be explained by the adhesive and cohesive properties of the water molecules.2. Xylem vessels are long, narrow and hollow tubes. 3. Joined end to end, they provide a continuous column of water from the roots,through the stem and to the leaves. 4. The narrow diameters of the xylem vessels increase the forces generated by capillarity.5. Capillarity is the cohesive and adhesive forces which enables the liquid to enter and move along very niurow spaces. 6. Capillarity is also known as capillary action. 7. Water molecules adhere to one another by cohesive forces. 8. They adhere to the walls of the xylem vessels by adhesive forces. 9. The cohesion and adhesion of water molecules are due to hydrogen bonding. 10. Capillary action only makes a small contribution to the upward movement of water. 11. Although root pressure and capillary action are not enough to drive water to the top of a tall tree, both effects are nonetheless important to water movement in plants.

Movement of water from the leaves to the atmosphere

1. The movement of water from the leaves to the atmosphere is due to the process of transpiration.2. Transpiration in the leaves is the main driving force for the movement of water from the soil up the stem. 3. We learnt that transpiration causes the evaporation of water molecules from thesurface of mesophyll cells. 4. This replaces the water vapour that is lost from the leaf's air spaces. 5. As water molecules evaporate into the atmosphere, more water evaporates from the thin film of water on mesophyll cells. 6. Due to its cohesive properties, the loss of water creates a tension or transpirational pull in the water column. 7. The transpirational pull draws water from the xylem in the leaves and stems and eventually from the xylem in the roots.8. As water is pulled upwards, the cohesion of water due to hydrogen bonding holds the water molecules together.9. This prevents the water column in the xylem from breaking apart as it is pulled upwards.10. At the same time, the adhesion of water molecules to the walls of xylem vessels and tracheids prevents water from sipping down due to gravitational pull.11. Root pressure, the adhesive and cohesive properties of water (cappilarity action) and the trasnpirational pull, all contribute to the movement of water from the roots to the leaves.

12

The regulation of transpiration by the stomata

1. Transpiration is required for the upward movement of water and minerals from the roots to the leaves. 2. However, if the rate of transpiration is faster than the rate of absorption of water by theroots, the leaves will wilt. 3. If the loss of water through transpiration is excessive, the plant will eventually die. 4. If you examine the lower epidermis of a dicotyledonous leaf under the microscope, you will notice the presence of small pores or stomata in the leaf's surface.5. Each stoma is surrounded by two guard cells.

13

6. The two guard cells regulate gaseous exchange by opening and closing the stoma.7. If the plant is to obtain suffrcient carbon dioxide for photosynthesis, it is necessary that the stomata are open. 8. However, when the stomata are open, water can be lost through these stomatathrough transpiration. 9. Closing the stomata stops transpiration and reduces water loss, but prevents carbon dioxide from entering the leaf.10. In order to balance the need for photosynthesis and at the same time to prevent the excessive loss of water, a plant usually opens its stomata in response to an increase in light intensity, and to a decrease in the levels of carbon dioxide in the air spaces of the leaf. 11. In general, stomata are open during the day and closed at night. 12. During the day, light stimulates photosynthesis in the guard cells. 13. The guard cells start producing glucose. 14. This makes energy available for active chtoroplast - transport. 15. The guard cells accumulate potassium ions (K+) from adjacent cells through active transport.16. The guard cells vacuole become hypertonic and water moves in by stoma- osmosis. 17. As a result, they swell up and become turgid. 18. Since the inner cell walls which form the guard cells are thicker than the outer walls, the guard cells bend outward and the stoma opens. 19. During the night, when photosynthesis stops, potassium ions exit the guard cells and water moves out by osmosis. 20. The guard cells become flaccid and the stoma closes.

14

15