<p> Transport in Angiosperms</p><p>Assessment Statement</p><p>9.2.1 Outline how the root system provides a large surface area for mineral ion and water uptake by means of branching and root hairs 9.2.2 List ways in which mineral ions in the soil move to the root 9.2.3 Explain the process of mineral ion absorption from the soil into rots by active transport 9.2.4 State that terrestrial plants support themselves by means of thickened cellulose, cell tugor and lignified xylem 9.2.5 Define transpiration 9.2.6 Explain how water is carried by the transpiration stream, including the structure of xylem vessels, transpiration pull, cohesion, adhesion, and evaporation 9.2.7 State that guard cell can regulate transpiration by opening and closing of stomata 9.2.8 State that the plant hormone abscisic acid causes the closing of stomata 9.2.9 Explain how the abiotic factors light, temperature, wind and humidity, affect the rate of transpiration in a typical terrestrial plant 9.2.10 Outline four adaptations of xerophytes that help to reduce transpiration 9.2.11 Outline the role of phloem in active translocation of sugars (sucrose) and amino acids from source (photosynthetic tissue and storage organs) to sink (fruit, seeds, roots) Root system, absorption and uptake</p><p>The roots provide a huge surface area to draw up essential ions and water. Contact with the soil is vastly increased by root hairs that occur just behind the growing tip of each root. The water flows in due to osmosis. The uptake of minerals is done via active transport.</p><p>1. Water uptake</p><p>Water uptake occurs from the soil in contact with the root hairs by osmosis. Uptake is largely by mass flow through the interconnecting free spaces in the cellulose cell walls to the xylem. There are three possible routes of water movement through the plant cells and tissues. The cortex structure of the root also facilitates the water uptake.</p><p> Apoplast Pathway (Mass Flow)</p><p> Water does not enter the cell. It moves through the cell walls until it reached the endodermis. Cells of the endodermis have a Casparian Strip around them that is impermeable to water. The water is diverted to the spaces of dead cells, eventually to the xylem. The Casparian Strip is thought to be a measure of control so water cannot move directly into the xylem, so the cell can divert it out of the cell, if it is not good.</p><p> Symplast Pathway</p><p> o Water enters the cytoplasm but not the vacuole. It passes from cell to cell via connections between cellular cytoplasm of adjacent cells, called plasmodesmata. The organelles are packed together in cells, and as a result, block significant progress of water. It is not the major pathway for water. Minerals mainly move through this pathway.</p><p> Vacuolar Pathway</p><p> o Water enters the cell and move into the vacuole. It then travels through the cytoplasm and the cell wall to the next cell.</p><p>Once in the endodermis, water can be actively secreted into the xylem or pulled via transpiration forces. 2. Uptake of Minerals</p><p>There are three ways that minerals can move from the soil to the root.</p><p>1. Active Transport using protein pumps in the plasma membranes of root cells. As the concentration outside the cells decreases, a concentration gradient is built up and then the ions flow in by diffusion. 2. Mass Flow, or the water carrying the ions goes into the root. 3. Fungal hyphae – fungus grows on the surface of roots and sometimes into the cells of the roots. The hyphae grow into the soil and absorb minerals ions and phosphate from the surface of soil particles. The ions are supplied to the roots and allow the plant to grow in mineral poor soil. Some plants supply sugars, and other nutrients to the fungus. This is an example of a mutualistic relationship. Transpiration</p><p>Transpiration – the loss of water vapour from the leaves and stems of plants.</p><p>As water is lost, the amount of water in the plant decreases. A pull is created in the plant to “pull” water up the plant. This is similar to maintaining homeostasis.</p><p>Transport up the stem</p><p>Water is taken up the stem, from the roots by the Mass Flow or Apoplast Pathway, into the xylem. </p><p>In the root, the xylem is centrally located, in the stem, the xylem occurs in the ring of vascular tissue. Xylem, therefore runs from the roots to stem and then to leaf. </p><p>Xylem begins as elongated cells with cellulose walls and living contents, connected end to end. During development, the end walls dissolve away, and the mature xylem is a long hollow tube. As the tubes extend, the tissue dies. Therefore, xylem is dead tissue, and is composed of two elements:</p><p> Tracheids – narrow cells arranged in columns, overlapping at tapered ends giving some support to the plant. The ends have pits for water to move rapidly from one cell to the next. Due to their structure, they are less efficient that xylem vessels.</p><p> Xylem vessels – larger columns of cells. When the dead cell walls disappear, they become wider and transport water more efficiently. Since they are so wide, they are reinforced by ligin for support of the plant. This only occurs in angiosperms. Mechanism of water transport </p><p>Transpiration controls the flow of water. But, how does water move against gravity?</p><p>First, in the leaf are stomata. These open and close to control the amount of water present in the leaf. Around each stomata are guard cells, which open and close, depending on the tugor of the cells.</p><p>When the plant is well hydrated, the guard cells are swollen, causing them to open, due to the pressure on the cells walls. When the plant dries out, the guard cells sag and the stomata close. Water loss is stopped and gas exchange is halted.</p><p>Other external factors that affect the opening and closing of stomata are:</p><p> Light causes stomata to open</p><p> Low CO2 levels in the air spaces in the cause the stomata to open  Shortage of water causes the stomata to close</p><p>When leaves are deficient in water, they synthesize a hormone called abscisic acid. This closes the stomata and overrides any external stimuli – the stomata close. This allows the plant to avoid dehydration and death.</p><p>When the stomata are open, water evaporates out of the leaves, diffusing water vapour out. This maintains a concentration gradient that requires more water. </p><p>The water in the stem, which is connected by xylem, moves up to replace the water lost by transpiration. As a result, the water is pulled up the plant. Water is polar and is held together by its cohesive forces. Therefore, the water molecules stick together and flow up the stem together. This is called transpiration pull.</p><p>Factors affecting Transpiration</p><p>The rate of transpiration is the amount of water vapour that a plant loses from its leaves and stems per unit of time. The rate depends on the:</p><p> Size of the plant  The thickness of the cuticle  How widely spaced the stomata are  Whether the stomata are open or closed</p><p>These are the biotic factors. These are the factors the plant can control. There are four abiotic factors that affect the rate of transpiration.</p><p>1. Temperature – affects the rate at which water evaporates from the surfaces inside the leaf. At higher temperatures, evaporation increases. The higher temperatures also increase the rate of diffusion between the inside of the leaf and the outside. The increase in temperature allows the air to hold more moisture and reduces the relative humidity of air outside the leaf. The concentration gradient increases, doubling the rate for every 10oC increase in temperarture</p><p>2. Humidity – is the water vapour content of the air. It is measured as a percentage of the maximum amount of water vapour the air can hold. The humidity inside the leaf is always around 100%. The lower humidity outside the leaf causes the evaporation to be higher, increasing transpiration. Evaporation is much higher in dry air, so on hot / cold, dry days, transpiration increases dramatically</p><p>3. Wind – air currents take water vapour away from the leaf surface, keeping the concentration gradient large, and increasing the rate of transpiration. On calm days, the humidity around the leaf increases, slowing down transpiration.</p><p>4. Light – in light, photosynthesis increases and stoma open to allow CO2 in, but also water out. Therefore, transpiration increases. In darkness, there is no need to absorb CO2, and water can be conserved, as the stoma close and photosynthesis decreases. What else does water do?</p><p>The take up of water, allows plants to grow very tall. </p><p>Plants do not have a skeleton. Woody trees and shrubs have xylem to support them.</p><p>Herbaceous plants have xylem, but depend on water to provide turgor pressure. Turgor Pressure is the swelling of cells with water. As the water is absorbed into the cells, from the xylem, the vacuole takes in water. As a result, the vacuole swells until its membrane presses against the cell wall. The pressure provides that stiffness, or crispness, that we see in vegetables, like celery. Moving Food in Plants</p><p>Translocation is the movement of manufactured food (sugars and amino acids). This occurs in the phloem tissues of the vascular bundles. </p><p>Sugars are made in the leaves (in the light) by photosynthesis and transported as sucrose. The first formed leaves transport sugars to sites of new growth (new stems, new leaves, and new roots). In older plants, sucrose is increasingly transported to sites or storage, such as the cortex of roots or stems, and in seeds and fruits.</p><p>Amino acids are mostly made in the root tips. Here is where the absorption of nitrates takes place. After they are made, amino acids are transported to sites where protein synthesis is occurring. These are mostly in buds, young leaves and young roots, and in developing fruits.</p><p>Translocation is not just limited to organic compounds. Chemicals that are applied to plants by spraying, and are then absorbed by the leaves may be carried all over the organism. Pesticides are called systemic for this reason.</p><p>Phloem tissue consists of sieve tubes and companion cells. </p><p>Sieve tubes are narrow, elongated elements connected end to end to form tubes. The end walls, know as sieve plates, are perforated by pores. The cytoplasm of mature sieve tubes has no nuclei, or many of the other organelles in a cell. But each sieve tube is connected to a companion cell by strands of cytoplasm passing through gaps (called pits) in the walls. The companion cells are believed to service and maintain the cytoplasm of the sieve tube, which has lost its nucleus.</p><p>Phloem is a living tissue, and has a relatively high rate of aerobic respiration during transport. In fact, transport of manufactured food in the phloem is an active process, using energy from metabolism.</p><p>Phloem transport may occur in either direction in stem leaves and roots, and is believed to move by mass flow. How this works is:</p><p> Solutes are loaded into the phloem sieve tubes, requiring ATP and then the solutes flow through the phloem from a region of high hydrostatic pressure to low hydrostatic pressure.</p><p> Hydrostatic pressure is high around photosynthetic cells in the light (mesophyll of the leaf), and in the phloem sieve tubes nearby. It is the presence of the sugars, which concentrate the fluid and creates a high osmotic pressure. Water flows in, raising the hydrostatic pressure further. This is called a source area.  Hydrostatic pressure is low in cells where sugar is converted to starch and stored. Areas of storage are the cortex of the root, stem, seeds and in the nearby phloem tissue. Here the removal of sugars lowers the osmotic pressure, and water flows away. These storage areas are called sink areas. Below summarizes the sources (areas where sugars and amino acids are loaded into the phloem) and sinks (where the sugars and amino acids are unloaded and used).</p><p>Sources Sinks Photosynthetic tissues: Roots that are growing or absorbing  Mature green leaves mineral ions using energy from cell  Green stems respiration Storage organs that are unloading their Parts of the plant that are growing or stores: developing food stores:  Storage tissues in germinating seeds  Developing fruit  Tap roots or tubers at the start of  Developing seeds the growth season  Growing leaves</p><p>Sometimes sinks turn into sources and visa versa, and therefore, phloem must be able to transport in both directions. Unlike the vessels in animals, there are no valves or a central pump. However, they are similar because in both, fluid flows inside tubes due to pressure gradients. Energy is needed for both, and therefore, both are active processes. The movement of substances in phloem is called active translocation for this reason. Adaptations of xerophytes</p><p>Xerophytes are plants that have adapted to arid climates. Examples are cacti are an example.</p><p>In order to adapt to dry climates, xerophytes must decrease water loss due to transpiration. Therefore, the plants have adapted by:</p><p> Small, thick leaves reducing the water loss by deceasing surface area (needles or green stems)  Reducing the number of stomata  Having the stomata located in crypts or pits on the leaf surface, which causes higher humidity near the stomata  Having a thickened, waxy cuticle  Having hair-like cells on the surface to trap water vapour  Becoming dormant in the dry months  Storing water in the fleshy stems and restore the water in the rainy season  Using alternative photosynthetic processes called CAM photosynthesis (Crassulacean acid metabolism) and C4 photosynthesis. CAM plants close stomata during the day and incorporate carbon dioxide at night. C4 plants have stomata open during the day, but take in carbon dioxide more rapidly than non- specialized plants.</p>