Plant Transport Plants are mostly made up of water. Lettuce is about 94% water and a potato is about 77% water. Plants need water for three main purposes: ——photosynthesis, -support -transport of chemicals Water is a raw material for photosynthesis. If water is in short supply, the rate of photosynthesis will be limited. Plants need water for support, otherwise they wilt. Water is needed to transport many chemicals within plants. Chemicals, such as mineral salts, dissolve in water and they can then be moved within the plant to the cells that need them.
Plants obtain water from the soil through their roots. The roots are adapted to absorb large volumes of water by having many tiny root hairs, which increase the surface area of the roots. Each root hair is an extension of an individual cell on the outside of a root, called a root hair cell. Water enters the root hair cell by osmosis. Osmosis is the movement of water from a dilute solution to a more concentrated solution, through a partially permeable membrane.
The solution inside the root hair cell is more concentrated than that of the soil water, so water moves from the soil into the cell.
The cell membrane is partially permeable. It has tiny holes which allow water molecules to pass through. These holes are too small for larger solute particles to pass through. Once inside the root hair cell, the water dilutes the solution in the cytoplasm, so the solution is less concentrated than that of cells closer to the centre of the root. Water therefore passes from cell to cell by osmosis across the root, until it reaches the xylem vessel at the centre of the root. The xylem vessel carries water up the plant. Plants need minerals to stay healthy.
Minerals are found dissolved in the soil water around the plant roots. When minerals dissolve, they form ions. These are very small and can pass through holes in the cell membrane. Some mineral ions diffuse from the soil into the root hair cells, because their concentration in soil water is higher than that in cell sap. Diffusion is a passive process. This means it does not use energy. When the concentration of minerals is higher in soil water than in root hair cells, the minerals can diffuse into the cells passively. An example of this is when a farmer puts fertilizer onto the soil.
Sometimes minerals are in a higher concentration inside the cells than they are in the soil water. This would favor diffusion of minerals out of the plant into the soil. However, this doesn’t happen. Instead, the plant pumps minerals into the root hair cells against the concentration gradient by a process called active transport. Active transport uses energy. This energy is provided by respiration. The site of respiration inside cells is the mitochondria. Root hair cells contain many mitochondria. If soil becomes waterlogged the plants may die.
This is because water takes the place of air in the soil, so the roots cannot obtain oxygen. Without oxygen, plants cannot respire to release energy for active transport, so they cannot take in minerals. Glucose sugar is produced when a plant photosynthesises. Some of the sugar is used for respiration to release energy. The sugar is also used to make different types of chemicals needed by the plant. Many sugar molecules are joined together to form starch, which is an insoluble carbohydrate stored in the cells. Starch can later be broken down into glucose again when the plant is not photosynthesising.
The glucose molecules can join together in a slightly different arrangement to form cellulose molecules. These are used to make cell walls. Glucose can be converted to fatty acids and glycerol. These make up fat molecules that are stored in many types of seeds. Glucose is combined with nitrogen and other elements to make amino acids, the building blocks of proteins. These other elements are obtained from the soil as minerals. Energy from respiration is needed for all these reactions. Plants require different minerals in varying amounts for a range of purposes. The major elements are needed in quite large amounts.
They are nitrogen, which is absorbed in the form of nitrate ions, phosphorus, which is absorbed in the form of phosphate ions, potassium ions, and magnesium ions. Other elements, called trace elements, are needed in very small amounts. If plants do not absorb sufficient minerals for their needs, they show deficiency symptoms. These relate to the specific use of each element in the plant. Nitrogen is needed to make amino acids, which are the building blocks of proteins. Proteins are an important part of cell membranes and enzymes. Without proteins, plants cannot grow properly or function efficiently.
Plants lacking nitrogen have yellow leaves and show stunted growth. Phosphorus plays an important role in the reactions involved in photosynthesis and respiration. It is also needed to make DNA and cell membranes. A shortage of phosphorus results in poor root growth and purple leaves. Potassium helps the enzymes involved in photosynthesis and respiration to work. It is also important for the production of flowers and fruit. Without potassium, plants have yellow leaves with dead spots and show poor fruit and flower growth. Magnesium is needed to make chlorophyll, which is essential to absorb light energy for photosynthesis.
If magnesium is lacking, the leaves turn yellow. Farmers and gardeners add fertilisers to the soil to provide more minerals. Some of these are called NPK fertilisers because they are rich in nitrogen, phosphorus, and potassium. By law, packaging must show the amounts of each mineral present. You can set up some water culture experiments to investigate the effects of mineral deficiency in plants. These experiments are left alone for several weeks. Black paper is used to prevent light entering. Light would stimulate algae to grow in the solution, and algae would use up some of the minerals.
The tube allows air containing oxygen to enter, so that the root cells can respire. A and F are used as controls. A contains a complete culture solution, which has all the minerals added. This allows the seedling to grow as well as possible. F contains only distilled water, which won’t contain any minerals at all. The seedling will show the poorest growth. The controls are used for comparison with the other plants, so you can see the effect of a shortage of each mineral. Flowering plants have two separate transport systems for water and nutrients.
Water, containing dissolved minerals, is transported upwards from the roots to the stems and leaves in xylem vessels. Nutrients, such as sugars and amino acids, are transported both upwards and downwards through the plant in phloem tubes. Glucose is made in the leaves of plants by the process of photosynthesis. The sugar is needed by all the cells of the plant for respiration, as well as for other purposes. It is carried from the leaves to all parts of the plant in phloem tissue. Phloem is a living tissue. The phloem tubes form a continuous system for the transport of soluble sugars and amino-acids throughout the leaves, stems and roots.
The nutrients are used by the growing tips of shoots and roots to make new cells, or they may be transported to the roots for storage. This movement of food is called translocation. It is a complex process which requires energy. The phloem tubes are positioned towards the outside of a stem. If a ring of tissue is removed from the outside of a stem, translocation cannot occur. Sugar solution accumulates above the ring and can be seen as a bulge. Sugar cannot reach the roots, so they die. Xylem vessels form a continuous system throughout the leaves, stems, and roots of plants.
Water, which is absorbed from the soil by osmosis, moves upward only through the plant. This is because water evaporates from the leaves, reducing the pressure at the top of the plant. The pressure is less than in the roots, and this causes water to be pushed up the plant. The evaporation of water from leaves is called transpiration. The upward flow of water through a plant is called the transpiration stream. The xylem contains long tubular cells which are dead. They have no cytoplasm, but their side walls are thickened with lignin to make them waterproof and to give the plant support.
The end walls of the vessels have pores through which water can travel. The phloem tubes and xylem vessels are in close association with each other in structures called vascular bundles. In a leaf, the vascular bundles are found within the veins. Plants have to be able to support themselves so that the leaves are held up towards the light, and the flowers are held open for pollinating insects to enter them. Water is very important in supporting a plant. Water enters cells by osmosis if the concentration of chemicals inside the cell is greater than that of the solution around them.
This makes the cell swell and become firm, due to the increased pressure inside the cell, which pushes against the cell wall. The cell wall is strong enough to withstand this pressure, so it does not burst. The cell is said to be turgid or firm. When lots of plant cells are turgid they push against each other and support the plant. If a plant is short of water it begins to wilt. This is because the cells lose water and become soft or flaccid. The vacuole is smaller and the pressure inside the cell is low so the cells do not push against each other to provide support.
Transpiration is the loss of water vapour from the surface of leaves. Water evaporates from cells within the leaf and then diffuses out through tiny holes called stomata. This loss of water from the leaves draws more water up through the stem from the roots in the transpiration stream. The flow of water through a plant is important for several reasons; it supplies water to the leaves for photosynthesis, transports dissolved minerals, keeps the cells turgid so that the plant is supported, and cools the leaves in hot weather. The rate of transpiration is affected by several factors.
It is fastest in hot, dry and windy conditions because the water will evaporate more quickly, just like washing drying on a line. The rate is also faster during the day when it is light. The stomata are fully open so that carbon dioxide, which is needed for photosynthesis, can enter the leaf, and oxygen can diffuse out. More water will also diffuse out of the leaf when the stomata are open. Transpiration slows down on cold, damp, dull days, and when there is a short supply of water. Many leaves have a thick waterproof waxy cuticle on the upper surface of the leaf to prevent too much water being lost.
For the same reason, most stomata are found on the lower epidermis where it is cooler and more humid because there is less air movement. The size of the stomata is controlled by the guard cells that surround them. Each stoma is surrounded by a pair of guard cells. The guard cells are the only cells of the epidermis which contain chloroplasts. This means that they can photosynthesise and make sugar when it is light. The sugar inside the guard cells causes water to enter by osmosis and the cells swell. The cell wall on the stomatal side of the guard cells is thicker than the wall on the outer surface.
This causes the cell to bend as it swells with water, opening the stoma. Try blowing up a long thin balloon that has sellotape stuck on one side. It curves as you blow it up. This is how the guard cells control the size of the stomata and therefore the rate of transpiration. In the dark, photosynthesis cannot occur, so the concentration of sugar in the guard cells falls and they lose water. This closes the stomata as the guard cells lose their turgidity. Similarly, if the plant is short of water, the guard cells become less curved and close the stomata, slowing down the loss of water from the plant to prevent wilting.
There are several methods that can be used to measure the rate of transpiration. The most common one to use is a potometer. The apparatus is assembled underwater and all the joints are sealed with grease to make sure that it is air-tight. The rate at which the plant takes up water is measured by timing how far an air bubble travels along the capillary tube in a given time. If we assume that the rate of water flow through the potometer equals the rate of loss from the leaves, then this gives us the transpiration rate.
The apparatus can be used to compare the rate of transpiration under different conditions. For example, by placing the potometer in different temperatures, in the wind or in places of differing humidities. This graph shows the results of an experiment to compare the amount of water lost in light and darkness. It shows that the water loss is greater in the light. Another way of measuring transpiration is to record the change in mass of a potted plant. The polythene bag prevents water being lost from the soil, so any change in mass must be due to water being lost through the leaves.
Pot B is the control. Its mass shouldn’t change. Plants that live in dry conditions are adapted to reduce water loss even more. Cacti have few stomata and a very thick waxy cuticle. In the course of evolution their leaves have been reduced to spines so there is less surface area for the evaporation of water. Marram grass grows on sand dunes where fresh water is very scarce. Its leaves are rolled up to trap a humid atmosphere around the stomata, which reduces water loss. Other plants have hairy leaves. The hairs trap water vapour creating a humid atmosphere to reduce transpiration.
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