Which structure is necessary for the Symplastic pathway of water transport in a plant?
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Plants need water for a number of reasons - for photosynthesis, hydrolysis reactions and to keep their cells turgid. Water moves from the soil into the roots then up the xylem to the rest of the plant against the force of gravity. In the leaves, it evaporates out of the stomata in a process called transpiration. Show
Movement of water into rootsWater moves from the soil into root hair cells by osmosis. There will always be a higher concentration (or water potential) in the soil compared to the plant because water is constantly being lost through the leaves in transpiration. Water moves down its concentration gradient into root hair cells then travels through the root cortex and the endodermis before reaching the xylem. Water can get into xylem vessels by two routes - the symplast pathway and the apoplast pathway. If water travels via the symplast pathway then it travels from cell to cell through the cytoplasm. Neighbouring cells are connected by small channels in the cell walls called plasmodesmata. However, if water is travelling via the apoplast pathway then it passes from cell to cell through the cell walls. Plant cell walls are very absorbent and water can travel through them easily, making this the main pathway that water uses to move from the roots to the xylem.  The problem with the apoplast pathway, however, is that the water (and the substances dissolved in it) is bypassing the cell membrane, who’s job it is to control what is going into the cell and ensure harmful substances such as toxins do not make their way in. The root, therefore, has something called the Casparian strip which is a waxy strip within the cell walls which is impermeable to water. This forces the water to go through a cell membrane which can then control which substances are allowed to enter the plant. Once water has passed through the Casparian strip, it can then reach the xylem. Movement through xylemOnce water is in the xylem, it travels upwards - against the force of gravity - towards the rest of the plant. Water is able to move against gravity due to two forces: tension and cohesion. Tension is a ‘sucking force’ which is created when water evaporates from leaves (transpiration), pulling more water into the leaf. You can think of this in the same way as drinking water through the straw - any water which is lost from the top of the straw (as you drink) is immediately replaced with water molecules below it. Cohesion describes how water molecules are attracted towards each other. The strong hydrogen bonds between water molecules causes them to ‘stick’ together, creating a column of water. This means that when tension pulls water up the xylem, the whole column of water moves upwards. Another force which facilitates the movement of water up the xylem is adhesion. Adhesion describes the attraction of water to non-water molecules (such as the molecules which make up the xylem walls). The attraction of water to the walls of the xylem help water to rise up through the vessel. TranspirationTranspiration is the loss of water vapour through evaporation from a plant’s surface. It mainly happens through gaps in the leaf called the stomata, which need to open during the daytime to allow gas exchange. Plants need to take in carbon dioxide for photosynthesis and get rid of oxygen, which happens through the stomata. A side-effect of this is that water vapour can also diffuse out of the leaf through the stomata - this is known as transpiration. Plants will close their stomata at night (because they are not photosynthesising so gas exchange does not need to take place) which minimises transpiration. The following factors affect the rate of transpiration:
Measuring transpiration rate using a potometerYou can investigate the rate of transpiration using a piece of apparatus called a potometer. A potometer actually measures the amount of water uptake by a plant. The assumption is that the amount of water uptake by a plant is equivalent to the amount of water lost through transpiration - this is not completely true since some water will be used to keep cells turgid, for photosynthesis and other chemical reactions and some water will be created from aerobic respiration. However, it is a good way of estimating the amount of transpiration taking place. You can carry out the experiment using the following method:
XerophytesXerophytes are plants which are adapted to living in regions where water is scarce. Examples include cacti, pineapple and marram grass. They have a number of adaptations which enables them to survive in such harsh conditions:
HydrophytesHydrophytes are plants which live on water, such as water lilies. Because oxygen does not dissolve well in water, hydrophytes need adaptation to enable them to cope with low oxygen levels. They are adapted in the following ways:
Did you know… Plants make up the oldest living organisms found on the plant. Some species of cacti can live to be three hundred years old. The oldest plant recorded is the bristlecone pine tree which is estimated to be almost five thousand years old. What is the symplastic pathway in a plant?The pathways of ion and water created by symplast are known as the symplastic pathway. This pathway offers resistance to the flow of water since the selective plasma membrane of the root cells handles the intake of ions and water.
What is the pathway for water transport in a plant?Xylem transports water, minerals, and nutrients from the soil to all the plant parts. There are two types of "transport" tissues in plants- xylem and phloem. Water and solutes are transported by the xylem from the roots to the leaves, and food is transported from the leaves to the rest of the plant by the phloem.
Which structure or compartment is part of the symplast?Symplast is the living cells and the connections between living cells. Starch is stored in the symplast. Axial parenchyma, ray parenchyma, sieve tubes, companion cells, cork cambium, the cambium, and plasmodesmada make up the symplast.
Which cell layer filters water and solutes moving into or out of the vascular tissue of the roots?In most seed plants, especially woody types, an endodermis is absent from the stems but is present in roots. The endodermis helps regulate the movement of water, ions and hormones into and out of the vascular system.
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