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The Movement of water and Solutes in Plants

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The Movement of water and Solutes in Plants Plants, like most living things, need to exchange gasses and transport minerals and substances to different parts of their structures. Unlike Animals, plants don't have a circulatory system. Diffusion is adequate for plants to exchange gasses, as they have a high surface area to volume ratio. Exchanging gasses is not a problem. The problem is moving water up stems that can be 100's of meters tall without the aid of a pump (such as the heart). The answer is all to do with hydrostatic cohesion. Before we look at how plants transport water and solutes, we must firstly know about the principles of water potential. Water potential Water potential(?) indicates which way water will move in a system. Water will always move from a high ? to a low ?. ? can be measured in Kilopascals (kPa). Pure water has a ? of 0 kPa. As solute is added, ? is reduced (it becomes a negative number.) e.g. This is useful because you can add pressure to the equation. If a pressure of 400 kPa is added to B (due to squeezing), the equation is -1000 + 400 = -600 ?-600 = -600 Now that the forces are equal, there will be no water movement. We will measure ? using Molar Concentration which has a concentration of 0 - 1. ...read more.


E.g, as the potassium ions increase inside the guard cell, the water potential decreases, and water enters the guard cell. The more water that is drawn out due to the transpiration stream, the more that is drawn in from the roots, and similarly, if less water is being drawn in from the roots, less water will be transpired out. Factors affecting transpiration can be tested using a potometer set up (which in reality only measures the rate of water being taken up the stem). A photometer set up looks like this. From this you can find out via experiments, the many factors which affect the rate of transpiration. A higher temperature increases the rate of evaporation and can reduce humidity which causes increased transpiration. Light stimulates the stomata to open allowing gas exchange for photosynthesis to occur, which increases the rate of transpiration. If wind around the plant is faster, then the saturated air will be blown away. This will be replaced by drier air, and transpiration will occur at a faster rate. So we now know about the transpiration stream, how does the water flow through the stem? The stem has a number of vessels running down it which are used to transport water around the plant. These consist of the following two. The Xylem and the Phloem. Xylem Xylem vessels are composed of dead cells which have hardened and have formed long empty tubes. ...read more.


Finally, the best way to explain the movement of water and sugars in plants is to use the Munch model. This is self explanatory, and the best explanation of it is to simply label one of the tubes xylem, and another tube, phloem. From Sunflower Biology Indiana The diagram shows two water-filled compartments connected by a small tube. Another U-shaped tube with bags made of semipermeable membranes are attached to the ends. At the start, the tube and the bag on the left are filled with water while the bag on the right is filled with a concentrated sugar solution. The membrane that the bags are made from allow water but not sugar to diffuse across. Due to the difference in water potential between the contents of the bag on the left and the water it is immersed in, osmosis will occur and water will begin to accumulate in the bag. As this happens, pressure will build and a flow will develop that pushes the sugar solution through the tube to the other bag. This system will only function for a short time since the sugar will eventually be distributed evenly throughout the bag and tube. In the phloem, there is a continuous input of solute from source tissues and a continuous efflux at the sink. This input and output at the two ends will maintain a pressure differential that will keep liquid flowing. Thus, the driving force for solute transport is a pressure gradient between the source and sink regions. ?? ?? ?? ?? ...read more.

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