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to investigate the water potential of potato tuber cells.

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Introduction

Aim: The aim of this practical is to investigate the water potential of potato tuber cells. Introduction: Water potential (Yw, psi), which is a measure of the energy state of water is affected by dissolved solutes, pressure and matrix particles. The contribution to water potential by dissolved solutes, termed osmotic potential (Ys), is always negative in sign. In other words, solutes decrease the water potential. The contribution of pressure (Yp) may be positive, negative or zero, but is generally positive since most plant cells are turgid (turgor pressure). The contribution due to the binding of water to colloidal particles (matrix) and surfaces, termed matrix potential (Ym), also lowers the water potential. Although it is often small enough to be ignored, matrix potential is important when talking about soil water relations. Thus, the water potential of a plant system can be arithmetically represented by the equation: Yw = Ys + Yp + Ym Prediction: I predict that the higher the water concentration of sucrose solution the lower the water potential, and the lower the concentration of the solution the higher the water potential. Therefore, when the highest concentration of sucrose solution is immersed with a potato cylinder it has a low water potential and as a result the potato cylinder will lose water and also mass. However, when a potato cylinder is added to a solution of low concentration it has higher water potential and therefore, the potato cylinder gains water and mass. Thus, it could be stated that water potential is indirectly proportional to concentration. This means that when the concentration increases the water potential decreases and vice versa. Potato cylinder that is placed in the highest concentration of sucrose solution will have the lowest water potential, and as a result the potato cylinder will have the greatest loss in water and therefore, when weighed, you would observe there is a loss in mass compared to the other potato cylinders. ...read more.

Middle

The change in water potential caused by the external application of pressure or suction to the soil water is called the pressure potential component of the soil water potential. The constraint that the pressure must be applied externally distinguishes pressure potential from the tension applied in the development of the matric potential. The pressures exerted on the soil water can come from several sources, but the primary source considered is ponded water or hydrostatic pressure. Water is often ponded on the soil surface during irrigation or heavy rains. This standing water exerts a positive pressure on the water in the soil. When there is no standing water on a soil, the external pressure applied to the soil is limited to the pressure of the atmosphere. Therefore, the applied pressure is atmospheric pressure. This is the pressure condition specified in the reference state for soil water potential so the without water ponding the pressure potential is zero. When water is ponded on a soil, the applied pressure is increased by the weight of the ponded water. This increase in applied pressure increases the potential energy of the water in the soil so the pressure potential component of water potential is positive. Pressure can also be applied to soil water by increasing the pressure of the soil air and from the weight of soil laying on top of the location of interest. For example, when barometric pressures change, the pressure of the soil air near the surface is either higher or lower than the new atmospheric pressure until equilibrium can be re-established. This causes temporary changes in the pressure applied to the soil water and resultant changes in soil water potential. These changes are referred to as the pneumatic potential component of water potential. Pure water has the highest possible water potential. Water molecules will always move from a region of high water potential (pure water) ...read more.

Conclusion

Only one variant was kept: - the concentration. However, when it came to removing the potato cylinders from the boiling tubes some took longer to remove then others as it was a slightly difficult task which may have also caused the potato cylinders to be squeezed slightly, resulting in the loss of water and mass. The only real problem within this experiment was the apparatus. Within my apparatus and method I stated "Using a cork borer and razor blade, prepare six potato cylinders, each about 1mm in diameter and 50mm long" however, due to inefficient equipment I changed the measurements of my potato cylinders too, length of 20mm and a diameter of 7mm. If I was to perform this investigation again, the only thing I would change is the timing, because I had to hurry slightly, thus, may perhaps cause inaccurate results. In addition to a greater time period, more readings may be taken to make my experiment fairer. For further research or evidence of osmosis taken place refer to the onion percentage plasmolysis investigation. This is simply where small thin strips of onion are placed under a microscope; where their cells are counted. Then the onion strips are placed within different sucrose concentrations (different moralities). After a set period of time, the onion strips are removed and once again placed under a microscope. Next you are required to count the number of cells plasmolysed. This experiment is made possible due to osmosis. If you put a plant cell in a solution (e.g. a concentrated sucrose solution) that has a lower (more negative) water potential than the cell's cytoplasm and vacuole, water leaves the cell by osmosis. The vacuole shrinks and eventually the protoplast (the living part of the cell) becomes detached from the cell wall. The point at which the protoplast is just about to become detached is called incipient plasmolysis. When it has become detached, the cell is said to be plasmolysed. Therefore, from this happening you are able to count the cells within this onion strip that has been plasmolysed, hence, proving the principles of osmosis. By Jaikishan Sharma 12LVY ...read more.

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