Determination of molarity of cell sap in potato tubers.

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Determination of molarity of cell sap in potato tubers

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Plant and animal cells are made up of a cell membrane. The plant cells have the cell membrane line in the inside of the cell wall. The cell wall provides the turgidity to the cell. The cell wall is dead and allows all substances to pass freely through it whereas the cell membrane is alive and selective. This is the case also for the cell membrane of the animal cells. The only difference is however that the cell membrane is the only layer around the cell unlike the cell wall that is part of the plant cells. The cell membrane is alive and selective as we know. This means that it is semi-permeable. This is called a semi permeable membrane. Semi permeable means that it is a layer or a surface that allows only certain particles to pass through but not any others. A completely permeable membrane means that it is a membrane that allows any particles to pass through like is the case for the dead cell wall.

This cell membrane is a thin layer of cytoplasm and allows the cell to control the products that come through and go out. For example the cell membrane allows only substances like water, glucose and water are allowed to enter and waste products are removed. However the other way round is not possible because the membrane does not allow the wastes to enter and the vital particles to exit. The membrane is very important to organisms.

The movement of gas molecules is called diffusion. The diffusion of other substances such as liquids across a semi permeable membrane is called osmosis. The process of osmosis will be studied in detail here. There is another type of movement and it is called active transport. To understand both of these it is important to know what the concentration gradient is. It is also called diffusion gradient because it also applies to gases. Diffusion or concentration gradient shows the molecules or the particles as from less concentrated to more concentrate. This is demonstrated below.

               A- higher concentration                        B- molecules moving                    C- Less concentration

From this diagram we can see that there are more molecules on the left and the molecules are moving from section A through the section B and moves to C where there are fewer molecules. This way we can see that the molecules are diffusing through the empty space. The arrow points towards the diffusion gradient. For the active transport it will be the other way which shows it is against the concentration gradient. This shows the movement of particles for gases. The difference for osmosis is that in osmosis there is a semi permeable membrane. This semi permeable membrane has tiny pores that allow only certain products to pass through.

Active transport is different from osmosis because osmosis is according to the concentration gradient and active transport is against the concentration gradient.

Factors of osmosis

Osmosis depends on the concentration and the temperature of the surroundings in which it takes place. The concentration of the solution is very important because then it will be according to the number of molecules in the water. If we take a sugar solution we know that there is sugar dissolved in it but also it is important to know what is the concentration of the solution. The higher the concentration, the more the amount of sugar molecules and so the amount of water is less. Therefore the amount of water that moves towards it will be more. If we take the above diagram and we place the semi permeable membrane in the middle or have a semi permeable membrane full of the solution we will see this change. This is demonstrated below.

 

 

 

For the sake of simplicity, the water molecule has been shown as small and monoatomic where its actual formula is H20 and the sugar molecule is also shown bigger so that it is easier to distinguish and its actual formula is C6H12O6. As we can see form the above diagram that the reason that the sugar molecule cannot pass through is that it is too big to fit through the pore. Even if the gap is large enough the sugar molecule passes though very slowly. The arrows in between show the movement of the particles. As we can see that the reason that the water molecules are moving is that there are less water molecules on the left and so the solution is concentrated and the solution on the right is dilute. The solution on the left is more concentrated because the sugar molecules are now hydrated and so have stopped the water molecules from moving freely and thus there are less water molecules present in the solution. This is usually called water of crystallisation. So we can define osmosis as the movement of water molecules from a place of higher concentration to a place of lower concentration. As we will observe the level of water on the right will decrease and the level of water on the left increases until both will be the same. So they will be in balance and proportion. In plants the following occurs so that the plants may take in fresh water and if seawater were added the reverse would happen, the cells would lose water. This we will see later. Osmosis can take place in any solution, whether it is a salt solution or even a sugar solution.

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The water potential is the measure of whether the solution is likely to gain or lose water and by how much. So it is concluded that pure water has the highest water potential because it has the highest number of free molecules and can diffuse from one solution to the other no matter how concentrated or dilute it may be. This again repeats the fact that the water moves from a high potential to a low potential. Once again it is important that the substance that is added to the water is soluble in water.

Osmosis is also used industrially, ...

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