Factors Affecting Osmosis.

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FACTORS AFFECTING OSMOSIS

AIM

The aim of this experiment is to investigate the factors affecting osmosis. I have chosen to investigate the effect varying concentration of sucrose solution has on the amount of osmotic activity between the sucrose solution and a plant cell. The plant cell I have chosen to use is a potato tuber from which I plan to cut potato chips of equal length which I will place in test tubes filled with varying concentrations at equal volumes of sucrose solution. After a set amount of time I will remove the potato chips and record the change in length.

BACKGORUND KNOWLEDGE

High water potential is a solution which has a high concentration of water and a low concentration of solute (e.g. sugar, sucrose or salt) or no solute at all. Therefore it can either be a very dilute solution of something like sucrose or pure water, however in each case there is a lot of water.  Whereas low water potential is the opposite of high water potential being a solution of low concentration of water and high concentration of solute (sugar, sucrose or salt). Hence it can be a concentrated solution of something like sucrose, however in this case there is much less water.

       When to solutions one of high water potential and one of low water potential are divided by a semi-permeable membrane water molecules will move from the solution of high water potential through the partially permeable membrane to the solution of low water potential. This will continue until both sides have reached a state of equilibrium or isotonic point, which is when both solutions wither side of the partially permeable membrane have equal water potential neither having higher or lower potential, they are said to be isotonic. When this occurs there is no net movement of water across the membrane only random diffusion of particles.

      What I have discussed in the previous paragraph is the process of osmosis, however in short osmosis is defined as the net movement of water molecules from a high water potential through a partially permeable membrane to a low water potential.

      As mentioned earlier the net movement of water must take place across a partially permeable membrane (e.g. cell wall) which is a very thin layer of material which allows small molecules such as Oxygen, water, Carbon Dioxide, Ammonia, Glucose and amino-acids to pass through. But does not allow larger molecules such as Sucrose, Starch and protein to pass through.

     

  • Hypertonic solutions are solutions which contain a high concentration of solute which means they contain low concentration of water (solvent).

  • Hypotonic solutions are solutions which contain a low concentration of solute which means they contain a high concentration of water. 

Osmosis is essential for the survival of living things, plants absorb most of their water by osmosis, while in animals osmosis helps regulate the flow of water and nutrients between body fluids and cells. Therefore osmosis occurs in both plant and animal cells.

 

OSMOSIS IN PLANT CELLS

When a plant cell is placed in a hypertonic solution (e.g. a concentrated sugar solution), the water potential outside the cell is lower than inside the cell causing the plant cell to loose water and become flaccid (shrunken and soft) or plasmolysed. When what I have stated above occurs the plant cell decreases in mass because water has left the cell. An everyday example of this involves ocean water which generally contains higher concentrations of solutes (roughly 3.5 grams of salt for every litre of water), therefore most land plants when placed in ocean water will effectively dehydrate because cells in the plant (high water potential) will loose water to the surrounding salt water (low water potential).

     However when a plant cell is placed in a hypertonic solution (e.g. a dilute sugar solution) or pure water the water potential outside the cell is higher than inside the cell causing water to move into the cell so that it becomes turgid (swollen and hard). Turgidity in plant cells is very important to plants because this is what makes the plant "stand up" into the sunlight. When water moves into the plant cell the pressure inside the cell rises, eventually the internal pressure of the cell is so high that no more water can enter the cell. When this occurs the cell is prevented from bursting by the strong cell wall which surrounds the plant cell of the plant cell. When what I have discussed above occurs the plant cell increases in mass as there is water moving into it.

     When plant cells are placed in solution which has exactly the same water potential as the plant cells only random diffusion of molecules takes place and the plant cells are in a state between turgidity and flaccidity. This can be called incipient plasmolysis, incipient meaning about to be.

OSMOSIS IN ANIMAL CELLS

Osmosis in animal cells is irrelevant to my investigation but nethertheless I will explain briefly what occurs in animal cells during osmosis

     When an animal cell is placed in a hypertonic solution (e.g. a concentrated sugar solution) the same thing happens as with plant cells. The plant cell looses water and becomes flaccid (shrunken and soft) because the water potential outside the cell is lower than inside the cell.

     The majour difference between plant cells and animal cells is that plant cells have no cell walls which is why they behave differently when placed in hypotonic solutions. When a animal cell is placed in a hypotonic solution (e.g. a dilute sugar solution) the water potential outside the cell is higher than inside the cell causing water to move into the cell, however animal cells do not become turgid instead they burst because there is no cell wall to support the cell membrane.

I have already mentioned that osmosis is essential for animals and plants, however osmosis is also important to humans because industries use reverse osmosis for such purposes as water purification and food preservation. Reverse osmosis is the opposite of naturally occurring osmosis and can be explained in the following way: when osmosis occurs the flow of water molecules from a high water potential through a partially permeable membrane to low water potential produces a measurable pressure which is called osmotic pressure. If pressure is applied to the solution of low water potential, and if that pressure exceeds the osmotic pressure water flows through the partially permeable membrane from the solution of low water potential to the solution of high water potential.

PILOT

I carried out a pilot, which is a preliminary experiment to test my method and prediction and see if they are both correct and accurate, if not I made modifications.

PREDICTION FOR PILOT

I predict that as the concentration of the sucrose solution increases the length of the potato cylinders will decrease. I can predict this because during osmosis water moves down a diffusion gradient from high water potential through a partially permeable membrane to a low water potential.  Therefore the more concentrated the sucrose solution the lower the water potential hence the water will move from high water potential in the potato cylinder through a partially permeable membrane to low water potential in the sucrose solution.  I can predict that the largest decrease in length will be in the potato cylinder in the most concentration sucrose solution (2M) because the potato will have to loose the largest volume of water so that both sides can reached a state of equilibrium, which is when the water potential is the same in both the potato cylinder and in the sucrose solution. I can further predict that the potato chip in 0.0M sucrose solution which is distilled water will not decrease in length but increase in length because the distilled water has a higher water potential than the potato. Hence the water will be moving from a high water potential in the 0.0M sucrose solution (distilled water) through a partially permeable membrane to low water potential in the potato chip. I can quantify my prediction by saying that, as concentration doubles the decrease in length will also double until a limiting factor operates.

METHOD FOR PILOT

  1. Take an averaged sized potato which is hard and healthy, and on a cutting board cut one end of the potato off using a scalpel.
  2. Next with care push a cork borer (size 5) down vertically through the potato, using a glass rod to push the potato cylinder out. Repeat this until you have bored out five potato cylinders.
  3. Now cut all the ends of the potato cylinders square and using plastic covered graph paper measure the cylinders to the nearest mm so that all five are exactly 2cm/20mm in length cutting any excess potato off on a cutting board and using a scalpel.
  4. Now place each potato cylinder in a separate test tube.
  5. Then using a 5cm3 syringe cover each chip in different concentrations of sucrose solution (0.0M; 1.5M; 1.0M; 1.5M and 2M).
  6. Cover each test tube with cling film and leave for 30 minutes timing the time with a stopwatch.
  7. After 30 minutes remove the potato cylinders from the test tubes wiping away any excess water using a paper towel.
  8. Measure the potato cylinders to the nearest mm using plastic covered graph paper and record any change in length in a table.
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RESULTS FROM PILOT

ANALYSIS OF PILOT

I obtained 2 anomalous results from my pilot experiment; the two anomalous results were for the 1.0M and 0.5M sucrose solutions. Even thought the potato cylinders in these two sucrose solutions were left in the sucrose solutions for the same amount of time as the other 3 potato cylinders, they did not change in length like the other 3 potato cylinders by remained the same length. Theoretically speaking the two potato cylinders in 0.5M and 1M sucrose solution should have decreased in length which is why I decided they were anomalous ...

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