Potato Osmosis - Assignment Example

Diffusion & Osmosis Introduction All living organisms including plants and animals have evolved from the simplest unicellular organisms like the planktons and amoebae to the much more complex living forms with intricate cellular differentiation and organization. But even the simplest unicellular protozoan needs to communicate with the external environment for survival because it needs to ingest food, eliminate waste products and toxins of metabolism and exchange gases through respiratory mechanisms. This communication is further developed in a more intricate manner in the multicellular organisms where exchanges take place within and between the cells in carrying out their assigned physiological functions. All cells are bound on the outside by a limiting membrane akin to a boundary wall which separates and protects the cellular components from the external environment. This membrane is known as the cell wall and the plasma membrane in plants and animals respectively. This membrane is a specialized component which selectively permits passage of water and other components across it either inward or outward depending upon the gradient of the fluid environment and internal contents or due to selectivity for particular molecules through specialized pores or channels. Two of the processes by which such exchanges take place are known as diffusion and osmosis. Diffusion Diffusion is simply the movement of molecules from a region of higher concentration to a lower concentration (intranet.landmark.edu). Diffusion occurs within a single compartment or environment where molecules move from a region of higher concentration to a lower concentration until equilibrium is reached. Such diffusive activity can take place inside a single cell. Osmosis Osmosis too is also movement of components within a solution either from a higher gradient to a lower one or selectively under active transport systems controlled by cellular physiology, but this diffusion occurs across a semi permeable membrane such as the plant cell wall or the animal plasma membrane. The plant or the animal cell walls are selectively permeable for certain molecules and do not allow other molecules to pass through them. Experiments Three experiments can be conducted in the laboratory by which the process of osmosis can be better understood by creating artificial environments at a gradient which are separated by a semi permeable membrane. The movement of selective molecules can then be studied by measuring their amounts prior to and after the two solutions with different concentrations are brought in proximity bys separating them by a known material which is capable of semi permeability. Materials and Methods 1. A 30 cm piece of dialysis tubing with one end tied to impart it the semblance of a bag. 2. 15% Glucose and 1% Starch Solutions. 3. Distilled Water and different concentrations of sucrose solutions [0.2, 0.4, 0.6, 0.8 & 1.0 M (Molar) solutions]. 4. Potato rods of roughly the same size prepared from a single potato. Three experiments were conducted in the laboratory using the above materials to study the processes of diffusion and osmosis. Experiment I Procedure A 30 cm piece of 2.5 cm. dialysis tubing which had been immersed in water prior to the actual experiment was taken and one end tied to give it the shape of a bag. 15 mL of 15% glucose and 1% starch solutions were filled in the bag and the open end was tied leaving enough space for expansion. The colour of the solution was recorded. The glucose content of the solution was recorded using glucose Testape@. A 250 mL beaker was filled 2/3rd with distilled water and the glucose content of this solution was also recorded. 4 mL of Lugol’s solution was added to the distilled water and the colour intensity was recorded. The dialysis tubing bag was immersed in the above solution and allowed to stand for thirty minutes. Later the final colour intensity of solutions, the one inside and the other outside the bag were recorded. Both liquids were also tested for their glucose concentration. Experiment II Procedure As in the above experiment, six pieces of 30 cm long dialysis tubing pre immersed in water were obtained and one end of each was tied up to obtain six bags. 25 mL each of the sucrose solutions of different concentrations (0.2, 0.4, 0.6, 0.8 & 1.0 M) were filled one each in the above dialysis bags. After removing the excess air all bags were closed by tying up the other end leaving enough space for the expected expansion. Each bag was rinsed thoroughly with distilled water to remove traces of sucrose from outside. After carefully blotting each bag to remove the excess moisture, their initial masses were recorded. Six 250 mL beakers were filled 2/3rd with distilled water and the bags with different concentrations of sucrose solution suspended in them. Beakers were labelled to indicate the molarity of the respective sucrose solutions. After an elapse of 30 minutes each bag was carefully removed, blotted and their masses recorded once again. Experiment III Procedure Five pieces of potato rods were cut from a single potato taking care to approximately keep the size of the pieces equal so that they had similar exposed surface areas. The pieces were thoroughly washed with distilled water and soaked overnight in different concentrations of sucrose solutions (0.2, 0.4, 0.6, 0.8 & 1.0 M). The initial mass before soaking and the final mass after 24 hour soaking were recorded. Results Experiment I The changes in colour and the amounts of glucose (concentration) observed before and after soaking are depicted in Table I Table I Initial Contents Initial Solution Colour Final Solution Colour Initial Presence of Glucose Final Presence of Glucose Bag 15% Glucose & 1% Starch Clear Dark Purple Positive Positive Beaker H2O + IKI Clear Orange Brown Negative Positive Experiment II The changes in mass of each bag which was suspended in different concentrations of sucrose are given in Table II Table II Contents in Dialysis Bag Initial Mass (gms.) Final Mass (gms.) Mass Difference (gms.) %age Change in Mass Distilled Water 24.74 24.69 - 0.05 0.202 Sucrose 0.02M 23.15 23.92 - 0.77 3.33 Sucrose 0.04M 24.49 26.27 1.78 7.27 Sucrose 0.06M 27.28 29.99 2.71 9.93 Sucrose 0.08M 30.25 34.03 3.78 12.50 Sucrose 1.0M 25.64 29.51 3.87 15.09 Uncertainties = 0.01 gm The following graph (Figure I) shows the changes in the dependent variable, the mass in relation to the dependent variable, the molarity of the sucrose solutions. Fig. I Experiment III The percentage change in the potato rods after 24 hours immersion in varying strengths of sucrose solutions is given in Table III Table III Potato Cores Distilled Water 0.02 M Sucrose 0.04 M Sucrose 0.06 M Sucrose 0.08 M Sucrose 1.0 M Sucrose Initial Mass 0.97 0.96 0.96 0.97 0.97 0.98 Final Mass 1.22 1.04 0.92 0.80 0.74 0.84 Change in Mass 0.25 0.08 - 0.05 - 0.17 - 0.24 - 0.14 %age Change in Mass 25.8 8.3 5.2 17.5 24.7 14.3 The following figure (Figure II) shows the relationship between the independent variable molarity to the change in mass (shrinkage) of the potato cores: Analysis Experiment I In the first experiment, from the results it was obvious that the exchange of some components was evident due to the observation of colour as well glucose in the inside of the bag and the outside solution respectively. The glucose from the bag passed through the semi permeable membrane and entered the solution in the beaker and correspondingly the IKI entered the bag from the beaker. The IKI is responsible for the colour change as it reacts with glucose. The starch molecule stays inside as it cannot cross the semi permeable membrane as shown by the dark purple colour developed inside the bag. It is obvious from the experiment that glucose and IKI due to their smaller molecular size move between the two compartments in this experiment. In order to change the character of this experiment from a qualitative to quantitative one, parameters like the size of the bag (diameter) and the optical density of the solution in the beaker can be measured to express the data in numerical form. Based on the observed result, the rank of different constituents in the experiment in terms of their molecular size from smaller to larger is water molecule < Glucose < Starch < IKI < Membrane. If the experiment was modified to include glucose and IKI inside the bag and starch and water outside, it would have the opposite result, i.e. the concentration would move from low to high inside the bag. Experiment II From Table II, it is evident that higher the molar concentration of sucrose solution more was the pressure gradient for the water molecules to move from the distilled water in the beaker into the bags with the sucrose solution. In the first bag in which only distilled water was present, there was no change in mass after the elapsed time because there was no pressure gradient. In the weak solution of 0.02M, still negligible change was recorded probably due to the short time of 30 minutes. However, in the bags containing > 0.04 M sucrose solution a definite increase in the mass was recorded for the bags which ranged from 7.27% in the 0.04M sucrose bag to a maximum value of 15.09% in the bag containing the highest concentration (1.0M) of sucrose. This shows that the gradient affects the passage of the molecules in an attempt to stabilize both solutions across a semi permeable membrane. Experiment III In the third experiment involving same sized potato cores obtained from a single tuber which were placed in varying concentrations of sucrose, the results confirm that water molecules move from an area of higher water potential to that with a lower one. When placed in a hypertonic solution, the potatoes will shrink in size, swell when placed in a hypotonic solution and stay the same when placed in an isotonic solution. In the distilled water, the potato core attained the maximum %age change in mass (25.8%) due to the unrestricted movement of pure water into the starch rich potato core. With increasing intensities (concentration) of sucrose, this free movement of water molecules was restricted as depicted by the low figures of %age change in mass at 0.02. A 0.04M sucrose concentration as sugar itself attracts the water molecules. At higher concentrations of sucrose (0.06, 0.08 and 1.0M) however again variable changes in the potato core masses were observed due to the competition between the sucrose solution and the potato starch for the movement of water molecules. Conclusion The above three experiments serve to satisfy the criteria on the basis of which movement of molecules across a semi permeable membrane takes place. All experiments show qualitatively as well as quantitatively that the processes of diffusion and osmosis take place across concentration gradients until a state of equilibrium is reached. Molecules with smaller size pass easily through a semi permeable membrane as compared to larger molecules although the larger molecules may contribute in enhancing the concentration gradient. Read More