Osmosis in Potato Tissue

Biology Aim: My objective is to plan and conduct an  experiment from which I should be able to draw a firm conclusion that  will either prove or disprove any predictions I make. This essay aims to assess and investigate the effect of various solution concentrations on the activity of osmosis in plant tissue. Background scientific theory: Plants exchange gases (CO2 and O2) in maintaining vital respiratory processes and in carrying out photosynthesis; they absorb certain minerals and sugars so to use as a source of energy and eradicate wastes in order to maintain specific requirements for survival.Large amounts of water are absorbed by root hairs and are then distributed across the cells of plants by the process of osmosis; water being essential to life, assists cells in executing crucial chemical processes. Molecules travel by two means; active transport or passive transport. Active transport is the movement of a substance from a low to high concentration against the norm concentr ation gradient. Hence, the process requires expenditure of energy, and the support of a carrier protein. Passive transport, however, does not require energy but occurs spontaneously instead.It is a form of transport by which molecules move along a concentration gradient, from an area of higher concentration to an area of lower concentration. Passive transport includes osmosis and facilitates diffusion. Osmosis is a special case of diffusion; it describes the passage of a solvent from a weaker solution, where there is higher water potential, to that of a more concentrated solution that has a lower water potential through a partially permeable membrane in order to achieve the state of equilibrium. A partially permeable membrane acts as a barrier to some substances but allows others to penetrate through freely.Within any plant cell the cytoplasm and cell sap within the vacuoles are of a variety of substances such as salt, sugars and proteins. In theory, water will diffuse into the cell by osmosis if the solution surrounding the cell is weaker but when enclosed by a stronger more concentrated solution than its contents than water is drawn from it by the same process. As a result it becomes flaccid; the turgor pressure falls, the vacuole collapses and the cytoplasm shrink away from the cell wall. This may result in its wilting or death. Plant cells have a cell wall as well as a plasma membrane.The cell wall is a strong and rigid structure that is used by the cell to create osmotic pressure within the cell. This pressure can build because of the rigidity of the cell wall. The cells within a plant that contain high water pressure act as the plant supportive structure, helping to give it its shape. When gaining, the vacuole will expand and press outwards on the cytoplasm and cell wall but since this cannot be over stretched there is a resistance on the inflow of water by the un-stretchable cell wall. This resistance results in turgor pressure exerted by the vacuole on the cell walls.When the vast majority of the cells within the leaves and stem of plant are turgid, its stem will be firm and upright and the leaves straight therefore providing mechanical strength. Similarly a flaccid celled leaf will be limp and the stem will droop; such plants are thought to be wilting. Factors that affect the rate of osmosis (potential variables) The rate of osmosis is much dependent upon a number of factors; the temperature, the nature of the solute, the difference in concentration of solute on either side of the membrane, and any external pressure applied against the direction of flow.Concentration: – The  concentration  of a chemical  solution  refers to the amount of solute  that is dissolved in a  solvent. The concentration of any solution or plan tissue is directly linked to its water potential; the higher the concentration the lower it water potential. Water potential is a measure of the potential of water to enter or leave. Within a di lute solution there is a higher proportion of free water molecules hence water will flow from the dilute to concentrated solution; it is said to have a higher water potential.The maximum water potential is nil; an example of which is within pure water hence water molecules will flow from it to any other aqueous solution regardless of how dilute it may be. There are three possible concentrations of solution. The first, a hypertonic solution, has a higher solute concentration than that of the cell and as a result the water will leave the cell resulting in collapsed vacuoles in plant cells. A hypotonic solution, however, has a lower solute concentration than the solute concentration inside the cell and hence the net movement of water inside the cell is at its maximum.Whilst an isotonic solution is when the concentration of solutes is the same inside the cell as it is outside of the cell; in such as case the cell would not lose nor gain any water. This state is otherwise recognized as d ynamic Equilibrium where the molecules are randomly distributed despite there still being a rapid net movement of water across the cell membrane in both directions; water movement in opposing directions occur at the same rate and thereby ‘cancel’ each other out.Larger differences between the concentrations or water potential of the solutions results in a steeper concentration gradient, meaning a faster rate of osmosis. In effect the concentration outside of any cell is proportional to osmosis rates and therefore the higher the concentration of the solution the higher the net movement of water. Concentration is the factor which is to be assessed in the following experiment; it can be altered by diluting the solution with distilled water whilst keeping the volume constant, (i. e. a 0. 8 molar sucrose solution would consist of 2cm? f distilled water with 8cm? of sucrose solution equivalent to 10cm? of a mixture. ) Temperature: – The rate of Osmosis is also reliant u pon the kinetic energy of the particles being diffused. Since energy is directly related to heat, temperature governs the amount of kinetic energy the particles have. The general temperature of the room in which the experiment takes place will influence measurements of mass of the potato; hence it is important that we ensure that it remains ambient throughout our experiment in order to increase the reliability of our measurements.Ultimately as temperature increases or decreases, the rate of osmosis should also increase or decrease in relative, causing the potato chips to lose/gain mass at a faster/ slower rate. Temperature is therefore directly proportional to the rate of osmosis. Surface area to volume ratio- Additionally the surface area of the plant tissue or in this case the potato will also influence osmosis; the larger the surface area, the more cells are directly exposed to the liquid solution; therefore more water can transfer through the membranes of the cells at any one ti me resulting in a more massive net movement of water by osmosis.Size of Potato;- A large sized potato is likely to contain more  substances. Therefore it is likely to contain more water, which can  affect osmosis as there will be an increasing concentration of water,  which, as the potato's size increases affecting the overall movement  of water depending on the concentration of solution that the potato is  kept in. All potato chips shall be approximately at the same mass initially (1. 6g ± 0. 2g) Plant Tissue:-Where the potato was sourced from will also greatly affect the osmosis rates that are observed by its change in mass also whilst some may be old others may be fresh. In theory osmosis rates will be considerably more efficient in newer than older potato strips for the reason that in older ones its cells may be more damaged or be on the verge of decay. Hence the permeability of the potato can too be determined by its age; its age and permeability to the larger exten t are in inverse proportion therefore.Type of solution:- Different solution types ( i. e. sucrose, glucose, potassium chloride and sodium chloride) will differ from one another in that the size of the molecules of the solute may be larger in one that the other. this will affect osmosis rates within the plant tissue in that smaller molecules will be able to penetrate through the partially permeable membrane with far more ease than larger ones ; hence the smaller the molecules of the solute the faster the rate of osmosis. Time; –The time given in which the plant tissue was immersed within the solution would also affect the experiment, as we would expect that with a longer duration, there would be more time for osmosis to occur. Pressure; – The pressure on one side of the membrane can increase or decrease the rate of osmosis by pushing the solution against the membrane. Selecting a factor â€Å"An independent variable is that which is presumed to affect or determine a de pendent variable[1]†. It can be changed as required, and its values do not represent a problem requiring explanation in an analysis, but are taken simply as given.More generally, the independent variable is what someone actively changes; while the dependent variable is what changes as a result. The constant variable or otherwise known as controlled variable, however, is never changed during an experiment as it remains ‘constant’. During my experiment I shall be measuring the change in mass of the potato chips as the dependant variable in order to investigate the effect of different concentration of sucrose solution (independent variable) on osmosis.Other factors such as the room temperature, type and volume of solution, form and size of plant tissue shall remain ambient through out my investigation as controlled variables or otherwise ‘constants’. Qualitative, Quantitative and Testable Hypothesis Osmosis is defined as being the net movement of water f rom a region of high concentration to that of a low concentration. This  movement must take place across a partially permeable membrane such as  a cell wall, which allows only explicit molecules to penetrate through but obstructs the pathway of others.Diffusion will continue until the area in which the molecules are found  reaches a state of equilibrium where molecules are  randomly distributed throughout solution. By my scientific background knowledge I can make a number of predications; I hypothesize that the rate of osmosis will differ in all test tubes depending on the concentration of solution, resulting in some either gaining mass in becoming turgid, others remaining the same whilst the rest lose mass and being flaccid after being immersed in solution subsequent to a fixed period of time.I hypothesize that plant tissue immersed in solution of higher concentration than that within the cell itself (hypertonic solution) will loose mass because there will be a net movement of water through the partially permeable membrane outside the cell where there is a considerably lower water potential. The plant cell becomes flaccid and as a result of the ell membrane shrinking and pulling away from its cells walls, it said to have been plasmolysed. Likewise, I predict that if the plant cell is placed in a hypotonic solution where the concentration of solution is lower than that of the cell than water is absorbed by osmosis.The plant cell swells and shall become turgid to the extent that the pressure within the cell matches the internal or turgor pressure. The turgor pressure shall prevent further uptake of Water preventing it from rupturing. More generally the concentration of the sucrose solution in the flasks is inversely proportional with the plant tissues change in mass; as you increase the concentration of the solution, both the mass and the length of the potato chip will decrease.This I shall prove by measuring the mass of the plant tissue before it being and subsequent to being submerged in solution. Data collected shall be handled and substituted into the following equation . . . . Final mass (g) – initial mass (g) X 100 = Percentage Change Initial mass (g) A negative percentage change will signify that water has been lost by its net movement through the partially permeable membrane whilst a positive one will suggest that there has been a gain. I predict that my findings when transferred onto a graph will have a similar trend as in the one shown below.Preliminary The main objective of carrying out four preliminary experiments was to inform me as to which instruments would be most efficient in its use, gain first hand practice and recognize suitable methods in order to conclude specified measures which I will be using in ensuring that my final investigation is of highest standards in its reliability. |Concentration (Mole)|Mass(g) |Percentage change of | | | |mass (% to 1 d. ) | | |Before |After |Change | | |1. 000 |2. 32 |1. 97 |0. 35 |15. 0 | |0. 500 |2. 30 |2. 24 |0. 06 |2. 60 | |0. 250 |2. 28 |2. 23 |0. 05 |2. 20 | |0. 125 |2. 32 |2. 59 |-0. 27 |-11. 60 | Preliminary experiment 1 – Equipment: ? cork bores (5mm) potato ? glucose solution ? ruler ? tissue ? distilled water ? scalpel ? pipette ? test tubes x4 ? measuring cylinder ? scales Key: outlier Experiment 1 varied the concentration of the glucose solution; from the readings observed a clear correlation can be identified as a result of measurements being wide ranging- as the concentration of the glucose solution reduced by half each time the mass of the potatoes reduced too and hence there is a proportional correlation between the independent and dependant variable.Such readings are scientifically explanatory and correspond with my initial prediction produced in the previously discussed section of the hypothesis. One major setback of this investigation was that I used too few solutions with varying concentrations and hence the readings ava ilable in giving evidence of any judgments on my findings or when proving/disproving my initial predictions are limited.Hence, if I were to further develop this investigation as my final one I intend on using 5 different concentrated solutions that range from 0. 2 to 1 molar with increments of 0. 2moles. In this investigation we failed to use distilled water but instead opted for tap water because we had forgotten; this is much likely to have reduced the reliability of my results because any dissolved substances within the water will have affected the rate of osmosis by changing the solutions proposed concentration.In the final experiment we shall therefore use distilled water. However, I believe that my readings from the first preliminary experiment are reasonably fair and reliable because no outliers which contradict the key identified trend can be found and the mass of the potatoes previous to and subsequent to the experiment was accurate to 0. 01 of a decimal place. The potato w ith which we worked with was . . . . . . Preliminary Experiment 2 Equipment: cork bores (5mm) ? Swede ? sucrose solution ? ruler |Temperature (C °) |Length (mm) |Percentage change in | | | |length (% to 1d. p ) | | |Before |After |change | | |45 |3. 5 |3. 3 |0. 2 |5. 7 | |0 |3. 5 |3. |-0. 2 |-5. 7 | |25 |3. 5 |3. 5 |0. 0 |0. 0 | ? tissue ? distilled water ? scalpel ? pipette ? test tubes x3 ? measuring cylinder Key: outlier The second of my preliminary experiments was to see the effect of change in temperature on the rate of osmosis; the cell membrane remained at constant as Swede which was immersed in a 10ml sucrose solution at 1 mole.It is different from other preliminary experiments by the fact the Swede cylinders were 40 mm in length; however as suggested by the data there is no significant change in my measurements. The measurements that were collected were few; by 45, 0 and 25 degrees acting thereby as a disadvantage because we are restricted in the amount of data available in proving the trend. It is suggested that the higher the temperature the higher the osmosis rates; our observations are fully explainable, expected and scientifically conventional.It is known that temperature has a direct relationship to the movement of particles across a membrane; therefore as temperature increases, the rate at which particles move across the membrane should increase too as its molecules will be more excited in having more kinetic energy. Nonetheless if the temperature were to rise too high the selective permeability of the membrane can be damaged due to becoming denatured hence permanently collapsing the process of osmosis; this could be investigated by experimenting with temperature at or over 60 degrees.If I were to further develop this investigation with temperature being the independent variable I would take 4 measurements with increments of 20 degrees with minimum values of -20 to 60 degrees. My results here are less accurate by my use of only an average the rmometer as it is corrected to the nearest centigrade measured; consequently if I were to develop this experiment I would prefer to use a data logger which will give me values that have been rounded to the nearest decimal place with a considerably smaller error tolerance.In all of our preliminary investigation there was a reaction time of 30 minutes given; this however in my opinion is unreasonably low because the process of osmosis may have not yet reached its maximum capacity or equilibrium; hence in the final experiment there will be 45 minutes given for each set of vegetable cylinders. We did not use a stop watch or any other means of measuring and setting the reaction time other than the classroom clock and our best estimates; this may have hindered the reliability of our measurements so I shall use an stop watch in the final experiment which is accurate to