De-starching the Leaves of a Plant

Brilliant Biology Student

Diffusion & Osmosis Investigating Osmosis Using Potato Strips

Investigate Osmosis using Potato Strips

Osmosis can be easily demonstrated in biological systems using  potato strips, water and salt or sugar solution. Remember that osmosis is defined a net movement of particles of solvent along its concentration gradient, across a selectively permeable membrane, until equilibrium is established. In this experiment the solvent particles are water particles, the selectively permeable membrane are the cell membranes of the cells in the potato tissue. The concentration gradient is  the result of the difference in the concentration of solute in the cytoplasm of the potato cells and the solutions. Apparatus & Materials 3 Potato strips (5 x  1 x 1) cm  (or potato cylinders/cores  of equal length e.g. 5 cm  diameter, made with a cork borer) 2 Petri dishes with equal volumes of solutions A and B  ; A = sucrose or salt solution (hypertonic solution); B = distilled water ( hypotonic solution) 1 empty petri dish  (control) paper towels Stop watch or timing device top pan balance ruler Procedure Observe each strip by feeling it, noting whether it is turgid or flaccid. Record this. Weigh and measure each potato strip, recording the initial mass and length Place a strip in each petri dish, taking care not to mix the strips.  Start the timer. Remove the strips after 15 minutes and dab on tissue. Weigh each strip and measure each potato strip, recording the final mass and length. Observe each strip by feeling it, noting whether it is turgid or flaccid. Record this. Perform % difference calculations for the mass and length using the formula: (final - initial) x 100% / initial   Record your results in a table in your book like the one below: TABLE SHOWING RESULTS OF EXPERIMENT  TO DEMONSTRATE OSMOSIS USING POTATO STRIPS Discussion Define osmosis and identify the semi-permeable membrane in the experiment Osmosis is the movement of solvent particles e.g. water along its concentration gradient  - from high to low concentration - via a semi-permeable membrane. In this experiment the semi-permeable membrane is the cell (surface) membrane. Account for the changes - if any  - in the mass and length of the strips in water,  salt solution and air. Strips in water Description - Turgid Mass - The increase in mass of the potato strip in water is due to the movement of water molecules into the plant cells via osmosis. Water molecules in the petri dish are at a higher concentration  than water molelcules in the cytoplasm of cells and so move along  their concentration gradient into the cells. As more water molecules are present in cells - and water  has mass - the final mass of the potato strip will be greater than the first. Length - the decrease in length is also due to the movement of water molecules via osmosis. More water molecules occupy more space - volume - and push against the cell membrane and the proximate cellulose cell wall.  The push of the water molecules result in expansion in all dimensions, including length.  The expansion is limited - and regulated  by the cellulose cell is inelastic. The push/force acting upon the surface area of the rigid cell produces turgor pressure (recall pressure is force acting per unit area), and hence strips are turgid. Strips in  Salt Solution Description - Flaccid Mass - the decrease in the mass of the potato strip is due to the movement of water molecules outside of the plant cells.  Water molecules in the cytoplasm are at a higher concentration than in the salt solution in the petri dish and so move along their concentration gradient out of the cells. Water molecules have mass and so the decreased number of water molecules results in decreased mass. Length - the decrease in the number of water molecules results in loss of volume. The turgor pressure exerted by the water molecules against the cell membrane and  consequently  the cellulose cell wall is less. The cell may even shrink if the turgor pressure dminishes further. Strips in Air Description -  Slightly flaccid. This strip as the control and theoretically does not change in texture. However in reality it loses water by evaporation via the cut surfaces and becomes dehydrated.  By the end of the experiment the strip in air is slightly flaccid, but certainly not as much as the strip in salt solution. Mas - mass changes are minimal as  minimal water was lost. Length - Length changes were minimal as minimal water was lost.