Wallace Footman/Audrey Elite
100989648
BIO 1103
A08
Introduction
Osmosis is the diffusion of free water across a selectively permeable membrane, and diffusion is the spontaneous movement of a substance down its concentration or electrochemical gradient, from a region where it is more concentrated to a region where it is less concentrated (Reece et al., 2014). The role of osmosis to the cell is to transport water across cell membranes, balancing the water between the cell and its environment optimizing the cells ability to survive (Mao et al., 2008). Diffusion allows substances to pass in and out of the cell using no energy, such as oxygen, or carbon dioxide (Reece et al., 2014).
Osmosis occurs in both, animal and plant cells. Most animal cells favor an isotonic environment, as the do not have a cell wall. The isotonic environment allows water to diffuse across the membrane at an equal rate, keeping the volume of the cell stable. Plant cells have a cell wall, which favors a hypotonic environment. The cell swells as water enters through osmosis, however the cell wall can only expand so much, the wall will then exerts a back pressure on cell, called turgor pressure, which prevents further uptake of water, making the cell turgid (Reece et al., 2014). Uptake of molecules by diffusion is based on the principal that as the surface area of a cell is large in comparison to its volume, the percentage weight gain of material uptake will be larger (Hoffman et al., 2009).
The purpose of the lab was to: study osmolarity by a change in weight, study the effect of surface area to volume ratios, test for the presence of starch using iodine potassium iodide (IKI), test for the presence of sugar using Benedict’s reagent, and to examine the effects of osmosis in plant cells, using a microscope.
Methods
The first portion of the experiment required that varying concentrations of glucose solution (0, 0.25, 0.65, 0.85 M) were placed in test tubes. Potato squares were cut, removing any skin and measured for volume, mass, and surface area, using a Denver Instrument MXX-212 scale, uncertainty of 0.01g and a ruler with an uncertainty of 0.5mm, the squares were then placed into one of the four solutions. The squares were left in each solution for one hour, then taken out and measured again.
Another portion of the experiment involved two square pieces of potato, one large (2cm sides), and one small (1cm sides). Each were measured and incubated in distilled water for one hour, then were measured again.
In the next portion of the experiment a 15cm sausage casing was obtained and a knot was tied at one end. 2ml of a 20% glucose solution were then transferred into the sausage casing, using a 5ml pipette. Using a different 5ml pipette 2ml of a 2% starch solution was transferred into the casing, the casing was then held closed and the solutions were mixed by inverting the casing three times. Each end of the casing was draped over the top of a 50ml plastic tripour beaker, filled with 50ml of distilled water, the middle portion of the sausage casing was submerged in the distilled water. The casing remained in the water for 30 minutes, and then 1ml of iodide potassium iodide was added to the water in the beaker. Four cuvettes were obtained and the solution taken and placed into each, were from: inside of the sausage casing (two cuvettes), the beaker and distilled water. One of the cuvettes with solution from the sausage casing had iodine potassium iodine placed into it and the remaining three cuvettes had Benedict’s reagent placed into them. The three cuvettes with Benedict’s reagent were placed in a 60 C Fisher Tissuemat Water bath for 5 minutes. Once water bath is finished, each solutions colour recorded.
The final portion of the experiment involved the immersion of red onion skin (allium haematochiton) in an isotonic solution, and a mystery solution for 5 minutes. The effects were observed on an Olympus CX31 microscope using the 10x
