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Observing diffusion through the cell membrane by adding a starch solution & NaCl to dialysis tubing, leaving it in water and testing with iodine & silver nitrate. Introduction: This lab experiments with the movement of molecules across the cell membrane. A cell membrane, in any living organism, consists of a bilayer made from phospholipids. A phospholipid is made from a hydrophilic phosphate “head” and a hydrophobic “tail”, made from two fatty acids, one saturated, and one unsaturated. These are both joined together by glycerol, a three carbon alcohol (Refer to Appendix B Figure 1). When surrounded by water, the phospholipids will be rearranged so the hydrophilic heads interact with water and the hydrophobic tails repel away from it. All these phospholipids form a bilayer, which is essentially a cell membrane (Refer to Appendix B Figure 2) (OpenStax, 2013). This makes up roughly 50% of the membrane’s mass. Membranes also contain proteins, which make up for about 49% of the mass of the membrane (Cooper, Sunderland, 2000). Proteins act as ‘channels’, allowing small polar molecules and ions to pass through. Proteins become channels by embedding themselves through the lipid bilayer. This is because proteins have polar and nonpolar parts of the structures. That means that the nonpolar parts of the protein can embed itself in the middle part of the lipid bilayer, which is non polar, and the polar parts of the protein can extend outside of the membrane to allow small polar molecules and ions to pass through. The bilayer and protein channels makes the cell membrane semi-permeable to ions and polar molecules such as 𝐻 𝑂. It also makes the 2

cell membrane a flexible, strong barrier (Refer to Appendix B Figure 3) (Black, Jevkinar, 2016). Molecules can travel through the membrane through passive transport or active transport. Passive transport means that molecules will travel from an area with a high concentration to an area with a low concentration. This includes regular diffusion, where small nonpolar molecules and very small polar molecules travel straight through the bilayer, and facilitated diffusion, where small polar molecules and ions go through a protein channel in order to pass through the cell membrane. Since passive transport is an immediate process, it does not require any energy, or ATP. Active transport, on the other hand, is where molecules move in the opposite direction, which is achieved by transporting through protein carriers and with some ATP (Refer to Appendix B Figure 4) (Albert, Johnson, Louis, 2002). This lab uses a dialysis tubing as a substitute for a cell membrane, specifically for passive transport. The reason why is because the plastic that dialysis tubing is made from is semipermeable like a cell membrane. This means that once the dialysis tubing is inside the beaker with distilled water, whatever solution is added into the dialysis tubing, in this case, starch and sodium chloride, will diffuse into the distilled water after approximately fifteen minutes. This lab also uses a pipette in order to add the starch solution and NaCl into the dialysis tubing. The reason why both solutions are being titrated is because it will ensure that 3mL of starch solution and 4mL of sodium chloride are added into the dialysis tubing, or at the very least, a volume as close to these two values as possible. After 15 minutes of the dialysis tubing being soaked in distilled water, the water from the beaker, and the solution in the dialysis tubing,

will be tested with iodine and silver nitrate. Iodine is being used to detect any traces of starch. If there is starch in the solution, it will turn black and opaque. If there are no traces of starch, the solution will simply turn yellow/gold and transparent. Silver nitrate will detect any traces of salts in a solution. If there is salt present, it will form a white precipitate, and make the solution more opaque. If there are no salts then there will be no visible changes. A real life application for testing diffusion in the cell membrane, is with diseases such as cystic fibrosis. The cystic fibrosis gene makes a damaged version of a protein called the cystic fibrosis transmembrane conductance regulator (CFTR). That protein becomes a protein channel and, like with most proteins, gets embedded through the membranes of cells. These proteins target cells that produce any mucus located in the liver, lungs, pancreas and other organs. If this were a normal protein, it would facilitate passive transport, allowing small polar molecules or ions, especially chloride ions, to pass through the membrane from an area with a high concentration to an area with a low concentration. Because this is an abnormal protein however, it has no control over how many chloride ions go into or out of the cell which can cause a severe imbalance in the ratio between the amount of salt vs the amount of water in the body. This imbalance will cause the mucus to get really viscous and start to stick to germs in the organs that will begin to cause infections. (Winikates, 2012). That is why some people dealing with cystic fibrosis take mucus thinning inhalation medication like hypertonic saline. By inhaling hypertonic saline, the patient is allowing a higher concentration of sodium chloride into the lungs. When there is enough sodium chloride, it will attract the water into the airways. By adding more water into the airways, the ratio between the amount of water and the amount of salt become more balanced, which thins the mucus in the lungs, making it easier for patients to cough the mucus out (Cystic Fibrosis Foundation, 2020). The purpose of this experiment is to see if, after adding sodium chloride and starch solution inside the dialysis tubing and then adding that dialysis tubing into a beaker with water, if it will diffuse after 15 minutes. If the dialysis tubing is semi-permeable then the solution in the dialysis tubing (NaCl and starch) will diffuse into water after 15 minutes. This is because, according to passive transport, the solution will move from an area with a higher concentration to an area with a lower concentration. If the dialysis tubing is semi permeable like a membrane, after 15 minutes, the distilled water may not have the same concentration of starch or sodium chloride as the dialysis tubing but there should be at least a small concentration of the solutions in the distilled water. Analysis: From a translucent, colourless solution, with silver nitrate, the beaker water needed 5 drops of silver nitrate for parts of the solution to turn an opaque white and 7 drops for the entire solution to turn completely white/blue and opaque. The solution in the dialysis tubing, in comparison, needed 1 drop of silver nitrate for the solution to achieve the same results as the beaker water after 7 drops of silver nitrate. With iodine, the beaker water turned gold/yellow and remained transparent, even with 10 drops added. With the dialysis tubing solution, after one drop of iodine, the entire solution turned black and opaque (Refer to Appendix A Figures 1 & 2)

Appendix A: Observation Tables Table 1: Testing the Water From The Beaker and the Solution in The Dialysis Tubing For Traces of Starch and Sodium Chloride Substance

Number of Drops Added

Positive or Negative

Silver Nitrate ( Beaker Water)

7

Positive

Silver Nitrate (Dialysis Tubing)

1

Positive

Iodine ( Beaker Water)

10

Negative

Iodine (Dialysis Tubing)

1

Positive

Table 2: Qualitative Observations From Testing the Water From The Beaker and the Solution in The Dialysis Tubing For Traces of Starch and Sodium Chloride. Silver Nitrate (Beaker Water)

Silver Nitrate (Dialysis Tubing)

Iodine (Beaker Water)

Iodine (Dialysis Tubing)

Before Experiment

● Translucent ● Colourless ● Odourless

● Translucent ● Colourless ● Odourless

● Translucent ● Colourless ● Odourless

● Translucent ● Colourless ● Odourless

After Experiment

● After 5 drops, there are little bits of a white solid. ● After 7 drops, the entire solution turns blue/white and opaque.

● After a single drop, the entire solution turns blue/white and opaque. ● This happened a lot faster than with the beaker water.

● With 10 drops (far more than with the silver nitrate experiment with the beaker water), the solution turns gold/yellow and transparent

● After one drop, the entire solution turns black and opaque.

Appendix B: Images

Figure 1: This image shows what a phospholipid looks like both visually and through its chemical structure. On the left side of the photo, there is a yellow circle. That yellow circle is used to highlight the phosphate part of the hydrophilic head. Below the yellow circle on the is C3H8O3, otherwise known as glycerol. Glycerol is what connects the hydrophilic head to the hydrophobic tails. The reason why one of the hydrophobic tails looks bent is because it is an unsaturated fat. An unsaturated fat is when there is at least one double carbon bond. This results in the unsaturated hydrocarbon chain being slightly bent. The saturated tail (the left tail) shares only single carbon bonds, resulting in the tail being straight (Black, Jevkinar, 2016).

Figure 2: This image shows a bilayer that is formed by fatty acids. When phospholipids interact with water, the hydrophilic heads will interact with water and the hydrophobic tails, since the tails are nonpolar, will avoid water. This creates either micelles (best seen with how soap forms bubbles) or a bilayer. The bilayer, pictured above. A bilayer, as mentioned before, is basically the basis for a cell membrane. Without any proteins being embedded into the membrane, the membrane will only allow small nonpolar molecules and very small polar molecules to pass through (Albert, Johnson, Louis, 2002).

Figure 3: This image shows the protein channel and a carrier protein. If polar molecules want to go past the bilayer, because polar molecules can’t go through the middle of the bilayer, it has to go through the protein channel. As mentioned in the background, passive transport travels from highest concentration to lowest concentration. When it goes the other way around, with a little bit of ATP, the carrier protein can transfer from the area with the lowest concentration to the area with the highest concentration. The difference between a protein channel and a carrier protein is that a carrier protein can either open its inner gate or its outer gate, both gates cannot be open simultaneously. With a protein channel, both ends are open for molecules to pass through. This is because a carrier protein is very specific to what kinds of molecules are allowed at once. For example, if a protein channel lets in 1000 molecules at the same time, a carrier protein will only allow 10 to enter (Cooper, Sunderland, 2000).

Figure 4: This image shows the difference between passive and active transport. The left side shows passive transport, that is when molecules move from a higher concentration, to an area across the cell membrane where there is a lower concentration, until both sides have reached equilibrium. This can be achieved through simple diffusion, where nonpolar or small polar molecules pass through the bilayer, or facilitated diffusion, where polar molecules move through a protein channel. Passive transport requires no energy, or ATP. With active transport, because molecules are moving the opposite direction, from an area with a lower concentration to one with a high concentration, it requires ATP for Active transport to occur (Albert, Johnson, Lewis, 2002).

Works Cited: Alberts, B., Johnson, A., & Lewis, J. (2002, January 1). Principles of Membrane Transport. Retrieved March 15, 2020, from https://www.ncbi.nlm.nih.gov/books/NBK26815/ Black, B. L., & Jevnikar, E. (2016, April 12). Retrieved March 15, 2020, from http://projects.ncsu.edu/project/bio183de/Black/membranes/membranes.html Cooper, G. M., & Sunderland, M. A. (2000, January 1). Cell Membranes. Retrieved March 15, 2020, from https://www.ncbi.nlm.nih.gov/books/NBK9928/ Cystic Fibrosis Foundation (2020, February 9). Mucus Thinners. Retrieved March 19, 2020, From https://www.cff.org/Life-With-CF/Treatments-and-Therapies/Medications/Mucus-Thinne rs/ OpenStax. (2013, March 6). 3.1 The Cell Membrane. Retrieved March 15, 2020, from https://opentextbc.ca/anatomyandphysiology/chapter/the-cell-membrane/ Winikates, K. (2012, January 1). The Embryo Project Encyclopedia. Retrieved March 15, 2020, from https://embryo.asu.edu/pages/cystic-fibrosis-transmembrane-conductance-regulator-cf...