4. Permeability Testing You perform a permeability lab where you tested the perm

Question

4. Permeability Testing You perform a permeability lab where you tested the permeability of various molecules in two different systems. System 1 contains artificial liposomes, as described in Section 6.2 of your text. The liposomes in this system are created with phospholipids only System 2 is a simple cell culture You determine the time is takes for each the molecules (V-Z) to pass through both the liposome and cell membrane. You placed your results in the table below, where 5 represents the fastest transit time, and 1 represents the slowest transit time Transport/Permeability Transport/Permeability in Artificial Liposomes in Cell Molecule Properties Small, non-polar uncharged Water Medium, polar Uncharged Small, polar uncharged Large, charged 4 4 a. Explain (in the context of membrane permeability) why molecule V moves the fastest through both membranes, and why molecule Z moves the slowest. 3 pts b. Explain one possible reason for why molecule W (water), passes through the membrane relatively quickly in the cell model, but passes relatively slowly in the artificial liposome system. 3 pts 5. Too Much Water Background: On the morning of January 12, 2007, a Sacramento, CA radio station held a "Hold Your Wee for a Wi" contest. The radio station, KDND, promised to give the highly-sought Wii game system to the winner of their contest. Contestants were asked to drink as much water as they could without urinating. The contestant able to hold the most water would be named the winner. One of the contestants, Jennifer Strange, 28 consumed nearly two gallons of water in three hours time. She died shortly after taking part in the contest. a. Discuss how an excessive increase in water can affect a person's cells, making sure to include tonicity and osmosis as part of your discussion. 3 pts b. Speculate how the changes you describe in part a could potentially lead to death

Explanation / Answer

4. a. Liposome is a vesicle composed of one or more concentric phospholipid bilayers. The lipid bilayer is selectively permeable which allows only certain substances to pass through it. Small non-polar molecules move across the lipid bilayer quickly, whereas large molecules and charged substances cross the membrane slowly. This is because, the small non-polar molecule does not interfere with the hydrophobic/hydrophilic nature of the bilayer.
Lipid bilayer membrane is made of polar phosphate head and nonpolar lipid part. Even though the head is hydrophilic the tail part is hydrophobic. So, when a polar/charged molecule tries to pass through the membrane it is repelled. Hence, permeability of large, charged molecules is very less.

b. Basically, water moves across lipid bilayers from regions of high concentration to regions of low concentration by osmosis. The lipid bilayer moiety of is considered to be the primary barrier to free movement of water because of its hydrophobic head and hydrophilic tail. Hence, transport of water molecules through this lipid membrane is comparitively lower than the transport in the biological cell membrance. This is because the biological cell membrance contains specific transport proteins called Aquaporins that help in the diffusion of water molecules in and out of the cell.

5. a. Water moves readily across cell membranes through special protein-lined channels, and if the total concentration of water and all dissolved solutes is not equal on both sides, there will be net movement of water molecules into or out of the cell. This movement of water from a region of higher concentration to a region of lower concentration through a semi-permeable membrane is called osmosis. During excessive intake of water, the concentration of water outside the cell is greater than the concentration of water inside the cell. This condition is called as Hypotonicity. When a cell is exposed to such hypotonic conditions, there is net water movement into the cell. Cells without walls will swell and may burst (lyse) if excess water enters into the cell.

b. When too much water is taken, the electrolyte concentration within the cell will be greater than the concentration outside the cell. In order to attain the equilibrium, water enters into the cell. The cells eventually swells up with increased concentration of water. This leads to transport of high amounts of water to the brain. Increased water concentration in the brain cells will increase the intracrainal pressure and prevents the flow of blood which leads to oxygen deprivation and eventually death of the brain cells.

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