A paper published in 2008 wanted to confirm the hypothesis that carbon dioxide m
ID: 300407 • Letter: A
Question
A paper published in 2008 wanted to confirm the hypothesis that carbon dioxide moves into cells through the cell membrane and not through water channels (aquaporins). Researchers compared the diffusion rate through the membrane in cells containing aquaporins (MDCK-AQ) in their membranes and cells without aquaporins (MDCK).
a. First let’s think about water movement through these cells. If we place MDCK-AQ cells in a hypertonic solution which way will water move and why? Rubric (2): Explain the relative concentration of water on the inside and outside of the cell (1) and why water would move in the direction you picked.(1)
In a hypertonic solution, the dissolved solute concentration is higher on the outside of the cell and therefore the concentration of water outside of the cell would be lower. This means that the water inside the cell will flow out of the cell and into the external environment to attain equilibrium.
b. Will the cell in the question above swell, shrink or remain the same? (1pts)
The cell will shrink due to the loss of water inside of the cell.
c. Water will move more slowly through the membranes of MDCK cells. Explain why based on the chemical character of water and the chemical character of the lipid bilayer. Rubric (3): the correct reason for slower movement(1) in MDCK cells is linked to the chemical characteristics of water (1) and bilayer (1)
d. Aquaporin is a channel protein. Use the definition for a channel protein to explain why water moved more quickly into MDCK-AQ cells. Be sure to explain what a channel is, what is the rate of transport for a channel protein, and whether it experiences saturation kinetics. Rubric (3): explanation why water moves faster by explaining conformation of channel in the membrane (1), how cargo is transported (1), and saturation kinetics (1).
e. Again, the research question was whether carbon dioxide moved through the lipid bilayer or aquaporins. The result is shown below. Obviously MDCK cells with and without Aquaporin show similar diffusion of carbon dioxide. Therefore, carbon dioxide does not require the channel to cross the membrane. Given the chemical and physical characteristics of carbon dioxide and the cell membrane, explain why the channel is not needed to increase CO2 diffusion across the membrane. Rubric (4): chemical (2) and physical characteristics (1) of carbon dioxide and lipid bilayer are used to explain ease of carbon dioxide diffusion through a cell membrane (1).
Explanation / Answer
a) If we place MDCK-AQ cells in hypertonic solution the water will move out of the cell. This is because the solute concentration in the hypertonic solution is high and the water concentration is less as compared to that inside the cell. Consequently the water will move out of the cell into the environment with less water concentration in order to attain equilibrium.
b)The cell will shrink when placed in the hypertonic solution as the water moves out of the cell into the environment.
c)The water will move slowly through the membrane of MDCK cell as compared to MDCK-AQ cells due to the presence of aquaporins. Aquaporins are integral membrane proteins that act as channels which help in water transfer across the cell membrane.
The cell membrane has a phospholipid bilayer structure that is permeable to small polar molecules, and water can slowly pass through it by simple diffusion. Presence of specialized integral membrane protein channels can further facilitate the efficiency of permeation of water across the membrane.
d) Aquaporins are integral membrane proteins that act as channels which help in water transfer across the cell membrane. A channel protein is basically a protein that allows the transport of specific substances across a cell membrane. Hence water moved more quickly into MDCK-AQ cells.
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