Question

This week take a single regular sheet of paper (or the tablet equivalent) and draw a line down the middle (length or width it doesn't matter).

On the left of the divide: Draw a picture of a lipid bilayer and somewhere in the bilayer draw a model for a protein transporter that can transport NEGATIVELY charged ions from HIGH concentration (top) to LOW concentration (bottom). Represent respective ion concentrations. Try to represent what kinds of functional groups you expect to see on the inside and outside of the protein.

On the right of the divide: Draw a picture of a lipid bilayer and somewhere in the bilayer draw a model for a protein transporter that can transport POSITIVELY charged ions from LOW concentration (top) to HIGH concentration (bottom). Represent respective ion concentrations. Try to represent what kinds of functional groups you expect to see on the inside and outside of the protein.

Next: Draw an ion trying to cross the membrane unassisted by the transporter - one for each side of the page.

Next: Below the drawing on the LEFT, draw a reaction coordinate diagram representing the movement of the ion from high to low concentration unassisted by the protein. On that same reaction coordinate diagram also draw a trace for the transport of the ion through the protein. Label both traces as "unassisted" and "catalyzed". Feel free to identify different ∆G values that are relevant to telling the story of the ion transport.

Next: Below the drawing on the RIGHT, draw a reaction coordinate diagram representing the movement of the ion from high to low concentration unassisted by the protein. On that same reaction coordinate diagram also draw a trace for the transport of the ion through the protein. Label both traces as "unassisted" and "catalyzed". Feel free to identify different ∆G values that are relevant to telling the story of the ion transport.

Use this as a study tool and try to figure out how many learning goals are incorporated in or relevant to this one drawing.

Take a picture of your drawing and submit it before the deadline.

Answer 1

A protein transporter is a multipass transmembrane protein, which assists in transport of various molecules. A ion transporter is generally made of protein which has multiple beta sheet structure forming a barrel to create a tunnel like region for ion transport within plasma mebrane.

Panel A shows an ion transporter which transports negatively charged ions from high concentration gradient to low concentration. Since it transports negatively charged ions, the interior of the tunnel should be decorated mostly with positively charged amino acid residues (such as lysine, histidine and arginine) so that it does not repel any ion and assist during the passage of a ion. The other side of the barrel which is facing plasma membrane should be lying with hydrophobic amino acids residues (such as Leucine, Isoleucine, Valine etc.) so that it can maintain a hydrophobic interaction with the phospholipids of the membrane. The extracellular and intracellular portion of the transporter should have polar residues (such as Serine, Threonine etc.) so that it can maintain its structure in aqueous environment.

SImilarly on Panel B, an ion transporter is made which transports positively charged ions from a gradient of low concnetration to high concentration. Since, it transports positively charged ions, the amino acids facing the tunnel side should be mostly negatively charged (such as Aspartic acid and Glutamic acid) for the same region mentioned above. The rest should be same.

These are called Passive Diffusion method of membrane transport. On the other hand, an unassisted mode of transport of ions happens when an ion tries to diffuse through the plasma membrane. However, the later is very rare event. Since, plasma membrane is made of phospholipids, where the phospho group is facing the aqueous environment where the interior of the plasma membrane is highly hydrophobic, so an ion can not pass through such environment; it will be repelled. On the contrary, a transporter can easily allow the passing of an ion, given that it fits with the property of the ion transporter.

[** The panel A' is an extension to the panel A and panel B' is an extension to the panel B ]

Now for the transporter in Panel A, the ions move from high concentration to low concentration, so the it will be highly favorable than the transporter of Panel B, where the ions move from low concentration to high concentration region. The later will need assistance from external energy source. So, the later mode of membrane transport is called Active transport.

So, in panel A', the activation energy (G++) of unassisted will be much higher than the activation energy (G1++) of catalyzed transport, since the ions does not have to break the barrier of repulsion by membrane phospholipid. Similarly this will remain true for the difference in activation energy in panel B'. However, the activasion energy by the transporter in panel A will be lower than the activation energy of the transporter in panel B; since the ions in panel B transporter has to overpass the opposite concentration gradient. So G1++ < G2++.

Now, from this exercise, you will be able to learn more about membrane transport. How there are different kinds of membrane transport mechanisms. Also you will be able to learn more about energy profile of a chemical reaction. You wiill learn how to draw a reaction cordinate diagram. And because of this exercise you will learn to know the different energy profile of various kinds of membrane transport.

-ve +ve -ve +ve -ve -ve -ve +ve -ve +ve +ve -ve +ve -Ve -ve +ve anahanan dann naman JU Concentration Gradient NIMIRUMANI +ve Concentration Gradient -ve IN N TIS MUULUU UUURIVUL MUUUUUUU ULUI +ve +ve -ve +ve +ve +ve +ve #ve +ve +ve +ve +ve +ve +ve +ve Positively charged amino acids Polar amino acids Hydrophobic Amino Acids Negatively charged amino acids Polar amino acids Hydrophobic Amino Acids

BY AGT LAGH AG LAGH - - - - - - + Unassisted Unassisted Energy Energy - Catalyzed - Catalyzed Reaction Cordinate Reaction Cordinate

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