Question

(1) Pre Lab Question: You are working in the lab and are told to create an artificial membrane from phospholipids. You are then instructed to add certain solutes to the membrane to see if it works appropriately.

Rank the permeability for the following across a phospholipid membrane for the following:

lysine, urea, sodium ion, insulin, water, indole

Answer 1

ANSWER :-

1. Indole is basically an organic compound that can be seen in the urine of the animals. It is not soluble in water but it is soluble in organic solvents as well as can pass through the plasma membrane or the lipid bilayer. It should be taken into consideration that as indole is soluble in organic solvents but relatively bigger in size than the water molecule, there tends to be permeability value which is less than water molecules.. Although certain amino acids are charged and lysine can be considered as one example, their entry through the lipid bilayer is restricted.
2. The plasma membrane is made up of hydrophobic regions positioned inside the two layers and the hydrophilic region positioned outside and inside the cell. So, mainly certain smaller and polar molecules such as water can easily diffuse through the cell membrane. Water does not carry any charge but is a very small molecule and hence can get easily diffused through the lipid bilayer. But, amino acids on the other hand may be polar, but the size of the molecules restrict their movement through the lipid bilayer.
3. Sodium ion carries a positive charge and hence needs a transporter such as the NA+/K+ transporter channel in order to mediate its transport through the cells. But once activated, there is immediate migration or influx following opening of the ion channels taking into consideration their small size.
4. Urea is very much similar to water taking into consideration that is is uncharged and hence it is permeable through water. Now, urea has a permeability co-efficient of around 10¹ nm per second whereas on the other hand an indole molecule has a permeability co-efficient slightly higher than urea of around 10³ nm per second. Water shows the maximum value in terms of all the components mentioned above. Sodium ion has minimum permeability. Amino acids tends to have a permeability co-efficient of around 1 nm per second and on the basis of all the points mentioned above, the following conclusion can be made.

Water > Indole > Urea > Lysine > Na ion > Insulin

(Insulin is added at the last position with less permeability as it is not transported within the cells during glucose transport and thus the mechanism is very much restricted to the cell membrane mainly due to them being a part of peptide hormone family.