The plasma membrane of E. coli is approximately 75% protein and 25% phospholipid
ID: 557280 • Letter: T
Question
The plasma membrane of E. coli is approximately 75% protein and 25% phospholipid by weight. it is known that the average membrane protein molecular weight is 50,000 Da and an average membrane phospholipid molecular weight is 750 Da. Calculate the number of membrane phospholipid molecules present per molecule of membrane protein. Number phospholipid molecules per protein molecule Select the additional information that one would need to calculate the fraction of membrane surface covered by phospholipids. the surface area of the E. coli cell the average cross-sectional areas of a 50,000 Da protein and a 750 Da phospholipid in a bilayer the average densities of a 50,000 Da protein and a 750 Da phospholipid the number of moles of protein and phospholipid in the membraneExplanation / Answer
Ans. #1. Let the total mass of plasma membrane = X Da
Total mass of protein = 75 % of X Da = 0.75X Da
Total mass of phospholipid = 25 % of X Da = 0.25X Da.
# Number of protein molecules =
Total mass of protein / Average molar mass of protein molecule
= 0.75 X Da / 50000 Da
= 0.000015 X
# Number of phospholipid molecules =
Total mass of phospholipid / Average molar mass of phospholipid molecule
= 0.25 X Da / 750 Da
= 0.0003333333 X
# Now,
No. of phospholipid molecules per protein molecule =
No. of phospholipid molecule / No. of protein molecule
= 0.0003333333 X / 0.000015 X
= 22.22
= 22 (nearest whole number, number of molecules shall be a whole number)
#2. Correct options A, B.
# Fraction of membrane surface covered by phospholipids =
Surface area of membrane / Total surface area of phospholipids
So, the surface area of membrane must be provided.
The average cross-sectional area of a 50000 Da protein and a 750 Da phospholipid. The cross-section area provides the area of protein (would not be required actually) and phospholipid molecules. Using the cross-section area of molecules, the total area covered by protein and phospholipid can be calculated. From here, the fraction of membrane surface area covered by phospholipids can be calculated, if the surface area of membrane is provided.
## Density = Mass / Volume. It does not relate surface area of molecules.
## The number of molecules does not relate to the surface area of a molecule.
Note: Option A or B alone is NOT enough to calculate the FRACTION. As shown in formula above, both A and B must be provided.