Question

In the membrane of a human red blood cell, the ratio of the mass of protein (average molecular weight 50,000) to phospholipid (average molecular weight 800) to cholesterol (molecular weight 386) is about 2:1:1. How many lipid molecules (phospholipid + cholesterol) are there for every protein molecule?

1) About 400
2) About 50
3) About 200
4) About 100

Answer 1

The ratio of the mass of protein to phospholipid to cholesterol is 2:1:1. After calculating the number of molecules for both lipids (phospholipid and cholesterol) by dividing their mass by the molecular weight, we find there are about 200 lipid molecules for every protein molecule in a human red blood cell membrane.

Step-by-step explanation:

The ratio of the mass of protein to phospholipid to cholesterol in the membrane of a human red blood cell is about 2:1:1. Given the average molecular weights, we can calculate the number of lipid molecules (phospholipid + cholesterol) for every protein molecule. The average molecular weight of protein is given as 50,000, phospholipid is 800, and cholesterol is 386. Since the ratio is 2:1:1, we have twice as much protein mass as we do of each lipid. To find the number of molecules, we divide the mass of proteins by its molecular weight and do the same for phospholipids and cholesterol.

For one part protein (with a weight of 50,000), we have:

• 1 part phospholipid = 50,000/800 = 62.5 molecules
• 1 part cholesterol = 50,000/386 = approximately 129.5 molecules

Combining both phospholipid and cholesterol parts, we get:

62.5 (phospholipid molecules) + 129.5 (cholesterol molecules) = approximately 192 lipid molecules per part protein.

Approximating to the nearest option provided, we get about 200 lipid molecules for every protein molecule, which corresponds to answer number 3.