PHYSICS · Multi-part · Fermi University Physics · Biophysics & Medical Physics · Physical foundations of the living cell · Macromolecules, membranes and cell architecture
Life at Thermal Energy, Ch The scales of life and the thermal energy
Problem
For an order-of-magnitude estimate, model a human red blood cell as a sphere of diameter 8μm, its measured width. Each lipid molecule in the bilayer occupies about 0.6nm2 of membrane area, and the bilayer has two leaflets.
((a)) List the assumptions this estimate makes.
((b)) Estimate the surface area of the modelled sphere, in μm2.
((c)) Estimate the number of lipid molecules in the whole membrane, to one significant figure.
((d)) A real red blood cell is a biconcave disc with a measured membrane area of about 140μm2 and a volume of about 90fL. Recount the lipids using the measured area, and say by what factor, and in which direction, the sphere model was off.
((e)) A biconcave disc is often said to have more surface than a sphere, yet your sphere has more surface than the real cell. Find the area of a sphere holding 90fL, and use it to explain how both statements can be true.