What determines how fluid a membrane is
Cell membranes are built from a lipid bilayer whose flexibility depends partly on the fatty acids in those lipids. Saturated fatty acids pack tightly and tend to make a membrane more rigid, while unsaturated fatty acids have kinks that prevent tight packing and keep the membrane more fluid. Cholesterol also modulates fluidity, and temperature plays a role.
Membrane fluidity is not just structural; it influences how proteins embedded in the membrane move and function, including some receptors and transporters. Cells actively adjust their lipid composition to maintain workable fluidity, a process called homeoviscous adaptation.
What the dietary-fat link does and does not show
Dietary fats can influence the fatty acid makeup of cell membranes over time, and this is an active area of nutritional science. However, simple claims that specific oils make membranes rigid and block receptor signaling overstate the evidence. Membrane composition is regulated by the cell, effects vary by tissue and fat type, and links to health outcomes are complex and not settled.
Treat broad good-oil versus bad-oil statements about membranes as oversimplifications rather than established fact. This is general educational information, not medical or dietary advice; consult a qualified professional for individual guidance.