Final answer:
The non-uniformity of lipid bilayers is primarily due to the mix of saturated and unsaturated fatty acid tails in the phospholipids, with unsaturated tails increasing fluidity by hindering close packing. Other factors include cholesterol presence and embedded proteins.
Step-by-step explanation:
The main reason for the non-uniformity of lipid bilayers is due to variations in the composition of the phospholipids. Specifically, the non-uniformity arises from the presence of both saturated and unsaturated fatty acid tails in the membrane's phospholipids. The saturated fatty acids have tails that are relatively straight because they lack double bonds. In contrast, unsaturated fatty acids contain one or more double bonds between adjacent carbon atoms, creating kinks in their tails. These bends hinder close packing of the fatty acid tails, thus increasing the fluidity of the membrane.
Furthermore, the presence of cholesterol and embedded proteins contribute to the non-uniformity. Cholesterol can stabilize and increase fluidity depending on the temperature, while the various proteins have different functions and interactions with the lipid components. An organism can adjust its membrane composition in response to temperature changes, for instance by modifying the proportion of unsaturated fatty acids, to maintain the necessary membrane fluidity for survival.