Question

At low temperature, the membrane fluidity of cell is maintained by increasing/decreasing the unsaturated fatty acids???

Answer 1

increase!
When thinking about unsaturated fatty acid, I think about the "kinking" C=C bond, this cause them to pack poorly together, and therefore, increase fluidity if you have more of them :D !

Answer 2

phuongbinh123 is exactly right. If you think about the lipid-lipid interactions between two halves of the bilayer, layers composed entirely of straight, saturated lipid tails aren't going to offer much resistance against one another.

However, in the case of a bilayer that is partially composed of saturated fatty acids, the C=C bond creates a 'kink' where the carbons contain double bonds rather than a second carbon-hydrogen bond. These 'kinks' cause the bylayer to encounter greater resistance to flow than if all the carbons were saturated