Final answer:
Saturated fatty acids and cholesterol molecules are two factors that cause a significant decrease in membrane fluidity.
Step-by-step explanation:
Membrane fluidity is influenced by various factors, but one significant aspect that causes a decrease in membrane fluidity is the presence of saturated fatty acids. Saturated fatty acids have straight tails, which can compress and make the membrane dense and rigid at lower temperatures. On the other hand, unsaturated fatty acids with kinks in their tails maintain some space between phospholipid molecules, contributing to membrane fluidity. Cholesterol molecules also decrease membrane fluidity by filling in the gaps between phospholipids.