Question

In lipid bilayers, there is an order-disorder transition similar to the melting of a crystal. In a lipid bilayer in which most of the fatty acids are unsaturated, would you expect this transition to occur at a higher temperature, a lower temperature, or the same temperature as it would in a lipid bilayer in which most of the fatty acids are saturated? Why?

Short answer

The transition occurs at a lower temperature for unsaturated fatty acids.

Step-by-step solution

Understanding the properties of fatty acids

First, recognize that saturated fatty acids have no double bonds, leading to straight chains which can pack tightly together. Unsaturated fatty acids contain one or more double bonds which introduce kinks in the chain, preventing tight packing.

Effect of fatty acid saturation on membrane fluidity

Tightly packed membranes (those with more saturated fatty acids) are more rigid and have a higher melting point. Membranes with unsaturated fatty acids are more fluid at a given temperature due to the kinks introduced by the double bonds.

Determining the order-disorder transition

Since saturated fatty acids pack more tightly and have a higher melting point, the transition from ordered (rigid) to disordered (fluid) states will occur at a higher temperature compared to membranes with unsaturated fatty acids.

Comparing transition temperatures

Therefore, the lipid bilayer with unsaturated fatty acids will have an order-disorder transition at a lower temperature compared to a lipid bilayer with saturated fatty acids.

Key concepts

saturated fatty acids

Fatty acids are long chains of carbon atoms. When we talk about saturated fatty acids, we mean fatty acids that do not have any double bonds between the carbon atoms. Their carbon chains are straight. Because they are straight, these fatty acids can pack tightly together. This tight packing makes the lipid bilayers in which they are present more rigid. So, the more saturated fatty acids we have, the less fluid the membrane tends to be. Saturated fatty acids are commonly found in animal fats and dairy products.

unsaturated fatty acids

Unlike saturated fatty acids, unsaturated fatty acids have one or more double bonds in their carbon chains. These double bonds create bends or 'kinks' in the chain. These kinks prevent the fatty acids from packing tightly together. This loose packing makes the membrane more fluid. Unsaturated fatty acids are usually found in plant oils and fish oils. The more unsaturated fatty acids present in a lipid bilayer, the more fluid the membrane becomes at a given temperature.

membrane fluidity

Membrane fluidity is how easily the lipid bilayer of a cell membrane moves and flexes. This property is essential for numerous cell functions, such as allowing proteins to move within the membrane and facilitating cell signaling. The structure of the fatty acids in the membrane greatly affects fluidity. Saturated fatty acids, with their straight chains, make the membrane more rigid. On the other hand, unsaturated fatty acids, with their kinks, increase fluidity because they cannot pack closely. By changing the ratio of saturated to unsaturated fatty acids, cells can regulate their membrane fluidity in response to various environmental conditions.

melting point

The melting point is the temperature at which a substance changes from a solid to a liquid. In the context of lipid bilayers, it refers to the temperature at which the membrane transitions from a more ordered, rigid state to a more disordered, fluid state. Saturated fatty acids have a higher melting point because their straight chains enable them to pack tightly and form a solid structure at a lower temperature. Unsaturated fatty acids, with their kinked chains, do not pack as well and thus have a lower melting point. Consequently, lipid bilayers with more unsaturated fatty acids will undergo the order-disorder transition at a lower temperature compared to those with more saturated fatty acids.