IMAT 2014 Q32 [Plant Bonds]

In cells of winter wheat (Triticum aestivum), the fluidity of the cell membrane can be varied by the plant, depending upon the environmental temperature. As the weather becomes colder, the cell membrane changes to maintain its fluidity.

Which type of bond would be more common in cell membranes of winter wheat plant cells as their growing conditions become colder?

A. C­N
B. C=C
C. C­H
D. C­O
E. C­C

The answer here is choice B, or the increase of C=C bonds.

Before we rationalize the answer, let’s get to the bottomline: A plant in cold weather will maintain its membrane fluidity by increasing the amount of unsaturation within the phospholipids of its membranes.

Let’s us then be reminded of the difference between saturated and unsaturated lipids and integrate it to our answer:

  • Saturated lipids: a fatty acid is said to be “saturated” when there are only single bonds that exist between the carbons. This “saturation” can be attributed to the increase in hydrogen, since increasing the single bonds simultaneously increases the hydrogen as well. What is the consequence of saturation in terms of structure? Because of this saturation, fatty acids are able to create a linear structure which allows strong interactions with other saturate fatty acid chains and produces a dense structure with a high melting point! This ultimately makes the structure LESS fluid.
  • Unsaturated lipids: As the name suggests, this is the opposite of saturation. Instead of adding single bonds in between the carbons, you will be adding double bonds! What is the consequence of decreasing saturation in terms of structure? In contrast with saturation, the double bonds do not create a linear structure, but instead produces a fatty acid that has “bends”. This prevents the strong interaction with other fatty acids, reducing the melting point and decreasing the density, making it more fluid. Thus, the only way for plants to maintain their fluidity, is to decrease saturation, which can be achieved if you add more double bonds (C=C) to the fatty acid chains of the phospholipids present in the membrane!