Winter wheat is planted in the early fall, grows over the winter when the weather is colder, and is harvested in the spring. As the temperature drops from fall to winter, the makeup of the plasma membrane of winter wheat changes. Unsaturated fatty acids replace saturated fatty acids in the phospholipids of the membrane. Why would colder weather select for the different lipid makeup in the plasma membrane? An abstract in the Canadian Journal of Botany states, “The changes in total lipid, fatty acid, and lipid phosphorus contents were followed during germination and early growth of Rideau wheat seedlings at 24 degrees C for 72 h and 2 degrees C for 5 weeks.
At both temperatures the changes in total lipids and total fatty acids were similar for seedlings at comparable stages of morphological development. In contrast, the phospholipid content of seedlings grown at 2 degrees C was considerably higher than that of seedlings grown at 24 degrees C. A marked increase in linolenic acid was also observed during growth at 2 degrees C, and this change occurred entirely in the embryo.
The results indicate an increased synthesis and unsaturation of membranes in seedlings grown at 2 degrees C. The changes may contribute to the increased resistance to freezing temperatures exhibited by seedlings grown at 2 degrees C. ” The phospholipid bilayer is flexible or has fluidity. This prevents it from solidifying at colder temperatures. Cholesterol in the plasma membrane affects the fluidity.
At lower temperatures cholesterol helps prevent the plasma membrane from freezing by not allowing contact between the phospholipid tails.
Saturated fats are solid at cooler temperatures, while unsaturated fats are not. So it is only logical to say that a higher percent of unsaturated fats in the phospholipid bilayer will keep winter wheat or any other living organisms from freezing in the winter. Fluidity of the plasma membrane is inversely correlated with the percent of saturated fatty acids in the phospholipids.
A certain degree of rigidity is desirable to maintain the structure of the membrane, but when the temperature drops a membrane made of predominantly saturated fats might become so rigid that transport movement of molecules across the membrane would be impaired. Decreasing saturation of the fatty acid portion of the phospholipids would keep the membrane fluid enough to allow substances to enter and exit cells, even in colder weather (Chapter 4 (31 May 2011)
Reference
Thinking Scientifically Mader, S. S. (2010) Biology 10th edition. New York, NY: McGraw Hill).
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