| MadSci Network: Botany |
Seeds often contain starch, which is composed of thousands of glucose molecules joined end to end in long chains. Starch is a good way to store carbohydrate because it is less reactive than sugars, basically insoluble, and does not attract water by osmosis. Because sugars, like glucose and sucrose, attract water by osmosis, a fresh seed with a lot of sugars has a higher water content. When it dries out, it loses more water and wrinkles. Sweet corn (Zea mays) is a seed with a high sugar content, hence the sweet taste. Smooth corn seeds are higher in starch so would not be eaten fresh like sweet corn. Similarly, smooth peas (Pisum sativum) are less sweet and are used in soups and were once even used to make flour. Wrinkled peas are sweeter so are eaten whole as a vegetable. The wrinkled pea seeds have a much reduced starch content compared to smooth seeds (30% vs. 50%) and a higher sugar content (10% vs. 6%). The reason why seeds are wrinkled or smooth is rather complex as the websites cited explain. The molecular basis of the round (RR and Rr) and wrinkled (rr) pea seeds involves the gene that codes for a starch-branching enzyme I (SBEI). Peas that are rr have an inactive form of the enzyme due to a gene defect. Peas that are Rr have smaller amounts of the enzyme than RR seeds. If the enzyme is present, the starch grains in the pea are large and spherical. These RR and Rr starch grains allow the seeds to retain water and shrink uniformly. Starch retains more water because it is a very large molecule (up to 2 million glucose per molecule) and can hold water molecules on its surface. Starch attracts less water than sugars but retains what water it does attract more strongly than sugars. Branched starch, called amylopectin, retains more water than unbranched starch, called amylose. In rr seeds, the starch grains are irregular in shape they lose water too rapidly and the seeds shrink unevenly, causing the wrinkled seed. Another way of looking at it in rr peas is that they have less starch and more sugars, which attract more water in the fresh seed. When the seed dries, there is more water lost, which causes the wrinkling. You should notice several differences between round and wrinkled seeds with your tests. 1. You did notice that the wrinkled seeds absorb more water. That explains why they shrink more and wrinkle. 2. The wrinkled seeds should test positive for sugar, but maybe not glucose. For example, sweet corn mainly contains sucrose so maybe your test was for the wrong sugar. 3. You did notice some differences in starch grains. Wrinkled seeds should have fewer, smaller starch grains. You should be able to distinguish between RR, Rr, and rr seeds based on the size and shape of the starch grains as well. References Molecular Basis of Smooth versus Wrinkled Pea Why are Mendel's Seeds Wrinkled? More Peas,Please! Starch
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