Evo-Ed: Integrative Cases in Evolution Education

Cases for Evolution Education

Cell Biology

Although pea sweetness and pea shape may seem unrelated, they are not. While growing, sweeter peas accumulate higher water content because the sugar in them makes them hypertonic. As a result they draw more water into the seed. When they later dry out, the sweeter peas lose that extra water making them wrinkle. The starchy peas have more starch and comparatively little water so when they dry the high starch content keeps the shape round.

Illistration of pea cells as part of the pea plant

How to make sugar in a plant

In plants, the sugar glucose is one of the end products of photosynthesis.  The processes of photosynthesis convert carbon dioxide and water to oxygen and three-carbon sugars, which subsequently are transformed into the six-carbon sugar, glucose. The light reactions of photosynthesis use the sun’s energy to split water into oxygen gas and hydrogen ions.  By way of proton gradients and redox reactions, the hydrogen ions drive the production of chemical potential energy, in the form of ATP and NADPH, needed for the Calvin Cycle reactions.  The Calvin Cycle reactions occur in three phases. In the first phase, carbon dioxide is incorporated into a five-carbon chain of sugar called ribulose bisphosphate (RuBP). This sugar is transformed into 3-phosphoglycerate. In the second phase of the Calvin Cycle, the chemical energy in ATP and NADPH is utilized to transform 3-phosphoglycerate into glyceraldehyde-3-phosphate or triose-phosphate*. This triose-phosphate goes through an additional five transformations to form ADP-glucose (see diagram). ADP-glucose is transformed into amylose starch by the starch synthetase enzyme; amylose is transformed into amylopectin starch by the starch branching enzyme.

Note: in the third phase of the Calvin Cycle triose-phosphate is converted back into RuBP. For the purpose of this case study we are interested in what happens to the portion of triose-phosphate that takes a different pathway getting converted (eventually) into starch rather than completing the cycle back into RuBP. For more on the Calvin Cycle visit: http://en.wikipedia.org/wiki/Calvin_cycle.

How to make starch from sugar in a plant

Like sugar, there are multiple types of starch. The first type, amylose, is unbranched starch. It is a polysaccharide made up of between 300 and 3000 glucose subunits. It typically comprises around 20-30% of starch found in plants. The second type, amylopectin, is branched starch. It is a polysaccharide made up of between 2000 and 200000 glucose subunits. Branching occurs every 20-30 subunits. It typically comprises around 70-80% of the starch found in the seeds of plants. The conversion of amylose to amylopectin requires the use of a starch branching enzyme.

Each step of the above process with a diagram of each moledule: Fructose 6-P, Glucose 6-P, Glucode 1-P, ADP-Glucose, Amylose, amyloprctin.

In wrinkled peas, the starch branching enzyme is generated from the r allele and is non-functional. This means that amylopectin cannot be produced, resulting in less total starch. Since less of the sugar is converted to starch there is excess simple sugar. Some of this sugar, Glucose 1-P is converted into UDP-Glucose which, when combined with Fructose 6-P, forms sucrose.