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Sweet Today, Starchy Tomorrow: Corn Kernels After the Harvest

Follow the sugar in picked sweet corn as kernel cells rearrange it and break it down.

Answer Key

The storage test

Read the passage and Table 1, then answer the questions.

The Farm Stand Mystery

1Every August, the garden club at Westbrook Middle School picks sweet corn from its raised beds and sells it at a farm stand on the first day of school. The club starts its corn seedlings in the school greenhouse each spring and moves them outside in May. Last year, several customers said that ears bought on the morning they were picked tasted much sweeter than ears left over from the day before. Seventh grader Tobias Okafor wanted to find out why, so he planned a test with the club's adviser, Ms. Ingrid Valdés.

2Ms. Valdés gave the club a model. While an ear is on the plant, the leaves make sugar and send it to the kernels. The kernel cells are alive, and they keep working for days after the ear is picked. They can do two things with sugar. They can link many sugar units together into starch, a much larger molecule that does not taste sweet. Building starch takes energy. The cells can also break sugar down in cellular respiration: sugar combines with oxygen to form carbon dioxide and water, and energy is released that the cells use for their work. In both processes, the atoms in the sugar are rearranged into new molecules.

3The club picked 18 ears of a standard sweet corn variety from one bed at 7 a.m. Kernels from 2 ears were tested right away. Eight ears were stored in a storeroom at 25 °C, and eight went into the cafeteria's walk-in refrigerator at 2 °C. All of the ears stayed in their husks inside loosely closed bags, so they did not dry out. After 12, 24 and 48 hours, kernels from 2 ears in each group went to the county extension lab, which measured the sugar and starch in them. The last 2 ears from each group were sealed in the lab's chambers to measure the carbon dioxide they gave off. No mold or rot was seen on any ear.

4The club also grew a supersweet variety in the next bed. Supersweet plants carry a gene change that blocks most of the step in which kernel cells link sugar into starch. Supersweet ears picked the same morning were stored at 25 °C and tested the same way.

Table 1. Standard Kernels (grams in each 100 g of kernels; average of 2 ears)
Hours after pickingSugar at 25 °C (g)Starch at 25 °C (g)Sugar at 2 °C (g)Starch at 2 °C (g)
08.06.08.06.0
125.87.97.76.2
244.29.27.46.4
482.610.46.96.8
1.

Use Table 1. In the 48 hours at 25 °C, the sugar in each 100 g of kernels fell by 5.4 g, and the starch rose by 4.4 g.

2.

For the 25 °C kernels, subtract the starch gained from the sugar lost. In each 100 g of kernels, 1 g of the lost sugar did not end up as starch.

3.
The starch in the 25 °C kernels rose while the ears sat in a dark storeroom. Which explanation fits the model in paragraph 2 best?
  • A. The kernel cells linked sugar units that were already in the kernels into long starch molecules.
  • B. The warm air in the storeroom melted the sugar, and the melted sugar slowly hardened into starch.
  • C. Starch moved into the kernels from the cob and the husks after the ear was cut off the plant.
  • D. The kernels built starch from carbon dioxide they took in from the air of the storeroom.
4.
Where did the sugar that did not end up as starch most likely go?
  • A. It was turned into energy for the kernel cells to use, so its atoms no longer exist.
  • B. It was eaten by mold growing on the ears in the warm storeroom.
  • C. It moved out of the kernels into the cob, where the lab did not measure it.
  • D. It combined with oxygen in respiration, and its atoms left as carbon dioxide and water.
5.
Compare the 2 °C kernels with the 25 °C kernels in Table 1. Which statement do the data support best?
  • A. The cold kernels lost sugar and gained starch too, but much more slowly than the warm kernels.
  • B. The cold kernels made starch faster than the warm kernels, but they lost less sugar overall.
  • C. The cold kernels lost sugar faster than the warm kernels during the first 12 hours.
  • D. The cold kernels kept their sugar near 8.0 g for 48 hours, because their cells rested in the cold.

Carbon dioxide and a supersweet variety

Use Tables 2 and 3 and paragraphs 2 to 4.

Table 2. CO₂ Given Off by the Stored Ears (per kg of kernels, first day)
Storage temperatureCO₂ (mg per kg per hour)
25 °C300
2 °C100
Table 3. Supersweet Kernels Stored at 25 °C (grams in each 100 g of kernels)
Hours after pickingSugar (g)Starch (g)
018.03.0
2417.43.1
4816.83.2
6.

Use Table 2. The ears stored at 25 °C gave off carbon dioxide 3 times as fast as the ears stored at 2 °C.

7.
Why did the ears at 25 °C give off carbon dioxide faster than the ears at 2 °C?
  • A. Their kernel cells turned starch into carbon dioxide without using any oxygen.
  • B. Warm air pushed carbon dioxide out of the husks that had been stored there.
  • C. Their kernel cells broke down sugar faster in cellular respiration.
  • D. Mold on the warm ears gave off the extra carbon dioxide that the lab measured.
8.
For each case, compare the sugar lost with the starch gained (Tables 1 and 3). The sugar that did not become starch was broken down in respiration. Sort each case by where most of its lost sugar went.
Standard corn at 25 °C, from 0 to 48 hoursSupersweet corn at 25 °C, from 0 to 48 hoursStandard corn at 2 °C, from 0 to 48 hoursSupersweet corn at 25 °C, from 0 to 24 hoursStandard corn at 25 °C, from 24 to 48 hours
More than half of the lost sugar became starchMore than half of the lost sugar was broken down in respiration
Standard corn at 25 °C, from 0 to 48 hours
Standard corn at 2 °C, from 0 to 48 hours
Standard corn at 25 °C, from 24 to 48 hours
Supersweet corn at 25 °C, from 0 to 48 hours
Supersweet corn at 25 °C, from 0 to 24 hours
9.
At 25 °C, the supersweet kernels lost only 1.2 g of sugar in 48 hours, while the standard kernels lost 5.4 g. Which explanation fits paragraph 4 and the data best?
  • A. The supersweet kernels made new sugar from the carbon dioxide in the storeroom air.
  • B. Their cells linked little sugar into starch, but they still broke down about 1 g in respiration.
  • C. The supersweet kernel cells stopped respiring after picking, so they kept their sugar.
  • D. The supersweet ears were kept colder than the standard ears, which slowed the reactions in their cells.
10.
This year, the club will pick standard sweet corn at 7 a.m. and sell it when the stand opens at 3 p.m. the next day. Based on the data, which plan would most likely keep the kernels sweetest?
  • A. Keep the ears in the 25 °C storeroom overnight, so the kernels have time to make more sugar
  • B. Put the ears into the 2 °C refrigerator right after picking and keep them cold until 3 p.m.
  • C. Husk the ears and keep them at 25 °C, so more oxygen reaches the kernels
  • D. Keep the ears at 25 °C for the first 12 hours, then move them into the refrigerator
11.
A customer says, “Picked corn loses its sweetness because the sugar slowly dries up and disappears.” Do you agree? Write a claim, use evidence from Tables 1, 2 and 3, and use the model in paragraph 2 to explain what happens to the sugar molecules and their atoms.

I disagree. The sugar does not disappear; the kernel cells rearrange it. In standard corn at 25 °C, the sugar fell by 5.4 g per 100 g of kernels in 48 hours, but the starch rose by 4.4 g, so most of the sugar was linked into starch, which does not taste sweet. The other 1.0 g was broken down in cellular respiration: it combined with oxygen and left as carbon dioxide and water, and the energy was used by the cells. Table 2 shows the warm ears giving off 300 mg of carbon dioxide per kg per hour. The supersweet kernels, which make very little starch, lost only 1.2 g of sugar, and almost all of that loss came from respiration. The atoms were not destroyed; they ended up in starch, carbon dioxide and water.