Starch Clues in the Tortoise Greenhouse

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Starch Clues in the Tortoise Greenhouse

Use leaf starch tests to explain how photosynthesis moves matter and energy into plants and tortoises.

The starch tests

Read the passage and study Table 1. Then answer the questions.

Food for Young Tortoises

1Wild desert tortoises have become rare in many places, so the Pinyon Hills Zoo runs a head-start program. Keepers raise young tortoises safely for a few years until their shells are hard enough to protect them, and then the tortoises are released into protected desert land. The young tortoises eat fresh plants every day, so the zoo grows their food in a greenhouse: desert globemallow, wild grasses and other plants they would find in the wild.

2Seventh graders Wen Zhao and Elif Demir volunteer in the greenhouse. Biologist Dr. Marisol Quintero told them that many plants store some of the sugar they make as starch, and that a drop of iodine solution turns starch blue-black. Where there is no starch, the iodine stays brown. To test leaves, the zoo's education lead, Mr. Anand Rao, first dipped each leaf in hot water and then in warm alcohol to remove its green color, so that any color change would be easy to see. Then he added iodine. He handled all of the hot liquids and the iodine, and the students recorded the results.

3Two days before the test, Wen wrapped a strip of foil around part of one globemallow leaf, while the rest of the leaf stayed in the light. Elif chose a leaf from a variegated hibiscus, a plant whose leaves have green patches and white patches. The white patches lack chlorophyll, the green substance in chloroplasts.

4The students also wanted to know how starch in globemallow leaves changes during a day. On a sunny day in May, and again on a cloudy day in the same week, Mr. Rao tested leaves picked at six times. The students gave each leaf a starch score from 0 (stays brown) to 4 (very dark blue-black).

5Each young tortoise eats several grams of fresh greens a day. Over the summer, the tortoises grow larger, and their shells thicken. In the afternoons, they often bask under a warm lamp before they feed.

Table 1. Iodine Tests on a Sunny Afternoon
Sample	Kept in light?	Color before test	Iodine result
1. Globemallow, open part	Yes	Green	Blue-black
2. Globemallow, under foil	No (2 days)	Green	Brown
3. Hibiscus, green patch	Yes	Green	Blue-black
4. Hibiscus, white patch	Yes	White	Brown

1.

Compare samples 1 and 2. What do their results show?

A. The leaf made its starch in the stem, then sent it to the open part of the leaf.B. The part under the foil lost its chlorophyll, so it turned white in two days.C. The part of the leaf kept in the dark had not made and stored starch like the rest.D. The foil gave off cold, which turned the starch in that part of the leaf into water.

2.

Samples 3 and 4 came from the same leaf in the same light. Which conclusion do their results support?

A. Iodine turns brown on white leaves and blue-black on green ones.B. Leaf cells need chlorophyll to capture the light used for making food.C. White leaf cells make starch faster, then use it up before the test.D. Hibiscus leaves store their food as oxygen instead of as starch.

3.

Underline each statement that Table 1 and paragraphs 2 and 3 support.

Starch through a day

Use paragraph 4 and Table 2 to answer the questions.

Table 2. Starch Scores of Globemallow Leaves (0 = none, 4 = a lot)
Time	Sunny day	Cloudy day
6 a.m.	1	1
10 a.m.	2	1
2 p.m.	3	1
6 p.m.	4	2
10 p.m.	2	1
2 a.m.	1	0

4.

On the sunny day, the starch score rose by points from 6 a.m. to 6 p.m.

5.

On the sunny day, the starch score fell from 4 at 6 p.m. to 1 at 2 a.m. Which explanation fits best?

A. The cool night air made the starch harder, so the iodine could not color it as well.B. At night the leaves gave their starch back to the air as carbon dioxide for the next day.C. At night the leaves switched to making starch from oxygen, which takes much longer.D. In the dark, no new sugar was made, and stored starch was broken down, moved and used.

6.

By 6 p.m., the leaves on the cloudy day had a starch score of only 2. Which explanation fits the data best?

A. The cloudy-day leaves were picked from plants that had been kept under foil for days.B. The cloudy-day leaves stored their extra sugar as chlorophyll instead of starch.C. With less light energy reaching the leaves, they made less sugar to store as starch.D. Clouds held back the carbon dioxide in the air, so the leaves had less to use.

7.

The zoo picks greens for the tortoises once a day. Based on Table 2, when do the globemallow leaves most likely hold the most stored food?

A. At 6 p.m., at the end of a sunny dayB. At 6 a.m., after a long, cool night of restC. At 10 a.m., soon after the Sun has come upD. At 2 a.m., when the starch score is the lowest

From the greenhouse to the tortoise

Use the passage, both tables and your answers above. Photosynthesis can be modeled as: carbon dioxide + water + light energy → sugar + oxygen.

8.

Match each input or output of photosynthesis in a globemallow leaf to its role.

9.

Which path could one carbon atom take into a young tortoise's growing shell?

A. Light from the Sun → sugar in a leaf → tortoise's food → shellB. Carbon dioxide in the air → sugar in a leaf → tortoise's food → shellC. Warm air from the basking lamp → tortoise's lungs → blood → shellD. Soil in the greenhouse pot → globemallow root → leaf → tortoise's food → shell

10.

Which statement best describes how matter and energy move differently in the greenhouse and the tortoise pens?

A. Carbon atoms can return to the plants, but energy leaves as heat, so new light is needed.B. Atoms and energy are both used up by the tortoises, so the greenhouse needs new soil and light.C. Energy cycles back to the plants as heat, but carbon atoms are used up by the tortoises.D. Atoms and energy both cycle back to the plants, so the greenhouse needs no new light.

11.

A zoo visitor says, “The globemallow gets its food from the potting soil, and the tortoises get the energy they need to grow from their warm basking lamp.” Do you agree? Write a claim, use evidence from Table 1 and Table 2, and use the photosynthesis model to explain where the plants' food and the tortoises' energy come from.