Bean Seedlings in the Dark Box

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Bean Seedlings in the Dark Box

Use seedling data and leaf slides to model how cell parts work together.

The seedling test

Read the passage and study the table. Then answer questions 1–5.

Pale Stems Under the Board

1Each spring, Kenji Varga, the naturalist at Fennimore Park, helps a middle school club start bean plants for the park's community garden. This year, club member Odalys Brennan found a bean that had sprouted under a flat board at the edge of the garden. It had a long, pale yellow stem and tiny yellow leaves. The beans in the open beside it were short, sturdy and green. The club set up a test to find out what was happening inside the seedlings' cells.

2The club planted 30 bean seeds of one variety, each in its own cup of the same moist potting mix. They put 15 cups on a sunny windowsill and 15 cups inside a large dark box in the same room, so both groups had the same temperature. Every cup got the same amount of water every other day. Every 4 days, the club pulled 3 seedlings from each group, washed off the potting mix and measured each stem. Mr. Varga then dried those seedlings overnight in a low oven so that only their dry matter was left, and the club weighed them. The dry seeds they started with had a mean mass of 0.50 g.

3On Day 12, the club looked at thin slices of a leaf from each group at 400×. Both kinds of leaf cells had a cell wall, a cell membrane and a nucleus. The window leaf cells were packed with green chloroplasts, but the dark-box leaf cells had no green chloroplasts at all. The club then moved two dark-box seedlings to the windowsill. Within two days their leaves had turned green, and their leaf cells now held green chloroplasts.

4Mr. Varga explained that a bean seed holds a store of food in its two thick seed leaves. Mitochondria in the seedling's cells release energy from that food, and the food is used up as they do. Chloroplasts use light energy to make sugar from carbon dioxide and water. The cell membrane controls what enters and leaves each cell, the stiff cell wall supports it, and the nucleus holds the instructions that direct the cell's activities.

Bean Seedlings (mean of 3 seedlings)
Day	Window height (cm)	Window dry mass (g)	Dark box height (cm)	Dark box dry mass (g)
0	0	0.50	0	0.50
4	3	0.46	5	0.46
8	8	0.43	14	0.41
12	12	0.48	22	0.36
16	15	0.62	27	0.32

1.

How much dry mass did the dark-box seedlings lose, on average, from Day 0 to Day 16? g

2.

How much dry mass did the window seedlings gain, on average, from Day 8 to Day 16? g

3.

Both groups lost dry mass from Day 0 to Day 8. Which explanation fits the table and paragraph 4?

A. Their mitochondria released energy from the seed's stored food, using some of it up.B. Their chloroplasts broke the seed's stored sugar into water to make the stems longer.C. The seedlings dried out in their cups, so they lost water before they were weighed.D. Their roots gave food back to the potting mix, since young roots take in nothing yet.

4.

After Day 8, the window seedlings gained dry mass, but the dark-box seedlings kept losing it. Which explanation fits the evidence best?

A. The dark-box cells had no cell walls, so their food leaked out into the box.B. The window cells had no mitochondria, so they kept all of the seed's food.C. The dark-box cells had no nucleus, so they could not use the seed's food.D. The window cells had green chloroplasts that used light to make new sugar.

5.

Odalys says, "The dark-box seedlings grew almost twice as tall, so they made more new food than the window seedlings." Which evidence best shows she is mistaken?

A. By Day 8, the dark-box dry mass was rising faster than the window dry mass.B. By Day 16, the dark-box stems were 12 cm shorter than the window stems.C. By Day 12, the dark-box leaf cells held more green chloroplasts than the window cells.D. By Day 16, the dark-box dry mass was 0.32 g, but the window dry mass was 0.62 g.

A model of a seedling's cells

Use the passage, the table and your answers above to answer questions 6–10.

6.

Why did the club keep both groups in the same room, in the same potting mix, with the same watering?

A. So the dark box would stay warmer than the windowsillB. So light would be the main difference between the groupsC. So both groups would grow to the same height by Day 16D. So the window seedlings would get more water than the others

7.

Suppose the club had left the other dark-box seedlings in the dark until Day 24. Which prediction fits the model best?

A. Their dry mass would fall below 0.32 g, since their cells would keep using stored food up.B. Their dry mass would jump, since their mitochondria would start making sugar in the dark.C. Their dry mass would rise to about 0.6 g, since their stems would keep getting taller.D. Their dry mass would stay at 0.32 g, since their cells would stop using energy in the dark.

8.

Two dark-box seedlings moved to the windowsill had green leaves within two days. Which explanation fits paragraph 3 best?

A. Green chloroplasts moved up from the roots into the leaves once the stems were in light.B. Light colored their cell walls green, though their cells still had no chloroplasts.C. Their leaf cells built green chloroplasts once light reached them, so they could make food.D. Their mitochondria turned into chloroplasts once the leaves were warmed by the sun.

9.

Complete the club's model of a window seedling on Day 16. Use words from the word bank. One word is not used.

Light reaches the leaf cells, where make sugar. The sugar travels down to the root cells and passes through each cell's . There, release energy from the sugar so the root can grow. A stiff cell around each stem cell helps hold the stem up, and the directs what each cell does.

10.

By Day 16, the window seedlings were heavier than the seeds they came from, but the dark-box seedlings were lighter. Explain this difference. Write a claim, give evidence from the table and paragraph 3, and use at least three cell parts to explain how they work together.