The Breathing Marsh: Photosynthesis at a Wildlife Refuge

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The Breathing Marsh: Photosynthesis at a Wildlife Refuge

Use day-and-night carbon dioxide data and cattail growth to trace matter and energy.

Carbon dioxide over the marsh

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

A Marsh That Breathes

1Every summer, volunteers at the Cold Spring Wildlife Refuge count the animals that live in its cattail marsh. Seventh grader Nadia Petrovic joined the July survey with refuge biologist Dr. Imani Castellanos. They counted Canada geese grazing on young shoots, muskrats chewing on cattail stems, and red-winged blackbirds nesting in the tall leaves. Dr. Castellanos explained that almost all of these animals depend on the marsh plants for food.

2Cattails, like other green plants, carry out photosynthesis in the chloroplasts of their leaves. Photosynthesis can be summarized as: carbon dioxide + water → sugar + oxygen, using light energy. Plants use some of the sugar for energy and build the rest into larger molecules, such as the cellulose in their stems. Plants, animals and microorganisms in the mud all carry out cellular respiration, day and night, which uses sugar and oxygen and gives off carbon dioxide.

3A sensor on a tower above the marsh measures the carbon dioxide (CO₂) in the air, in parts per million (ppm). Table 1 shows its readings over one clear July day.

4Each month, Dr. Castellanos clips all the cattails in a few 1-square-meter plots, dries them and weighs them. Drying removes the water, so the dry mass is mostly the plant's sugar, cellulose and other molecules. About 45% of a dry cattail's mass is carbon. The graph shows her results for this year.

5In the 1600s, a scientist named Jan Baptist van Helmont did a famous experiment. He planted a small willow tree with a mass of about 2 kg in a tub holding about 91 kg of dry soil, covered the tub and gave the tree only water. After 5 years the tree had a mass of about 77 kg, but the dried soil had lost only about 60 g. Van Helmont concluded that the tree's new mass came from the water.

Table 1. CO₂ Above the Marsh on a Clear July Day
Time	Sunlight	CO₂ (ppm)
2 a.m.	Dark	445
6 a.m.	Dim, sunrise	440
10 a.m.	Bright	410
2 p.m.	Bright	395
6 p.m.	Low sun	405
10 p.m.	Dark	430

1.

How much lower was the CO₂ reading at 2 p.m. than at 2 a.m.? ppm

2.

Which explanation best fits the drop in CO₂ from 6 a.m. to 2 p.m.?

A. Sunlight broke the carbon dioxide in the air apart into oxygen gas.B. The geese and muskrats flew or swam away from the marsh during the day.C. The marsh plants stopped respiring once the sun came up.D. In bright light, the plants took in CO₂ for photosynthesis faster than living things gave it off.

3.

Why did the CO₂ reading rise again from 6 p.m. to 2 a.m.?

A. At night, the cattails switched from taking in CO₂ to making new CO₂ from sunlight they had stored.B. Without light, photosynthesis stopped, but plants, animals and microorganisms kept giving off CO₂.C. At night, the plants took in oxygen and turned it directly into carbon dioxide.D. Cooler night air pulled extra carbon dioxide down from high in the sky over the marsh.

4.

Predict the CO₂ reading at 2 p.m. on a dark, cloudy day in July.

A. Higher than 395 ppm, because less light means less photosynthesisB. Lower than 395 ppm, because plants respire less in shadeC. About 395 ppm, because the time of day is the sameD. Lower than 395 ppm, because clouds keep the marsh cooler

Where the cattails' mass comes from

Use paragraphs 2, 4 and 5 and the graph to answer.

5.

Use the graph and paragraph 4.

About how many grams of carbon were in the cattails in one square meter in July? g

6.

Sort each event in the marsh. Is it about matter (atoms) cycling or about energy flowing?

7.

Use paragraph 5.

About how many kilograms of mass did the willow tree gain in 5 years? kg

8.

Modern scientists explain van Helmont's results differently. Which explanation does the evidence on this page best support?

A. The tree's mass came from sunlight, which the leaves turned into new atoms of wood.B. The tree's mass came from the water, because water was the only thing van Helmont added.C. Much of the tree's new mass came from carbon dioxide in the air, combined with water.D. Most of the tree's mass came from minerals in the soil, which is why the soil lost 60 g.

9.

The geese at the refuge graze on cattail shoots. Where did the energy stored in a goose's body first come from?

A. Light from the Sun, captured by the marsh plants during photosynthesisB. Water from the marsh, which the goose drinks while it feedsC. Oxygen in the air, which the goose breathes in all day and nightD. Minerals in the marsh mud, taken up by the roots of the plants

10.

A refuge visitor says, “The cattails grew from 150 to 900 grams per square meter by pulling matter out of the mud.” Do you agree? Write a claim, use evidence from Table 1, the graph and van Helmont's experiment, and explain your reasoning with the photosynthesis model.