Oxygen in Tide Pools by Day and by Night

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Oxygen in Tide Pools by Day and by Night

Use oxygen readings to model how chloroplasts and mitochondria work together.

Part 1: Two pools at low tide

Read the passage and Table 1, then answer questions 1–4.

When the Tide Goes Out

1Nalani Akana's sixth-grade class spent two days at the Gull Rock Marine Station, beside a rocky shore. At low tide, the sea pulls back and leaves pools of seawater in hollows of the rock. Pools high on the shore are cut off from the sea for several hours at each low tide. On the day of the trip, the low tides came at about noon and about midnight, so the high pools were cut off from about 9 AM to 3 PM and again from about 9 PM to 3 AM.

2The marine educator, Ms. Beatrix Holloway, led the class to two high pools of about the same size and depth. Pool A was crowded with seaweed: bright green sea lettuce and brown rockweed. Pool B was mostly bare rock with very little seaweed. Each pool held about 40 snails and a few sea anemones. The snails graze on seaweed. Using an oxygen probe, the class measured the oxygen dissolved in each pool every 3 hours while the pool was cut off. They came back with flashlights for the night readings. The first day was sunny.

3Back at the station, Nalani's group set up a bottle test with seawater taken from the sea at high tide. They filled four clear bottles, added a piece of sea lettuce or one small snail to some of them and wrapped one bottle in foil so that no light could get in. All four bottles sat in sunlight for 2 hours. Ms. Holloway explained that a sheet of sea lettuce is only two cells thick, so every one of its cells touches the water around it.

Table 1: Dissolved Oxygen While the Pools Were Cut Off (mg/L)
Time	Pool A: lots of seaweed	Pool B: little seaweed
9 AM	8	8
12 PM	12	7
3 PM	16	6
9 PM	8	8
12 AM (midnight)	6	7
3 AM	4	6

1.

Use Table 1. Look at the daytime readings, from 9 AM to 3 PM.

The oxygen in Pool A rose by mg/L, while the oxygen in Pool B fell by mg/L.

2.

Which explanation best fits the daytime readings for Pool A?

A. The snails in Pool A made oxygen as they grazed, so the snails added more than the seaweed did.B. Waves splashed fresh oxygen into Pool A, since it was closer to the sea than Pool B.C. The seaweed's mitochondria made oxygen from sunlight, and the snails' chloroplasts used it up.D. Chloroplasts in the seaweed cells used sunlight and released oxygen faster than it was used.

3.

Look at the night readings, from 9 PM to 3 AM. The oxygen fell by 4 mg/L in Pool A but by only 2 mg/L in Pool B. Which explanation fits the data best?

A. The flashlights warmed Pool A more than Pool B, so oxygen bubbled out of the water.B. The seaweed's own mitochondria used oxygen too, and with no light its chloroplasts released none.C. The seaweed's chloroplasts switched to taking in oxygen at night instead of taking in light.D. The snails in Pool A were twice as large, so their cells needed twice as much oxygen.

4.

Ms. Holloway gave the class a simple model. At night, with no light, the oxygen in Pool A fell by about 2 mg/L every 3 hours. Suppose the living things in Pool A use oxygen at that same night rate during the day too.

From 9 AM to 12 PM, Pool A's oxygen rose by mg/L. So the seaweed's chloroplasts released about mg/L of oxygen in those 3 hours.

Part 2: The bottle test

Use Table 2, Table 1 and paragraph 3 to answer questions 5–10.

Table 2: Bottle Test (2 hours in sunlight)
Bottle	Added to seawater	Wrapped in foil?	Oxygen change (mg/L)
1	Sea lettuce	No	+3.0
2	Sea lettuce	Yes	−0.4
3	One small snail	No	−0.6
4	Nothing	No	0

5.

Why did the group include Bottle 4?

A. To check that the seawater by itself did not change the oxygen levelB. To show that seawater makes oxygen when it sits in bright sunlightC. To give the snail a second bottle in case the first one got too warmD. To give the sea lettuce more room so its cells could take in light

6.

Bottles 1 and 2 held the same kind of sea lettuce. Why did the oxygen go up in Bottle 1 but down in Bottle 2?

A. The foil blocked the nucleus from sending out instructions, so the cells stopped working.B. The foil trapped the oxygen inside the sea lettuce cells instead of letting it into the water.C. In the dark, the chloroplasts released no oxygen, but the mitochondria kept using it.D. The sea lettuce in Bottle 2 had no mitochondria, so its cells broke down and used up oxygen.

7.

Circle each result from Table 1 or Table 2 that is evidence that seaweed cells use oxygen.

8.

The snail's cells have mitochondria but no chloroplasts. Where does the sugar that the snail's mitochondria use most likely come from?

A. From the oxygen in the pool, which the snail's mitochondria turn into sugarB. From the seaweed it grazes on, which was made by chloroplasts in the seaweed's cellsC. From sunlight that passes through its shell and into the nucleus of each cellD. From the seawater, which holds sugar that the snail's shell soaks up at low tide

9.

Use paragraph 3. Oxygen released inside a sea lettuce cell has to cross the cell membrane to reach the water. Why does it help that the sheet is only two cells thick?

A. A thin sheet holds more chloroplasts in each cell than a thick seaweed like rockweed does.B. The thin sheet has no cell walls, so oxygen can leave without passing through anything.C. Each cell's membrane touches the water, so oxygen and carbon dioxide pass in and out easily.D. The water can reach the nucleus directly, so the nucleus can send the oxygen out.

10.

The next day was cloudy, with much less sunlight. Using the class's model, which reading is most likely for Pool A at 3 PM, if it again started at 8 mg/L at 9 AM?

A. About 10 mg/L, since the chloroplasts would release less oxygen in dimmer lightB. About 16 mg/L, the same as the sunny day, since the seaweed and snails were the sameC. About 24 mg/L, since cooler cloudy weather lets the chloroplasts work fasterD. About 2 mg/L, since the mitochondria use extra oxygen when the sky is cloudy

Argue from the data

Use both tables and the passage to answer question 11.

11.

A classmate says, "Seaweed makes oxygen, and animals use it up. Seaweed cells do not need oxygen themselves." Do you agree? Write a claim, give at least two pieces of evidence from the tables, and explain how the chloroplasts and mitochondria in seaweed cells work together.