Plankton, Kelp and Sea Foam: A Day at the Marine Station

Use microscope and jar evidence to tell one-celled life, many-celled life and things that are not made of cells.

Part 1: The samples and the jar

Read the passage and use Tables 1 and 2 to answer questions 1–4.

From the Pier to the Microscope

1Ms. Halldóra Price's sixth-grade class spent a day at the Gull Rock Marine Station, a small lab at the end of a long pier. In the morning, marine biologist Dr. Pilar Venkataraman showed the class how to tow a plankton net, a cone of fine mesh that strains tiny living things out of the water. Oren Fairbanks and his group lowered the net from the pier rail and pulled it back up three times.

2Before lunch, the class walked the beach beside the pier. Along the high-tide line lay a strip of washed-up seaweed called a wrack line. The class collected a piece of kelp that had been lying on the sand for about two days, a scoop of white sea foam that the waves had piled up and a few clear plastic beads from the sand.

3Back in the lab, the class looked at each sample at 400× and recorded what they saw (Table 1). The copepod, a tiny animal from the plankton tow, had jointed legs, an eye spot, bands of muscle and a see-through gut, each built of many small cells. Inside its gut, the class could see several of the glassy shells from the tow.

4Before they left, Dr. Venkataraman put a drop of the plankton water and a scoop of the foam into a jar of filtered seawater on a sunny windowsill. For the next three days, she counted the glassy cells in one drop of water and the foam bubbles in one microscope view, and she sent the counts to the class (Table 2). Each drop is 0.05 mL, so 20 drops make 1 mL.

Table 1: The Class Microscope Log (400×)
Sample	What the class saw	Nucleus seen?	Over 3 days in the jar
Glassy specks from the tow	One cell in a glassy shell; some split	Yes	More each day (Table 2)
Copepod from the tow (about 1 mm long)	Legs, eye spot, muscles, gut: many cells	Yes	Not put in the jar
Kelp blade from the wrack line	Small brown boxy cells packed in a sheet	Yes	Not put in the jar
Kelp holdfast (the root-like base)	Long cells with thick walls in strands	Yes	Not put in the jar
Sea foam from the beach	Round bubbles packed together; no parts	No	Fewer each day (Table 2)
Plastic bead from the sand	Smooth clear ball; no parts inside	No	Not put in the jar

Table 2: Counts From the Windowsill Jar
Day	Glassy cells in one drop	Foam bubbles in one view
0	6	80
1	12	35
2	25	9
3	49	0

Use Table 2 and paragraph 4. Glassy cells in 1 mL = glassy cells in one drop × 20.

On Day 0, the jar held about glassy cells in each mL. On Day 3, it held about glassy cells in each mL.

Under the microscope, the sea foam looked a little like a sheet of cells. Use Tables 1 and 2. Which evidence best shows that the foam is not made of cells?

A. Its bubbles were round, but the cells of living things are box-shaped like the kelp cells.
B. Its bubbles had no nucleus or parts inside, and over 3 days they popped instead of splitting.
C. Its bubbles were full of air, which made the foam too light to be made of any cells.
D. Its bubbles were packed side by side, but the cells of living things float apart from each other.

Circle each observation that is evidence that the glassy specks are living things.

The number in one drop about doubled each day in the jar.Each one sits inside a clear, glassy shell.Some were splitting in two.They were strained out of the water by the plankton net.A copepod had several of their shells in its gut.

Use Table 1 and the passage. Sort each sample.

One glassy speck from the towThe copepodThe kelp bladeThe sea foamThe plastic beadThe kelp holdfast

A one-celled organism	From a many-celled organism	Not made of cells

Part 2: One cell or many kinds of cells

Use the passage and both tables to answer questions 5–9.

Which observation is the best evidence that a kelp is made of more than one kind of cell?

A. The blade has small boxy cells, while the holdfast has long, thick-walled cells.
B. The kelp holdfast is made of strands that have no nucleus in them.
C. The kelp blade is one long cell that runs from the holdfast to its tip.
D. The kelp blade's cells come in many different shapes and sizes.

Use paragraph 3 and Table 2. Are the glassy cells inside the copepod's gut cells of the copepod?

A. No, because glassy cells live and split on their own, so the copepod likely ate them.
B. Yes, because anything inside an animal's body counts as one of its own cells.
C. Yes, because the copepod's gut cells grow glassy shells as the copepod gets older.
D. No, because the glassy shells are bits of sand that the copepod swallowed by mistake.

The kelp blade showed cells with a nucleus, but the kelp had been lying on the sand for about two days. What can the class conclude from the slide alone?

A. The kelp is a one-celled organism, because each of its cells has a nucleus.
B. The kelp is a nonliving thing like the bead, because it was found lying on dry sand.
C. The kelp is still alive, because it is made of cells that each have a nucleus.
D. The kelp came from a many-celled living thing, but the slide does not show it is alive now.

On a rock by the pier, Oren finds tiny white specks and is not sure whether they are living. Which test would give the best evidence?

A. Weigh the specks, since living things are heavier than nonliving bits of the same size.
B. Compare their color with the foam and the kelp, since white things are not made of cells.
C. Look at them at 400× for cells, then check them in seawater for new cells over a few days.
D. Drop them in seawater, since cells sink and nonliving specks float to the top.

Oren says, "The sea foam is a colony of tiny cells, because its bubbles are packed together just like the boxes in the kelp blade." Do you agree? Write a claim, give at least three pieces of evidence from the tables and the passage, and explain your reasoning about what living things are made of.