Algae, Oxygen and Salamander Eggs

Model how cell parts in two kinds of cells work together.

Answer Key

The egg test

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

Green Eggs at Ironwood Marsh

1Ironwood Marsh National Wildlife Refuge protects a forest dotted with vernal pools: shallow ponds that fill with snowmelt and spring rain and dry up by late summer. The refuge, the people and the numbers here are made up, but they are based on real studies. Every March, refuge biologist Dr. Lucinda Okafor surveys the egg masses that spotted salamanders lay in these pools. This year, sixth grader Bastian Kowalczyk joined her crew.

2A spotted salamander egg mass is a clump of clear jelly about the size of a fist, holding roughly 100 eggs. Within a week or two, many masses turn green. Under a microscope at 400×, Bastian saw why. Tiny round cells of a green alga were living in the jelly right next to each embryo. Each algal cell had a cell wall, a nucleus and one large chloroplast. The embryo's cells had a nucleus but no cell wall and no chloroplasts, and they were packed with yolk, the stored food the embryo lives on until it hatches.

3Dr. Okafor gave the crew a model of the cell parts involved. Every cell is surrounded by a cell membrane, which lets oxygen and carbon dioxide pass in and out. A cell wall, found in the algal cells, is a stiff layer outside the membrane that supports the cell and gives it its shape. In light, chloroplasts use light energy to make sugar from carbon dioxide and water, and they give off oxygen. Mitochondria, found in both kinds of cells, use oxygen to release the energy stored in food such as sugar or yolk, and they give off carbon dioxide. The nucleus directs the cell's activities, including when it divides to make new cells. The embryo does not breathe air yet, so the oxygen it uses has to spread to it through the jelly.

4To test whether the algae help, the crew collected four egg masses on the day they were laid, before any algae had moved in. From each mass they took four clumps of 10 eggs and gave one clump to each of four groups, so every group had 40 eggs. Groups 1 and 2 sat in pond water, where algae soon moved into the jelly. Groups 3 and 4 sat in pond water passed through a fine filter that removes algae. Groups 1 and 3 got 12 hours of lamp light each day, and Groups 2 and 4 were kept dark. Every tank stayed at 15 °C. On day 20, at the end of a light period, a thin probe measured the oxygen in the fluid next to the embryos.

Table 1. Lab Test Results (40 eggs in each group)
Group	Algae in jelly?	Light	O₂ near embryo (mg/L)	Hatched (of 40)	Days to hatch (avg.)
1	Yes	12 h a day	9	36	33
2	Yes	Dark	3	24	42
3	No	12 h a day	4.5	28	39
4	No	Dark	4.5	27	40

Use Table 1.

The oxygen near the embryos in Group 1 was 4.5 mg/L higher than in Group 3, and 3 times as high as in Group 2.

Which two groups, compared with each other, show that lamp light by itself, without algae, did little for the embryos?

A. Groups 1 and 3
B. Groups 3 and 4
C. Groups 1 and 2
D. Groups 2 and 4

Which explanation of the high oxygen level in Group 1 fits the model in paragraph 3 best?

A. The embryos' own chloroplasts made oxygen faster once the lamp light reached them.
B. The algal cell walls pulled oxygen out of the pond water and held it near the embryos.
C. The jelly soaked up energy from the lamp and turned it into oxygen for the embryos.
D. Chloroplasts in the algal cells used the lamp light and gave off oxygen into the fluid.

Group 2 had algae, yet the oxygen near its embryos was lower than in Group 4, which had no algae. Which explanation fits the model best?

A. In the dark, the algal cells' chloroplasts took in oxygen and stored it as sugar.
B. The filter in Group 4's water added extra oxygen as the water passed through it.
C. In the dark, the algal cells' mitochondria used up oxygen while their chloroplasts made none.
D. The Group 2 embryos grew the fastest, so they used up more of the oxygen.

Use Table 1.

In Group 1, 90% of the eggs hatched. In Group 2, 60% of the eggs hatched.

The Group 1 embryos hatched about a week sooner than those in Groups 3 and 4. Which chain of events explains this best?

A. The jelly around the embryos grew thicker in the light, so the embryos were pushed out of their eggs sooner.
B. More oxygen reached the embryo cells, so their mitochondria released more energy from yolk for growing and dividing.
C. The algae passed some of their own yolk into the embryo cells, so the embryos had more food to grow with.
D. More light reached the embryo cells, so their chloroplasts made extra sugar and the embryos used less yolk.

Two kinds of cells working together

Use paragraphs 2 and 3 and Table 1.

Match each cell part to the job it does in the egg mass.

Chloroplast in an algal cellB
Mitochondrion in an embryo cellE
Cell membrane of an embryo cellA
Cell wall of an algal cellD
Nucleus of an embryo cellC

A. Lets oxygen in and carbon dioxide out
B. Captures light energy to make sugar and gives off oxygen
C. Directs the cell, including when it divides
D. Gives the cell a stiff layer that holds its shape
E. Releases energy from yolk using oxygen

Sort each part or feature by the kind of cell that has it, based on Bastian's microscope view and Dr. Okafor's model.

ChloroplastMitochondriaCell wallStored yolkCell membraneNucleus

Algal cell only	Embryo cell only	Both kinds of cells
Chloroplast Cell wall	Stored yolk	Mitochondria Cell membrane Nucleus

In one pool, a fallen pine shades the green egg masses for most of each day for two weeks. Based on Table 1, which prediction for the embryos in those masses fits best?

A. They would get less oxygen in the daytime and develop more slowly, more like Group 2.
B. They would develop at the Group 1 rate, since the algae would still live in the jelly.
C. They would grow faster, because the algae would stop taking light away from them.
D. They would make their own sugar from yolk in the shade and develop at the same speed.

The pool survey

In April the crew checked the egg masses in three of the refuge's pools. Use Table 2 and Table 1 to answer.

Table 2. Egg Mass Survey in Three Pools
Pool	Open sky above pool	Egg masses	Green masses	Eggs hatched (%)
Fern Pool	Most of it	40	36	85
Ash Hollow	About half	30	18	74
Deep Shade Pool	Very little	25	5	61

10.

Use Table 2.

At Fern Pool, 90% of the egg masses were green. At Deep Shade Pool, 20% were green.

11.

Dr. Okafor says the survey fits the lab test but is weaker evidence that the algae help the embryos. Why?

A. The pools may also differ in other ways, such as water temperature or predators.
B. The survey measured oxygen with a different probe than the one used in the lab.
C. The survey counted egg masses in one pool, while the lab test used three pools.
D. In the survey, the pool with the least open sky had the most green egg masses.

12.

A visitor sees the green egg masses and says, "That green stuff is a mold growing on the eggs. Rinse it off before the eggs go into the nature center's display tank, which has a daytime lamp." Do you agree? Write a claim, give evidence from Table 1, and explain how chloroplasts, mitochondria and cell membranes in the two kinds of cells work together.

I disagree: the green cells are algae that help the embryos, so they should stay. Evidence: in Table 1, eggs with algae in the light (Group 1) had 9.0 mg/L of oxygen near the embryos, compared with 4.5 mg/L without algae (Group 3). In Group 1, 36 of 40 eggs (90%) hatched after 33 days, but only 28 of 40 (70%) hatched after 39 days in Group 3. Reasoning: in the light, the chloroplasts in the algal cells make sugar and give off oxygen, which passes out through the algal cell membranes, through the jelly and in through the embryo's cell membranes. The embryo's mitochondria use that oxygen to release energy from the yolk, so its cells can grow and divide faster. The carbon dioxide the mitochondria give off passes back out through the membranes, where the algae's chloroplasts can use it. Rinsing off the algae would make the eggs like Group 3, so they would likely hatch later and fewer would hatch, even under the lamp.