Worksheet

Safety in Clumps: Pond Algae and Hungry Paramecia

Grade 8 natural selection practice: use pond microscope video and a jar test to explain how inherited clumping helps some algae survive and reproduce.

Grade 8ScienceInteractive · Printable

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11 questions
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Format: Interactive · Printable
Grade: Grade 8
Subject: Science
Question types: Fill in the blank, Multiple choice, Circle the answer, Sorting, Short answer

About this worksheet

What students practice

Students use video counts and jar test data to compare how two inherited forms of algae survive and reproduce.
Students explain why the advantage of a trait depends on whether predators are present.

How to use

Read the pond passage together and make sure students know that cells inside clumps were counted one by one. Let students work through the data on their own, then discuss the jars without paramecia before the final explanation.

Curriculum

NGSS standard covered

MS-LS4-4

Explain how genetic variation in traits makes some individuals more likely to survive and reproduce.

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Safety in Clumps: Pond Algae and Hungry Paramecia

Use microscope video and a jar test to explain which algae survive and reproduce.

Part 1: Under the microscope

Read the passage, then answer questions 1 to 4.

Green Water From the School Pond

1In September, Tamsin Okwuosa's eighth grade class at Willow Bend Middle School took water samples from the pond behind the school. Water from the shallow edge looked pale green. Under a microscope at 400 times magnification, the class saw that the color came from a one-celled green alga, a tiny plantlike organism that makes its own food from sunlight. Some algae cells floated alone, while others sat in tight clumps of 4 to 16 cells. Swimming among them were paramecia, one-celled organisms that move with tiny hairs and sweep bits of food into a groove on one side of the body.

2Their teacher, Mr. Lorenzo Haugland, works with a university lab that raised two strains of this alga from the same pond. Each strain grew from a single cell, so all the cells in one strain carry the same genes. In both strains, a cell reproduces by dividing into 4 or 8 new cells inside its wall. In the single strain, the new cells break free and drift apart. In the clump strain, they stay stuck together. The lab grew both strains for six months in the same water, light and temperature, with no paramecia, and each strain kept its own form, generation after generation. A single cell is about 10 µm (micrometers) wide, and a clump is 25 to 40 µm across.

3To see how paramecia feed, the class mixed both strains in one drop with a few paramecia and filmed the drop for 10 minutes. In the video, four paramecia swallowed 46 single cells and 2 small clumps of 4 cells each. Larger clumps bumped into a paramecium's groove and were swept away again.

4Next, the class ran a jar test. They set up four jars with the same filtered pond water, kept at 22 °C under a lamp for 14 hours a day. Each jar started with 1,000 single-strain cells and 1,000 clump-strain cells in every milliliter (mL); cells inside clumps were counted one by one. Two jars also got 50 paramecia per mL, and two jars got none. After 7 days, the students counted the algae cells in a small sample from each jar with a counting slide and worked out the number of cells per mL. Table 1 shows the average for each pair of jars.

5Mr. Haugland pointed out one more difference between the strains. A clump is heavier than a single cell, so in still water clumps sink faster, and the jars were never stirred. Light from the lamp was dimmer near the bottom of each jar, just as light is dimmer in deeper pond water.

Use paragraph 3. Which observation is the best evidence that paramecia have a harder time eating clumps than single cells?

A. Both strains of the alga were raised from cells taken from the same pond.
B. In 10 minutes the paramecia swallowed 46 single cells but just 2 clumps.
C. The paramecia swam with tiny hairs that cover the outside of the cell.
D. The water from the shallow edge of the pond looked pale green.

Use paragraph 3. Count every algae cell the paramecia swallowed in the video, including the cells inside the 2 clumps.

In all, the paramecia swallowed algae cells. Rounded to the nearest whole percent, % of the cells they swallowed were single cells.

Use paragraph 2. Why does it matter that the lab grew both strains for six months in the same conditions with no paramecia?

A. It gave the clump strain six months to learn to stick together before the test.
B. It let the single strain catch up to the clump strain in its total number of cells.
C. It shows the clumping form is passed from cell to new cells, not caused by paramecia.
D. It let the clumps grow wider than 40 µm, too wide for a paramecium to swallow.

Sort each difference between algae cells by whether it is inherited variation or a difference caused by the cell's surroundings.

Its new cells stay stuck together, like the cells of its whole strain.Its wall was dented when a paramecium bumped into it.Its new cells drift apart, like the cell it came from.It was torn out of its clump when a student pressed on the slide.

Inherited variation	Caused by surroundings

Part 2: The jar test

Use Table 1 and paragraphs 4 and 5 to answer questions 5 to 11.

Table 1: Algae Cells per mL (average of 2 jars)
When and which jars	Single-strain cells	Clump-strain cells
Day 0, all jars	1,000	1,000
Day 7, no paramecia	9,000	6,000
Day 7, with paramecia	1,200	4,800

Use Table 1. Look only at the jars with paramecia. Do not round.

From day 0 to day 7, the single-strain cells grew to times their starting number, and the clump-strain cells grew to times their starting number.

Use Table 1. On day 0, half of the algae cells in every jar were clump-strain cells. Find the share of clump-strain cells on day 7.

In the jars with paramecia, % of the algae cells were clump-strain cells. In the jars with no paramecia, % were clump-strain cells.

In the jars with no paramecia, the single strain reached 9,000 cells per mL but the clump strain reached 6,000. Which explanation fits paragraph 5 best?

A. Clumps sank into the dimmer light near the bottom, so they made less food.
B. Clump cells broke apart in those jars and were counted as single cells.
C. Small animals in the filtered water ate the clumps but left the single cells.
D. The single cells in those jars sat closer to the lamp than the clump cells.

Circle the two results that, taken together, best show that clumping is an advantage when paramecia are present but a drawback when they are absent.

Every jar started with 1,000 cells of each strain per mL.With paramecia, the clump strain reached 4,800 cells per mL and the single strain 1,200.The jars were kept at 22 °C under a lamp for 14 hours a day.Without paramecia, the single strain reached 9,000 cells per mL and the clump strain 6,000.A single cell is about 10 µm wide, and a clump is 25 to 40 µm across.

Each summer, paramecia become very common in the school pond for several weeks. Over those weeks, how will the mix of algae in the pond most likely change?

A. The share of clump-forming algae will likely rise, since clumps survive and keep dividing.
B. Single cells will start forming clumps as they sense the paramecia around them.
C. The mix will stay near half and half, since both forms of the alga came from this pond.
D. The share of single cells will likely rise, since single cells divide faster than clumps.

10.

A classmate says, "The paramecia made the algae form clumps to protect themselves." Which evidence from the page best shows a problem with this idea?

A. In the video, the paramecia swallowed 46 single cells in 10 minutes.
B. With paramecia, the clump strain grew to 4,800 cells per mL in a week.
C. A clump is 25 to 40 µm across, several times as wide as a single cell.
D. The clump strain kept forming clumps for six months with no paramecia.

11.

Explain how variation in clumping affected which algae cells in the jars survived and reproduced. Write a claim, use at least two numbers from the page as evidence, and explain your reasoning. Include the jars with no paramecia in your answer.