Practice questions · Topic 8.1

Unit 8 · Practice for the Topic 8.1 end-of-topic test

You’ve gone through everything in this topic. The summary video below recaps it all, so you’re ready for the questions.

Watch first: Responses to the environment, summed up

Video coming soon

A stimulus and a response at the scale of the whole organism; behavioral or physiological; taxis, kinesis and the plant’s two versions; a signal as a stimulus made for another organism, its five channels and four common jobs; innate or learned; cooperation and kin; the choice-chamber test and its chi-square.

These are practice questions in the shape of the topic test. Work through them before you take the test; every question tells you what it wanted.

Answer every question. For each multiple-choice question, pick one option and press Check; the feedback gives the reasoning. For the free-response questions, write one short sentence for each step of your reasoning, each on its own line, and make every link clear (so, because, therefore). That is what the exam’s ‘paragraph form’ means for you: linked sentences, not bullet points. The first free-response question walks you through one choice-chamber test one part at a time, and you can open a hint for each part; the second is at the level of the test. When you finish a question, open the scoring guide and mark your own work against it. Where a question needs a critical value, the chi-square table from the formula sheet is printed beside it.

Question 1

A student reads that ecologists study the badgers of one wood and says: “Ecology is the study of wild animals in their natural surroundings.”

Which of the following corrects the student’s statement?

A. Ecology studies wild animals in the laboratory as well as in the wild, but only animals B. Ecology is the care of wild places, so it is a job rather than a branch of biology C. Ecology studies how living things of every kind interact with one another and with their surroundings D. Ecology studies the organs and cells inside wild animals rather than the animals’ surroundings

Question 2

Field mice at two air temperatures: minutes of 30 spent in the insulated shelter, oxygen used, and body temperature after 30 minutes.

Researchers let field mice choose between an insulated shelter and an open chamber for 30 minutes, at two air temperatures. The table gives the results.

Which of the following is a physiological response by the mice to the cold air?

A. The 18 minutes the mice spent in the insulated shelter B. The rise in the oxygen the mice used C. The fall in the air temperature to 8 °C D. The mice’s body temperature staying near 37 °C

Question 3

Four small animals each respond to a change around them.

Which of the following movements is a taxis?

A. Mosquito larvae swim straight down whenever a shadow falls on the water above them B. Earthworms turn more often on warm soil than on cool soil, in no fixed direction C. Pond snails crawl faster in water with little oxygen and slower in oxygen-rich water, whichever way they are heading D. Water fleas dart about more in warm water than in cool water, heading nowhere in particular

Question 4

In a kind of wasp, some young females stay at the nest they hatched in and spend their lives feeding their mother’s later young, their own sisters, instead of laying eggs of their own. A student says: “The helpers stay because raising sisters is good for the wasp species.”

Which of the following corrects the student’s reason?

A. The helpers stay because feeding the young passes their own alleles into the young with the food B. The helpers stay because staying at the nest costs them nothing C. The helpers stay because their own alleles change to match their sisters’ while they feed them D. The helpers stay because their sisters share many of their alleles, and the fed sisters survive to breed

Question 5

Two male electric fish before and while facing each other over a worm: pulses per second and what each fish did.

Two males of one kind of electric fish live in one shoal. A worm sinks to the river bed between them, and both turn toward it. Each fish gives off a steady train of weak electric pulses, and a researcher records the pulses before and while the two face each other. The table gives the record.

Which of the following best describes what the larger male’s change in pulses did for it?

A. It showed dominance B. It marked the river bed around the worm as its own patch C. It led the smaller male to the worm D. It won the smaller male as a mate

Question 6

Nestmates that left for the food each hour after an ant returned, across four barriers, with what each barrier let through.

When an ant of one kind that forages at night finds food, it returns to the nest and its nestmates then leave for the food. Researchers place a barrier between a returning ant and its nestmates and count the nestmates that leave for the food each hour. The table gives the results and what each barrier lets through.

Which kind of signal do the returning ants use to bring their nestmates to the food?

A. A visual signal B. An audible signal C. A tactile signal D. A chemical signal

Question 7

The defensive chemical in an undamaged bean seedling’s leaves after 24 hours beside a damaged or undamaged seedling, arranged four ways.

Suppose caterpillars feed on a bean seedling for an hour. Researchers then remove the caterpillars and place the damaged seedling near an undamaged one in four ways, and after 24 hours measure a defensive chemical in the undamaged seedling’s leaves. The table gives the results.

Which conclusion do the results support?

A. The damaged seedling passes a chemical through the shared soil to the undamaged seedling B. The damaged seedling gives off a chemical into the air that the undamaged seedling detects C. The caterpillars must touch the undamaged seedling before it makes the defensive chemical D. An undamaged seedling makes the defensive chemical whenever another seedling stands near it

Question 8

The share of a feeding flock of finches that flew into cover within 30 seconds of each of four recorded sounds.

Researchers play one recorded sound to each of several feeding flocks of finches and record the share of each flock that flies into the cover of a shrub within 30 seconds. The table gives the results.

Which conclusion do the results support?

A. The finches fly into cover when they hear any loud sound B. The finches fly into cover after they see a hawk, whatever sound they hear C. The sharp three-note call, in its usual order, sends the flock into cover D. The finches fly into cover after the low two-note feeding call

Question 9

Ten male bowerbirds with undecorated bowers and ten with bowers decorated with blue objects: mean females that visited and mean females that mated with each male over a season.

Suppose male bowerbirds of one kind each build a bower of sticks on the ground and display in front of it to visiting females. Some males decorate the bower with blue objects. Researchers watch 10 males of each kind through a season and count the females that visit and mate with each. The table gives the means.

Which claim do the results support?

A. Decorating the bower keeps the male alive longer B. Every male mates with about the same number of females, whatever his bower C. Females that visit a decorated bower lay more eggs D. Decorated males mate with more females, so they leave more offspring

Question 10

Caterpillars that thrash and caterpillars that drop off the leaf on their first attack by a parasitic wasp: the share parasitized and the share that survived to become adults.

A parasitic wasp attacks caterpillars of one kind for the first time in their lives, and each caterpillar does one of two things: it thrashes, or it drops off the leaf on a thread of silk. Every caterpillar does its behavior perfectly on this first attack, with no caterpillar to copy, and its young later do as it did. Researchers record what happens to the caterpillars of each kind. The table gives the results.

Which of the following predicts what happens to dropping over the generations, and why?

A. Thrashing becomes more common, because dropping costs the caterpillar its place on the leaf and its meal B. Dropping becomes more common, because more droppers survive to breed and their young drop too C. Dropping becomes more common, because the young caterpillars copy the droppers they see survive D. Dropping becomes more common, because the parasitized caterpillars change their alleles to dropping

How to tackle the free-response questions. Read the verb first: describe asks what you see or know; explain asks why or how, so name the mechanism; predict asks what will happen and why; justify asks for the evidence that supports a claim. Each point is earned by one idea, stated in a sentence that names the thing and the mechanism. Extra words earn nothing; a wrong extra can lose the point. If there is a figure or table, use what it shows. When you finish, check the box for each point your answer earns and compare your sentences with the full-credit answer.

Free response 1 · Scientific Investigation · 6 points

A student tests whether brine shrimp gather in the light. He shades one end of a narrow trough of sea water and lights the other end, tips 26 brine shrimp into the middle, and after five minutes counts the shrimp at each end. The counts are drawn below.

The trough from above after five minutes: 7 brine shrimp at the shaded end and 19 at the lit end.

(a) Identify the independent variable in the student’s test. (1 point)

Hint: Which one condition did the student make different between the two ends of the trough?

A full-credit answer: The independent variable is the light at an end of the trough, lit or shaded, because it is the one condition the student deliberately changed.

Check the box for each point your answer earns

light or shade at an end as the independent variable.

Common slip: Naming the number of shrimp at each end. That is what was counted: the dependent variable.

(b) Identify the dependent variable in the student’s test. (1 point)

Hint: What did the student count to see the effect?

A full-credit answer: The dependent variable is the number of brine shrimp at each end, the quantity the student counted.

Check the box for each point your answer earns

the number of shrimp at each end as the dependent variable.

Common slip: Naming the 26 shrimp tipped in. That number was the same for the whole trough: a condition kept the same.

(c) State the null hypothesis for the test. (1 point)

Hint: What would the two counts look like if the light made no difference to the shrimp?

A full-credit answer: The null hypothesis is that the shrimp have no preference between the lit end and the shaded end, so any gap from an even split is due to chance.

Check the box for each point your answer earns

no preference (an even split), any difference due to chance.

Common slip: Writing “the shrimp prefer the light”. That is the claim the test might support; the null hypothesis is no preference.

(d) Calculate the expected count at each end under the null hypothesis. (1 point)

Hint: Under the null hypothesis, how do 26 shrimp share two ends?

shrimp

Write down the values in the question:

total = 26 shrimp

ends = 2

Write down the equation:

e = \frac{total}{number of ends}

Substitute the values into the equation:

e = \frac{26}{2} = 13 shrimp at each end

A full-credit answer: Under no preference the expected count at each end is 26 ÷ 2 = 13 shrimp.

(e) Calculate chi-square for the observed counts against the expected counts, to three significant figures. (1 point)

Hint: The formula sheet gives chi-square; o is each count you were given, and e is your answer to part (d).

Write down the values in the question:

o = 19 (lit end), 7 (shaded end)

e = 13 at each end

Write down the equation:

χ^{2} = \sum \frac{{(o - e)}^{2}}{e}

Substitute the values into the equation, one end per line:

lit end: \frac{{(19 - 13)}^{2}}{13} = \frac{36}{13} = 2.77

shaded end: \frac{{(7 - 13)}^{2}}{13} = \frac{36}{13} = 2.77

Add the ends:

χ^{2} = 2.77 + 2.77 = 5.54

A full-credit answer: Lit end: $\frac{{(19 - 13)}^{2}}{13} = 2.77$ .
Shaded end: $\frac{{(7 - 13)}^{2}}{13} = 2.77$ .
So $χ^{2} = 2.77 + 2.77 = 5.54$ .

(f) Determine whether the null hypothesis is rejected at p = 0.05, and justify the verdict with the two numbers it rests on. (1 point)

Hint: How many degrees of freedom do two ends give? Read the p = 0.05 row in that column and compare your chi-square with it.

The chi-square table from the formula sheet.

A full-credit answer: Two ends give one degree of freedom.
The p = 0.05 row reads 3.84 at one degree of freedom.
5.54 is larger than 3.84.
So the null hypothesis is rejected: the counts support a preference for the lit end.

Check the box for each point your answer earns

reject the null hypothesis, because 5.54 is larger than 3.84 at one degree of freedom.

Common slip: Reading the table at two degrees of freedom because there are two ends. Degrees of freedom are one fewer than the number of classes.

Free-response score: 0 of 6

Free response 2 · Scientific Investigation · 4 points

A student studies a roundworm of one kind that lives in soil and eats soil bacteria. The bacteria give off a chemical, diacetyl, into the air spaces of the soil. The student puts 100 worms in the middle of a dish, a drop of diacetyl at one edge and a drop of water at the opposite edge, and after 60 minutes counts the worms near each drop and in the middle. The table gives the counts.

Where 100 roundworms were after 60 minutes in a dish with a drop of diacetyl at one edge and a drop of water at the other.

(a) Describe what the counts show about the worms’ response to diacetyl. (1 point)

A full-credit answer: The worms detect the diacetyl and gather where it is: 85 of the 100 were near the diacetyl drop after 60 minutes, and 5 near the water.

Check the box for each point your answer earns

the worms gather near the diacetyl (most of the 100 end up there), so they detect it and respond to it.

Common slip: Saying the diacetyl pulled the worms across the dish. The worms moved; the chemical was the change they detected.

(b) Explain how the response you described in part (a) helps a worm survive. (1 point)

A full-credit answer: The bacteria the worm eats give off diacetyl.
So where the diacetyl is, the bacteria are.
A worm that gathers where the diacetyl is ends up among its food.
So it eats, and it survives.

Check the box for each point your answer earns

diacetyl comes from the bacteria the worm eats, so gathering where it is brings the worm to its food and it survives.

Common slip: Saying the worm likes the smell. The point is what the response does for survival: it brings the worm to food.

(c) A classmate says: “The counts show the worms crawled straight toward the diacetyl, so the movement is a taxis.” Evaluate the classmate’s claim. (1 point)

A full-credit answer: The claim is not supported by the counts alone.
A taxis is movement toward the stimulus; a kinesis is a change in speed or turning with no fixed direction.
Worms that slow down and turn less wherever diacetyl is would also pile up near the drop.
So the same counts fit a kinesis, and only the worms’ paths, not their end positions, can settle which it is.

Check the box for each point your answer earns

the judgement (the claim is not supported by the counts alone) AND the ground (a kinesis, slowing or turning less near the chemical, would give the same end counts; the paths would have to be watched to tell).

Common slip: Agreeing because most worms ended up near the diacetyl. Where the animals finish is the same for a taxis and a kinesis; how they move differs.

(d) The student spreads diacetyl evenly over the whole surface of a second dish and tips in 100 worms. Predict where the worms are after 60 minutes, and justify your prediction. (1 point)

A full-credit answer: The worms spread about evenly across the dish, or they stay near the middle.
Diacetyl is everywhere, so no part of the dish differs from another in the chemical.
With no difference to detect, the worms have nothing to gather toward, so they spread by chance.
If instead the worms slow down wherever diacetyl is strong, they slow down everywhere, so they stay near the middle, where they were tipped in.

Check the box for each point your answer earns

EITHER prediction with its matching reason: an even (chance) spread, because the diacetyl is the same everywhere so there is no difference for the worms to detect; OR the worms stay near the middle, because a worm that slows wherever diacetyl is strong slows everywhere, so it stays near where it was tipped in. A prediction paired with the other prediction’s reason does not earn the point.

Common slip: Predicting that the worms all gather at one edge. With no difference between the parts of the dish, there is nothing to choose between.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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