Practice questions · Topic 1.2

Unit 1 · Practice for the Topic 1.2 end-of-topic test

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 choose one answer and press Check; the feedback tells you what a wrong choice assumed. For the free-response questions, write your answer in full sentences. Use a hint if you are stuck, then open the full-credit answer and score your own work against it.

Question 1

A fungus grows on a fallen log in the dark beneath a thick layer of dead leaves. Over a year the fungus gains 400 g of new material, and the log loses more than 400 g of its dry mass.

Where did the atoms in the fungus’s new material come from?

A. From the molecules of the log, which the fungus took in as its food B. From the energy in the small amount of light reaching the log C. From atoms the fungus made for itself as it grew D. From the air alone, since a fungus takes in carbon dioxide to build its molecules

Question 2

A caterpillar eats 2.0 g of dry leaf material in a week and gains 0.5 g of new body mass. A student says the other 1.5 g of atoms were destroyed as the caterpillar used them up.

What happened to the atoms in the other 1.5 g?

A. They were destroyed, as the student says; used-up atoms disappear B. They left the caterpillar in its droppings and in the carbon dioxide it gave off C. They were turned into the energy the caterpillar used to move D. They are still in the caterpillar, packed more tightly into the 0.5 g

Question 3

A student burns dried samples of four foods, each rich in one class of biological molecule: pasta (carbohydrate), olive oil (lipid), egg white (protein) and a yeast extract rich in nucleic acids. She lists the elements in each.

Which elements should appear on every one of her four lists?

A. Carbon, hydrogen and nitrogen B. Carbon, hydrogen, oxygen and phosphorus C. Carbon and hydrogen only D. Carbon, hydrogen and oxygen

Question 4

A soil bacterium takes in sulfate, a source of sulfur, and builds the sulfur into its own molecules.

Where in the bacterium’s molecules does the sulfur end up?

A. In the phosphate groups of its nucleotides B. In the R groups of some of the amino acids in its proteins C. In the –OH groups of the sugar units in its carbohydrates D. In the hydrocarbon tails of its fatty acids

Question 5

A growing yeast cell is fed phosphate containing a radioactive form of phosphorus, and the cell builds that phosphorus into new molecules. Afterward each class of molecule is tested for radioactivity.

Which molecules will be radioactive?

A. Its proteins and its nucleic acids B. Its proteins only C. Its phospholipids and its nucleic acids D. Its glucose stores and its fats

Question 6

A cell has plenty of carbon, hydrogen, oxygen and sulfur but has run out of nitrogen. It tries to build a new amino acid.

Which part of the amino acid needs the missing element?

A. The central carbon B. The carboxyl group, –COOH C. The amine group, –NH₂ D. The hydrogen atom on the central carbon

Question 7

Sheep’s wool is made of keratin, a protein whose chains are held in shape by many disulfide bridges between sulfur-containing R groups. Sheep grazing a pasture whose soil is very low in sulfur grow wool slowly and the wool is weak.

Why does a shortage of sulfur slow the growth of wool?

A. Sulfur is used to build proteins, in the R groups of some amino acids, so the sheep can build less new keratin B. Sulfur is used to build the phosphate groups that hold the wool’s protein chains together in bridges C. Sulfur is used to build the sugar units that the wool’s chains are made of, so fewer chains form D. Sulfur supplies the energy that the sheep’s cells use to build wool, so with less sulfur they build less

Question 8

A microbe is moved into water that supplies every element it needs except nitrogen. It goes on taking in sugar.

Which two classes of molecule does the microbe stop building?

A. Proteins and carbohydrates B. Proteins and lipids C. Proteins and nucleic acids D. Nucleic acids and carbohydrates

Question 9

A researcher wants to feed a growing cell one element in a radioactive form so that every class of biological molecule the cell builds becomes radioactive.

Which element should the researcher choose?

A. Nitrogen B. Phosphorus C. Sulfur D. Carbon

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 · Conceptual Analysis · 5 points

Lettuce plants are grown with their roots in tanks of water instead of soil. Tank 1 supplies every element the plants need. Tank 2 is identical except that it supplies no nitrogen. Both tanks receive the same light and the same carbon dioxide. Within two weeks the plants in Tank 2 have stopped growing and their new leaves are pale, while the plants in Tank 1 are still adding leaves. When nitrate, a source of nitrogen, is added to Tank 2, its plants begin growing again.

(a) Identify the three elements that make up most of the atoms in all four classes of biological molecule. (1 point)

Hint: Think of what a sugar and a fatty-acid tail are built from.

A full-credit answer: The three most common elements in carbohydrates, lipids, proteins and nucleic acids are carbon, hydrogen and oxygen.

Check the box for each point your answer earns

carbon, hydrogen and oxygen.

Do not award: a list that adds nitrogen or phosphorus to the three, or a list of two.

Common slip: Adding nitrogen because proteins and nucleic acids need it. Nitrogen is missing from carbohydrates and fats, so it is not one of the three that every class shares.

(b) Describe where the lettuce gets the atoms it builds into new leaves, and state why it must get them there. (1 point)

Hint: Where were the atoms in a new leaf a month ago?

A full-credit answer: The lettuce takes its atoms in from its surroundings, as carbon dioxide from the air and as water and dissolved substances from the tank, because a plant, like every organism, cannot make atoms.

Check the box for each point your answer earns

the atoms come in from the surroundings (air and the tank water), because an organism cannot make atoms.

Accept: ‘atoms are never created, so every atom in a new leaf was taken in’.

Do not award: atoms made from light, or ‘from the tank’ with no reason given.

Common slip: Saying the plant makes its new material from sunlight. Light is energy, not matter; it cannot become a carbon or a nitrogen atom.

(c) Identify the two classes of biological molecule that need nitrogen, and state where the nitrogen sits in the monomer of each. (1 point)

Hint: Picture an amino acid and a nucleotide: which group in each holds the nitrogen?

A full-credit answer: Nitrogen is needed to build proteins, where it sits in the amine group (–NH₂) of every amino acid, and nucleic acids, where it sits in the nitrogenous base of every nucleotide.

Check the box for each point your answer earns

proteins (nitrogen in the amine group of every amino acid) and nucleic acids (nitrogen in every nitrogenous base).

Accept: ‘–NH₂’ for the amine group; ‘the bases A, T, G, C and U’ for the nitrogenous bases.

Do not award: nucleic acids alone, or lipids or carbohydrates named as needing nitrogen.

Common slip: Naming nucleic acids only. Every amino acid carries an amine group, so every protein contains nitrogen too.

(d) Predict which classes of biological molecule the Tank 2 plants can still build while nitrogen is missing. (1 point)

Hint: What are the molecules of each class built from?

A full-credit answer: The Tank 2 plants can still build carbohydrates and fats, because those molecules are built from carbon, hydrogen and oxygen only, all of which the plants still receive.

Check the box for each point your answer earns

carbohydrates and lipids (fats), because they need no nitrogen.

Accept: ‘sugars, starch and cellulose, and fats’.

Do not award: proteins or nucleic acids named as still buildable, or ‘nothing’.

Common slip: Predicting that all building stops. Carbon dioxide and water still supply carbon, hydrogen and oxygen, so the two classes made only from those keep being built.

(e) Explain why the Tank 2 plants stop growing even though they can still build some classes of molecule, and why adding nitrate restarts growth. (1 point)

Hint: What must a plant build before it can add a single new cell to a leaf?

A full-credit answer: A new leaf is made of new cells, and every new cell needs new proteins and new nucleic acids, which the plants cannot build while nitrogen is missing. Sugar and fat alone cannot make a cell, so growth stops. Nitrate supplies nitrogen, so the plants can again build amine groups and nitrogenous bases, and new proteins and nucleic acids, and new cells, are built.

Check the box for each point your answer earns

new cells need proteins and nucleic acids, which need nitrogen, so growth stops; nitrate restores the nitrogen, so those classes, and growth, resume.

Accept: ‘no new proteins or DNA means no new cells’ with the nitrate link.

Do not award: ‘the plants run out of energy’, or nitrate described as food that is burned.

Common slip: Treating nitrate as the plant’s food or energy supply. Nitrate supplies an element, nitrogen, that two classes of molecule are built from; the energy comes from light.

Free-response score: 0 of 5

Free response 2 · Conceptual Analysis · 4 points

Bread mold is a fungus. Spores land on a slice of bread, which is mostly starch with some protein, and within days a gray mat of mold covers it. A student grows the same mold on two plates. Plate 1 holds a slice of bread. Plate 2 holds a block of pure cellulose, a polymer of glucose, moistened with water that contains no other substances. This mold can break cellulose down into free glucose and take the glucose in. Both plates sit in the same warm, dark cupboard. The mold spreads across Plate 1 but barely grows on Plate 2.

(a) Describe where the mold on Plate 1 gets the atoms it builds into new mold, and explain why those atoms had to come from there. (1 point)

A full-credit answer: The mold gets its atoms from the bread, whose molecules it takes in as food, and from the water. They had to come from there because an organism cannot make atoms: every atom in a new molecule was taken in from the surroundings.

Check the box for each point your answer earns

the atoms come from the bread (and water) taken in as food, because an organism cannot make atoms.

Accept: ‘every atom in the mold was in the bread or the water first’.

Do not award: atoms from light or from the air alone, or ‘from the bread’ with no reason.

Common slip: Saying the mold makes its own material as it grows. Growth is new molecules, and every atom in them was taken in.

(b) New mold is built from all four classes of biological molecule. Identify the elements, beyond carbon, hydrogen and oxygen, that its proteins and its nucleic acids need, and name the part of each monomer that holds each element. (1 point)

A full-credit answer: Proteins need nitrogen, in the amine group of every amino acid, and sulfur, in the R groups of some amino acids. Nucleic acids need nitrogen, in every nitrogenous base, and phosphorus, in the phosphate group of every nucleotide.

Check the box for each point your answer earns

nitrogen in the amine group of every amino acid (proteins), and nitrogen in the nitrogenous base plus phosphorus in the phosphate group (nucleic acids).

Accept: sulfur in the R groups of some amino acids as an addition for proteins; sulfur is not required for the point. Nitrogen alone for proteins earns it.

Do not award: phosphorus placed in proteins, sulfur placed in nucleic acids, or nitrogen missing from either class.

Common slip: Giving nitrogen to nucleic acids only. Every amino acid has an amine group, so proteins need nitrogen too.

(c) Predict which classes of molecule the mold on Plate 2 can build from cellulose and water, and which classes it stops building. (1 point)

A full-credit answer: From cellulose and water the mold receives carbon, hydrogen and oxygen only. It can build carbohydrates and fats, which need nothing else, but it cannot build new proteins or new nucleic acids, which need nitrogen, and sulfur or phosphorus as well.

Check the box for each point your answer earns

carbohydrates and lipids can be built; proteins and nucleic acids cannot, for lack of nitrogen (and sulfur, phosphorus).

Accept: ‘only the classes made of carbon, hydrogen and oxygen’ for the first half, provided proteins and nucleic acids are named as the ones that cannot be built.

Do not award: ‘nothing can be built’, or proteins named as buildable.

Common slip: Predicting that the mold can build nothing at all. Cellulose supplies carbon, hydrogen and oxygen, which is enough for carbohydrates and fats.

(d) Explain why the mold barely grows on Plate 2 even though it takes in glucose from the cellulose. (1 point)

A full-credit answer: Growing means making new cells, and every new cell needs new proteins and new nucleic acids as well as carbohydrates and lipids. The glucose from the cellulose supplies carbon, hydrogen and oxygen, but Plate 2 supplies no nitrogen, phosphorus or sulfur, so those two classes cannot be built, and with no new proteins or nucleic acids no new cells can be made. Glucose alone is not enough to build a cell.

Check the box for each point your answer earns

new cells need proteins and nucleic acids, which need elements Plate 2 lacks, so the mold cannot make new cells even with glucose available.

Accept: ‘a cell needs all four classes, and two of them cannot be built here’.

Do not award: ‘cellulose is too hard to digest’ (the mold breaks cellulose into glucose and takes it in, as the description states), or ‘the mold has no energy’.

Common slip: Answering that the mold starves for energy. Glucose is available; what is missing is the nitrogen, phosphorus and sulfur that two of the four classes are built from.

Free-response score: 0 of 4

Multiple choice checked: 0 of 9 correct.

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