Unit 1: 55 extra questions

One per lesson part, in course order. Green = correct answer. Orange edge = a part with no question today (8). All 55 pass the rules checker, a blind answer check (55/55) and a taught-content check (everything needed is taught at or before that part). Adds 58.75 XP. Open a question's details to see where each idea was taught.
1U1-L01-P1 · Two atoms sharing electrons Multiple choiceNew✓ taught (fixed 9 Nov)
Tests: Recognize a covalent bond as a pair of electrons shared by two atoms (new molecule, Cl₂).

In Cl₂, one pair of electrons lies between the two chlorine atoms and is held by both. The atoms are joined by…

  1. A. a covalent bond
    Right: the two atoms share the pair, and a shared pair is a covalent bond.
  2. B. one atom taking the pair completely
    Neither atom takes the pair; both hold it, so it is shared.
  3. C. no bond at all
    A pair of electrons held by both atoms does join them: that is a bond.
Explanation: The pair of electrons is held by both chlorine atoms, so it is shared. A shared pair of electrons holding two atoms together is a covalent bond.
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Relies on (where it was taught):

  • covalent bond = shared pair — u1-l01-p1: “a shared pair of electrons holds two atoms together”
  • electron pair held by both atoms (Cl2 detail) — stem: “held by both”
  • distractor: ionic bond — NOT IN UNIT 1: “(term never named; ions only at u1-l01-p2)”
  • distractor: hydrogen bond — LATER: u1-l02-p3: “we call the attraction a hydrogen bond”

Taught-check fix: ionic/hydrogen bond (not yet taught) replaced with taught ideas

Already in this part:

  • Two pairs of atoms are drawn, P and Q. Which pair is joined by a covalent bond?

Changed: fixed: stem trimmed (C1); example, options and key kept; taught-check 9 Nov: ionic/hydrogen bond (not yet taught) replaced with taught ideas

Blind check: answered A; key A; ok

2U1-L01-P2 · An electron moves completely: ions Multiple choiceNew✓ taught
Tests: The atom that loses an electron completely becomes a positive ion (new pair, K and Br).

A potassium atom loses one electron completely to a bromine atom. The potassium is now…

  1. A. a negative ion
    The bromine gained the electron and became negative; the potassium lost it.
  2. B. a positive ion
    Right: losing a negative electron leaves potassium with one more proton than electrons, so it is K⁺.
  3. C. an uncharged atom
    An atom that has lost a whole electron is no longer neutral; it carries a full charge.
Explanation: The potassium has lost one negative electron, so it now has one more proton than electrons. It carries a full positive charge: it is a positive ion, K⁺.
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Relies on (where it was taught):

  • losing an electron makes an atom positive — u1-l01-p2: “The sodium has lost one electron, so it is now positive.”
  • ion = atom with full charge after losing/gaining electron — u1-l01-p2: “we call it an ion”
  • K and Br as atoms (new example) — stem: “loses one electron completely to a bromine atom”

Already in this part:

  • A lithium atom gives one electron to a fluorine atom. The electron moves across completely. Wha…

Changed: fixed: K/Br replaces Na/Cl, the article's own example (A4)

Blind check: answered B; key B; ok

3U1-L01-P4 · Partial charges Multiple choiceBank question u1-t11-q04✓ taught
Tests: Mark δ− on the atom with the stronger attraction for the shared pair and δ+ on the other (H–F).

In H–F, the shared pair of electrons is much more strongly attracted to fluorine than to hydrogen. The charges are…

  1. A. F is δ+, H is δ−
    This is reversed: the atom the pair sits closer to becomes δ−, and that is fluorine.
  2. B. F⁻ and H⁺, a full charge on each
    Full charges form only when an electron moves across completely; here the pair is still shared, so the charges are partial.
  3. C. F is δ−, H is δ+
    Right: the pair sits closer to fluorine, so fluorine is δ− and hydrogen is δ+.
  4. D. no charge on either atom
    Shared electrons can still sit unevenly; the uneven pair gives each atom a partial charge.
Explanation: The shared pair sits closer to fluorine, and electrons are negative, so fluorine is slightly negative, δ−. Hydrogen, with the pair held farther from it, is slightly positive, δ+. The pair is still shared, so the charges are partial, not full.
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Relies on (where it was taught):

  • δ−/δ+ partial charges on unequal sharing — u1-l01-p4: “We mark them δ− and δ+”
  • F attracts the pair more strongly — stem: “much more strongly attracted to fluorine”
  • full charge (ion) vs partial charge — u1-l01-p4: “Neither atom in a polar bond is an ion.”
  • ion notation F⁻, H⁺ — u1-l01-p2: “We write the sodium ion Na⁺ and the chloride ion Cl⁻.”

Already in this part:

  • Oxygen has a stronger attraction for shared electrons than carbon does. Which labels fit a C–O …

Changed: fixed: stem and options trimmed (C1, D4); same example, task and four options

Blind check: answered C; key C; ok

4U1-L02-P1 · Why a water molecule is polar Multiple choiceNew✓ taught
Tests: In a polar molecule, each hydrogen bonded to a more strongly attracting atom is δ+ (applied to NH₃).

In ammonia, NH₃, nitrogen attracts the shared electrons more strongly than hydrogen does. Each hydrogen carries…

  1. A. a partial positive charge, δ+
    Right: the pair sits closer to nitrogen, which leaves each hydrogen slightly positive.
  2. B. a partial negative charge, δ−
    This is reversed: the atom with the stronger attraction, nitrogen, is the δ− one.
  3. C. a full positive charge, H⁺
    A full charge needs a whole electron to move across; in N–H the pair is still shared.
Explanation: In each N–H bond the shared pair sits closer to the nitrogen. So the nitrogen is δ− and each hydrogen, with the pair held farther from it, is δ+. Ammonia, like water, is a polar molecule.
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Relies on (where it was taught):

  • N attracts shared electrons more than H — stem: “nitrogen attracts the shared electrons more strongly”
  • atom with weaker pull is δ+ — u1-l01-p4: “The hydrogen end... is slightly positive.”
  • full positive charge H⁺ (distractor) — u1-l01-p2: “Each now carries a whole unit of charge, a full charge.”

Already in this part:

  • Why is the oxygen in a water molecule slightly negative?

Changed: fixed: ammonia replaces water, whose hydrogens' charges the article states outright (A4)

Blind check: answered A; key A; ok

5U1-L02-P4 · Weak one at a time, strong together Multiple choiceNew✓ taught
Tests: A change of state in water breaks or forms only hydrogen bonds between molecules; the O–H bonds stay intact.

Steam condenses into droplets on a cold mirror. The bonds that form are…

  1. A. covalent O–H bonds inside water molecules
    The O–H bonds inside each molecule never broke when the water boiled, so none form now.
  2. B. hydrogen bonds between water molecules
    Right: condensing brings whole water molecules back into hydrogen bonds with one another.
  3. C. covalent bonds between water molecules
    Water molecules in a liquid are held to one another by hydrogen bonds, not by shared pairs of electrons.
Explanation: Steam is made of whole water molecules, because boiling breaks only the hydrogen bonds between molecules. As steam cools and condenses, the molecules slow down and hydrogen bonds form between them again. The O–H bonds inside each molecule stay intact the whole time.
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Relies on (where it was taught):

  • steam = whole water molecules; boiling breaks only H bonds — u1-l02-p4: “The covalent O–H bonds inside each molecule stay intact.”
  • hydrogen bond = δ+ H attracted to δ− O/N — u1-l02-p3: “we call the attraction a hydrogen bond”
  • hydrogen bonds break and re-form between molecules — u1-l02-p4: “hydrogen bonds break and new ones form”
  • intramolecular covalent vs intermolecular H bond — u1-l02-p3: “A covalent bond acts within one molecule.”
  • 'condenses' (gas to liquid droplets) — stem: “condenses into droplets”

Already in this part:

  • In liquid water, nearly every molecule is hydrogen-bonded to neighbors at any instant, yet each…

Changed: fixed: 'both' option removed (D3); condensing steam replaces the article's boiling example (A4)

Blind check: answered B; key B; ok

6U1-L03-P2 · Water is attracted to other things: adhesion TypedNew✓ taught
Tests: Name water's attraction to a different surface that carries partial charges as adhesion, apart from cohesion.

Water climbs up the fibers of a paper towel. Is the attraction of water to the fibers adhesion or cohesion?

Typed answer, accepted: adhesion, adhesive, adhesive force, adhesive forces, adhesion force, it is adhesion, it's adhesion, its adhesion
If wrong: Did you choose cohesion? Cohesion is water attracted to water; here water is attracted to the paper fibers, a different substance.
Explanation: The fibers are a different substance from water. Water attracted to a different surface that carries partial charges is adhesion. Cohesion is water attracted to water.
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Relies on (where it was taught):

  • adhesion = water sticking to something other than water — u1-l03-p2: “When water sticks to something other than water, we call it adhesion.”
  • cohesion = water to water — u1-l03-p1: “When water sticks to water like this, we call it cohesion.”
  • fibers are a different substance from water — stem: “the fibers of a paper towel”

Already in this part:

  • Two identical narrow tubes stand in a dish of water. One is glass. The inside of the other is c…

Changed: fixed: two-option MCQ became typed (B2); paper towel replaces glass, the article's surface (A4)

Blind check: answered adhesion; key ['adhesion', 'adhesive', 'adhesive force', 'adhesive forces', 'adhesion force', 'it is adhesion', "it's adhesion", 'its adhesion']; ok

7U1-L03-P3 · The surface holds together: surface tension Multiple choiceNew✓ taught (fixed 9 Nov)
Tests: Surface tension holds a dense object on an unbroken water surface (new example, paper clip).

A steel paper clip sinks if pushed under the water. Laid flat and gently on still water, it stays on top. It is held up by…

  1. A. adhesion to the steel
    Adhesion is water attracted to another surface; it does not explain why the surface resists being broken.
  2. B. its buoyancy in water
    Steel is denser than water, so buoyancy alone cannot hold it up; pushed under, the clip sinks.
  3. C. surface tension
    Right: the tightly held surface resists being broken, so it supports the clip.
Explanation: Water molecules at the surface are attracted sideways and inward to their neighbors, so the surface is held tight. The clip presses on that tight surface without breaking through, so surface tension holds it up. Steel is denser than water, so the clip sinks once the surface breaks.
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Relies on (where it was taught):

  • surface tension holds up an object pressing on the surface — u1-l03-p3: “The tightly held surface holds it up.”
  • adhesion (distractor) — u1-l03-p2: “we call it adhesion”
  • distractor: buoyancy — NOT IN UNIT 1
  • steel is denser than water / sinks once surface breaks — LATER: u1-l05b-p1: “we say the substance is less dense”

Taught-check fix: stem now says steel sinks when pushed under, so buoyancy is ruled out by taught facts

Already in this part:

  • Why is a water molecule at the surface attracted inward, toward the rest of the liquid?

Changed: fixed: two-option MCQ (B2) now has three options with the buoyancy misconception; paper clip replaces needle; taught-check 9 Nov: stem now says steel sinks when pushed under, so buoyancy is ruled out by taught facts

Blind check: answered C; key C; ok

8U1-L04-P1 · Water is slow to warm: specific heat capacity Multiple choiceNew (replaces bank u1-t11-q13)✓ taught
Tests: The substance that warms less from the same energy per gram has the higher specific heat capacity.

50 g each of water and dry sand take in the same energy. The sand ends up much hotter. The higher specific heat capacity belongs to…

  1. A. the water
    Right: the same energy warmed the water less, so water needs more energy per gram per degree.
  2. B. the sand
    The sand warmed more from the same energy, so it needs less energy per degree, not more.
  3. C. neither; they are equal
    Taking in the same energy does not mean the same specific heat capacity; the different temperature rises show they differ.
Explanation: Specific heat capacity is the energy needed to warm one gram of a substance by one degree. The water and the sand took in the same energy, but the water warmed less. So water needs more energy per gram per degree: its specific heat capacity is higher.
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Relies on (where it was taught):

  • specific heat capacity definition — u1-l04-p1: “energy needed to warm one gram of a substance by one degree”
  • same energy, smaller rise = higher specific heat — u1-l04-p1: “water needs far more energy to warm than oil, rock or sand”

Already in this part:

  • Equal masses of water and of rubbing alcohol, in identical sealed flasks, sit on identical hot …

Changed: replaced: table removed (A3) and the article's own water/oil pair swapped for sand (A4); checker's 'water echoed in key' is a false alarm, both substances are named in the stem

Blind check: answered A; key A; ok

9U1-L04-P3 · A steady body temperature: homeostasis Multiple choiceNew✓ taught
Tests: A body with more water changes temperature more slowly when it loses thermal energy.

Two animals of equal mass lose the same thermal energy. One is 80% water, the other 60%. Compared with the other, the 80%-water animal's temperature drops…

  1. A. more
    Water slows temperature change, so more water gives a smaller drop, not a larger one.
  2. B. less
    Right: more water means more energy per degree, so the same energy loss gives a smaller drop.
  3. C. by the same amount
    Equal energy loss gives equal drops only if both bodies need the same energy per degree; more water means more energy per degree.
Explanation: Water needs far more energy than most other substances to change temperature by one degree. The animal with more water in its body changes temperature more slowly. So, losing the same energy, its temperature drops less.
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Relies on (where it was taught):

  • water changes temperature slowly (high specific heat) — u1-l04-p1: “Water’s specific heat capacity is unusually high.”
  • organism mostly water so temperature changes slowly — u1-l04-p3: “its temperature changes only slowly”
  • thermal energy — u1-l04-p1: “how much thermal energy the substance takes in”

Already in this part:

  • A person rests in a warm room for an hour. Thermal energy transfers from the room into their bo…

Changed: fixed: options X/Y/Neither reworded so each stands without labels (D2)

Blind check: answered B; key B; ok

10U1-L05-P1 · Water is slow to evaporate: heat of vaporization Multiple choiceNew✓ taught
Tests: Water's heat of vaporization is high: a gram of water takes more energy to vaporize than a liquid without hydrogen bonds.

Hexane forms no hydrogen bonds. Compared with turning 1 g of hexane into gas, turning 1 g of water into gas takes…

  1. A. less energy
    Molecules held by hydrogen bonds take more energy to break free, not less.
  2. B. the same energy
    Equal masses do not take equal energy; how strongly the molecules attract one another decides it.
  3. C. more energy
    Right: water molecules must break their hydrogen bonds to escape, so each gram takes more energy.
Explanation: Every water molecule that escapes into the gas must first break its hydrogen bonds to its neighbors, and that takes energy. Hexane molecules attract one another far more weakly. So water's heat of vaporization is higher: each gram takes more energy to turn into gas.
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Relies on (where it was taught):

  • hexane forms no hydrogen bonds — stem: “Hexane forms no hydrogen bonds.”
  • vaporizing water must break hydrogen bonds; takes energy — u1-l05-p1: “must first break its hydrogen bonds to its neighbors. That takes energy.”
  • weaker attractions = less energy to vaporize — u1-l05-p1: “each one breaks free with less energy”

Already in this part:

  • Rubbing alcohol evaporates from skin faster than water does, yet each gram of alcohol that evap…

Changed: fixed: 'both the same' option removed (D3); a real liquid (hexane) named

Blind check: answered C; key C; ok

11U1-L05-P2 · Evaporative cooling Multiple choiceNew✓ taught
Tests: Water evaporating from a surface cools that surface (evaporative cooling), on a new example.

An elephant sprays water over its back. As the water evaporates, the elephant's skin…

  1. A. cools
    Right: the escaping molecules carry energy away, so the skin left behind cools.
  2. B. warms
    Evaporation removes energy from the surface; it adds none.
  3. C. stays at the same temperature
    Each escaping molecule takes energy with it, so the skin's temperature falls.
Explanation: Only the fastest water molecules break their hydrogen bonds and escape as vapor, and they carry their energy away. The water left on the skin is cooler, and so is the skin. This is evaporative cooling.
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Relies on (where it was taught):

  • evaporating water cools the surface — u1-l05-p2: “we call it evaporative cooling”
  • fastest molecules leave, carrying energy — u1-l05-p2: “Each escaping molecule takes its energy with it.”

Already in this part:

  • On a very humid day, sweat runs off your skin in drops instead of evaporating, and you feel muc…

Changed: fixed: elephant replaces the panting dog, which the article uses (A4)

Blind check: answered A; key A; ok

12U1-L05B-P1 · Ice floats Multiple choiceNew✓ taught
Tests: The same mass of water takes up more room as ice than as liquid.

A sealed plastic bottle, full to the brim with liquid water, freezes solid overnight. The bottle…

  1. A. caves inward
    Ice takes up more room than liquid water, not less, so the bottle cannot cave in.
  2. B. bulges outward
    Right: the same mass takes up more room as ice, so the bottle bulges.
  3. C. keeps its shape
    The mass stays the same, but the room it takes up grows as the water freezes.
Explanation: As water freezes, each molecule settles into four hydrogen bonds that hold the molecules farther apart than in the liquid. So the same mass of water takes up more room as ice. The bottle is full, so the ice presses it outward.
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Relies on (where it was taught):

  • same mass takes more room as ice — u1-l05b-p1: “the same mass, takes up more room as ice”
  • bottle full and sealed — stem: “full to the brim”

Already in this part:

  • Two drawings, P and Q, show water molecules. Which drawing shows ice?

Changed: fixed: the 'because' clause cut from each option so each tests one thing (A1)

Blind check: answered B; key B; ok

13U1-L06-P2 · Joining two units Multiple choiceBank question u1-t13-q04✓ taught
Tests: After dehydration synthesis, a covalent bond holds the two monomers together.

Two monomers have just been joined by dehydration synthesis. They are now held together by…

  1. A. a hydrogen bond
    A hydrogen bond is a weak attraction between partial charges; the join made here is a shared pair of electrons.
  2. B. the released water molecule
    The water molecule leaves; it is no part of the link between the monomers.
  3. C. a covalent bond
    Right: the monomers are now joined through the remaining oxygen by a shared pair of electrons.
Explanation: In dehydration synthesis, an –OH leaves one monomer and an H leaves the other, and they go off together as one water molecule. The oxygen that stayed now shares a pair of electrons with a carbon of the other monomer. That shared pair is a covalent bond.
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Relies on (where it was taught):

  • dehydration synthesis joins monomers — u1-l06-p2: “is called dehydration synthesis”
  • joined through oxygen by a covalent bond — u1-l06-p2: “joined through that oxygen by a covalent bond”
  • monomer — u1-l06-p1: “is called a monomer”
  • hydrogen bond = δ+ H attracted to δ− O/N — u1-l02-p3: “we call the attraction a hydrogen bond”

Already in this part:

  • Two sugar units are joined by dehydration synthesis. The pieces removed from them join as one w…

Changed: fixed: option D cut (it overlapped the hydrogen-bond option, D2); stem now an incomplete sentence (C1)

Blind check: answered C; key C; ok

14U1-L06-P3 · Breaking the bond with water Multiple choiceNew (replaces bank u1-t13-q07)✓ taught
Tests: In hydrolysis a water molecule is taken in at each bond that breaks.

In the gut, lactose is split into its two sugar units, glucose and galactose. At the bond that breaks, a water molecule is…

  1. A. taken in
    Right: hydrolysis adds a water molecule to the bond it breaks.
  2. B. released
    Water is released when units join, by dehydration synthesis; splitting them takes water in.
  3. C. neither taken in nor released
    The bond breaks only when a water molecule is added to it, so water does take part.
Explanation: Splitting the bond between two sugar units is hydrolysis. A water molecule joins the bond and splits into an H and an –OH. The –OH goes to one unit and the H to the other, so the water is taken in.
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Relies on (where it was taught):

  • splitting a bond between units adds water (hydrolysis) — u1-l06-p3: “adding water to a bond is called hydrolysis”
  • digestion in the gut is hydrolysis — u1-l06-p3: “Digestion in your gut is hydrolysis.”
  • lactose = glucose + galactose — stem: “split into its two sugar units, glucose and galactose”

Already in this part:

  • A water molecule joins the bond between monomers M and N, and the bond breaks. Afterwards, N ca…

Changed: replaced: v1 asked two things (A1) on the article's bread example (A4); now one thing, on lactose

Blind check: answered A; key A; ok

15U1-L07-P2 · Where the mass goes Multiple choiceBank question u1-t13-q10✓ taught
Tests: When two monomers join, the missing mass leaves as one water molecule; no atoms are destroyed.

Two glucose units join into maltose. Maltose weighs less than the two glucose units did together. The missing mass…

  1. A. was destroyed as the bond formed
    Atoms are rearranged in a reaction, never destroyed; the mass left in a water molecule.
  2. B. left as one water molecule
    Right: one join releases one water molecule, and its mass is the mass the product lacks.
  3. C. left as two water molecules
    One join releases one water molecule, an –OH from one unit and an H from the other, not a whole water from each.
Explanation: When two monomers join by dehydration synthesis, an –OH from one and an H from the other leave together as one water molecule. Atoms are never destroyed in a reaction. The missing mass is the mass of that one water molecule.
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Relies on (where it was taught):

  • product weighs less by one water molecule — u1-l07-p2: “weighs less, by exactly one water molecule”
  • atoms never destroyed — u1-l07-p2: “atoms are rearranged, never created or destroyed”
  • one join releases one water — u1-l07-p1: “one water molecule moves for every join”
  • maltose = two glucose units joined — stem: “Two glucose units join into maltose.”

Already in this part:

  • A polymer is fully broken apart into its monomers. Compared with the polymer, the pile of monom…

Changed: fixed: named a real pair (glucose to maltose) so the stem no longer echoes the article (A4); options trimmed

Blind check: answered B; key B; ok

16U1-L07-P3 · Polymerization, and its reverse Multiple choiceBank question u1-ut1-q12✓ taught
Tests: Whatever the monomer, water is released when a polymer is built and taken in when it is broken down.

A bacterium builds a protein from amino acids, then later breaks it back down. Water is…

  1. A. taken in as the protein is built, released as it is broken down
    This is reversed: building is dehydration synthesis, which releases water.
  2. B. released in each reaction
    Breaking the protein down is hydrolysis, which takes water in.
  3. C. released as the protein is built, taken in as it is broken down
    Right: joining releases water and hydrolysis takes it back in, whatever the monomer.
  4. D. taken in by each reaction
    Building the protein is dehydration synthesis, which releases water.
Explanation: Each amino acid joins the chain by dehydration synthesis, and one water molecule leaves. Breaking the protein down is hydrolysis, and each bond broken takes one water molecule back in. The rule is the same for every kind of monomer.
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Relies on (where it was taught):

  • building any polymer releases water; breaking takes it in — u1-l07-p3: “out when units join, in when they separate”
  • rule holds for any kind of monomer — u1-l07-p3: “That is true whatever kind of unit is being joined.”
  • protein is built from amino acid units — stem: “builds a protein from amino acids”
  • protein / amino acid as terms — LATER: u1-l17-p1: “Amino acids are the monomers of proteins.”

Already in this part:

  • Inside one cell, one chain is being extended while an identical chain nearby is being taken apa…

Changed: fixed: 'both' options reworded (D3), stem trimmed (C1); bank id kept, but u1-ut1 item: confirm no unit test draws it (G2)

Blind check: answered C; key C; ok

17U1-L08-P1 · Glucose is a monosaccharide Multiple choiceNew (replaces bank u1-t14-q02)✓ taught
Tests: A single sugar unit is a monosaccharide (new example, fructose).

Fructose, the sugar in fruit, is a single sugar unit. Fructose is a…

  1. A. monosaccharide
    Right: one sugar unit is a monosaccharide, mono for one.
  2. B. polysaccharide
    A polysaccharide is many sugar units joined into a chain; fructose is just one.
  3. C. disaccharide
    A disaccharide is two sugar units joined together; fructose is a single unit.
Explanation: A monosaccharide is a single sugar unit that cannot be split into a smaller sugar. Fructose is one sugar unit, so it is a monosaccharide, like glucose. Monosaccharides are the monomers of carbohydrates.
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Relies on (where it was taught):

  • monosaccharide = single sugar unit — u1-l08-p1: “A single sugar unit that cannot be split into a smaller sugar”
  • fructose is one sugar unit — stem: “is a single sugar unit”
  • distractor: polysaccharide — LATER: u1-l08-p2: “form a polysaccharide”
  • distractor: disaccharide — NOT IN UNIT 1

Already in this part:

  • Every bond in a carbohydrate is broken. What is left is many small, identical molecules, each a…

Changed: replaced: v1 reused the article's blood-glucose example (A4) and the key was the longest option (D4)

Blind check: answered A; key A; ok

18U1-L08-P2 · Sugars joined into a chain Multiple choiceNew (replaces bank u1-t14-q06)✓ taught (fixed 9 Nov)
Tests: The sugar units of a polysaccharide are joined to one another by covalent bonds.

Cellulose, in cotton, is a chain of thousands of glucose units. Each unit is joined to the next by…

  1. A. a hydrogen bond
    Hydrogen bonds hold neighboring cellulose chains side by side, but the units along one chain share electrons.
  2. B. a covalent bond
    Right: each join is made by dehydration synthesis, which forms a covalent bond.
  3. C. an attraction between ions
    No electron moves across completely, so no ions form; the two atoms share a pair of electrons.
Explanation: Each glucose unit joins the next by dehydration synthesis. That reaction leaves the two units sharing a pair of electrons through an oxygen. A shared pair is a covalent bond, so a polysaccharide is monosaccharides joined by covalent bonds.
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Relies on (where it was taught):

  • polysaccharide units joined by covalent bonds — u1-l08-p2: “Many monosaccharides joined to one another by covalent bonds”
  • cellulose = chain of glucose — stem: “a chain of thousands of glucose units”
  • hydrogen bond = δ+ H attracted to δ− O/N — u1-l02-p3: “we call the attraction a hydrogen bond”
  • distractor: ionic bond — NOT IN UNIT 1

Taught-check fix: 'ionic bond' (never named in Unit 1) -> 'an attraction between ions'

Already in this part:

  • A plant cell joins 500 glucose units end to end into one molecule. What does the cell make, and…

Changed: replaced: v1 options combined label entries (D3); now one bond type, in cellulose; taught-check 9 Nov: 'ionic bond' (never named in Unit 1) -> 'an attraction between ions'

Blind check: answered B; key B; ok

19U1-L08-P3 · Linear or branched TypedBank question u1-t14-q08✓ taught
Tests: Classify a polysaccharide with chains coming off its sides as branched.

A polysaccharide is a chain of 300 sugar units, with shorter chains joined to its sides at 20 places. Is it linear or branched?

Typed answer, accepted: branched, it is branched, it's branched, its branched, branched polysaccharide, a branched polysaccharide, branched chain, a branched chain, branching
If wrong: Did you call it linear because most of it is one long chain? Any side chain makes a polysaccharide branched.
Explanation: A polysaccharide with side chains along its length is branched, however long its main chain is. Linear means nothing comes off the sides.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • branched = side chains along its length — u1-l08-p3: “side chains coming off along its length is a branched polysaccharide”
  • linear = nothing off sides — u1-l08-p3: “nothing coming off its sides is a linear polysaccharide”

Already in this part:

  • Look at the two polysaccharides, X and Y. Which statement is correct?

Changed: fixed: options mixed the answer with a reason (A1); now a typed two-way judgment (B2), same description

Blind check: answered branched; key ['branched', 'it is branched', "it's branched", 'its branched', 'branched polysaccharide', 'a branched polysaccharide', 'branched chain', 'a branched chain', 'branching']; ok

20U1-L08-P4 · Many ends, quick release Multiple choiceNew✓ taught (fixed 9 Nov)
Tests: Sugar units come off a stored polysaccharide only at chain ends, so many branches give fast release.

During a sprint, a muscle cell breaks down glycogen, its branched store of glucose. It removes glucose units…

  1. A. from the branch points
    Branch points are where side chains join; units are removed from the ends of the chains.
  2. B. from anywhere along a chain
    Units come off one at a time from chain ends, never from the middle of a chain.
  3. C. from the ends of chains
    Right: units come off only at chain ends, and glycogen's many branches give it many ends.
Explanation: Glucose units come off a polysaccharide only at the ends of its chains, one unit at a time. Glycogen is branched, and every branch adds another end. So many units can come off at once.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • units come off only at chain ends — u1-l08-p4: “takes sugar units off a polysaccharide at the ends of its chains”
  • glycogen is a branched glucose polysaccharide — LATER: u1-l09-p1: “Their store is glycogen: chains of glucose branched even more heavily”
  • branch adds working ends — u1-l08-p4: “Every branch adds another working end.”

Taught-check fix: glycogen glossed (taught only at l09-p1)

Already in this part:

  • Two glucose stores hold the same number of glucose units. Store A is one linear chain. Store B …

Changed: fixed: new context (muscle glycogen); 'only from the middle' replaced by the branch-point misconception so options are exclusive (D2); taught-check 9 Nov: glycogen glossed (taught only at l09-p1)

Blind check: answered C; key C; ok

21U1-L10-P2 · What water is attracted to Multiple choiceBank question u1-ut1-q18✓ taught
Tests: A hydrocarbon offers water no partial charges, so it separates; molecules with –OH groups mix in.

Each liquid is stirred into water. The one that separates into its own layer is…

  1. A. octane, C₈H₁₈
    Right: octane is a hydrocarbon with no partial charges, so water excludes it.
  2. B. ethanol, C₂H₅OH
    Ethanol's –OH group carries partial charges, and water is attracted to them, so ethanol mixes in.
  3. C. glycerol, C₃H₅(OH)₃
    Glycerol has three –OH groups, each attracted to water, so it mixes in.
Explanation: Octane is a hydrocarbon: it has only C–H and C–C bonds, so it carries no partial charges for water to be attracted to. The water molecules stay attracted to one another and exclude it. Ethanol and glycerol carry polar –OH groups, so they mix in.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • hydrocarbon (only C and H) carries no partial charges — u1-l10-p2: “carbon and hydrogen atoms and nothing else is called a hydrocarbon”
  • –OH groups are polar; water attracted, mixes in — u1-l10-p2: “Each –OH group is polar, so it carries partial charges”
  • hydroxyl group –OH — u1-l06-p2: “is called a hydroxyl group”
  • hydrophobic substance separates into its own layer — u1-l10-p1: “each separates into a layer of its own”
  • formulas show octane has no O, others have OH — stem: “octane, C₈H₁₈ / ethanol, C₂H₅OH”

Already in this part:

  • Two molecules are about the same size. Molecule P carries several –OH groups. Molecule Q is bui…

Changed: fixed: four-liquid stem cut to 14 words (A3, C1); oil swapped for octane so its structure shows; bank id kept, u1-ut1 item: confirm no unit test draws it (G2)

Blind check: answered A; key A; ok

22U1-L10-P3 · The fatty acid TypedNew✓ taught
Tests: A fatty acid is mostly hydrophobic because its hydrocarbon tail is most of the molecule.

Lauric acid has an 11-carbon hydrocarbon tail and a –COOH group at one end. As a whole, is it mostly hydrophobic or mostly hydrophilic?

Typed answer, accepted: hydrophobic, mostly hydrophobic, it is mostly hydrophobic, it's mostly hydrophobic, it is hydrophobic, it's hydrophobic, hydrophobic mostly
If wrong: Did you go by the –COOH group alone? It is one small end; the long hydrocarbon tail makes the molecule mostly hydrophobic.
Explanation: Water is attracted to the partial charges on the –COOH group. But the hydrocarbon tail is most of the molecule, and it offers water nothing to be attracted to. So the fatty acid as a whole is mostly hydrophobic.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • fatty acid mostly hydrophobic: tail is most of molecule — u1-l10-p3: “So a fatty acid is mostly hydrophobic”
  • carboxyl group carries partial charges — u1-l10-p3: “The carboxyl group carries partial charges”
  • lauric acid structure — stem: “11-carbon hydrocarbon tail and a –COOH group”

Already in this part:

  • Here are two molecules, numbered 1 and 2. Each molecule is a long chain of carbon and hydrogen …

Changed: fixed: two-way judgment became typed (B2); 'fully charged' distractor dropped; a real fatty acid named

Blind check: answered hydrophobic; key ['hydrophobic', 'mostly hydrophobic', 'it is mostly hydrophobic', "it's mostly hydrophobic", 'it is hydrophobic', "it's hydrophobic", 'hydrophobic mostly']; ok

23U1-L11-P2 · Molecules attract when they lie close Multiple choiceNew✓ taught
Tests: Weak attractions between nonpolar molecules are stronger the more of their length is in contact.

Pentane has 5 carbons in a chain; hexadecane has 16. Both are nonpolar. Compared with two pentane molecules, two hexadecane molecules side by side attract each other…

  1. A. less strongly
    Longer chains have more length in contact, which makes the attraction stronger, not weaker.
  2. B. more strongly
    Right: longer chains touch along more of their length, so the weak attractions add up.
  3. C. equally strongly
    Neither carries a charge, but the attraction still grows with the length in contact.
Explanation: Any two molecules attract weakly where they lie close. Hexadecane is longer, so two of its molecules side by side touch along more of their length. More contact means a stronger attraction.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • more length in contact = stronger attraction — u1-l11-p2: “the more of their length is in contact, the stronger the attraction”
  • nonpolar molecules still attract weakly — u1-l11-p2: “even when neither carries a charge or a partial charge”
  • chain lengths, both nonpolar — stem: “Pentane has 5 carbons... hexadecane has 16”

Already in this part:

  • Two hexane molecules drift close together inside a drop of oil. Both molecules are nonpolar. Wh…

Changed: fixed: new example (pentane vs hexadecane); 'are far apart' was no misconception (D1)

Blind check: answered B; key B; ok

24U1-L11-P4 · Why the kink decides solid or liquid Multiple choiceNew✓ taught
Tests: Straight (saturated) tails pack closer together than kinked ones.

Coconut oil's fatty-acid tails are mostly saturated and straight; sunflower oil's are mostly kinked. Compared with sunflower oil's tails, coconut oil's tails pack…

  1. A. farther apart
    This is reversed: kinks keep tails apart, so it is sunflower oil's tails that pack farther apart.
  2. B. equally close
    Kinks limit how close tails can get, so straight and kinked tails cannot pack equally close.
  3. C. closer together
    Right: straight tails line up close along their whole length; kinks keep tails apart.
Explanation: A saturated tail has only single bonds, so it runs straight. Straight tails can lie close against their neighbors along their whole length. Kinked tails cannot, so coconut oil's tails pack closer together.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • straight tails pack close; kinked cannot — u1-l11-p4: “Kinked tails cannot lie close against their neighbors”
  • saturated = straight — u1-l11-p1: “joined only by single bonds, so that its tail runs straight”
  • which oil is straight vs kinked — stem: “mostly saturated and straight; sunflower oil's are mostly kinked”

Already in this part:

  • A manufacturer treats a liquid vegetable oil so that most of the double bonds in its tails beco…

Changed: kept: same example and key; minor trim

Blind check: answered C; key C; ok

25U1-L12-P2 · Why the products hold less energy part had no questionTypedNew✓ taught
Tests: Breaking a chemical bond takes energy in; energy is released only when stronger bonds form.

Splitting an O₂ molecule into two oxygen atoms breaks a covalent bond. Does breaking that bond take in energy or release energy?

Typed answer, accepted: take in, takes in, take in energy, takes in energy, takes energy in, take energy in, it takes in energy, absorb, absorbs, absorbs energy, absorb energy, it absorbs energy, requires energy, uses energy, takes energy, needs energy, endothermic, is endothermic
If wrong: Did you answer 'release'? Energy is not stored in a bond, ready to come out; breaking any bond takes energy in, and energy is released only when stronger bonds form.
Explanation: Atoms joined by a covalent bond are held together, so separating them takes energy in. Energy comes out of a reaction only when the new bonds that form are stronger than the bonds broken.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • breaking bonds takes energy in — u1-l12-p2: “Breaking bonds on its own only takes energy in.”
  • covalent bond = shared pair — u1-l01-p1: “a shared pair of electrons holds two atoms together”

Already in this part:

  • none

Changed: fixed: two-option MCQ became typed (B2); a specific bond (O₂) named

Blind check: answered take in; key ['take in', 'takes in', 'take in energy', 'takes in energy', 'takes energy in', 'take energy in', 'it takes in energy', 'absorb', 'absorbs', 'absorbs energy', 'absorb energy', 'it absorbs energy', 'requires energy', 'uses energy', 'takes energy', 'needs energy']; ok

26U1-L12-P3 · Fat or glucose part had no questionTypedNew✓ taught
Tests: Gram for gram a fat gives more than twice a sugar's energy, so an animal that carries its fuel stores fat.

A monarch butterfly flies thousands of kilometers on its migration, carrying its own fuel. Does it store that fuel mainly as fat or as glycogen?

Typed answer, accepted: fat, fats, as fat, mainly fat, mainly as fat, lipid, lipids, fat (lipid), triglyceride, triglycerides, oil
If wrong: Did you pick glycogen because cells use glucose quickly? For a long flight the store is carried, and fat holds more than twice the energy per gram.
Explanation: Gram for gram, a fat gives more than twice the energy of a sugar store such as glycogen. So the butterfly carries the same energy in less mass, and has less mass to lift on every wingbeat.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • fat gives more than twice the energy per gram — u1-l12-p3: “Gram for gram, a fat gives more than twice the energy”
  • flyer stores fat: less mass to lift — u1-l12-p3: “Less mass stored in its body means less mass to lift”
  • glycogen = animal glucose store — u1-l09-p1: “Their store is glycogen”
  • monarch actually stores fat (real-world) — stem: “answerable from the principle; fact not required”

Already in this part:

  • none

Changed: fixed: two-option MCQ became typed (B2); monarch replaces dandelion seed (safer science, F7); glycogen replaces starch for an animal

Blind check: answered fat; key ['fat', 'fats', 'as fat', 'mainly fat', 'mainly as fat', 'lipid', 'lipids', 'fat (lipid)', 'triglyceride', 'triglycerides', 'oil']; ok

27U1-L12B-P1 · Three fatty acids on a glycerol NumberNew✓ taught
Tests: One water molecule leaves at each fatty-acid join, so three leave per fat.

A cell builds 4 fat molecules, each from three fatty acids and one glycerol. How many water molecules leave in all?

Number answer, accepted: 12, twelve, 12.0
If wrong: Did you count one water per fat (4) instead of one per fatty acid joined (3 per fat)? Or did you count the glycerols as joins too (16)?
Explanation: Each fatty acid joins the glycerol by dehydration synthesis, and one water molecule leaves at each join. One fat has three joins, so three water molecules leave. Four fats: 4 × 3 = 12 water molecules.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • fat = 3 fatty acids + glycerol; 3 waters leave — u1-l12b-p1: “three water molecules leave when a fat is built”
  • multiply by number of fats — stem: “builds 4 fat molecules”

Already in this part:

  • Here is a fat. What is a fat made of?

Changed: fixed: v1 asked the article's own worked count of 3 (A4); now 4 fats, key 12

Blind check: answered 12; key ['12', 'twelve', '12.0']; ok

28U1-L12B-P2 · Two jobs for a fat part had no questionMultiple choiceNew✓ taught
Tests: Besides storing energy, a fat layer under the skin slows heat escaping (insulation).

A harbor seal swims in icy water. Besides storing energy, the thick blubber under its skin…

  1. A. slows heat escaping from its body
    Right: blubber is insulation, so heat escapes the body more slowly.
  2. B. speeds heat escaping from its body
    Fat slows heat escaping; it does not speed it up.
  3. C. has no effect on heat escaping
    A thick fat layer is insulation, so it does slow heat loss.
Explanation: A thick layer of fat under the skin is insulation. Heat made inside the body escapes more slowly through it, so the seal stays warm in icy water.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • fat layer under skin slows heat escaping — u1-l12b-p2: “A thick layer of fat under the skin slows heat escaping”
  • blubber = fat layer — u1-l12b-p2: “the whale’s blubber does two jobs”

Already in this part:

  • none

Changed: fixed: combined 'and'/'only' options removed (D3); now asks the second job alone

Blind check: answered A; key A; ok

29U1-L12C-P2 · Many steroids are hormones Multiple choiceNew✓ taught (fixed 9 Nov)
Tests: A steroid made in one place and carried in the blood to change another part acts as a hormone.

Progesterone is a steroid made in the ovaries and carried in the blood to the uterus, where it changes the uterine lining. Progesterone acts as…

  1. A. an energy store
    Fats store energy; progesterone is carried in the blood to change another organ.
  2. B. a hormone
    Right: it is made in one place and changes another, so it is a chemical signal, a hormone.
  3. C. insulation under the skin
    Fat under the skin slows heat escaping; progesterone is a signal carried in the blood.
Explanation: A hormone is a chemical signal made in one part of the body that changes what happens somewhere else. Progesterone is made in the ovaries and acts on the uterus, so it is a steroid hormone.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • hormone = chemical signal made in one place acting elsewhere — u1-l12c-p2: “made in one part of the body to change what happens”
  • progesterone made in ovaries, acts on uterus — stem: “made in the ovaries and carried in the blood to the uterus”
  • fat as energy store (distractor) — u1-l12b-p2: “A fat is a store of energy.”
  • distractor: cell membrane — LATER: u1-l13-p2: “It is also called the cell membrane”

Taught-check fix: cell membrane (taught later) -> insulation (taught at l12b)

Already in this part:

  • A person’s adrenal glands stop releasing their steroid hormones. The level of glucose in their …

Changed: fixed: progesterone replaces testosterone, the article's own example (A4); taught-check 9 Nov: cell membrane (taught later) -> insulation (taught at l12b)

Blind check: answered B; key B; ok

30U1-L13-P1 · The phospholipid: one molecule, two sides Multiple choiceNew✓ taught
Tests: A phospholipid has two fatty-acid tails, where a fat has three.

Lecithin, a phospholipid in egg yolk, carries…

  1. A. three fatty-acid tails
    Three tails on a glycerol make a fat; a phospholipid has a phosphate head in place of the third.
  2. B. one fatty-acid tail
    A phospholipid has two tails; the phosphate head takes only the third position.
  3. C. two fatty-acid tails
    Right: a phospholipid has two tails, with the phosphate head in place of a third.
Explanation: A phospholipid is glycerol joined to two fatty-acid tails, with a head containing a phosphate group where a fat's third tail would be. Lecithin is a phospholipid, so it has two tails.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • phospholipid has two fatty-acid tails — u1-l13-p1: “two fatty-acid tails and a head containing a phosphate group”
  • lecithin is a phospholipid — stem: “Lecithin, a phospholipid”
  • fat has three tails (distractor) — u1-l12b-p1: “Three fatty acids are joined to one small three-carbon molecule.”

Already in this part:

  • A single phospholipid is placed in water. Which parts of it are attracted to the water?

Changed: fixed: a real phospholipid (lecithin) as the new example; incomplete-sentence stem (C1)

Blind check: answered C; key C; ok

31U1-L13B-P1 · Cholesterol in the membrane Multiple choiceNew✓ taught
Tests: Most of a cholesterol molecule sits among the phospholipid tails because it is nonpolar.

In a red blood cell's membrane, the four carbon rings of each cholesterol molecule lie…

  1. A. among the phospholipid tails
    Right: the nonpolar rings stay among the nonpolar tails, away from the water.
  2. B. among the phospholipid heads
    The heads face the water; the nonpolar rings stay among the tails, and only cholesterol's –OH reaches the heads.
  3. C. in the water outside the cell
    Cholesterol is mostly nonpolar, so water excludes it; it stays inside the membrane.
Explanation: Cholesterol's four rings are nonpolar: they carry no charge or partial charge for water to be attracted to. So they sit among the phospholipid tails, in the part of the membrane water does not reach. Only its single –OH sits beside the heads.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • cholesterol rings sit among phospholipid tails — u1-l13b-p1: “cholesterol sits inside the membrane, wedged among the phospholipid tails”
  • rings nonpolar, stay where water does not reach — u1-l13b-p1: “stay in the part of the membrane that water does not reach”
  • heads face water — u1-l13-p2: “The heads are hydrophilic, so they face the water.”

Already in this part:

  • Two batches of animal cells are compared. Cholesterol has been removed from the membranes of on…

Changed: fixed: red blood cell as new example; asks about the rings so the –OH near the heads is no second right answer (D1)

Blind check: answered A; key A; ok

32U1-L13B-P2 · Heat and cold in the membrane Multiple choiceNew✓ taught
Tests: More double bonds (kinks) in the tails keep a membrane fluid at a lower temperature.

An arctic fish's membrane tails have more double bonds than a tropical fish's. At 2 °C, compared with the tropical fish's membranes, the arctic fish's membranes are…

  1. A. less fluid
    This is reversed: double bonds add kinks, which keep tails apart and the membrane fluid.
  2. B. more fluid
    Right: more kinks mean looser packing and weaker attractions, so the membrane stays fluid in the cold.
  3. C. equally fluid
    At the same temperature, tails with more kinks pack more loosely, so the fluidity differs.
Explanation: Each double bond puts a kink in a tail. Kinked tails cannot pack close, so the attractions between them stay weak and the molecules keep sliding past one another. So the arctic fish's membranes stay more fluid in the cold.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • more double bonds keep membrane liquid at lower temperature — u1-l13b-p2: “The membrane stays liquid at a lower temperature.”
  • double bond kinks tail — u1-l11-p1: “At the double bond the tail bends.”
  • membrane fluidity — u1-l13b-p1: “the membrane becomes more fluid”

Already in this part:

  • A phospholipid bilayer is measured at 37 °C and then at 5 °C. Nothing about its molecules has c…

Changed: fixed: 'both the same' option removed (D3); new context (arctic and tropical fish)

Blind check: answered B; key B; ok

33U1-L18-P1 · Joining two amino acids Multiple choiceNew✓ taught (fixed 9 Nov)
Tests: A peptide bond is a covalent bond between two amino acids.

The peptide bond joining glycine to alanine in a dipeptide is…

  1. A. a hydrogen bond
    Hydrogen bonds form between partial charges along a folded chain; the peptide bond is a shared pair.
  2. B. an attraction between ions
    No electron moves across completely, so no ions form; the two atoms share a pair of electrons.
  3. C. a covalent bond
    Right: the carboxyl carbon and the amine nitrogen share a pair of electrons, so the bond is covalent.
Explanation: A peptide bond forms when the carboxyl group of one amino acid loses its –OH and the amine group of the next loses an H. The carbon and the nitrogen then share a pair of electrons. A shared pair is a covalent bond.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • peptide bond is a covalent bond — u1-l18-p1: “This covalent bond between two amino acids is called a peptide bond.”
  • glycine/alanine are amino acids; dipeptide = two joined — stem: “in a dipeptide”
  • hydrogen bond = δ+ H attracted to δ− O/N — u1-l02-p3: “we call the attraction a hydrogen bond”
  • distractor: ionic bond — NOT IN UNIT 1

Taught-check fix: 'ionic bond' (never named in Unit 1) -> 'an attraction between ions'

Already in this part:

  • A cell adds one more amino acid to the end of a chain it is building. Which two groups join, an…

Changed: fixed: a specific dipeptide (glycine–alanine) as the new example; taught-check 9 Nov: 'ionic bond' (never named in Unit 1) -> 'an attraction between ions'

Blind check: answered C; key C; ok

34U1-L18-P2 · The chain that results Multiple choiceNew✓ taught
Tests: A polypeptide is one unbranched chain of amino acids.

The 76 amino acids of ubiquitin, a small protein, are joined by peptide bonds into…

  1. A. one unbranched chain
    Right: each amino acid joins only the one before and the one after it, so the chain has no branches.
  2. B. a chain with branches
    Glycogen branches; amino acids join only end to end, so a polypeptide has no branches.
  3. C. a closed ring of units
    A polypeptide has a free end at each end of the chain; it does not close into a ring.
Explanation: Each amino acid joins the one before it and the one after it by a peptide bond, and nothing branches off. So a polypeptide is one unbranched chain, unlike glycogen.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • polypeptide = one unbranched chain — u1-l18-p2: “The chain is one line... nothing branches off.”
  • ubiquitin is a protein of 76 amino acids — stem: “The 76 amino acids of ubiquitin, a small protein”

Already in this part:

  • A cell builds a polypeptide of twelve amino acids. How many peptide bonds hold the chain togeth…

Changed: fixed: ubiquitin as the new example; glycogen hint cut from the distractor

Blind check: answered A; key A; ok

35U1-L18-P3 · The order decides Multiple choiceNew✓ taught (fixed 9 Nov)
Tests: The order of amino acids in a polypeptide is its primary structure.

A database lists the amino acids of a polypeptide in order: Met-Ala-Ser-Lys… This list gives the polypeptide's…

  1. A. three-dimensional shape
    A list of amino acids shows their order, not how the chain folds.
  2. B. primary structure
    Right: the order of the amino acids is the primary structure.
  3. C. total charge
    A list shows which amino acids come in what order, not the overall charge.
Explanation: The order of amino acids in a polypeptide, read from the first to the last, is its primary structure. A list of the amino acids in order is that primary structure.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • primary structure = amino acid order — u1-l18-p3: “The order of amino acids in a polypeptide... is its primary structure.”
  • list of amino acids in order — stem: “lists the amino acids of a polypeptide in order”
  • distractors: secondary / tertiary structure — LATER: u1-l19-p2 / u1-l19b-p2: “we call that folding the chain’s secondary structure”

Taught-check fix: secondary/tertiary structure (taught later) replaced

Already in this part:

  • Two polypeptides contain exactly the same twenty amino acids, one of each, but joined in a diff…

Changed: fixed: a listed sequence as the example, instead of restating the definition (A4); taught-check 9 Nov: secondary/tertiary structure (taught later) replaced

Blind check: answered B; key B; ok

36U1-L19-P1 · The backbone bonds to itself Multiple choiceBank question u1-ut1-q01✓ taught
Tests: Backbone N–H and C=O hydrogen-bond because the δ+ hydrogen and δ− oxygen attract.

In silk protein, a backbone N–H on one stretch lies beside a backbone C=O on the next. They form a hydrogen bond because…

  1. A. the H and the O carry full opposite charges
    No electron moves across, so the charges are partial, not full.
  2. B. the N and the C share a pair of electrons
    A shared pair between atoms would be a covalent bond; a hydrogen bond is an attraction between partial charges.
  3. C. the H is δ+ and the O is δ−
    Right: the δ+ hydrogen of N–H and the δ− oxygen of C=O attract each other.
Explanation: Nitrogen attracts shared electrons more strongly than hydrogen, so the hydrogen of N–H is δ+. Oxygen attracts them more strongly than carbon, so the oxygen of C=O is δ−. The δ+ hydrogen and the δ− oxygen attract: that attraction is a hydrogen bond.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • backbone N–H hydrogen δ+, C=O oxygen δ− — u1-l19-p1: “The hydrogen of each backbone N–H is δ+.”
  • δ+ H attracted to δ− O = hydrogen bond — u1-l19-p1: “This is a hydrogen bond, the same attraction as between two”
  • full charge vs partial charge (distractor A) — u1-l01-p4: “A partial charge is much smaller than a full charge.”
  • silk is a protein — stem: “In silk protein”

Already in this part:

  • A polypeptide chain folds into a coil. Which attraction holds the turns of the coil in place?

Changed: fixed: 70-word stem cut to 22 (A3, C1), 'both nonpolar' option cut (D3 wording); silk as new example; bank id kept, u1-ut1 item: confirm no unit test draws it (G2)

Blind check: answered C; key C; ok

37U1-L19-P2 · Coils and sheets TypedNew✓ taught (fixed 9 Nov)
Tests: A coil held turn to turn by backbone hydrogen bonds is an alpha helix.

Much of the keratin in hair is coiled, each turn bonded to the one below by backbone hydrogen bonds. Is each coil an alpha helix or a beta pleated sheet?

Typed answer, accepted: alpha helix, an alpha helix, α helix, α-helix, alpha-helix, alpha, helix, an α helix
If wrong: Did you mix them up? A beta pleated sheet is stretches of chain lying side by side; a coil held turn to turn is an alpha helix.
Explanation: A stretch of chain coiled and held turn to turn by hydrogen bonds between backbone N–H and C=O groups is an alpha helix. It is part of the chain's secondary structure.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • alpha helix = backbone H-bonded coil — u1-l19-p2: “we call the coil an alpha helix”
  • beta pleated sheet (distractor) — u1-l19-p2: “we call the flat arrangement a beta pleated sheet”
  • keratin is a protein — u1-l17-p1: “Hair and fingernails consist mostly of keratin.”
  • distractor: double helix — LATER: u1-l16-p3: “we call the shape a **double helix**”

Taught-check fix: double helix (taught later) dropped; two-way choice -> typed (rule B2)

Already in this part:

  • Two stretches of one polypeptide are drawn below, lying side by side, with dashed lines between…

Changed: fixed: keratin in hair as the new example instead of a bare definition; taught-check 9 Nov: double helix (taught later) dropped; two-way choice -> typed (rule B2)

Blind check: answered A; key A; ok

38U1-L19B-P2 · Tertiary structure Multiple choiceNew✓ taught
Tests: An interaction between R groups (here a disulfide bridge) belongs to tertiary structure.

A disulfide bridge joins two cysteine R groups that lie far apart along one chain. This bridge is part of the chain's…

  1. A. secondary structure
    Secondary structure is held only by hydrogen bonds between backbone N–H and C=O groups.
  2. B. tertiary structure
    Right: it joins R groups and helps hold the whole chain's shape, so it is tertiary structure.
  3. C. primary structure
    Primary structure is the order of amino acids, held by peptide bonds.
Explanation: Tertiary structure is the whole chain's three-dimensional shape, held by interactions between R groups. A disulfide bridge joins two R groups, so it belongs to tertiary structure.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • disulfide bridge links two cysteine R groups — u1-l19b-p1: “sulfur atoms can form a covalent bond... disulfide bridge”
  • tertiary structure held by R-group interactions — u1-l19b-p2: “held by interactions between R groups”

Already in this part:

  • One folded chain is drawn below with two hydrogen bonds marked. Bond X joins a backbone N–H to …

Changed: kept: example, options and key unchanged; stem lightly trimmed

Blind check: answered B; key B; ok

39U1-L19C-P1 · Water sorts the R groups TypedNew✓ taught (fixed 9 Nov)
Tests: A charged R group stays on the surface of a folded protein, facing the water.

In a protein dissolved in the watery fluid inside a cell, lysine's R group carries a full positive charge. Is it more likely on the surface or in the core?

Typed answer, accepted: surface, the surface, on the surface, on the surface of the protein, surface of the protein, outside, on the outside, the outside, exterior, outer surface, facing the water, facing water
If wrong: Did you think charged groups hide inside? Water is attracted to charges, so a charged R group stays on the surface, facing the water.
Explanation: Water is attracted to a charged R group, so the R group stays in contact with the water. Nonpolar R groups are left together in the core. So a charged lysine R group is most likely on the surface.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • charged R groups stay on the surface touching water — u1-l19c-p1: “The polar and charged R groups stay in contact with the water”
  • lysine R group charged — u1-l17-p2: “ending in –NH₃⁺”
  • 'cytosol' = watery fluid of the cell — NOT IN UNIT 1
  • 'core' = middle of the fold — u1-l19c-p1: “left together, away from the water: in the middle of the fold”

Taught-check fix: cytosol (untaught) -> watery fluid inside a cell

Already in this part:

  • A protein sits in the watery inside of a cell. One stretch of its chain is buried in the middle…

Changed: fixed: two-option MCQ became typed (B2); lysine named; taught-check 9 Nov: cytosol (untaught) -> watery fluid inside a cell

Blind check: answered surface; key ['surface', 'the surface', 'on the surface', 'on the surface of the protein', 'surface of the protein', 'outside', 'on the outside', 'the outside', 'exterior', 'outer surface', 'facing the water', 'facing water']; ok

40U1-L20-P2 · Shape is the job Multiple choiceNew✓ taught (fixed 9 Nov)
Tests: A protein's shape lets it do its job, so changing the shape impairs the job even with the same order.

Heating changes the folded shape of insulin, a protein that works by fitting onto a receptor molecule. Its order of amino acids stays the same. Its ability to fit the receptor most likely…

  1. A. stays the same
    The order is unchanged, but the job depends on the folded shape, and that has changed.
  2. B. increases
    A changed shape fits the receptor worse, not better.
  3. C. decreases
    Right: insulin's job depends on its shape, so a changed shape fits the receptor less well.
Explanation: A protein's shape is what lets it do its job. Insulin works by fitting its receptor. Change the shape and it fits less well or not at all, even with the same order of amino acids.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • protein shape lets it do its job — u1-l20-p2: “the shape is what lets the protein do its job”
  • heating changes shape, order same — stem: “Heating changes insulin's folded shape”
  • primary structure (order) vs shape — u1-l18-p3: “The primary structure decides the overall shape”
  • insulin; 'binding its receptor' = its job, depends on fit — NOT IN UNIT 1

Taught-check fix: insulin/receptor explained in the stem

Already in this part:

  • Collagen, a protein of skin and tendons, is built from three long chains wound around one anoth…

Changed: fixed: 'not' removed from the stem (C2); options cut to the outcome alone, reasons removed (A1); taught-check 9 Nov: insulin/receptor explained in the stem

Blind check: answered C; key C; ok

41U1-L14-P3 · Four bases in DNA, and a fifth in RNA Multiple choiceNew✓ taught
Tests: RNA carries uracil in place of DNA's thymine.

In RNA nucleotides, uracil takes the place of DNA's…

  1. A. thymine
    Right: uracil replaces thymine, so thymine is found only in DNA.
  2. B. adenine
    Adenine is in both DNA and RNA; uracil pairs with adenine but replaces thymine.
  3. C. cytosine
    Cytosine is in both DNA and RNA; it is thymine that RNA lacks.
Explanation: RNA has no thymine. In its place it carries uracil. So DNA's bases are A, T, G and C, and RNA's are A, U, G and C.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • RNA has uracil in place of thymine — u1-l14-p3: “RNA has no thymine. In its place is a fifth base, uracil”
  • DNA bases A, T, G, C — u1-l14-p1: “adenine (A), thymine (T), guanine (G) or cytosine (C)”

Already in this part:

  • A nucleotide is taken from a cell. Its sugar is ribose and its base is uracil. Which nucleic ac…

Changed: fixed: 'not' removed from the stem (C2); adenine replaces guanine as a distractor (real pairing confusion)

Blind check: answered A; key A; ok

42U1-L14B-P1 · Joining two nucleotides Multiple choiceNew (replaces bank u1-ut1-q23)✓ taught (fixed 9 Nov)
Tests: Along a nucleic acid strand, each sugar is joined to the next phosphate by a covalent bond.

Along one strand of RNA, the sugar of each nucleotide is joined to the next phosphate group by…

  1. A. a hydrogen bond
    Hydrogen bonds are weak attractions between partial charges; the backbone is held by shared pairs.
  2. B. a covalent bond
    Right: sugar and phosphate share electrons along the backbone, so the bond is covalent.
  3. C. an attraction between ions
    No electron moves across completely, so no ions form; the two atoms share a pair of electrons.
Explanation: Nucleotides join by a covalent bond from carbon 3 of one sugar to the phosphate group of the next. This gives the sugar-phosphate backbone, in RNA as in DNA.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • sugar to next phosphate is a covalent bond — u1-l14b-p1: “Two nucleotides join by a covalent bond”
  • RNA is a chain of nucleotides — u1-l14-p3: “RNA is also a chain of nucleotides”
  • distractor: ionic bond — NOT IN UNIT 1

Taught-check fix: 'ionic bond' (never named in Unit 1) -> 'an attraction between ions'

Already in this part:

  • A single strand of DNA is cut into two shorter pieces. Which bond was broken?

Changed: replaced: v1 needed a figure and an 80-word stem (A3) on the article's DNA strand; RNA strand is the new example; peptide-bond option cut; taught-check 9 Nov: 'ionic bond' (never named in Unit 1) -> 'an attraction between ions'

Blind check: answered B; key B; ok

43U1-L14B-P3 · Which way a strand grows TypedNew (replaces bank u1-ut1-q22)✓ taught
Tests: Each new nucleotide joins a growing strand at its 3′ end.

A cell adds one more nucleotide to a growing RNA strand. Does it join at the 3′ end or the 5′ end?

Typed answer, accepted: 3′, 3', 3’, 3, 3′ end, 3' end, 3’ end, 3 end, the 3′ end, the 3' end, the 3’ end, the 3 end, three prime, three prime end, the three prime end, 3 prime, 3 prime end
If wrong: Did you pick the 5′ end? The free phosphate at the 5′ end never gains a nucleotide; new nucleotides bond to the free –OH at the 3′ end.
Explanation: Every new nucleotide joins at the 3′ end. A covalent bond forms between the –OH on carbon 3 of the last sugar and the phosphate group of the new nucleotide. The new nucleotide then carries the strand's free –OH, so a strand grows from 5′ to 3′.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • new nucleotides join at the 3 end — u1-l14b-p3: “it adds every new nucleotide at the 3 end”
  • prime notation 3′ = 3 end — u1-l14b-p2: “the 3′ end is the 3 end”
  • RNA is a nucleotide strand — u1-l14-p3: “RNA is also a chain of nucleotides”

Already in this part:

  • A new DNA strand grows one nucleotide at a time until it is 500 nucleotides long. Where is the …

Changed: replaced: v1 needed a figure and long stem (A3) and was a two-way choice (B2); now typed, on an RNA strand

Blind check: answered 3′; key ['3′', "3'", '3’', '3', '3′ end', "3' end", '3’ end', '3 end', 'the 3′ end', "the 3' end", 'the 3’ end', 'the 3 end', 'three prime', 'three prime end', 'the three prime end', '3 prime', '3 prime end']; ok

44U1-L14B-P4 · Reading a strand TypedNew✓ taught
Tests: A strand's information is the order of its bases read 5′ to 3′, not which bases it holds.

Do the strands 5′-C-A-T-G-3′ and 5′-G-T-A-C-3′ carry the same information or different information?

Typed answer, accepted: different, different information, they carry different information, they are different, different info, differ, they differ
If wrong: Did you go by which bases are present? Both strands hold the same four bases, but read 5′ to 3′ their order differs, and the order is the information.
Explanation: The information is in the order of the bases, read from the 5′ end to the 3′ end. Both strands hold one each of C, A, T and G, but in a different order from 5′ to 3′, so they carry different information.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • information is in base order 5′→3′ — u1-l14b-p4: “the information in DNA is in the order of its bases”
  • strands written 5 to 3 — u1-l14b-p4: “A strand is written from its 5 end to its 3 end.”

Already in this part:

  • Two stretches of DNA from different places in a cell contain exactly the same number of each ki…

Changed: fixed: two-option MCQ became typed (B2); new sequences

Blind check: answered different; key ['different', 'different information', 'they carry different information', 'they are different', 'different info', 'differ', 'they differ']; ok

45U1-L15-P2 · Which base pairs with which Multiple choiceNew✓ taught
Tests: Name the base opposite a given base in DNA from memory (thymine pairs with adenine).

In double-stranded DNA, the base lying opposite thymine is…

  1. A. guanine
    Guanine pairs with cytosine; guanine and thymine do not fit each other.
  2. B. uracil
    Uracil is an RNA base; DNA has no uracil.
  3. C. adenine
    Right: thymine pairs with adenine.
Explanation: In DNA, adenine pairs with thymine and guanine pairs with cytosine. So opposite every thymine lies an adenine.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • A pairs with T — u1-l15-p2: “Adenine pairs with thymine.”
  • uracil (distractor) — u1-l14-p3: “a fifth base, uracil (U)”

Already in this part:

  • In a double-stranded DNA molecule, one base lies opposite a guanine. Use the table. Which base …

Changed: fixed: asks the base opposite thymine so its options and key do not mirror the RNA item that follows (D6)

Blind check: answered C; key C; ok

46U1-L15-P3 · RNA: one strand, and one different base Multiple choiceNew✓ taught
Tests: In RNA, uracil takes thymine's place opposite adenine.

An RNA strand pairs with a stretch of DNA. Opposite each adenine on the DNA, the RNA carries…

  1. A. uracil
    Right: in RNA, uracil takes thymine's place opposite adenine.
  2. B. thymine
    RNA has no thymine; uracil takes its place.
  3. C. guanine
    Guanine pairs with cytosine, not with adenine.
Explanation: RNA has no thymine; uracil takes its place. So where an RNA base pairs with adenine, that base is uracil.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • in RNA, adenine pairs with uracil — u1-l15-p3: “adenine pairs with uracil”
  • RNA has no thymine — u1-l15-p3: “RNA has no thymine.”

Already in this part:

  • A nucleic acid taken from a cell is made of two strands held together along their whole length.…

Changed: fixed: new context (RNA pairing with DNA) instead of restating the article's rule (A4)

Blind check: answered A; key A; ok

47U1-L15B-P1 · Count first: ten pairs Multiple choiceNew✓ taught
Tests: In double-stranded DNA the number of adenines equals the number of thymines.

A stretch of double-stranded DNA from a yeast cell holds 40 adenines. So it must also hold 40…

  1. A. guanines
    Guanine pairs with cytosine, so the guanine count matches the cytosine count, not the adenine count.
  2. B. thymines
    Right: every adenine is paired with a thymine, so their numbers match.
  3. C. cytosines
    Cytosine pairs with guanine; the adenine count tells you only the thymine count.
Explanation: Every adenine in double-stranded DNA is paired with a thymine on the other strand. So the number of adenines always equals the number of thymines. Guanines and cytosines are paired with each other.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • number of A equals number of T — u1-l15b-p1: “the number of A’s always equals the number of T’s”

Already in this part:

  • A stretch of double-stranded DNA from a bacterium holds 80 bases in all, and 30 of them are thy…

Changed: fixed: a counted example (40 adenines) instead of restating the rule (A4)

Blind check: answered B; key B; ok

48U1-L15B-P2 · Millions of pairs: use percentages part had no questionNumberBank question u1-ut1-q26✓ taught
Tests: Work out one base's percentage from another's using the pairs and the 100% total.

Double-stranded DNA from a fungus is 27% thymine. What percentage of its bases is cytosine?

Number answer, accepted: 23, 23%, 23 %, 23.0, 23.0%
If wrong: Did you give 27%, the adenine value? Or 46%, guanine and cytosine together before halving? Or 73%, which subtracts thymine alone from 100%?
Explanation: Thymine pairs with adenine, so adenine is also 27%. Together A and T make 54%. The four bases add up to 100%, so G and C make 46%. Guanine equals cytosine, so cytosine is 46% ÷ 2 = 23%.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • percentages sum to 100; T=A, C=G — u1-l15b-p2: “%A + %T + %G + %C = 100%”

Already in this part:

  • none

Changed: fixed: calculation turned from MCQ into numeric entry (B1), stem trimmed; bank id kept, u1-ut1 item: confirm no unit test draws it (G2)

Blind check: answered 23; key ['23', '23%', '23 %', '23.0', '23.0%']; ok

49U1-L16-P1 · Two strands, opposite ways TypedNew✓ taught (fixed 9 Nov)
Tests: DNA strands are antiparallel: opposite one strand's 5′ end lies the other's 3′ end.

A double-stranded DNA molecule stands upright with strand 1's 5′ end at the top. At the top, does strand 2 have its 3′ end or its 5′ end?

Typed answer, accepted: 3′, 3', 3’, 3, 3′ end, 3' end, 3’ end, 3 end, the 3′ end, the 3' end, the 3’ end, the 3 end, three prime, three prime end, the three prime end, 3 prime, 3 prime end
If wrong: Did you pick 5′? Two strands with their 5′ ends together would run the same way; DNA strands are antiparallel, so the 3′ end lies opposite.
Explanation: The two strands of DNA are antiparallel: they run in opposite directions. So where one strand has its 5′ end, the other has its 3′ end.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • strands antiparallel: opposite 5′/3′ ends — u1-l16-p1: “they run in opposite directions”
  • prime notation — u1-l16-p1: “Your exam prints these ends as 5′ and 3′”
  • 'double helix' term — LATER: u1-l16-p3: “we call the shape a **double helix**”

Taught-check fix: double helix (taught at l16-p3) -> double-stranded DNA molecule

Already in this part:

  • Four drawings of two paired strands are lettered W, X, Y and Z, with every end labeled. Which d…

Changed: fixed: two-option MCQ became typed (B2); taught-check 9 Nov: double helix (taught at l16-p3) -> double-stranded DNA molecule

Blind check: answered 3′; key ['3′', "3'", '3’', '3', '3′ end', "3' end", '3’ end', '3 end', 'the 3′ end', "the 3' end", 'the 3’ end', 'the 3 end', 'three prime', 'three prime end', 'the three prime end', '3 prime', '3 prime end']; ok

50U1-L16-P3 · A zipper, twisted part had no questionTypedNew✓ taught
Tests: In the double helix the sugar-phosphate backbones are on the outside and the paired bases inside.

In a DNA double helix, are the two sugar-phosphate backbones on the outside or on the inside?

Typed answer, accepted: outside, on the outside, the outside, outer, on the outer side, the outer side, exterior, outside of the helix, on the outside of the helix, out
If wrong: Did you place the backbones inside? The bases point inward to pair, so the backbones run along the outside.
Explanation: The bases hang off the sugars and point inward, where paired bases are held by hydrogen bonds. So the two sugar-phosphate backbones are on the outside of the double helix.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • backbones outside, bases inside — u1-l16-p3: “the two backbones are on the outside”
  • double helix — u1-l16-p3: “we call the shape a **double helix**”
  • sugar-phosphate backbone — u1-l14b-p1: “This repeating chain is called the sugar-phosphate backbone”

Already in this part:

  • none

Changed: fixed: two-option MCQ became typed (B2)

Blind check: answered outside; key ['outside', 'on the outside', 'the outside', 'outer', 'on the outer side', 'the outer side', 'exterior', 'outside of the helix', 'on the outside of the helix', 'out']; ok

51U1-L16A-P1 · Start at the 5 end part had no questionTypedNew✓ taught
Tests: A strand is written 5′ to 3′, so the first base written sits at the 5′ end, with the free phosphate.

A strand is written 5′-C-A-G-T-3′. Which base sits at the end with the free phosphate group?

Typed answer, accepted: c, cytosine, the c, c (cytosine), cytosine (c), the cytosine
If wrong: Did you pick T? T is at the 3′ end, which carries the free –OH; the free phosphate is at the 5′ end, where writing starts.
Explanation: A strand is written from its 5′ end to its 3′ end. The 5′ end has the free phosphate, so the first base written, C, sits there.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • strand written 5′ to 3′; first base at 5′ end — u1-l16a-p1: “G is at the 5 end, the end with the free phosphate”
  • 5′ end carries free phosphate — u1-l14b-p2: “This end is called the 5 end”

Already in this part:

  • none

Changed: fixed: two-option MCQ became typed (B2)

Blind check: answered C; key ['c', 'cytosine', 'the c', 'c (cytosine)', 'cytosine (c)', 'the cytosine']; ok

52U1-L16B-P1 · Two nucleic acids part had no questionMultiple choiceNew✓ taught
Tests: Every DNA and RNA nucleotide has a phosphate group, a five-carbon sugar and a base.

A nucleotide from a cell's RNA and a nucleotide from its DNA both contain…

  1. A. the base thymine
    Thymine is found only in DNA; RNA carries uracil instead.
  2. B. the sugar deoxyribose
    Deoxyribose is DNA's sugar; RNA's sugar is ribose.
  3. C. a phosphate group
    Right: every nucleotide of DNA and of RNA has a phosphate group.
Explanation: Every nucleotide, in DNA or in RNA, has a phosphate group, a five-carbon sugar and a nitrogenous base. The sugar differs, deoxyribose in DNA and ribose in RNA, and thymine is found only in DNA.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • every nucleotide has phosphate, sugar, base — u1-l16b-p1: “the same three parts: a phosphate group, marked P”
  • deoxyribose only in DNA — u1-l14-p2: “DNA is built with deoxyribose.”
  • thymine only in DNA — u1-l14-p3: “RNA has no thymine.”

Already in this part:

  • none

Changed: kept: same options and key; stem lightly reworded

Blind check: answered C; key C; ok

53U1-L16C-P1 · The model, one part at a time part had no questionTypedNew✓ taught
Tests: In a DNA model, the dashed lines between paired bases are hydrogen bonds; the backbone lines are covalent.

A DNA model shows solid lines along each backbone and dashed lines between paired bases. Which kind of bond do the dashed lines show?

Typed answer, accepted: hydrogen, hydrogen bond, hydrogen bonds, a hydrogen bond, h bond, h bonds, h-bond, h-bonds, hydrogen bonding
If wrong: Did you answer covalent? The solid lines along each backbone are covalent bonds; the dashed lines between paired bases are hydrogen bonds.
Explanation: Paired bases on the two strands are held together by hydrogen bonds, drawn as dashed lines. The solid lines along each backbone are covalent bonds.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • dashed lines = hydrogen bonds between paired bases — u1-l16c-p1: “Between the two strands the dashed lines are hydrogen bonds.”
  • solid lines = covalent along backbone — u1-l16c-p1: “the solid lines are covalent bonds”

Already in this part:

  • none

Changed: fixed: two-option MCQ became typed (B2)

Blind check: answered hydrogen; key ['hydrogen', 'hydrogen bond', 'hydrogen bonds', 'a hydrogen bond', 'h bond', 'h bonds', 'h-bond', 'h-bonds', 'hydrogen bonding']; ok

54U1-L21-P1 · Every atom in you came in from outside Multiple choiceNew (replaces bank u1-t12-q02)✓ taught
Tests: An organism builds new molecules only from atoms it takes in (nitrogen comes in as food protein).

A calf gains 100 kg in its first year, drinking milk and eating grass. The nitrogen atoms it built into new tissue came from…

  1. A. the protein in its food
    Right: an animal's nitrogen comes in as protein in its food.
  2. B. the water it drank
    Water is made of hydrogen and oxygen only; it carries no nitrogen.
  3. C. carbon atoms its cells turned into nitrogen
    Cells cannot turn one element into another; every atom came in from outside.
Explanation: An organism cannot make atoms, so every atom in a new molecule came in from outside. Water has no nitrogen, and cells cannot turn one element into another, so the calf's nitrogen came from the protein in its milk and grass.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • organisms cannot make atoms; all come from food, water, air — u1-l21-p1: “The tree cannot make atoms, and neither can any other organism.”
  • water is H2O (no nitrogen) — u1-l01-p4: “one oxygen atom joined to two hydrogen atoms”
  • proteins contain nitrogen (amine group) — u1-l17-p1: “a nitrogen bonded to two hydrogens, –NH₂”
  • milk and grass contain protein (real-world) — stem: “answerable by elimination of B and C”

Already in this part:

  • Over one summer a young tree roughly doubles in mass. Where did the atoms in all that new mater…

Changed: replaced: the athlete's muscle repeated the article's own example (A4); now a calf; water option cut; blind review: N2-from-air option replaced with water (only one defensible answer)

Blind check: answered A; key A; fix

55U1-L21-P2 · Carbon, hydrogen and oxygen: the common core Multiple choiceNew (replaces bank u1-t12-q09)✓ taught
Tests: Carbohydrates are built from carbon, hydrogen and oxygen.

Cellulose is built entirely from glucose units. Every atom in cellulose is carbon, hydrogen or…

  1. A. nitrogen
    Nitrogen is found in proteins and nucleic acids, not in a chain of glucose units.
  2. B. oxygen
    Right: glucose holds only carbon, hydrogen and oxygen, so cellulose does too.
  3. C. phosphorus
    Phosphorus is found in nucleic acids and phospholipids; glucose has none.
Explanation: Glucose is C₆H₁₂O₆: carbon, hydrogen and oxygen, and nothing else. Joining glucose units removes only water, so cellulose is built from carbon, hydrogen and oxygen too.
Taught where · already in this part · what changed · blind check

Relies on (where it was taught):

  • glucose is only C, H, O — u1-l21-p2: “Glucose is C₆H₁₂O₆... and nothing else.”
  • joining removes only water — u1-l06-p2: “The two pieces leave together as one water molecule.”
  • cellulose built only from glucose — stem: “Cellulose is built entirely from glucose units.”

Already in this part:

  • Which three elements are the most common in all four classes of biological molecule?

Changed: replaced: v1 option 'carbon, hydrogen and oxygen only' combined options (D3) and the fat was the article's example (A4); now cellulose

Blind check: answered B; key B; ok