The Chemistry of Life — Science

  1. Grade 10 — Atoms, Bonds & Water
  2. Grade 10 — Carbon & the Macromolecules of Life
  3. Grade 10 — Enzymes
  4. Grade 10 — Biochemical Reactions & Energy

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Problem 1

A cell links 150 amino acids together into a single polypeptide chain. Each new peptide bond forms by a condensation (dehydration synthesis) reaction that releases one water molecule. How many water molecules are released in building this chain?
Hints

Think about how many BONDS are needed to join 150 beads into one chain.

Joining n monomers in a row takes n − 1 links.

One water is released per bond.

Answer and explanation

149 molecules

Linking 150 amino acids in a chain requires 150 − 1 = 149 peptide bonds.

Each bond forms by releasing one water molecule (dehydration synthesis).

So the number of water molecules released is 149 molecules.

Problem 2

Sweating cools you down because water absorbs a large amount of heat as it evaporates from your skin. Which property of water, and its cause, best explains this cooling?
  1. Its solid form is less dense than its liquid (ice floats).
  2. Its ability to dissolve many substances (the 'universal solvent').
  3. Its high heat of vaporization: hydrogen bonds between water molecules must absorb a lot of energy to break as water evaporates.
  4. Its cohesion, which lets water form beads and produce surface tension.
Hints

All four are genuine properties of water — pick the one specifically about absorbing heat during evaporation.

Evaporating means molecules escape the liquid; that requires breaking the attractions holding them together.

Which attractions between water molecules must absorb energy to break?

Answer and explanation

Its high heat of vaporization: hydrogen bonds between water molecules must absorb a lot of energy to break as water evaporates.

Evaporation requires breaking the hydrogen bonds that hold liquid water together, which absorbs a great deal of heat — water's high heat of vaporization.

Ice floating, solvent power, and cohesion are real properties of water but none explains evaporative cooling.

High heat of vaporization from hydrogen bonding is the cause, therefore C.

Problem 3

An unknown biological molecule is analyzed. It contains only carbon, hydrogen, and oxygen in a ratio close to 1 : 2 : 1, dissolves easily in water, and tastes sweet. Which type of macromolecule is it most likely to be?
  1. A carbohydrate.
  2. A lipid, such as a fat or oil.
  3. A protein.
  4. A nucleic acid.
Hints

The 1 : 2 : 1 ratio of C : H : O is a fingerprint of one macromolecule family.

Proteins contain nitrogen, and nucleic acids contain nitrogen and phosphorus — but this molecule has only C, H, and O.

Which C, H, O molecules are typically water-soluble and sweet (sugars)?

Answer and explanation

A carbohydrate.

A C : H : O ratio near 1 : 2 : 1, water solubility, and sweetness all point to a sugar.

Proteins (contain N) and nucleic acids (contain N and P) are ruled out by the elements present; fats are not water-soluble and share no 1 : 2 : 1 pattern.

The molecule is a carbohydrate, therefore A.

Problem 4

An enzyme works best at 37°C. A student heats the reaction to 60°C; the reaction stops completely and does NOT restart even after the mixture is cooled back to 37°C. What is the best explanation?
  1. The enzyme was consumed as a reactant, so it ran out.
  2. The heat denatured the enzyme, permanently changing the shape of its active site so substrate no longer fits.
  3. The substrate all evaporated at 60°C, leaving nothing to react.
  4. Enzymes always work more slowly at higher temperature, so it merely slowed down.
Hints

The key clue is that cooling back down does NOT restore activity — the damage is permanent.

Enzymes are catalysts: they are not used up, and something that merely slowed would speed back up on cooling.

What does high heat do to a protein's three-dimensional shape?

Answer and explanation

The heat denatured the enzyme, permanently changing the shape of its active site so substrate no longer fits.

High temperature breaks the bonds that hold an enzyme's 3-D shape, denaturing it and deforming the active site.

Because denaturation is permanent, activity does not return on cooling — ruling out 'used up,' 'just slowed,' and (the enzyme, not the substrate, is the issue) evaporation.

Permanent denaturation of the active site fits, therefore B.

Problem 5

Which statement correctly describes HOW an enzyme speeds up a biochemical reaction?
  1. It adds energy to the reactants, pushing them over the energy barrier.
  2. It increases the total amount of energy the reaction releases.
  3. It permanently raises the temperature of the whole solution.
  4. It lowers the activation energy — the energy barrier — so the reaction proceeds faster.
Hints

Enzymes change the RATE of a reaction without changing how much energy the reaction gives off overall.

Picture the energy 'hill' reactants must climb to react — what does the enzyme do to that hill?

It provides an easier path, not extra energy or extra heat.

Answer and explanation

It lowers the activation energy — the energy barrier — so the reaction proceeds faster.

An enzyme offers an alternative pathway with a lower activation energy, so more collisions succeed and the reaction speeds up.

It does not supply energy to reactants, change the reaction's overall energy release, or heat the solution.

Lowering activation energy is the mechanism, therefore D.

Problem 6

Building large molecules from small subunits (for example, joining amino acids into proteins) is energy-REQUIRING (endergonic). How do cells typically power such reactions?
  1. By cooling down, which releases stored heat into the reaction.
  2. By coupling them to the energy released when ATP is broken down into ADP and phosphate.
  3. By simply waiting, since endergonic reactions happen on their own given time.
  4. By adding more enzymes, because enzymes supply the energy the reaction needs.
Hints

Endergonic reactions need an input of energy — where do cells store readily usable energy?

Think about the molecule cells 'spend' to get cellular work done.

Enzymes speed reactions but do not provide energy; heat is not how cells drive synthesis.

Answer and explanation

By coupling them to the energy released when ATP is broken down into ADP and phosphate.

Cells drive endergonic reactions by coupling them to the exergonic breakdown of ATP → ADP + phosphate, which supplies the needed energy.

Endergonic reactions do not proceed on their own, enzymes are not an energy source, and cooling does not power synthesis.

Energy coupling with ATP is the answer, therefore B.

Problem 7

In winter, a lake freezes at the top but stays liquid underneath, letting fish survive. Which explanation of WHY ice floats is correct?
  1. In ice, hydrogen bonds lock water molecules into an open lattice that is LESS dense than liquid water, so ice floats and insulates the water below.
  2. Solid ice is denser than liquid water, so the ice sinks and shields the bottom.
  3. Ice floats because it is warmer than the water beneath it.
  4. Ice floats only because air bubbles trapped inside make it lighter.
Hints

Floating means the solid is less dense than the liquid — unusual, since most solids sink in their own liquid.

Think about how hydrogen bonds arrange water molecules when it freezes.

The open, spaced-out lattice of ice is the key to its lower density.

Answer and explanation

In ice, hydrogen bonds lock water molecules into an open lattice that is LESS dense than liquid water, so ice floats and insulates the water below.

When water freezes, hydrogen bonds hold the molecules in an open, widely spaced lattice, making ice less dense than liquid water, so it floats.

Ice is not denser than water (that idea is backwards), not warmer than the water below, and it floats because of its open lattice structure, not trapped air.

The open hydrogen-bonded lattice explains floating ice, therefore A.

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