Investigating the Chemistry of Life — Chapter quiz · Life Science: Biology, Grade 10

Use fair comparisons and molecular models to explain reactions in living systems.

Problem 1

Which type of bond forms when two atoms share a pair of electrons?
  1. Ionic bond
  2. Covalent bond
  3. Hydrogen bond
  4. Metallic bond
Answer and explanation

Covalent bond

Sharing a pair of electrons is a covalent bond — the kind holding oxygen to hydrogen in water.

Problem 2

Water dissolves many substances and clings to itself mainly because
  1. it is a polar molecule that forms hydrogen bonds
  2. it is completely nonpolar
  3. it is a single element
  4. it carries no charge anywhere in the molecule
Answer and explanation

it is a polar molecule that forms hydrogen bonds

Water's polarity — slightly negative oxygen, slightly positive hydrogens — lets it form hydrogen bonds, giving it both cohesion and its power as a solvent.

Problem 3

How many times more acidic is a solution of pH 3 than a solution of pH 6?
Answer and explanation

1000 times

Each pH unit is a factor of 10, and 6 − 3 = 3 units, so 10³ = 1000 times more acidic.

Problem 4

The monomer (building block) of a protein is a(n)
  1. fatty acid
  2. nucleotide
  3. amino acid
  4. glucose molecule
Answer and explanation

amino acid

Proteins are polymers of amino acids; nucleotides build nucleic acids and glucose builds many carbohydrates.

Problem 5

Which macromolecule makes up cell membranes and stores energy for the long term?
  1. Protein
  2. Carbohydrate
  3. Nucleic acid
  4. Lipid
Answer and explanation

Lipid

Lipids (fats and phospholipids) store the most energy per gram and form the cell membrane.

Problem 6

An enzyme speeds up a reaction by
  1. lowering the activation energy the reaction needs to get started
  2. raising the activation energy
  3. removing the need for any reactants
  4. using itself up so the reaction can proceed
Answer and explanation

lowering the activation energy the reaction needs to get started

Enzymes lower the activation energy — the starting 'hump' — so the reaction proceeds quickly at body temperature, and the enzyme is not used up.

Problem 7

Explain why each enzyme usually works on only one specific substrate.
Answer and explanation

An enzyme folds into a shape that gives it an active site matching one particular substrate, like a lock that fits only one key. A molecule with a different shape can't fit into the active site, so the enzyme is specific to its own substrate.

Problem 8

A cell uses 2 ATP molecules to build one small molecule. How many ATP molecules are needed to build 45 of them?
Answer and explanation

90 ATP

2 ATP per molecule × 45 molecules = 90 ATP.

Problem 9

Which best describes the role of ATP in a cell?
  1. It stores the cell's genetic information
  2. It is the cell's rechargeable energy currency
  3. It is the main building block of proteins
  4. It forms the rigid wall around plant cells
Answer and explanation

It is the cell's rechargeable energy currency

ATP carries energy in a small, spendable form and cycles between ATP and ADP as it is used and recharged.

Problem 10

A cell needs to run a synthesis reaction that would be far too slow and needs energy. Explain how an enzyme and ATP work together to make it happen.
Answer and explanation

The enzyme lowers the activation energy so the synthesis reaction can happen quickly at body temperature, while ATP supplies the energy the reaction needs by breaking down into ADP + phosphate. Together, the enzyme makes the reaction fast enough and the ATP pays its energy cost — so the reaction that couldn't happen alone now proceeds.

Problem 11

Reaction rates are 12, 15, and 18 units/min. Find the mean.
Answer and explanation

15 units/min

45 ÷ 3 = 15.

Problem 12

One treatment has a higher mean than another. Name two things to examine before claiming an effect.
Answer and explanation

Examine spread, replicate independence, sample size, controlled conditions, measurement accuracy, and whether the result repeats.

Problem 13

Independent chapter check: Two trays receive different nutrient treatments. Each tray contains one plant measured six times. Explain the true replication and redesign the comparison.
Answer and explanation

There is only one treated biological/treatment unit per condition; six readings are repeated observations. Use several independently assigned plants or trays per condition, keep light/water/species comparable and preserve all time records.

There is only one treated biological/treatment unit per condition; six readings are repeated observations. Use several independently assigned plants or trays per condition, keep light/water/species comparable and preserve all time records.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 14

Blood pH stays near 7.4 even after exercise releases acid into it. The property responsible is
  1. cohesion
  2. buffering
  3. evaporative cooling
  4. capillary action
Answer and explanation

buffering

A buffer accepts or releases hydrogen ions and resists pH change. Cohesion and cooling are also water properties but do not stabilize pH.

Problem 15

A student claims that water is "the universal solvent, so it dissolves everything." Correct the claim and give the evidence a cell provides against it.
Answer and explanation

Water dissolves polar and ionic substances well because its partial charges surround and separate solute particles, but it does not dissolve nonpolar substances. The cell membrane is the evidence: its nonpolar lipid interior stays intact in a watery cell, which it could not do if water dissolved everything.

Response criteria

  • State a biologically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 16

Independent chapter check: Compare hydrogen-ion concentrations at pH 5 and pH 8 using a tenfold change per unit. Explain why a buffer cannot be described as preventing every pH change.
Answer and explanation

The pH 5 sample has 10³ = 1,000 times the hydrogen-ion concentration of the pH 8 sample in the dilute-solution comparison. A buffer has a finite working range and capacity; sufficiently large additions can exceed it.

The pH 5 sample has 10³ = 1,000 times the hydrogen-ion concentration of the pH 8 sample in the dilute-solution comparison. A buffer has a finite working range and capacity; sufficiently large additions can exceed it.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 17

Two proteins have the same number of amino acids but different functions. The most direct explanation is that they differ in
  1. the number of peptide bonds
  2. the sequence of amino acids, and therefore the folded shape
  3. the element carbon
  4. the amount of water used to assemble them
Answer and explanation

the sequence of amino acids, and therefore the folded shape

Sequence determines folding, folding determines shape, and shape determines which molecules the protein can bind. The bond count and elements are the same.

Problem 18

Explain why the claim "all large biological molecules are polymers of a repeating monomer" needs qualification, using lipids as the case.
Answer and explanation

Carbohydrates, proteins, and nucleic acids are polymers assembled from monosaccharides, amino acids, and nucleotides respectively. Lipids are large and built partly by dehydration synthesis, but a triglyceride is a glycerol joined to three fatty acids rather than a chain of identical repeating units, so it is not a polymer in the same sense. The generalization holds for three of the four classes.

Response criteria

  • State a biologically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 19

Independent chapter check: A treatment unfolds an enzyme but leaves its amino-acid chain largely intact. Explain what may change, what need not change, and what evidence would test retained function.
Answer and explanation

Folding and active-site arrangement may change while amino-acid sequence remains largely intact. Compare catalytic rates under matched conditions and examine whether the fold/function can recover; unfolding is not automatically complete peptide hydrolysis.

Folding and active-site arrangement may change while amino-acid sequence remains largely intact. Compare catalytic rates under matched conditions and examine whether the fold/function can recover; unfolding is not automatically complete peptide hydrolysis.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 20

An enzyme's rate drops to zero at pH 2 and recovers no activity when returned to pH 7. The best explanation is
  1. the substrate was used up
  2. the enzyme was denatured and did not refold
  3. the reaction became energetically unfavourable
  4. the enzyme was consumed by the reaction
Answer and explanation

the enzyme was denatured and did not refold

Extreme pH disrupts the interactions holding the folded shape. Enzymes are not consumed by the reactions they catalyse, so consumption cannot explain the loss.

Problem 21

Explain why reporting "the enzyme works best at 37 °C" from a single trial at each of 15, 25, 35, and 55 °C overstates what the data support.
Answer and explanation

The highest tested value was 35 °C, not 37 °C, so the stated optimum lies outside the measured points; the true optimum could be anywhere between 35 and 55 °C. One trial per temperature also gives no estimate of variation, so a small difference between two means cannot be distinguished from measurement noise. A supported claim names the highest tested temperature and calls for replicates and intermediate temperatures.

Response criteria

  • State a biologically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 22

Independent chapter check: An enzyme has the same reactant and product energy levels with and without a catalyst, but a lower pathway peak with it. Explain rate, equilibrium and energy implications.
Answer and explanation

The lower barrier can increase the approach rate in both directions. It does not by itself change the equilibrium or net free-energy change, and an unfavorable isolated process still needs suitable coupling or conditions.

The lower barrier can increase the approach rate in both directions. It does not by itself change the equilibrium or net free-energy change, and an unfavorable isolated process still needs suitable coupling or conditions.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 23

An enzyme is added to an endergonic reaction in a test tube with no other energy source. The result is
  1. the reaction proceeds to completion
  2. the reaction becomes exergonic
  3. the reaction still does not proceed appreciably
  4. the enzyme is consumed
Answer and explanation

the reaction still does not proceed appreciably

An enzyme lowers activation energy but supplies no energy and does not change the overall energy balance, so an endergonic reaction still needs a coupled energy source.

Problem 24

A metabolic pathway converts A to D through B and C, with each step catalysed by a different enzyme. Explain two advantages of using several steps rather than one, and what regulation becomes possible.
Answer and explanation

Several small steps mean each activation barrier is lower, so each reaction can proceed at body temperature, and the energy released can be captured in usable amounts rather than lost as a single large burst of heat. Multiple steps also give multiple control points: inhibiting the first enzyme shuts the whole pathway down, and feedback inhibition by product D on that first enzyme lets the cell match production to need.

Response criteria

  • State a biologically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 25

Independent chapter check: A coupled synthesis needs +18 free-energy units and its linked hydrolysis contributes −26. Calculate the net, then explain why a catalyst alone cannot replace the coupling.
Answer and explanation

The net is −8 units under the supplied conditions. A catalyst can lower the activation barrier but does not make the isolated +18 process favorable by changing its endpoint free-energy balance.

The net is −8 units under the supplied conditions. A catalyst can lower the activation barrier but does not make the isolated +18 process favorable by changing its endpoint free-energy balance.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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