Investigating the Chemistry of Life — Problem set, Grade 10

Covalent (atoms share electrons), ionic (one atom transfers an electron to another, forming ions that attract), and hydrogen (a weak attraction between polar molecules).

A covalent bond forms when two atoms share a pair of electrons — the strong bonds inside molecules like water. An ionic bond forms when one atom transfers an electron to another, creating oppositely charged ions (like Na⁺ and Cl⁻) that attract. A hydrogen bond is a much weaker attraction between the slightly positive part of one polar molecule and the slightly negative part of another.

Its electrons are shared unequally, so the oxygen end is slightly negative and the hydrogen ends slightly positive; those charged ends let it surround and dissolve other charged or polar substances.

In water, oxygen pulls the shared electrons closer than the hydrogens do, leaving the oxygen end slightly negative and the hydrogen ends slightly positive — that is what 'polar' means. Because it has charged ends, water can cluster around the charged parts of other substances (like the Na⁺ and Cl⁻ in salt) and pull them apart, dissolving them. That's why water is called the 'universal solvent.'

1000 times more acidic.

Each step on the pH scale is a tenfold change in acidity. The difference is 5 − 2 = 3 pH units, so the lemon juice has 10 × 10 × 10 = 10³ = 1000 times more hydrogen ions than the coffee.

Carbohydrates (quick energy and structure), lipids (long-term energy storage and cell membranes), proteins (do most of the cell's work, including acting as enzymes), and nucleic acids (store and carry genetic information).

Carbohydrates supply quick energy (glucose) and structure (cellulose). Lipids store energy long-term and build cell membranes. Proteins carry out most of the cell's jobs — structure, transport, defense, and catalysis as enzymes. Nucleic acids (DNA and RNA) store and carry the genetic instructions for building proteins.

19 bonds and 19 water molecules.

In a single chain, the number of bonds is always one less than the number of monomers: 20 − 1 = 19 bonds. Because dehydration synthesis releases one water molecule for each bond formed, 19 bonds release 19 water molecules.

An enzyme is a protein catalyst that speeds up a specific reaction by lowering its activation energy, without being used up.

An enzyme is a protein that acts as a biological catalyst. It lowers the activation energy — the 'starting hump' — of a specific reaction, so the reaction can happen quickly at the mild temperatures inside a cell. Because the enzyme is not consumed, it can be reused again and again.

At 5°C it works slowly but is undamaged; at 70°C it stops working because it is denatured.

At 5°C the molecules move and collide slowly, so the reaction is slow — but the enzyme keeps its shape and would speed up again if warmed. At 70°C the excess heat denatures the enzyme: its folding unravels, the active site loses its shape, and the substrate can no longer bind, so activity drops sharply and usually does not recover.

The wrong pH disrupts the attractions that hold the enzyme's shape, distorting the active site so the substrate no longer fits.

An enzyme works only when its active site keeps the right shape, and that shape is held together by chemical attractions that are sensitive to H⁺ concentration (pH). If the pH moves away from the enzyme's optimum, the extra or missing H⁺ ions disrupt those attractions, the active site changes shape, and the substrate can no longer bind — so the enzyme slows down or stops, and may denature completely.

ATP (adenosine triphosphate) is the molecule cells use to store and deliver energy; it's called 'currency' because it's a small, standard, spendable packet of energy the cell can use anywhere.

ATP is an energy-transfer molecule. Its complete hydrolysis reaction can be coupled to work; breaking a bond alone does not release energy. Metabolism supplies energy to regenerate ATP.

(a) synthesis; (b) decomposition.

A synthesis reaction builds a larger molecule from smaller ones, so (a) — linking glucose into glycogen — is synthesis. A decomposition reaction breaks a larger molecule into smaller ones, so (b) — breaking a protein into amino acids — is decomposition.

About 120 ATP.

Multiply the ATP per glucose by the number of glucose molecules: 30 ATP × 4 = 120 ATP. (In a real cell, those ATP are then broken back to ADP as they power the cell's work, and recharged again — the number just tells you how much energy the food released.)

(a) amino acids are the monomers linked into the protein; (b) an enzyme speeds up the bond-forming reaction; (c) ATP supplies the energy the synthesis reaction needs.

Amino acids supply building material. Catalytic machinery assembles the chain, with energy-dependent steps supported by nucleotide reactions. Reactions must be coupled; an enzyme alone does not supply the needed energy.

Either factor or their interaction could contribute. Change one factor with the other controlled, or use a planned factorial comparison.

Light and fertilizer both differ between the two plants, so light is a confounding variable and the growth difference cannot be assigned to fertilizer. A supported design keeps light, water volume, pot size, soil, and temperature the same and varies only fertilizer, with several plants per group.

Response criteria

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

13 cm

(11 + 14 + 12 + 15) / 4 = 52 / 4 = 13 cm. Report the range 11–15 cm alongside the mean so the spread stays visible.

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.

6780 J

3.0 g × 2,260 J/g = 6,780 J. The high heat of vaporization is why transpiration cools a leaf effectively.

Water molecules are polar and hydrogen-bond with each other, while oil molecules are nonpolar and cannot. Water molecules associate with one another rather than surrounding the oil, so the two form separate layers. The same behaviour organizes the cell membrane: nonpolar fatty-acid tails are excluded from water and face inward, while polar phosphate heads face the watery surroundings.

Response criteria

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

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.

The glucose units are linked in different orientations, giving starch and cellulose different bond geometry and overall shape. Human digestive enzymes have active sites that fit the linkage in starch but not the one in cellulose, so cellulose passes through undigested. Enzyme specificity therefore depends on the precise arrangement of a substrate, not only on its chemical formula.

Response criteria

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

119

A chain of 120 units contains 119 bonds, so 119 water molecules are used.

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.

Fresh pineapple contains a protein-digesting enzyme that hydrolyses the gelatin protein, so the chains are too short to form a gel. Canning heats the fruit, which denatures the enzyme and destroys its active-site shape, so the gelatin sets normally. The comparison isolates the enzyme as the cause, since the fruit is otherwise the same.

Response criteria

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

8 μmol/min

(7 + 8 + 9) / 3 = 8 μmol/min, with an observed range of 7–9 that should be reported alongside it.

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.

2 ATP

One ATP supplies about 30 kJ/mol, which is not enough; two supply about 60 kJ/mol, which covers 46 kJ/mol. The excess is released as heat rather than stored.

Synthesis and decomposition describe whether molecules get larger or smaller, not the energy balance. The net energy change depends on the relative stability of all bonds in the reactants and products and on the conditions. Some syntheses release energy and some decompositions require it, so the direction of size change is not a reliable guide.

Response criteria

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

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.