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NSSCAS Biology 8223 practice questions

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Question 1Cell structure and function

Which statement best describes the function of the rough endoplasmic reticulum in a eukaryotic cell?

  1. It synthesises proteins on attached ribosomes and folds and modifies them for transport. Correct answer.
  2. It digests worn-out organelles using hydrolytic enzymes at low pH.
  3. It packages proteins into vesicles for secretion after they have been modified.
  4. It produces ATP by oxidative phosphorylation on its inner membrane.

Explanation

The rough ER is studded with ribosomes, so it is the site where proteins destined for membranes, secretion or other organelles are synthesised and then folded and modified inside its lumen. The other options describe the mitochondrion, the Golgi apparatus and the lysosome respectively.

Question 2Cell structure and function

A student examines a cell and finds a rigid wall outside the plasma membrane, a large permanent vacuole and chloroplasts. Which conclusion is most consistent with these observations?

  1. The cell is a plant cell because of the wall, vacuole and chloroplasts together. Correct answer.
  2. The cell is an animal cell because it has a vacuole.
  3. The cell is a prokaryote because it has a rigid wall.
  4. The cell is a fungal cell because chloroplasts are absent from plants.

Explanation

A cellulose wall, a large permanent vacuole and chloroplasts occurring together are diagnostic features of a photosynthetic plant cell. Prokaryotes and fungi may have walls but lack chloroplasts and a large central vacuole.

Question 3Cell structure and function

Which feature is shared by all prokaryotic cells?

  1. Circular DNA free in the cytoplasm Correct answer.
  2. A membrane-bound nucleus containing linear DNA
  3. A cell wall made of cellulose
  4. Membrane-bound organelles such as mitochondria

Explanation

Prokaryotes keep their genetic material as circular DNA in the cytoplasm, not enclosed by a nuclear envelope. They also lack membrane-bound organelles such as mitochondria, and their walls are not made of cellulose.

Question 4Cell structure and function

A cell is placed in a solution with a water potential of -800 kPa. The cell contents have a water potential of -500 kPa. What is the expected net movement of water?

  1. Water moves equally in both directions.
  2. Water moves into the cell by active transport.
  3. Water enters the cell by osmosis. Correct answer.
  4. Water leaves the cell by osmosis.

Explanation

Water moves by osmosis from the higher (less negative) water potential to the lower (more negative) one. The cell at -500 kPa has the higher water potential, so water enters it from the -800 kPa solution.

Question 5Cell structure and function

What is the main role of the Golgi apparatus in a secretory cell?

  1. Synthesising lipids for the cell membrane
  2. Transcribing DNA into messenger RNA
  3. Modifying proteins and packaging them into vesicles Correct answer.
  4. Generating ATP for protein synthesis

Explanation

The Golgi apparatus receives proteins from the ER, further modifies them (for example by glycosylation) and packages them into vesicles for secretion or delivery. Lipid synthesis and ATP production belong to other organelles.

Question 6Cell structure and function

Which statement correctly describes the fluid mosaic model of the plasma membrane?

  1. A protein layer enclosing a core of lipid-soluble pigments
  2. A phospholipid bilayer with proteins and glycoproteins dispersed within and across it Correct answer.
  3. A rigid double layer of lipids with proteins fixed on the outer surface
  4. A single layer of phospholipids coated on both sides by protein

Explanation

The model pictures the membrane as a phospholipid bilayer in which proteins float and are partly or fully embedded, giving a mosaic pattern within a fluid lipid environment. This allows lateral movement and membrane flexibility.

Question 7Cell structure and function

Which observation provides direct evidence that a structure is a membrane-bound organelle rather than a fold of the plasma membrane?

  1. It is bounded by a single membrane and contains hydrolytic enzymes.
  2. It is visible under a light microscope.
  3. It is surrounded by a double membrane and lies free in the cytoplasm. Correct answer.
  4. It contains DNA.

Explanation

A structure enclosed by its own double membrane and separate from the plasma membrane is a distinct organelle, as seen with mitochondria and chloroplasts. Being visible or containing DNA does not by itself prove separation from the plasma membrane.

Question 8Cell structure and function

An electron micrograph shows a cell with many mitochondria and extensive rough endoplasmic reticulum but no chloroplasts or cell wall. Which cell type is most likely?

  1. A root hair cell
  2. A palisade mesophyll cell
  3. A metabolically active animal cell such as a liver cell Correct answer.
  4. A bacterial cell

Explanation

Numerous mitochondria and abundant rough ER indicate high rates of ATP production and protein synthesis, typical of active animal cells. The absence of a wall and chloroplasts rules out plant and bacterial cells.

Question 9Cell structure and function

Which combination of features is found in a typical plant cell but not in a typical animal cell?

  1. Cellulose cell wall and chloroplasts Correct answer.
  2. Mitochondria and ribosomes
  3. Nucleus and rough endoplasmic reticulum
  4. Golgi apparatus and lysosomes

Explanation

A cellulose wall and chloroplasts are characteristic of plant cells and absent from animal cells. Mitochondria, ribosomes, a nucleus, ER, Golgi and lysosomes occur in both.

Question 10Cell structure and function

A red blood cell is placed in distilled water and bursts. Which explanation is correct?

  1. Solutes diffuse out of the cell, causing it to swell.
  2. Water leaves the cell because the cell has a higher water potential than pure water.
  3. Water enters the cell by osmosis because the cell contents have a lower water potential than pure water. Correct answer.
  4. Water is actively pumped into the cell against a concentration gradient.

Explanation

Pure water has a water potential of zero, higher than the negative water potential of the cytoplasm, so water enters by osmosis. With no cell wall to resist the pressure, the cell swells and bursts.

Question 11Cell structure and function

A plant cell with a pressure potential of 300 kPa and a solute potential of -700 kPa is placed in pure water. What is its water potential, and will water enter?

  1. -1000 kPa; water will enter
  2. -400 kPa; water will leave
  3. -400 kPa; water will enter Correct answer.
  4. +400 kPa; water will leave

Explanation

Water potential equals pressure potential plus solute potential, so 300 + (-700) = -400 kPa. Pure water has a water potential of 0 kPa, which is higher, so water enters the cell by osmosis.

Question 12Cell structure and function

Which statement about the relationship between surface area to volume ratio and cell size is correct?

  1. Surface area to volume ratio stays constant regardless of cell size.
  2. As a cell increases in size, its surface area to volume ratio decreases. Correct answer.
  3. Surface area to volume ratio is unrelated to the rate of diffusion into a cell.
  4. As a cell increases in size, its surface area to volume ratio increases.

Explanation

Surface area increases with the square of the linear dimension while volume increases with the cube, so the ratio falls as the cell grows. A smaller ratio limits the rate at which materials can be exchanged relative to the cell's needs.

Question 13Cell structure and function

Which feature of the small intestine epithelium increases the rate of absorption of glucose?

  1. A thick layer of keratin on the surface cells
  2. A low density of mitochondria in the epithelial cells
  3. A single flat layer of cells with no folds
  4. Numerous microvilli on the apical surface Correct answer.

Explanation

Microvilli greatly increase the surface area of the apical membrane, raising the rate of absorption. Active transport of glucose also relies on many mitochondria supplying ATP.

Question 14Cell structure and function

Which process allows a large polar molecule to enter a cell without passing directly through the phospholipid bilayer?

  1. Bulk flow through the cell wall
  2. Osmosis through the bilayer
  3. Simple diffusion through the hydrophobic core
  4. Facilitated diffusion through a channel or carrier protein Correct answer.

Explanation

Large polar molecules cannot cross the hydrophobic core freely, so they pass through membrane proteins by facilitated diffusion. This is passive but depends on specific transport proteins.

Question 15Cell structure and function

A cell is treated with a drug that blocks ATP synthesis. Which transport process would be most directly affected?

  1. Osmosis of water across the membrane
  2. Active transport of sodium ions out of the cell Correct answer.
  3. Diffusion of oxygen into the cell
  4. Facilitated diffusion of glucose down its gradient

Explanation

Active transport moves substances against their gradient and requires ATP, so blocking ATP synthesis directly stops sodium pumps. Diffusion, osmosis and facilitated diffusion are passive and do not depend on ATP.

Question 16Cell structure and function

Which structure is responsible for maintaining cell shape and enabling movement of organelles within a eukaryotic cell?

  1. The nucleolus
  2. The smooth endoplasmic reticulum
  3. The cytoskeleton Correct answer.
  4. The lysosome

Explanation

The cytoskeleton, made of microtubules, microfilaments and intermediate filaments, supports the cell and provides tracks for moving organelles and vesicles. The other structures have different specialised roles.

Question 17Cell structure and function

In a cell that secretes digestive enzymes, which sequence correctly traces the pathway of an enzyme from synthesis to release?

  1. Golgi apparatus, rough ER, lysosome, plasma membrane
  2. Smooth ER, lysosome, secretory vesicle, plasma membrane
  3. Nucleus, mitochondrion, Golgi apparatus, plasma membrane
  4. Rough ER, Golgi apparatus, secretory vesicle, plasma membrane Correct answer.

Explanation

Secretory proteins are made on ribosomes of the rough ER, modified and packaged by the Golgi, carried in secretory vesicles and released when the vesicles fuse with the plasma membrane.

Question 18Cell structure and function

Which statement about the resolving power of microscopes is correct?

  1. A light microscope resolves finer detail than an electron microscope.
  2. Staining a specimen increases the resolving power of the lens.
  3. The resolution of a microscope is limited by the wavelength used to form the image. Correct answer.
  4. Magnification and resolution mean exactly the same thing.

Explanation

Resolution depends on the wavelength of the radiation used; shorter wavelengths allow finer detail to be distinguished, which is why electron microscopes resolve better than light microscopes. Magnification is the degree of enlargement and is distinct from resolution.

Question 19Cell structure and function

A cell has a solute potential of -600 kPa and is at incipient plasmolysis. What is its pressure potential?

  1. +600 kPa
  2. 0 kPa Correct answer.
  3. +1200 kPa
  4. -600 kPa

Explanation

At incipient plasmolysis the protoplast just begins to pull away from the wall, so the pressure potential is zero. The water potential then equals the solute potential, -600 kPa.

Question 20Cell structure and function

Which property of the phospholipid bilayer directly explains why small nonpolar molecules cross the membrane more readily than small ions?

  1. The bilayer is rigid and impermeable to everything.
  2. The bilayer contains only proteins and no lipid.
  3. The bilayer is hydrophilic on the inside.
  4. The hydrophobic core repels charged particles. Correct answer.

Explanation

The interior of the bilayer is hydrophobic, so nonpolar molecules dissolve in it and pass through, whereas charged ions are repelled and need protein channels or carriers.

Question 21Cell structure and function

A cell is placed in a solution whose water potential is equal to that of the cell contents. Which prediction is correct?

  1. There is no net movement of water, so the cell volume remains constant. Correct answer.
  2. Water leaves the cell until it becomes flaccid.
  3. Solutes move across the membrane until the cell shrinks.
  4. Water enters the cell until it bursts.

Explanation

When the external and internal water potentials are equal, water molecules still cross the membrane but the rates in each direction are equal, so there is no net movement and volume stays constant.

Question 22Cell structure and function

A researcher finds that a substance enters a cell only when its external concentration is higher than its internal concentration and the process stops when ATP synthesis is inhibited. Which conclusion is best supported?

  1. The substance enters by simple diffusion.
  2. The substance enters by osmosis.
  3. The substance enters by facilitated diffusion.
  4. The substance enters by active transport. Correct answer.

Explanation

Entry only down a concentration gradient could be passive, but the dependence on ATP shows energy is used, which is the defining feature of active transport. The gradient condition alone is not decisive, but the ATP requirement is.

Question 23Cell structure and function

Two cells have the same volume, but cell A is spherical and cell B is a long thin cylinder. Which statement about their exchange with the environment is correct?

  1. Cell B has a larger surface area to volume ratio and can exchange materials more efficiently. Correct answer.
  2. Cell B has a smaller surface area to volume ratio because it is elongated.
  3. Both cells have identical surface area to volume ratios because their volumes are equal.
  4. Cell A has a larger surface area to volume ratio because a sphere is compact.

Explanation

For equal volumes, an elongated cylinder exposes more surface area than a compact sphere, so its surface area to volume ratio is larger. A higher ratio allows faster diffusion relative to the cell's needs.

Question 24Cell structure and function

An electron micrograph shows a cell with a nucleus, many chloroplasts and a thick wall, and also numerous mitochondria. Which interpretation is most valid?

  1. The cell is a prokaryote because it has a wall.
  2. The cell is a photosynthesising plant cell that also respires actively. Correct answer.
  3. The cell is an animal cell because it has mitochondria.
  4. The cell cannot be eukaryotic because it has a wall.

Explanation

A nucleus, chloroplasts and a thick wall identify a eukaryotic plant cell, and abundant mitochondria show it also respires at a high rate. Photosynthesis and respiration commonly occur together in illuminated plant cells.

Question 25Biological molecules

Which bond links two monosaccharides together in the formation of a disaccharide?

  1. Peptide bond
  2. Glycosidic bond Correct answer.
  3. Ester bond
  4. Phosphodiester bond

Explanation

A condensation reaction between the hydroxyl groups of two monosaccharides releases one water molecule and forms a glycosidic bond, the characteristic linkage of carbohydrates such as sucrose and lactose.

Question 26Biological molecules

A solution of a reducing sugar is heated with Benedict's reagent. Which observation indicates a positive result?

  1. A purple colour develops
  2. A yellow flame is produced
  3. A brick-red precipitate forms Correct answer.
  4. A white emulsion appears

Explanation

Reducing sugars such as glucose reduce the blue copper(II) ions in Benedict's reagent to insoluble copper(I) oxide, which appears as a brick-red precipitate after heating in a water bath.

Question 27Biological molecules

Which statement correctly describes the structure of an amino acid?

  1. A central carbon bonded to two amino groups, a hydrogen atom and an R group
  2. A central carbon bonded to an amino group, a carboxyl group, a hydrogen atom and an R group Correct answer.
  3. A phosphate group bonded to a pentose sugar and a nitrogenous base
  4. A glycerol backbone bonded to three fatty acid chains

Explanation

Every amino acid has a central (alpha) carbon attached to four groups: an amine group, a carboxyl group, a hydrogen atom and a variable side chain (R group). The R group differs between the twenty standard amino acids and determines their properties.

Question 28Biological molecules

Which reagent is used to test a food sample for the presence of starch?

  1. Benedict's solution
  2. Iodine in potassium iodide solution Correct answer.
  3. Biuret reagent
  4. Ethanol

Explanation

Iodine solution (iodine dissolved in potassium iodide) turns from yellow-brown to blue-black in the presence of starch because iodine molecules become trapped within the coiled amylose helix.

Question 29Biological molecules

Which property of water results directly from hydrogen bonding between water molecules?

  1. Its high specific heat capacity Correct answer.
  2. Its inability to act as a solvent
  3. Its ability to dissolve non-polar gases readily
  4. Its low boiling point compared with similar molecules

Explanation

Hydrogen bonds between water molecules require considerable energy to break, so water absorbs and releases large amounts of heat with little temperature change. This high specific heat capacity helps organisms maintain stable internal temperatures.

Question 30Biological molecules

Which elements are present in all carbohydrates?

  1. Carbon, hydrogen and oxygen Correct answer.
  2. Carbon, hydrogen, oxygen and phosphorus
  3. Carbon, hydrogen, oxygen and nitrogen
  4. Carbon, hydrogen, oxygen and sulfur

Explanation

Carbohydrates are composed of carbon, hydrogen and oxygen, typically in the ratio C:H:O of approximately 1:2:1, as seen in glucose (C6H12O6). Nitrogen and phosphorus are characteristic of proteins and nucleic acids rather than carbohydrates.

Question 31Biological molecules

A student tests a food sample and obtains a purple colour with biuret reagent, but a negative result with Benedict's solution. Which conclusion is best supported?

  1. The sample contains reducing sugar but no protein
  2. The sample contains starch but no reducing sugar
  3. The sample contains protein but no reducing sugar Correct answer.
  4. The sample contains lipid but no protein

Explanation

A purple colour with biuret reagent indicates peptide bonds and therefore protein. A negative Benedict's result means reducing sugar is absent, so the combined evidence supports protein without reducing sugar.

Question 32Biological molecules

Sucrose gives a negative result with Benedict's reagent before hydrolysis but a positive result after boiling with dilute hydrochloric acid and neutralising. Which explanation is correct?

  1. Hydrolysis converts sucrose into starch, which is a reducing sugar
  2. Hydrolysis breaks the glycosidic bond, releasing glucose and fructose which are reducing sugars Correct answer.
  3. The acid itself reduces the copper(II) ions in Benedict's reagent
  4. Sucrose is a reducing sugar that is destroyed by boiling

Explanation

Sucrose is a non-reducing disaccharide because both anomeric carbons are involved in the glycosidic bond. Acid hydrolysis splits this bond to give glucose and fructose, both of which are reducing sugars that react with Benedict's reagent.

Question 33Biological molecules

Which structural feature of a triglyceride makes it an efficient energy storage molecule?

  1. It is highly reduced and contains little oxygen, releasing much energy per gram on oxidation Correct answer.
  2. It is highly branched and therefore rapidly hydrolysed
  3. It contains many peptide bonds that store energy
  4. It contains phosphate groups that store high-energy bonds

Explanation

Triglycerides consist of long hydrocarbon chains that are highly reduced, so oxidation releases roughly twice the energy per gram compared with carbohydrates. They are also stored anhydrously, adding little mass as water.

Question 34Biological molecules

In a phospholipid bilayer, which part of the phospholipid molecule faces the aqueous environment on each side of the membrane?

  1. The ester bonds between glycerol and fatty acids
  2. The glycerol backbone only
  3. The phosphate-containing hydrophilic head Correct answer.
  4. The hydrocarbon hydrophobic tails

Explanation

Phospholipids are amphipathic: the phosphate-containing head is polar and hydrophilic, so it orientates towards water, while the non-polar tails face inwards away from water. This arrangement forms the bilayer of cell membranes.

Question 35Biological molecules

Which level of protein structure is stabilised mainly by hydrogen bonds between the C=O of one amino acid and the N-H of another amino acid four residues away along the same chain?

  1. Secondary structure Correct answer.
  2. Primary structure
  3. Tertiary structure
  4. Quaternary structure

Explanation

Secondary structure refers to local folding such as the alpha helix and beta pleated sheet, held together by hydrogen bonds between backbone carbonyl and amine groups. These regular hydrogen-bonding patterns do not involve the R groups directly.

Question 36Biological molecules

A mutation changes a single amino acid in the active site of an enzyme. Which outcome is most likely?

  1. The primary structure remains unchanged
  2. The substrate no longer fits the active site, so enzyme activity decreases Correct answer.
  3. The enzyme becomes more efficient because all mutations improve function
  4. The enzyme is converted into a carbohydrate

Explanation

The active site shape is determined by the precise sequence and folding of amino acids. Changing one residue can alter the shape or charge of the active site, preventing the substrate from binding and reducing activity.

Question 37Biological molecules

Which pair of molecules are both polymers of glucose?

  1. Glycogen and glycerol
  2. Sucrose and lactose
  3. Starch and cellulose Correct answer.
  4. Maltose and fructose

Explanation

Starch and cellulose are both polysaccharides formed by the condensation of many glucose monomers, differing in the type of glycosidic linkage and the arrangement of chains. Both are therefore glucose polymers.

Question 38Biological molecules

Which feature of cellulose allows it to provide strength in plant cell walls?

  1. It contains many unsaturated fatty acid chains
  2. Alternating beta-glucose units form straight chains cross-linked by many hydrogen bonds Correct answer.
  3. It is composed of amino acids joined by peptide bonds
  4. Branched alpha-glucose chains form compact granules

Explanation

In cellulose, beta-glucose molecules alternate in orientation, producing straight unbranched chains. Adjacent chains are held by numerous hydrogen bonds, forming strong microfibrils that resist tension in plant cell walls.

Question 39Biological molecules

Why does a high body temperature above about 45 °C reduce the activity of most human enzymes?

  1. The substrate molecules are converted into enzymes
  2. Peptide bonds are hydrolysed, breaking the polypeptide into amino acids
  3. The activation energy of the reaction increases permanently
  4. Hydrogen and ionic bonds holding the tertiary structure break, denaturing the active site Correct answer.

Explanation

Above the optimum temperature, increased vibration disrupts the weak hydrogen and ionic bonds that maintain the precise tertiary structure. The active site loses its shape, substrates can no longer bind and the enzyme is denatured.

Question 40Biological molecules

Which statement best explains why ice floats on water and insulates the water beneath it?

  1. Hydrogen bonds hold water molecules in a regular lattice in ice, making it less dense than liquid water Correct answer.
  2. Ice has a higher density than liquid water because its molecules move faster
  3. Ice is denser than water but is held up by surface tension
  4. Ice contains no hydrogen bonds, so its molecules are more closely packed

Explanation

As water freezes, hydrogen bonds fix each molecule to four neighbours in an open hexagonal lattice. This arrangement occupies more volume than liquid water, so ice is less dense and floats, insulating the water below and allowing aquatic organisms to survive.

Question 41Biological molecules

A polypeptide has a quaternary structure. Which observation is consistent with this?

  1. It is a single chain folded into an alpha helix
  2. It consists of two or more polypeptide chains associated together Correct answer.
  3. It contains only one amino acid repeated many times
  4. It consists of a sequence of amino acids joined by peptide bonds

Explanation

Quaternary structure exists only when two or more separate polypeptide chains associate to form a functional protein, as in haemoglobin with its four globin subunits. A single folded chain has, at most, tertiary structure.

Question 42Biological molecules

A sample of DNA from a virus contains 28% adenine. Assuming standard complementary base pairing, what percentage of the bases is guanine?

  1. 28%
  2. 56%
  3. 22% Correct answer.
  4. 44%

Explanation

By complementary base pairing, adenine equals thymine, so thymine is also 28% and A plus T together make 56%. The remaining 44% is shared equally between guanine and cytosine, giving 22% guanine.

Question 43Biological molecules

An enzyme is added to a substrate solution and the initial rate of reaction is measured at increasing substrate concentrations until the rate plateaus. Which explanation accounts for the plateau?

  1. All active sites are occupied, so enzyme concentration becomes the limiting factor Correct answer.
  2. The substrate has been completely used up before the measurement begins
  3. The enzyme molecules are converted into product during the reaction
  4. The activation energy of the reaction has increased to its maximum

Explanation

At high substrate concentration the rate levels off because the number of active sites is finite. Once all active sites are saturated, adding more substrate cannot increase the rate, and the enzyme concentration becomes the limiting factor.

Question 44Biological molecules

Two solutions are separated by a partially permeable membrane. Solution X contains 0.5 mol dm⁻³ sucrose; solution Y contains 0.5 mol dm⁻³ glucose. Which statement best predicts the relative osmotic effects of the two solutions?

  1. Solution Y exerts greater osmotic pressure because glucose has smaller molecules
  2. Neither solution exerts osmotic pressure because the solutes are uncharged
  3. Solution X exerts greater osmotic pressure because sucrose has a larger molecular mass
  4. They exert the same osmotic pressure because they contain equal numbers of dissolved particles per unit volume Correct answer.

Explanation

Osmotic pressure depends on the number of dissolved particles per unit volume, not on the size or mass of the solute molecules. Equal molar concentrations of two non-dissociating solutes therefore produce equal osmotic effects.

Question 45Enzymes

Which statement best describes the induced fit model of enzyme action?

  1. The substrate changes shape to match the rigid active site before catalysis occurs.
  2. The active site has a fixed shape that only substrates of exactly matching shape can enter.
  3. Enzymes lower activation energy by binding substrates covalently at all times.
  4. The active site is flexible and changes shape slightly as the substrate binds, improving the fit. Correct answer.

Explanation

The induced fit model proposes that the active site is not rigidly pre-shaped but adjusts slightly around the substrate as it binds, tightening the fit and straining bonds. This contrasts with the older lock-and-key idea of a fixed complementary shape. The conformational change helps bring catalytic groups into position and destabilise substrate bonds.

Question 46Enzymes

A student adds a competitive inhibitor to an enzyme-catalysed reaction. Which change is expected when the substrate concentration is increased greatly?

  1. The maximum rate stays the same but a higher substrate concentration is needed for half-maximal rate. Correct answer.
  2. The maximum rate falls because the inhibitor permanently denatures the enzyme.
  3. The reaction stops completely because the inhibitor binds irreversibly to the active site.
  4. Both the maximum rate and the substrate affinity increase above the uninhibited values.

Explanation

A competitive inhibitor binds at the active site, so its effect can be overcome by raising substrate concentration. At very high substrate concentration the inhibitor is outcompeted and the same maximum rate is reached, but more substrate is needed to achieve half-maximal rate. The apparent affinity for substrate is therefore reduced.

Question 47Enzymes

Which factor explains why an enzyme-catalysed reaction slows and then stops as temperature rises well above the optimum?

  1. Substrate molecules move too slowly to collide with active sites at high temperature.
  2. The activation energy of the reaction increases as temperature rises.
  3. The enzyme is converted into a product that cannot catalyse the reaction.
  4. Hydrogen bonds and other weak interactions maintaining tertiary structure break, distorting the active site. Correct answer.

Explanation

Above the optimum, increased kinetic energy disrupts the hydrogen bonds, ionic interactions and other weak forces holding the enzyme's tertiary structure. The active site loses its complementary shape, so substrate can no longer bind and the rate falls sharply. This denaturation is usually irreversible.

Question 48Enzymes

An enzyme has an optimum pH of 2. Which statement about its behaviour at pH 7 is most likely correct?

  1. It will be unaffected because pH influences only the substrate and not the enzyme.
  2. It will catalyse the reaction faster because neutral pH suits all enzymes.
  3. It will show maximum activity because pH 7 allows the most hydrogen bonds to form.
  4. It will lose activity because changes in pH alter the ionisation of groups in the active site. Correct answer.

Explanation

Each enzyme has an optimum pH at which the ionisation of amino acid side chains in the active site favours substrate binding and catalysis. A large shift away from the optimum, as from pH 2 to pH 7, alters these charges and can disrupt the tertiary structure. The result is reduced activity and eventually denaturation.

Question 49Enzymes

Which statement correctly describes the role of enzymes in lowering activation energy?

  1. Enzymes supply extra heat energy to reactant molecules.
  2. Enzymes increase the equilibrium constant of the reaction they catalyse.
  3. Enzymes reduce the free energy change of the reaction.
  4. Enzymes provide an alternative reaction pathway with a lower activation energy. Correct answer.

Explanation

Enzymes catalyse reactions by providing an alternative pathway, often through stabilising the transition state, that requires less activation energy. They do not alter the overall free energy change or the position of equilibrium. The rate increases because a larger proportion of molecules can overcome the lower energy barrier.

Question 50Enzymes

In an experiment, the initial rate of an enzyme reaction increases with substrate concentration and then plateaus. Which explanation is correct?

  1. The activation energy of the reaction increases at high substrate concentration.
  2. The enzyme is gradually denatured as substrate concentration rises.
  3. The substrate begins to act as a competitive inhibitor at high concentration.
  4. All active sites become saturated with substrate, so adding more substrate cannot increase the rate further. Correct answer.

Explanation

At low substrate concentration the rate rises roughly in proportion because collisions with active sites are limiting. As concentration increases, more active sites become occupied until all are working at maximum turnover. Further substrate then has no effect, producing the plateau known as the maximum rate.