IB Biology Quiz: Apply Proteins
20 questions · exam conditions
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Apply ProteinsQuestion 1 of 20

The sodium-potassium pump is a transmembrane protein essential for neural signalling. It actively transports three sodium ions out of the cell and two potassium ions into the cell, a process that requires ATP. This protein must therefore possess binding sites for which combination of molecules?

Sodium ions, potassium ions, and ADP.
Sodium ions and potassium ions only.
Sodium ions, potassium ions, and ATP.
Sodium ions, potassium ions, ATP, and chloride ions.
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IB Biology Quiz

IB Biology Quiz: Apply Proteins

Practice Apply Proteins in IB Biology with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Apply Proteins, giving you a quick way to practice the rules, question types, and explanations that matter most for IB Biology.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

The sodium-potassium pump is a transmembrane protein essential for neural signalling. It actively transports three sodium ions out of the cell and two potassium ions into the cell, a process that requires ATP. This protein must therefore possess binding sites for which combination of molecules?

  1. Sodium ions, potassium ions, and ADP.
  2. Sodium ions and potassium ions only.
  3. Sodium ions, potassium ions, and ATP. (correct answer)
  4. Sodium ions, potassium ions, ATP, and chloride ions.
Explanation: The correct answer is C. The function of the sodium-potassium pump involves binding to its transport substrates (sodium and potassium ions) and also binding to ATP to power the transport. The hydrolysis of ATP to ADP and Pi provides the energy for the conformational change that moves the ions against their concentration gradients. Therefore, the protein must have specific binding sites for Na⁺, K⁺, and ATP. A is incorrect because the pump binds ATP, not ADP; ADP is a product of the reaction. B is incorrect because it omits the essential energy source, ATP. D is incorrect as chloride ions are not directly involved in the action of this specific pump.

Question 2

A point mutation in a gene results in the substitution of glutamic acid (a negatively charged amino acid) with valine (a non-polar amino acid) on the surface of a globular protein that functions as an enzyme in the cytoplasm. Which of the following is the most likely consequence for the protein?

  1. The primary structure of the protein will be significantly altered, causing immediate denaturation and loss of function.
  2. The protein is likely to aggregate with other copies of itself, as the new hydrophobic patch on its surface will seek to avoid the aqueous environment. (correct answer)
  3. The protein's catalytic activity will increase because the removal of a charged residue will reduce electrostatic repulsion with the substrate.
  4. The protein will become a fibrous protein because the change in amino acid sequence will disrupt its ability to fold into a globular shape.
Explanation: The correct answer is B. Replacing a hydrophilic, charged amino acid (glutamic acid) on the surface with a hydrophobic, non-polar one (valine) creates a 'hydrophobic patch'. In the aqueous cytoplasm, this patch will be unstable and will tend to associate with hydrophobic regions on other molecules, leading to protein aggregation. This is the underlying principle of sickle cell anemia, where a similar mutation occurs in hemoglobin. A is incorrect because a single amino acid substitution alters the primary structure but does not necessarily cause immediate, complete denaturation. C is incorrect as this change is more likely to disrupt the active site or overall conformation, decreasing activity rather than increasing it. D is incorrect as a single point mutation is extremely unlikely to convert a globular protein into a fibrous one; this involves a fundamental change in the entire polypeptide's folding pattern and repetitive sequence.

Question 3

A researcher discovers a novel protein that shows remarkable stability at high temperatures (above 90°C). Analysis of its tertiary structure reveals an unusually high number of specific covalent bonds stabilizing its shape. Which bond type is most likely responsible for this thermostability?

  1. Hydrogen bonds between R-groups of polar amino acids.
  2. Ionic bonds between positively and negatively charged R-groups.
  3. Disulfide bridges between cysteine residues. (correct answer)
  4. Hydrophobic interactions between non-polar R-groups in the core.
Explanation: The correct answer is C. Disulfide bridges are covalent bonds, which are significantly stronger than the other interactions listed. This covalent cross-linking makes the protein's tertiary structure much more resistant to thermal denaturation. A, B, and D are all weaker, non-covalent interactions that are more easily disrupted by the increased kinetic energy at high temperatures. While all these interactions contribute to the protein's overall structure, the presence of strong covalent disulfide bridges is a key adaptation for thermostability.

Question 4

The protein collagen is composed of three polypeptide chains twisted into a triple helix, forming a strong, rope-like structure. This arrangement describes which level(s) of protein structure?

  1. Tertiary structure only.
  2. Secondary and tertiary structures only.
  3. Quaternary structure only.
  4. Secondary, tertiary, and quaternary structures. (correct answer)
Explanation: The correct answer is D. Collagen exhibits all three higher levels of structure. The individual polypeptide chains are coiled into a helical shape (secondary structure). Each of these chains then folds into its specific three-dimensional conformation (tertiary structure). Finally, the three separate polypeptide chains associate to form the functional triple helix, which is the definition of quaternary structure (the interaction of multiple polypeptide subunits). A, B, and C are incorrect because they fail to account for all the levels of organization present in the final functional protein.

Question 5

A competitive inhibitor and a non-competitive inhibitor were tested on an enzyme. Both inhibitors were found to decrease the rate of the enzyme-catalyzed reaction. A researcher then significantly increased the substrate concentration in both experiments.

Based on the principles of protein-inhibitor interactions, what is the expected outcome of increasing the substrate concentration?

  1. The effect of both inhibitors will be overcome, and the reaction rate will approach its maximum velocity (Vmax).
  2. The inhibition by the competitive inhibitor will be reduced, but the inhibition by the non-competitive inhibitor will remain largely unaffected. (correct answer)
  3. The inhibition by the non-competitive inhibitor will be reduced, but the inhibition by the competitive inhibitor will remain largely unaffected.
  4. The effect of both inhibitors will be enhanced, as higher substrate concentration increases the chances of inhibitor binding.
Explanation: The correct answer is B. Competitive inhibitors bind to the active site, directly competing with the substrate. By increasing the substrate concentration, the substrate molecules are more likely to outcompete the inhibitor for the active site, thus reducing the effect of the inhibition. Non-competitive inhibitors bind to an allosteric site, changing the enzyme's conformation and reducing its efficiency. Since they do not compete for the active site, increasing substrate concentration does not overcome their effect. A is incorrect because the effect of the non-competitive inhibitor is not overcome. C is incorrect as it reverses the roles of the two inhibitor types. D is incorrect as higher substrate concentration specifically counteracts competitive inhibition, it does not enhance it.

Question 6

[HL] The proteome of a human cell is vastly more diverse than its genome. Which biological process provides the most significant contribution to this phenomenon by generating multiple different protein products from a single gene sequence?

  1. The occurrence of point mutations during DNA replication.
  2. Alternative splicing of pre-mRNA before translation. (correct answer)
  3. The degeneracy of the genetic code allowing multiple codons for one amino acid.
  4. The action of chaperone proteins during post-translational folding.
Explanation: The correct answer is B. Alternative splicing allows different combinations of exons from a single pre-mRNA transcript to be joined together, creating different mature mRNA molecules. These different mRNAs are then translated into distinct protein isoforms, significantly expanding the proteome from a limited number of genes. A is incorrect because mutations are generally random events and not a programmed mechanism for generating protein diversity. C is incorrect because degeneracy refers to the redundancy of the code (multiple codons for one amino acid), which ensures fidelity but does not create different proteins from one gene. D is incorrect because chaperone proteins assist in the correct folding of a polypeptide chain, but they do not alter its primary amino acid sequence to create a different protein.

Question 7

Prions are infectious agents composed solely of protein. The pathogenic form (PrPSc) has the same primary amino acid sequence as the normal cellular form (PrPC) but a different three-dimensional structure with a higher proportion of beta-pleated sheets. How does the PrPSc protein cause disease?

  1. It degrades the normal PrPC protein, leading to a loss of function in neural cells.
  2. It acts as a template, inducing the misfolding of normal PrPC proteins into the pathogenic PrPSc form. (correct answer)
  3. It inserts into the host cell's genome and directs the synthesis of more pathogenic protein.
  4. It blocks the active sites of essential enzymes, leading to metabolic failure in the brain.
Explanation: The correct answer is B. The mechanism of prion disease is based on a conformational change. The pathogenic PrPSc protein physically interacts with the normal PrPC protein and catalyzes its conversion into the misfolded PrPSc conformation. This sets off a chain reaction, leading to the accumulation of insoluble PrPSc aggregates that cause neurodegeneration. A is incorrect because PrPSc does not degrade PrPC; it converts it. C is incorrect because prions are proteins and contain no nucleic acid, so they cannot alter the host's genome. D is incorrect because the primary pathology is protein aggregation and conversion, not enzyme inhibition.

Question 8

Some proteins require a non-protein component, called a prosthetic group, to be functional. Hemoglobin is an example of such a conjugated protein. Which of the following correctly identifies the prosthetic group in hemoglobin and its direct function?

  1. The globin chain, which changes conformation to bind oxygen.
  2. The heme group, which contains an iron atom that reversibly binds oxygen. (correct answer)
  3. The iron atom, which catalyzes the formation of carbonic acid.
  4. The alpha-helix, which provides a binding site for carbon dioxide.
Explanation: The correct answer is B. The prosthetic group is the non-protein part, which in hemoglobin is the heme group. Each heme group contains a central iron ion (Fe²⁺) that is the actual site of reversible oxygen binding. A is incorrect because the globin chain is the polypeptide (protein) part, not the prosthetic group. C is incorrect because while the iron atom is part of the heme group, its function here is oxygen binding, not catalysis of carbonic acid formation (which is done by carbonic anhydrase). D is incorrect because the alpha-helix is a level of secondary structure within the globin protein, not a prosthetic group.

Question 9

Proline is an amino acid with a unique cyclic R-group that restricts bond rotation. What is the most likely structural consequence of placing a proline residue in the middle of a polypeptide sequence that would otherwise form an alpha-helix?

  1. It will introduce a kink or break in the alpha-helix due to its rigid structure. (correct answer)
  2. It will strengthen the alpha-helix by forming additional hydrogen bonds.
  3. It will cause the polypeptide to form a beta-pleated sheet instead of an alpha-helix.
  4. It will have no effect on the secondary structure but will alter the tertiary structure.
Explanation: The correct answer is B. The rigid ring structure of proline's R-group prevents it from adopting the phi and psi bond angles required for a regular alpha-helix. Furthermore, its amino group, being part of the ring, cannot act as a hydrogen bond donor in the standard alpha-helical pattern. For these reasons, proline is known as a 'helix breaker' and its presence typically introduces a kink or terminates an alpha-helical segment. A is incorrect as it does the opposite. C is incorrect as its presence disrupts helices but doesn't necessarily induce sheet formation. D is incorrect because it has a profound and direct effect on the secondary structure.

Question 10

A researcher identifies two globular proteins of similar molecular mass. Protein A is an intracellular enzyme, while Protein B is a secreted hormone. What difference would be expected in the amino acid composition of their surfaces?

  1. Protein A will have more non-polar amino acids on its surface than Protein B.
  2. Protein B will have more hydrophilic amino acids on its surface than Protein A.
  3. Both proteins will have a similar proportion of polar and non-polar amino acids on their surfaces.
  4. Protein B is more likely to have cysteine residues on its surface to form disulfide bridges for stability in the extracellular environment. (correct answer)
Explanation: The correct answer is D. Secreted proteins like hormones must be stable in the harsh extracellular environment (e.g., bloodstream), which is oxidizing and has fluctuating conditions. Disulfide bridges, which are covalent bonds formed between cysteine residues, provide significant stability to the protein's tertiary structure and are commonly found in secreted proteins. The cytoplasm (intracellular environment) is a reducing environment, which disfavors disulfide bond formation. Both proteins will be soluble in their respective aqueous environments (cytoplasm and blood plasma), so both will have predominantly hydrophilic surfaces (making A and B incorrect). However, the need for enhanced stability through covalent cross-linking is a key feature of many secreted proteins.

Question 11

The protein ubiquitin is a small, highly conserved globular protein that tags other proteins for degradation by the proteasome. What property of globular proteins makes ubiquitin well-suited for this role within the crowded cellular cytoplasm?

  1. Their general insolubility, which causes tagged proteins to precipitate out of the cytoplasm.
  2. Their high water solubility and compact shape, allowing them to diffuse freely and interact with target proteins. (correct answer)
  3. Their repetitive primary structure, which provides multiple identical binding sites for the proteasome.
  4. Their tendency to form long, structural filaments that can guide tagged proteins to the proteasome.
Explanation: The correct answer is B. Globular proteins are characterized by their compact, roughly spherical shape and their solubility in aqueous environments like the cytoplasm. These properties are essential for proteins like ubiquitin that need to move around the cell (diffuse) and interact specifically with a wide range of other soluble or membrane-bound proteins. A is incorrect because globular proteins are typically soluble, not insoluble. C describes a feature of fibrous proteins, not globular ones. D also describes a property of fibrous structural proteins, whereas ubiquitin functions as a mobile signalling molecule.

Question 12

[HL] In epigenetics, histone proteins can be chemically modified. The acetylation of lysine residues on histone tails reduces their positive charge. What is the most likely effect of this modification on DNA packaging and gene expression?

  1. It neutralizes the charge attraction between histones and DNA, leading to a more open chromatin structure and increased transcription. (correct answer)
  2. It causes the DNA to bind more tightly to the histones, leading to gene silencing.
  3. It causes the histone proteins to denature, releasing the DNA completely and stopping all gene expression.
  4. It recruits DNA methyltransferase enzymes, which add methyl groups to the DNA and permanently inactivate the genes.
Explanation: The correct answer is B. DNA has a negatively charged phosphate backbone. Histone proteins are rich in positively charged amino acids like lysine, which allows them to bind tightly to DNA. Acetylation adds an acetyl group to the lysine, neutralizing its positive charge. This weakens the electrostatic interaction between the histones and DNA, causing the chromatin to decondense into a more open state (euchromatin). This open structure allows transcription factors and RNA polymerase to access the DNA, generally leading to an increase in gene expression. A is incorrect as it describes the opposite effect. C is an overstatement; it modifies but does not denature the histones. D confuses two different epigenetic mechanisms; histone acetylation and DNA methylation are distinct processes, though they can influence each other.

Question 13

Lysozyme is an enzyme that functions in an acidic environment (pH ~5.0). If it is moved to a solution with a pH of 10.0, it loses all catalytic activity. This loss of function is primarily due to the disruption of which type of bonds that stabilize its active site conformation?

  1. Peptide bonds between adjacent amino acids.
  2. Ionic bonds and hydrogen bonds between R-groups. (correct answer)
  3. Hydrophobic interactions within the protein's core.
  4. Disulfide bridges between cysteine residues.
Explanation: The correct answer is B. A significant change in pH alters the ionization state of acidic and basic R-groups (like those on aspartic acid, glutamic acid, lysine, and arginine). This change disrupts the ionic bonds (salt bridges) and hydrogen bonds that are critical for maintaining the specific tertiary structure of the protein, including the precise shape of the active site. A is incorrect because peptide bonds, which form the primary structure, are covalent and not broken by pH changes. C, while important for overall structure, is less directly affected by pH than the charged R-groups. D is incorrect as disulfide bridges are covalent and also resistant to changes in pH.

Question 14

In the induced-fit model of enzyme action, the binding of the substrate to the enzyme's active site causes a change in the protein's conformation. What is the primary purpose of this conformational change?

  1. To release the products from the active site once the reaction is complete.
  2. To increase the activation energy of the reaction, ensuring it only proceeds under specific conditions.
  3. To orient the substrate molecules optimally and strain their chemical bonds, facilitating catalysis. (correct answer)
  4. To permanently alter the enzyme's primary structure, adapting it to the specific substrate.
Explanation: The correct answer is C. The induced-fit model proposes that the active site is flexible. The initial binding of the substrate induces a subtle change in the enzyme's shape, which brings catalytic R-groups into the proper position to act on the substrate. This optimal alignment and the physical strain placed on the substrate's bonds lower the activation energy, making the reaction more likely to occur. A describes a later step in the catalytic cycle, not the purpose of the initial binding change. B is incorrect as enzymes lower, not increase, activation energy. D is incorrect as the conformational change is temporary and does not alter the covalent primary structure.

Question 15

An experiment is conducted to determine the isoelectric point (pI) of a protein, which is the pH at which the protein has no net electrical charge. At a pH below its pI, what would be the net charge of the protein and why?

  1. Net positive, because the carboxyl groups (–COOH) gain protons.
  2. Net negative, because the carboxyl groups (–COOH) lose protons.
  3. Net negative, because the amino groups (–NH2) lose protons.
  4. Net positive, because the amino groups (–NH2) gain protons. (correct answer)
Explanation: The correct answer is B. At a pH below the isoelectric point, the solution is relatively acidic, meaning there is a high concentration of H⁺ ions (protons). These protons will be accepted by the basic amino groups (–NH₂), converting them to their protonated form (–NH₃⁺). While some carboxyl groups will also be protonated (–COO⁻ to –COOH), the predominant effect is the protonation of amino groups, resulting in a net positive charge on the protein. A is incorrect because while carboxyl groups do gain protons, the main effect on net charge comes from the amino groups becoming positive. C and D describe what happens at a pH above the pI, where the protein would have a net negative charge due to deprotonation.

Question 16

A researcher uses a chemical that breaks all hydrogen bonds in a protein but leaves peptide and disulfide bonds intact. Which level of protein structure would be completely disrupted?

  1. Secondary structure. (correct answer)
  2. Primary structure.
  3. Tertiary structure, but not secondary.
  4. Quaternary structure, but not tertiary or secondary.
Explanation: The correct answer is B. Secondary structures, such as alpha-helices and beta-pleated sheets, are formed and stabilized exclusively by hydrogen bonds between the atoms of the polypeptide backbone. Breaking all hydrogen bonds would completely eliminate these structures. A is incorrect because primary structure is the sequence of amino acids linked by peptide bonds, which are left intact. C and D are incorrect because while tertiary and quaternary structures would also be severely disrupted (as they rely on hydrogen bonds among other interactions), the secondary level is the one defined by hydrogen bonding in the backbone and would therefore be the most completely and directly eliminated.

Question 17

A patient is treated with a drug that acts as an allosteric activator for a specific enzyme in a metabolic pathway. How does this drug most likely exert its effect on the enzyme's protein structure?

  1. It binds covalently to the active site, permanently increasing its affinity for the substrate.
  2. It binds to a site other than the active site, inducing a conformational change that stabilizes the enzyme in its active form. (correct answer)
  3. It acts as a coenzyme, directly participating in the catalytic reaction within the active site.
  4. It cleaves a small peptide from the enzyme, a process known as proteolytic activation, to expose the active site.
Explanation: The correct answer is B. This is the definition of allosteric activation. Allosteric modulators (activators or inhibitors) bind to a regulatory site (the allosteric site) distinct from the active site. This binding causes a change in the protein's shape (conformation) that either increases (activation) or decreases (inhibition) the enzyme's affinity for its substrate. A is incorrect because allosteric modulators do not bind to the active site, and their binding is typically non-covalent. C describes the function of a coenzyme, not an allosteric activator. D describes a different mechanism of enzyme activation (e.g., zymogen activation), not allosteric regulation.

Question 18

[HL] During muscle contraction, the protein troponin undergoes a conformational change upon binding to Ca²⁺ ions. What is the direct functional consequence of this protein shape change?

  1. It pulls on tropomyosin, exposing the myosin-binding sites on the actin filament. (correct answer)
  2. It causes ATP to hydrolyze, providing energy for the power stroke.
  3. It directly pulls the actin filament towards the M-line of the sarcomere.
  4. It causes the myosin head to detach from the actin filament, allowing the muscle to relax.
Explanation: The correct answer is B. In a resting muscle, the protein tropomyosin blocks the sites on the actin filament where myosin heads can bind. When calcium ions are released, they bind to troponin. This binding causes troponin to change shape, and this change in conformation pulls the attached tropomyosin molecule away from the binding sites. This exposure allows the myosin heads to bind to actin and initiate the cross-bridge cycle. A is incorrect because ATP hydrolysis occurs on the myosin head. C is incorrect as the pulling is done by the myosin heads, not troponin. D is incorrect as myosin detachment is caused by the binding of a new ATP molecule, not the action of troponin.

Question 19

Antibodies are proteins that play a critical role in the adaptive immune system. The specificity of an antibody for its antigen is determined by which feature of its structure?

  1. The sequence of amino acids in the constant regions of the heavy and light chains.
  2. The overall quaternary structure formed by the four polypeptide chains.
  3. The three-dimensional shape and chemistry of the variable regions at the tips of the 'Y' shape. (correct answer)
  4. The type of disulfide bridges that link the heavy and light chains together.
Explanation: The correct answer is C. The antigen-binding site of an antibody is formed by the variable regions of one heavy chain and one light chain. The unique amino acid sequence in these regions creates a specific 3D shape and distribution of charges that is complementary to a particular epitope on an antigen. This is the basis of antibody specificity. A is incorrect because the constant regions are similar among antibodies of the same class and determine the antibody's effector function, not its antigen specificity. B is incorrect because while the quaternary structure is essential, it is the specific variable region within that structure that confers specificity. D is incorrect because the disulfide bridges provide structural integrity but do not determine which antigen will be bound.

Question 20

Which statement best contrasts the structural properties of keratin, a fibrous protein, with those of myoglobin, a globular protein?

  1. Keratin has a complex quaternary structure and is soluble in water, while myoglobin is a single polypeptide and is insoluble.
  2. Keratin is primarily composed of beta-pleated sheets for strength, while myoglobin is composed of alpha-helices and functions in oxygen storage.
  3. Keratin's function depends on its tertiary structure being easily denatured, while myoglobin's structure is stabilized by numerous disulfide bridges.
  4. Keratin consists of long polypeptide chains with a repetitive amino acid sequence, making it insoluble, while myoglobin folds into a compact, soluble shape. (correct answer)
Explanation: The correct answer is B. This statement accurately captures the key differences. Fibrous proteins like keratin have elongated, repetitive structures (often rich in alpha-helices or beta-sheets) that assemble into fibers, making them strong and insoluble, suitable for structural roles. Globular proteins like myoglobin have complex tertiary (and sometimes quaternary) structures that fold into a compact, spherical shape, which makes them soluble in the cytoplasm and suitable for metabolic roles like transport or catalysis. A is incorrect because keratin is insoluble and myoglobin is soluble. C is incorrect because keratin's function depends on stability, not denaturation, and myoglobin does not have disulfide bridges. D is incorrect because keratin is primarily composed of alpha-helices, not beta-pleated sheets.