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Biochemistry - MCAT

Amino Acids

Which of the following amino acids is the only one that lacks chirality at its α-carbon?

A. Alanine
B. Glycine
C. Cysteine
D. Serine

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The stereochemistry of the α-carbon in all naturally occurring eukaryotic amino acids (except glycine) is:

A. D
B. R
C. L
D. (S)

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All chiral amino acids, except cysteine, possess which absolute configuration at the α-carbon?

A. (R)
B. (S)
C. (D)
D. (L)

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The designations L and D indicate __________ configuration, whereas S and R indicate __________ configuration.

A. Absolute; Relative
B. Relative; Absolute
C. Spatial; Geometric
D. Optical; Magnetic

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Chemical Properties and Groups

Amino acids are organic molecules that contain both a(n) __________ group and a(n) __________ group.

A. Hydroxyl; methyl
B. Carboxyl; amino
C. Carbonyl; sulfhydryl
D. Amide; phosphate

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Amphoteric molecules are capable of acting as:

A. Reducing or oxidizing agents
B. Acids or bases
C. Polar or nonpolar compounds
D. Hydrophilic or hydrophobic molecules

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The pH at which half of a molecule's acidic groups are deprotonated is referred to as the:

A. pH
B. pKa
C. Ka
D. Isoelectric point

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At very low pH, amino acids will exist predominantly in which form?

A. Fully deprotonated
B. Fully protonated
C. Zwitterionic
D. Negatively charged

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The pH at which an amino acid carries no net electrical charge is known as the:

A. Isoelectric point
B. pKa
C. Neutralization point
D. Protonation constant

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A molecule that contains both positive and negative charges, yet is overall neutral, is termed a:

A. Cation
B. Dipole
C. Zwitterion
D. Anion

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Isoelectric Point Calculations

For an amino acid with no ionizable side chain, the isoelectric point (pI) can be calculated as:

A. (pKa₁ + pKa₂) / 2
B. (pKa₂ + pKa₃) / 2
C. (pKa₁ + pKa₃) / 2
D. (pKa₁ + pKa₂ + pKa₃) / 3

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For an amino acid with a basic side chain, the isoelectric point is best estimated as:

A. (pKa₁ + pKa₂) / 2
B. (pKa₂ + pKa₃) / 2
C. (pKa₁ + pKa₃) / 2
D. (pKa₂ − pKa₁) / 2

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For an amino acid with an acidic side chain, the isoelectric point is best estimated as:

A. (pKa₁ + pKa₂) / 2
B. (pKa₂ + pKa₃) / 2
C. (pKa₁ + pKa₃) / 2
D. (pKa₁ − pKa₂) / 2

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Amino Acid Identification

Which of the following amino acids is hydrophobic and lacks a side-chain chiral center?

A. Glycine
B. Alanine
C. Proline
D. Serine

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Which amino acid has the side chain –CH₃ and is nonpolar?

A. Alanine
B. Valine
C. Glycine
D. Proline

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Which amino acid is branched-chain and hydrophobic?

A. Serine
B. Valine
C. Asparagine
D. Threonine

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Which amino acid contains a thioether group in its side chain?

A. Methionine
B. Cysteine
C. Serine
D. Threonine

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Which amino acid disrupts α-helices and introduces kinks in secondary structure?

A. Glycine
B. Proline
C. Valine
D. Tyrosine

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Which aromatic amino acid absorbs ultraviolet light and has a benzyl side chain?

A. Tryptophan
B. Phenylalanine
C. Tyrosine
D. Histidine

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Which aromatic amino acid contains an indole functional group?

A. Tryptophan
B. Tyrosine
C. Phenylalanine
D. Histidine

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Which aromatic amino acid serves as a precursor to catecholamines such as dopamine and epinephrine?

A. Tryptophan
B. Tyrosine
C. Phenylalanine
D. Histidine

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Polar and Charged Amino Acids

Which amino acid is polar, uncharged, and contains a hydroxyl group?

A. Serine
B. Threonine
C. Cysteine
D. Tyrosine

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Which amino acid is polar, uncharged, and contains both a hydroxyl and methyl group?

A. Serine
B. Threonine
C. Tyrosine
D. Glutamine

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Which amino acid is polar, uncharged, and forms disulfide bonds in protein structure?

A. Cysteine
B. Methionine
C. Serine
D. Asparagine

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Which amino acid is the amide derivative of aspartic acid?

A. Asparagine
B. Glutamine
C. Serine
D. Glutamate

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Which amino acid is the amide derivative of glutamic acid?

A. Glutamine
B. Asparagine
C. Tyrosine
D. Cysteine

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Charged Amino Acids

Which amino acid has a positively charged side chain at physiological pH and a terminal amino group?

A. Lysine
B. Arginine
C. Histidine
D. Glutamate

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Which amino acid contains a guanidinium functional group?

A. Arginine
B. Lysine
C. Histidine
D. Serine

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Which amino acid contains an imidazole functional group capable of acting as both an acid and base near physiological pH?

A. Histidine
B. Arginine
C. Tyrosine
D. Cysteine

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Which amino acid is negatively charged and structurally related to asparagine?

A. Aspartate
B. Glutamate
C. Glutamine
D. Serine

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Which amino acid is negatively charged and contains one additional methylene group compared to aspartate?

A. Glutamate
B. Aspartate
C. Glutamine
D. Histidine

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Protein Structure Levels

The primary structure of a protein refers to:

A. Hydrogen bonding between backbone atoms
B. The sequence of amino acids linked by peptide bonds
C. The 3-D folding of a polypeptide chain
D. The arrangement of multiple polypeptide subunits

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Which type of bond stabilizes the primary structure of a protein?

A. Hydrogen bonds
B. Ionic bonds
C. Peptide (covalent) bonds
D. Disulfide bonds

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The secondary structure of a protein is primarily stabilized by:

A. Hydrogen bonding between backbone atoms
B. Peptide bonds between amino acids
C. Disulfide linkages between cysteines
D. Hydrophobic interactions among R groups

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α-helices and β-pleated sheets are examples of which protein structure level?

A. Primary
B. Secondary
C. Tertiary
D. Quaternary

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The tertiary structure of a protein is stabilized by all of the following EXCEPT:

A. Hydrophobic interactions
B. Peptide bonds
C. Hydrogen bonds
D. Disulfide bonds

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Which type of interaction is most responsible for the quaternary structure of a protein?

A. Covalent bonding only
B. Non-covalent interactions such as hydrophobic and ionic forces
C. Hydrogen bonding exclusively
D. Peptide bonds between chains

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Which of the following proteins exhibits quaternary structure?

A. Myoglobin
B. Hemoglobin
C. Insulin monomer
D. A single polypeptide enzyme

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Secondary Structure Details

α-helices are characterized by:

A. Parallel or antiparallel strands
B. Clockwise coils around a central axis
C. Random coiling without hydrogen bonds
D. Rigid, planar peptide bonds preventing coiling

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β-pleated sheets differ from α-helices because they:

A. Are stabilized by disulfide bonds
B. Are formed from strands that can run parallel or antiparallel
C. Contain only hydrophobic amino acids
D. Require proline at regular intervals

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Why does proline often disrupt α-helices and β-sheets?

A. It forms strong hydrogen bonds
B. It introduces a kink due to its cyclic structure
C. It is too polar to fit in hydrophobic regions
D. It carries a positive charge at physiological pH

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Protein Bonds and Characteristics

Peptide bonds form between:

A. The R groups of adjacent amino acids
B. The α-carboxyl group of one amino acid and the α-amino group of another
C. The side chains of hydrophobic amino acids
D. Two hydroxyl groups

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Disulfide bonds form through:

A. Reduction of cysteine residues
B. Oxidation of two cysteine molecules to form cystine
C. Hydrogen bonding between sulfur atoms
D. Ionic attraction between cysteines

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The hydrophobic effect primarily contributes to which level of protein structure?

A. Primary
B. Secondary
C. Tertiary
D. Quaternary

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What drives the hydrophobic effect in protein folding?

A. Attraction between polar side chains
B. Formation of covalent bonds between nonpolar residues
C. Increase in entropy of water molecules when hydrophobic residues cluster
D. Decrease in protein entropy

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Denaturation

Denaturation of a protein results in the loss of which structural levels?

A. Only 4°
B. 2°, 3°, and 4°
C. 1° and 2°
D. All levels

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Which bonds typically remain intact after denaturation?

A. Hydrogen bonds
B. Ionic bonds
C. Peptide bonds
D. Disulfide bonds

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Complex and Conjugated Proteins

A conjugated protein is best described as:

A. A protein composed only of amino acids
B. A protein that includes a non-protein component
C. A denatured polypeptide chain
D. A protein with only quaternary structure

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The prosthetic group of a conjugated protein is:

A. Always a carbohydrate
B. The non-amino acid component, such as a metal ion or vitamin
C. The peptide backbone of the protein
D. Formed by peptide bond condensation

Enzyme Structure & Action Theories

What is the active site of an enzyme?

A. The site where cofactors bind
B. The location of regulatory molecules
C. The region where catalysis occurs
D. The part that binds irreversibly to substrates

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The lock and key model assumes:

A. Enzyme undergoes major conformational changes upon substrate binding
B. Enzyme and substrate have a perfect fit from the start
C. Enzyme is permanently altered by the substrate
D. Allosteric sites are used for catalysis

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According to the induced fit model:

A. Enzymes are rigid structures
B. Substrate binds irreversibly
C. Enzyme and substrate change shape to better interact
D. Catalysis only occurs after full saturation

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Which model is more accurate in explaining enzyme-substrate interactions?

A. Lock and Key
B. Induced Fit
C. Allosteric Binding
D. Covalent Catalysis

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Which of the following best explains why the induced fit model is preferred?

A. It supports random binding
B. It explains enzyme reuse
C. It accounts for conformational changes
D. It includes cofactor involvement

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Enzyme Cofactors & Coenzymes

What is a cofactor?

A. A competitive inhibitor
B. An enzyme-substrate complex
C. A required metal ion for enzyme function
D. A secondary active site

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A coenzyme differs from a cofactor because it is:

A. A protein
B. An inorganic ion
C. An organic molecule required for enzyme activity
D. A lipid-based molecule

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Enzyme Kinetics

What is V_max?

A. The rate when no substrate is present
B. The rate when all enzyme active sites are saturated
C. The minimum rate of reaction
D. The concentration of enzyme

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The Michaelis-Menten equation is:

A. V = V_max × K_m / [S]
B. V = [S] / (K_m + V_max)
C. V = (V_max × [S]) / (K_m + [S])
D. V = (K_m × [S]) / (V_max + [S])

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What is the effect of cooperative binding on a rate vs. [S] graph?

A. Linear curve
B. Hyperbolic curve
C. Sigmoidal curve
D. Inverse curve

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Which type of enzyme usually displays cooperativity?

A. Competitive
B. Allosteric
C. Ligase
D. Transferase

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Cooperative binding means:

A. All active sites bind substrate simultaneously
B. Binding of one substrate increases affinity for others
C. Binding is purely random
D. The enzyme only works with cofactors

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When substrate concentration is equal to K_m, the rate of the reaction is:

A. Equal to V_max
B. Half of V_max
C. Zero
D. Dependent on enzyme concentration

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Which plot helps distinguish different types of inhibition?

A. Michaelis-Menten plot
B. Lineweaver-Burk plot
C. Sigmoid curve
D. Bar graph

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Enzyme Inhibition & Regulation

Allosteric effectors bind:

A. At the active site
B. At the coenzyme pocket
C. At a different site than the active site
D. Covalently to the substrate

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A positive allosteric effector:

A. Decreases enzyme activity
B. Prevents substrate binding
C. Increases enzyme activity
D. Acts as a competitive inhibitor

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A homotropic effector is:

A. An enzyme cofactor
B. A different molecule from the substrate
C. The substrate itself acting as an allosteric regulator
D. An irreversible inhibitor

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What is feedback inhibition?

A. Activation of the first enzyme by a product
B. Inhibition of an enzyme by its own substrate
C. Inhibition of an enzyme by a product of the same pathway
D. Inhibition caused by excess enzymes

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In competitive inhibition, the inhibitor binds:

A. To the allosteric site
B. To the enzyme-substrate complex
C. To the active site
D. Covalently to the product

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How does competitive inhibition affect V_max and K_m?

A. ↓ V_max, ↓ K_m
B. No change in V_max, ↑ K_m
C. ↓ V_max, no change in K_m
D. Both increase

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What is true about uncompetitive inhibition?

A. It increases both V_max and K_m
B. It has no effect on kinetics
C. It decreases both V_max and K_m
D. It can be reversed by more substrate

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In noncompetitive inhibition, what is the effect on enzyme kinetics?

A. K_m increases, V_max stays the same
B. K_m stays the same, V_max decreases
C. Both decrease
D. No changes occur

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Which type of inhibitor cannot be overcome by adding more substrate?

A. Competitive
B. Reversible
C. Noncompetitive
D. Weak inhibitor

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What defines suicide inhibition?

A. Reversible binding to active site
B. Substrate analog forms an irreversible complex
C. Inhibitor binds enzyme-substrate complex
D. Product binds the active site

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A Lineweaver-Burk plot with lines intersecting on the Y-axis indicates:

A. Competitive inhibition
B. Noncompetitive inhibition
C. Uncompetitive inhibition
D. Irreversible inhibition

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Carbohydrates & Stereochemistry

What is the general formula for a carbohydrate?

A. C_nH_nO_n
B. C_m(H₂O)_n
C. C_nH_2n+1O_n
D. C_nH_2nO_2n

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Natural carbohydrates are usually in which configuration?

A. L
B. D
C. R
D. S

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Enantiomers are:

A. Identical isomers with different chemical formulas
B. Superimposable mirror images
C. Non-superimposable mirror images
D. Isomers that differ at two or more carbons

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Epimers differ at:

A. Every carbon atom
B. Only one chiral carbon
C. The carbonyl group
D. The anomeric carbon only

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Anomers are a subtype of epimers that differ at which carbon?

A. C-2
B. C-3
C. The anomeric carbon (C-1)
D. The highest-numbered carbon

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Which of the following pairs of monosaccharides are epimers?

A. D-glucose and D-fructose
B. D-glucose and D-galactose
C. D-fructose and D-ribose
D. D-glucose and L-glucose

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Biological Molecules & Processes

What is glycosylation?

A. Cleavage of peptide bonds
B. Addition of a metal cofactor
C. Addition of a carbohydrate to a molecule
D. Removal of a phosphate group

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What is a zymogen?

A. An inactive form of an enzyme that requires activation
B. A cofactor that activates enzymes
C. A carbohydrate-bound enzyme
D. A mutated form of an enzyme

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In the Michaelis-Menten scheme, what does K₂ represent?

A. Enzyme binding constant
B. Enzyme inhibition constant
C. Catalytic rate / rate-limiting step
D. Cofactor dissociation rate

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What does the enzyme-substrate complex (ES) represent in the Michaelis-Menten equation?

A. An irreversible product
B. A stable waste molecule
C. A temporary complex that can form product
D. An enzyme inhibitor

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Which step in the Michaelis-Menten equation is rate-limiting?

A. E + S ⇌ ES
B. ES → E + P
C. E + P → ES
D. ES → E + S

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What happens to the enzyme after the product is released?

A. It is degraded
B. It remains bound to the product
C. It is reused in the reaction
D. It binds a coenzyme permanently

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Protein Structure & Function

Which protein is a major component of the extracellular matrix?

A. Myosin
B. Collagen
C. Kinesin
D. Albumin

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Which protein is responsible for elasticity in connective tissues?

A. Actin
B. Keratin
C. Elastin
D. Tubulin

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Motor proteins convert:

A. Mechanical energy into chemical energy
B. Chemical energy into mechanical work
C. Glucose into ATP
D. Heat into chemical energy

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Which motor protein is involved in muscle contraction?

A. Kinesin
B. Dynein
C. Myosin
D. Tubulin

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Kinesin typically moves along:

A. Actin filaments toward the minus end
B. Microtubules toward the plus end
C. Keratin fibers
D. Collagen fibrils

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Binding proteins serve what primary role?

A. Hydrolyze ATP
B. Join two or more molecules
C. Activate zymogens
D. Break down proteins

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What type of cell adhesion molecule requires calcium and connects similar cells?

A. Integrin
B. Selectin
C. Cadherin
D. Albumin

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Which CAM allows cells to bind to carbohydrates on other cells, especially in immune function?

A. Selectin
B. Integrin
C. Cadherin
D. Keratin

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Biosignaling & Ion Channels

Which type of ion channel is always open?

A. Voltage-gated
B. Ligand-gated
C. Ungated
D. Second messenger gated

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Voltage-gated channels respond to:

A. Ligand binding
B. Hormones
C. Membrane potential changes
D. Presence of ATP

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Ligand-gated ion channels open when:

A. Membrane potential is high
B. A specific molecule binds
C. ATP is hydrolyzed
D. Temperature increases

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Which receptor type initiates second messenger cascades?

A. Ligand-gated channel
B. Enzyme-linked receptor
C. Voltage-gated channel
D. Ion pump

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What do GPCRs (G protein-coupled receptors) detect?

A. Only light stimuli
B. Only ions
C. External molecules triggering intracellular pathways
D. Carbohydrate polymers

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In GPCR signaling, the first messenger is:

A. ATP
B. The intracellular signaling molecule
C. The extracellular ligand
D. The enzyme-linked protein