- The Fluid Mosaic Model of the cell membrane was proposed by:
(a) Watson and Crick
(b) Singer and Nicolson
(c) Robertson
(d) Danielli and Davson
Answer: (b) Singer and Nicolson
Explanation: Singer and Nicolson (1972) described the membrane as a fluid lipid bilayer with embedded proteins. - The primary lipid component of eukaryotic cell membranes is:
(a) Triglycerides
(b) Phospholipids
(c) Cholesterol
(d) Sphingolipids
Answer: (b) Phospholipids
Explanation: Phospholipids (e.g., phosphatidylcholine) form the bilayer’s structural backbone. - Phospholipids in membranes are amphipathic due to:
(a) Covalent bonds between lipids
(b) Hydrophilic heads and hydrophobic tails
(c) Presence of cholesterol
(d) Glycosylation
Answer: (b) Hydrophilic heads and hydrophobic tails
Explanation: Polar heads face water; nonpolar tails face inward, enabling bilayer formation. - Cholesterol in animal cell membranes functions to:
(a) Generate energy
(b) Modulate fluidity and stability
(c) Facilitate active transport
(d) Synthesize proteins
Answer: (b) Modulate fluidity and stability
Explanation: Cholesterol stiffens membranes at high temperatures and prevents crystallization at low temperatures. - Integral membrane proteins are characterized by:
(a) Loose attachment to lipid heads
(b) Transmembrane domains with hydrophobic regions
(c) Covalent bonding to carbohydrates
(d) Peripheral localization
Answer: (b) Transmembrane domains with hydrophobic regions
Explanation: Hydrophobic α-helices span the bilayer, anchoring proteins permanently. - The asymmetry of the cell membrane refers to:
(a) Identical lipid composition in both leaflets
(b) Unequal distribution of lipids and proteins between leaflets
(c) Random protein orientation
(d) Symmetrical carbohydrate placement
Answer: (b) Unequal distribution of lipids and proteins between leaflets
Explanation: Outer leaflet: Sphingomyelin/glycolipids; inner leaflet: Phosphatidylethanolamine/phosphatidylserine. - Glycocalyx is composed of:
(a) Nucleic acids
(b) Glycolipids and glycoproteins
(c) Cholesterol clusters
(d) Phospholipid micelles
Answer: (b) Glycolipids and glycoproteins
Explanation: Carbohydrate chains on lipids/proteins form a protective, recognition-enabled layer. - FRAP (Fluorescence Recovery After Photobleaching) demonstrates:
(a) Lateral mobility of membrane components
(b) DNA replication
(c) Protein synthesis
(d) Ion channel gating
Answer: (a) Lateral mobility of membrane components
Explanation: Measures lipid/protein diffusion rates by tracking fluorescence recovery post-bleaching. - Lipid rafts are enriched in:
(a) Phosphatidylserine
(b) Cholesterol and sphingolipids
(c) Unsaturated fatty acids
(d) Peripheral proteins
Answer: (b) Cholesterol and sphingolipids
Explanation: Cholesterol/sphingolipid microdomains organize signaling proteins (e.g., GPI-anchored). - Which is NOT a function of membrane proteins?
(a) Transport
(b) Signal transduction
(c) DNA replication
(d) Cell adhesion
Answer: (c) DNA replication
Explanation: Membrane proteins mediate transport, signaling, adhesion, but not DNA replication. - The “fluid” nature of the membrane is primarily due to:
(a) Protein rigidity
(b) Weak hydrophobic interactions between lipid tails
(c) Covalent bonds
(d) Carbohydrate cross-linking
Answer: (b) Weak hydrophobic interactions between lipid tails
Explanation: Van der Waals forces allow lipids/proteins to diffuse laterally. - Peripheral membrane proteins bind via:
(a) Transmembrane helices
(b) Electrostatic interactions with lipid heads
(c) Hydrophobic anchors
(d) Glycosidic linkages
Answer: (b) Electrostatic interactions with lipid heads
Explanation: Attach via H-bonds/ionic bonds to polar heads or integral proteins. - Phosphatidylserine externalization signals:
(a) Cell division
(b) Apoptosis
(c) Endocytosis
(d) Glycolysis
Answer: (b) Apoptosis
Explanation: Flipped to the outer leaflet by scramblases during programmed cell death. - Saturation of fatty acyl chains affects membrane:
(a) Carbohydrate content
(b) Fluidity
(c) Protein synthesis
(d) DNA binding
Answer: (b) Fluidity
Explanation: Saturated chains pack tightly (less fluid); unsaturated chains increase fluidity via kinks. - Aquaporins are:
(a) Enzymes
(b) Water-channel integral proteins
(c) Glycolipids
(d) Peripheral receptors
Answer: (b) Water-channel integral proteins
Explanation: Tetrameric channels facilitating rapid water transport (e.g., AQP1 in erythrocytes). - The thickness of the plasma membrane is approximately:
(a) 3–4 nm
(b) 7–10 nm
(c) 15–20 nm
(d) 50 nm
Answer: (b) 7–10 nm
Explanation: ~8 nm thick, including hydrophobic core (3–4 nm) and hydrophilic surfaces. - Which technique visualizes membrane protein distribution?
(a) PCR
(b) Freeze-fracture electron microscopy
(c) Western blot
(d) DNA sequencing
Answer: (b) Freeze-fracture electron microscopy
Explanation: Freeze-fracture splits the bilayer, revealing intramembrane protein particles. - Na⁺/K⁺-ATPase is a:
(a) Voltage-gated channel
(b) P-type ATPase pump
(c) G-protein coupled receptor
(d) Ligand-gated channel
Answer: (b) P-type ATPase pump
Explanation: Transmembrane pump hydrolyzing ATP to maintain Na⁺/K⁺ gradients. - Caveolae are:
(a) Enzyme complexes
(b) Cholesterol-rich invaginations for endocytosis
(c) Nuclear pores
(d) Mitochondrial cristae
Answer: (b) Cholesterol-rich invaginations for endocytosis
Explanation: Flask-shaped invaginations enriched in caveolin proteins. - GPI anchors attach proteins to the membrane via:
(a) Transmembrane domains
(b) Covalent linkage to phosphatidylinositol
(c) Hydrophobic loops
(d) Carbohydrate bridges
Answer: (b) Covalent linkage to phosphatidylinositol
Explanation: GPI anchors tether proteins to the outer leaflet’s lipid bilayer. - The role of flippases is to:
(a) Degrade lipids
(b) Transport phospholipids from outer to inner leaflet
(c) Synthesize cholesterol
(d) Flip carbohydrates
Answer: (b) Transport phospholipids from outer to inner leaflet
Explanation: ATP-dependent enzymes maintain lipid asymmetry (e.g., move phosphatidylserine inward). - Membrane fluidity decreases when:
(a) Unsaturated lipids increase
(b) Saturated lipids or cholesterol increase
(c) Temperature rises
(d) Glycolipids are added
Answer: (b) Saturated lipids or cholesterol increase
Explanation: Saturated chains/cholesterol restrict lipid movement, reducing fluidity. - Lipid droplets are surrounded by a:
(a) Single protein layer
(b) Phospholipid monolayer
(c) Bilayer with cholesterol
(d) Glycocalyx coat
Answer: (b) Phospholipid monolayer
Explanation: Neutral lipids (e.g., triglycerides) are enclosed in a phospholipid monolayer. - Which lipid is negatively charged and localized to the inner leaflet?
(a) Sphingomyelin
(b) Phosphatidylserine
(c) Glycolipid
(d) Cholesterol
Answer: (b) Phosphatidylserine
Explanation: Phosphatidylserine contributes to the negative charge of the cytoplasmic face. - Bacteriorhodopsin in archaea functions as a:
(a) Receptor
(b) Light-driven proton pump
(c) Ion channel
(d) Adhesion protein
Answer: (b) Light-driven proton pump
Explanation: Retinal-containing protein generating proton gradients using light. - The “membrane skeleton” is formed by:
(a) Integral proteins
(b) Spectrin and ankyrin networks
(c) Cholesterol clusters
(d) Glycolipids
Answer: (b) Spectrin and ankyrin networks
Explanation: Spectrin-ankyrin meshwork underlies the plasma membrane (e.g., in erythrocytes). - Lipid bilayers are permeable to:
(a) Ions
(b) Glucose
(c) Small nonpolar molecules (O₂, CO₂)
(d) Amino acids
Answer: (c) Small nonpolar molecules (O₂, CO₂)
Explanation: Nonpolar molecules diffuse freely; polar/charged molecules require transporters. - Tight junctions prevent:
(a) Cell adhesion
(b) Paracellular diffusion of solutes
(c) Membrane fluidity
(d) Endocytosis
Answer: (b) Paracellular diffusion of solutes
Explanation: Claudin/occludin seals block intercellular solute movement. - The “unit membrane” model was proposed by:
(a) Singer and Nicolson
(b) Robertson
(c) Danielli and Davson
(d) Gorter and Grendel
Answer: (b) Robertson
Explanation: Robertson (1959) described a trilaminar (dark-light-dark) EM structure. - Glycosphingolipids are:
(a) Found only in inner leaflet
(b) Carbohydrate-attached sphingolipids in outer leaflet
(c) Cholesterol derivatives
(d) Peripheral proteins
Answer: (b) Carbohydrate-attached sphingolipids in outer leaflet
Explanation: Sphingosine-based lipids with sugars (e.g., cerebrosides, gangliosides). - Membrane fusion during exocytosis requires:
(a) Cholesterol removal
(b) SNARE proteins (v-SNAREs/t-SNAREs)
(c) Glycocalyx degradation
(d) Ion gradients
Answer: (b) SNARE proteins (v-SNAREs/t-SNAREs)
Explanation: SNAREs mediate vesicle-target membrane docking/fusion. - The “mosaic” aspect of the fluid mosaic model refers to:
(a) Lipid homogeneity
(b) Protein icebergs in a lipid sea
(c) Carbohydrate symmetry
(d) DNA fragments
Answer: (b) Protein icebergs in a lipid sea
Explanation: Proteins are interspersed like mosaics in the fluid lipid bilayer. - Phosphatidylinositol 4,5-bisphosphate (PIP₂) is involved in:
(a) Glycolysis
(b) Signal transduction and vesicle trafficking
(c) DNA replication
(d) Protein synthesis
Answer: (b) Signal transduction and vesicle trafficking
Explanation: PIP₂ recruits signaling proteins (e.g., PLC cleaves it to IP₃/DAG). - Which factor increases membrane fluidity?
(a) High cholesterol
(b) Unsaturated fatty acids
(c) Low temperature
(d) Saturated lipids
Answer: (b) Unsaturated fatty acids
Explanation: Cis-double bonds in unsaturated lipids create kinks, enhancing fluidity. - Porins are transmembrane proteins with:
(a) Single α-helix
(b) β-barrel structure
(c) GPI anchors
(d) Disulfide bonds
Answer: (b) β-barrel structure
Explanation: β-Sheets form hydrophilic pores in outer membranes of bacteria/mitochondria. - The “fence-and-picket” model describes:
(a) DNA organization
(b) Cytoskeleton corralling membrane proteins
(c) Lipid raft formation
(d) Glycocalyx function
Answer: (b) Cytoskeleton corralling membrane proteins
Explanation: Actin fences restrict protein diffusion to membrane compartments. - Cardiolipin is found in:
(a) Plasma membrane
(b) Mitochondrial inner membrane
(c) Lysosomes
(d) Golgi apparatus
Answer: (b) Mitochondrial inner membrane
Explanation: Diphosphatidylglycerol stabilizes ETC complexes in mitochondria. - Membrane asymmetry is established by:
(a) Glycosyltransferases
(b) Flippases, floppases, and scramblases
(c) Cholesterol synthases
(d) Proteases
Answer: (b) Flippases, floppases, and scramblases
Explanation: Flippases (inward), floppases (outward), and scramblases (bidirectional) maintain lipid asymmetry. - FRAP experiments show that lipids diffuse:
(a) Slower than proteins
(b) Faster than most transmembrane proteins
(c) Only vertically
(d) Via active transport
Answer: (b) Faster than most transmembrane proteins
Explanation: Lipids diffuse rapidly (~1 µm²/s); proteins are slower due to size/anchoring. - The “liquid-ordered” state in lipid rafts has:
(a) Disordered lipid tails
(b) Extended lipid tails with lateral mobility
(c) Immobile proteins
(d) No cholesterol
Answer: (b) Extended lipid tails with lateral mobility
Explanation: Cholesterol promotes ordered yet fluid lipid packing in rafts. - Gangliosides are:
(a) Phospholipids
(b) Sialic acid-containing glycosphingolipids
(c) Cholesterol esters
(d) Peripheral proteins
Answer: (b) Sialic acid-containing glycosphingolipids
Explanation: Gangliosides (e.g., GM1) act as receptors in neuronal membranes. - Membrane curvature is induced by:
(a) Glycolipids
(b) BAR domain proteins and amphipathic helices
(c) Cholesterol
(d) Peripheral proteins only
Answer: (b) BAR domain proteins and amphipathic helices
Explanation: BAR domains sense/bend membranes during vesicle formation. - The inner mitochondrial membrane has high amounts of:
(a) Glycolipids
(b) Cardiolipin
(c) Phosphatidylcholine
(d) Sphingomyelin
Answer: (b) Cardiolipin
Explanation: Cardiolipin constitutes ~20% of mitochondrial inner membrane lipids. - Which is NOT a function of the glycocalyx?
(a) Cell adhesion
(b) Protection
(c) ATP synthesis
(d) Immune recognition
Answer: (c) ATP synthesis
Explanation: Glycocalyx mediates adhesion, signaling, protection, but not energy production. - ABC transporters utilize:
(a) Light energy
(b) ATP hydrolysis
(c) Proton gradients
(d) Sodium symport
Answer: (b) ATP hydrolysis
Explanation: ATP-Binding Cassette pumps (e.g., MDR1) export substrates via ATP hydrolysis. - Lipid-anchored proteins include:
(a) Glycophorin
(b) Ras (farnesylated)
(c) Na⁺/K⁺-ATPase
(d) Aquaporin
Answer: (b) Ras (farnesylated)
Explanation: Ras is anchored via farnesyl groups; others use GPI or myristoyl/palmitoyl chains. - The “permeability barrier” function primarily relies on:
(a) Carbohydrates
(b) Hydrophobic lipid core
(c) Membrane proteins
(d) Cholesterol
Answer: (b) Hydrophobic lipid core
Explanation: Hydrophobic interior blocks diffusion of hydrophilic solutes/ions. - Membrane domains enriched in sphingolipids/cholesterol are called:
(a) Caveolae
(b) Lipid rafts
(c) Tight junctions
(d) Gap junctions
Answer: (b) Lipid rafts
Explanation: Lipid rafts are dynamic platforms for signaling and trafficking. - Which lipid is a precursor for steroid hormones?
(a) Phosphatidylcholine
(b) Cholesterol
(c) Sphingomyelin
(d) Phosphatidylserine
Answer: (b) Cholesterol
Explanation: Cholesterol is converted to cortisol, estrogen, testosterone, etc. - The membrane structure least permeable to water is:
(a) Phospholipid bilayer with aquaporins
(b) Pure phospholipid bilayer without proteins
(c) Membrane with cholesterol
(d) Glycolipid-rich membrane
Answer: (b) Pure phospholipid bilayer without proteins
Explanation: Pure lipid bilayers have low water permeability; aquaporins enhance it 10-fold.
Molecular structure of mitochondria.
- Mitochondria are surrounded by:
(a) Single membrane
(b) Double membrane
(c) Triple membrane
(d) No membrane
Answer: (b)
Explanation: Mitochondria have a double-membrane structure: an outer permeable membrane and an inner selectively permeable membrane with cristae. - The space between the outer and inner mitochondrial membranes is called:
(a) Matrix
(b) Cristae
(c) Intermembrane space
(d) Cytoplasm
Answer: (c)
Explanation: The intermembrane space lies between the outer and inner membranes and contains protons for ATP synthesis. - Cristae in mitochondria are:
(a) Infoldings of the outer membrane
(b) Infoldings of the inner membrane
(c) DNA particles in the matrix
(d) Ribosomes in the matrix
Answer: (b)
Explanation: Cristae are inward folds of the inner mitochondrial membrane, increasing surface area for ATP production. - Which complex of the electron transport chain is exclusively part of the mitochondrial matrix?
(a) Complex I
(b) Complex II
(c) Complex III
(d) Complex IV
Answer: (b)
Explanation: Complex II (succinate dehydrogenase) is embedded in the inner membrane but faces the matrix, participating in the Krebs cycle. - Mitochondrial DNA is:
(a) Linear and double-stranded
(b) Circular and double-stranded
(c) Linear and single-stranded
(d) Circular and single-stranded
Answer: (b)
Explanation: Mitochondrial DNA (mtDNA) is circular and double-stranded, resembling bacterial DNA. - Porin proteins present in the outer mitochondrial membrane form:
(a) Voltage-gated channels
(b) Ligand-gated channels
(c) Large aqueous channels
(d) Proton pumps
Answer: (c)
Explanation: Porins create large channels (1–3 nm) allowing passive diffusion of molecules <5 kDa. - The mitochondrial matrix contains all EXCEPT:
(a) Ribosomes
(b) tRNA
(c) Enzymes for Krebs cycle
(d) Cytochrome c
Answer: (d)
Explanation: Cytochrome c is loosely bound to the inner membrane, not free in the matrix. - F₁ particles of ATP synthase are located:
(a) In the intermembrane space
(b) On the matrix side of the inner membrane
(c) On the outer membrane
(d) In the cristae junctions
Answer: (b)
Explanation: The F₁ subunit (catalytic head) projects into the matrix, synthesizing ATP. - Mitochondria divide by:
(a) Mitosis
(b) Binary fission
(c) Budding
(d) Meiosis
Answer: (b)
Explanation: Mitochondria replicate via binary fission, similar to bacteria. - Cardiolipin is a phospholipid found in:
(a) Outer mitochondrial membrane
(b) Inner mitochondrial membrane
(c) Nuclear envelope
(d) Golgi apparatus
Answer: (b)
Explanation: Cardiolipin is abundant in the inner membrane, enhancing impermeability to protons. - The permeability transition pore (mPTP) spans:
(a) Outer membrane only
(b) Inner membrane only
(c) Both membranes
(d) Cristae junctions
Answer: (c)
Explanation: mPTP forms at contact sites between outer and inner membranes during apoptosis. - Which protein import machinery is specific to the mitochondrial matrix?
(a) TOM complex
(b) TIM22 complex
(c) TIM23 complex
(d) SAM complex
Answer: (c)
Explanation: TIM23 imports proteins with N-terminal presequences into the matrix. - Mitochondria originated from:
(a) Cyanobacteria
(b) Alpha-proteobacteria
(c) Archaea
(d) Rickettsia
Answer: (b)
Explanation: Endosymbiotic theory posits mitochondria evolved from alpha-proteobacteria. - Mitochondrial ribosomes are:
(a) 80S
(b) 70S
(c) 55S
(d) 40S
Answer: (c)
Explanation: Mammalian mitochondrial ribosomes are 55–60S; bacterial are 70S. - The inner mitochondrial membrane lacks:
(a) Cholesterol
(b) Cardiolipin
(c) Phosphatidylcholine
(d) Sphingomyelin
Answer: (a)
Explanation: The inner membrane is cholesterol-poor but rich in cardiolipin. - Cytochrome c is involved in:
(a) Krebs cycle
(b) Glycolysis
(c) Apoptosis
(d) Protein import
Answer: (c)
Explanation: Cytochrome c release from mitochondria triggers caspase activation in apoptosis. - The Krebs cycle occurs in the:
(a) Intermembrane space
(b) Outer membrane
(c) Inner membrane
(d) Matrix
Answer: (d)
Explanation: Enzymes for the Krebs cycle are soluble in the mitochondrial matrix. - Contact sites between mitochondrial membranes facilitate:
(a) Protein synthesis
(b) Phospholipid exchange
(c) DNA replication
(d) Ribosome assembly
Answer: (b)
Explanation: Contact sites allow lipid transfer proteins to shuttle phospholipids between membranes. - Which complex pumps protons into the intermembrane space?
(a) Complex I
(b) Complex II
(c) Complex III
(d) Both (a) and (c)
Answer: (d)
Explanation: Complexes I, III, and IV pump protons; Complex II does not. - Mitochondria are absent in:
(a) Muscle cells
(b) Erythrocytes
(c) Neurons
(d) Hepatocytes
Answer: (b)
Explanation: Mammalian erythrocytes lack mitochondria (and nuclei) to maximize hemoglobin. - The mitochondrial genome encodes:
(a) All ETC proteins
(b) rRNA and tRNA only
(c) 13 ETC proteins, rRNA, tRNA
(d) DNA polymerase
Answer: (c)
Explanation: Human mtDNA encodes 13 ETC subunits, 22 tRNAs, and 2 rRNAs. - Oxysomes refer to:
(a) Mitochondrial granules
(b) ATP synthase particles
(c) Cristae junctions
(d) mtDNA nucleoids
Answer: (b)
Explanation: Oxysomes (or F₁-F₀ particles) are ATP synthase complexes on cristae. - Mitochondria can store:
(a) Glycogen
(b) Calcium ions
(c) Sodium ions
(d) Potassium ions
Answer: (b)
Explanation: Mitochondria buffer cytosolic Ca²⁺, storing it in the matrix. - The inner boundary membrane is:
(a) Part of the outer membrane
(b) Flat region of the inner membrane
(c) Cristae membrane
(d) Matrix protein network
Answer: (b)
Explanation: The inner boundary membrane is the flat section adjacent to the outer membrane, while cristae are tubular invaginations. - MICOS complex stabilizes:
(a) Outer membrane
(b) Cristae junctions
(c) Matrix enzymes
(d) mtDNA
Answer: (b)
Explanation: MICOS (Mitochondrial Contact Site) maintains cristae junctions and inner membrane architecture. - Mitochondrial fusion requires:
(a) Drp1
(b) OPA1
(c) Fis1
(d) Mff
Answer: (b)
Explanation: OPA1 mediates inner membrane fusion; mitofusins (Mfn1/2) fuse outer membranes. - Which state teaching exam featured a question on mitochondrial permeability transition pores in 2021?
(a) Role in necrosis
(b) Role in apoptosis
(c) ATP synthesis
(d) Protein import
Answer: (b)
Explanation: Pore opening releases pro-apoptotic factors like cytochrome c. - The outer mitochondrial membrane is permeable to molecules up to:
(a) 1 kDa
(b) 5 kDa
(c) 10 kDa
(d) 50 kDa
Answer: (b)
Explanation: Porins allow passive diffusion of solutes ≤5 kDa. - Mitochondrial matrix pH is:
(a) Lower than intermembrane space
(b) Higher than intermembrane space
(c) Same as cytoplasm
(d) Acidic
Answer: (b)
Explanation: Proton pumping creates an acidic intermembrane space (pH ~7.0) and alkaline matrix (pH ~8.0). - Inherited mitochondrial disorders follow:
(a) Autosomal dominant pattern
(b) Autosomal recessive pattern
(c) X-linked pattern
(d) Maternal inheritance
Answer: (d)
Explanation: mtDNA is maternally inherited due to degradation in sperm mitochondria post-fertilization. - The TIM22 complex imports:
(a) Matrix proteins
(b) Inner membrane carrier proteins
(c) Outer membrane proteins
(d) Intermembrane space proteins
Answer: (b)
Explanation: TIM22 imports multi-pass inner membrane proteins (e.g., metabolite carriers). - Cristae junctions are regulated by:
(a) ATP synthase dimers
(b) MIC60 protein
(c) Porins
(d) Cytochrome c
Answer: (b)
Explanation: MIC60 (mitofilin) is a core MICOS component that stabilizes cristae junctions. - Mitochondrial RNA polymerase is encoded by:
(a) Mitochondrial genome
(b) Nuclear genome
(c) Both
(d) Neither
Answer: (b)
Explanation: Nuclear genes encode mitochondrial transcription machinery, including POLRMT. - Which vitamin derivative is part of the ETC?
(a) Vitamin A
(b) Vitamin C
(c) Vitamin K
(d) Vitamin B₂
Answer: (d)
Explanation: Vitamin B₂ (riboflavin) is a precursor for FAD (Complex II). - The “powerhouses of the cell” analogy was coined by:
(a) Robert Hooke
(b) Carl Benda
(c) Philip Siekevitz
(d) Lynn Margulis
Answer: (c)
Explanation: Philip Siekevitz described mitochondria as “powerhouses” in a 1957 paper. - Mitochondria are most abundant in:
(a) Skin cells
(b) Cardiac muscle cells
(c) Adipocytes
(d) Osteocytes
Answer: (b)
Explanation: Cardiac muscle has high mitochondrial density (up to 5,000/cell) for constant energy. - Superoxide dismutase in mitochondria converts O₂⁻ to:
(a) H₂O₂
(b) H₂O
(c) OH⁻
(d) O₂
Answer: (a)
Explanation: Mn-SOD in the matrix converts superoxide (O₂⁻) to hydrogen peroxide (H₂O₂). - Mitochondrial fission protein Drp1 is recruited by:
(a) OPA1
(b) Mff and Fis1
(c) Mitofusins
(d) MIC60
Answer: (b)
Explanation: Drp1 is recruited to the outer membrane by adaptors Mff, Fis1, and MiD49/51. - The malate-aspartate shuttle transfers:
(a) Pyruvate
(b) NADH
(c) ATP
(d) Acetyl-CoA
Answer: (b)
Explanation: This shuttle transfers reducing equivalents (NADH) from cytosol to mitochondrial matrix. - Which is NOT a function of mitochondria?
(a) Heme synthesis
(b) Steroid synthesis
(c) Urea cycle
(d) Glycolysis
Answer: (d)
Explanation: Glycolysis occurs in the cytosol; mitochondria handle heme synthesis, steroidogenesis, and urea cycle. - The mitochondrial genetic code differs from nuclear DNA in:
(a) Using different stop codons
(b) Encoding methionine with AUA
(c) Both
(d) Neither
Answer: (c)
Explanation: mtDNA uses AUA for methionine (instead of isoleucine) and AGA/AGG as stop codons. - Which complex contains heme groups?
(a) Complex I
(b) Complex II
(c) Complex III
(d) Complex IV
Answer: (d)
Explanation: Complex IV (cytochrome c oxidase) contains heme a and heme a₃. - Mitochondrial diseases often affect:
(a) Muscles and nerves
(b) Bones
(c) Skin
(d) Kidneys
Answer: (a)
Explanation: Tissues with high energy demands (muscles, nerves) are vulnerable to mitochondrial dysfunction. - The SAM complex assembles:
(a) Matrix proteins
(b) Inner membrane proteins
(c) β-barrel outer membrane proteins
(d) Cristae proteins
Answer: (c)
Explanation: SAM (Sorting and Assembly Machinery) folds/inserts β-barrel proteins (e.g., Tom40) into the outer membrane. - Coenzyme Q is:
(a) Water-soluble
(b) Embedded in Complex I
(c) Mobile in the lipid bilayer
(d) A protein
Answer: (c)
Explanation: CoQ (ubiquinone) is lipid-soluble and shuttles electrons between complexes in the inner membrane. - Mitochondria-targeted antioxidants target:
(a) Matrix
(b) Outer membrane
(c) Intermembrane space
(d) Cristae
Answer: (a)
Explanation: Compounds like MitoQ conjugate to lipophilic cations, accumulating in the matrix. - The Krebs cycle enzyme aconitase is inactivated by:
(a) Superoxide
(b) Nitric oxide
(c) Both
(d) Neither
Answer: (c)
Explanation: Aconitase is inactivated by O₂⁻ (damaging Fe-S clusters) and NO (forming iron-nitrosyl complexes). - Mitophagy is mediated by:
(a) Atg5
(b) PINK1/Parkin
(c) p53
(d) Caspases
Answer: (b)
Explanation: PINK1 accumulates on damaged mitochondria, recruiting Parkin to trigger autophagy. - How many protons are pumped per NADH oxidized?
(a) 4
(b) 6
(c) 10
(d) 12
Answer: (c)
Explanation: Complex I (4H⁺), III (4H⁺), and IV (2H⁺) pump 10 H⁺ per NADH. - Mitochondrial biogenesis is regulated by:
(a) mTOR
(b) PGC-1α
(c) HIF-1α
(d) AMPK
Answer: (b)
Explanation: PGC-1α co-activates transcription factors (e.g., NRF-1, TFAM) for mitochondrial gene expression.
Enzymes – Properties, mechanism and kinetics of action, regulation of enzyme action, enzyme inhibitors.
- Enzymes accelerate reactions by:
(a) Increasing equilibrium constant
(b) Decreasing activation energy
(c) Increasing substrate concentration
(d) Shifting reaction equilibrium
Answer: (b)
Explanation: Enzymes lower activation energy (Eₐ) via transition-state stabilization, without altering equilibrium. - The active site of an enzyme contains:
(a) Allosteric regulators
(b) Cofactors only
(c) Substrate-binding and catalytic residues
(d) Competitive inhibitors
Answer: (c)
Explanation: The active site has substrate-binding pockets and catalytic residues (e.g., serine in chymotrypsin). - Km (Michaelis constant) represents:
(a) Enzyme-substrate affinity
(b) Maximum reaction velocity
(c) Turnover number
(d) Dissociation constant for ES complex
Answer: (d)
Explanation: Km = (k₋₁ + k₂)/k₁; low Km indicates high affinity. - A non-competitive inhibitor binds to:
(a) Active site
(b) Substrate
(c) Enzyme-substrate complex
(d) Allosteric site
Answer: (d)
Explanation: It binds allosterically, reducing V_max without affecting Km. - Zymogens are:
(a) Vitamin-derived coenzymes
(b) Inactive enzyme precursors
(c) RNA enzymes
(d) Competitive inhibitors
Answer: (b)
Explanation: Zymogens (e.g., pepsinogen) are activated by proteolytic cleavage. - Lineweaver-Burk plot is used to determine:
(a) V_max and K_m
(b) k_cat
(c) Enzyme specificity
(d) Optimal pH
Answer: (a)
Explanation: Double-reciprocal plot (1/V vs. 1/[S]) gives -1/K_m (x-intercept) and 1/V_max (y-intercept). - Allosteric enzymes exhibit:
(a) Michaelis-Menten kinetics
(b) Sigmoidal kinetics
(c) Linear kinetics
(d) Ping-pong kinetics
Answer: (b)
Explanation: Cooperative substrate binding leads to sigmoidal V vs. [S] curves (e.g., ATCase). - Competitive inhibition can be reversed by:
(a) Decreasing [S]
(b) Increasing [S]
(c) Adding allosteric activator
(d) Denaturing enzyme
Answer: (b)
Explanation: High [S] outcompetes inhibitor for active site. - The “catalytic triad” in chymotrypsin consists of:
(a) Ser-His-Asp
(b) Cys-His-Glu
(c) Lys-Arg-Glu
(d) Tyr-Thr-Ser
Answer: (a)
Explanation: Ser-195, His-57, Asp-102 form the charge-relay system. - k_cat (turnover number) is:
(a) V_max / [E]_total
(b) K_m / V_max
(c) [S] at half V_max
(d) K_m / [S]
Answer: (a)
Explanation: k_cat = V_max / [E]_total (molecules converted per enzyme per second). - Feedback inhibition typically involves:
(a) Competitive inhibition
(b) End-product inhibiting the first enzyme
(c) Zymogen activation
(d) Covalent modification
Answer: (b)
Explanation:g., CTP inhibiting aspartate transcarbamoylase (ATCase) in pyrimidine synthesis. - Metal ions in metalloenzymes function as:
(a) Competitive inhibitors
(b) Co-substrates
(c) Catalytic cofactors
(d) Allosteric regulators
Answer: (c)
Explanation:g., Zn²⁺ in carbonic anhydrase; Fe²⁺ in catalase. - An enzyme without its cofactor is called:
(a) Holoenzyme
(b) Apoenzyme
(c) Isozyme
(d) Ribozyme
Answer: (b)
Explanation: Apoenzyme (inactive) + cofactor = holoenzyme (active). - Enzymes in glycolysis regulated by phosphorylation:
(a) Hexokinase
(b) Pyruvate kinase
(c) Phosphofructokinase-1
(d) Enolase
Answer: (b)
Explanation: Pyruvate kinase is inhibited by phosphorylation (glucagon signaling). - Suicide inhibitors are:
(a) Reversible competitive
(b) Irreversible covalent
(c) Non-competitive
(d) Allosteric
Answer: (b)
Explanation:g., Aspirin acetylates COX; penicillin binds transpeptidase. - The induced-fit model was proposed by:
(a) Emil Fischer
(b) Leonor Michaelis
(c) Daniel Koshland
(d) Maud Menten
Answer: (c)
Explanation: Koshland (1958) proposed conformational changes upon substrate binding. - Which is NOT a mechanism of enzyme regulation?
(a) Genetic expression control
(b) Covalent modification
(c) Denaturation
(d) Compartmentalization
Answer: (c)
Explanation: Denaturation is irreversible loss of function, not regulatory. - Alkaline phosphatase is an example of:
(a) Transferase
(b) Hydrolase
(c) Lyase
(d) Ligase
Answer: (b)
Explanation: EC 3.1.3.1; hydrolyzes phosphate monoesters. - In sequential kinetics:
(a) Substrates bind randomly
(b) Products release before all substrates bind
(c) A ternary complex forms
(d) Enzyme is covalently modified
Answer: (c)
Explanation: All substrates bind before products release (e.g., lactate dehydrogenase). - Isoenzymes differ in:
(a) Amino acid sequence
(b) Catalytic mechanism
(c) Substrate specificity
(d) All of the above
Answer: (a)
Explanation:g., Lactate dehydrogenase (LDH) has H (heart) and M (muscle) subunits. - Phosphorylation of enzymes occurs on:
(a) Ser/Thr/Tyr residues
(b) His residues
(c) Asp residues
(d) Cys residues
Answer: (a)
Explanation: Kinases phosphorylate Ser/Thr/Tyr (e.g., glycogen phosphorylase). - Competitive inhibition alters:
(a) Kₘ only
(b) Vₘₐₓ only
(c) Both Kₘ and Vₘₐₓ
(d) kₐₜ
Answer: (a)
Explanation: Kₘ increases; Vₘₐₓ unchanged (Lineweaver-Burk: slopes increase). - The enzyme with the highest catalytic efficiency has:
(a) Low Kₘ, high kₐₜ
(b) High Kₘ, low kₐₜ
(c) Low Kₘ, low kₐₜ
(d) High Kₘ, high kₐₜ
Answer: (a)
Explanation: Catalytic efficiency = kₐₜ / Kₘ. - Ribozymes are enzymes made of:
(a) Proteins
(b) RNA
(c) DNA
(d) Carbohydrates
Answer: (b)
Explanation:g., RNase P cleaves tRNA precursors. - Which is a key regulator of glycolysis?
(a) Citrate synthase
(b) Phosphofructokinase-1 (PFK-1)
(c) Succinate dehydrogenase
(d) Pyruvate carboxylase
Answer: (b)
Explanation: PFK-1 is inhibited by ATP/citrate and activated by AMP/F-2,6-BP. - Enzyme commission (EC) number for oxidoreductases is:
(a) EC 1
(b) EC 2
(c) EC 3
(d) EC 4
Answer: (a)
Explanation: EC 1: Oxidoreductases (e.g., dehydrogenases). - Covalent catalysis involves:
(a) Metal ions
(b) Transient covalent enzyme-substrate intermediate
(c) pH changes
(d) Allosteric modulation
Answer: (b)
Explanation:g., Schiff base formation in aldolase. - Which enzyme follows ping-pong kinetics?
(a) Lactate dehydrogenase
(b) Hexokinase
(c) Chymotrypsin
(d) Aspartate aminotransferase
Answer: (d)
Explanation: Aminotransferases form enzyme-bound intermediates (ping-pong Bi-Bi). - Urea denatures enzymes by disrupting:
(a) Disulfide bonds
(b) Hydrophobic interactions
(c) Hydrogen bonds
(d) Ionic bonds
Answer: (c)
Explanation: Urea H-bonds with peptide groups, unfolding proteins. - An allosteric activator:
(a) Binds active site
(b) Decreases Kₘ
(c) Increases Kₘ
(d) Reduces cooperativity
Answer: (b)
Explanation:g., ATP inhibition vs. CTP activation of ATCase. - The enzyme inhibited by cyanide is:
(a) Hexokinase
(b) Cytochrome c oxidase
(c) Catalase
(d) Carbonic anhydrase
Answer: (b)
Explanation: CN⁻ binds Fe³⁺ in cytochrome a₃ (Complex IV). - Optimal pH for pepsin is:
(a) 2.0
(b) 7.4
(c) 8.5
(d) 9.0
Answer: (a)
Explanation: Pepsin (stomach) acts at pH 1.5–2.5; trypsin (intestine) at pH 7.5–8.5. - Enzyme kinetics at high [S] is limited by:
(a) Kₘ
(b) Vₘₐₓ
(c) [E]ₜₒₜₐₗ
(d) kₐₜ
Answer: (b)
Explanation: V → Vₘₐₓ when [S] ≫ Kₘ. - Which is a reversible covalent modification?
(a) Proteolytic cleavage
(b) Phosphorylation
(c) Glycosylation
(d) Ubiquitination
Answer: (b)
Explanation: Phosphorylation (kinases) and dephosphorylation (phosphatases) are reversible. - The lock-and-key model fails to explain:
(a) Substrate specificity
(b) Catalytic action
(c) Conformational changes
(d) Competitive inhibition
Answer: (c)
Explanation: Fischer’s rigid model doesn’t account for induced fit. - Which is NOT a serine protease?
(a) Trypsin
(b) Chymotrypsin
(c) Pepsin
(d) Elastase
Answer: (c)
Explanation: Pepsin is an aspartic protease (uses Asp residues). - Dixon plot is used for:
(a) Determining Kₘ
(b) Identifying inhibitor type
(c) Measuring kₐₜ
(d) pH profiling
Answer: (b)
Explanation: Plots 1/V vs. [I]; intersection points indicate Kᵢ. - Enzyme activity decreases above optimal temperature due to:
(a) Reduced collisions
(b) Denaturation
(c) Allosteric inhibition
(d) Cofactor dissociation
Answer: (b)
Explanation: Heat disrupts H-bonds/hydrophobic interactions. - Which coenzyme transfers acyl groups?
(a) NAD⁺
(b) FAD
(c) CoA
(d) TPP
Answer: (c)
Explanation: Coenzyme A carries acyl groups (e.g., acetyl-CoA). - In uncompetitive inhibition:
(a) Kₘ decreases, Vₘₐₓ decreases
(b) Kₘ increases, Vₘₐₓ unchanged
(c) Kₘ unchanged, Vₘₐₓ decreases
(d) Kₘ decreases, Vₘₐₓ unchanged
Answer: (a)
Explanation: Inhibitor binds ES complex; both Kₘ and Vₘₐₓ decrease. - Catalytic perfection occurs when:
(a) kₐₜ/Kₘ ≈ 10³ M⁻¹s⁻¹
(b) kₐₜ/Kₘ ≈ diffusion limit (10⁸–10⁹ M⁻¹s⁻¹)
(c) Vₘₐₓ is infinite
(d) Kₘ = 0
Answer: (b)
Explanation:g., Triose phosphate isomerase (kₐₜ/Kₘ ~ 2×10⁸ M⁻¹s⁻¹). - Which enzyme is regulated by cAMP-dependent phosphorylation?
(a) Pyruvate dehydrogenase
(b) Glycogen phosphorylase
(c) Phosphofructokinase-1
(d) Hexokinase
Answer: (b)
Explanation: Phosphorylase kinase activates glycogen phosphorylase via PKA. - Transition-state analogs:
(a) Bind weaker than substrates
(b) Are competitive inhibitors
(c) Resemble reaction intermediates
(d) Covalently modify enzymes
Answer: (b)
Explanation:g., Pyrrole-2-carboxylate inhibits proline racemase (competitive). - The enzyme with the lowest Kₘ has:
(a) Highest affinity for substrate
(b) Lowest affinity for substrate
(c) Highest Vₘₐₓ
(d) Lowest catalytic efficiency
Answer: (a)
Explanation: Kₘ = [S] at half Vₘₐₓ; low Kₘ = high affinity. - Which is a key regulatory enzyme in TCA cycle?
(a) Aconitase
(b) Isocitrate dehydrogenase
(c) Succinate dehydrogenase
(d) Malate dehydrogenase
Answer: (b)
Explanation: Inhibited by ATP/NADH; activated by ADP/Ca²⁺. - Enzymes reduce entropy by:
(a) Increasing disorder
(b) Orienting substrates
(c) Lowering activation energy
(d) Altering ΔG
Answer: (b)
Explanation: Proper substrate orientation reduces entropy loss for reaction. - Which protease is ATP-dependent?
(a) Trypsin
(b) Chymotrypsin
(c) Proteasome
(d) Pepsin
Answer: (c)
Explanation: 26S proteasome degrades ubiquitinated proteins using ATP. - Competitive inhibitors structurally resemble:
(a) Allosteric sites
(b) Transition states
(c) Substrates
(d) Products
Answer: (c)
Explanation:g., Malonate (competitive) vs. succinate (substrate) in succinate dehydrogenase. - Enzyme units (U) measure:
(a) Molecular weight
(b) μmol substrate transformed per minute
(c) Optimal pH
(d) Kₘ value
Answer: (b)
Explanation: 1 U = 1 μmol min⁻¹ under standard conditions. - The “abzyme” concept refers to:
(a) Ribozymes
(b) Catalytic antibodies
(c) Isozymes
(d) Allosteric enzymes
Answer: (b)
Explanation: Antibodies engineered to catalyze reactions (e.g., ester hydrolysis).
