60 free MCQs on Cell: The Unit of Life with worked answers and explanations. Cell theory, prokaryotic vs eukaryotic cells, organelles and their functions. Foundation of all biology.
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Animal cell structure with major organelles labeled. Image: LadyofHats (Mariana Ruiz), Public Domain, via Wikimedia Commons.
Q1.
Who proposed the cell theory?
Answer: B. Schleiden and Schwann
Why: Matthias Schleiden (plants) and Theodor Schwann (animals) proposed cell theory in 1838-1839.
Q2.
The powerhouse of the cell is the:
Answer: C. Mitochondria
Why: Mitochondria produce ATP through aerobic respiration. They are called the powerhouse of the cell.
Q3.
Which organelle is responsible for protein synthesis?
Answer: C. Ribosome
Why: Ribosomes are the sites of protein synthesis (translation) in all cells.
Q4.
Cell wall in plant cells is made of:
Answer: C. Cellulose
Why: Plant cell walls are made of cellulose (polysaccharide). Fungal cell walls contain chitin; bacterial walls have peptidoglycan.
Q5.
Which organelle is called the post office of the cell?
Answer: C. Golgi apparatus
Why: Golgi apparatus modifies, packages, and ships proteins to their destinations: like a post office.
Q6.
The fluid mosaic model describes the structure of:
Answer: B. Plasma membrane
Why: Singer and Nicolson's fluid mosaic model (1972): plasma membrane is a fluid phospholipid bilayer with embedded proteins.
Q7.
Lysosomes are called the suicide bags because:
Answer: B. Their enzymes can destroy the cell if released
Why: Lysosomes contain hydrolytic enzymes. If these enzymes leak, they can digest the cell itself: hence suicide bags.
Q8.
Which type of cell lacks a membrane-bound nucleus?
Answer: C. Prokaryotic
Why: Prokaryotic cells (bacteria, archaea) have no membrane-bound nucleus. Their DNA is in the nucleoid region.
Q9.
Chloroplasts are found in:
Answer: C. Only plant cells and algae
Why: Chloroplasts are present in plant cells and algae. They are the sites of photosynthesis.
Q10.
The nucleus contains:
Answer: B. DNA and proteins
Why: The nucleus contains DNA (genetic material) along with proteins (histones) and RNA. It is the control center of the cell.
Q11.
Osmosis is the movement of water from:
Answer: B. Lower solute to higher solute concentration (i.e., across semi-permeable membrane)
Why: Osmosis: water moves from low solute concentration (high water potential) to high solute concentration (low water potential) through a semi-permeable membrane.
Q12.
Which organelle contains digestive enzymes in animal cells?
Answer: B. Lysosome
Why: Lysosomes contain hydrolytic (digestive) enzymes. They digest worn-out organelles and foreign particles.
Q13.
The endoplasmic reticulum with ribosomes attached is called:
Answer: B. Rough ER
Why: Rough ER has ribosomes on its surface. It is involved in synthesis and processing of secretory proteins.
Q14.
Plasmolysis occurs when a plant cell is placed in:
Answer: C. Hypertonic solution
Why: In a hypertonic solution, water leaves the cell by osmosis. The cell membrane shrinks away from wall: this is plasmolysis.
Q15.
The basic unit of life is:
Answer: C. Cell
Why: The cell is the basic structural and functional unit of all living organisms (Cell Theory).
Q16.
ATP is synthesized mainly in the:
Answer: C. Mitochondria
Why: The majority of ATP is produced in mitochondria during aerobic respiration (oxidative phosphorylation).
Q17.
Which of these organelles is NOT found in animal cells?
Answer: C. Chloroplast
Why: Chloroplasts are only in plant cells and algae. Animal cells lack chloroplasts.
Q18.
The nuclear membrane is present in:
Answer: B. Eukaryotes only
Why: Nuclear membrane (nuclear envelope) is a defining feature of eukaryotic cells. Prokaryotes have no such membrane.
Q19.
Ribosomes in prokaryotes are:
Answer: B. 70S
Why: Prokaryotic ribosomes are 70S (50S + 30S subunits). Eukaryotic cytoplasmic ribosomes are 80S.
Q20.
Cell membrane is primarily made of:
Answer: B. Phospholipids
Why: The cell membrane is a phospholipid bilayer with embedded proteins (fluid mosaic model).
Q21.
Which theory best explains the origin of mitochondria and chloroplasts?
Answer: B. Endosymbiont theory
Why: Endosymbiont theory (Lynn Margulis): mitochondria and chloroplasts were once free-living prokaryotes engulfed by ancestral eukaryotic cells.
Q22.
The process by which cells engulf foreign particles is called:
Answer: C. Phagocytosis
Why: Phagocytosis: cell engulfs solid particles (eat cell). Pinocytosis: cell engulfs liquid droplets (drink cell). Both are endocytosis.
Q23.
Centrioles are involved in:
Answer: B. Formation of spindle fibers in cell division
Why: Centrioles (in animal cells) form asters and organize the spindle apparatus during cell division.
Q24.
The nucleolus is responsible for:
Answer: C. rRNA synthesis and ribosome assembly
Why: The nucleolus synthesizes ribosomal RNA (rRNA) and assembles ribosome subunits.
Q25.
Active transport requires:
Answer: C. ATP (energy) and carrier proteins
Why: Active transport moves substances against the concentration gradient using ATP energy and specific carrier proteins.
Q26.
Which organelle is the site of lipid synthesis?
Answer: B. Smooth ER
Why: Smooth ER (no ribosomes) is the site of lipid and cholesterol synthesis, and also involved in detoxification.
Q27.
Turgor pressure in plant cells is generated by:
Answer: B. Entry of water into vacuole by osmosis
Why: Water enters the vacuole by osmosis, pushing the cell membrane against the rigid cell wall. This creates turgor pressure.
Q28.
Na+/K+ pump is an example of:
Answer: B. Active transport (primary)
Why: The Na+/K+ ATPase pump transports 3 Na+ out and 2 K+ in per ATP, against concentration gradients: primary active transport.
Q29.
Peroxisomes function in:
Answer: C. Breaking down hydrogen peroxide (catalase enzyme)
Why: Peroxisomes contain catalase enzyme that converts toxic H2O2 to water and oxygen.
Q30.
The unit membrane model proposes that all membranes have a:
Answer: C. Fluid mosaic of lipids and proteins
Why: Robertson's unit membrane model proposed a protein-lipid-protein sandwich structure. Modern understanding is the fluid mosaic model.
Q31.
Microtubules are made of the protein:
Answer: B. Tubulin
Why: Microtubules are hollow tubes made of alpha and beta tubulin protein subunits. They form the cytoskeleton and spindle fibers.
Q32.
Plasmodesmata are:
Answer: A. Pores in plant cell wall connecting adjacent cells
Why: Plasmodesmata are cytoplasmic channels through plant cell walls that connect neighboring cells for communication and transport.
Q33.
The Fluid Mosaic Model was proposed by:
Answer: C. Singer and Nicolson
Why: Singer and Nicolson (1972) proposed the Fluid Mosaic Model for cell membrane structure.
Q34.
Tonoplast is the membrane surrounding the:
Answer: B. Vacuole
Why: Tonoplast is the membrane that surrounds the central vacuole in plant cells.
Q35.
Which cell structure maintains osmotic balance in Amoeba?
Answer: B. Contractile vacuole
Why: Contractile vacuole in Amoeba and other protists pumps out excess water, maintaining osmotic balance.
Q36.
Ribosomes inside mitochondria are:
Answer: B. 70S
Why: Mitochondrial ribosomes are 70S (like prokaryotes), supporting the endosymbiont theory of their origin.
Q37.
Gap junctions in animal cells are analogous to plant:
Answer: B. Plasmodesmata
Why: Gap junctions (animal) and plasmodesmata (plant) are both channels allowing direct cytoplasmic communication between adjacent cells.
Q38.
The inner membrane of mitochondria is folded into:
Answer: C. Cristae
Why: The inner mitochondrial membrane folds into cristae, greatly increasing surface area for ATP synthesis.
Q39.
Which substance is NOT found in the cell membrane?
Answer: D. Cellulose
Why: Cellulose is found in plant cell walls, not in the cell membrane. Membranes contain phospholipids, proteins, and cholesterol.
Q40.
Pinocytosis differs from phagocytosis in that:
Answer: B. It involves engulfing liquids, not solids
Why: Pinocytosis = cell drinking (liquid engulfed). Phagocytosis = cell eating (solid particles engulfed). Both are endocytosis.
Q41.
The Z-scheme in photosynthesis describes:
Answer: C. Non-cyclic electron transport in light reactions
Why: Z-scheme: electrons from water (via PS II) and from PS I flow in a Z-shaped path, generating ATP and NADPH in non-cyclic photophosphorylation.
Q42.
Transmembrane proteins in the fluid mosaic model span:
Answer: C. Both phospholipid layers completely
Why: Integral (transmembrane) proteins span the entire phospholipid bilayer, with portions exposed on both sides.
Q43.
KDEL sequence on a protein indicates it should be:
Answer: C. Retained in the ER lumen
Why: KDEL (Lys-Asp-Glu-Leu) is the ER retention signal. Proteins with KDEL are retrieved from the Golgi and retained in the ER.
Q44.
Connexons are protein units that form:
Answer: B. Gap junctions
Why: Gap junctions consist of connexons (hemichannels). Two cells each contribute a connexon; together they form a channel for ion/molecule transfer.
Q45.
The signal hypothesis (Blobel) explains how:
Answer: B. Secretory proteins are directed to the rough ER
Why: Blobel's signal hypothesis: N-terminal signal sequence on secretory proteins directs ribosomes to rough ER; protein enters the ER lumen.
Q46.
Autophagy is the process of:
Answer: B. Self-digestion of organelles by lysosomes
Why: Autophagy: worn-out organelles or cytoplasmic components are enclosed in autophagosomes, then fused with lysosomes for degradation.
Q47.
The voltage across cell membrane (resting potential) is maintained by:
Answer: B. Na+/K+ ATPase pump and selective ion permeability
Why: Resting potential (-70 mV in neurons) is maintained by Na+/K+ ATPase pump (active) plus the much higher permeability of K+ than Na+ (passive).
Q48.
Matrix of the mitochondria contains:
Answer: B. Krebs cycle enzymes, mitochondrial DNA, and ribosomes
Why: Mitochondrial matrix contains Krebs cycle enzymes, circular mitochondrial DNA, 70S ribosomes, and other soluble components.
Q49.
Glycosylation of proteins occurs in the:
Answer: C. Rough ER and Golgi apparatus
Why: N-linked glycosylation begins in the rough ER; O-linked glycosylation occurs in the Golgi. Both add carbohydrate groups to proteins.
Q50.
Which technique separates cell organelles by density?
Answer: C. Centrifugation (ultracentrifuge)
Why: Ultracentrifugation separates organelles by size and density. Different organelles pellet at different centrifuge speeds.
Q51.
Apoptosis differs from necrosis in that apoptosis:
Answer: C. Is programmed, orderly cell death without inflammation
Why: Apoptosis = programmed cell death: orderly, no inflammation, DNA fragmented, cell pieces cleared by phagocytes. Necrosis is uncontrolled damage-caused death.
Q52.
The proton gradient across inner mitochondrial membrane drives:
Answer: B. ATP synthesis by ATP synthase (chemiosmosis)
Why: Electron transport chain pumps H+ across inner membrane. H+ flow back through ATP synthase (chemiosmosis) drives ATP production.
Q53.
Which protein is responsible for maintaining chromosome condensation during mitosis?
Answer: B. Condensin
Why: Condensin complex compacts chromosomes during mitosis. Cohesins hold sister chromatids together until anaphase.
Q54.
Which cellular structure is the site of the electron transport chain?
Answer: A. Inner mitochondrial membrane
Why: The electron transport chain complexes (I-IV) are embedded in the inner mitochondrial membrane. ATP synthase (Complex V) is also there.
Q55.
SDS-PAGE separates proteins by:
Answer: B. Size (molecular weight)
Why: SDS-PAGE: SDS denatures proteins and gives them uniform negative charge proportional to size. Proteins separate by size in polyacrylamide gel.
Q56.
Thylakoid membranes in chloroplasts are the site of:
Answer: B. Light reactions (photo-phosphorylation)
Why: Thylakoid membranes contain photosystems I and II, electron transport chain, and ATP synthase: all light reaction components.
Q57.
Clathrin-coated vesicles are involved in:
Answer: B. Receptor-mediated endocytosis
Why: Clathrin-coated pits and vesicles mediate receptor-mediated endocytosis (e.g., LDL uptake, transferrin internalization).
Q58.
The fluid mosaic model proposed that membrane lipids can:
Answer: B. Move laterally within the bilayer (lateral diffusion) but rarely flip
Why: Phospholipids diffuse rapidly laterally within their leaflet but rarely flip (transverse diffusion/flip-flop). Proteins also move laterally.
Q59.
What is the function of the signal recognition particle (SRP)?
Answer: B. Directs ribosome-mRNA complex to rough ER
Why: SRP recognizes the signal sequence on a growing polypeptide and targets the ribosome-mRNA complex to the rough ER membrane.
Q60.
Voltage-gated ion channels open in response to:
Answer: B. Changes in membrane potential
Why: Voltage-gated channels (e.g., Na+ channels in neurons) open when membrane potential reaches a threshold (depolarization).