USMLE Step 1 Cell Biology Practice Questions

A researcher is studying protein transport within a eukaryotic cell. They observe a protein destined for secretion being synthesized on ribosomes, then entering the endoplasmic reticulum, moving to the Golgi apparatus for modification and packaging, and finally being released via vesicles. Which cellular structure is directly responsible for the synthesis of proteins destined for secretion, insertion into membranes, or delivery to organelles like lysosomes?

• (A) Ribosomes attached to the Rough Endoplasmic Reticulum (RER)
• (B) Free ribosomes in the cytosol
• (C) Smooth Endoplasmic Reticulum (SER)
• (D) Peroxisomes

Lysosomal storage diseases result from defects in lysosomal enzymes, leading to the accumulation of undigested substrates within lysosomes. Tay-Sachs disease, characterized by progressive neurodegeneration and a 'cherry-red spot' on the macula, results from a deficiency in which enzyme, leading to the accumulation of GM2 gangliosides?

• (A) Hexosaminidase A
• (B) Glucocerebrosidase
• (C) Sphingomyelinase
• (D) Alpha-galactosidase A

Microtubules, microfilaments (actin filaments), and intermediate filaments are the three main components of the eukaryotic cytoskeleton. Which of these components is primarily responsible for forming the structural core of cilia and flagella, and also forms the spindle fibers during mitosis?

• (A) Microtubules
• (B) Microfilaments (Actin filaments)
• (C) Intermediate filaments
• (D) Myosin filaments

The fluid mosaic model describes the plasma membrane as a dynamic structure composed of a phospholipid bilayer with embedded proteins. Which type of lipid molecule is amphipathic (having both hydrophilic and hydrophobic regions) and forms the basic bilayer structure of cell membranes?

• (A) Phospholipids
• (B) Triglycerides
• (C) Cholesterol
• (D) Glycolipids

Which cellular organelle is primarily responsible for synthesizing lipids (including steroids), detoxifying drugs and poisons, and storing calcium ions?

• (A) Smooth Endoplasmic Reticulum (SER)
• (B) Rough Endoplasmic Reticulum (RER)
• (C) Golgi apparatus
• (D) Mitochondria

Zellweger syndrome is a peroxisomal biogenesis disorder characterized by severe neurological defects, liver dysfunction, and craniofacial abnormalities. This condition results from defects in the import of proteins into which organelle?

• (A) Peroxisome
• (B) Mitochondrion
• (C) Lysosome
• (D) Endoplasmic Reticulum

Which type of cell junction forms a strong mechanical link between adjacent epithelial cells, connecting their intermediate filament cytoskeletons and providing resistance to shearing stress?

• (A) Desmosomes (Macula adherens)
• (B) Tight junctions (Zonula occludens)
• (C) Gap junctions (Nexus)
• (D) Adherens junctions (Zonula adherens)

The process by which cells engulf large particles, such as bacteria or cellular debris, by extending pseudopods is known as:

• (A) Phagocytosis
• (B) Pinocytosis
• (C) Receptor-mediated endocytosis
• (D) Exocytosis

Which motor protein moves cargo along microtubules towards the minus end (typically located near the cell center or microtubule organizing center)?

• (A) Dynein
• (B) Kinesin
• (C) Myosin
• (D) Actin

The nucleolus, a dense structure within the nucleus, is primarily involved in the synthesis and assembly of which cellular components?

• (A) Ribosomal RNA (rRNA) and ribosomal subunits
• (B) Messenger RNA (mRNA)
• (C) Transfer RNA (tRNA)
• (D) Histone proteins

Which component of the cytoskeleton is composed of various protein types depending on the cell type (e.g., keratins in epithelial cells, vimentin in mesenchymal cells, desmin in muscle, neurofilaments in neurons) and primarily functions to provide mechanical strength and resist stress?

• (A) Intermediate filaments
• (B) Microtubules
• (C) Actin filaments (Microfilaments)
• (D) Spectrin filaments

The sodium-potassium pump (Na+/K+-ATPase) actively transports ions across the plasma membrane to maintain electrochemical gradients. For each molecule of ATP hydrolyzed, how many sodium ions are pumped out of the cell, and how many potassium ions are pumped into the cell?

• (A) 3 Na+ out, 2 K+ in
• (B) 2 Na+ out, 3 K+ in
• (C) 3 Na+ in, 2 K+ out
• (D) 1 Na+ out, 1 K+ in

Apoptosis, or programmed cell death, is a tightly regulated process essential for development and tissue homeostasis. Which family of cysteine proteases plays a central role in executing the apoptotic program by cleaving specific cellular substrates?

• (A) Caspases
• (B) Metalloproteinases
• (C) Serine proteases
• (D) Cathepsins

Which organelle contains enzymes like catalase and urate oxidase, and is involved in breaking down fatty acids (beta-oxidation), detoxifying harmful substances (like ethanol), and synthesizing certain lipids (plasmalogens)?

• (A) Peroxisome
• (B) Lysosome
• (C) Golgi apparatus
• (D) Mitochondrion

What is the primary function of the Golgi apparatus (Golgi complex)?

• (A) Modification, sorting, and packaging of proteins and lipids for secretion or delivery to other organelles
• (B) Synthesis of ATP via oxidative phosphorylation
• (C) Synthesis of proteins
• (D) Degradation of cellular waste materials

Actin filaments (microfilaments) are dynamic polymers essential for cell movement, muscle contraction, and maintaining cell shape. Which protein motor interacts with actin filaments to generate force, for example, in muscle contraction and cytokinesis?

• (A) Myosin
• (B) Kinesin
• (C) Dynein
• (D) Tubulin

Which type of cell junction allows direct passage of small ions and molecules (like cAMP, Ca2+) between the cytoplasm of adjacent cells, enabling rapid communication and metabolic coupling?

• (A) Gap junctions
• (B) Tight junctions
• (C) Desmosomes
• (D) Adherens junctions

Mitochondria are often referred to as the 'powerhouses' of the cell because they are the primary site of:

• (A) ATP synthesis via oxidative phosphorylation
• (B) Protein synthesis
• (C) Lipid synthesis
• (D) Glycolysis

The process by which a cell takes up specific molecules (ligands) from the extracellular environment after they bind to specific receptors on the cell surface is called:

• (A) Receptor-mediated endocytosis
• (B) Phagocytosis
• (C) Pinocytosis
• (D) Autophagy

Which cellular structure acts as the main Microtubule Organizing Center (MTOC) in animal cells, from which microtubules typically radiate?

• (A) Centrosome
• (B) Nucleolus
• (C) Basal body
• (D) Kinetochore

I-cell disease (Mucolipidosis II) is a severe lysosomal storage disorder where lysosomal enzymes are incorrectly secreted outside the cell instead of being targeted to lysosomes. This is caused by a defect in the enzyme responsible for adding which specific tag to lysosomal hydrolases in the Golgi apparatus?

• (A) Mannose-6-phosphate (M6P)
• (B) Ubiquitin
• (C) N-linked oligosaccharide
• (D) GPI anchor

Which component of the extracellular matrix (ECM) is a large glycoprotein that binds to integrins on the cell surface and also interacts with other ECM components like collagen and proteoglycans, playing a key role in cell adhesion, migration, and wound healing?

• (A) Fibronectin
• (B) Elastin
• (C) Laminin
• (D) Collagen Type IV

What is the term for the process by which a cell engulfs portions of its own cytoplasm or organelles within a double membrane, delivering them to the lysosome for degradation and recycling?

• (A) Autophagy
• (B) Phagocytosis
• (C) Endocytosis
• (D) Apoptosis

Which cytoskeletal filament system is responsible for forming the contractile ring during cytokinesis, leading to the physical separation of daughter cells at the end of mitosis?

• (A) Actin filaments (Microfilaments) and Myosin II
• (B) Microtubules
• (C) Intermediate filaments
• (D) Septins

The inner mitochondrial membrane is folded into cristae, which significantly increase its surface area. This large surface area is critical for housing which essential components of cellular respiration?

• (A) Electron transport chain complexes and ATP synthase
• (B) Citric acid cycle enzymes
• (C) Glycolysis enzymes
• (D) Fatty acid synthesis enzymes

Which organelle modifies proteins synthesized on the RER by adding N-linked oligosaccharides (glycosylation)?

• (A) Rough Endoplasmic Reticulum (RER)
• (B) Golgi apparatus
• (C) Lysosome
• (D) Peroxisome

Cellular senescence is a state of stable cell cycle arrest, often triggered by factors like telomere shortening or DNA damage. Which tumor suppressor protein plays a critical role in initiating and maintaining the senescent state, often by activating cyclin-dependent kinase inhibitors?

• (A) p53
• (B) Ras
• (C) Myc
• (D) Cyclin D

Which type of transport across a cell membrane requires energy (usually from ATP hydrolysis) to move a substance against its concentration gradient?

• (A) Active transport
• (B) Facilitated diffusion
• (C) Simple diffusion
• (D) Osmosis

What is the name of the protein complex that recognizes ubiquitin tags on proteins and targets them for degradation?

• (A) Proteasome
• (B) Lysosome
• (C) Ribosome
• (D) Spliceosome

Which family of cell surface receptors binds to components of the extracellular matrix (like fibronectin, laminin) and also transmits signals from the ECM to the cell interior, influencing cell adhesion, migration, proliferation, and survival?

• (A) Integrins
• (B) Cadherins
• (C) Selectins
• (D) Immunoglobulin superfamily receptors

The process of synthesizing proteins from an mRNA template is called:

• (A) Translation
• (B) Transcription
• (C) Replication
• (D) Transduction

Which structure within the nucleus is responsible for synthesizing ribosomal RNA (rRNA)?

• (A) Nucleolus
• (B) Nuclear pore complex
• (C) Nuclear lamina
• (D) Heterochromatin

What type of filament forms the core of microvilli, the finger-like projections on the apical surface of some epithelial cells that increase surface area for absorption?

• (A) Actin filaments (Microfilaments)
• (B) Microtubules
• (C) Intermediate filaments
• (D) Keratin filaments

Which cellular process involves the fusion of vesicles containing neurotransmitters with the presynaptic membrane to release their contents into the synaptic cleft?

• (A) Exocytosis
• (B) Endocytosis
• (C) Phagocytosis
• (D) Pinocytosis

Tight junctions (Zonula occludens) are primarily responsible for which function in epithelial tissues?

• (A) Preventing leakage of solutes and water between cells (paracellular barrier)
• (B) Anchoring intermediate filaments between cells
• (C) Allowing direct communication between adjacent cells
• (D) Anchoring actin filaments between cells

Which protein acts as a 'cap' on the plus end of actin filaments, preventing further polymerization or depolymerization at that end?

• (A) CapZ
• (B) Tropomyosin
• (C) Profilin
• (D) Cofilin

The nuclear envelope, which surrounds the nucleus in eukaryotic cells, is composed of:

• (A) Two concentric membranes (inner and outer) perforated by nuclear pores
• (B) A single membrane continuous with the endoplasmic reticulum
• (C) A meshwork of intermediate filaments
• (D) A layer of glycoproteins

Which molecule is often referred to as the 'second messenger' that is generated by adenylyl cyclase in response to activation of G protein-coupled receptors (GPCRs) linked to Gs?

• (A) Cyclic AMP (cAMP)
• (B) Calcium ions (Ca2+)
• (C) Diacylglycerol (DAG)
• (D) Inositol trisphosphate (IP3)

What is the primary role of chaperone proteins (e.g., Hsp70, Hsp60)?

• (A) Assisting in the proper folding of other proteins and preventing aggregation
• (B) Degrading misfolded proteins
• (C) Synthesizing proteins
• (D) Transporting proteins across membranes

Which cytoskeletal element provides tracks for the movement of chromosomes during mitosis and meiosis?

• (A) Microtubules
• (B) Actin filaments
• (C) Intermediate filaments
• (D) Spectrin