Ultrastructure of cells Practice Questions
21 free Ultrastructure of cells practice questions for the USMLE Step 1. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
What does the term “cell ultrastructure” refer to in histology and cell biology?
- A The gross anatomy of whole organs as seen under a light microscope
- B The fine internal architecture of cells and their organelles
- C The pattern of gene expression within an individual cell
- D The spatial distribution of tissues throughout an organ
Correct answer: The fine internal architecture of cells and their organelles
Ultrastructure means the detailed internal organisation of a cell — membranes, organelles, cytoskeleton — at a scale below the roughly 200 nm resolving limit of the light microscope, so it is studied by electron microscopy. Gross anatomy and tissue distribution are questions of larger-scale structure, and gene expression describes molecular activity rather than architecture.
Which microscopy technique is most commonly used to study the ultrastructure of internal cellular organelles (e.g., mitochondria, ER)?
- A Light microscopy with H&E stain
- B Confocal fluorescence microscopy
- C Transmission electron microscopy (TEM)
- D Scanning electron microscopy (SEM)
Correct answer: Transmission electron microscopy (TEM)
Transmission electron microscopy (TEM) uses electrons passed through ultrathin sections and heavy-metal staining to visualize internal cell architecture at high resolution, enabling clear visualization of organelles such as mitochondria, nucleus, and Golgi.
Which of the following organelles is characterized ultrastructurally by a double membrane and inner folds called cristae?
- A Nucleus
- B Rough endoplasmic reticulum
- C Mitochondrion
- D Golgi apparatus
Correct answer: Mitochondrion
Mitochondria are double-membraned organelles whose inner membrane folds into cristae, maximizing surface area for oxidative phosphorylation and ATP production.
In electron micrographs, which cellular structure appears as a system of flattened membrane-bound sacs (cisternae) typically located near the nucleus and involved in protein modification and sorting?
- A Lysosome
- B Golgi apparatus
- C Peroxisome
- D Mitochondrion
Correct answer: Golgi apparatus
The Golgi apparatus appears as stacks of flattened cisternae in TEM images; it receives proteins from the ER, modifies and packages them into vesicles for secretion or delivery to other organelles.
Which organelle is characterized ultrastructurally by a network of membrane-bound tubules and cisternae, some of which are studded with ribosomes?
- A Smooth endoplasmic reticulum (sER)
- B Mitochondria
- C Rough endoplasmic reticulum (rER)
- D Peroxisome
Correct answer: Rough endoplasmic reticulum (rER)
Rough endoplasmic reticulum (rER) is a membranous network whose cisternae have ribosomes on their cytosolic surface, giving a 'rough' appearance under EM; it is involved in synthesis of secretory and membrane proteins.
What ultrastructural feature distinguishes rough ER from smooth ER in electron microscopy images?
- A Presence of inner-membrane cristae folds
- B Ribosomes attached to the cytosolic surface
- C Presence of DNA molecules within the lumen
- D A characteristic double-membrane structure
Correct answer: Ribosomes attached to the cytosolic surface
Rough ER has ribosomes bound to its cytosolic surface, which are visible as electron-dense dots under TEM — smooth ER lacks these ribosomes and hence appears smoother.
Which of the following organelles is not bound by a membrane according to ultrastructural studies but is abundant in the cytosol and involved in protein synthesis?
- A Mitochondria
- B Ribosome
- C Golgi apparatus
- D Lysosome
Correct answer: Ribosome
Ribosomes are non-membrane-bound macromolecular complexes visible under EM as dense particles, present free in cytosol or attached to rER; they are essential for protein synthesis.
Which ultrastructural component provides mechanical support to maintain cell shape and organizes intracellular structure, including positioning of organelles?
- A Endoplasmic reticulum
- B Cytoskeleton
- C Golgi apparatus
- D Lysosome
Correct answer: Cytoskeleton
The cytoskeleton — composed of microtubules, intermediate filaments, and microfilaments — forms a dynamic framework within the cytosol that maintains cell shape, organizes organelles, and allows intracellular transport.
Which ultrastructural feature is most likely to be seen in a cell with active protein secretion (e.g., a plasma cell producing antibodies)?
- A Large numbers of mitochondria and few ribosomes
- B Prominent rough ER with abundant Golgi cisternae
- C Many peroxisomes together with abundant lysosomes
- D An expansive smooth endoplasmic reticulum network
Correct answer: Prominent rough ER with abundant Golgi cisternae
Cells specialized for protein secretion typically show extensive rough ER (for synthesis) and an expanded Golgi apparatus (for protein processing and packaging), visible under EM as abundant cisternae.
Which structure, visible under electron microscopy, regulates transport between the cell nucleus and cytoplasm?
- A Cytosolic free ribosomes
- B Pores within the nuclear envelope
- C Smooth endoplasmic reticulum
- D The peroxisomal membrane
Correct answer: Pores within the nuclear envelope
The nuclear envelope is perforated by nuclear pore complexes that control the bidirectional transport of RNA, proteins, and other molecules between nucleus and cytoplasm; these pores are visible under high-resolution EM.
Which organelle appears as a dark, dense spherical region within the nucleus on electron micrographs and is the site of ribosomal subunit assembly?
- A Nucleolus
- B Centrosome
- C Peroxisome
- D Lysosome
Correct answer: Nucleolus
The nucleolus is a dense region within the nucleus that lacks a membrane; under electron microscopy it appears as a dark sphere and is the site where ribosomal RNA is transcribed and ribosomal subunits assemble.
Which ultrastructural organelle is primarily responsible for cellular degradation of worn-out organelles and macromolecules, visible as membrane-bound vesicles in EM images?
- A Golgi apparatus
- B Smooth endoplasmic reticulum
- C Lysosomes
- D Mitochondria
Correct answer: Lysosomes
Lysosomes are membrane-bound vesicles containing hydrolytic enzymes; in EM they appear as dense bodies and are responsible for intracellular digestion and recycling of cell components.
Which of the following statements about mitochondria is correct based on ultrastructural and functional properties?
- A They have a single membrane and carry out photosynthesis like chloroplasts
- B They contain their own DNA and generate ATP by oxidative phosphorylation
- C They lack internal membranes entirely, much like free cytosolic ribosomes
- D They digest worn-out macromolecules using acidic hydrolytic enzymes
Correct answer: They contain their own DNA and generate ATP by oxidative phosphorylation
Mitochondria are double-membraned organelles carrying their own circular DNA and 70S ribosomes, a legacy of their bacterial ancestry. The electron transport chain sits in the folded inner membrane (cristae) and drives ATP synthesis by oxidative phosphorylation. Photosynthesis belongs to chloroplasts and hydrolytic digestion to lysosomes.
Which structure is most directly involved in lipid synthesis and detoxification and would appear under EM as a network of smooth tubular membranes lacking ribosomes?
- A Rough endoplasmic reticulum
- B Smooth endoplasmic reticulum
- C Mitochondria
- D Golgi apparatus
Correct answer: Smooth endoplasmic reticulum
Smooth ER consists of membrane tubules without ribosomes; it synthesizes lipids and detoxifies certain compounds — this ultrastructural appearance helps distinguish it from rough ER.
Which ultrastructural feature would you expect to see in a cell that is highly active in secretion of glycosylated proteins (e.g., secretory epithelial cell)?
- A A prominent nucleolus but sparse rough ER
- B Expanded rough ER and Golgi with many vesicles
- C A predominant abundance of lysosomes only
- D Numerous peroxisomes with little surrounding cytosol
Correct answer: Expanded rough ER and Golgi with many vesicles
Secretory cells typically have large amounts of rER for protein synthesis, a well-developed Golgi for post-translational modification (e.g., glycosylation) and many vesicles for secretion — features clearly visible on electron micrographs.
What advantage does TEM (transmission electron microscopy) provide over light microscopy for studying cell ultrastructure?
- A Ability to view live cell processes in real time
- B True-color photographs of all cellular components
- C Resolution far greater than light microscopy, down to ~0.1 nm
- D Visualization limited to the external cell surface
Correct answer: Resolution far greater than light microscopy, down to ~0.1 nm
TEM provides vastly higher resolution than light microscopy — on the order of tenths of a nanometer — allowing visualization of fine cell structures (membranes, organelles, ribosomes) not resolvable by light microscopy.
Which of the following correctly matches organelle and its primary function, based on ultrastructural knowledge?
- A Peroxisome — cellular protein synthesis
- B Nucleus — cellular ATP production
- C Golgi apparatus — packaging of proteins and lipids
- D Mitochondrion — digestion of macromolecules
Correct answer: Golgi apparatus — packaging of proteins and lipids
The Golgi apparatus processes, modifies, and packages proteins and lipids for secretion or delivery to other organelles — a function consistent with its ultrastructural organization of stacked cisternae and associated vesicles.
Which organelle would you expect to be sparse or small in a cell that does not engage in lipid synthesis or detoxification (e.g., mature red blood cell)?
- A Ribosomes
- B Smooth endoplasmic reticulum
- C Mitochondria
- D Golgi apparatus
Correct answer: Smooth endoplasmic reticulum
In cells that do not synthesize lipids or detoxify substances, smooth ER is typically limited or absent, because its functions (lipid metabolism, detoxification) are not required.
Which ultrastructural region of a eukaryotic cell is continuous with the perinuclear space and forms an extensive network throughout the cytoplasm?
- A Golgi cisternae
- B Mitochondrial matrix
- C Endoplasmic reticulum
- D Lysosomal lumen
Correct answer: Endoplasmic reticulum
The endoplasmic reticulum (ER) is continuous with the perinuclear space of the nuclear envelope and extends throughout the cytoplasm as a network of tubules and cisternae — visible clearly under electron microscopy.
Which ultrastructural observation would most strongly suggest a cell is highly metabolically active and producing large amounts of ATP?
- A Numerous prominent lysosomes
- B Abundant mitochondria with cristae
- C Few ribosomes and a small nucleus
- D An expanded smooth ER network
Correct answer: Abundant mitochondria with cristae
A high number of mitochondria with well-developed cristae under EM indicates robust capacity for oxidative phosphorylation and ATP generation, characteristic of metabolically active cells.
Which ultrastructural feature is most likely to be disrupted in a genetic defect impairing nuclear pore complex formation?
- A Inner cristae folds of the mitochondria
- B Stacked cisternae of the Golgi apparatus
- C Nuclear envelope pores between nucleus and cytoplasm
- D Ribosomal attachment sites on the rough ER
Correct answer: Nuclear envelope pores between nucleus and cytoplasm
Nuclear pore complexes are channels spanning the nuclear envelope that mediate transport between nucleus and cytoplasm; defects would impair nucleocytoplasmic transport and would be evident as abnormal or missing nuclear pores on EM.