Questions: Cell Structure, Organelles, and Function

Question 1

A researcher examines two cell types under electron microscopy. Cell A has an enormous rough endoplasmic reticulum and a large, prominent Golgi apparatus. Cell B has dense clusters of mitochondria throughout but relatively little ER or Golgi. What is the most reasonable inference?

• A. Cell A is a muscle cell; Cell B is a secretory gland cell
• B. Cell A is specialized for protein synthesis and secretion; Cell B is specialized for high-energy-demand work such as contraction or active signaling
• C. Both cells are undifferentiated stem cells with identical organelle complements
• D. Cell A is more evolutionarily advanced than Cell B because it has greater organelle diversity

Answer: (B)

Organelle abundance directly reflects cellular function — this is the central insight of organelle biology applied to anatomy. The rough ER is studded with ribosomes and synthesizes proteins; the Golgi modifies and packages them for secretion. A cell dominated by these organelles is a secretory cell (like a pancreatic acinar cell or an antibody-secreting plasma cell). Mitochondria produce ATP; a cell dominated by mitochondria performs work requiring continuous energy (muscle, neuron, hepatocyte). Reading organelle profiles is how cell biologists infer function from structure.

Question 2

A mature red blood cell has no nucleus and no mitochondria. The most accurate explanation for this is:

• A. Red blood cells are too small to physically contain a nucleus — it is a size constraint
• B. Red blood cells are prokaryotic in origin and never possessed these organelles
• C. Losing the nucleus and mitochondria during development maximizes interior space for hemoglobin, optimizing the cell's gas-transport function
• D. Red blood cells receive energy from neighboring white blood cells through gap junctions

Answer: (C)

Red blood cells actively eject their nucleus and mitochondria during maturation in the bone marrow — this is not damage or error, it is a programmed specialization. Every unit of interior volume freed from organelles can be filled with hemoglobin, the oxygen-carrying protein. The result is a cell that is essentially a membrane sac packed with hemoglobin — maximally efficient for gas transport. The tradeoff is that mature RBCs cannot synthesize proteins, repair themselves, or respire aerobically, which is why they have a limited lifespan (~120 days) and must be continuously replaced.

Question 3

Most eukaryotic cells contain the same organelles in roughly the same proportions — differences between cell types arise from gene expression, not from organelle number or abundance.

• T. True
• F. False

Answer: False

This is incorrect. While all nucleated eukaryotic cells contain the same basic set of organelles, the relative abundance of each organelle varies enormously between cell types based on specialized function. A pancreatic acinar cell has proportionally enormous rough ER and Golgi (for enzyme secretion); a skeletal muscle cell has proportionally massive numbers of mitochondria and a specialized ER (sarcoplasmic reticulum); a red blood cell has neither nucleus nor mitochondria. Organelle complement is itself regulated by gene expression during differentiation, so the statement confuses cause and effect — gene expression determines which organelles are abundant, producing dramatic differences in organelle profiles.

Question 4

The plasma membrane's structure — with hydrophilic phosphate heads facing outward and hydrophobic fatty acid tails sandwiched inward — explains why small, uncharged molecules like O₂ and CO₂ cross freely while ions and large polar molecules require protein channels.

• T. True
• F. False

Answer: True

The hydrophobic core of the phospholipid bilayer is an effective barrier to polar and charged molecules because they cannot dissolve into a nonpolar environment. Small, uncharged, lipid-soluble molecules can partition into the hydrophobic interior and diffuse across; O₂ and CO₂ are small and nonpolar, crossing freely. Ions (Na⁺, K⁺, Cl⁻) are charged and heavily hydrated — they cannot enter the hydrophobic core. Large polar molecules like glucose cannot cross either. These molecules require specific membrane protein channels or transporters, giving the membrane its selective permeability and the cell control over its internal environment.

Question 5

Explain why a pancreatic acinar cell (which secretes digestive enzymes) has a vastly different organelle profile than a skeletal muscle cell, even though both cells contain the same DNA.

Model answer: Both cells carry identical genomes, but differential gene expression during development determines which genes are active and which proteins are produced, resulting in different organelle abundances. Pancreatic acinar cells are secretory: they synthesize large amounts of digestive enzymes (proteins) and package them for export. This function requires enormous rough ER (protein synthesis by ribosomes) and a large Golgi apparatus (protein modification, sorting, and packaging into secretory vesicles). Skeletal muscle cells perform high-force mechanical work requiring enormous and continuous ATP production; they are therefore packed with mitochondria. Their ER (sarcoplasmic reticulum) is specialized for rapid calcium release triggering contraction, not for protein secretion. Organelle specialization IS cellular specialization.

This principle — that structure reflects function at the organelle level — is the key analytical tool for anatomy and physiology. When studying any organ or tissue, the question 'which organelles does this cell type have in abundance, and why?' immediately reveals the cell's dominant function and metabolic demands. It also predicts clinical consequences: cells with many mitochondria are most vulnerable to mitochondrial toxins; secretory cells are most vulnerable to ER stress; cells without nuclei cannot repair DNA damage.