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Epithelium, Skin and Exocrine Glands Quiz
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Which type of epithelial cell junction most directly limits the passage of ions and molecules between adjacent cells, thereby helping to maintain a controlled barrier?
- A Tight junctions forming a continuous seal around the apical end of epithelial cells You picked ✓
- B Adherens junctions linking the actin cytoskeletons of adjacent cells You picked ✕
- C Gap junctions allowing direct cytoplasmic communication between cells You picked ✕
- D Desmosomes providing strong mechanical attachment between cells You picked ✕
- E Hemidesmosomes anchoring epithelial cells to the basement membrane You picked ✕
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Incorrect
The answer is A: Tight junctions forming a continuous seal around the apical end of epithelial cells
Explanation
Why A is correct
See Introduction to Epithelia. Option A is correct. Epithelial cells are packed tightly together and joined by specialised junctions. Tight junctions sit at the top (apical) end of the cells and firmly join them to form an almost impermeable barrier that limits the passage of molecules and ions between the cells (the paracellular route). Substances therefore have to cross through the cells themselves, which allows tight control over what is absorbed, as in the kidney tubules. They also stop proteins in the cell membrane from drifting between the top and side surfaces, which keeps the two surfaces different. The other junction types do different jobs: desmosomes anchor cells together and resist stretching, gap junctions let small substances pass between cells, and hemidesmosomes anchor the cells to the basement membrane. A memory aid: tight junctions are the "tight seals".
- Tight junctions: sit at the apical end of the cells and join them into an almost impermeable barrier that limits passage between the cells
- Consequence: substances must cross through the cells, which allows tight control over absorption, as in the kidney tubules
- Cell polarity: they stop membrane proteins drifting between the top and side surfaces
- Other junctions: desmosomes anchor cells together, gap junctions let small substances pass and hemidesmosomes anchor cells to the basement membrane
Why the other options are incorrect
- B. Adherens junctions linking the actin cytoskeletons of adjacent cells: Adherens junctions link the internal supporting filaments of neighbouring cells and give mechanical strength. They are not the main seal against the passage of ions and molecules. Your answer
- C. Gap junctions allowing direct cytoplasmic communication between cells: Gap junctions form channels that let ions and small molecules pass directly between cells, which is the opposite of a barrier. They are for communication. Your answer
- D. Desmosomes providing strong mechanical attachment between cells: Desmosomes are strong spot-like attachments that resist stretching and twisting. They hold cells together but do not seal the gap against ions and molecules. Your answer
- E. Hemidesmosomes anchoring epithelial cells to the basement membrane: Hemidesmosomes anchor epithelial cells to the basement membrane beneath them. They do not control what passes between neighbouring cells. Your answer
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1
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2
Which statement best distinguishes exocrine glands from endocrine glands?
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