Which of the following statements about collagen is false? The collagen proteinase cleaves the termini of pro collagen. Cells bind collagen directly through cell surface integrin proteins. Single collagen polypeptide chains are assembled into triple stranded helices. Which of the following is a function of gap junctions? To adhere cells to one another through adherin proteins. To allow the passage of ions between cells. To prevent the lateral diffusion of proteins and lipids. To adhere cells to the basal lamina. The transmembrane integrin proteins can be found in which of the following types of adhesion junctions? A) Tight junctions. B) Adherens junctions. C) Hemidesmosomes. D) Desmosomes. Which of the following is not true of the basal lamina? A) It is a structure upon which epithelial cells rest. B) It contains osteoblasts. C) It is composed of collagen and laminin. D) Epithelial cells attach to it through hemidesmosomes.
The Correct Answer and Explanation is:
Correct Answers:
- False statement about collagen:
Cells bind collagen directly through cell surface integrin proteins. ← False - Function of gap junctions:
To allow the passage of ions between cells. - Integrins are found in:
C) Hemidesmosomes - Not true of the basal lamina:
B) It contains osteoblasts.
Detailed Explanation:
1. Collagen – False Statement
The incorrect statement is: “Cells bind collagen directly through cell surface integrin proteins.”
This is false because integrins do not bind directly to collagen. Instead, they usually bind to collagen indirectly through bridging molecules like fibronectin. Fibronectin binds to collagen and also contains a binding site for integrins, forming a connection between the extracellular matrix and the cytoskeleton inside the cell. Collagen proteinase does cleave the N- and C-terminal ends of procollagen, and three collagen polypeptide chains form a stable triple-stranded helix, both of which are true.
2. Gap Junction Function
Gap junctions are specialized intercellular connections that form channels allowing the passage of ions and small molecules between adjacent cells. This communication is essential in many tissues, such as cardiac muscle, where ion movement coordinates muscle contraction. Gap junctions are not involved in adhesion, blocking diffusion, or basal lamina attachment—those are roles of other junction types like tight junctions, adherens junctions, or hemidesmosomes.
3. Integrins in Adhesion Junctions
Integrins are transmembrane receptor proteins that mediate the attachment of a cell to the extracellular matrix, particularly in hemidesmosomes. Hemidesmosomes connect the basal surface of epithelial cells to the basal lamina. Tight junctions and desmosomes do not involve integrins; instead, they use other adhesion molecules such as claudins, occludins, and cadherins.
4. Basal Lamina – False Statement
The statement “It contains osteoblasts” is false. The basal lamina is a specialized sheet of the extracellular matrix composed primarily of collagen (especially type IV) and laminin, and it supports epithelial cells, separating them from the underlying connective tissue. Osteoblasts are bone-forming cells found in bone tissue, not in the basal lamina.
