1921 a the flow experiments done in the presence of a

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19–21 A. The flow experiments done in the presence of a P-selectin antibody, or with EDTA present, show that the cells bind to the bilayer surface in a way that is expected for P-selectin-mediated binding. Antibody to P-selectin blocks cell binding, as expected. And EDTA, which binds divalent metal ions such as A440 Chapter 19: Cell Junctions, Cell Adhesion, and the Extracellular Matrix
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Ca 2+ , prevents interaction, as it should because P-selectin binding is Ca 2+ dependent. These two controls encourage the idea that the interactions be- tween the cells and the synthetic lipid bilayer are due to the specific binding of P-selectin to its ligand. B. Two aspects of the data are consistent with the idea that transient tethering of cells to the bilayer surface is due to single P-selectin–neutrophil interac- tions. First, the number of tethering events increases in direct proportion to the density of P-selectin in the bilayer. Second, the off-rate is unaffected by the density of P-selectin. If tethering depended on a pair of interactions, for example, the number of tethering events would be expected to increase as the square of the density of P-selectin in the bilayer. Similarly, the off rate would be expected to decrease substantially with increasing density of P- selectin. C. Increasing the flow rate might be expected to increase the off rate by strain- ing the noncovalent bonds holding P-selectin to its ligand. Imagine hanging onto a cliff by your fingertips. If someone else is hanging onto your foot, exerting an additional force by their weight, you will not be able to hold on as long: your off-rate will increase. Reference: Alon R, Hammer DA & Springer TA (1995) Lifetime of the P- selectin-carbohydrate bond and its response to tensile force in hydrody- namic flow. Nature 374, 539–542. TIGHT JUNCTIONS AND THE ORGANIZATION OF EPITHELIA DEFINITIONS 19–22 Polarized 19–23 Septate junction 19–24 Tight junction 19–25 Apical TRUE/FALSE 19–26 True. The polarity of essentially all epithelia is the same: the basal surface is anchored in the basal lamina, which abuts other tissues, while the apical surface is exposed. 19–27 True. The barriers formed by tight junction proteins restrict the flow of molecules between cells and the diffusion of proteins (and lipids) from the apical to the basolateral domain and vice versa. 19–28 False. All paracellular transport is passive; it results from the movement of material down an electrochemical gradient. 19–29 False. Septate junctions are a special variety of occluding junction that is found in invertebrates, including insects, but not restricted to them. THOUGHT PROBLEMS 19–30 Epithelial sheets are a common architectural feature in animal bodies, lining all its surfaces and cavities. A few examples include the lining of the lumen of the intestine, the outside of the intestine that separates it from the body cavity, the lining of the bladder, the lining of the lungs and the outside of the lungs, and the epidermis.
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