__________ is a sticky. gelatinous coating external to the cell wall composed of polysaccharides and sometimes protein.

A. Microvilli: Microvilli are small, finger-like projections on the apical surface of epithelial cells that increase surface area for absorption and secretion. They are abundant in the intestines and kidney tubules but do not play a primary role in clearing mucus or trapped particles from the respiratory tract.

B. Flagella: Flagella are long, whip-like structures used for motility in certain single-celled organisms, such as sperm cells or bacteria. Human respiratory epithelial cells do not possess flagella, so they are not involved in airway mucus clearance.

C. Cilia: Cilia are hair-like projections on the apical surface of respiratory epithelial cells that beat in a coordinated manner to move mucus, trapped dust, and pathogens toward the pharynx for clearance. Chronic cigarette smoking damages ciliary structure and function, impairing mucociliary clearance and leading to mucus accumulation, chronic productive cough, and recurrent infections.

D. Desmosomes: Desmosomes are intercellular junctions that provide mechanical adhesion between epithelial cells. While important for tissue integrity, they do not facilitate movement of mucus or debris along the airway epithelium.

E. Tight junctions: Tight junctions are specialized connections between epithelial cells that prevent paracellular leakage of fluids and solutes. They maintain barrier function but are not involved in the active transport of mucus or particulate matter along the respiratory tract.

The outer membrane of gram-negative bacteria contains lipopolysaccharide (LPS), a complex molecule with three components: lipid A, a core polysaccharide, and an O-antigen. Lipid A is embedded in the outer membrane and acts as an endotoxin. When released during bacterial lysis or multiplication, lipid A triggers a strong immune response, leading to fever, inflammation, and in severe cases, septic shock. This property makes gram-negative infections potentially life-threatening. The presence of LPS also contributes to the structural integrity of the outer membrane and provides resistance to certain antibiotics.