What is apical-basal polarity and name two proteins that are enriched apically vs basolaterally?

• A. It refers to distinct membrane domains with different protein compositions; apical region enriched with transporters and tight junction proteins (claudins/occludins); basolateral enriched with E-cadherin and Na+/K+-ATPase.
• B. It describes the nuclear orientation within a cell; apical enriched with histones; basolateral enriched with DNA polymerase.
• C. It is the gradient of mineral content across the cell; apical enriched with calcium channels; basolateral with chloride pumps.
• D. It is the alignment of mitochondria within the cytoplasm; apical enriched with ATP synthase; basolateral with cytochrome c.

Answer: (A)

Apical-basal polarity refers to the organization of an epithelial cell’s surface into two functional membrane domains with distinct protein makeups. The apical surface faces the lumen and typically harbors transporters and tight junction components that regulate selective passage and seal the space between cells. Tight junction proteins such as claudins and occludins are classic apical features that establish that barrier. The basolateral surface, on the other hand, supports adhesion and nutrient/ion handling from the side that interfaces with neighboring cells and the underlying tissue; here you find proteins like E-cadherin at adherens junctions for cell–cell adhesion and the Na+/K+-ATPase pump that maintains ion gradients and drives vectorial transport. This separation enables directional absorption and maintains tissue integrity. Other options describe cellular features not tied to this polarity pattern, such as nuclear organization, mineral transport gradients, or mitochondrial distribution, which do not define apical–basal polarity or the specific protein localization described.