Final answer:
The active sites of brush border enzymes are oriented toward the lumen of the small intestine, playing a critical role in the final stages of carbohydrate and protein digestion.
Step-by-step explanation:
The active sites of brush border enzymes are oriented toward the lumen of the small intestine. These enzymes are vital in the digestive process as they help complete the digestion of carbohydrates and proteins. The small intestine is highly adapted for this function with its many microvilli, which appear as a brush border under the microscope. Each microvillus is a cylindrical extension of the mucosa's epithelial cell's plasma membrane, containing these enzymes. The sheer number of microvilli increases the surface area, significantly enhancing the absorption capabilities of the intestine. The complex structure comprising the circular folds, villi, and microvilli increases the efficiency of the small intestine in nutrient digestion and absorption.