Final answer:
Microvilli on the plasma membrane in the small intestine drastically increase the absorptive surface area for nutrient uptake. They sit on the villi and together with circular folds contribute to a greatly expanded surface that improves digestion and absorption. This adaptation is crucial for maximizing nutrient absorption from digested food.
Step-by-step explanation:
The absorptive surface of the small intestine is significantly increased by the presence of microvilli, which are tiny, finger-like projections on the plasma membrane of the cells lining the small intestine. These structures are specialized for absorption and effectively maximize the area available for the uptake of nutrients. The villi are larger projections on which the microvilli sit, and together with the circular folds, they work to amplify the surface area for absorption. An individual cell may have thousands of microvilli, and given that each square inch of mucosa in the small intestine can have around 20,000 villi and well over 100 billion microvilli, the total area for absorption is massive in scale. Consequently, this large surface area created by microvilli greatly enhances the efficiency of nutrient uptake by providing extensive space for diffusion and absorption of nutrients, such as bile salts and vitamin B12.
Microvilli are important because they amplify the surface area of the plasma membrane where absorption takes place, allowing for a greater amount of nutrients to pass through. When food passes through the small intestine as chyme, the enlarged surface created by microvilli allows for it to come into contact with more digestive enzymes and for a more extensive area for absorption. This is essential for thorough digestion and effective absorption of nutrients into the body's circulatory system. The increase in absorptive surface area due to microvilli is a critical adaptation for maximizing nutrient uptake from food.