H 3 Absorption of Digested Foods H 3

H 3 Absorption of Digested Foods H 3. 1 Draw and label a diagram showing a transverse section of the ileum as seen under a light microscope

H 3. 2 Explain the structural features of an epithelium cell of a villus as seen in electron micrographs including microvilli, mitochondria, pinocytotic vesicles and tight junctions • Villi are fingerlike projections of the mucosa, over 1 mm high, that give it a velvety texture, much like the soft nap of a towel

The epithelial cells of the villi are chiefly absorptive columnar cells called enterocytes or simply intestinal cells These cells are bound by tight junctions which are connections between cells to maintain the integrity of the tissue, and richly endowed with microvilli to further extend the absorptive area available Mitochondria are present to provide the energy required for active transport by which some nutrients are absorbed

Vesicles may be present since some food is taken up by pinocytosis The epithelia also have many mucus-secreting goblet cells In the core of each villus is a dense capillary bed and a wide lymph capillary called a lacteal Digested food stuffs are absorbed through the epithelial cells into both the capillary blood and the lacteal

H 3. 3 Explain the mechanisms used by the ileum to absorb and transport food, including facilitated diffusion, active transport, and endocytosis • The monosaccharides glucose and galactose, liberated by the breakdown of starch and disaccharides, are transported into the epithelial cells by common protein carriers (secondary active transport coupled to sodium ions) and then move by facilitated diffusion into the capillary blood; fructose moves entirely by facilitated diffusion

Secondary Active Transport

Several types of carriers actively transport different dipeptides and tripeptides resulting from protein digestion and then after further digestion in the epithelial cell, the amino acids enter capillary blood by diffusion

Most nutrients are absorbed through the mucosa of the intestinal villi by active transport processes driven directly or indirectly (secondarily) by metabolic energy (ATP) The exception is some of the lipid digestion products which are absorbed passively by diffusion, and then enter the lacteal in the villus to be carried to the blood by lymphatic fluid

Micelles: aggregations of biles salts, monoglycerides, and fatty acids in intestinal lumen Chylomicrons: large fat droplets made of triglycerides, cholesterol, proteins, and phospholipids

The products of nucleic acid digestion are actively transported across the epithelium by special carriers in the villus epithelium after which they enter the blood Most water soluble vitamins are absorbed easily by diffusion with the exception of vitamin B 12 which is very large and has a charge It is bound to intrinsic factor produced in the stomach and this complex binds to specific mucosal receptor sites in the terminal ileum and triggers its active uptake by endocytosis

H 3. 4 List the materials that are not absorbed and are egested The primary concerns of the large intestine are propulsive activities that force fecal material toward the anus and then eliminate it from the body (defecation) Fecal material includes cellulose, lignin (combines with cellulose; found in cell walls), bile pigments, bacteria, and intestinal cells