All of these are the components of the catabolic pathway or using the nutrients to provide energy from it. The breakdown of food molecules begins in the mouth and continues to the small intestine. The nutrients are absorbed through the wall of the small intestine which. The surface of the intestine wall is specially modified (contains a huge number of hair-like structures-microvilli) which increase nutrient absorption. (more area for nutrients to be absorbed). The digestive tract is lined with mucosa which consists of simple columnar epithelial cells. Monomer subunits of the food, like glucose are than absorbed and diffused down a concentration gradient into capillary blood. Glucose is converted into pyruvate molecules through the process of glycolysis. Catabolism ends in the major energy-converting organelle, the mitochondrion, where the ATP is produced.