5.2 Nutrient Absorption in Animals

Animals obtain their nutrition from the consumption of other organisms. Depending on their diet, animals can be classified into the following categories: plant eaters (herbivores), meat eaters (carnivores), and those that eat both plants and animals (omnivores). Once consumed, nutrients and macromolecules present in food are not immediately accessible to the cells. There are a number of processes that modify food within the animal body in order to make the nutrients and organic molecules usable for cellular function. As animals evolve in  form and function, their digestive systems have also evolved to accommodate their various dietary needs. Digestion involves more than just material – it is a complex system of breakdown and nutrient transfer that will fuel or fail the body.

Once the macromolecules in the food are converted into monomers, these monomers can be absorbed into the bloodstream so that they can be distributed to all cells in the body to support cell maintenance and growth.

The stomach is the major site for protein digestion in animals other than ruminants. Protein digestion is mediated by an enzyme called pepsin in the stomach chamber. Pepsin is secreted by the chief cells in the stomach where it breaks peptide bonds and cleaves proteins into smaller polypeptides and amino acids. Another cell type—parietal cells—secrete hydrogen and chloride ions, which combine in the lumen to form hydrochloric acid, the primary acidic component of the stomach juices. The highly acidic environment kills many microorganisms in the food and, combined with the action of the enzyme pepsin, results in the hydrolysis of protein in the food.

We can look at the parietal cell, which is responsible for producing the acid (HCl) in the stomach to infer the type of membrane transport responsible for ensuring that H⁺ and Cl^– are in high concentrations in the stomach lumen (Figure 1).

Figure 1. Production of hydrochloric acid by a parietal cell.

Inferring concentration of H⁺ in the parietal cell: The enzyme carbonic anhydrase converts one molecule of carbon dioxide and one molecule of water indirectly into a bicarbonate ion (HCO₃^–) and a hydrogen ion (H⁺). This leads to a high concentration of H+ in the cytoplasm of the parietal cell.

Primary active transport of H+ into the gastric lumen: In the stomach ion exchange is used to move H⁺ ions out the cells and into the lumen of the stomach. The H⁺/K⁺ ATPase uses the energy from the hydrolysis of ATP to push H⁺ out of the cell and pull K⁺ into the cell. This ensures that there is no net charge build up across the membrane and the highest concentration of H⁺ is found in the gastric lumen.

Secondary active transport of Cl^– into the parietal cell: The bicarbonate ion (HCO₃^–), which builds up in the parietal cell cytoplasm, is then exchanged for a chloride ion (Cl^–) on the basal side (away from the lumen) of the cell. The transport of HCO₃^– down its concentration gradient provides the energy to drive Cl^– ions into the parietal cell cytoplasm. Cl^– ions can then diffuse through a channel into the gastric lumen side of the cell.

When digesting protein and some fats, the stomach lining must be protected from getting digested by pepsin. The stomach lining protects itself by 1)  synthesizing the pepsin enzyme in the inactive form to protect chief cells, and 2)  maintaining a thick mucus lining that protects the underlying tissue from digestive juices.

Small Intestine

Chyme moves from the stomach to the small intestine. The small intestine is the organ where the digestion of protein, fats, and carbohydrates is completed. The small intestine is a long tube-like organ with a highly folded surface containing finger-like projections called the villi. The apical surface of each villus has many microscopic projections called microvilli. These structures, illustrated in Figure 2, are lined with epithelial cells on the luminal side (facing the interior of the small intestine) and allow for the nutrients to be absorbed from the digested food and absorbed into the bloodstream on the other side. The villi and microvilli, with their many folds, increase the surface area of the intestine and increase absorption efficiency of the nutrients. Absorbed nutrients in the blood are carried into the hepatic portal vein, which leads to the liver. There, the liver regulates the distribution of nutrients to the rest of the body and removes toxic substances, including drugs, alcohol, and some pathogens.

Figure 2. Villi are folds on the small intestine lining that increase the surface area to facilitate the absorption of nutrients.

Large Intestine

The large intestine, illustrated in Figure 3, reabsorbs the water from the undigested food material and processes the waste material. The human large intestine is much smaller in length compared to the small intestine but larger in diameter. The colon is home to many bacteria or “intestinal flora” sometimes refered to as “gut flora” that aid in the digestive processes. The main functions of the colon are to extract the water and mineral salts from undigested food, and to store waste material. Carnivorous mammals have a shorter large intestine compared to herbivorous mammals due to their diet (Figure 3).

Figure  3  The large intestine reabsorbs water from undigested food and stores waste material until it is eliminated.