How does the food change in consistency and form?
> The food changes in form and consistency the moment it enters our mouth and the moment we chew it (Ingestion). Chewing food may be referred to as mechanical digestion, in which the food is broken down into smaller pieces. The chewed food then mixes with the salivary amylase, digesting the starch of the food and breaking it down to maltose. This may be referred to as chemical digestion.
The tongue will then force the food down the pharynx, where food (now a bolus) will continue to go to the distal end of the esophagus, where the cardioesophageal sphincter opens for the food to enter the stomach. In the stomach, the bolus will turn into chyme. Stomach squirts chyme into the small intestine where its nutrients will be absorbed.
How could the body absorb the nutrients from the foods we eat?
> There are three mechanisms in which the body absorb the nutrients:
Absorption of Carbohydrates
Carbohydrates are absorbed in the form of monosaccharides. The monosaccharides glucose and galactose are transported by common proteins into the cells through a secondary active transport mechanism. When they leave the cell, they do so through facilitated diffusion. Monosaccharides fructose, however, is transported and absorbed by facilitated diffusion only.
Absorption of proteins
Proteins are broken down into smaller polypeptides by the hydrochloric acid and pepsin in the stomach. Then, in the small intestine, the pancreatic enzymes act on the bonds of amino acid sequences whilst the brush border cells’ enzymes further breakdown the peptide chains. The duodenum and jejunum of the small intestine absorbs the nutrients of the protein through an active transport mechanism. The protein molecule is also now small enough to enter the bloodstream.
Lipid Absorption
The most common lipid is the triglycerides, which is bonded to three fatty acid chains. Absorption of its nutrients happens in the small intestine. The small sized and short chain fatty acids, through diffusion, are absorbed by enterocytes. Afterwards, they will enter the blood capillary of a villus through an active transport mechanism. On the other hand, large, long, and hydrophobic chain of fatty acids (and monoacylglycerols) are enclosed in a micelle by the bile salts and lecithin in order to come in contact with the absorptive surface of epithelial cells. When the micelle containing the fatty acid has squeezed through the microvilli of the cell and is near the cell surface, the lipid substances exit the micelle and are diffused into the cell to be absorbed. Once in the cell, fatty acids are reincorporated back into triglycerides, which will turn into chylomicron as it gets mixed with phospholipids and cholesterol. The chylomicrons will then be transported into the lymphatic vessels and empties in the left thoracic duct. The triglyceride is finally broken down in the bloodstream, passing through capillary walls so cells can use them for energy.