Question

Briefly depict how carbohydrates, fats, and proteins are digested and absorbed. Start with the events that occur in the mouth and work your way through the gastrointestinal (GI) tract by explaining what happens to each macronutrient in the stomach, small intestine, and large intestine. Include the contributions of the liver, gallbladder, and pancreas to digestion, and be specific about which molecules are absorbed.

Answer 1

Answer:

When a meal is put into the mouth, enzymes such as salivary amylase starts the digestion of Carbohydrates.

When the food is swallowed,Stomach enzymes containing pepsin and Hcl starts in the digestion of Protein.

The food then moves Into the duodenum which is a part of the small intestine. The protein is then again acted on by trypsin to form amino acid polypeptides.

Digestion of fats and oil starts here too and it is aided by bile which is produced by the liver and secreted by the gall bladder. Pancreatic juice which contains amylase and lipase acts on the starch and lipids respectively for further breakdown into glucose and glycerol respectively.

Nutrient and water reabsorption takes place in the large intestine the waste is expelled as faeces through the rectum.

Answer 2

Answer:

Carbohydrate digestion begins in the mouth with enzyme salivary  amylase which converts starch to maltose, maltotriose  and ∝-limit dextrins. These are   moved  by peristalsis into the stomach. The low pH inactivated  amylase, thus the partially digesting  food(chyme) is moved to the small intestine.

The acidity of the stomach acid is neutralize by the bicarbonate ions secreted in the pancreas from secretin hormone stimulation. This makes alpa amylase to be activated.

In the intestinal broader are maltase,a-dextrinase, sucrase. These enzymes hydrolyze these  oligosaccharides to glucose. (maltose, maltotriose are oligosaccharides).

Assuming the diet contains sucrose,lactose, trehalose, these diasacchrides are converted  to  respective monosaccharides by lactase,trehalase,sucrase. Thus  sucrose to glucose and fructose; trehalose to glucose and lactose to glucose and galactose. These product release from the membrane walls of the duodenum are aided by  alpha-amylase on the brush borders.

These end products except fructose are transported into the cells from the lumen, via Na+ dependent co-transport (SGL1)  in the epithelial lining  membranes of the duodenum walls. Fructose is transported  via facilitated diffusion from the duodenal lumen  into the cells. The sugar are transported against the concentration gradient into the cells(,Na+ moves passively into the cells) and later into the blood by facilitated diffusion.

Protein digestion begins in the stomach, and completed in the small intestine.

The acidic  medium  of the stomach  activated pepsin precursor pepesinogen to pepsin. The protein is converted to peptones and proteoses. These are moved into the duodenum where the pancreatic bicarbonate neutralizes  the acidity  of the stomach, thus inactivating pepsin.

In the duodenum, digestion of protein is completed.Pancreatic and intestinal juice are secreted. Inactive proteases-Trypsin, chymotrypsin,elastase, Carboxypeptidase A and B, are converted into active forms by trypsin. These proteoses hydrolyse the protein to amino acids.

These amino acids units are transported across the luminal walls by Na+ dependent co-transport into the cells,. Dipeptide and tripeptides are transported via H+ dependent co-transport  into the  cells, They are latter hydrolyse into amino acids by  peptidases. They are moved into the blood via facilitated diffusion, and absorbed like glucose.

Digestion of Fats

In the mouth, fragments of ingested triglycerides are converted to monoglycerides and fatty acids by lingual lipases.

This is moved by peristalsis  down into the stomach, where stomach churning   breaks down the lipids to increase  the surface area for  pancreatic enzymes  actions.

In  the small intestine, bile secrets bile acids. The latter emulsify the lipids breaks them down into tiny insolube  fat globules, and added water. This is now creamy and  fluid in nature, it is called chyle. They are made soluble as micelles by the bile acids.  

Pancreatic lipase, cholesterol ester hydrolase  and phospholipase A2 hydrolyse lipids to fatty acids, monoglycerides and cholesterol. These are brought to the intestinal membranes  by micelles, ( except  glycerol which is hydrophlic  )where they diffused across the membrane walls into the cells.

In the intestinal cells,  these products are re-esterfied; thus  triglycerides, phospholipids,  chylomicrons and cholesterol  are formed. Chylomicrons are too large to pass through the capillaries wall, they are absorbed by the lacteal and transported  into the lymphatic vessels. From here they enter the blood through the thoracic duct completing digestion

Explanation:

Explanation: