Biochemistry of digestion Turning the macromolecules into their building blocks

You probably know that food goes in your mouth…it’s chewed up, swallowed, goes down the esophagus into the stomach, enters the sm intestine, then the large intestine, and then any left over material exits. BUT…what’s really happening at a chemical level?

Our diet consists of proteins, carbohydrates and fats that have to be digested (hydrolysis) and absorbed (diffusion) in the intestines. Most of these substances are macromolecules which cannot diffuse directly between intestinal cells into the blood stream. The digestive process consists of breaking down these food materials into smaller molecules aka hydrolysis. NOTE: a small amount of energy is released as heat during hydrolysis but none of it is stored directly for later use. That’s a separate process within cells called cellular respiration.

Protein! In the stomach there are enzymes for breaking the bonds between amino acids (hydrolysis) in proteins This reaction occurs in the presence of protein specific enzymes. Hydrolysis of proteins initially yields peptones and proteoses, which are further broken down into dipeptides and tripeptides and finally into individual amino acids. These amino acids are used to then build proteins in your body…not usually for energy unless you are starving (called ketosis).

Carbohydrates! The breakdown (hydrolysis) of starch begins in the mouth with amylase enzyme. It finishes in the beginning of the small intestine with other carbohydrate specific enzymes. Hydrolysis of complex carbohydrates and polysaccharides yields disaccharides and finally monosaccharides, mainly glucose. Carbs are for quick “easy” energy. The polysaccaride starch breaks down into many many monosaccharides which will be used directly in cellular energy and/or can be stored as glycogen (a source of stored quick energy). Fiber (cellulose) from plants does not break down in our digestive system but is an important component of a healthy digestive system.

Lipids! The digestion of triglycerides fats also requires hydrolysis. Fat hydrolysis occurs in the beginning of the small intestine in the presence of enzymes called lipases. Lipids also are for energy and storage. If the fats are not used for immediate fuel they will be stored in fat cells.

Nucleic Acids? The nucleotides needed to make RNA and DNA can be synthesized using specialized biosynthetic pathways: there are no “essential nucleotides” that must be provided in the diet.

Once the bonds of a macromolecule are broken… The monomers (or smaller parts of lipids) enter the blood stream through the small intestine (via the microvilli) by squeezing between cells.

REMEMBER: this hydrolysis is not providing you with energy! At the most maybe a little heat. The process of releasing and storing energy to power your cells is called cellular respiration and we’ll get to that…..

To visualize the idea of food and: activation energy, potential energy, energy release by bonds breaking (thermal) & conversion of materials and transfer of energy… We’ll we will literally BURN items to calculate calories in food items. Calories are the “energy” available in food