Korean biochem

STARCH

Starch becomes soluble in water when heated.
Gelatinization: The starch granules swell, and small amylose molecules leach out, forming a network that holds water, increasing the viscosity of the mixture. This process is termed as starch gelatinization.
This property enables it to be used as a thickening agent in cooking or as a paste due to its viscosity.
The most common test to identify starch is the iodine test. The iodine TS is composed of iodine dissolved in potassium iodide, forming a triiodide ion.
This ion forms a complex with the starch, resulting in a blue to violet color of the solution.
The intensity of the color depends on the amount of starch molecules present in a solution.

Fast-releasing carbohydrates stimulate the sweetness taste sensation, which is the most sensitive of all taste sensations. Extremely low concentrations of sugars in foods will stimulate the sweetness sensation.
Fructose is the top naturally occurring sugar in sweetness value.
Less-sweet whole grains (starches and fiber) take longer to chew and get sweeter the more you chew them.

The chyme formed in the stomach is gradually expelled into the duodenum. Upon entry of the chyme into the small intestine, the pancreas releases pancreatic juice through a duct.
Pancreatic juice contains pancreatic amylase, which starts breaking down dextrin into shorter carbohydrate chains. Enzymes secreted by the intestinal cells lining the villi include:
- Sucrase: hydrolyzes sucrose into glucose and fructose
- Maltase: hydrolyzes maltose into two glucose molecules
- Lactase: hydrolyzes lactose into glucose and galactose
Once carbohydrates are chemically broken down into single sugar units, they are transported into the inside of intestinal cells.

The small intestine cells have membranes containing transport proteins to get monosaccharides and other nutrients into the blood for distribution to the rest of the body.
Fructose enters the bloodstream through facilitated diffusion, while glucose and galactose enter through active transport.
The monosaccharides absorbed are then transferred to the liver, where galactose is converted to glucose, fructose is broken down into smaller carbon-containing units, and glucose is stored as glycogen or exported back to the blood.
The utilizable form of sugar in the body is glucose; fructose and galactose must be converted for the body to utilize them.

Mechanical digestion: Chewing or mastication crumbles carbohydrate foods into smaller pieces.
Chemical digestion: Involves salivary amylase, breaking the bonds between monomeric sugar units of disaccharides, oligosaccharides, and starches, breaking down amylose and amylopectin into dextrin and maltose. Only about 5% of starches are broken down in the mouth.

Amylose: Linear, insoluble in water, 250 g0 units, dark blue in iodine test, enzyme amylase.
Amylopectin: Branched (a-1,4 & a-1,6), soluble in water, >1000 units, blue-violet in iodine test, enzymes a-glucosidase & dextrinase.

Remaining indigestible carbohydrates (fiber-rich food) are broken down by enzymes released by bacteria in the large intestine. Products include short-chain fatty acids and gases (causing smelly flatulence).
Short-chain fatty acids are used by bacteria for energy, eliminated in feces, or absorbed by colon cells, with some transported to the liver.
Dietary fiber is digested less than other carbohydrate types, linked to decreased weight gain and chronic disease risk.

The first step involves chewing. The teeth mechanically break down food into smaller pieces.
Saliva aids swallowing and the passage of mashed pieces through the esophagus.
In the stomach, acidity unfolds proteins, and pepsin begins breaking them into smaller fragments. HCl also aids digestion.
Protein digestion in the stomach takes longer than carbohydrates but shorter than fats.
High-protein meals increase the time food remains in the stomach, prolonging the feeling of fullness.

The stomach empties chyme into the small intestine, where most protein digestion occurs.
Pancreas secretes digestive juice containing enzymes like chymotrypsin and trypsin to further break down protein fragments.
Small intestine enzymes finally break peptides into individual amino acids.
The goal is to break proteins into dipeptides and amino acids for absorption.
Over 90% of ingested protein is broken down into amino acid monomers; little protein reaches the large intestine.

Proteins are absorbed as tripeptides, dipeptides, or amino acids in the small intestine.
Peptides/amino acids pass through the brush border by facilitated diffusion or active transport, before entering the bloodstream.
This process is part of the enterohepatic circulation, where amino acids are transported to the liver.

Lipid digestion begins in the mouth with saliva.
Chewing and emulsifiers aid digestive enzymes, making fats more accessible.
In the stomach, gastric lipase starts breaking down fats, though little fat digestion occurs here.

In the small intestine, bile emulsifies fats for efficient digestion. Bile contains emulsifiers like bile salts and lecithin that help integrate fats with watery fluids.
Pancreatic lipase breaks fats down into free fatty acids and monoglycerides.
Bile salts form micelles with these lipids for transport across the intestinal lining.

Micelles allow fats to pass through the mucous layer of the digestive lining and enter intestinal cells.
Inside, monoglycerides and fatty acids reform into triacylglycerols, cholesterol, and phospholipids, forming chylomicrons that enter the lymphatic system.
Cholesterol absorption is facilitated by dietary fats but hindered by high-fiber diets. High fiber can block absorption of bile salts and cholesterol.
If fat malabsorption occurs, it may lead to steatorrhea, characterized by high fat content in stool, often due to conditions like Crohn's disease or cystic fibrosis.