Biochemistry: Digestive System Notes

DIGESTION
Cooking vs. Digestive Processes
Cooking Processes:
Grilling
Microwaving
Frying
Roasting
Barbeque
Boiling
Steaming
Poaching
Digestive Processes:
Ingestion
Propulsion
Mechanical digestion
Chemical digestion
Absorption
Defecation
Salivary Digestion
SALIVARY DIGESTION
Salivary Glands the salivary glands are a collection of accessory organs surrounding the mouth that secrete a fluid called saliva.
Saliva consists of 99.5 % water and 0.5 % solutes. medium for dissolving soluble food molecules solutes include mucus and enzymes one enzyme (lysozyme) destroys bacteria second enzyme known as salivary amylase, begins the chemical digestion of starchy foods. produced by three pairs of salivary glands parotid , submandibular, and sublingual glands.
PAROTID GLANDS largest salivary glands. located in front of and slightly below each ear, between the skin of the cheek and the masseter muscle. secretes a clear, watery fluid that is rich in salivary amylase.
SUBMANDIBULAR GLANDS located along the inner surface of the jaw in the floor of the mouth. secrete a more viscous fluid than the parotids, owing to the presence of mucus in their secretions. SUBLINGUAL GLANDS lies in front of the submandibular glands under the tongue. the fluid they secrete contains an abundance of mucus, and so is thick and stringy.
DIGESTION in the MOUTH: digestion begins in the mouth. Mastication + mixing, form of mechanical digestion
BOLUS: formation of mixing food + mastication.

Gastric Digestion
Gastric Digestion (Stomach Digestion) is the process of breaking down proteins by the action of the gastric juices in the stomach.
The Stomach is located on the left side of the body above the waist and behind the ribs. It performs four major functions: (1) the bulk storage of ingested food. (2) the mechanical breakdown of ingested food, (3) the disruption of chemical bonds in food material through the action of acids and enzymes, and (4) the production of intrinsic factor, a glycoprotein whose presence in the digestive tract is required for the absorption of vitamin B-12 in the small intestine.
About 2-4 L of food can be stored in the stomach. Bolus (chewed food) enters the stomach and leaves as chyme (liquid food) to the small intestine. Like the walls of esophagus, the walls of stomach are strong and muscular. They contract with enough force to break food into smaller pieces as glands in the stomach walls release stomach juices. There are two distinct processes in the stomach: (a) Mechanical, the churning of food, contraction of walls by the stomach, and breaking down the food into smaller pieces. (b) Chemical, the secretion of hydrochloric acid (HCl) and pepsinogen from glands along the walls of the stomach, conversion of pepsinogen to pepsin to break down proteins.
The layers of muscle on the stomach contain the glands and gastric pits which consist of mucous cells, parietal cells, and chief cells. Mucous cells give off mucus. Parietal cells give off HCl and intrinsic factor. Chief cells give off enzymes that break down proteins.

Gastric Secretion
Gastric mucous/ Stomach mucous secretes 1.2 to 1.5 L of gastric juice per day. Gastric juice renders food particles soluble, initiate digestion (particularly of proteins), and converts the gastric contents to a semiliquid mass called chyme, thus preparing it for further digestion in small intestine. It is a variable mixture of water, HCl, electrolytes, and organic substances. This juice is highly acidic because of its HCl contents and rich in enzymes. Stomach walls are protected from digestive juices by the membrane on the surface of the epithelial cells bordering the lumen of the stomach; this membrane is rich in lipoproteins, which are resistant to attack by acid.
The process of gastric digestion can be divided into three (3) phases: Cephalic, controlled by the central nervous system, Gastric, regulated by short reflexes of the enteric nervous system, coordinated in the wall of stomach, and Intestinal, regulated by hormones of the digestive tract.

The Cephalic phase -begins when you see, smell, taste, or think of food. This stage, which is directed by the CNS, prepares your stomach to receive food.

The Gastric phase -begins with the arrival of food in the stomach and builds on the stimulation provided during the cephalic phase. The stimuli that initiate the gastric phase are (1) distension of the stomach, (2) an increase in the pH of the gastric contents, and (3) the presence of undigested materials in the stomach, especially proteins and peptides.

The Intestinal phase -begins when chime starts to enter the small intestine. When the chyme contains lipids from the digestion of fats or contains enough HCl to bring its pH below 2, gastric secretion is inhibited.

INTESTINAL DIGESTION -the part of digestion carried on in the intestine, affecting all foodstuffs, including starches, fats, and proteins.
Small Intestine
-is the portion of the digestive system most responsible for absorption of nutrients from food into the bloodstream.
-has an expanded surface area with inner folds, called plicae, villi and microvilli(brush border of the small intestine).
Large intestine
-is the portion of the digestive system most responsible for absorption of water from the indigestible residue of food.
Intestinal Digestion Process.
1. When the work of digestion is completed in the stomach the food is poured through the pyloric orifice into the small intestine where it undergoes further changes.
2. In the small intestine majority of digestion and absorption occurs where the milky chyme enters the duodenum.
Duodenum
part of the small intestine that is closest to the stomach. a neutralization chamber in which the chyme from the stomach is mixed with bicarbonate. iron and calcium, are taken up most efficiently in the duodenum

3. It then enters the jejunum, is the place where most nutrients are actively absorbed.

Jejunum the middle section of the small intestine. place where most nutrients are actively absorbed. amino acids as well as most vitamins and minerals are absorbed in the jejunum. *Active fat absorption also occurs in the duodenum and the jejunum
*The majority of starch is also digested in the duodenum and jejunum

4. Then goes to the ileum, which is responsible for completing the digestion of nutrients and for reabsorbing the bile salts that have helped to solubilize (keep in solution), the fats.

Ileum the final part of the small intestine. responsible for completing the digestion of nutrients and for reabsorbing the bile salts. the place where vitamin B12 is selectively absorbed into your body.

5. After passing through the small intestine, food passes into the large intestine.

Large Intestine food, which at this point is primarily fiber food stays longer in the large intestine - spends six to 72 hours in your large intestine before final removal contains an ecosystem of bacteria that can ferment much of this fiber, producing many nutrients necessary for the health of the colon cells. - probiotic bacteria
- first part of the large intestine is called the cecum.
- Food then travels upward in the ascending colon.
-the food travels across the abdomen in the transverse colon.
- goes back down the other side of the body in the descending colon.
-and then through the sigmoid colon.
PANCREATIC JUICE
Definition:
-It is a clear, colorless, alkaline secretion of the exocrine part of the pancreas necessary for digestion.

Composition:
- It is composed of two secretory products critical to proper digestion:
1.) Bicarbonates
- It is secreted by the epithelial cells lining the small pancreatic ducts.
- This made the pancreatic acid to be alkaline.
- It neutralizes the acid in chyme to protect the intestinal wall and to create the proper environment for the functioning of pancreatic enzymes.

2.) Digestive Enzymes
- These enzymes are synthesized and secreted by the exocrine acinar cells. These enzymes help in the digestion of essential macromolecules into their absorbable forms.
-The enzymes are classified according to their substrates:
Proteases –proteins
Amylases –carbohydrates
Lipases and phospolipases –lipids
Nucleases –nucleic acids
The secretion of Pancreatic Juice is controlled by two hormones: secretin (for bicarbonates) and cholecystokinin (digestive enzymes)
*Pancreatic Enzymes:
Proteases
These enzymes are responsible for protein catabolism that is by the hydrolysis of the peptide bonds that link amino acids together in the polypeptide chain forming the protein.
Two major pancreatic proteases:
1.) Trypsin
–secreted as an inactive precursor trypsinogen.
*Trypsinogen is a single polypeptide chain with molecular weight of 25,000 and activated by a duodenal mucosal enzyme enterokinase or enteropeptidase which cleaves hydrolytically a terminal hexapeptide.
- Formation of sufficient trypsin activates the rest of the precursor autocatalytically.
- Trypsin acts on peptide bonds where carboxy groups are contributed by arginine or lysine present deep inside the polypeptide chain (endopeptidase) at an optimum pH of 8.0.
- Certain inorganic ions, chiefly calcium, help activation and also accelerate the action of trypsin. 2.) Chymotrypsin
-secreted as an inactive precursor chymotrypsinogen.
*Chymotrypsinogen is a single polypeptide chain of molecular weight approximately 25,700 and is activated by trypsin.
-It prefers large hydrophobic residues such as the amino acids.
- It acts preferentially on peptide bonds where carboxyl groups belong to aromatic amino acids like tyrosine and phenylalanine, at an optimum pH of 8.0.
-It also acts readily upon amides and esters of susceptible amino acids.
- Unlike trypsin, chymotrypsin coagulates milk.

* Carboxypeptidase
-is the other proteolytic enzyme of the pancreatic juice, secreted as procarboxypeptidase and activated by trypsin.
- Being an exopeptidase, it cleaves off terminal amino acids with free carboxyl groups.
LIPASES
- is a powerful lipolytic enzyme which hydrolyses triglycerides to lower triglycerides and free fatty acids.

NUCLEASES
- The nucleases- ribo-nuclease and deoxyribonucleases, which break up nucleic acid to nucleotides, are also present in the pancreatic juice.
ELASTASES
-cuts peptide bonds next to small, uncharged side chains such as those of alanine and serine.
AMYLASES
- is an α-amylase (similar to ptyalin) of molecular weight 45,000 and requires chloride ions for activation.
- This enzyme continues the starch digestion process. It hydrolyses starch to maltose by splitting the 1-4 glycosidic linkages at an optimum pH of 6.5 to 7.2.
-It completes digestion of carbohydrate, producing glucose, a small molecule that is absorbed into your blood and carried throughout your body.
- It also digests glycogen.
* Amylopsin
- It is a starch-digesting amylase that converts starch into sugar.
- It is used therapeutically as a digestive aid and as an anti-inflammatory agent.
Terms:
Zymogen – an inactive precursor that prevents cell’s self-digestion
Endopeptidase – cleaves off amino acids inside the polypeptide chain
Exopeptidase – cleaves off terminal amino acids with free carboxyl groups

Intestinal Juice
Intestinal Juices
The complex enzymes which break down food in all the main food groups.
They are clear to pale yellow, watery secretion composed of hormones, digestive enzymes, mucus, and neutralizing substances released from the glands and mucous-membrane lining of the small and large intestines.
Neutralize hydrochloric acid coming from the stomach; release gastrointestinal hormones into the bloodstream.
The main function of intestinal juice is to dissolve fibrin and to emulsify fats. Intestinal juice is capable of being extracted with glycerin and is closely related to the properties of the secretion of the pancreas.

Intestinal Enzymes
All digestive enzymes are hydrolases in which they control the hydrolysis. They speed up the breakdown (hydrolysis) of food molecules into their ‘building block’ components.
How the enzyme does works?

There are four kinds of Intestinal Enzymes
Sucrase
Lactase
Maltase
Peptidases
Sucrase - is the name given to a number of enzymes that catalyze the hydrolysis of sucrose to fructose and glucose.
It is also called as Invertase.
Present in yeast and in the intestine mucosa of animals. Localize in the brush border (a chemical barrier through which food is absorbed) that coats the intestinal villi.

Lactase - any of a group of enzymes found in the small intestine, liver, and kidney of mammals that catalyze the breakdown of lactose (milk sugar) into the simple sugars glucose and galactose.
Particularly abundant during infancy. Produced by the mucous membrane cells that line the intestinal walls; granules localize in the brush border (a chemical barrier through which food must pass to be absorbed) that coats the intestinal villi.

Peptidases - also called protease or proteinase, is a type of enzyme that helps to break down proteins in the body.
This type of enzyme occurs naturally in the living things and forms part of many metabolic processes Break down proteins by destroying the chains between their amino acids, and many can usually be found in the digestive tract. When protein enters the body, it needs to be digested and broken down into smaller molecules so that it can be used. This type of enzyme is responsible for this catabolic process.
They were the first enzymes to be studied because of their abundance in the digestive system.
They are the most important group of proteolytic enzymes in succus entericus. Because they bring about the final breakdown polypeptides to amino acids.
Maltase - enzyme that catalyzes the hydrolysis of the disaccharide maltose to the simple sugar glucose. is found in plants, bacteria, and yeast; in humans and other vertebrates. it is thought to be synthesized by cells of the mucous membrane lining the intestinal wall.
The glucose so produced is either utilized by the body or stored in the liver as glycogen (animal starch).

BILIARY SYSTEM
MAIN FUNCTIONS: To drain waste products from the liver into the duodenum To help in digestion with the controlled release of bile
BILE is a bitter-tasting, dark green to yellowish brown fluid, produced by the liver, it is stored in the gallbladder and upon eating is discharged into the duodenum.
2 FUNCTIONS:
To break down fats during digestion To carry away waste

Bile salts act as emulsifying agents in the digestion and absorption of fats. Cholesterol and bile pigments from the breakdown of hemoglobin are excreted from the body in the bile.

PHYSICAL PROPERTIES: 1. Hepatic Bile: pH 7.4, golden yellow 2.Bladder Bile: pH 6.8, color become darker
Volume of bile produced reaches to one liter of bile per day (depending on body size).

BILE ACIDS are two compound acids, glycocholic and taurocholic.
BILE SALTS are bile acids compounded with a cation, usually sodium
BILE ACIDS/BILE SALTS
FUNCTIONS:
- to facilitate the formation of micelles - eliminating cholesterol from the body - driving the flow of bile to eliminate catabolites from the liver - emulsifying lipids and fat-soluble vitamins in the intestine to form micelles
BILE PIGMENTS
- are biological pigments formed in many organisms as a metabolic product of certain porphyrins.
SECRETION OF BILE
- major route for eliminating cholesterol
Bile is secreted in TWO STAGES: Stage one; The liver hepatocytes will secret an initial secretion that is rich in bile salts , cholesterol, and other organic components, the initial secretion will drain through the many minute bile canaliculi that penetrate the liver. Stage Two ; The initial secretion will flow towards the bile ducts , during its flow in the ducts a secondary secretion is added to the initial bile which is a watery solution of sodium bicarbonate ions.
Role of Bile Acids in Fat Digestion and Absorption
Cholesterol is converted into the bile acids cholic and chenodeoxycholic acids. HYDROPHOBIC (methyl groups) HYDROPHILIC (hydroxyl groups
Their amphipathic nature enables bile acids to carry out two important functions:
1. Emulsification of lipid aggregates: Bile acids have detergent action on particles of dietary fat which causes fat globules to break down or be emulsified into minute, microscopic droplets.
2. Solubilization and transport of lipids in an aqueous environment: Bile acids are lipid carriers and are able to solubilize many lipids by forming micelles .

Enterohepatic Circulation
-95% of the bile acids delivered to the duodenum
- Venous blood goes straight into the portal vein and through the liver
-Hepatocytes extract bile acids from sinusoidal blood
- Bile acids are transported across the hepatocytes to be resecreted into canaliculi
Pattern and Control of Bile Secretion
Cholecystokinin: The name of this hormone describes its effect on the biliary system - cholecysto = gallbladder and kinin = movement. The most potent stimulus for release of cholecystokinin is the presence of fat in the duodenum.
Secretin: This hormone is secreted in response to acid in the duodenum. Its effect on the biliary system is very similar to what was seen in the pancreas
Abnormalities associated with bile
Gall stone ; The majority of gall stones are made up of cholesterol , (cholesterol tends to accrete into lumps in the gallbladder)
Causes of gall stones; - Too much absorbtion of water from the bile . - Too much cholesterol in bile. - Inflammation of the epithelium.