Digestive System

Topic 6.1

6.1.1• Basics of Digestion

– Ingestion: you eat the food– Digestion: a series of chemical reactions that

convert ingested food to smaller and smaller molecular forms

– Absorption: small molecular forms are absorbed through cells of digestive system, pass into nearby blood or lymphatic vessels

– Transport: circulatory system delivers the small molecular nutrients to your body cells.

6.1.1• Digestion of macromolecules

– Most food molecules are large polymers and insoluble

• Too large to pass through cell membranes of the intestines and the capillaries

– They must first be digested to smaller soluble molecules (hydrolysis) before they can be absorbed into the blood

6.1.1• Molecular type of some food categories

Molecule Type Molecular form ingested

Molecular form after digestion

Protein Protein Amino acids

Lipids Triglycerides Glycerol, fatty acids

Carbohydrates Polysaccharides, disaccharides, monosaccharides

Monosaccharides

Nucleic acids DNA, RNA nucleotides

6.1.1• Digestion of macromolecules

– Different organisms have different characteristic molecules that they create.

– Digestion allows organisms to disassemble macromolecules from their food sources and use the ‘building blocks’ to create their own macromolecules.

6.1.2• Digestive Enzymes

– Are specific for a specific food type.– Secreted into the lumen of the gut – Increase the rate of reaction of the hydrolysis of

insoluble food molecules to soluble end products – Function at body temperature (which is why it is

so important for your body to maintain a stable temperature)

6.1.3– General types of digestive enzymes

• Amylases–break down complex sugars into

monosaccharides• Proteases

–break down proteins into amino acids• Lipases

–break down lipids into fatty acids

6.1.3– Examples of digestive enzymes

• Pancreatic amylase–Source: Pancreas cells–Optimal pH: 7.5-7.8 –Substrate: starch (amylose) –End product: disaccharide maltose –Action: hydrolysis of 1-4 glycosidic

bonds

6.1.3

6.1.3• Pepsin (a Protease: enzyme that digests proteins)

– Source: stomach cells– Optimal pH: is 2 – Substrate: polypeptide chains (proteins)– End product: amino acids– Action: hydrolysis of peptide bonds within

the polypeptide chain

6.1.3

6.1.3• Pancreatic lipase

–Source: pancreas cells –Optimal pH: 7.2 –Substrate: triglyceride lipid –The product: glycerol and fatty acid chains –Action: hydrolysis of ester bonds between

the glycerol molecules and the fatty acid chains.

6.1.3– The action of pancreatic amylases also

requires the presence of bile salts that emulsify the lipid. This emulsification has two effects:

» Increases the surface area of the lipid for the digestion of fat

» Exposes the glycerol 'head' structure to the enzyme

6.1.3

6.1.4• Organs of the Digestive System

– Food passes through the digestive tract in the following sequence:• Mouth, pharynx (throat), esophagus, stomach, small

intestine, large intestine, rectum, anus– The liver, pancreas, and gallbladder are accessory

digestive organs that produce digestive enzymes for secretion into the digestive tract

6.1.4

6.1.5• Digestion

– Mouth• specialized for ingestion and the beginning of the digestion

process• Mechanical digestion begins as you bite, grind, and chew the

meat and bun with your teeth• While food is being mechanically disassembled by the teeth, it is

also moistened by saliva• Three pairs of salivary glands secrete about a liter of saliva into

the mouth cavity each day• Saliva contains an enzyme, salivary amylase, which initiates the

chemical digestion of starch into sugar

6.1.5– Throat

• After a bite of food has been chewed and fashioned into a lump called a bolus, it is swallowed and moved through the pharynx and into the esophagus

• The pharynx, or throat, is a muscular tube that serves as a hallway of the respiratory system as well as the digestive system

• During swallowing, the opening to the airway is closed by a small flap of tissue called the epiglottis

6.1.5

• Waves of peristalsis (muscle contraction) sweep the bolus through the pharynx and esophagus to the stomach

• Circular muscle fibers in the wall of the esophagus contract around the top of the bolus, pushing it downward

• Almost at the same time, longitudinal muscles around the bottom of the bolus and below it contract, shortening the tube

6.1.5

– Stomach• The entrance to the large, muscular

stomach is normally closed by a ring of muscle at the lower end of the esophagus

• When a peristaltic wave passes down the esophagus, the muscle relaxes, permitting the bolus to enter the stomach

• When empty, the stomach is collapsed and shaped almost like a hot dog

6.1.5

• Folds of the stomach wall, called rugae, give the inner lining a wrinkled appearance

• As food enters, the rugae gradually smooth out, expanding the capacity of the stomach to more than a liter

• The stomach is lined with an epithelium that secretes large amounts of gastric juice, composed of

• Tiny pits mark the entrances to the millions of gastric glands, which extend deep into the stomach wall

6.1.5• Parietal cells in the gastric glands secrete

hydrochloric acid and a substance known as an intrinsic factor,needed for adequate absorption of vitamin B12

• Chief cells in the gastric glands secrete pepsinogen, an inactive enzyme precursor

• When pepsinogen comes in contact with the acidic gastric juice in the stomach, it is converted to pepsin, the main digestive enzyme of the stomach

• Pepsin hydrolyzes proteins, converting them into small polypeptides

6.1.5• Several protective mechanisms prevent the gastric juice from

digesting the wall of the stomach:– Cells of the gastric mucosa secrete an alkaline mucus that

coats the stomach wall and neutralizes the acidity of the gastric juice along the lining

– Epithelial cells of the lining fit tightly together, preventing gastric juice from leaking between them and into the tissue beneath

– These mechanisms sometimes malfunction, and part of the stomach lining is digested, leaving an open sore, or ulcer

– The bacterium heliobactor pylori has been implicated as a causative factor in ulcers because it causes a reduction in the amount of mucus secreted

6.1.5• The actual process of digestion in the stomach is as follows:

– The stomach churns and chemically degrades the food so that it assumes the consistancy of thick soup; this partially digested food is called chyme

– Protein digestion then begins, and any proteins are degraded into polypeptides

– Digestion of starch to small polysaccharides continues until salivary amylase is inactivated by the acidic pH of the stomach

– Over a period of several hours, peristaltic waves release the chyme in spurts through the stomach exit, the pylorous, and into the small intestine

6.1.5– Small Intestine

• Digestion of food is completed in the small intestine, and nutrients are absorbed through its wall

• The small intestine, which is 5-6 m (about 17 ft) in length, has three regions:

– Duodenum– Jejunum– Illium

6.1.5

• Most chemical digestion takes place in the duodenum, not in the stomach

• Bile from the liver and enzymes from the pancreas are released into the duodenum and act on the chyme

• Enzymes produced by the epithelial cells lining the duodenum catalyze the final steps in the digestion of major types of nutrients

6.1.7• The lining of the small intestine appears

wrinkly because of its millions of tiny finger-like projections called villi

– The villi increase surface area for digestion and absorption of nutrients

– The intestinal surface is further expanded by thousands of microvilli, folds of cytoplasm on the exposed surface of the epithelial cells of the villi

6.1.7

– Absorption of nutrients is primarily the job of the intestinal villi

– The wall of a villus consists of a single layer of epithelial cells

– Inside each villus is a network of capillaries and a central lymph vessel, called a lacteal

– To reach the blood (or lymph), a nutrient molecule must pass through an epithelial cell of the intestinal lining and through a cell of the blood or lymph vessel lining

6.1.7–Absorption occurs by a combination

of simple diffusion, facilitated diffusion, and active transport

–Fructose is absorbed by facilitated diffusion

–Amino acids and glucose are absorbed by active transport and then transported directly to the liver by the hepatic portal veins

Intestinal Villi

6.1.7

6.1.5– Liver

• The liver, the largest internal organ as well as one of the most complex, lies in the upper right part of the abdomen just under the diaphragm

• The liver’s food processing functions include the following:

– Secretes bile, which is important in the mechanical digestion of fats

– Helps maintain homeostasis by removing or adding nutrients to the blood

– Converts excess glucose to glycogen and stores it– Converts excess amino acids to fatty acids and urea– Detoxifies alcohol and other drugs and poisons

6.1.5– Gallbladder

• Bile is stored in the pear shaped gallbladder, which concentrates the bile and releases it into the duodenum as needed

• Bile consists of water, bile salts, bile pigments, cholesterol, salts, and lecithin (a phospholipid).

• Bile mechanically digests fats by a detergent like action

• Because it contains no digestive enzymes, bile does not enzymatically digest food

6.1.5– Pancreas

• The pancreas is an elongated gland that secretes both digestive enzymes and hormones that help regulate the level of glucose in the blood

• Among its enzymes are:– trypsin and chymotrypsin, which digest polypeptides

to dipeptides;– pancreatic lipase, which degrades fats– pancreatic amylase, which breaks down almost all

types of carbohydrates except cellulose to disaccharides– ribonuclease and deoxyribonucleas which split the

nucleic acids to free nucleotides

6.1.5– Large Intestine

• Undigested material, such as the cellulose of plant foods, along with unabsorbed chyme, passes into the large intestine

• The small intestine joins the large intestine, forming a blind pouch called the cecum

• From the cecum to the rectum, the large intestine is known as the colon

6.1.5• The regions of the large intestine are the

cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus – the opening for elimination of wastes

• As the chyme passes slowly through the large intestine, water and sodium are absorbed from it, and it gradually assumes the consistancy of normal feces

• Bacteria inhabiting the large intestine are nourished by the last remnants of the meal and benefit their host by producing vitamin K and certain B vitamins that can be absorbed and used

6.1.5• Absorption vs. Assimilation

– Insoluble food molecules are digested to soluble products in the lumen of the gut.

– Absorption:• The soluble products are first taken up by various

mechanisms into the epithelial cells that line the gut. • These epithelial cells then load the various absorbed

molecules into the blood stream. – Assimilation:

• The soluble products of digestion are then transported to the various tissues by the circulatory system.

• The cells of the tissues then absorb the molecules for use within this tissues