What Is Digestion

• Process whereby the body breaks down food into absorbable nutrients.

• Digestive tract is a long tube surrounded by muscles.

• Digestion is a process involving the

hydrolysis of large and complex organic

molecules of foodstuffs into smaller and

preferably water-soluble molecules which can

be easily absorbed by the GIT for utilization

by the organism

• Digestion of macromolecules also promotes

the absorption of fat soluble vitamins and

certain minerals

Mouth

• Teeth grind food to reduce the size.• Saliva released to help moisten food.• Some carbohydrate digestion begins.• Tongue pushes food to the back of the mouth

to start swallowing reflex.• Food passes through the esophagus and

enters stomach.

Stomach

• Distended pouch. Strong circular muscles at each end control food entering and leaving.

• Mixes food by muscular contractions causing food to break up further.

Small Intestine

• Most digestion and absorption occurs in small intestine.

• Bile released to emulsify (break up) fat.• Pancreatic enzymes released to digest

carbohydrates, proteins and fats.• Final digestive enzymes in intestinal lining

break down carbohydrates, proteins and fats into absorbable units.

Final Digestion Product

• Final digestion products absorbed by cells lining small intestine.– Carbohydrates:• Monosaccharides

– Proteins:• Amino acids• Chains of 2 or 3 amino acids

– Fats: • Fatty acids• Glycerol• Monoglycerides

Absorption

• Water-soluble nutrients and short fatty acids released directly into the bloodstream.

• Longer fatty acids, fat-soluble vitamins, and fat-like compounds such as cholesterol are not water-soluble.

Indigestible Matter

• After digestion and absorption of nutrients, indigestible matter, such as fiber moves into the large intestine.

• Indigestible matter is compacted by removing water.

• Little nutrient absorption occurs in large intestine.

Metabolism

• Chemical reactions that occur in the body:– Building and maintaining body tissues– Regulating body functions– Supplying energy

• For metabolism to occur the body needs:– Water– Energy– Oxygen– Nutrients

Biological Importance

large molecules small molecules

small molecules

Food

Digestion

Absorption

vitamins, minerals, monosaccharides and free amino acids

BLOOD

Digestion and absorption of carbohydratesCarbohydrates present in the diet

PolysaccharidesDisaccharides Monosaccharides

StarchGlycogen

LactoseMaltoseSucrose

GlucoseFructosePentose

In GIT, all complex carbohydrates are converted to simpler monosaccharide form which is the absorbable form.

• Complex carbohydrates into simple

• Polysaccharide into monosaccharide

Details of digestion of carbohydrates2 Types of enzymes are important for the digestion of carbohydrates

Amylases Disaccharidases

Salivary Amylase

Pancreatic Amylase

convert polysaccharides to disaccharidesConvert disaccharides to monosaccharides which are finally absorbed

Maltase

Sucrase-Isomaltase

Lactase

Trehalase

Digestion in

mouth

Digestion in stoma

ch

Digestion in

small intestine

DIGESTION OF CARBOHYDRATES

Digestion of Carbohydrate starts in the mouth, upon contact with saliva during mastication.Saliva contains a carbohydrate splitting enzyme called salivary amylase , also known as ptylin.

Digestion in the Mouth

Action of ptylin (salivary amylase)

• Location: mouth• It is α-amylase and requires Cl− ion for

activation with an optimum pH of 6.7 (Range 6.6 to 6.8).

• However, ptylin action stops in the stomach when the pH falls to 3.0.- acidic pH

Starch, Glycogen and dextrins(Large polysaccharide molecules)

α- Amylase

Glucose,Maltose, Dextrin, maltose and Maltotriose.

(Smaller molecules)

Drawbacks of this method

• Shorter duration of food in mouth. • Thus it is incomplete digestion of starch or

glycogen in the mouth

Digestion in the Stomach• Stomach receives partially digested &

Partially indigested food from mouth mixed with salivary enzyme

• No CHO digesting enzymes, no digestion• However, HCl present in the stomach causes

hydrolysis of sucrose to fructose and glucose. Sucrose Fructose + GlucoseHCl

Digestion in Duodenum

• Complete digestion, SI, longer time -stay• Food bolus reaches the duodenum from the

stomach where it meets the pancreatic juice.• Enzymes: pancreatic amylase & intestinal

amylase• Includes: maltase• Isomaltose• Limit dextrinase• Sucrase• lactase

Digestion in Duodenum

• Pancreatic juice contains a carbohydrate splitting enzyme, pancreatic amylase

(amylopsin) similar to salivary amylase.

Action of pancreatic amylase

• It is an α- Amylase• Optimum pH=7.1• Like ptylin, it requires Cl− ion for its activity.• It hydrolyses α-1→ 4 glycosidic linkages

situated well inside polysaccharide molecules.• Note: Pancreatic amylase, an isoenzyme of salivary

amylase, differs only in the optimum pH of action. Both the enzymes require Chloride ions for their actions (Ion activated enzymes).

Reaction catalyzed by pancreatic amylase

Starch/Glycogen

Maltose/ Isomaltose +

Dextrins and oligosaccharides

Pancreatic Amylase

Starch/glycogen Pancreatic/intestinal amylase

Dextrin + limit + dextrin + maltose + Isomaltose

Isomaltose Isomaltase 2 glucose units

Maltose Maltase 2 glucose units

Limit dextrin Limit dextrinase 2 glucose units

Sucrose Sucrase Glucose + fructose

Lactose Lactase Glucose + galactose

MOUTH

STOMACH

COMPLEX CARBOHYDRATES

SMALL % OF DIGESTION

INTESTINE

NO DIGESTION

DEXTRIN + LIMIT DEXTRIN + MALTOSE + ISOMALTOSE

2 GLUCOSE UNITS

ABSORPTION

• Occurs in small intestine• In blood stream, occurs in form of simple

sugar-monosaccharide-glucose, galactose, fructose

• Simple sugar enter the portal circulation via capillaries of villi & are transported to liver

• In the liver fructose and galactose are converted to glycogen for storage

• The glycogen stored gets reconverted to glucose during fasting

Absorption of carbohydrates

3 mechanisms

Passive diffusionFacilitated diffusion/Carrier mediated

Active transport

Glucose absorption

GluT4- Glucose transport in cells

Features Passive diffusion Facilitated diffusion Active transport

Concentration gradient

Down the concentration gradient from high to low.

Down the concentration gradient from high to low.

Against a concentration gradient from low to high

Energy expenditure none none Energy expenditure is in the form of ATP

Carrier protein/ transporter

Not required required required

Speed Slowest mode Fast Fastest mode

Absorption of Glucosefrom the small intestinal lumen

by carrier mediated mechanism involving transporter proteins 1) Na+-dependent transporter

by secondary active transport and to a less extent by

2) Na+-independent transporter by passive transport

into the intestinal epithelial cells

Monosaccharides, the end

products of carbohydrate

digestion, enter the capillaries of

the intestinal villi

In the liver, galactose & fructose are converted to glucose.

Small intestine Monosaccharides

travel to the liver via the portal vein.

Diagram showing absorption of monosaccharides

Factors affecting rate of absorption of Monosaccharides

• The absorption is faster through intact mucosa. The absorption is decreased if there is some inflammation or injury to the mucosa.

• Thyroid hormones ↑ the rate of absorption of glucose.

• Mineralocorticoid, i.e Aldosterone ↑ the rate of absorption.

• Vitamin B6,B12, pantothenic acid, folic acid are required for absorption of glucose.

• With advancing age, rate of absorption declines.

Uptake of glucose in peripheral cells

• Mechanism: facilitated diffusion.• There are 7 important glucose transporter for

uptake of glucose into special cells.• They have been numbered from 1 to 7 (GLUT

1 to GLUT 7).• They are biologically important.

METABOLISM

• Simple carbohydrates which reach the liver through portal circulation is now used up by the body

Energy production

• Glucose obtained – directly utilized by the body cells to obtain energy

• Glucose broken down to release energy

Glycolysis• Glucose 2 pyruvate

• Pyruvate enters TCA cycle ( tricarboxylic acid ) also known as kreb cycle to produce energy in form of ATP

Pyruvate 2 acetyle CoA Kreb cycle

Energy( 36 ATP Mol )+Carbondioxide(6)+Water(6)

Storage products

Conversion into glycogen• Primarily glucose is used for energy production• Extra glucose is converted into glycogen in

presence of insulin• And stored in liver & muscle• Glycogenesis

Insulin• Glucose glycogen

Conversion into fat• If glucose level exceeds beyond the storage

space available in liver & mucle• Then rest of glucose is stored in form of fat in

adipose tissue• Lipogenesis

Maintenance of blood glucose levels

• After complete digestion – glucose enters blood stream – helps normal functioning – all blood cells

• FBS (Fasting Blood Glucose) : 80-110 mg/dl

HORMONES EFFECT ON BLOOD GLUCOSE LEVEL

Insulin Decreases

Glucogen Increases

Epinephrine ( adrenaline)

Increases

Glucocorticoids Increases

Thyroxine Increases

Growth hormone Increases