Carbohydrate Metabolism Digestion

8/20/2019 Carbohydrate Metabolism Digestion

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CARBOHYDRATE

METABOLISM

İmge Kunter PhD.


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CONTENTS

Digestion and absorptionof carbohydrates

Glucose Metabolism

Glycogen Metabolism


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Carbohydrates present in the

diet

PolysaccharidesDisaccharides Monosaccharides

StarchGlycogen

Lactose

MaltoseSucrose

Glucose

Fructose

Pentose


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Details of digestion of carbohydrates

2 Types of enzymes are important for the digestion

of carbohydrates

Amylases Disaccharidases

Salivary

Amylase

Pancreatic

Amylase

convert polysaccharides

to disaccharides

Convert disaccharides tomonosaccharides which are

finally absorbed

Maltase

Sucrase-Isomaltase

Lactase

Trehalase


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Digestion inmouth

Digestion instomach

Digestionin smallintestine


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Saliva contains salivary amylase .

Digestion in the Mouth

The enzyme hydrolyses α-1→ 4 glycosidic linkages.

However, its action stops in the stomach when the

pH falls to 2.0-3.0.


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Starch, Glycogen and dextrins

(Large polysaccharide molecules)

α- Amylase

Glucose,Maltose and Maltotriose.(Smaller molecules)


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Digestion in the Stomach

• HCl present in the stomach causes hydrolysis

of sucrose to fructose and glucose.

Sucrose Fructose + GlucoseHCl


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Digestion in Duodenum

• Food mixture reaches the duodenum, meets thepancreatic juice.

•Pancreatic juice contains pancreatic amylase

similar to salivary amylase.

• Optimum pH=7.1

• It hydrolyses α-1→ 4 glycosidic linkagessituated well inside polysaccharide molecules.


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© 2008 Thomson - Wadsworth


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Summary of Carb digestion

• *In the mouth, the salivary enzyme amylase ---starch to polysaccharides.

• *In the stomach, acid hydrolyze starch and fiberdelays gastric emptying

•

*In the small intestine, pancreatic amylase andother enzymes hydrolyzes starches todisaccharides and monosaccharides.


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Clinical significance of Digestion

• Lactose intolerance is the inability to digest

lactose due to the deficiency of Lactase

enzyme.

• Causes

Congenital Acquired during lifetime


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Monosaccharides, the end products of carbohydrate digestion,

enter the capillaries of the intestinal villi.

In the liver,galactose andfructose areconverted toglucose.

Small intestine

Monosaccharides travel to

the liver via the portal vein.

Stepped Art


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Absorption of carbohydrates

3 mechanisms

Passive diffusion

Facilitated

diffusion/Carriermediated

Active transport


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Glucose transporters

Glucose transporters

Na+

dependenttransporter

Na+ independent

transporter

2 types

SGLT GLUT

Also called Also called


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Na+ dependent transporter

• Type of co-transport

• Na+ and glucose binding sites

• After Na + binding…. conformational changes …..

glucose can bind.• Glucose and galactose -sodium-dependent

• They are carried by the same transport protein (SGLT

1),


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Coupled

transport

1. A sodium-potassium exchange pump maintains aconcentration of Na that is higher outside the cell than inside.

2. Na moves back into the cell by a carrier protein that alsomoves glucose. The concentration gradient for Na provides theenergy required to move glucose against its concentrationgradient.

3. Na + is transported across cell membrane, down the

concentration gradient and glucose goes against aconcentration gradient.


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http://www.namrata.co/wp-content/uploads/2012/07/glucose-active-transport.jpg


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Na+ independent transporters

• Used for facilitated transport.

• These transporters are numbered from 1 to

14 GLUT.


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Uptake of glucose in peripheral cells

• Mechanism: facilitated diffusion.

• There are 7 important glucose transporter for uptake

of glucose into special cells.

Tissue specific Tissue distribution Functions


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Tissue specific

glucose

transporter

Tissue distribution Functions

GLUT-1

(great affinity for

glucose)

Present in almost all

cells with an abundance

in RBC.

Na-independent

GLUT-2

(low affinity for

glucose)

Present in intestine,

liver and pancreas.

Acts as a sensor for the release of insulin

by pancreas.

Promotes uptake of glucose in liver cells,

lowering down blood glucose.

GLUT 3 Brain cells, all othercells of body

GLUT 4 Adipose tissue, skeletal

muscles, cardiac

muscles

The only transporters which are under the

influence of insulin.

Insulin promotes uptake of glucose in the

tissues by mobilizing the transporters to thecell surface whenever there is high glucose

concentration in the blood.


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Blood Glucose Homeostasis

• Sources of glucose in the blood

– Diet

– Glycogenolysis (breakdown of glycogen)

– Gluconeogenesis (synthesis of glucose fromnoncarbohydrate substances)


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A Preview of Carbohydrate Metabolism

– The body uses glucose for energy

– The body stores glucose as glycogen in liver and muscle.

–

If glycogen stores are depleted, the body makes glucosefrom protein.

• Gluconeogenesis is the conversion of protein to glucose.

–The body can use glucose to make body fat whencarbohydrates are consumed excessively.


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• Low blood

glucose may

cause

dizziness,sweating and

weakness.


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• High blood

glucose

may cause

extremethirst and

urge to

urınete also

fatigue.


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The Constancy of Blood Glucose

– The Regulating Hormones

• Insulin moves glucose into the cells and helps tolower blood sugar levels.

•

Glucagon brings glucose out of storage andraises blood sugar levels.

• Epinephrine acts quickly to bring glucose out ofstorage during times of stress.

– Blood glucose can fall outside the normal rangewith hypoglycemia or diabetes.


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• Diabetes health problems are serious.

The complications can be deadly!


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– Diabetes

• Type 1 diabetes is the less common type with no insulinproduced by the body. (Insulin Dependent)

• Type 2 diabetes is the more common type where fat

cells resist insulin. (Non Insulin Dependent)

• Prediabetes is blood glucose that is higher than normal

but below the diagnosis of diabetes.


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Table 1. Chart summarizing differences between

Type I and Type II diabetes


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– Diabetes

– Hypoglycemia is low blood glucose and can often

be controlled by dietary changes.


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Glycemic response

– Glycemic response is how quickly the blood

glucose rises and elicits an insulin response.

• Glycemic index classifies foods according to theirpotential for raising blood glucose.


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• http://www.medbio.info/horn/time%203-

4/homeostasis_2.htm

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Carbohydrate Metabolism Digestion