MLAB 2401: Clinical Chemistry
Keri Brophy-Martinez
CARBOHYDRATE
OVERVIEW
Introduction
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 Organisms rely on the oxidation of complex
organic compounds to obtain energy
 Three general types of compounds provide
chemical energy to our cells
Lipids=Fats
 Amino acids = Proteins
 Carbohydrates= Sugars, starches

Carbohydrates
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 Major food source & energy supply of body
 Primary source of energy for brain, erythrocytes, retinal cells

Depending on individual diet, 50-90% of the body's carbohydrate
intake is in the form of





Grains - cereals, bread
Starchy vegetables - potatoes
Legumes - beans, peanuts
other sources = sugar, molasses, lactose from milk, fructose from fruit
Stored primarily as liver and muscle glycogen
Description and Classification of Carbohydrates
 Contain C, H and O molecules
 Contain a C=O (ketone) and an –OH(aldehyde)
functional group
 Classification
 Based on certain properties
The size of the base carbon chain
 Location of the CO functional group
 Number of sugar units
 Stereochemistry of compound

Chemical Properties
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 Some ( not all ) carbs are reducing substances (donate electrons)


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Chemical reduction of other substances
These sugars must contain an aldehyde or ketone group
Reducing sugars
Glucose
 Maltose
 Lactose
 Fructose
 Galactose


Sucrose is not a reducing substance
Carbohydrate Metabolism
 Glucose is primary energy source
 Nervous tissue can not concentrate or store
carbohydrates, so a steady supply of glucose is
needed
 Once the level of glucose falls below a certain range,
normal function is impaired
Carbohydrate Breakdown
Dietary
Carbohydrates
Dextrins/
Maltose
• Mouth
• Salivary amylase
• Stomach/Intestines
• Pancreatic amylase
• Absorption into intestinal mucosa
• Delivered to liver
Monosaccharide
Carbohydrate Breakdown
 Ultimate Goal
 Convert glucose to CO2 and water with ATP as a by-product
 Possible channels
 Converted to liver glycogen and stored
 Metabolized to CO2 and H2O
 Converted to keto-acids, amino acids, and proteins
 Converted to fats and stored in adipose tissue
Biochemical Pathways in Carbohydrate Breakdown

Embden-Meyerhoff pathway
Converts glucose to pyruvate/lactate
 Primary energy source for humans


Hexose monophosphate shunt
Oxidizes glucose to ribose and CO2
 Produces NADPH as an energy source


Glycogenesis

Converts glucose to glycogen
Carbohydrate Metabolism
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 Glycolysis – the conversion of glucose and other hexoses into
lactate or pyruvate
 Breakdown of glucose for energy production
 Glycogenesis – the conversion of glucose to glycogen
usually in liver & muscle
 Excess glucose is converted and stored as glycogen
 High concentrations of glycogen in liver and skeletal muscle
 Glycogen is a quickly accessible storage form of glucose

Carbohydrate Metabolism
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 Glycogenolysis – the breakdown of glycogen to form glucose
Glycogenolysis occurs when plasma glucose is decreased
 Occurs quickly if additional glucose is needed
 Controlled by hormones & enzymes

 Gluconeogenesis – the formation of glucose from non-carbohydrate
sources, such as amino acids, glycerol & fatty acids into glucose
 Occurs mainly in the liver
Glycolysis
Gluconeogenesis
Glucose
Glycogenesis
Glycogenolysis
Carbohydrate Metabolism
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 Also related:
 Lipogenesis – the conversion of carbohydrates to fatty acids


Fat is another energy storage form, but not as quickly accessible as
glycogen
Lipolysis – the decomposition of fat
 The sum or net of all of these processes determines
the level of blood glucose.
Regulation of Plasma Glucose
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 Organs / systems involved in glucose regulation

Liver :

Muscle

Glucose
Glycogen
Glucose
Skeletal & heart

Pancreas
 Synthesizes hormones Insulin and Glucagon, somatostatin

Other Endocrine glands
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Anterior pituitary gland ( growth hormone)
Adrenal gland (epinephrine and cortisol)
Thyroid gland (thyroxine)
Regulation of Plasma Glucose
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 If plasma glucose is decreased :

Glycogenolysis


The liver releases glucose into the plasma (quick response)
Gluconeogenesis and lipolysis
 If plasma glucose is increased :

Glycogenesis


Liver stores glucose as glycogen
Lipogenesis

Formation of lipids
Hormones that Regulate Glucose
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 Insulin
 Most important & only one to decrease glucose
level
 Synthesized in the Beta cells of the Islets of Langerhans
(in the pancreas)
 Released when plasma glucose is increased
Action / Effects of insulin
 Facilitates glucose entry into cells

cell membranes need insulin to be present for glucose to enter
 Promotes liver glycogenesis

glucose to glycogen
 Promotes glycolysis

speeds up utilization of glucose in cells
 Promotes synthesis of lipids from glucose

Such as the formation of Triglycerides
 Promotes amino acid synthesis from glucose intermediates
 Decreases / inhibits glycogenolysis and gluconeogenesis
Insulin Control
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Insulin secretion controlled by:


Blood glucose level
Certain Amino Acids ie. leucine, & arginine
Counterregulatory Hormones
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 Glucagon
 2nd most important glucose regulatory hormone

Referred to as a hyperglycemic agent

Synthesized in alpha cells of the islets of Langerhans
Action/Effect of Glucagon
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 Stimuli – decreased plasma glucose
 Action
 Increases glycogenolysis & gluconeogenesis
 Promotes breakdown of fatty acids
 Promotes breakdown of proteins to form amino acids
 Increases plasma glucose concentration
Other Regulatory Hormones
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 Epinephrine

One of two glucose regulating hormones from the adrenal gland
 Origin – adrenal medulla
 Action/effect
 Inhibits insulin secretion & release
 Promotes lipolysis
 Stimulates glycogenolysis
 Immediate release of glucose

Stimuli
 Neurogenic - based on physical / emotional stress.
 Adrenal tumors
Other Regulatory Hormones
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 Glucocorticoids - such as cortisol
 Origin – adrenal cortex

Effect – antagonistic to insulin
increases blood glucose
 promotes gluconeogenesis from breakdown of proteins
 inhibits the entry of glucose into muscle cells


Stimuli – anterior pituitary’s ACTH
Other Regulatory Hormones
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 Growth Hormone (GH) and Adrenocorticotropic
Hormone (ACTH)


Origin – anterior pituitary gland
Effect – antagonistic to insulin


Increases plasma glucose levels
 inhibits insulin secretion
 inhibits entry of glucose into muscle cells
 inhibits glycolysis
 inhibits formation of triglycerides from glucose
Stimuli
decreased glucose stimulates its release
 increased glucose inhibits its release

Other Regulatory Hormones
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 Thyroid hormones (such as thyroxine)
 Origin – thyroid gland
 Effect
increases absorption of glucose from intestines
 Promotes comversion of liver glycogen to glucose
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
Stimuli – pituitary gland’s TSH
Other Regulatory Hormones
 Somatostatin
 Origin-Delta cells of the islets of Langerhans in the pancreas
 Effect - increase plasma glucose
 Actions
antagonistic to insulin,
 inhibits endocrine hormones including glucagon & growth
hormone

References
 Bishop, M., Fody, E., & Schoeff, l. (2010). Clinical Chemistry:
Techniques, principles, Correlations. Baltimore: Wolters
Kluwer Lippincott Williams & Wilkins.
 Sunheimer, R., & Graves, L. (2010). Clinical Laboratory
Chemistry. Upper Saddle River: Pearson .
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