Carbohydrates and Glycogen Metabolism Quiz

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Questions and Answers

What is the primary source of carbohydrates in the diet?

Simple sugars only

Plant-derived starch (correct)

Synthetic glucose supplements

Animal products like glycogen

Which process describes the breakdown of glycogen into glucose?

Glycolysis

Gluconeogenesis

Glycogenolysis (correct)

Glycogenesis

What happens to blood glucose levels during prolonged fasting?

They rise significantly

They fluctuate widely

They remain constant

They drop significantly (correct)

Under what condition might the body resort to gluconeogenesis?

Liver inhibition in nitrogen processing Signup and view all the answers

What is the normal fasting glucose level range in mmol/L?

4.4-5.6 Signup and view all the answers

What by-product results from increased lipolysis during gluconeogenesis?

Ketone bodies Signup and view all the answers

Which carbohydrates should be approached with caution due to their impact on glycemic index?

Fructose-containing products Signup and view all the answers

What is the role of the debranching enzyme in glycogen metabolism?

It facilitates cleavage of α-1,6 glycosidic bonds. Signup and view all the answers

Which hormonal change primarily activates glycogen synthesis during high blood glucose levels?

Elevation of insulin concentration. Signup and view all the answers

What is the effect of phosphorylation on glycogen phosphorylase?

It converts phosphorylase to its active form. Signup and view all the answers

Which statement accurately describes the process of glycogenolysis in the liver?

It involves glucagon promoting cAMP pathways. Signup and view all the answers

What is the primary function of glucose-6-phosphatase?

To convert glucose-6-P back to glucose for export. Signup and view all the answers

What mechanism is primarily used for the absorption of monosaccharides from the intestinal lumen into enterocytes?

Secondary active transport with Na+ Signup and view all the answers

Which monosaccharide follows the highest transport velocity during absorption?

Galactose (Gal) Signup and view all the answers

What is the role of the basolateral membrane Na+, K+-ATPase in monosaccharide absorption?

Maintains sodium gradient for absorption via antiport mechanism Signup and view all the answers

Which substances are classified as resistant to human digestive enzymes and are often referred to as 'ballast bodies'?

Cellulose and hemicellulose Signup and view all the answers

What effect do incretins have on insulin secretion?

They stimulate insulin secretion Signup and view all the answers

Which of the following incretins inhibits glucagon secretion?

GLP-1 Signup and view all the answers

Which physiological state promotes the secretion of incretins?

High glucose levels in the intestine Signup and view all the answers

What is the primary function of bacterial fermentation in the human intestine?

Creating short-chain fatty acids which aid in cancer prevention Signup and view all the answers

What distinguishes the incretin effect from other forms of insulin response?

It is stronger for oral glucose compared to intravenous glucose Signup and view all the answers

Which of the following is a significant effect of GLP-1 on metabolism?

Lowers blood glucose levels Signup and view all the answers

Which type of polysaccharide is branched and primarily composed of α-1,6 bonds?

Glycogen Signup and view all the answers

What is the primary absorptive form of saccharides in the intestines?

Monosaccharides Signup and view all the answers

Which enzyme is responsible for the initial hydrolysis of α-1,4 glycosidic bonds in the mouth?

Salivary α-amylase Signup and view all the answers

Which hormone is secreted by the pancreas and plays a role in starch digestion?

Cholecystokinin Signup and view all the answers

What are the products of the enzymatic cleavage of amylopectin and glycogen?

Maltose and maltotriose Signup and view all the answers

Which process involves glucose being converted into storage forms of energy?

Glycogenesis Signup and view all the answers

Which saccharide is a product of the cleavage by the enzyme maltase?

Glucose Signup and view all the answers

What is the role of glucose in metabolic pathways?

Releases energy Signup and view all the answers

What are the potential effects of an increased intake of maltodextrins?

High insulin release Signup and view all the answers

What is the primary enzyme involved in the degradation of glycogen to produce glucose 1-phosphate?

Glycogen phosphorylase Signup and view all the answers

What is the main substrate converted during glycogenogenesis?

Glucose 6-phosphate Signup and view all the answers

Lactose intolerance is caused by the deficiency of which enzyme?

Lactase Signup and view all the answers

What type of bond is formed during the synthesis of glycogen?

α-1,4 glycosidic bond Signup and view all the answers

Which substance accumulates due to the absence of aldolase B in fructose intolerance?

Fructose-1-P Signup and view all the answers

What is the storage form of glucose in animals?

Glycogen Signup and view all the answers

What is the role of the branching enzyme in glycogen synthesis?

Formation of α-1,6 bonds Signup and view all the answers

Which of the following symptoms is NOT associated with fructose intolerance?

Diarrhoea Signup and view all the answers

Which organ has the highest concentration of glycogen storage?

Liver Signup and view all the answers

What is the energy requirement for the elongation of glycogen chains?

Endergonic reaction Signup and view all the answers

Study Notes

Carbohydrate Metabolism I

Carbohydrate digestion and absorption are covered.
Glycogenolysis and glycogenogenesis, and their regulation, are also discussed.

Glycaemia

Blood glucose (Glc) level is 3.3-5.8 mmol/L.
This level is a result of tissue metabolism and hormonal regulation.
Glycosylated Hb (HbA1c) levels are determined if blood glucose is higher than normal for prolonged periods.

Normal Blood Glucose Levels

Fasting: 4.4-5.6 mmol/L (80-100 mg/dL)
12 hours: 4.4 mmol/L (80 mg/dL)
3 days: 3.9 mmol/L (70 mg/dL)
5-6 weeks: 3.6 mmol/L (65 mg/dL)

Sources of Blood Glucose

Blood glucose levels are affected by dietary glucose, glycogenolysis, and gluconeogenesis.
Dietary glucose leads to a rapid increase in blood glucose.
Glycogenolysis contributes to maintaining blood glucose during fasting.
Gluconeogenesis is vital during prolonged fasting to maintain blood glucose levels.

Carbohydrates in Food

Carbohydrates are the primary energy source for organisms.
Developed countries typically consume 40% carbohydrates (250-800 g/day).
The average intake of carbohydrates is approximately 300 g/day.
Carbohydrates originate primarily from plants (starch) with a smaller contribution from animals (glycogen).
Lactose is an exception.

Carbohydrates - Clinical Notes

Gluconeogenesis can be synthesized from amino acids (glucogenic amino acids).
This synthesis helps with protein-sparing in the body.
Liver issues can affect nitrogen processing.
Lipolysis can lead to the production of compounds known as ketone bodies, causing ketoacidosis.
The Atkinson keto diet is used therapeutically.
The glycemic index must be considered when substituting glucose with other sugars.
Fructose has a cautionary note on use.

Saccharides in the Human Diet

Polysaccharides:
- Starch (amylose, amylopectin)
- Glycogen
- Non-starch polysaccharides (cellulose, pectins)
Free saccharides:
- Glucose, fructose, mannose, ribose, deoxyribose
Disaccharides:
- Sucrose, lactose, mannose
Oligosaccharides:
- Raffinose, fructans
Saccharidic alcohols:
- Sorbitol, manitol, gulcitol, inositol

Intake of Saccharides: Digestion, Absorption, and Transport

Only monosaccharides are absorbed in the intestines.
Monosaccharides are transported to the liver before being distributed throughout the body.
Glucose is a vital substrate for metabolic processes.
Glucose can be used to release energy via glycolysis or for the synthesis of glycogen.
Glucose can be converted into other saccharides or utilized in the synthesis of other essential compounds (lipids and amino acids).
Glucose is crucial in the production of glycoproteins, proteoglycans, and glycolipids.

Saccharide Digestion

Enzymes from salivary glands, pancreas, and intestine are involved in carbohydrate digestion.
Salivary a-amylase (ptyalin) initiates carbohydrate digestion by hydrolyzing alpha-1,4 glycosidic bonds.
Pancreatic α-amylase continues the process of starch digestion.
Products of starch cleavage include maltose, maltotriose, and maltodextrins.

Intestinal Mucosal Cells and Brush Border Membranes

Specific enzymes, located in brush border membranes, break down disaccharides like sucrose, lactose, and maltose into their component monosaccharides.
Saccharase, lactase, and maltase are the key enzymes responsible for this digestion.

Clinical Note - Maltodextrins

Maltodextrins, used as food additives, have a high glycemic index.
This leads to large insulin releases and potential GIT issues.

Absorption of Monosaccharides

Monosaccharides are absorbed by active transport (involving Na+) in the intestinal brush border membranes.
These substances are then released into the bloodstream through passive transport mechanisms.
Different monosaccharides have varying transport velocities in the body.

GLUT Transport Proteins

GLUT transporters are proteins responsible for glucose transport across cell membranes.
Different tissues express various GLUT isoforms.
GLUT 1, 2, 3 are high-affinity transporters
GLUT 4, is an insulin-sensitive transporter.
GLUT 5 specializes in fructose transport.

Non-Digested Carbohydrates

Cellulose, hemicellulose, inulin, and pectin are non-digestible carbohydrates (fibers).
These substances are partially metabolized by gut bacteria in processes that produce probiotics and prebiotics.
Bacterial breakdown of these substances produces substances such as hydrogen, methane, and various organic acids, which are important in preventing some types of cancer.

Hormonal Regulation of Blood Glucose

High blood glucose levels trigger insulin release (incretins are also involved).
Low blood glucose prompts the release of hormones like glucagon, epinephrine, and growth hormone.

Incretins

Incretins are hormones released from the GIT to stimulate insulin secretion based on the level of glucose in the intestine.
GIP (gastric inhibitory peptide) is among these incretins, along with GLP-1.
These hormones play a significant role in controlling digestion and sugar metabolism.
Incretins enhance glucose-dependent insulin secretion and regulate lipid metabolism.

Clinical Note - Incretin Effect

Oral glucose intake results in a different insulin response compared to intravenous glucose.
The incretin effect helps achieve euglycemia in diabetics.

Hypoglycemia

Low blood glucose levels (hypoglycemia) can be life-threatening.
The body has counter-regulatory hormones (cortisol, epinephrine, norepinephrine, glucagon) to address low glucose levels.

Pathologies of Carbohydrate Digestion and Metabolism

Lactose intolerance is a condition caused by lactase deficiency.
Fructose intolerance stems from the lack of an enzyme, leading to fructose buildup.

Glycogen Metabolism

Glycogen is the primary storage form of glucose in animals.
Glycogen is stored in the liver and muscle tissues.
Glycogenolysis is the process of breaking down glycogen to release glucose.
Glycogenesis is the process of synthesizing glycogen from glucose.
Glycogen synthesis begins with glucose 6-phosphate to glucose 1-phosphate conversion, followed by formation of UDP-glucose and the elongation and branching of glycogen chains.
Glycogen breakdown begins with glycogen phosphorylase breaking down α-1,4 glycosidic bonds in the glycogen chain.
Debranching enzyme cleaves α-1,6 glycosidic bonds.
Glucose 6-phosphatase in the liver catalyzes the removal of phosphate from glucose 6-phosphate, releasing free glucose into the bloodstream.

Regulation of Glycogen Metabolism

Blood glucose concentration governs glycogen synthesis and degradation.
High glucose activates glycogen synthesis (mostly in the liver) and inhibits glycogen breakdown;
Low glucose activates glycogen breakdown (in the liver and muscles).
Hormones like insulin and glucagon control glycogen metabolism.

Mechanism of Glycogen Enzyme Action

Glycogen enzymes are regulated through covalent modification (phosphorylation/dephosphorylation) of their structures.
Phosphorylation activates enzymes involved in glycogen breakdown (e.g., phosphorylase) and deactivates those involved in synthesis (e.g., synthase).

High Glucose and Insulin

When blood glucose is high, insulin increases glycogen synthesis and inhibits glycogen phosphorylase kinase, hence, no glycogenolysis.

Regulation of Glycogen Synthesis and Degradation

Blood glucose concentration is the key regulator. High glucose prefers glycogen synthesis, and low glucose promotes glycogenolysis.

Glycogen Storage Diseases

Genetic deficiencies in glycogen metabolism cause glycogen storage diseases.
Affected organs and symptoms depend on the specific enzyme deficiency.

Clinical notes - Glycogen Storage Diseases

Various glycogen storage diseases are associated with specific enzymatic defects and organ-specific symptoms.

Further Information

Further detail on specific enzyme defects and their roles in glycogen synthesis and degradation is also provided in the given notes.

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Description

Test your knowledge on carbohydrates, glycogen metabolism, and their roles in nutrition and energy production. This quiz covers topics such as glycogen breakdown, blood glucose levels, and enzyme functions. See how well you understand the biochemical processes involved in carbohydrate metabolism!