Biochemistry Quiz: Carbohydrate Metabolism

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Questions and Answers

What primary role does salivary α-amylase play in carbohydrate digestion?

It denatures proteins in the stomach.

It absorbs monosaccharides in the bloodstream.

It converts glucose to pyruvate.

It catalyzes the hydrolysis of glycosidic bonds in carbohydrates. (correct)

What happens to salivary α-amylase in the stomach?

It gets denatured by stomach acid. (correct)

It is activated by stomach acid.

It begins breaking down monosaccharides.

It continues to digest proteins.

Which of the following enzymes plays a role in breaking down disaccharides?

Phosphofructokinase

Lactase (correct)

Phosphoglucose isomerase

Hexokinase

In glycolysis, what is the product of the first reaction where ATP is used?

Glucose-6-phosphate Signup and view all the answers

What is the end product of glycolysis from one glucose molecule?

Two molecules of pyruvate Signup and view all the answers

Which enzyme catalyzes the conversion of glucose-6-phosphate to fructose-6-phosphate?

Phosphoglucose isomerase Signup and view all the answers

What is a potential consequence of lactose intolerance?

Fermentation of lactose in the large intestine Signup and view all the answers

What is the total ATP cost for glucose synthesis in the liver?

4 ATP Signup and view all the answers

Which of the following is true about glycolysis?

It occurs in the cytosol of the cell. Signup and view all the answers

Which hormone is released by the pancreas after meals to lower blood glucose levels?

Insulin Signup and view all the answers

During intense physical activity, which substance is primarily depleted after available ATP?

Creatine phosphate Signup and view all the answers

What happens during the anaerobic threshold in muscle activity?

ATP supply comes only from glycolysis. Signup and view all the answers

What role does the Cori Cycle play during prolonged exercise?

It recycles lactate to pyruvate in the liver. Signup and view all the answers

What effect does high levels of glucose-6-phosphate have on hexokinase?

It inhibits hexokinase Signup and view all the answers

Which enzyme is activated by high levels of AMP?

Phosphofructokinase Signup and view all the answers

Under anaerobic conditions, what is the primary result of lactate dehydrogenase activity on pyruvate?

Pyruvate is reduced to lactate Signup and view all the answers

What is the net gain of ATP from glycolysis under anaerobic conditions?

2 ATP Signup and view all the answers

What happens to pyruvate under aerobic conditions?

It is oxidized to acetyl-CoA Signup and view all the answers

What is the main purpose of gluconeogenesis?

To synthesize glucose from other metabolites Signup and view all the answers

Which of the following statements about the regulation of gluconeogenesis is true?

It replaces three irreversible reactions of glycolysis with new enzymes Signup and view all the answers

How many new reactions are introduced in gluconeogenesis to replace the regulated reactions from glycolysis?

Four new reactions Signup and view all the answers

What are the products of the enzyme aldolase in glycolysis?

Dihydroxyacetone phosphate and Glyceraldehyde-3-phosphate Signup and view all the answers

Which enzyme catalyzes the isomerization of dihydroxyacetone phosphate?

Triose phosphate isomerase Signup and view all the answers

What happens to the aldehyde group of glyceraldehyde-3-phosphate in glycolysis?

It is oxidized to a carboxyl group Signup and view all the answers

How many molecules of ATP are produced as a net gain during glycolysis?

Two Signup and view all the answers

What is the role of phosphoglycerate kinase in glycolysis?

Transferring a phosphate group from 1,3-bisphosphoglycerate to ADP Signup and view all the answers

During which reaction does dehydration occur in glycolysis?

Reaction 9 Signup and view all the answers

What intermediate do galactose and fructose form to enter the glycolysis pathway?

Dihydroxyacetone phosphate Signup and view all the answers

Which enzyme catalyzes the transfer of a phosphate group from phosphoenolpyruvate to ADP?

Pyruvate kinase Signup and view all the answers

What does metabolism encompass?

All chemical reactions in the body Signup and view all the answers

What characterizes catabolic reactions?

Breaking down complex molecules Signup and view all the answers

Which stage of catabolism involves the breakdown of polymers to monomers?

Digestion and hydrolysis Signup and view all the answers

Which component of ATP is responsible for storing energy?

Phosphate groups Signup and view all the answers

What occurs to a coenzyme when it gains hydrogen ions and electrons?

It is reduced Signup and view all the answers

Which vitamin is associated with coenzyme A (CoA)?

Vitamin B5 Signup and view all the answers

What is the primary function of coenzyme A in metabolism?

To prepare small acyl groups for enzyme reactions Signup and view all the answers

In the context of catabolic reactions, what happens during the citric acid cycle?

ATP energy is produced through oxidation Signup and view all the answers

Study Notes

Chapter Twenty Two: Metabolic Pathways for Carbohydrates

This chapter discusses metabolic pathways for carbohydrates.
Homework assignments are included (numbers 1-7, 15, 16, 20-31, 33, 35, 37-44, 47, 49, 51-78, 85, 89, 125, 126, 127, 129, 131, 133, and 135).

Metabolism

Metabolism encompasses all chemical reactions supplying energy and substances for cell growth.
Two types of metabolic reactions exist:
- Catabolic reactions: Break down complex molecules into smaller ones, releasing energy.
- Anabolic reactions: Use energy to build larger molecules.

Stages of Catabolism

Catabolic reactions are divided into stages:
- Stage 1: Digestion and hydrolysis break down polymers into monomers, which enter the bloodstream.
- Stage 2: Cellular degradation breaks down molecules into two and three-carbon compounds.
- Stage 3: Oxidation of all small molecules within the citric acid cycle and electron transport to produce ATP energy.

Stages of Catabolism: From Digestion to Cell

This diagram illustrates the process of catabolism, outlining the breakdown of proteins, polysaccharides, and lipids into smaller molecules within the mitochondria.
The process ultimately leads to the production of ATP, the primary energy currency of the cell.

ATP, Adenosine Triphosphate

ATP is composed of adenine, ribose sugar, and three phosphate groups.
Hydrolysis of ATP to ADP and AMP releases energy; more phosphates equate to more stored energy.

ATP Drives Reactions

Catabolic reactions release energy, storing it as ATP.
Anabolic reactions use ATP energy to build molecules.
ATP hydrolysis releases 7.3 kcal/mole (-31 kJ/mole) of energy.

Metabolic Reactions

Coenzymes can gain or lose hydrogen ions and electrons.
Reduced coenzymes store energy, while oxidized ones release it.

Structure of Coenzyme NAD+

NAD+ is nicotinamide adenine dinucleotide.
It has ADP attached to a niacin derivative (vitamin B3).
It collects H⁺ and 2 electrons.

Coenzyme NAD+ or NADP+

NAD+ and its phosphorylated form, NADP+, are coenzymes essential for metabolic reactions.
NADH is the reduced form, while NAD+ is the oxidized form.

Coenzyme FAD

FAD is flavin adenine dinucleotide.
It contains ADP and riboflavin (vitamin B2).
It collects 2H⁺ atoms (+2e¯) to reduce to FADH₂.

Structure of Coenzyme A

Coenzyme A (CoA) has three components: pantothenic acid (vitamin B5), phosphorylated ADP, and aminoethanethiol.

Function, Coenzyme A

CoA prepares acyl groups (e.g., acetyl) for reactions.
CoA produces the energy-rich thioester acetyl CoA.

Digestion of Carbohydrates

Stage 1 of catabolism is digestion, breaking down food into small molecules.
In the mouth, physical and chemical digestion occurs.
Salivary α-amylase hydrolyzes glycosidic bonds in carbohydrates, continuing to act in the stomach until denatured by stomach acid.

Digestion: In Small Intestine

Pancreatic α-amylase breaks down polysaccharides to glucose.
Sucrase, maltase, and lactase break disaccharides.
Monosaccharides are absorbed into the bloodstream.

Digestion of Carbohydrates (Diagram)

This diagram illustrates the breakdown of polysaccharides, maltose, lactose, and sucrose into monosaccharides (glucose, fructose, and galactose) by specific enzymes in the digestive system and absorption into the bloodstream and cells.

Lactose Intolerance

Lactase production can cease after childhood.
Undigested lactose in the large intestine may cause discomfort due to bacterial fermentation.
Lactose-free products are available as a solution for intolerance.

In The Cell Stage 2: Glycolysis

Glucose from the bloodstream enters cells and undergoes degradation (Stage 2 catabolism).
Glycolysis converts 1 glucose (6C) into 2 pyruvate (3C).
ATP is initially used but ultimately gained in glycolysis.
Glycolysis happens in the cell's cytosol.

Glycolysis: Reaction 1

Phosphorylation occurs, transferring a phosphate group from ATP to glucose.
Glucose-6-phosphate and ADP are produced.
Hexokinase catalyzes the reaction.

Glycolysis: Reaction 2

Isomerization converts glucose-6-phosphate to fructose-6-phosphate.
Phosphoglucose isomerase catalyzes the reaction.

Glycolysis: Reaction 3

Phosphorylation adds another phosphate group to fructose-6-phosphate (via ATP), producing fructose-1,6-bisphosphate.
Phosphofructokinase catalyzes the reaction.

Glycolysis: Reaction 4

Cleavage splits fructose-1,6-bisphosphate into dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P).
Aldolase catalyzes this reaction.

Glycolysis: Reaction 5

Isomerization converts DHAP to G3P.
Triose phosphate isomerase catalyzes the reaction.

Glycolysis: Reaction 6

Oxidation and phosphorylation of G3P, producing 1,3-bisphosphoglycerate, reducing NAD⁺ to NADH and H⁺ (x2).
Glyceraldehyde-3-phosphate dehydrogenase catalyzes this reaction.

Glycolysis: Reaction 7

Phosphate transfer from 1,3-bisphosphoglycerate to ADP produces ATP (x 2).
Phosphoglycerate kinase catalyzes the reaction.

Glycolysis: Reaction 8

Isomerization converts 3-phosphoglycerate to 2-phosphoglycerate by phosphoglycerate mutase.

Glycolysis: Reaction 9

Dehydration converts 2-phosphoglycerate to phosphoenolpyruvate by enolase.

Glycolysis: Reaction 10

Phosphate transfer by pyruvate kinase converts phosphoenolpyruvate to pyruvate with ATP production (x 2).

Glycolysis: Overall Reaction

Two ATP initialize phosphorylation of glucose and fructose-6-phosphate.
Four ATP are formed through direct phosphate group transfers to ADP.
A net gain of 2 ATP and 2 NADH molecules from glucose.

Fructose and Galactose

Galactose and fructose undergo intermediary reactions to enter the glycolysis pathway.

Regulation of Glycolysis

Hexokinase activity is inhibited by high glucose-6-phosphate amounts.
Phosphofructokinase activity is inhibited by high ATP levels, but activated by high AMP levels.
Pyruvate kinase is inhibited by high ATP levels and acetyl CoA.

Fates of Pyruvate: Fermentation or Oxidation

Pyruvate's fate depends on oxygen availability.
In anaerobic conditions, it ferments to lactate or ethanol and CO₂.
In aerobic conditions, it's oxidized into Acetyl-CoA and CO₂.

Pyruvate: Anaerobic Conditions

High NADH and low NAD⁺ prevent glycolysis continuation.
Reduction of pyruvate to lactate regenerates NAD⁺.
Production of a small amount of ATP in glycolysis continues.

Pyruvate: Aerobic Conditions

Pyruvate moves from the cytosol to the mitochondria where it's oxidized, converting to acetyl-CoA, CO₂, and reducing NAD⁺ to NADH.
Acetyl-CoA enters the citric acid cycle and NADH moves to electron transport for ATP production.

Gluconeogenesis

Glucose is a primary energy source for the brain and other tissues.
Gluconeogenesis is crucial during fasting and starvation, producing glucose from pyruvate, amino acids, and glycerol when glycogen stores are depleted.

Glycolysis and Gluconeogenesis (Diagram)

Diagram illustrates the relationship between glycolysis and gluconeogenesis, highlighting the key enzymes and reaction steps.
Bypassing regulated reactions in glycolysis (i.e. 1,3,10), gluconeogenesis uses different enzymes.

Energy Cost of Gluconeogenesis

Glucose synthesis during gluconeogenesis costs 4 ATP, 2GTP, and 2 NADH.
This process is necessary for maintaining glucose levels in the body.

Regulation of Blood Glucose

Insulin and glucagon, hormones released by the pancreas, regulate blood glucose.
Insulin promotes glucose uptake by cells after meals.
Glucagon signals the liver to release glucose during fasting.

The Biochemistry of Extreme Activity

Intense activity initially uses available ATP and creatine phosphate and then glycolysis.
Anaerobic glycolysis occurs until lactate buildup causes muscle fatigue.

The Biochemistry of Longer Exercise

Maintaining oxygen supply is crucial for long-duration exercise.
Aerobic glycolysis produces ATP.
ATP is generated solely through glycolysis during the anaerobic threshold.

Lactate and the Cori Cycle

Lactate from muscles travels to the liver.
The liver converts pyruvate to glucose, which returns to the muscles.

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Description

Test your knowledge on carbohydrate digestion and metabolism, focusing on the roles of various enzymes like salivary α-amylase and glycolysis. This quiz covers reactions, hormones involved, and consequences of disorders such as lactose intolerance. Challenge yourself with these key concepts of biochemistry!