Biochemistry Chapter on Carbohydrate Digestion

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Questions and Answers

What enzyme is responsible for hydrolyzing lactose?

Dextrinase

Sucrase

Lactase (correct)

Maltase

Which of the following statements about cellulose is true?

Humans can digest cellulose due to β(1-4) glucosidase.

Cellulose is fully digested and not excreted.

Cellulose is digested by microbes in ruminants. (correct)

Cellulose contributes to harder feces by absorbing water.

Which disaccharidase hydrolyzes maltose?

Isomaltase

Maltase (correct)

Lactase

Sucrase

In which part of the digestive system does the final carbohydrate digestion take place?

Small intestine Signup and view all the answers

What is the primary end product of carbohydrate digestion that is absorbed into the portal veins?

Monosaccharides Signup and view all the answers

Which mechanism is NOT involved in the absorption of monosaccharides?

Osmosis Signup and view all the answers

What does α-dextrinase hydrolyze?

Isomaltose Signup and view all the answers

How do fructose and galactose transform into glucose in the liver?

Through conversion Signup and view all the answers

What is the primary fate of absorbed fructose and galactose in the liver?

They are converted to glucose. Signup and view all the answers

What is produced during the hexose monophosphate pathway?

Ribose, deoxyribose, and NADPH + H+ Signup and view all the answers

Which of the following is a major pathway for glucose oxidation?

Glycolysis Signup and view all the answers

In which tissues is glycolysis considered physiologically important?

Mature RBCs and intestines Signup and view all the answers

What is the initial energy cost in the first stage of glycolysis?

2 ATP molecules Signup and view all the answers

What is the primary function of transport proteins?

To transport specific molecules or ions across membranes Signup and view all the answers

What byproducts are formed during anaerobic glycolysis?

Lactate Signup and view all the answers

How does the sodium-dependent glucose transporter (SGLT-1) facilitate glucose transport?

By coupling sodium transport with glucose transport using energy Signup and view all the answers

Which product is formed through lipogenesis from absorbed sugars?

Fat Signup and view all the answers

Which substance is not a product of carbohydrate metabolism?

Triglycerides Signup and view all the answers

What is the role of the enzyme adenosine triphosphatase (ATPase) in active transport?

To catalyze the hydrolysis of ATP for sodium pump energy Signup and view all the answers

What type of transport does GLUT-5 specifically facilitate?

Facilitated passive transport for fructose Signup and view all the answers

What happens when insulin is present in tissues with insulin receptors?

Insulin increases the number of glucose transporters Signup and view all the answers

Which molecule is primarily expelled outside the cell by the sodium pump?

Sodium Signup and view all the answers

What is an effect of ouabain on active transport?

It inhibits the sodium pump by blocking ATPase Signup and view all the answers

What characterizes passive transport as facilitated diffusion?

It allows molecules to move from high to low concentration without energy Signup and view all the answers

What is the affinity of liver cells to glucose?

Low affinity, it acts only in high blood glucose Signup and view all the answers

Which substrates can enter all tissue cells for metabolism?

Glucose, galactose, and fructose Signup and view all the answers

How does insulin affect glucokinase in liver cells?

Induces synthesis of glucokinase Signup and view all the answers

What effect does glucose-6-phosphate have on hexokinase in all tissue cells?

Inhibits hexokinase allosterically Signup and view all the answers

What function does glucose serve in the liver after meals?

It is converted into glucose-6-phosphate for metabolic processes Signup and view all the answers

What is the net ATP yield from anaerobic glycolysis?

2 ATP Signup and view all the answers

Which of the following is NOT a function of glycolysis?

Storage of fatty acids Signup and view all the answers

During aerobic glycolysis, how many ATP molecules are ultimately produced?

6 ATP Signup and view all the answers

Which intermediate produced from glycolysis is directly involved in lipogenesis?

Dihydroxyacetone phosphate Signup and view all the answers

How does 2,3 bisphosphoglycerate affect hemoglobin's affinity for oxygen?

Decreases affinity Signup and view all the answers

Which of the following statements regarding pyruvate is true?

It gives rise to acetyl CoA in the Krebs cycle. Signup and view all the answers

What are the two pathways through which NAD+ can be regenerated during glycolysis?

Respiration and lactate formation Signup and view all the answers

Which enzyme is primarily involved in the phosphorylation of glucose to glucose-6-phosphate?

Hexokinase Signup and view all the answers

What is the primary function of SGLT-1?

Absorption of glucose by active transport Signup and view all the answers

What characterizes congenital lactase deficiency?

It is a rare condition that occurs soon after birth Signup and view all the answers

Which statement about GLUT-2 transporters is true?

They transport glucose out of intestinal and renal cells to circulation Signup and view all the answers

What is a common effect of lactose remaining undigested in the intestine?

Increased osmotic pressure leading to diarrhea Signup and view all the answers

Which condition is characterized by a slower absorption of glucose and galactose due to a defect in the carrier mechanism?

Monosaccharide malabsorption Signup and view all the answers

What is the typical treatment for lactase deficiency?

Removal of lactose from diet Signup and view all the answers

What causes increased fermentation of lactose by intestinal bacteria?

Presence of undigested lactose Signup and view all the answers

In which locations are GLUT-5 transporters primarily active?

Intestines and sperm cells Signup and view all the answers

Study Notes

Metabolism & Bioenergetics Overview

This topic covers carbohydrate metabolism, glucose metabolism, the citric acid cycle, and oxidative phosphorylation.

Carbohydrate Metabolism Learning Outcomes

Differentiate various carbohydrate metabolism pathways.

Carbohydrate Metabolism Topics

Introduction to metabolism
Digestion, Absorption, and Transport of carbohydrates
- Different types of digestive enzymes
- Different types of transport systems in absorption
- Types of defects and abnormalities in these systems
Glycolysis
Gluconeogenesis
Pentose Phosphate Pathway

Cellulose Digestion

Cellulose contains β(1-4) bonds between glucose molecules.
Humans lack the β(1-4) glucosidase to digest these bonds, so cellulose passes through the digestive system.
Cellulose promotes water retention in the intestine, producing larger, softer feces, and preventing constipation.
Ruminants and some monogastric animals can digest cellulose with the help of microbial cellulase.

Carbohydrate Absorption

The end products of carbohydrate digestion (glucose, galactose, and fructose) are absorbed from the jejunum to portal veins and the liver.
Fructose and galactose are converted to glucose.
Absorption mechanisms include active transport (against the concentration gradient) and facilitated diffusion.
Transport proteins are integral membrane proteins facilitating molecule or ion transport across biological membranes.

Mechanisms of Glucose Absorption

Active transport:
- Sodium-dependent glucose transporter (SGLT-1) uses energy from the sodium-potassium pump to transport glucose into cells from the intestinal lumen.
- Sodium is then transported out of the cell against its concentration gradient, maintaining the driving force for glucose transport.
- Insulin increases glucose transporter numbers in insulin-sensitive tissues.
Inhibitors of active transport:
- Ouabain (cardiac glycoside): inhibits adenosine triphosphatase (ATPase), thus affecting the sodium pump.
- Phlorhizin: inhibits the sodium binding to the carrier protein.
Passive transport (facilitated diffusion): Utilizes concentration gradient, no energy required.
- Sodium-independent GLUT transporters (GLUT-2, GLUT-5) facilitate fructose and other monosaccharide transport
The end products are transported out of the cell into the circulation.

Glucose Transporter Summary

SGLT-1: Absorption of glucose by active transport (uses Na⁺-K⁺ pump). Located in the intestines and renal tubules.
GLUT-5: Transports fructose and, to a lesser extent, glucose and galactose. Located in the intestines and sperm.
GLUT-2: Transports glucose out of intestinal and renal cells to circulation. Located in the intestines, renal tubules, β cells of islets and liver.

Carbohydrate Digestion Defects

Lactase deficiency (lactose intolerance):
- Deficiency of lactase, which digests lactose into glucose and galactose
- Can be congenital or acquired.
- Results in lactose in the large intestine, where bacteria ferment it causing increased osmotic pressure (water shifts), abdominal cramps, and diarrhea.
Sucrose deficiency: A rare condition resembling lactase deficiency, affecting the digestion and absorption of sucrose
Monosaccharide malabsorption: A congenital condition results in slow absorption of glucose and galactose due to a transport defect. Fructose absorption and digestion is unaffected.

Fate of Absorbed Sugars

Oxidation: Glucose is oxidized via glycolysis and the Krebs cycle to produce energy.
Storage: Glycogen storage and lipogenesis (fat storage)
Conversion: Glucose converts to other vital molecules like ribose, deoxyribose, RNA, DNA, glucosamine, galactosamine, mucopolysaccharides, glucoronic acid, and fructose, a compound present in semen.

Glycolysis (Embden-Meyerhof Pathway)

Breakdown of glucose to pyruvate under aerobic or anaerobic conditions.
Location: Cytoplasm of all cells.
Importance in tissues without/poor mitochondria, high oxygen demand tissues like muscles, and cells with no mitochondria like mature RBCs.
Steps:
- Energy investment phase (needs energy, 2 ATP consumed)
- Energy payoff phase (produces ATP and energy, 4 ATP produced).
Energy production: Net gain of 2 ATP and 2 NADH per glucose molecule.

Anaerobic Respiration (Fermentation)

Absence of oxygen.
Lactate fermentation: Occurs in muscle, producing lactate as a by-product.
Alcohol fermentation: Occurs in yeast and some bacteria, producing ethanol and CO2.

Pyruvate Oxidation

Pyruvate is converted to Acetyl CoA, which enters the Krebs cycle (aerobic conditions)
A carboxyl group is removed as CO₂
An electron pair from the remaining two-carbon fragment is transferred to NAD⁺, forming NADH.
The resulting acetate combines with coenzyme A (CoA) into acetyl CoA.

Aerobic vs. Anaerobic Glycolysis

Aerobic glycolysis ends with pyruvate, which enters the mitochondria to produce more ATP in the Krebs cycle and electron transport chain.
Anaerobic glycolysis ends with lactate formation, which regenerates NAD+ to sustain glycolysis.

Glucokinase and Hexokinase Comparison

Glucokinase: Primarily in liver, low affinity for glucose, induced by high blood glucose, doesn't inhibit glucose-6-phosphate.
Hexokinase: A more general enzyme throughout tissues, high affinity for glucose, inhibited by glucose-6-phosphate.

Key takeaway

Glycolysis is a fundamental metabolic pathway for energy production in all cells, with different outcomes depending on oxygen availability.

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Description

Test your knowledge on enzyme functions, carbohydrate digestion, and absorption processes with this quiz focused on biochemistry. Answer questions about lactose, maltose, glucose, and the metabolic pathways involved. This quiz is perfect for students studying biochemistry and nutrition.