Carbohydrates in Human Biology

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Questions and Answers

What type of linkage in cellulose makes it indigestible for humans?

γ-glucosidic linkage

β-glucosidic linkage (correct)

δ-glucosidic linkage

α-glucosidic linkage

Which enzyme starts the digestion of starch in the mouth?

Salivary amylase (correct)

Sucrase

Pancreatic amylase (correct)

Lactase

What is the primary fate of excess glucose in the body?

Conversion to triacylglycerols (correct)

Conversion to amino acids

Conversion to lactic acid

Conversion to ribose

What process occurs after glycolysis in aerobic conditions?

Krebs' cycle Signup and view all the answers

Which disaccharide is broken down by lactase?

Lactose Signup and view all the answers

Where is glycogen primarily stored in the body?

Liver and muscles Signup and view all the answers

What condition leads to glucose being detectable in urine?

Exceeding renal threshold Signup and view all the answers

How is glucose absorbed in the intestines?

Active transport Signup and view all the answers

What is the primary function of liver glycogen?

Maintains blood glucose levels between meals Signup and view all the answers

Which process breaks down glycogen into glucose?

Glycogenolysis Signup and view all the answers

What is the main function of gluconeogenesis?

To produce glucose from non-carbohydrate sources Signup and view all the answers

What is the starting molecule for the Krebs cycle provided by glycolysis?

Pyruvate Signup and view all the answers

Which enzyme is responsible for glycogenesis?

Glycogen synthase Signup and view all the answers

What happens to liver glycogen after approximately 12-18 hours of fasting?

It is completely depleted Signup and view all the answers

Which of the following is a gluconeogenic precursor?

Glycerol Signup and view all the answers

Muscle glycogen is primarily utilized during which of the following activities?

Prolonged exercise Signup and view all the answers

Which process reverses glycolysis to synthesize glucose?

Gluconeogenesis Signup and view all the answers

What is the role of pyruvate in metabolism?

It leads to the formation of acetyl CoA Signup and view all the answers

What is the function of maltase, sucrase, and lactase in digestion?

They are enzymes that break down carbohydrates. Signup and view all the answers

What is one of the fates of absorbed glucose?

Conversion to glycogen for storage. Signup and view all the answers

What defines glycolysis?

The breakdown of glucose into pyruvate, producing ATP. Signup and view all the answers

What is the primary importance of glycolysis?

It provides energy production from carbohydrates. Signup and view all the answers

What is glycogenesis?

The formation of glycogen from glucose. Signup and view all the answers

What enzyme is responsible for glycogenesis?

Glycogen synthase. Signup and view all the answers

What is glycogenolysis?

The breakdown of glycogen into glucose. Signup and view all the answers

What enzyme is responsible for glycogenolysis?

Glycogen phosphorylase. Signup and view all the answers

Where are the two main sites of glycogen storage in the body?

Liver and muscles. Signup and view all the answers

What is gluconeogenesis?

The synthesis of glucose from non-carbohydrate sources. Signup and view all the answers

Study Notes

Dietary Carbohydrates

Include sucrose (table sugar), lactose (in milk) and starch (found in potatoes, rice, corn and wheat)
Cellulose is present in the cell wall of some plants but is not digested by humans because it contains β-glucosidic linkage, which human enzymes cannot hydrolyze
Cellulose increases the bulk of stools and prevents constipation.

Digestion of Carbohydrates

Mouth: Salivary amylase starts the digestion of starch
Stomach: HCl partially hydrolyzes carbohydrates to monosaccharides.
Small Intestines: Pancreatic amylase completes the digestion of starch
Intestinal disaccharidases digest disaccharides:
- Maltase digests maltose into 2 glucose molecules
- Lactase digests lactose into glucose and galactose
- Sucrase digests sucrose into glucose and fructose

Absorption of Carbohydrates

Absorbed by active transport, which requires energy (ATP) and a transporter.

Fate of Absorbed Glucose

Uptake and Utilization by Different Tissues:
- Oxidation to produce energy: glycolysis, then Krebs’ cycle, and electron transport chain
- Conversion to other substances, including:
  - Other carbohydrates: such as ribose for nucleic acids synthesis
  - Lipids
  - Proteins
Storage of Excess Glucose:
- As glycogen in liver and muscles (glycogenesis)
- Excess glucose is converted to triacylglycerols in the adipose tissue (lipogenesis)
Excretion in Urine:
- Normally, there is no glucose detectable in urine
- If the blood glucose exceeds the maximum capacity of the kidneys to reabsorb glucose (>180 mg/dL), it will be excreted in urine

Glycolysis

A group of reactions by which glucose is converted to pyruvate (in aerobic conditions) or lactate (in anaerobic conditions).
Glucose is the major source of energy for all cells.
Glycolysis itself liberates energy from glucose oxidation, and it also provides the mitochondria with pyruvate, which gives acetyl CoA to start the Krebs' cycle.

Glycogen Metabolism

Introduction:
- Glycogen is the main storage form of carbohydrates in animals.
- It is stored mainly in the form of liver glycogen and muscle glycogen.
Liver glycogen:
- Maintains blood glucose, especially between meals
- Is depleted after 12-18 hours of fasting
Muscle glycogen:
- Supplies glucose within muscles during contraction
- Is only depleted after prolonged exercise
Definitions:
- Glycogenesis: synthesis of glycogen from glucose; by glycogen synthase
- Glycogenolysis: breakdown of glycogen to glucose; by glycogen phosphorylase

Gluconeogenesis

Synthesis of glucose from non-carbohydrate sources (mainly in the liver)
Occurs by reversal of glycolysis
Gluconeogenic precursors include lactate, glycerol, and some amino acids
The main function is to supply blood glucose in case of carbohydrate deficiency (fasting, starvation, and low carbohydrate diet) for more than 18 hours (after glycogen depletion)

Sources of Blood Glucose

Liver glycogen: Provides glucose between meals.
Diet: Provides glucose from carbohydrates.
Gluconeogenesis: Provides glucose from non-carbohydrate sources during prolonged fasting or starvation.

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Description

This quiz covers key concepts related to dietary carbohydrates, their digestion, and absorption in the human body. Explore the roles of different carbohydrates, enzymes involved, and the process of glucose utilization. Perfect for students looking to deepen their understanding of nutrition and human physiology.