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Carbohydrate Metabolism

Chapter 34

Carbohydrate Metabolism

Chapter 34

Hein * Best * Pattison * Arena

Colleen KelleyChemistry DepartmentPima Community College

© John Wiley and Sons, Inc.

Version 1.0

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Chapter Outline

34.1 Metabolic Pathways

34.2 Exercise and Energy Metabolism

34.3 The Bloodstream: A Metabolic Connection

34. 4 Anaerobic Sequence

34.5 Citric Acid Cycle

(Aerobic Sequence)

34.6 Gluconeogenesis

34.7 Overview of Complex Metabolic Pathways

34.8 Hormones

34.9 Blood Glucose and Hormones

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Metabolic PathwaysMetabolic Pathways

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A metabolic pathway is a series of biochemical reactions that serve a specific purpose.

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Exercise and Exercise and Energy MetabolismEnergy Metabolism

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• Everything we do requires metabolic energy.

• Metabolism is a complex interplay: chemical reactions within cells lead to chemical transport between cells.

• Carbohydrate catabolism is designed to release energy relatively quickly, so this form of catabolism is activated during strenuous muscular exercise.

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• When a muscle contracts, energy is consumed.

• Muscle contraction uses ATP; ATP is in short supply.

• Muscle tissue can contract for no more than several seconds before the supply of high-energy phosphate bonds is depleted.

Muscle Contraction

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• After the initial contraction, the muscle cells look for other energy sources.

• Muscle glycogen is the next available source.• This polymer breaks down to glucose, which is

oxidized to replenish the ATP supply.• Because glucose oxidation is a complex process,

muscle contraction must proceed at a slower rate.• This energy supply is only useful for about 2

minutes of work; muscles rapidly deplete their glycogen stores and build up lactic acid.

Muscle Contraction

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The Bloodstream: A The Bloodstream: A Metabolic ConnectionMetabolic Connection

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• The bloodstream transports chemicals from one cell to another.

• Nutrients (e.g. glucose, amino acids, and fatty acids) and oxygen are delivered; metabolic products (e.g. lactate) and carbon dioxide are removed.

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Figure 34.1 Overview of carbohydrate metabolism

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• When blood glucose is in excess, it is converted to glycogen in the liver and in muscle tissue.

• Glycogen is a storage polysaccharide; it quickly hydrolyzes to replace depleted glucose supplies in the blood.

• The synthesis of glycogen from glucose is called glycogenesis.

• The hydrolysis, or breakdown, of glycogen to glucose is known as glycogenolysis.

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Anaerobic SequenceAnaerobic SequenceAnaerobic SequenceAnaerobic Sequence

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• In the absence of oxygen, glucose in living cells is converted to a variety of end products, including lactic acid (in muscle) and alcohol (in yeast).

• At least a dozen reactions, many different enzymes, ATP, and inorganic phosphate (Pi) are required.

• Such a sequence of reactions from a particular reactant to end product is called a metabolic pathway.

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Embden-Myerhof pathway

• The anaerobic conversion of glucose to pyruvate is known as the Embden-Myerhof pathway.

• The sequence is a catabolic one in which glucose is oxidatively degraded.

– D-glucose 2 pyruvate

Embden-Meyerhof pathway

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Glycolysis• When lactate is the final product of

anaerobic glucose catabolism, the pathway is termed glycolysis.

• What glycolysis does for the cell can be summarized with the following net chemical equation:

C6H12O6 + 2 ADP + 2 Pi 2CH3CH(OH)COO- + 2 ATP + 150 kJ

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Citric Acid CycleCitric Acid Cycle(Aerobic Sequence)(Aerobic Sequence)

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• Since only a small fraction of the energy that is potentially available from glucose is liberated during anaerobic conversion to lactate (glycolysis), lactate remains valuable to the cells.

• The lactate formed may be:1) Circulated back to the liver and converted to

glycogen at the expense of some ATP 2) Converted back to pyruvate in order to enter

the citric acid cycle.

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Figure 34.3

The citric acid cycle (Krebs cycle). During one cycle, (1) the carbons marked in blue enter the cycle, and (2) the carbons marked in red are lost as CO2.

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Energy Summation

glucose

glycolysis

2 lactate

citric acid cycle, e- transport, oxidative phosphorylation

6 CO2

2 ATP

+ 30 ATP

32 ATP

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GluconeogenesisGluconeogenesis

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Gluconeogenesis

• The formation of glucose from noncarbohydrate sources is called gluconeogenesis.

• Most of the glucose formed during gluconeogenesis comes from lactate, certain amino acids, and the glycerol of fats.

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Figure 34.4 An overview of gluconeogenesis. All transformations except lactate to pyruvate require a series of reactions.

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Overview of Overview of Complex Metabolic Complex Metabolic

PathwaysPathways

Overview of Overview of Complex Metabolic Complex Metabolic

PathwaysPathways

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Figure 34.5 A single-step oxidation process compared with a multiple process: In the pathway, A, B, and C represent hypothetical pathway intermediates.

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HormonesHormonesHormonesHormones

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Hormones• Hormones are chemical substances that act

as control agents in the body, often regulating metabolic pathways.

• Hormones help to adjust physiological processes such as:– digestion– metabolism– growth– reproduction

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Hormones• Hormones are often called the chemical

messengers of the body.

• They do not fit into any single structural classification:

• Polypeptides or proteins• Steroids• Phenol or amino acid derivatives

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Blood Glucose and Blood Glucose and HormonesHormones

Blood Glucose and Blood Glucose and HormonesHormones

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• An adequate blood-glucose level must be maintained to ensure good health.

• To achieve this goal, hormones regulate and coordinate metabolism in specific organs.

• The hormones control selected enzymes, which, in turn, regulates the rates of reaction in the appropriate metabolic pathways.

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Figure 34.7

Conditions related to the concentration of glucose in the blood.

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Figure 34.8 Typical responses to a glucose-tolerance test.

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