Reading #1 Introduction to Science

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The Metabolic Mill

The Krebs cycle plays a more important role than simply degrading pyruvate produced during glucose catabolism. Fragments from other organic compounds formed from fat and protein breakdown provide energy during the Krebs cycle.

The “metabolic mill” (Figure 12) depicts the Krebs cycle as the essential "connector" between energy from food macronutrients energy and chemical energy of ATP. The Krebs cycle also serves as a metabolic hub to provide intermediates to synthesize bionutrients for maintenance and growth. For example, excess carbohydrates provide the glycerol and acetyl fragments to synthesize triglyceride. Acetyl–CoA also functions as the starting point for synthesizing cholesterol and many hormones. In contrast, fatty acids do not contribute to glucose synthesis because pyruvate's conversion to acetyl-CoA does not reverse (notice the one-way arrow in Figure 12). Many of the carbon compounds generated in Krebs cycle reactions also provide the organic starting points for synthesizing nonessential amino acids. Amino acids, particularly alanine with carbon skeletons resembling Krebs cycle intermediates after deamination becomes synthesized to glucose.

Fats Burn in a Carbohydrate Flame

Interestingly, fatty acid breakdown depends in part on a continual background level of carbohydrate breakdown. Recall that acetyl–CoA enters the Krebs cycle by combining with oxaloacetate to form citrate. Depleting carbohydrate decreases pyruvate production during glycolysis. Diminished pyruvate further reduces Krebs cycle intermediates, slowing Krebs cycle activity. Fatty acid degradation in the Krebs cycle depends on sufficient oxaloacetate availability to combine with the acetyl-CoA formed during b-oxidation. When carbohydrate level decreases, the oxaloacetate level may become inadequate. In this sense,fats burn in a carbohydrate flame.

For Your Information

Excess Protein Accumulates Fat

Athletes and others who believe that taking protein supplements add to muscle beware. Extra protein consumed above what the body requires ends up as body fat. If an athlete desires to become fat, excessive protein intake achieves this end. A protein excess does not contribute to the synthesis of muscle tissue.

Macronutrients In Excess Readily Convert To Fat

Excess energy intake from any fuel source can be counterproductive. Too much of any macronutrient results in accumulation of body fat. Surplus dietary carbohydrate first fills the glycogen reserves. Once these reserves fill, excess carbohydrate converts to triglycerides for storage in adipose tissue. Excess dietary calories as fat move easily into the body’s fat deposits as does any protein excess. Excess amino acids readily convert to fat.

Lecture #5 Workbook

Define Key Terms and Concepts

  1. Adenosine triphosphatase

  2. Aerobic

  3. Amino acid

  4. Anaerobic

  5. Coupled reactions

  6. Enzymes

  7. Free fatty acids

  8. Glycerol

  9. Glycolysis

  10. High-energy phosphate

  11. Krebs Cycle

  12. Lactate

  13. Metabolic mill

  14. Muscle glycogen

  15. PCr

  16. Pyruvate

  17. The metabolic mill

Study Questions

ATP and Phosphate Bond Energy

ATP – The Energy Currency

Complete the reaction:

ATP + H2O –––––––––>

ATP: A Limited Currency

Complete the following two equations.

ATP + H2O ––––––––>

PCr + H2O ––––––––>

Phosphocreatine (PCr): The Energy Reservoir

What main function does PCr play in energy metabolism?

Cellular Oxidation

“For every reaction involving cellular oxidation, there is a reaction involving __________________”.

Electron Transport

Briefly describe the main purpose of electron transport.

Oxidative Phosphorylation

Complete the following chemical equation:

NADH + H+ + 3ADP + 3 P + 1/2 O2 –––––––>

Role of Oxygen in Energy Metabolism

“The main role of oxygen in energy metabolism is to _______________________________________”.

Energy Release from Carbohydrate

Write the equation for the complete breakdown (hydrolysis) of one mole of glucose.

Anaerobic versus Aerobic

The two stages of carbohydrate breakdown are called ________________________ and _________________________.

Anaerobic Energy from Glucose: Glycolysis (Glucose Splitting)

Glycolysis occurs in what part of the cell?

Formation of Lactic Acid

Write the chemical formula for lactic acid.

Aerobic Energy From Glucose: The Krebs cycle

Give the most important function of the Krebs cycle.

Net Energy Transfer From Glucose Catabolism

Give the total ATP from glucose catabolism.

Energy Release From Fat

Complete the following equation:

Triglyceride + 3H2O –––––––––>

Breakdown of Glycerol and Fatty Acids

Which substance, glycerol or fatty acid, undergoes beta oxidation?


How many molecules of ATP synthesize when one glycerol molecule breaks down?

Fatty Acids

Of what importance is oxygen in fatty acids catabolism?

Total Energy Transfer From Fat Catabolism

How many molecules of ATP become synthesized in the complete combustion of a neutral fat molecule?

Energy Release From Protein

After nitrogen removal from an amino acid, what happens to the remaining “carbon skeleton” in energy metabolism?

The Metabolic Mill

Why is the Krebs cycle so important?

Fats Burn in a Carbohydrate Flame

Explain why “fats burn in a carbohydrate flame.”

Excess Macronutrients Convert To Fat

Can protein consumed in excess of the body’s energy requirement end up as stored fat? Explain.

Lecture #6

Energy Transfer During Exercise


Three major factors affect differences in the magnitude of energy transfer capacity:

In sprint running, cycling, and swimming, energy output can increase 120 times above resting metabolism. In contrast, during less intense but sustained marathon running, for example, energy requirements still exceed the resting level by 20 to 30 times. This chapter explains how the body’s diverse energy systems interact during rest and different exercise intensities.

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