Your Body’s Metabolism

© 2010 Pearson Education, Inc.

Your Body’s Metabolism


Metabolism

Sum of all chemical reactions in the body’s cells • Generation of energy from carbohydrates, proteins, and

fats- Anaerobically- Aerobically

• Production of biological compounds- Nonessential amino acids- Intermediate substances needed for metabolism


Metabolism

Never stops Adapts to individual needs and the environment Has several metabolic pathways

• Glycolysis• TCA cycle• Electron transport chain

Overview of Energy Metabolism

Figure 8.1


Chemical reactions involved in energy production Different cells perform different functions Each cell’s structure in similar

Metabolism in the Cell

Metabolism Takes Place within Cells

Figure 8.2


Cell Structure

Cell construction is similar for all cells Outside of cell

• Plasma membrane

- Holds in the cell contents Inside of cell

• Includes several special internal structures: organelles


Internal Cell Structure

Organelles

• Mitochondrion - “Powerhouse of the cell”

- Aerobic metabolism

• Ribosomes- Help manufacture proteins

• Smooth endoplasmic reticulum- Produces lipids

Cytosol

• Fluid portion of cell

• Anaerobic metabolism


Is the most metabolically active organ in the body First organ to metabolize, store, and distribute nutrients

after absorption Proteins, carbohydrates, and fats are absorbed as:

• Amino acids

• Monosaccharides

• Glycerol and fatty acids

Liver


Proteins, carbohydrates, and fats are in the liver converted to

• Usable forms of energy

• Storage forms

- Glycogen

- Triglycerides

Liver

The Metabolic Fate of Food

Figure 8.3


Metabolic Pathways

A sequence of reactions that convert compounds from one form to another in the production of energy

Different nutrients follow different pathways All pathways eventually converge into a pathway called the

TCA cycle

Anabolic and Catabolic Reactions

Figure 8.4


Enzymes and Hormones

Enzymes allow chemical reactions of metabolism to occur at rates sufficient to maintain normal body function

Coenzymes assist enzymes Hormones regulate anabolic and catabolic reactions


Quick Review

Metabolism is the sum of all metabolic processes that occur in the cells

Mitochondria is the site of most of the metabolic reactions Metabolic processes follows specific pathways

• Anabolic which use energy to build new substances

• Catabolic which produce energy by breaking down molecules

Enzymes and coenzymes catalyze reactions Hormones regulate reactions


Energy Drinks: Can They Alter Metabolism?

Main ingredient is caffeine

• Promotes lipolysis

• Overall excess can cause negative health effects

- Elevated heart rate and blood pressure

- Anxiety

- Diminished ability to concentrate

- Insomnia

Limit caffeine

intake to no more than

300 milligrams

per day


Energy Drinks: Can They Alter Metabolism?

Mixing energy drinks with alcohol doubles

• Risk of injury

• Need for medical attention

• Driving with intoxicated drivers


How Does the Body Fuel Metabolism?


Adenosine Triphosphate (ATP)

A high-energy molecule composed of adenine, ribose, and three phosphate molecules

Part of Figure 8.5


Adenosine Triphosphate (ATP)

Only source of energy used directly by the cell Energy is stored in the bonds that connect the phosphate

groups The body must continually produce ATP to provide a

constant supply of energy

Figure 8.5

ATP to ADP (Adenosine Diphosphate)


Creating ATP from ADP and Creatine Phosphate

Regenerating ATP from ADP requires inorganic phosphate Sources

• Inorganic phosphate produced from initial breakdown of ATP

• Inorganic phosphate in creatine phosphate (a.k.a. phosphocreatine or PCr)

Process requires energy


Creatine Phosphate

High-energy compound in muscle cells Creatine combine plus inorganic phosphate


Creatine Phosphate

Creatine monohydrate – a supplement sold in stores

• Marketed to athletes to maximize PCr stores

• Research

- Increased performance of short-duration, high-intensity activities

- Side effects


Anaerobic and Aerobic Metabolism

Anaerobic metabolism

• Produces more ATP per minute

• Limited in use, provides only 1–1.5 minutes of maximal activity

• Involved in high-intensity, short-duration activities, e.g., sprinting, heavy weight lifting


Anaerobic and Aerobic Metabolism

Aerobic metabolism

• Produces less ATP per minute

• Is able to produce ATP indefinitely

• Involved in low-intensity, long-duration activities

When demand for ATP is greater than the rate of metabolism the activity slows down


Quick Review

ATP – energy the body uses to fuel all metabolic reactions ATP is not stored

• Formed from ADP and inorganic phosphate

- Creatine phosphate can donate inorganic phosphate

• Produced during anaerobic metabolism

• Produced during aerobic metabolism


Carbohydrates to Energy

Carbohydrate metabolism is the backbone of energy production

Glucose

• Important energy source for the brain and red blood cells

• Generates energy anaerobically and aerobically

• Transforms to energy via four metabolic pathways

- Glycolysis

- Intermediate reaction pyruvate to acetyl CoA

- Tricarboxylic acid (TCA) cycle

- Electron transport chain


Glycolysis

Breakdown of glucose First step in forming ATP from glucose Takes place in the cytosol of the cell Ten-step catabolic process One six-molecule glucose process to two three-carbon

molecules of pyruvate and two molecules of ATP


Glycolysis

The other monosaccharides can also be used to produce ATP• Fructose enters glycolysis after going through seven

metabolic steps• Galactose enters after going through four metabolic steps


Glycolysis Summary

Ten-step process that produces • Two molecules of ATP• Two molecules of pyruvate• Two energized coenzyme molecules• Two hydrogen ions that are transported to the electron

transport chain• Two water molecules

The Fate of Pyruvate

Figure 8.7

The Cori Cycle

Figure 8.8


Glycolysis

• Process by which carbohydrates provide energy to the cell

• Backbone of metabolism

• One glucose molecule yields - Two pyruvate

- Two ATP

- Two energized coenzymes

Pyruvate

• Reduce to lactate during anaerobic metabolism

• Converted to acetyl CoA during aerobic metabolism

- Two hydrogen ions

- Two water molecules

Quick Review


Fats to Energy

Dietary fat (triglycerides) yields six times more energy Triglycerides

• Glycerol backbone

• Three fatty acids

• Stored in adipose tissue Glycerol and fatty acids can be used for fuel Glycerol produces little energy


Triglycerides

Hydrolyzed to fatty acids and glycerol during lipolysis

• Reaction catalyzed by an enzyme in the adipose tissue

- Glucagon during times of fasting or starvation

- Epinephrine or cortisol when under stress Once in the blood stream, they travel to the tissues and

enter the metabolic pathway


Fatty Acids and Glycerol

Glycerol

• Glucogenic – can be transformed to glucose Fatty acids

• Ketogenic – can be transformed to ketone bodies

• Ketones are the backup fuel for brain and nerve function when glucose is limited


Glycerol

In the liver

• Converted to glucose through gluconeogenesis

• Enters glycolysis to produce ATP and pyruvate

• Path entered depends on body’s need for glucose Brains and nerves prefer to use glucose for fuel Red blood cells can only use glucose for fuel When the diet is low in carbohydrates glucose must come

from other sources


Quick Review

Both the glycerol and fatty acid portions of triglycerides provide energy

Fatty acids are the more concentrated sources of energy and are ketogenic, nonglucose forming

Glycerol is glucogenic forming glucose through gluconeogenesis


How Does Protein Provide Energy?

Amino acids

• Primary use/most important function is building protein

• If the amino acid is not used to build protein the amine group must be removed through deamination

• Excess can be used for energy production, converted to glucose, or stored as fat

• Used, in a limited extent, for energy in diets low in kilocalories and/or carbohydrate


Ketogenic verses Glucogenic Amino Acids

Ketogenic amino acids

• Leucine

• Lysine Both ketogenic and glucogenic amino acids

• Isoleucine

• Tryptophan

• Phenylalanine

• Tyrosine Glucogenic amino acids

• The fourteen other amino acids

Acetyl CoA cannot be used to make glucoseFigure 8.10

Glucogenic and Ketogenic Amino Acid Metabolism


Amino Acids to Glucose

Glucogenic amino acids

• Major source of blood glucose when the diet is lacking in carbohydrate

• Can come from food

• Can come from the breakdown of muscle


Quick Review

Amino acids can

• Be used to produce energy

• Be used to produce glucose

• Convert to fatty acids and can be stored as triglycerides Must be deaminated to be used for energy Once deaminated can be transformed into:

• Pyruvate

• Acetyle CoA

• TCA cycle compounds Glucogenic amino acids can be converted to glucose


Where Do the Macronutrients Come Together?

Acetyl CoA

• “Gateway” molecule for aerobic metabolism

• Carbohydrates, proteins, and fat all eventual converted to acetyl CoA

• Alcohol also converted to acetyl CoA

• Macronutrients enter the TCA cycle as acetyl CoA


The Tricarboxylic Acid (TCA) Cycle

Gathers electrons from the carbons in the energy nutrients Transfers stored energy to two coenzyme hydrogen ion

carriers to be released in the electron transport chain One molecule of acetyl CoA enters the TCA cycle at a time

The TCA Cycle

Figure 8.11


The Tricarboxylic Acid Cycle

Each cycle produces

• Two carbons that are lost to CO2

• Eight hydrogen atoms and their electrons

• Small amount of energy as GTP Provides the starting material for creating nonessential

amino acids through transamination


Transamination

The transfer of an amino group from one amino acid to an alpha-keto acid to form a new nonessential amino acid

Carbon skeleton for nonessential amino acids come from

• Pyruvate from glycolysis

• Alpha-ketoglutarate from the TCA cycle

Figure 8.12

Transamination


The Electron Transport Chain

Final stage of metabolism when electrons are transferred from one complex to another, resulting in the formation of ATP and water

Generates 90% of the ATP used by the body for energy, growth, and maintenance

Takes place in the inner mitochondrial membrane of the cell


Figure 8.13



Flavoproteins

• Protein complexes that transfer the electrons through the electron transport chain

• Contain B vitamin riboflavin



Cytochromes

• Protein complexes that move electrons down the electron transport chain

• Contain iron and copper

• Iron deficiency can slow down this metabolic process

Though vitamins and minerals do not provide energy, they are essential for energy production

Figure 8.14

Overview of Energy Metabolism


Quick Review Acetyl CoA is the gateway molecule for all energy nutrients Acetyl CoA combines with oxaloacetate to form citrate in

the first step of the TCA cycle The TCA cycle produces

• Two energized coenzymes

• Two molecules of CO2

• Small amount of energy as GTP Electrons from hydrogen atoms in coenzymes enter the

electron transport chain Protons are used to form ATP during the electron transport

chain


What Happens if You Eat Too Much or Too Little

Metabolism adjusts

• Anabolic reactions store excess kilocalories as glycogen and adipose tissue

• Catabolic reactions use glycogen, adipose tissue, and proteins for energy

Metabolism Adapts during Feasting or Fasting

Figure 8.15


If You Don’t Eat Enough

With continued fasting

• Ketone bodies from fatty acids used as an alternative fuel source

• Ketogenesis peaks three days into fasting or limited carbohydrate intake

Ketoacidosis, often seen in type 1 diabetics, can result from a build-up of excess ketone bodies in the blood and cause

• Impaired heart activity

• Coma

• Death


Quick Review

Ingestion of excess kilocalories

• Anabolic metabolism

• Excess energy from carbohydrates, proteins, fats, and alcohol are converted to fat and stored

Fasting or starvation

• Catabolic metabolism

• Fat is broken down to fatty acids

• Glycerol and amino acids maintain blood glucose

• Lack of glucose leads to formation of ketone bodies for energy


How Does the Body Metabolize Alcohol?

Alcohol

• Contains 7 kilocalories per gram

• Absorbed directly through the stomach mucosa and intestinal lining

• Metabolized by the liver, about half an ounce per 1.5 hours

• Excess stored as fat in the adipose tissue and liver

Figure 8.17

The Metabolism of Alcohol


Quick Review

Alcohol is absorbed and metabolized in the liver by two enzyme systems

• ADH – most efficient

• MEOS

- Used when intake is high

- Also used for drugs Alcohol is also metabolized in the brain Excess energy from alcohol is converted to fatty acids and

stored as triglycerides


What Are Inborn Errors of Metabolism?

Genetic conditions in which an individual lacks an enzyme that controls a specific metabolic pathway

Results in the buildup of toxins Cannot be cured Can be controlled through diet


Quick Review

Inborn errors of metabolism are the result of a defective gene

Treatment includes strict dietary regimen to control symptoms while providing adequate nutrition

Your Body’s Metabolism

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