Chapter 7 – Metabolism: Transformations and Interactions
Learning Objectives
After completing Chapter 7, the student will be able to:
1.
Discuss the chemical reactions that occur within the body, including metabolism, anabolism, and
catabolism.
2. Describe how carbohydrates, proteins, and fats are used to meet the energy needs of the body.
3. Explain the process of glycolysis.
4. Explain the process of deamination and the synthesis of non-essential amino acids.
5. Discuss the TCA cycle and the electron transport chain.
6. Explain what happens in the body during feasting and fasting.
7. Discuss the term moderation in reference to alcohol consumption.
8. Explain how the body metabolizes alcohol.
9. Discuss the role of the liver in alcohol metabolism.
10. Discuss the short- and long-term effects of alcohol on health.
I.
Chemical Reactions in the Body
Plants use the sun’s energy to make carbohydrate from carbon dioxide and water. This is called
photosynthesis. Humans and animals eat the plants and use the carbohydrate as fuel for their bodies.
During digestion, the energy-yielding nutrients are broken down to monosaccharides, fatty acids,
glycerol, and amino acids. After absorption, enzymes and coenzymes can build more complex
compounds. In metabolism they are broken down further into energy (ATP), water, and carbon
dioxide.
A.
B.
C.
D.
Metabolic reactions take place inside of cells, especially liver cells.
Anabolism is the building up of body compounds and requires energy.
Catabolism is the breakdown of body compounds and releases energy.
The Transfer of Energy in Reactions—ATP
1. A high-energy compound called adenosine triphosphate (ATP) is made.
2. Coupled reactions are chemical reactions that occur simultaneously.
E. Enzymes and coenzymes are helpers in reactions.
1. Enzymes are protein catalysts that cause chemical reactions.
2. Coenzymes are organic molecules that function as enzyme helpers.
3. Cofactors are organic or inorganic substances that facilitate enzyme action.
II. Breaking Down Nutrients for Energy
The breakdown of glucose to energy starts with glycolysis to pyruvate. Pyruvate may be converted to
lactic acid anaerobically (without oxygen) and acetyl CoA aerobically (with oxygen). Eventually, all
energy-yielding nutrients enter the TCA cycle or tricarboxylic acid cycle (or Kreb’s cycle) and the
electron transport chain.
A. Glucose
1. Glucose-to-pyruvate is called glycolysis or glucose splitting.
2. Pyruvate’s Options—Anaerobic or Aerobic
a. Anaerobic – lactate.
b. Aerobic – acetyl CoA.
3. Pyruvate-to-Lactate
a. Oxygen is not available or cells lack sufficient mitochondria.
b. Lactate is formed when hydrogen is added to pyruvate.
c. Liver cells recycle muscle lactic acid through the Cori cycle.
4. Pyruvate-to-acetyl CoA is irreversible.
5. Acetyl CoA’s Options
a. Synthesize fats when the body has enough ATP.
b. Generate ATP when the cell is low in energy.
B. Glycerol and Fatty Acids
1. The conversion of glycerol to pyruvate is easy because they are both three-carbon
compounds.
2. Fatty acids-to-acetyl CoA reactions are called fatty acid oxidation.
3. Fatty acids cannot be used to synthesize glucose. Glucose must be available to provide energy
to the red blood cells, brain, and nervous system.
C. Amino Acids
1. Amino acids can be converted energy.
2. Amino acids are a fairly good source of glucose when carbohydrate is not available.
D. Breaking Down Nutrients for Energy—In Summary
1. Glucose and fatty acids are primarily used for energy, amino acids to a lesser extent.
2. Glucose is made from all carbohydrates, most amino acids, and the glycerol portion of fat.
3. Protein is made from amino acids.
4. Glucose can be made into nonessential amino acids if nitrogen is present.
5. All energy-yielding nutrients consumed in excess can contribute to fat storage.
E. The Final Steps of Catabolism
1. The TCA cycle contains a 4-carbon compound called oxaloacetate that has a critical role.
2.
3.
The Electron Transport Chain
a. Consumes oxygen.
b. Produces carbon dioxide and water.
c. Produces energy as ATP.
The kCalories-per-Gram Secret Revealed
a. Fat provides 9 kcal/gram.
b. Carbohydrate provides 4 kcal/gram.
c. Protein provides 4 kcal/gram.
d. Fat provides more energy because the bonds in fat molecules are easily oxidized and
result in more ATP.
III. Energy Balance
When energy intake exceeds energy output, there is a gain in weight. Excess energy can come from
protein, fat, or carbohydrate. Fat is the most efficiently stored as fat.
A. Feasting—Excess Energy
1. Excess protein is converted to fat but this is inefficient and indirect. Its priority is other roles.
2. Excess carbohydrate is converted to fat but this is inefficient and indirect. Its priority is
glycogen stores.
3. Excess fat is efficiently converted to fat.
B. The transition from feasting to fasting draws on reserves.
C. Fasting—Inadequate Energy
1. Glucose is needed for the brain and nerve cells.
2. Protein meets glucose needs through amino acids that provide pyruvate.
3. The shift to ketosis occurs when the brain becomes fueled by ketone bodies.
a. Ketones are produced when glucose is not available.
4. Ketosis causes a suppression of the appetite.
5. Hormones slow the metabolism.
6. Symptoms of Starvation:
a. Muscle wasting.
b. Decreased heart rate, respiratory rate, metabolic rate, and body temperature.
c. Impaired vision.
d. Organ failure.
e. Decreased immunity.
f. Depression, anxiety, and food-related dreams.
D. Low-Carbohydrate Diets
1. Result in changes in metabolism similar to what occurs during fasting.
2.
3.
Ketones will be present in the urine when glycogen depletion has occurred.
When a dieter returns to a well-balanced diet, the body will retain depleted nutrients.
IV. Highlight: Alcohol (ethyl alcohol, ethanol) and Nutrition
The metabolism of alcohol is handled differently in the body. Alcohol interferes with metabolism and
impairs health and nutrition. There are potential health benefits to consuming moderate amounts of
alcohol.
A. Alcohol in Beverages
1. Beer, wine, and distilled liquor (hard liquor)
2. Alcohol behaves like a drug, therefore altering body functions.
3. Moderation of drinks
a. 5 ounces of wine
b. 10 ounces of wine cooler
c. 12 ounces of beer
d. 1 ½ ounces distilled liquor (80 proof)
B. Alcohol in the Body
1. Quickly absorbed.
2. Carbohydrates decrease the absorption of alcohol.
3. Alcohol dehydrogenase breaks down alcohol in the stomach.
4. Women absorb more alcohol then men.
C. Alcohol Arrives in the Liver
1. Accumulation of fatty acids.
2. Alcohol dehydrogenase breaks down alcohol to acetaldehyde.
3. Alcohol abuse has damaging effects.
4. Coenzyme NAD.
D. Alcohol Disrupts the Liver
1. Development of a fatty liver is the first stage of liver deterioration.
2. Fibrosis is the second stage.
3. Cirrhosis is the most advanced stage of liver deterioration.
4. Microsomal ethanol-oxidizing system (MEOS) metabolizes alcohol and drugs.
E. Alcohol Arrives in the Brain
1. Alcohol acts as a narcotic, anesthetizes pain
2. Alcohol suppresses antidiuretic hormone (ADH), resulting in the loss of body water.
F. Alcohol and Malnutrition
1. Heavy drinkers may have inadequate food intake.
2. Impaired nutrient metabolism will result from chronic alcohol abuse.
3. Vitamin B6, folate, thiamin deficiencies
4. Wernicke-Korsakoff syndrome is seen in chronic alcoholism.
G. Alcohol’s Short-Term Effects
1. Accidents, fatalities, and breaking the law
2. Binge drinking or heavy drinking can result in death.
H. Alcohol’s Long-Term Effects
1. Abuse during pregnancy.
2. Third leading cause of preventable death.
3.
Health Effects of Heavy Alcohol Consumption
a. Arthritis
b. Cancer
c. Fetal alcohol syndrome
d. Heart disease
e. Hyperglycemia
f. Hypoglycemia
g. Infertility
h. Kidney disease
i. Liver disease
j. Malnutrition
I.
k. Nervous disorders
l. Obesity
m. Psychological disturbances
Personal Strategies
1. Serve and consume nonalcoholic beverages.
2. Drink slowly and consume alcohol moderately.
3. Do not drive.