Metabolism: Transformations and Interactions
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
a. Anaerobic – lactic acid
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.
1
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 concerted to acetyl CoA after deamination.
2. Amino Acids-to-Glucose – a fairly good source of glucose when carbohydrate is not
available
3. Deamination results in two products:
a. Keto acid
b. Ammonia
4. Transamination is the transfer of the amino group from an amino acid to a keto acid.
5. Ammonia is converted to urea—a much less toxic compound—in the liver.
6. Urea is excreted through the kidneys to rid the body of unused nitrogen.
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. The Electron Transport Chain
a. Consumes oxygen
b. Produces carbon dioxide and water
c. Produces energy as ATP
3. 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 efficient in being 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
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 Needed for the Brain
2. Protein Meets Glucose Needs
3. The Shift to Ketosis- produced when glucose is not available
a. Ketosis causes a suppression of the appetite.
b5. Slowing of 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
1.
Define metabolism, anabolism, and catabolism; give an example of each.
2. Name one of the body’s high-energy molecules, and describe how is it used.
3. What are coenzymes, and what service do they provide in metabolism?
4. Name the four basic units, derived from foods, used by the body in metabolic
transformations. How many carbons are in the “backbones” of each?
5. Define aerobic and anaerobic metabolism.
6. How does the body dispose of excess nitrogen?
7. Summarize the steps in the metabolism of glucose, fatty acids, and amino acids.
8. Describe how a surplus of the three energy nutrients contributes to body fat stores.
9. What adaptations does the body make during a fast? What are ketone bodies? Define
ketosis.
10. Distinguish between a loss of fat and a loss of weight, and describe how both might
happen.
3
Anabolic or Catabolic?
Metabolism explains how the cells in the body use nutrients to meet its needs. Cells may start
with small, simple compounds and use them as building blocks to form larger, more complex
structures (anabolism). These anabolic reactions involve doing work and so require energy.
Alternatively, cells may break down large compounds into smaller ones (catabolism). Catabolic
reactions usually release energy. Determine whether the following reactions are anabolic or
catabolic.
Anabolic Catabolic
1. A cracker becomes glucose.


2. Glucose becomes glycogen.


3. You consume more energy than your body expends.


4. Fasting.


5. A piece of ham becomes amino acids.


6. Amino acids become your muscles.


7. A cookie becomes fatty acids.


8. Fatty acids become body fat.


9. Fatty acids provide energy.


4