Section 16.1 – Objectives
By the end of this section you should be able to:
• List the building blocks of all the main food groups as well as their storage forms
in the body.
• Describe the three chemical reactions/metabolic pathways in the body that
produceATP. List how muchATP each can form, which one requires oxygen, and
where the reactions take place in the cell.
• Describe where each fuel source enters the metabolic pathways.
• Describe in detail the full metabolism of glucose by glycolysis and the citric acid
cycle (in the presence of oxygen and without oxygen).
• Describe in detail the full metabolism of fats and amino acids.
• Explain all the processes involved in the fed state of metabolism and how insulin
is involved in this process and all its functions.
• Explain all the processes involved in the fasted state of metabolism and how
glucagon is involved in this process and all its functions.
• List all the other hormones that help to regulate metabolism and explain how they
perform their function.
• Explain what would happen to digestion and metabolism if you had not eaten for
an extended period of time and were in a very stressful situation. List all the
hormones that would be involved. This will require you to combine everything
you know about digestion and metabolism.
Section 16.2 – Introduction
• In the last section, we saw how food is broken down and then absorbed into the
circulatory system. It is now time to see what happens to it next.
• Essentially, the fats, carbohydrates, and proteins are used for energy or to build
Defined as the chemical changes that occur within a cell to either
manufacture energy, store energy, or build new cell structures.
The two types of metabolism are listed below
• You should remember that the energy molecule used by the body is called
adenosine triphosphate, or ATP.
Section 16.3 – The Building Blocks
1 • Recall that all of the food we saw in the digestive module is made up of smaller
Make up most of the fat found in the body
Consist of three fatty acid chains attached to one molecule of glycerol.
Consist of long chains of amino acids linked together
Carbohydrates are long chains of monosaccharides (like glucose) joined
• We have seen how these larger molecules are broken down in the digestive tract
and then absorbed.
• Once in the body, some may partially reform while others stay as building blocks.
• These building blocks are circulating in the body and can be used to create new
structures in the cell or to formATP, or they can be stored for later use.
• Their storage form may not be the same as their circulating form.
• The carbohydrate glucose:
Stored inside muscle and liver cells in the form of glycogen.
Stored as triglycerides inside fatty tissue called adipose tissue.
• The amino acids become structural or functional proteins inside the muscle cells.
Section 16.4 – The Building Blocks: Energy Production
• All of the molecules we have just seen can be used to produce energy in the form
2 • Glucose:
Contributes to 1% of the total energy requirements of the body, and its
reserves (in the form of glycogen) can last for roughly a day.
• Fatty acids:
Contribute to 77% of the total energy produced and can last for up to two
months depending on the individual.
• Amino acids:
Stored as proteins are not usually used to create energy but can account for
22% of the body’s fuel requirements if absolutely necessary.
Section 16.5 – Metabolism:AGeneral Look
• There are three primary chemical reactions that take place within the cell to
produce energy from the breakdown of food molecules.
• Glycolysis (or the glycolytic pathway):
Is a series of reactions that occurs in the cytoplasm
It does not require oxygen and is therefore considered an anaerobic
Glycolysis can produce two molecules ofATP very quickly from one
molecule of glucose.
• The citric acid cycle (CAC, or Krebs cycle):
This reaction, which takes place inside the mitochondria, requires the
presence of oxygen and therefore is an aerobic reaction.
It can produce two molecules ofATP per molecule of glucose.
3 Section 16.6 – Metabolism:AGeneral Look (cont’d)
• Oxidative phosphorylation:
Sometimes referred to as the electron transport chain
Can produce 34 molecules ofATP from one molecule of glucose.
It also occurs in the mitochondria of the cell and also requires oxygen.
• Notice that all of these equations are linked together through various intermediate
steps that we will examine shortly.
4 Section 16.7 – Metabolism:AGeneral Look (cont’d)
• Each of the food molecules can enter one or more of these reactions to make
• For example:
• Glucose (as well as its storage form glycogen):
Can enter glycolysis at the beginning of this reaction.
• Amino acids (AA):
Can be converted to pyruvate to enter glycolysis or can be converted to
acetyl coenzyme A (acetyl CoA) to enter the citric acid cycle (CAC) to
• Fats (triglycerides):
Can be broken down to glycerol and free fatty acids.
• The glycerol molecules:
Can enter glycolysis, and the
• Fatty acids:
Can be converted to acetyl CoAto enter the CAC.
5 Section 16.8 – The Metabolism of Glucose
• As we saw in the digestive system, glucose is a monosaccharide, and is stored as
glycogen in most cells of the body.
However, most glycogen stores can be found in the liver and skeletal
• Glucose is a common fuel source for all the cells in the body, but it is one of the
only fuel sources for the brain – unlike most other cells in the body, which can use
fats and amino acids as well as glucose.
• The liver stores glucose for the brain, which cannot store any for when blood
glucose levels get low.
6 Section 16.9 – The Metabolism of Glucose: Glycolysis
• Glucose will enter a cell and will be almost immediately converted to glucose-6-
• From here the G-6-P can enter glycolysis to produceATP or it can be converted to
glycogen and then stored.
• When necessary, glycogen can be converted back to G-6-P to enter glycolysis to
Section 16.10 – The Metabolism of Glucose: Glycolysis (cont’d)
• In glycolysis, G-6-P will undergo a series of reactions that will result in the
production of ATP and the end product of pyruvate.
• The pyruvate can then undergo two possible reactions:
1. It can either enter the citric acid cycle (CAC) and produce lots ofATP
(with the help of oxidative phosphorylation) or
2. It can enter another shorter reaction to produce a small amount ofATP and
the by-product lactate (also called lactic acid).
• Which reaction will pyruvate take?
That depends on whether oxygen is present or not.
If there is sufficient oxygen (O 2, then most of the pyruvate will enter the
Remember that since it is an aerobic pathway, it requires O .2
If there is not enough O p2esent, then the CAC will not be running at full
capacity and the pyruvate will be converted to lactic acid.
Section 16.11 – The Metabolism of Glucose: LacticAcid
7 • During strenuous exercise, your cardiovascular system may not be able to supply
sufficient oxygen to your working muscle cells to produceATP via the citric acid
• However, since glycolysis is anaerobic, this reaction can continue without oxygen
to produceATP and the end product pyruvate.
• With insufficient oxygen, the CAC will not work at full capacity and pyruvate
will start to accumulate.
o If too much pyruvate accumulates, then even glycolysis will be slowed.
• In order to keep glycolysis working so it can produceATP for the muscles, the
pyruvate will be converted to lactic acid.
o The accumulation of lactic acid causes the "burning" sensation in the
muscle and is believed to interfere with the contractile proteins causing
• You should recall that the accumulation of lactic acid will cause the blood vessels
to dilate and the decreasing pH will unload more oxygen from hemoglobin.
• These mechanisms will help to increase blood flow and increase oxygen delivery
to this working tissue to reduce the buildup of lactic acid.
Section 16.12 – The Metabolism of Glucose: LacticAcid (cont’d)
• It is very important to note that most of the reactions we have seen so far are
reversible – that is, they can work in both directions.
For example: If oxygen is restored, lactic acid can be converted back to
pyruvate to enter the citric acid cycle.
Also, pyruvate can be turned back into glycogen, through G-6-P, and
stored for later use.
8 Section 16.13 – The Metabolism of Glucose: The CitricAcid Cycle
• If there is a sufficient supply of oxygen, then the citric acid cycle (CAC) can
function at full capacity.
• The pyruvate from glycolysis is converted to acetyl coenzymeA(acetyl CoA),
which then enters the CAC.
• As we mentioned earlier, the CAC in conjunction with oxidative phosphorylation
together produce a total of 36 molecules ofATP from one molecule of glucose (2
ATP from CAC and 34 from oxidative phosphorylation).
• They also produce the by-products CO2 and H2O.
The CO2 will diffuse into the blood and leaves the body at the lungs.
9 Section 16.14 – The Metabolism of Glucose: Review
• Glucose enters the cell where it can be converted to its storage form of glycogen,
or it can enter the glycolytic pathway where it will produceATP and pyruvate.
• Without sufficient oxygen, the pyruvate will be converted to lactic acid while
• In the presence of oxygen, the pyruvate will enter the citric acid cycle to produce
large quantities of ATP while giving off CO2 and H2O.
Section 16.15 – The Metabolism of Fats andAminoAcids
• Amino acids are stored in cells as proteins
• Fats are mostly stored as triglycerides.
• These molecules can also be metabolized to createATP by cells, but each one
enters the metabolic pathways at different points.
• Amino acids:
Can be converted to either pyruvate or acetyl CoA to enter glyc