CHAPTER 24-METABOLISM
I. NUTRITION
A. Much of what we eat is used in some way to generate ATP for body cells.
B. Nutrient-a substance in food used by the body to promote normal growth, maintenance
and repair. Most foods offer a mixture of the major nutrients needed by the body.
These major nutrients include carbohydrates, lipids, proteins, vitamins and minerals.
II. ESSENTIAL NUTRIENTS-nutrients that must be obtained from the diet. These cannot be
made by the body.
III. MAJOR NUTRIENTS NEEDED FOR THE HUMAN BODY TO FUNCTION PROPERLY
A. Carbohydrates-sugars and related compounds.
1. These are primarily used as energy sources in the human body. The
monosaccharide glucose is the major energy source in the human body. Cells
are capable of converting glucose into ATP.
2. We also ingest and utilize a large supply of the polysaccharide starch.
3. The bulk of the carbohydrates that humans consume are derived from plants
(fruits, sugar cane).
B. Lipids-fats
1. These are typically found in animal products.
2. There are obvious reasons to avoid fats in foods; however, lipids do play some
critical roles in the body. For example, they help the body absorb fat-soluble
vitamins and they help to build myelin sheaths and cell membranes.
3. Adipose or fat tissue also serves as a source of stored energy and it helps to
pad, cushion and protect body structures.
C. Proteins
1. These are composed of amino acids that are covalently bonded together.
2. Uses of Proteins in the Human Body:
a. They make up enzymes. What are these?
b. They serve as structural components in many tissues (muscle, bone).
c. They are found in cell membranes.
3. Humans obtain their proteins and essential amino acids from a variety of food
sources, including meat, fish, eggs and milk.
D. Vitamins-organic compounds that are needed to help the body use the above nutrients.
1. Most vitamins function as coenzymes which help enzymes accomplish their
specific task.
2. Vitamins are usually not produced in the body; therefore, they must be taken
via food or supplements. The exceptions to this are Vitamin D (which is made
in the skin), some B Vitamins and Vitamin K (which are produced by intestinal
bacteria).
3. Vitamins are found in all foods but no one food contains all vitamins.
4. Vitamins are classified as being either fat soluble or water soluble.
5. Water Soluble Vitamins-are absorbed with water from the gastrointestinal tract.
a. B Vitamins and Vitamin C are the primary Water Soluble Vitamins.
b. These vitamins are usually not stored in the body. So, any ingested
amounts of these vitamins not used within an hour are excreted via
the urine.
c. Some of the primary uses of the B Vitamins and Vitamin C are listed in
the textbook.
6. Fat Soluble Vitamins-these bind to ingested lipids and are absorbed along with
these compounds in the G.I. Tract. Vitamins A, D, E, and K are categorized as
Fat Soluble Vitamins.
a. These can be stored for future use in the body. Some fat soluble vitamins
can be toxic in large amounts (a condition known as Hypervitaminosis).
b. Again, some of the functions of these vitamins are listed in Table 24.2
of the textbook.
E. Minerals
1. Like vitamins, these are not used for fuel in the body, but they work with other
bodily compounds to ensure that the body functions properly.
2. The body requires moderate amounts of seven primary minerals and minute
amounts of other minerals to function properly.
3. The seven major minerals needed by the body are: calcium, phosphorous,
potassium, sulfur, sodium, chlorine, and magnesium.
4. As with vitamins, minerals must be ingested with the foods that we eat.
5. The primary functions of the vital minerals are listed in Table 24.3 of the book.
IV. METABOLISM-OVERVIEW
A. What is Metabolism?
1. Metabolic reactions are either anabolic or catabolic. What do the terms mean?
2. Many of the catabolic reactions that occur in the human body are someway
involved in generating ATP for body cells through the process of Aerobic
Cellular Respiration.
B. Carbohydrate Metabolism
1. Most carbohydrates contain glucose as a component of their structure; therefore,
cells in the body are essentially geared to be able to metabolize glucose.
2. Cells primarily use carbohydrates as a source of energy. Carbohydrates can be
used as structural components as well.
3. Glucose is metabolized via Aerobic Cellular Respiration.
a. Equation for Aerobic Cellular Respiration:
1) Notice that in aerobic cellular respiration, glucose is broken down
to produce ATP. Oxygen is required for this process to occur.
b. Aerobic Cellular Respiration occurs through three processes: Glycolysis,
The Kreb’s Cycle and Electron Transport.
C. Lipid Metabolism
1. Fats serve as a major source of stored energy for body cells. Energy yield from
lipids is extremely high.
2. Lipogenesis-lipid synthesis, this occurs when ATP and glucose levels are high in
the body.
3. Lipolysis-“fat-splitting.” This releases lipids into the blood. This fat can then be
metabolized to generate ATP. This occurs when ATP and glucose levels are low
in the blood.
a. Under certain conditions (when oxaloacetic acid is low), lipid breakdown
may be incomplete. This may result in a process known as ketogenesis or
the formation of ketones.
b. What is ketosis?
D. Protein Metabolism
1. Proteins typically have a short life span in the human body. Proteins and their
amino acids are usually recycled in the body and used to rebuild structural
or support compounds.
2. Proteins serve as major structural compounds in the body. They offer little in the
production of ATP. However, they can be used as a source of energy during
times of extreme need. Unfortunately, this is not a very effective process for
providing fuel to the body.
3. What is the idea behind protein diets? Are these diets safe?
V. ABSORPTIVE STATE-the time during and immediately after eating when nutrients are being
absorbed in the gastrointestinal tract. Catabolic reactions are most common during this period.
A. Carbohydrates are absorbed and carried to the liver for sugar conversions.
B. Lipids are absorbed and carried to the lymphatic system where they are converted into
fatty acids and glycerol which can move through capillaries into body tissues.
C. Proteins are carried directly to the liver after absorption.
D. Insulin-plays a key role in absorption; especially glucose absorption.
1. Increased blood sugar levels stimulate pancreatic cells to release insulin which
then increases cellular uptake of sugar.
2. Diabetes mellitus
VI. POSTABSORPTIVE STATE-is the period when the GI tract is empty and energy sources are
supplied by the breakdown of reserve compounds.
A. The release of fats and glycogen into the blood for conversion into glucose would be
examples of this state.
VII. REGULATION OF FOOD INTAKE
A. When energy inputs and outputs are balanced, the body remains constant.
B. The hypothalamus plays a key role in regulating feeding behaviors.
C. What Regulates Food Intake?
1. Nerve signals from the Digestive Tract.
2. Stored Energy Levels-increased levels of energy inhibits hunger.
3. Hormones
4. The release of insulin during food absorption, depresses hunger.
5. Body Temperature-increased temperature inhibits hunger.
6. Psychological factors
IX. CLINICAL TERMS-at the end of the chapter.