Download BIOLOGY I: CARBOHYDRATES, LIPIDS AND

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BIOLOGY I: CARBOHYDRATES, LIPIDS AND PROTEINS UNIT, #43
Objectives: To state three functions of lipids; compare the use of carbohydrates and lipids in energy storage.
Overview of Lipid Function. Read the following article and answer the questions that follow. (Adapted from:
http://www.elmhurst.edu/~chm/vchembook/620fattyacid.html) (65 pts)
Nearly all of the energy needed by the human body is provided by the oxidation of carbohydrates and lipids. Whereas carbohydrates
provide a readily available source of energy, lipids function primarily as an energy reserve. The amount of lipids stored as an energy reserve far exceeds
the energy stored as glycogen since the human body is simply not capable of storing as much glycogen compared to lipids. Lipids yield 9 kcal of energy
per gram while carbohydrates and proteins yield only 4 kcal of energy per gram.
It is interesting to compare the relative amounts of energy provided by various biochemicals in a typical 154 lb male. The free glucose in
the blood provides only a 40 kcal energy reserve -- only enough to maintain body functions for a few minutes. Glycogen remaining stored in the liver
and muscles after an overnight fast, amounts to about 600 kcal energy. Glycogen reserves can maintain body functions for about one day without new
inputs of food. Protein (mostly in muscle) contains a substantial energy reserve of about 25,000 kcal.
Finally, lipid reserves containing 100,000 kcal of energy can maintain human body functions without food for 30-40 days with sufficient
water. Lipids or fats represent about 24 pounds of the body weight in a 154 pound male. Lipids provide the sole source of energy in hibernating
animals and migrating birds. Fortunately, lipids are more compact and contain more energy per gram than glycogen; otherwise body weight would
increase approximately 110 pounds if glycogen were to replace fat as the energy reserve.
Lipids or fats are stored in cells throughout the body principle in special kinds of connective tissue called adipose tissue or depot fat.
Whereas many cells contain phospholipids in the bilayer cell membranes, adipose tissue cells consist of fat globules of triglycerides which may occupy
as much as 90% of the cell volume.
In addition to energy storage, depot fat provides a number of other functions. Fat serves as a protective cushion and provides structural
support to help prevent injury to vital organs such as the heart, liver, kidneys, and spleen. Fat insulates the body from heat loss and extreme
temperature changes. At the same time, fat deposits under the skin may be metabolized to generate heat in response to lower skin temperatures.
Lipids ingested as food are digested in the small intestine where bile salts are used to emulsify them and pancreatic lipase hydrolyzes lipids
into fatty acids, glycerol, soaps, or mono- and diglycerides. There is still some dispute about the lipid form that passes through the intestinal wall -whether as fatty acids or as glycerides. In either case, triglycerides are found in the lymph system and the blood.
Since lipids are not soluble in blood, they are transported as lipoproteins after reaction with water-soluble proteins in the blood. Fatty acids
are generally transported in this form as well. There is always a relatively constant supply of lipids in the blood, although of course, the concentration
increases immediately following a meal. Lipids in the blood are absorbed by liver cells to provide energy for cellular functions. The liver is responsible
for providing the proper concentrations of lipids in the blood. Some lipids are utilized by brain cells to synthesize brain and nerve tissue.
Excess lipids in the blood are eventually converted into adipose tissue. If lipid levels in the blood become too low, the body synthesizes
lipids from other foods, such as carbohydrates, or removes lipids from storage. The body also excretes some lipids in the form of fats, soaps, or fatty
acids as a normal component of feces.
Abnormally high levels of triglycerides and cholesterol are thought to be involved in hardening of the arteries. Lipids may be deposited on
the walls of arteries as a partial consequence of their insolubility in the blood.
1. How does the energy content in glycogen and lipids compare? (10 pts)
2. In the example of the male who weighs 154 lbs, what percentage of his weight comes from fat? (10 pts)
3. How do animals other than humans take advantage of the energy content in fat? (10 pts)
4. Besides storing a large amount of energy, state three other functions of lipids. (15 pts)
5. Using context clues, what does “emulsify” mean? (5 pts)
6. Where do most lipids go after digestion and what are they used for? (15 pts)
Fat’s Where It’s At! Many people incorrectly think that all fat is bad. Create a colorful mini-poster below, advertising
the benefits of lipids. Remember to be colorful, creative and factually correct. (35 pts)