BSAA – ANIMAL SCIENCE
ABSORPTION OF NUTRIENTS
Student Learning Objectives. Instruction in this lesson should result in
students achieving the following objectives:
1 Identify end products of food which are capable of being absorbed and how
they are utilized by animals.
2 Describe the role of the plasma membrane in animal cells and explain how a
selectively permeable membrane functions.
3 Explain how nutrient absorption is affected by food digestibility.
Anticipated Problem: What are the end products of food which are capable of being
absorbed and how are they utilized by animals?
I. Proteins, carbohydrates, and fats must be broken down into their ‘end products’ in
order to be digested.
A. These end products, mostly amino acids, move from the digestive canal into cells that
line the digestive tract, and then into the bloodstream.
B. Vitamins, minerals, and water are also absorbed from the digestive tract and
transported by the blood throughout the body.
C. These end products are used by individual cells to carry out daily processes of life,
growth, and reproduction.
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Anticipated Problem: What is the role of the plasma membrane in animal cells and how
does a selectively permeable membrane function?
II. Absorption is the movement of food nutrients from the digestive canal into the
bloodstream though the cell wall of the digestive system.
A. Absorption is aided by three processes: diffusion, assisted diffusion, and active
diffusion.
1. Diffusion is the movement of a substance from a high concentration to a low
concentration. Diffusion occurs when the concentration of nutrients outside a cell is
greater than the the concentration of nutrients inside the cell. This difference creates
what is known as a concentration gradient. Whenever a concentration gradient
exists, molecules will naturally move from high to low concentrations, in a slow
movement.
2. Assisted diffusion is when the rate of diffusion is increased by the use of carrier
molecules, or molecules that ‘carry’ nutrient molecules from a high to low concentration.
3. Nutrients may sometimes move against a concentration gradient (from a low to
high concentration) in a process called active transport. Active transport requires the
input of additional energy from inside the cell. Active transport is important for the
movement of cells through the intestinal lining.
B. Absorption in animals takes place because of a selectively permeable membrane
surrounding cells, called the plasma membrane.
1. The plasma membrane is a semi-permeable membrane, meaning some molecules are
allowed to pass through, while others are not.
2. Osmosis is the process of diffusion through a semi-permeable membrane.
C. Nutrients are absorbed in the digestive tract at specific locations. The location of
absorption is influenced by where nutrient digestion to end products occur.
1. Minerals, such as calcium, magnesium, and iron are absorbed in the upper-most
region of the small intestine.
2. Water soluble vitamins and simple sugars are absorbed from the alkaline environment
of the small intestine.
3. Fats and proteins are absorbed in the lower small intestine.
4. Water is primarily absorbed from the colon.
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Anticipated Problem: How is absorption affected by food digestibility?
III. Sources of nutrients for livestock feed can vary greatly in their digestibility.
A. Digestibility refers to the ability of the animal to digest the food into useful end
products that can be absorbed and utilized. Producers must consider the digestibility of a
food source as well as the nutrient content when planning a balanced ration.
B. Apparent digestibility is the difference in the total nutrients consumed and the
nutrients remaining in the feces. The goal of an animal nutritionist is to formulate feed
with high apparent digestibility.
C. Total digestible nutrients, abbreviated TDN, represents the relative energy value of
feed to livestock. There are four factors that will affect TDN:
1. Percentage of dry matter.
2. Digestibility of dry matter.
3. Amount of mineral matter.
4. Amount of fat.
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ABSORPTION OF NUTRIENTS
Part One: Matching
Instructions: Match the term with the correct response.
a. osmosis
b. diffusion
c. digestibility
d. active transport
g. apparent digestibility
e. plasma membrane h. carrier molecules
f. concentration gradient
_______1. A selectively permeable membrane surrounding animal cells.
_______2. Diffusion from a low to high concentration (requires energy).
_______3. Diffusion through a selectively permeable membrane.
_______4. Molecules that assist nutrients in passing the semi-permeable membrane.
_______5. Created by a difference in concentrations inside and outside a cell.
_______6. Movement of substance from a high to low concentration.
_______7. The difference between nutrients in the feces and nutrients in the feed.
_______8. The ability of an animal to breakdown feed into usable end products.
Part Two: Completion
Instructions: Provide the word or words to complete the following statements.
1. Minerals, such as calcium, are normally absorbed in the upper portion of the ___________
_____________.
2. Absorption is the movement of food nutrients from the digestive canal into the
_________________.
3. Active transport requires the additional input of _________ from outside the cell.
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Part Three: Multiple Choice
Instructions: Circle the letter of the correct answer.
_______1. Nutrients move against a concentration gradient by active transport in which way?
a. high to low concentration
b. low to high concentration
c. high to high concentration
d. low to low concentration
_______2. A semi-permeable membrane allows which molecules to pass through?
a. large molecules
b. all molecules
c. small molecules
d. semi-circular molecules
_______3. What does TDN stand for?
a. the daily nutrition
b. total daily nutrition
c. total daily nutrients
d. total digestible nutrients
_______4. What is the movement of water or other substances through a cell’s semi-permeable
membrane called?
a. transference
b. active transport
c. osmosis
d. absorption
_______5. In non-ruminants, such as pigs and poultry, most of the absorption takes place in the:
a. stomach
b. intestines
c. rumen
d. abomasums
_______6. Which is not a factor that influences TDN of animal feed?
a. % dry matter
b. amount of mineral matter
c. amount of fat
d. feed cost
Part Four: Short Answer
Instructions: Provide information to answer the following statement.
List and discuss the three types of diffusion.
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Assessment
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TS–A
Technical Supplement
ABSORPTION OF NUTRIENTS
1. What is a selectively permeable membrane? (Semi-permeable membrane)
In all animals the process of absorption is by osmosis through a semi-permeable
membrane which lines the digestive tract. The structure of the plasma membrane
does not permit molecules to diffuse through it freely. The membrane is selectively
permeable, meaning that some substances can pass through it more readily than
others can. The membrane even prevents the passage of certain kinds of molecules,
to which it is impermeable.
2. How do nutrients and waste products move in and out of a cell?
Protein, carbohydrates, and fats must be broken down into their “end products” in
order to move from the digestive canal into cells that line the digestive tract and then
into the blood. This movement of nutrients is called absorption. Vitamins, minerals,
and water are also absorbed from the digestive tract and transported by the blood
throughout the body. Absorption of nutrients is aided by three processes: diffusion,
assisted diffusion, and active transport. Diffusion occurs when the concentration of
nutrients outside a cell is greater than the concentration of nutrients within the cell. This
difference creates a concentration gradient. When a concentration gradient exists,
molecules naturally move from areas of high concentration to areas of low concentration.
The process of diffusion, by itself, is slow. The rate of nutrient absorption is increased by
the utilization of carrier molecules. When a carrier or transport molecule helps move
nutrients into/from a cell the process is called assisted diffusion. In assisted diffusion the
movement of nutrients is also due to the concentration gradient.
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Nutrients may move against a concentration gradient (from low to high concentration)
by a process called active transport. Active transport relies on additional energy
from inside the cell to move the nutrients. Active transport is important for the
movement of nutrients through the intestinal lining. Nutrients are absorbed from
the digestive tract, mostly the small intestine, at specific locations. The location of
absorption is primarily influenced by where nutrient breakdown to end products
occurs. Minerals, such as calcium, magnesium, and iron are absorbed in the uppermost
region of the small intestine. Water soluble vitamins and simple sugars are
absorbed from the alkaline environment of the small intestine and fats/proteins are
absorbed in the lower small intestine. Water is primarily absorbed from the colon.
3. When does osmosis occur in living cells?
The movement of water through a semi-permeable membrane (such as a cell's
plasma membrane) is called osmosis. Water moves from a region where it is present
in high concentration or pressure to one where it is present in low concentration or
pressure.
The red blood cell is a good example of the importance of diffusion and osmosis to
living cells. As it races through the blood vessels, the red blood cell is bathed in a
fluid that has a concentration of salts, sugars, and other molecules similar to that in
the pool of fluid inside the cell. Because the concentration of water molecules is
similar inside and outside the cell, as many water molecules move in as move out,
and the cell retains its normal disk shape. But what would happen if you placed red
blood cells in a beaker of pure distilled water? The pure water would have a vastly
lower concentration of ions, proteins, and other solutes than the cell's cytoplasm. At
the same time, however, the concentration of water molecules in the beaker would
be far greater than inside the cell, and so water would tend to move into the cell by
osmosis. This would cause the cell to swell or even burst from the extra water. One
could also prepare a solution with a higher concentration of salts, sugars, proteins,
and other solutes but a lower concentration of water than the fluid inside a red blood
cell. Since the concentration of water molecules would be lower outside the cell
than in, water would rush out of the cell by osmosis, leaving it seriously shriveled.
You may have noticed the effects of osmosis yourself; the day after you eat a big
helping of a very salty food—say, a large bag of potato chips—your weight may go up
2 or 3 pounds. The reason is that the salt concentration of your cells increases, and so
you tend to retain more of the water you drink as it moves into the cells to offset the
extra salt.
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4. What forms of nutrients are capable of being absorbed by cells?
Sources of nutrients for livestock feed can vary greatly in their digestibility. Producers
must consider the digestibility of a feed as well as the nutrient composition of the
feedstuff when planning a balanced ration. Animal scientists perform research
experiments to determine the apparent digestibility of feed samples. Apparent
digestibility is the difference between the total nutrients consumed and the nutrients
remaining in the feces. The goal of animal nutritionists is to formulate feeds
which have high apparent digestibility.
TDN is the abbreviation for total digestible nutrients and represents the relative
energy value of a feed to an animal. Factors that affect TDN are percentage of dry
matter, digestibility of dry matter, amount of mineral matter, and amount of fat.
Before animals can utilize these nutrients they must first be converted to end products
which are capable of being absorbed into the blood stream. In ruminant animals,
such as the cow, this absorption takes place through the rumen wall. In
nonruminants, most absorption takes place in the small and large intestines. In all
animals the process of absorption is by osmosis through a semi-permeable membrane
which lines the digestive tract.
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