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How would your body react if you stepped outside without a jacket on a cold day? Your
muscles would tighten and you would feel very uncomfortable. If you stayed outside for
more than a few moments, you would likely begin to shiver. What causes the tightening
of muscles and the shivering? Why does your body react this way? Does it help you get
In the example above, the body responded to
cold air and reacted in a certain way in order
to maintain homeostasis, which is the process
of maintaining a constant state of balance
within a normal range. The muscles tightened
to conserve heat and the body shivered to help
generate heat. All of this occurred in order to
help maintain a fairly constant internal body
While the word itself may sound complicated,
homeostasis is a fairly simple concept.
Cells and organisms must exist in a state of
balance. All living cells are constantly working
to maintain homeostasis. Your body is even
working to maintain homeostasis right now!
There are three general steps in a homeostatic
response. When something in the body is out of balance, it is sensed by a biological
“sensor.” This sensor sends a message to the “control center,” which is usually the nervous
system. The control center then sends a command to an effector, a muscle or gland, to
correct the imbalance. The regulation of thirst and the regulation of body temperature are
examples of homeostasis in the human body.
In order to maintain proper blood volume and fluid salinity (salt concentration), an animal
must continually drink and excrete an appropriate amount of water. If body fluids are high
in salinity (low in water), specific sensor cells in the brain are triggered. These cells can
send commands that trigger a thirst response. They may also send commands in the form
of hormones from a gland (effector) that reduces the further elimination of fluids. With
respect to temperature, the human body must be maintained close to 98.6°F (37°C). Body
temperature can rise, such as during physical exertion or fever. Body temperature can also
decrease, such as when you step outside on a cold day. Temperature sensors detect the
imbalance and the nervous system responds. On a warm day, a command could be sent
to the sweat glands (effectors) to secrete sweat. This helps the body to cool and restore
proper temperature. On a cold day, a signal might cause your muscles (effectors) to shiver,
increasing your body temperature.
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The examples of homeostasis discussed here exist at the level of the organism. What
about homeostasis at the cellular level? Individual cells must maintain their internal
environment in a balanced state. This includes maintaining a constant supply of cellular
energy as well as healthy cellular structures composed of biomolecules. Cells must also
regulate the passage of materials across their membranes. The internal environment of a
cell must maintain the proper salinity and pH. What are the cellular processes important for
maintaining homeostasis?
Energy Conversion in Cells
All of the physical and chemical processes
that take place inside a cell are referred
to as cellular metabolism. Many of the
metabolic reactions within a cell involve
the conversion of energy between different
forms. Cells require a constant supply of
usable chemical energy to perform various
functions. Some of these functions include
building cellular structures, transporting
materials, and eliminating wastes. The
ATP provides cells with the energy
molecule that supplies this cellular energy
needed to perform essential functions.
is adenosine triphosphate (ATP). ATP
is considered a high-energy molecule
because the bonds between the phosphate groups are unstable. When the bond between
the second and third phosphate groups of an ATP molecule is broken, energy is released.
This energy can be used to power a chemical reaction. The resulting molecule has two
phosphate groups and is called ADP, or adenosine diphosphate.
How does a cell obtain the ATP that it needs to perform cellular work? The process of
cellular respiration breaks down larger molecules such as glucose (a sugar) into smaller
molecules. Oxygen is commonly used during this process. Water, carbon dioxide, and
energy are released when respiration occurs in the presence
of oxygen. The energy released from this breakdown is used
heterotroph: an
to generate ATP from ADP and phosphate. For organisms
organism that must
like animals, the original chemical energy source must be
consume its energy
consumed. These organisms are called heterotrophs.
C6H12O6 + 6O2
6CO2 + H2O + energy
The formula for cellular respiration above
illustrates that carbon dioxide, water, and energy
are released.
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Autotrophs, such as plants, produce their own sugar
autotroph: an
molecules, generally during the cellular process of
organism that
photosynthesis. Sunlight is used to build glucose molecules
is capable of
during photosynthesis. Water and carbon dioxide are also
producing its own
required, and oxygen is released as a by-product. Therefore,
energy sources via
cellular respiration and photosynthesis are two complementary
processes that convert energy between sunlight, sugar
(glucose) molecules, and high-energy ATP molecules. Reactions
that build larger molecules from smaller ones, such as photosynthesis, are called anabolic
reactions. Catabolic reactions, on the other hand, break down larger molecules into smaller
ones. Cellular respiration involves catabolic reactions.
look out!
Many people are familiar with the fact that plant cells undergo photosynthesis. However, it
is important to remember that all living cells, including those that perform photosynthesis,
also perform cellular respiration. Photosynthesis does not produce energy in the form
of ATP. It yields the nutrient glucose, which is needed to generate ATP during cellular
respiration. For this reason, photosynthetic autotrophs form the base of most food chains.
Heterotrophs, such as animals, consume nutrients that are produced by autotrophs,
either by consuming them directly or eating other organisms that consume them.
Autotrophs, on the other hand, need not consume other species. In the case of plants,
they photosynthesize their nutrients from water, carbon dioxide, and sunlight. But even
these autotrophs must use cellular respiration to break down those nutrients in order to
make ATP.
Everyday Life: You Really Are What You Eat
Living cells are made up of four major types of organic biomolecules—carbohydrates,
lipids, proteins, and nucleic acids. Cells can synthesize these molecules from simpler
building blocks. However, for the most part, those building blocks must be consumed in
the food that organisms eat. Foods like bread, pasta, and fruit contain mostly carbohydrate
molecules. These may be small (simple) carbohydrates such as sugars, or complex
carbohydrates such as starch. As discussed in the previous sections, cells break down the
simple sugar glucose to yield energy in the form of ATP. Some carbohydrates are also used
for cellular structure, and others attach to the surface of cells and aid in cell recognition.
The carbohydrate cellulose provides structural support for plant cells.
Lipids are an important type of molecule consumed from fatty foods such as oils and animal
fats. There are many different types of lipids. Like carbohydrates, lipids can be used as
an energy source, and they can be broken down to generate ATP. Lipids also form the
membranes around cells and around many internal cellular organelles. Some lipids are
stored in the fat tissue of animals.
© 2013-2014 Accelerate Learning - All Rights Reserved
The animal tissue provides cushioning
and insulation. Lipids can also function as
signaling molecules by moving into cells and
triggering various processes to occur.
The next major biomolecule, protein, comes
from foods such as meat, dairy, nuts, and
beans. After consumption, proteins are
broken down into their constituent building
Biomolecules are important components of
blocks, called amino acids. Cells then use
cell membranes.
the amino acids to build the specific proteins
needed for cellular work. There are many
different kinds of proteins within cells. The functions of protein in a cell include structural
support, signaling molecules, and catalyzing chemical reactions. Proteins that catalyze
chemical reactions are called enzymes. Muscle tissue contains specific contractile proteins,
while other proteins are found within cell membranes and regulate the import and export of
cellular materials.
Finally, nucleic acids are the biomolecule responsible for “information storage” within a cell.
These molecules include DNA and RNA, the molecules that provide the genetic blueprint
for building and maintaining living cells.
Crossing the Cellular Membrane
One of the primary strategies for maintaining cellular homeostasis is regulating materials
that pass into and out of the cell. While some molecules can easily cross a cell membrane,
the passage of many materials is tightly controlled. This variability in whether a certain
substance can easily cross the membrane results from the fact that the cell membrane
is selective, or semipermeable. Substances that are small
and nonpolar are generally able to freely cross the cell
membrane. These substances are able to squeeze through the
nonpolar: lacking
nonpolar lipids that comprise the membrane. Examples of such
chemical polarity
substances are the gases oxygen and carbon dioxide.
© 2013-2014 Accelerate Learning - All Rights Reserved
When a substance can freely cross the membrane, it
moves from an area of higher concentration to one of
lower concentration. This type of movement is called
diffusion. Like a spray of perfume spreading throughout
a room, any dissolved substance will naturally diffuse
in this manner. When a molecule goes freely through
the lipid bilayer of the cell membrane it is called simple
diffusion. If a substance is too large or polar, it may
require the assistance of either a carrier protein or a
channel protein to diffuse across the membrane. This
type of movement is called facilitated diffusion.
polar: having a slightly
positive charge at one
side of a molecule and a
slightly negative charge
at the other side
carrier protein: a
protein in the cell
membrane that
changes shape to
allow a substance to
pass through
Neither simple diffusion nor facilitated diffusion requires
the input of energy. They are thus considered to be forms
of passive transport across membranes. The active
transport of substances, on the other hand, requires
the input of energy (from ATP). Why would energy be
required to move substances across a membrane?
Substances can only move passively (diffuse) down
their concentration gradient. The movement of a
substance from an area of lower concentration to one
of higher concentration is in a direction against, or up,
its concentration gradient. Movement in this direction
requires energy.
channel protein:
a protein in the cell
membrane that forms a
channel through which
a substance can pass
gradient: a difference in
the amount of substance
on two sides of a barrier
Sodium concentration gradient
Potassium concentration gradient
ADP + Pj
Facilitated diffusion, pictured on the left, does not require an input of energy. Active
transport, pictured on the right, requires energy to move substances against their
concentration gradient.
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what do you think?
Water (H2O) is a relatively small polar molecule. Water
solute: a substance
molecules, as with dissolved substances, must cross the
that is dissolved in
plasma membrane in order to regulate the tonicity of the
another substance
intracellular fluid. Tonicity refers to the relative concentration
(called the solvent)
of water and solutes in a solution. A hypertonic solution is
one in which there is a higher solute concentration compared
to another solution. A hypotonic solution has a lower solute
solution: a mixture in
concentration compared to another solution. Two solutions
which the molecules
that are isotonic have the same solute concentration. There
of one substance
are channel proteins within cell membranes through which
(solute) are dissolved
water molecules can pass. Osmosis refers to the diffusion
in another substance
of water molecules. Imagine that the fluid surrounding a cell
is hypertonic to the fluid inside the cell. Do you think water
molecules will move into or out of the cell? What if the fluid
outside the cell is hypotonic to the fluid inside? In each of these cases, is the movement of
water (osmosis) passive or active?
Tonicity of solution
around the cell
Solution around the cell
(as compared to the cell)
high (hyper) solute
low water concentration
Solution (cytosol) in the cell
low solute concentration
high water concentration
low solute (hypo) concentration high solute concentration
high water concentration
low water concentration
solute and water concentrations
solute and water concentration
are the same as that of the
are the same as that of the cell
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what do you think?
The chart below lists terms associated with cellular process in the left column. Match each
term on the left with the related image or phrase on the right. Write the letter(s) of the
matching image or phrase next to the term in the left column. Be sure to include all possible
correct answers.
Cellular Respiration
Active transport
D, H, K
Facilitated Diffusion
Simple Diffusion
A, L
Passive transport
G, H
A, L
B. Process that yields ATP
C. Balance of conditions within a cell or organism
D. Performs photosynthesis
F. Requires ATP to move substances against a
concentration gradient
G. Yields sugar (glucose)
I. Releases energy when phosphate bonds are broken
J. Olive oil
K. Performs cellular respiration
L. Involves a substance moving down its concentration
© 2013-2014 Accelerate Learning - All Rights Reserved
connecting with your child
Exploring Food Labels
To help students learn more about the major
biomolecules, have them study food labels
from some common foods in the house or
the grocery store. Examples of food labels
to research are peanut butter, packaged
lunchmeat, baby carrots, honey, fruit jam,
chocolate, vegetable oil, cheese, bread,
packaged pasta, canned beans, etc. Have
students guess the relative amounts of fat,
protein, and carbohydrates in each food.
Then, have them compare their guesses
with the information on the food label.
You may want to explain to students that
a calorie is a unit of energy: one gram
of fat contains nine calories of energy,
whereas one gram of protein or one gram
of carbohydrate contains four calories of
Here are some questions to discuss with
• Why is it important to eat a balanced diet
that contains sufficient carbohydrates,
protein, and fat? How do living cells use
each of these nutrients?
• Why does a tablespoon of honey
have so many fewer calories than a
tablespoon of olive oil?
• If protein, carbohydrates, and fat all
contain energy, why must a cell undergo
cellular respiration and break down
sugars (or other nutrients) to produce
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