Page 4
The Skeleton
Teacher’s Background………………………………..5
Teacher’s Guide to Activity……………………9
Student Activity……………………………………….11
Inner Space in Outer Space:
A Virtual Astronaut Teacher’s Guide
The Skeleton
Teacher Background
Page 5
Introduction
Think about the human body. Now compare it to the body of animals you
have seen. Do you notice anything different? The human body is the only one
that supports an upright, or
bipedal
, form of locomotion. Over millions of
years, the bones of the skeleton have developed in a way that allows us to
walk upright on two legs and function against the pull of the Earth’s gravity.
What happens, then, when we leave this source of gravity? Once on orbit,
astronauts live in
microgravity
: the force of gravity is much, much weaker
(virtually zero), and the body no longer needs to counteract it. As a result, the
body adapts to its new microgravity surroundings. The changes to bone in
particular hold consequences for astronauts once they return to Earth.
Before we can understand how and why bone changes in microgravity, we
need an understanding of how bone works on Earth.
Bone on Earth
The 206 bones in the human body come in many sizes and shapes
.
One of the
largest bones of the body, the femur, can be over a foot long
,
while some of
the smallest bones in the body—
the pisiform bone in the wrist, or
the stapes in the inner ear—are no
bigger than a pea.
What do your bones do for you?
The first answer that you might
come up with is “Bones support my
body”—and you would be correct.
The skeletal system provides the
framework for the human body—
and much more. We rely on our
bones for many things:
•
Support and protection: Bones
are an essential component of
the body’s support system. The
skeleton anchors soft tissues
like organs, muscles, ligaments,
and tendons. It also supports
the weight of these tissues. It is
easy to visualize the skeleton as
the concrete support structure
of a building.
Just like the
concrete of a building protects
its inhabitants from the outside
elements, the skeleton also
The Skeleton
Teacher Background
Page 6
protects some of our most vital organs. The thick bones of the skull, for
instance, protect the brain. Without this protection, the internal organs
would be in danger of injury from slight bumps and falls.
•
Storage: The skeleton houses several sources of nutrition and energy.
Bone contains
yellow marrow
, where the body stores a reserve of fat
cells. These fat cells can be utilized when the body needs a supply of
energy. In addition to fat, bones also store essential ions, like calcium,
phosphate, hydrogen, potassium, and magnesium. These minerals are
used in many systems of the body for a wide range of processes.
•
Cell Production: The skeleton is also responsible for the production of
blood cells. The soft spongy tissue inside bone called
red marrow
manufactures
red blood cells
, which transport oxygen throughout the
body, and
platelets
, which help the clotting process.
•
Movement: The skeleton also plays an important role in our ability to walk,
jump, sit, stand, and run. In order to create movement, our muscles
operate a system of levers—our bones. Without these levers, the muscles
will not move.
The structure of bone gives it the strength to protect and support our bodies
and the flexibility to absorb the forces generated by some types of
movement. The combination of strength and flexibility comes from the
composite
structure of bone.
This means that it has both an organic
component and an inorganic component.
The organic component is
composed mainly of
collagen
, long chains of protein that intertwine in
flexible, elastic fibers. Collagen is also so strong that a single fiber only one
millimeter in diameter can support the weight of a 10-
kilogram
load.
Hydroxyapatite,
the inorganic component, is a calcium-rich mineral that
stiffens and strengthens the collagen. Together, the interwoven organic and
inorganic components of bone create a sturdy yet flexible skeletal structure
able to support the body, absorb the shock of movement, counteract the pull
of gravity, and allow us to move.
Bone is a dynamic material; it is constantly changing throughout our lives.
The
remodeling
process replaces old bone with healthy new material and
also helps heal fractures and breaks. The remodeling process involves three
types of cells: osteoblasts, osteocytes, and osteoclasts.
Osteoblasts
form new
bone. These cells synthesize and deposit bone material.
Osteocytes
, which
are actually mature osteoblasts, maintain mature bone tissue. The third type
of bone cell,
osteoclasts
,
resorb
or “eats” older bone tissue. As the calcium
in mature bone ages, osteoclasts begin to resorb it, leaving a tiny tunnel.
About three weeks later, when they are finished with their meal, the
osteoclasts disappear, and osteoblasts enter the tunnel. The osteoblasts
The Skeleton
Teacher Background
Page 7
redeposit new calcium, leaving the bone healthy and strong. The osteoblasts
mature into osteocytes and become embedded in the bone. The bone
continues to age and the remodeling process starts again.
The key to the remodeling process is that osteoblasts and osteoclasts work at
an equal rate. Osteoblasts lay down the same amount of new bone as the
osteoclasts resorb. If the body were to absorb too much calcium from bones
(if osteoclasts work faster than osteoblasts), the skeleton would become thin
and weak. Consequently, a healthy body never loses more calcium than will
be replaced, and the bones maintain a constant strength. As we are about to
see, however, exposure to microgravity changes this balance.
Bone in Space
Bone begins to change after an astronaut has lived in microgravity for only a
few days.
Microgravity reduces the amount of weight that bones must
support to almost zero. At the same time, many bones that aid in movement
are no longer used as much as they are on Earth. For example, microgravity
allows astronauts to “float” effortlessly in one position, so they do not have to
use the bones in their legs, hips, or back to sit or stand. When a bone is not
used, a biomechanical trigger causes calcium normally stored in the bones to
be broken down and released into the bloodstream. This decrease in bone
mass density is called
osteoporosis
. Osteoporosis leaves bone weak and less
able to support the body's weight and movement upon return to Earth, placing
the astronaut at a higher risk of fracture.
Bone loss begins within the first few days in space. Some astronauts who spent
months aboard the Russian space station
Mir
lost as much as 20% of their bone
mass. The most severe loss occurs between the second and fifth months in
space, although the process continues throughout the entire time spent in
microgravity.
The exact mechanism that causes the loss of calcium in
microgravity is unknown. Many scientists believe that microgravity causes
osteoclasts to resorb bone much faster than osteoblasts lay down new bone.
The exact trigger for this change has not been found.
Astronauts do not feel the effects of bone loss while they are in space, but they
are affected upon their return to Earth. When the force of gravity returns, the
skeleton may no longer be strong enough to support the body’s weight or
counteract gravity. This leaves astronauts at a much higher risk of bone break
or fracture.
Microgravity-induced osteoporosis is a serious matter, so doctors and
scientists are researching ways to limit or prevent bone loss in astronauts.
Throughout the history of space flight, astronauts have always done special
exercises to keep their muscles and bones strong. In addition, flight surgeons
make sure that astronauts receive dietary and vitamin supplements that give
The Skeleton
Teacher Background
Page 8
them added protection against bone loss. In the future, it is possible that new
types of exercises, diets, or even medication may prevent bone mineral
loss—in astronauts and in people on Earth.
The Skeleton
Teacher Background
Page 9
Activity #1: Bag of Bones
Objective
Following this activity, the student will be able to
? Identify the effects of decreased bone mass (osteoporosis)
? Describe why healthy bones are important in space and on Earth
National Science Standards
Unifying Concepts and Processes in Science
? Evidence, models, and explanation
? Change, constancy, and measurement
Form and function
Science as Inquiry
? Abilities necessary to do scientific inquiry
? Understanding about scientific inquiry
Life Science
? Structure and function in living systems
? Diversity and adaptations of organisms
Science in Perspective
? Personal health
History and Nature of Science
? Nature of science
National Mathematics Standards
? Mathematics as problem solving
? Mathematics as reasoning
? Mathematical connections
? Computation and estimation
Materials Needed
? Corn puff cereal (approx. 4.5 oz. per group)
? Ziplock snack bags (6 5/8 inch x 3 ¼ inch) - 5 per group (larger bags hold
too much cereal to count in a reasonable amount of time)
? Permanent markers for labeling bags
? Heavy books (one per group)
? Student Activity Guide (one per student)
? Broom and dustpan (for clean-up)
Time Required
This activity may be spread out over a two- or three-day period. You may
wish to use the first day for discussion and baggie preparation, and the
second and third days for experimentation, data collection, and discussion.
The Skeleton
Teacher Background
Page 10
Procedure
1. Begin with a discussion of osteoporosis. Ask students if they know
anyone—grandparents, for example—who suffers from osteoporosis. Do
they know what osteoporosis is? Do they know what causes it?
2. Explain to students that astronauts experience a particular kind of
osteoporosis. Describe the effects of microgravity on bone.
3. Tell the students that they are going to investigate bone loss and the
effects that it may have. To do this, they will use baggies and cereal to
make their own “bones.” Explain that each baggie will represent a bone,
and the cereal inside the bone will represent the calcium and cells that
make the bone strong. Removing cereal from some of the bags will
simulate a bone that has lost some of its mass.
4. Distribute cereal, snack bags, and worksheets to students. In order to
expedite the experiment, students should work in groups of four; the
group can work on Bag 1 together, and then each student is responsible
for one additional bag.
5.
Students should follow the directions on the worksheet. Note about cereal
smashing: some of the cereal has natural holes in it. Explain to students
that they should examine the cereal
before
smashing it, so that they have a
reference point when counting unaffected pieces after the smashing step.
In addition,
one
student should be responsible for smashing
all
of the
bags, so that the amount of force will be the same on each bag.
Discuss what students should look for when they are counting “affected”
pieces of cereal. Pieces that have dust (from other smashed pieces) or
only a tiny flake taken off should not be counted as “affected.”
6. After the students have completed the activity, bring the group back
together. Ask each group to share their results with the class. Discuss the
results and the follow-up questions.
7.
If some groups’ results did not come out as expected (i.e., density did not
drop), discuss the possible reasons for this. Answers may include
miscounting, uneven force applied when smashing bags, etc. Can
students think of another way to test their hypotheses?
Extensions
1. Have students graph the data from their investigations to explore the
relationship between bone density and amount of damage.
2. Compare group outcomes to demonstrate that results always vary—and
are often unexpected. Emphasize that science is often indefinite, but
always involves
systematic
study.
3. Investigate other risk factors for osteoporosis, including health, ethnicity,
age, etc. Explore ways in which students can change their lifestyles now
so that they may avoid osteoporosis in the future.
Assessment
Student worksheets and classroom discussion may be used for assessment.
The Skeleton
Student Activity
Page 11
Bag of Bones
Osteoporosis
is a loss of bone mass. It
makes bones weak and fragile, which can
make it very easy to fracture or break a
bone. Low bone mass can be a problem
throughout a person’s life, but it may not be
until late in life that full-blown osteoporosis
develops.
In fact, many people do not
realize that they have osteoporosis until one
of their bones fractures after a minor slip or
fall.
Osteoporosis is a big problem in the world
today.
In the United States alone, two
million men and eight million women have osteoporosis, and another
eighteen million men and women are at risk for this disease. It is not limited
to one group of people, either. As many as 50% of Caucasian and Asian
women are at the highest risk. More than ten percent of Hispanic and African-
American women also run the risk of osteoporosis.
Astronauts who spend more than a week in space also suffer from a form of
osteorporosis. This type, called
disuse osteoporosis
, occurs when astronauts
do not use their bones in space in the same way that they do on Earth. For
example, the bones used in standing and walking on Earth are not used
nearly as much in space, because astronauts spend much of their days
“floating” and propelling themselves with their arms. By the end of their
mission, astronauts lose bone mass in their legs and hips, which leaves them
at risk for fracutures and breaks when they return to Earth’s gravity. Although
astronauts eventually regain
most of their bone mass, they
may not fully recover.
It is
important,
both
for
future
astronauts and for our health
here on Earth, that researchers
learn all that they can about why
osteoporosis occurs, and how
we can prevent it.
Using every-day bags and
cereal to represent bone, and a
heavy textbook to represent an
unexpected force (like a bump or a fall), you will see how low bone mass
affects bone, and why it is important that astronauts and people on Earth do
everything they can to prevent osteoporosis.
The Skeleton
Student Activity
Page 12
Get Ready
? 5 snack bags
? Corn puff cereal
? A very heavy book (like a dictionary)
? A broom and dustpan (for clean-up)
? Permanent marker
? Pen or pencil
Think about it
? Why is it important to have strong, healthy bones?
? What will happen if your bones become weak?
Formulate your hypothesis
What do you think will happen to a bone (in this case, represented by your
baggie and cereal) if force is suddenly applied to it? Will the results change if
the bone is progressively weakened?
Collect the data and test your hypothesis
1. Using a permanent marker, label the bags 1-5.
2. Bag 1 will represent a healthy bone on Earth. To build a “bone” you will
use pieces of cereal to represent individual units of bone mass. Fill the
bag with enough cereal so that the bag is very full and there is very little
air in it, but not so full that you cannot close it. Keep track of how many
pieces of cereal you put into the bag, and record this on your worksheet as
Normal Bone Density.
Close the bag, and make sure it is closed
tightly
—
otherwise you may wind up with a very big mess!
3. To represent a bone that has lost mass as a result of spaceflight or aging,
you now need to fill each bag with less cereal, or bone mass, than is in Bag
1.
Bag 1: 0% bone loss (normal bone)
Bag 2: 90% of original bone remains; 10% original bone lost
Bag 3: 80% of original bone remains; 20% original bone lost
Bag 4: 65% of original bone remains; 35% original bone lost
Bag 5: 50% of original bone remains; 50% original bone lost
The Skeleton
Student Activity
Page 13
To calculate the amount of cereal you need in Bag 2, you will need to
calculate 90% of
Normal Bone Density
. Fill Bag 2 with this amount. This
represents a loss of 10% of the bone mass.
4. Use a similar method to calculate 80%, 65%, and 50% of the Normal Bone
Density, and fill Bags 3, 4, and 5 with these amounts. Record the amount of
bone left in each bag on your worksheet.
5. Now you are ready to see what effects a sudden force may have on
weakened bones. Place Bag 1 on a hard surface. Then, quickly and
carefully, but forcefully, smash the heavy book onto the bag. Again using
the same amount of force, smash the remaining bags.
6. What happened to your bones? Count the number of
unaffected
cereal
pieces left in each bag, and record this on your worksheet.
7. How much of the bone was unaffected? To calculate this, use the formula
below and record your values on your worksheet.
8. How much of the bone was affected? To calculate this, subtract the
Unaffected Bone value from 100%. Record your values on your worksheet.
#
unaffected
remaining
in the bag
x
100
original
density of
÷
the bag
The Skeleton
Student Activity
Page 14
Bag of Bones Worksheet
Normal Bone Density=__________ pieces of cereal in Bag 1
Density of Bone 2 = 90% of Bag 1 = ________ pieces of cereal
Density of Bone 3 = 80% of Bag 1 = ________ pieces of cereal
Density of Bone 4 = 65% of Bag 1 = ________ pieces of cereal
Density of Bone 5 = 50% of Bag 1 = ________ pieces of cereal
Before the Experiment
After the Experiment
Bag
Bone
Loss
Represented
Density
(# of cereal
pieces in bag)
# of unaffected
pieces
% of
bone
unaffected
% of bone
affected
1
0%
2
10%
3
20%
4
35%
5
50%
Analyze the results
What happened as the amount of cereal decreased?
Now imagine that your baggie bone is actually a real bone. If a real bone
were built like your baggie bone, what would happen if a sudden force (like a
bump or fall) were applied to the bone?
Do your findings support your hypothesis? Why or why not?
How do you think we can prevent bone loss?
