Mastering Concepts
28.1
1. How do the skeletal and muscular systems interact?
Skeletal and muscular systems interact to move the body as muscles pull against the
skeleton.
2. Describe similarities and differences among the three main types of skeletons.
All three skeletons have attached muscles that pull against them. Differences between the
three skeletons are that only the exoskeleton is external, only the hydrostatic skeleton is
flexible, and only the endoskeleton allows an animal to become large.
3. How do vertebrate skeletons reveal common ancestry?
Vertebrate skeletons are composed of the same cell types, and many of the bones are
arranged in similar ways.
28.2
1. What are the two subdivisions of the human skeleton?
The major groups of bones that make up the human skeleton are the axial and
appendicular skeletons.
2. What are the locations of the pectoral and pelvic girdles?
The pectoral girdle is in upper part of the body and consists of the shoulder and collar
bones that attach to the arms. The pectoral girdle is in the lower part of the body and
consists of the hipbones that attach to the legs.
28.3
1. Describe the organization and functions of bone tissue and cartilage.
Bone and cartilage are both connective tissues that feature cells embedded in a solid
matrix. In bone, the matrix is mineralized collagen. The collagen gives bones flexibility,
elasticity, and strength. A cross section of bone reveals concentric rings of osteons. Bone
cells are held in osteons connected by canals and receiving nerve and blood supply. In
cartilage, a matrix of collagen is filled with protein fibers, water, and chondrocytes that
secrete cartilage. The fibers give cartilage resilience, strength, and elasticity, and the
water makes it a good shock absorber. A cross section of cartilage shows fibers and a
scattering of cells that secrete cartilage and elastin fibers. Cartilage has no blood supply.
2. List the names and functions of each of the three cell types in bone tissue.
Osteoclasts destroy bone tissue, osteoblasts build bone tissue, and osteocytes are former
osteoblasts that are located within osteons.
3. What are osteons, and in what type of bone tissue are they most concentrated?
Osteons are microscopic structures that look like concentric rings within bony tissue. In
an osteon, a space that contains a bone cell is connected to other such spaces by canals.
Blood vessels and nerves are the center of the osteon. Osteons are most abundant in
compact bone.
4. How do osteoclasts and osteoblasts remodel and repair bones throughout life?
Early in life, after cartilaginous bones have been formed, osteoblasts enter the
cartilaginous matrix and secrete the bony matrix. As the child grows, the replacement of
cartilage by bone is restricted to the ends of bones. Bones are fully-grown when the
person is in their early 20s. The bones become thicker and denser through heavy exercise
and become lighter and less dense from lack of exercise. Throughout life, osteoclasts
break down bone tissue if blood calcium concentrations dip too low. In addition, if a
bone is broken, osteoblasts repair the break.
5. How do bones participate in calcium homeostasis?
When blood levels of calcium are too low, bones release calcium ions into the
bloodstream in response to stimulation by parathyroid hormone. In contrast, when blood
levels of calcium are too high, bones absorb calcium ions from the bloodstream and
deposit it in the matrix in response to stimulation by calcitonin from the thyroid gland.
6. What structures form a synovial joint?
The structures in a synovial joint include two adjacent bones that are connected by
ligaments. The surfaces of these bones are covered with cartilage, and a capsule of
fibrous connective tissue surrounds the pair of bones. The inner membrane of the capsule
secretes a fluid that lubricates the joint, making it freely movable.
28.4
1. What is an antagonistic pair of muscles?
An antagonistic pair of muscles moves a bone around a joint in a back-and-forth motion.
Contraction of one muscle pulls a bone in one direction; contraction of the second muscle
pulls the bone in the opposite direction.
2. Describe the levels of organization of a vertebrate muscle.
Moving from the largest structure to the smallest, a muscle is an organ consisting of
several tissue types, including muscle tissue. Muscle tissue consists of parallel bundles
of cells called muscle fibers. Each muscle fiber contains hundreds of myofibrils, which
are in turn made of many filaments of the proteins actin and myosin.
3. Describe how sliding filaments shorten a sarcomere.
The pivoting head of the thick filament myosin swings out and connects with the thin
filament actin to form a cross bridge. The thin filament then slides between the thick
filaments, shortening the sarcomere.
4. How do motor neurons, calcium ions, and ATP participate in muscle contraction?
Motor neurons release the neurotransmitters that cause the electrical signal in muscle
cells. The electrical signal then causes Ca2+ to leave the endoplasmic reticulum and bind
to troponin so that tropomyosin can move aside. ATP allows for the release of the cross
bridges so that myosin can unbend and prepare for the next pull. ATP also allows for the
active transport of Ca2+ back into the endoplasmic reticulum so the muscle can relax.
28.5
1. Describe the role of creatine phosphate in muscle metabolism.
At the start of muscular activity creatine phosphate donates a high-energy phosphate to
ADP to quickly regenerate ATP.
2. What happens when a muscle cell cannot generate ATP by aerobic respiration?
Muscle cells switch to fermentation when they can’t generate ATP by aerobic respiration.
28.6
1. How can the same muscle generate both small and large movements?
The same muscle can generate both small and large movements, depending on how many
of its motor units are engaged.
2. How do slow- and fast-twitch muscle fibers differ?
Slow- and fast-twitch muscles differ in their supplies of oxygen from myoglobin
molecules. Fast-twitch muscles (larger white fibers) lack myoglobin and a rich blood
supply, so they often use anaerobic metabolic pathways and cannot sustain prolonged
contraction. Slow-twitch muscles (smaller red fibers) have ample supplies of myoglobin,
mitochondria and capillaries. They use aerobic pathways.
3. How does exercise strengthen muscles?
Through active exercise, muscle cells grow larger; blood supply to the muscles improves
as capillaries become more numerous. Also, exercise increases the concentration of
active enzymes and the abundance of mitochondria in muscle cells.
28.7
1. Summarize the hypothesized relationship between the myosin gene mutation and brain
size in humans and other primates.
The mutation of the myosin gene on chromosome 7 is only expressed in the muscles used
for chewing, making them smaller and weaker. The mutation, not found in other
primates, arose around 2.4 million years ago and coincides with the evolutionary trend
for increased brain size. Researchers hypothesize that the smaller chewing apparatus
allowed for increased brain size.
2. Use the genetic code in chapter 7 to predict the amino acid sequence corresponding to
the two gene fragments in figure 28.21. How does the deletion result in a truncated
protein?
The first sequence of amino acids would be: leu-val-asp-leu-phe-asp-tyr-trp-trp-glu-valser. The second sequence would be: ala-trp-gly-lys-thr-gly-asp-ile-ile-gly-val-thr-gln-ilearg-STOP. The loss of two bases causes a stop codon to appear in the DNA sequence
after only 15 amino acids in the second part of the sequence; whereas, in the non-human
primate DNA there is no stop codon in the genetic code portion given. This full code
results in 17 amino acids in the second sequence and a continuation of the code beyond
that.
Write It Out
1. Distinguish among a hydrostatic skeleton, an exoskeleton, and an endoskeleton. Give
an example of an animal with each.
A hydrostatic skeleton (hydro- means water) consists of fluid constrained within a layer
of flexible tissue. Combined with muscle action, a hydrostatic skeleton can provide
locomotion. An example of an animal with a hydrostatic skeleton is a jellyfish. An
exoskeleton (exo- means outside) protects an organism from the outside, much like a suit
of armor. A beetle has an exoskeleton. An endoskeleton (endo- means inner) is an
internal support structure. Humans have endoskeletons.
2. Explain the observation that animals with exoskeletons and endoskeletons are better
represented in the fossil record than are animals with hydrostatic skeletons. How might
this difference alter scientific interpretations of the fossil record?
Hard body parts fossilize more easily, and therefore sea animals with hydrostatic
skeletons are underrepresented in the fossil record. This can lead to several incorrect
interpretations of the record: that there were fewer soft bodied animals, that soft bodied
animals were less important in the evolutionary history of life, or that hard bodied
organisms evolved quite suddenly.
3. What are the components of the axial and appendicular skeletons?
The axial skeleton, located in the longitudinal central axis of the body, consists of the
skull, vertebral column, ribs, and sternum. The appendicular skeleton consists of the
appendages (upper and lower limbs), and the bones that support them (pectoral girdle and
pelvic girdle).
4. What role does cartilage play in the vertebrate skeletal system?
In the vertebral column, the vertebrae are separated by cartilage disks that cushion shocks
and enhance flexibility. The flexibility of the cartilage between the ribs and other bones
allows muscles to elevate the ribs, a movement important in breathing. At synovial joints,
cartilage reduces friction where bone moves against bone.
5. What are the differences between spongy bone and compact bone?
Compact bone and spongy bone differ in density. Compact bone is dense and consists of
closely packed osteons. Spongy bone is hard, but it has many large spaces between a web
of bony struts, which makes it lighter. Red marrow, a nursery for blood cells and
platelets, fills the spaces with spongy bone. Spongy bone tissue contains few osteons;
instead, the osteocytes in spongy bone acquire nutrients and oxygen directly from the
nearby bone marrow.
6. What are the two major components of bone matrix? How do they work together to
give bone its characteristics?
In bone matrix, collagen gives the bone flexibility, elasticity, and strength. The hardness
and rigidity of bone comes from calcium, phosphate, and other minerals that coat the
collagen fibers.
7. Describe the events of bone development from embryo through adulthood.
The matrix of the cartilage model in the embryo hardens with Ca2+ deposits in the fetus,
causing the matrix to degenerate and allowing blood and osteoblasts to enter and secrete
bone matrix. This change to bone continues in the ends of the bone in the newborn and
then is limited to the growth plate through childhood and into the early 20’s. Through
adulthood bone continually remodels with use or disuse, calcium imbalances, or breaks.
8. Would an overactive parathyroid prevent osteoporosis or contribute to it? Explain your
answer.
An overactive parathyroid gland would contribute to osteoporosis. Parathyroid hormone
(PTH) stimulates bones to release calcium when blood calcium levels are low.
9. How do antagonistic muscle pairs move bones? Give an example of such a pair.
A contracting muscle can pull a bone in only one direction; it cannot push the bone the
opposite way. The body can generate back-and-forth movements because many skeletal
muscles occur in antagonistic pairs whose members operate in opposite directions. For
example, when a person contracts the biceps, the arm bends at the elbow joint.
Contraction of the triceps muscle straightens the arm.
10. Describe four muscle proteins and their functions.
Actin (the thin filament) is two entwined protein strands that slide through myosin.
Myosin (the thick filament) has heads that form cross bridges and pull the actin.
Troponin is a small protein on tropomyosin that can change shape and can bind Ca2+.
Tropomyosin is a ribbon-like protein that covers actin and keeps myosin from binding.
11. What is the role of tetanus when you use your arms to lift a heavy box?
Tetanus results from a high rate of action potentials and allows your muscles to contract
smoothly for a prolonged time as you lift the box.
12. How do the effects of exercise (or lack thereof) illustrate homeostasis in bones and
muscles?
Less–used bones lose mass as the minerals slowly dissolve. For example, astronauts lose
bone density if they are in a prolonged weightless environment because their
musculoskeletal systems don’t have to work as hard as they do against Earth’s gravity.
Likewise, a muscle exercised regularly increases in size because each muscle cell
thickens. An unused muscle shrinks.
13. How does the muscular system interact with the nervous system? The skeletal
system? The respiratory system? The circulatory system?
The muscular system requires neurotransmitters from the nervous system to generate its
electrical signal. The muscles of the muscular system attach to bones for movement of
the body. The respiratory system brings in the O2 that muscles need for aerobic
respiration, and the circulatory system delivers it through the capillaries.
14. What roles does fluid play in hydrostatic skeletons, cartilage, and synovial joints?
The hydrostatic skeleton consists of fluid constrained within a layer of flexible tissue. In
cartilage, water cleanses the tissue and bathes it with dissolved nutrients from nearby
blood vessels. The high water content of the matrix also makes cartilage an excellent
shock absorber. A synovial joint consists of movable bones joined by a fluid-filled
capsule of fibrous connective tissue. A lubricating fluid inside the capsule, combined
with slippery cartilage on the bone ends, allows bones to move against each other in a
nearly friction-free environment.
15. Search the Internet for disorders of the skeletal or muscular system. Choose one such
illness to research in more detail. In the context of what you have learned in this chapter,
describe how the disorder interferes with bone or muscle function. What causes the
disorder? Is a treatment or cure available?
Possible disorders include, but are not limited to: rheumatoid arthritis, osteoporosis,
carpal tunnel system, fibromyalgia, Duchenne muscular dystrophy. Using rheumatoid
arthritis as an example answer: In rheumatoid arthritis the immune system attacks the
tissues of the synovial joints causing inflammation and potentially destruction of the
cartilage covering the ends of the bones. This causes severe pain and sometimes even
deformities in the joints as the joint surfaces no longer move smoothly and easily. While
there is no known cure for rheumatoid arthritis, it can be treated with pain relievers,
steroids or other anti-inflammatories, rest and joint strengthening exercises.
16. What is the role of calcium in bones? In muscle contraction?
Calcium hardens the matrix of the bones, which help regulate calcium homeostasis. In
muscle contraction, stimulation by a motor neuron triggers the release of calcium ions
from the muscle cell’s endoplasmic reticulum. These calcium ions bind to regulatory
proteins that normally block muscle contraction. As a result, the muscle can contract.
17. The following table shows recent men’s world-record times for various running
events. Graph the distance traveled against the average running speed, in meters per
second. How does muscle use of ATP over time explain the graph?
Initially, in the shorter duration races (100 and 200 m) creatine phosphate quickly
replenishes ATP. There is a sharp drop in average speed for the 400 m race as creatine
phosphate is exhausted and the muscles switch to the less efficient lactate fermentation.
The longer races (800 m or more) allow the muscles to use aerobic respiration to generate
ATP. Though the ATP doesn’t come in quick bursts, it is steady and continual and so the
average speeds no longer vary greatly.
Pull It Together
1. How do bones help maintain blood calcium concentrations?
Bones serve as a reservoir for calcium. Hormones from the parathyroid gland use
negative feedback to control how much calcium is shuttled between bone and blood.
2. How do actin and myosin interact in the sliding filament model of muscle contraction?
Muscle contraction occurs when thick and thin filaments move past one another. Myosin
cross bridges connect to actin filaments and pull them together with use of ATP.
3. Add acetylcholine, calcium ions, ATP, and creatine phosphate to this concept map.
“Acetylcholine” is released from “Motor neurons”. “Acetylcholine” stimulates the
contraction of “Muscle fibers”. “Calcium ions” expose binding sites for myosin on
“Actin”. “ATP” supplies the energy to ratchet “Myosin”. “Creatine phosphate” during
muscle activity replenishes “ATP”.
4. How do fast-twitch muscle fibers differ from slow-twitch muscle fibers?
Slow-twitch muscle fibers use ATP slowly and regenerate it using aerobic respiration.
Fast-twitch muscle fibers burn ATP quickly; they also have fewer capillaries and store
less oxygen than slow-twitch muscle fibers.