Essay#2 - Structure is critical to proper function. Discuss key structural characteristics, and the various functions provided, for three of the items listed below.

  • cell (cytoskeleton)
  • tree (vascular tissues, strengthening tissues, lignin, pectin, turgor, large central vacuole)
  • jelly fish (hydrostatic skeleton)
  • arthropod (exoskeleton, chitin)
  • mammal (endoskeleton, bone)

Skeletal System

Using your text book, online video, and the Internet for help, fill in the tables and answer the questions below.
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Many invertebrates lack any specialized skeletal structures. they instead have what is called a "Hydrostatic Skeleton".Organisms have fluid contained inside of their bodies that exerts a pressure and along with their musculature helps to establish a shape. Movement and changes in shape can be achieved by contracting specific muscle groups.

Assuming a constant fluid volume, a change in the width of the animal must be compensated for by a change in length, which is brought about by the increase in internal hydrostatic pressure during muscle contraction. The animal can only attain its initial shape if it is surrounded by a stiff, elastic covering that will spring back to its original shape upon relaxation of the longitudinal musculature (jellyfish, round worm, flat worm) or if it contains opposing muscle groups for the initial shape change (earthworm).

Thus, the “skeletal” system in our hypothetical organism is a fluid. Temporary increases in internal pressure are caused by the contraction of 1 set of muscles, and this temporary pressure increase elongates another set of muscles. Hydrostatic skeletons play a role in the movements made by representatives of nearly every invertebrate phylum.

Some invertebrates have developed the ability to produce a "Shell" over it 's soft tissues for protection (clams, muscles, snails). The makeup of this protective covering is usually composed of calcium carbonate (shell) and tends to limit movement because of it's weight.

Others have taken that idea of a "shell" a step further and produce an "Exoskeleton" that is segmented and composed of a lighter more flexible material called CHITIN. It completely covers all external surfaces and is composed of segments. Having segments offers greater flexibility while still offering some basic protection from potential predators and desiccation.

While there is more flexibility than the shell seen in mollusks the exoskeleton is still limiting. Terrestrial arthropods rarely exceed 10cm in length. This is a direct result of the weight imposed. Aquatic arthropods can get substantially larger than 10cm as a result of buoyancy. In addition, in order to grow the arthropod must molt its exoskeleton. The organism is very vulnerable during the molting period.

Many animals have developed the ability to produce an internal skeleton or "Endoskeleton". This offers protection of vulnerable internal organs while still enabling soft sensory organs to be on the external surface as well as offering even more flexibility. The most primitive internal skeletal structure is seen in Echinoderms (sea urchins, sand dollars, star fish). Their internal skeleton is composed of thick plates with limited or no flexibility.

The most recent skeletal system to have evolved is seen a a group of animals called Chordates. During their embryological development a dorsal rod of connective tissue is established that acts as a support structure establishing shape and acting as a place where muscles can attach. In some representatives the notochord disappears as the organism matures, in others it remains, and in the most advanced representatives it develops into the vertebrae of the spine (Vertebrates).

Sea Squirt

Skeletal System:

The skeletal system is obviously responsible for providing shape for the organism. In addition, the skeletal system provides a place of attachment for muscles. The greater the number of different surfaces upon which muscles can attach, the greater the range of movement. Cortical bone forms a dense protective outer layer, Trabecular bone forms a inner branched matrix that provides strength with minimal weight. Vertebrates will vary in the amount of cortical bone based on their specific needs. Rhino, lots of weight & stress to bone needs a large amount of cortical bone. Birds that fly have comparatively a much smaller percentage of cortical bone to insure minimal weight (why shouldn't you give a dog chicken bones?).

Being that the skeleton is a living tissue it requires a blood supply in order to bring the living cells vital nutrients like O2, amino acids, carbohydrates, vitamins, and essential minerals like Ca+. Calcium is removed from the blood by the bone cells in large amounts so that it may be deposited around the cells (analogy: building a brick fortress all around yourself). A rather unique arrangement of bone cells and blood vessels exists to insure proper exchange. Blood vessels occupy canals called the Haversian Canals. Bone cells are arranged in concentric rings around the blood vessel. Tiny extensions off of the bone cells (canaliculi) allow cells to exchange nutrients and wastes with each other.The concentric rings of bone cells around a haversian canal forms an Osteon or also known as a Haversian System.


Osteoblasts remove Ca+ from the blood and lay it down in the form of calcium salts that harden into bone. Osteoblasts that become encased in the dense bone (now called osteocytes) form tiny extensions off of their plasma membranes (canaliculi) that connect them together and allow sharing of nutrients while inside the dense bone.

Another type of bone cell does the opposite of osteoblasts. Osteoclasts break down the bone and put Ca+ back into the blood stream. Hormones produced by the parathyroid gland in your neck help regulate the activity of both osteoblasts and osteoclasts.


1- What situation might be responsible for increasing Osteoclast activity?
Why do elderly people tend to have Osteoporosis (pits forming in the bone making them weak and easily broken).

In many animals the skeletal system also acts as a place to store minerals and produce blood cells (marrow). Round bones possess yellow marrow which stores a variety of minerals. Flat bones possess red marrow which contains stem cells responsible for the formation of blood cells.

3- List at list three bones that would contain red marrow and three bones that would contain yellow marrow.

4- What forms the appearance of concentric rings of an Osteon?

5- What can be found in the Haversian canal?

6- What is the difference between a hydrostatic skeleton, shell, endoskeleton and an exoskeleton?

7- Arthropods possess an exoskeleton. Identify at least two advantages and and two disadvantages of having an exoskeleton.

8- Differentiate between tendon and ligament.

Cartilage can be found in many of the joints of our skeletal system especially in the knee (meniscus) and vertebrae (disc).

9- What are the functions of cartilage in the joints of the skeletal system?

A membrane can be found lining movable joints. This membrane secretes a lubricating fluid (synovial fluid) that limits friction between the bones. No synthetic lubricant made by humans equals its lubricating properties. If the fluid escapes the joint area it can compromise the lubrication of the joint leading to stiffness, pain, and in advanced cases significantly reduce mobility.

[Muscular System][Circulatory System]