The Skeletal System Study Guide (page 2)

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Updated on Sep 22, 2011

Bones Are Alive

When you see a bone, it certainly doesn't look alive. However, bones are actually living tissue with cells that require food molecules for cellular metabolism and that are served by the circulatory system. Small canals in the bone tissue allow the blood vessels to enter the interior of bones where a soft, pulpy tissue called marrow exists. Bone marrow is the tissue that produces blood cells and platelets (the clotting factors of the blood).

Because they are alive, bones can grow. They are initially formed from cartilage or other connective tissue. As the organism grows, much of this connective tissue is replaced with calcium phosphate mineral formations. At the ends of long bones in the arms and legs are growth plates that still consist of cartilage even in teenagers and some young adults. As new cartilage is made, the older part of the growth plate will ossify (turn to bone), lengthening the bone. In adulthood, this growth of new cartilage at the ends of the growth plates stops, and the bones are no longer able to grow longer. However, bone tissue is constantly being broken down and reformed. Calcium is deposited and removed from the reservoir stored in the bones. Thus, bones are not static; they are very much alive.


A joint is the place where two bones come together, and special connective tissues at the joint prevent the bones from damaging each other. Joints hold bones in place but allow them to be far enough apart for movement. Joints can be freely movable (such as the elbow or knee), slightly movable (such as the vertebrae in the back), or immovable (such as the joints that join the bones of the skull together).

But What about Plants?

Plants also need to support their tissues and give shape to their bodies. However, they do not do so with a skeletal system. Nonvascular plants do not have a great need for support because they don't grow very tall, but vascular plants need to be supported by rigid tissue. The cells that make up the vascular tissue of plants form a continuous system of tubes running from the roots through the stems and to the leaves. Water and nutrients flow to the leaves through vascular tissue called xylem, where they are used in the process of photosynthesis. Following that process, the products of photosynthesis then flow through vascular tissue called phloem back down to the roots.

In Short

The skeleton is the chief structural system, which, along with the skin, provides form and shape to the body. In human beings, the skeletal system has 206 bones, most of which are found in the hands and feet. The bones of the skeletal system are rigid, but the whole system is flexible because of the joints where bones are joined. This system of bones is also highly coordinated with the muscular system. Bones are actually living tissue with cells that require food molecules for cellular metabolism and that are served by the circulatory system. Plants do not have skeletal systems, but they do have a need for support. Nonvascular plants rely on the rigid walls of their cells because they don't grow very tall. Vascular plants, however, need to be supported by rigid tissue and have a system of tubes for transporting water and nutrients throughout the plant body, even to very great heights.


Practice problems of this concept can be found at: The Skeletal System Practice Problems

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