The Skeletal System Study Guide (page 2)
The skeletal systems of organisms work with the muscles to allow the movement of body parts and the locomotion of the whole organism. They also protect the soft internal organs from injury. The skeletal system also acts as a repository of minerals used in various aspects of metabolism. In vertebrates, the interiors of bones in the skeletal system are responsible for producing blood cells. Skeletal systems appear in arthropods (insects and crustaceans) and vertebrates (birds, fish, reptiles, and mammals). In arthropods, the skeletal system is an exoskeleton, and in vertebrates, the skeletal system is an endoskeleton.
Vertebrates and Invertebrates
Earlier, when classifying animals, it was mentioned that we separate multicelled animals into two overall, large groups. Those two groups are the vertebrates (those animals with backbones) and the invertebrates (those without backbones).Many invertebrates have no internal skeleton, but they have what is called an exoskeleton, which is a hard outer coating. Insects, crabs, and lobsters all have exoskeletons. Muscles can attach to the exoskeleton, which acts as protective body armor. The disadvantage to an exoskeleton is that it limits the organism's growth. Many invertebrates have worked around this problem by molting (or shedding) their exoskeletons at various stages in their life cycles so that they can grow their bodies and then grow a new exoskeleton.
In vertebrate animals, the skeleton is internal (an endoskeleton) and has evolved structures called vertebrae, which are hard, bony projections surrounding the spinal cord. This vertebral column is commonly known as the backbone. Although the exoskeleton of invertebrates is dead, excreted tissue (such as hair and nails), the endoskeleton of vertebrates is composed of living tissue that is known as the skeletal system. The cells that make up the skeletal system are called osteocytes, and they associate together to form connective tissue. This connective tissue, along with cartilage tissue (another type of connective tissue), comprises the skeletal system. Because the skeletal system is alive, it can grow with the organism and molting is not necessary.
The Skeletal System
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 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 highly coordinated with the muscular system. The nervous system is responsible for the coordination necessary for such actions as painting fine detail in a picture or participating in an Olympic decathlon. The skeletal system is a wonderful example of how specialized tissues, collected together into several different organ systems, are further organized into cooperative groups of interacting systems. The muscular and skeletal systems are so closely linked that they are often referred to as one large system called the musculoskeletal system.
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.
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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