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In this lesson, we will review the organization of organisms — from cells to organisms.
Animals have many different body forms, but all animals must have internal systems that meet a few basic needs: respiration, circulation, excretion, reproduction, and digestion. Comparative physiology is the study of the similarities and differences among animals and their various solutions to meeting those basic needs.
The basic structural unit in all organisms is the cell. In multicellular eukaryotes, cells are usually highly specialized and highly differentiated. For example, there are approximately 200 different cell types in the human body.
Cells in turn are grouped into tissues. Tissues are composed of cells that are similar in structure and function. In all but the simplest animals, there are four basic types of tissue: epithelial tissue, connective tissue, nervous tissue, and muscle tissue.
Epithelial tissue generally occurs in sheets of tightly packed cells. In vertebrates, it covers the outside of the body and lines body cavities. Epithelial tissue is organized into layers. The outside layer is bathed in air or some fluid, while the innermost layer is attached to a supportive membrane.
Connective tissue binds and supports other tissues. Connective tissue actually contains few cells that are dispersed through an extracellular matrix. The matrix is usually a web of fibers embedded in a uniform foundation. Collagenous fibers are non-elastic bundles of collagen molecules that have been twisted into rope-like structures. Elastic fibers are stretchy fibers made of a protein called elastin. Reticular fibers are thin, branched fibers made from collagen. They form a fabric that supports and connects other tissues. In vertebrates, connective fibers form adipose tissue, loose connective tissue, cartilage, bone, and blood.
Nervous tissue forms the nerve fibers that sense stimuli and transmit signals from one part of the body to another. Nervous tissue is composed of cells called neurons. The neuron consists of a cell body and two or more long extensions or processes called dendrites and axons. In humans a neuron can be over one meter in length.
Muscle tissue is the most abundant tissue in most animals. It is composed of long, thin muscle fibers. These fibers can contract when they receive a stimulus from a neuron bundle. The two contractile proteins actin and myosin are found within the cytoplasm of muscle cells. There are three types of muscle tissue found in vertebrates. Skeletal muscle is attached to bones by tendons (a form of connective tissue) and is responsible for voluntary movements of the body. Skeletal muscle is also called striated muscle because overlying layers of muscle give it a striped appearance. Cardiac muscle forms the contractile walls of the heart. Smooth muscle lacks striations. It is found in the digestive tract, bladder, and other internal organs.
Which tissue type contains few cells?
The correct answer is D; connective tissue is a matrix of collagen fibers. Muscle, nervous, and epithelial tissues have many more cells than connective tissue.
The next level of organization in all but the simplest animals is the organs. Organs are composed of groups of tissue (sometimes in layers) that work closely together to perform some function. For example, the stomach consists of four layers of tissue: epithelium, connective tissue, smooth muscle, and another layer of connective tissue.