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Cells, Tissues, and Organs: The Hierarchical Organization of Life
DIRECTIONS: Read the passage below. Use the information you gather to categorize the eye structures listed in the word
bank as connective tissue, muscle tissue, nervous tissue, or epithelial tissue on the associated graphical organizer.
WORD BANK: Ciliary Body, Optic Nerve, Choroid, Extrinsic Muscles, Sclera, Retina, Fatty Tissue, Cornea
Our bodies are made of cells, tissues, and organs. The cell is the smallest unit of life. All living things are made of
cells. One or more kinds of cells working together makes one kind of tissue. Our bodies have many different kinds of
cells because we have many different kinds of tissue. Tissues are groups of cells working together to do the same job.
The different kinds of tissues are classified into four groups, epithelial tissue, connective tissue, nerve tissue, and
muscle tissue. Within each group are many specific of tissues, but they are similar in the job they do.
Epithelial tissue’s job is to protect the body from injury and infection. This group includes the skin and the inner
surfaces of the body such as the surface of the lungs, stomach, intestines, blood vessels, and other organs. Connective
tissue supports our bodies. Bone, cartilage, and fatty tissue are in this group. These tissues are strong and yet usually can
stretch to hold our bodies together. Connective tissue gives our bodies a frame.
Nerve tissues are made of cells called neurons. They are the information networks for our bodies. Nerves send
information from one part of our bodies to another. They have long branching structures called dendrites and axons that
connect one cell to another. Impulses pass along the axon of one nerve cell to the dendrites of others. These neural
connections send sensory information to the brain, and also control muscle movement.
Three types of muscle cells are combined to make the muscle tissue group. All these muscle tissues contract and
relax. The voluntary muscles are striated muscle tissue. We move these muscles by choice. They are the muscles
attached to our skeletons. Smooth muscle tissue makes involuntary muscles. We do not choose to move these muscles,
but they move on their own. These muscles are inside our bodies in our organs. The muscles of our lungs, stomach, and
intestines are made of smooth muscle tissue. Cardiac muscle tissue is the third type of muscle tissue. Cardiac muscle
tissue makes our hearts.
Organs are groups of tissues that work together to do a job. Almost every organ is built at least in part from all
four tissue types. For example, our eyes are organs made of nerve tissues, epithelial tissues, muscle tissues, and
connective tissues working together as a unit to allow sight.
Our bodies are made of cells, which make tissues, which in turn make organs. This hierarchical structure can be
extended in both ways. For example, cells are composed of yet smaller parts called organelles, which are themselves
constructed from macromolecules like protein. In the larger direction, organs function together as an organ system,
which work together to keep an organism alive. Organisms live together in populations, which are part of biological
communities and ecosystems. At the very top of this hierarchical scheme is the biosphere: the space including Earth’s
soils, surface, water, atmosphere, and the life they contain.