Cells

Cell

make-animal-cell-using-food-800X800    anatomy

The cell is the basic unit of a living organism. In multicellular organisms (organisms with more than one cell), a collection of cells that work together to perform similar functions is called a tissue. In the next higher level of organization, various tissues that perform coordinated functions form organs. Finally, organs that work together to perform general processes form body systems.

Types of cells

Multicellular organisms contain a vast array of highly specialized cells. Plants contain root cells, leaf cells, and stem cells. Humans have skin cells, nerve cells, and sex cells. Each kind of cell is structured to perform a highly specialized function. Often, examining a cell’s structure reveals much about its function in the organism. For instance, certain cells in the small intestine have developed microvilli (hairs) that promote the absorption of foods. Nerve cells, or neurons, are another kind of specialized cell whose form reflects function. Nerve cells consist of a cell body and long attachments, called axons, that conduct nerve impulses. Dendrites are shorter attachments that receive nerve impulses.

Sensory cells are cells that detect information from the outside environment and transmit that information to the brain. Sensory cells often have unusual shapes and structures that contribute to their function. The rod cells in the retina of the eye, for instance, look like no other cell in the human body. Shaped like a rod, these cells have a light-sensitive region that contains numerous disks. Within each disk is embedded a special light-sensitive pigment that captures light. When the pigment receives light from the outside environment, nerve cells in the eye are triggered to send a nerve impulse in the brain. In this way, humans are able to detect light.

Cells, however, can also exist as single-celled organisms. The organisms called protists, for instance, are single-celled organisms. Examples of protists include the microscopic organism called Paramecium and the single-celled alga called Chlamydomonas.

Prokaryotes and eukaryotes. Two types of cells are recognized in living things: prokaryotes and eukaryotes. The word prokaryote literally means “before the nucleus.” As the name suggests, prokaryotes are cells that have no distinct nucleus. Most prokaryotic organisms are single-celled, such as bacteria and algae.

The term eukaryote means “true nucleus.” Eukaryotes have a distinct nucleus and distinct organelles. An organelle is a small structure that performs a specific set of functions within the eukaryotic cell. These organelles are held together by membranes. In addition to their lack of a nucleus, prokaryotes also lack these distinct organelles.

The structure and function of cells

The basic structure of all cells, whether prokaryote and eukaryote, is the same. All cells have an outer covering called a plasma membrane. The plasma membrane holds the cell together and permits the passage of substances into and out of the cell. With a few minor exceptions, plasma membranes are the same in prokaryotes and eukaryotes.

The interior of both kinds of cells is called the cytoplasm. Within the cytoplasm of eukaryotes are embedded the cellular organelles. As noted above, the cytoplasm of prokaryotes contains no organelles. Finally, both types of cells contain small structures called ribosomes. Ribosomes are the sites within cells where proteins are produced. (Proteins are large molecules that are essential to the structure and functioning of all living cells.) Ribosomes are not bounded by membranes and are not considered, therefore, to be organelles.

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