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(a) Nasal sinus cells (viewed with a light microscope), (b) onion cells (viewed with a light microscope), and (c) Vibrio tasmaniensis bacterial cells (seen through a scanning electron microscope) are from very different organisms, yet all share certain characteristics of basic cell structure.
Close your eyes and picture a brick wall. What is the basic building block of that wall? A single brick, of course. Like a brick wall, your body is composed of basic building blocks, and the building blocks of your body are cells.
Your body has many kinds of cells, each specialized for a specific purpose. Just as a home is made from a variety of building materials, the human body is constructed from many cell types. For example, epithelial cells protect the surface of the body and cover the organs and body cavities within. Bone cells help to support and protect the body. Cells of the immune system fight invading bacteria. Additionally, blood and blood cells carry nutrients and oxygen throughout the body while removing carbon dioxide. Each of these cell types plays a vital role during the growth, development, and day-to-day maintenance of the body. In spite of their enormous variety, however, cells from all organisms—even ones as diverse as bacteria, onion, and human— share certain fundamental characteristics.
A cell is the smallest unit of a living thing. A living thing, whether made of one cell (like bacteria) or many cells (like a human), is called an organism. Thus, cells are the basic building blocks of all organisms.
Several cells of one kind that interconnect with each other and perform a shared function form tissues, several tissues combine to form an organ (your stomach, heart, or brain), and several organs make up an organ system (such as the digestive system, circulatory system, or nervous system). Several systems that function together form an organism (like a human being). Here, we will examine the structure and function of cells.
To understand the structures of human cells, the subcellular structures will be discussed including cellular organelles. The word “organelle” means “little organ,” and organelles have specialized cellular functions, just as your body's organs have specialized functions.
Although the generalized characteristics commonly found in animal cells (human cells are animal cells) are examined in the following sections, keep in mind that the diversity of cell types found in the human body, including red blood cells, white blood cells, bones cells, skeletal muscle cells, and sperm cells, means that the structures of these cells are different from each other to suit their specific functions. Structure dictates function. This means that not only are their shapes and sizes different, but the organelles found in each type of cell differ to suit the specific function of that cell type. All multitude of cell types within a single human contain identical DNA (with some exceptions, particularly sperm and egg cells), but they use different parts of that DNA to suit their specific functions (cells of different types express different genes even though these cells contain the same genes as each other).
- "Biology 2e" by Mary Ann Clark, Matthew Douglas, Jung Choi, OpenStax is licensed under CC BY-NC 4.0