The cells the basic unit of structure and function

Cells are the structural and functional units of all organisms, including humans. An adult human body contains about 75 trillion cells. Most cells are composed of characteristic parts that work together to allow them to perform specific body functions.

There are approximately 200 different types of cells in the human body, but all of them share certain common characteristics:

■ All cells perform the general housekeeping functions necessary to sustain life. Each cell must obtain nutrients and other materials essential for survival from its surrounding fluids. Recall from chapter 1 that the total of all the chemical reactions that occur in cells is called metabolism.
■ Cells must dispose of the wastes they produce. If a cell didn’t remove its waste products, this waste would build up in the cell and lead to its death.
■ The shape and integrity of a cell is maintained by both its internal contents and its surrounding membrane.
■ Most cells are capable of undergoing cell division to make more cells of the same type.





The small size of cells is the greatest obstacle to determining their nature. Cells were discovered after microscopes were invented, and high-magnification microscopes are required to see the smallest human body cells. The dimensional unit often used to measure cell size is the micrometer (μm). One micrometer is equal to 1/10,000 of a centimeter (about 1/125,000 of an inch). For example, a red blood cell has a diameter of about 7–8 μm, whereas one of the largest human cells, an oocyte, has a diameter of about 120 μm.
 compares the size of the smallest unit of structure in the human body (an atom) to various cell types as well as to other macroscopic structures, such as an ostrich egg and a human.