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Digital illustration of human cell. Digital illustration of human cell. More than 100 trillion cells make up the human body. Most of these cells contain all the genes and other information needed to “build” a human being. Much of this genetic information is found in the nucleus of the cell, a “control center” that keeps all the material together in one place.

Cells: The Basic Units of Living Organisms
Before exploring how the human body works, it is necessary to understand the components of the body and their anatomical relationships to each other. The simplest structural units into which a complex multi–cellular organism can be divided and still retain the functions, characteristic of life are called cells.

More than 100 trillion cells make up the human body. Most of these cells contain all the genes and other information needed to “build” a human being. Much of this genetic information is found in the nucleus of the cell, a “control center” that keeps all the material together in one place. The nucleus stores its genetic material in packages called chromosomes. Humans have 46 chromosomes in each cell–23 from their mother and 23 from their father.

After fertilization, the two sets of chromosomes match up to form 23 pairs. The chromosomes in the 23rd pair are called the sex chromosomes (allosomes), X and Y; they determine a person’s sex. Males usually have one Y chromosome and one X chromosome; females usually have two X chromosomes. Each chromosome is made up of genes. Genes contain the information used by other parts of the cell to make proteins, the body’s building blocks. Proteins make up the structure of our organs and tissues; they are also needed for our body’s chemical functions. Each protein performs a specific job in different types of cells, and the information for making at least one protein is contained in a single gene.

Formation of zygote by fertilization Formation of zygote by fertilization Each human organism begins as a single cell, a fertilized egg, which divides to create two cells, each of which divides in turn to result in four cells, and so on
General aspects

One of the unifying and important generalizations of biology is that certain fundamental activities are common to almost all cells and represent the minimal requirements for maintaining cell integrity and life. Thus, for example, a human liver cell and an amoeba are remarkably similar in terms of how they exchange materials with their immediate environments, obtain energy from organic nutrients, synthesize complex molecules, duplicate themselves, and detect and respond to signals in their immediate environments.

Each human organism begins as a single cell, a fertilized egg, which divides to create two cells, each of which divides in turn to result in four cells, and so on. If cell multiplication were the only event occurring, the end result would be a spherical mass of identical cells.

During development, however, each cell becomes specialized for the performance of a particular function, such as producing force and movement or generating electric signals. The process of transforming an unspecialized cell into a specialized cell is known as cell differentiation, the study of which is one of the most exciting areas in biology today.

Cell differentiation Cell differentiation occurs in multicellular organisms Differentiation is a common process in adults as well: adult stem cells divide and create fully differentiated daughter cells during tissue repair and during normal cell turnover. Differentiation dramatically changes a cell′s size, shape, membrane potential, metabolic activity, and responsiveness to signals.
Cellular differentiation

Cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of tissues and cell types and finally into a complex organism.

A cell that is able to differentiate into all cell types of the adult organism is known as pluripotent. Such cells are called embryonic stem cells in animals and meristematic cells in higher plants. A cell that is able to differentiate into all cell types, including the placental tissue, is known as totipotent.

Totipotency is the ability of a single cell to divide and produce all the differentiated cells in an organism, including extraembryonic tissues. Totipotent cells include spores and zygotes. In some organisms, cells can dedifferentiate and regain totipotency.

For example, a plant cutting or callus can be used to grow an entire plant. Pluripotency (meaning having power) refers to a stem cell that has the potential to differentiate into any of the three germ layers: endoderm (interior stomach lining, gastrointestinal tract, the lungs), mesoderm (muscle, bone, blood, urogenital), or ectoderm (epidermal tissues and nervous system).

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