Cell Definition

The cell is the basic structural and functional unit of all living organisms. About 100 trillion cells make up one of the most complex of such organisms, the human.

Types of Cells in the Human Body

There are three basic types of cells in the body: stem cells, germ cells, and somatic cells.

The foundation of life: stem cells

Stem cells are the primal, undifferentiated cells that give rise to all other cells. They are primarily abundant and functional during early embryonic development (embryonic stem cells). These are the cells of the blastocyst, the earliest form of a new life, and at this stage are totipotent: They have the ability to become any other kind of cell. Genes instruct dividing stem cells how to differentiate or form specific kinds of cells that then develop into various organs and body structures.

Umbilical cord blood stem cells

The blood that remains in the umbilical cord and placenta at birth is an abundant source of multipotent blood stem cells. Cord blood transplantation is an emerging treatment for leukemia and other cancers as well as sickle cell disease and other blood disorders. Many people now opt to collect and store or donate the cord blood of their newborns after birth.

As the body takes shape stem cells become increasingly diffuse and specialized, transitioning to pluripotent (able to become cells of distinct body systems such as cardiovascular or gastrointestinal) and finally multipotent (able to become cells of specific kinds, such as blood or bone). The most versatile stem cell that remains when development is complete is the blood stem cell, which has the ability to differentiate into various types of blood cells throughout life. Other adult stem cells (also called somatic stem cells to distinguish them from embryonic stem cells) exist in most body tissues though are interspersed among other cells. Their role remains unclear though they appear responsible for large-scale regeneration of tissue such as can occur in the liver.

The cells of reproduction: germ cells

Germ cells, also called gametes, are the cells of reproduction: the ova or eggs (female) and the spermatozoa or sperm (male). Gametes are haploid cells; each gamete contains one-half the complement of chromosomes. When two gametes merge in conception, the resulting zygote acquires the full complement of genetic material.

The cells of the functioning body: somatic cells

All cells that are not stem cells or germ cells are somatic cells. Somatic cells make up more than 99 percent of the cells in the adult body. They are diploid cells; each somatic cell contains the full complement of chromosomes. Somatic cells make up the organs and structures of the body. They are the body’s primary working units, responsible for carrying out the myriad functions of METABOLISM that support life. Though similar in structure and function, somatic cells are broadly diverse in their activities and specializations.

Cell Structure

Most cells have standard, key structural components in common. These include

  • Plasma membrane, the cell’s outer wall made up of a protein layer and a lipid (fatty) layer, that separates the cell’s contents from its external environment yet permits interaction between the cell and the external environment
  • Cytoskeleton, a dynamic construct of filaments and fibers that support the cell’s shape and inner components
  • Cytoplasm, a watery fluid that suspends the inner structures of the cell, moves substances through the cell, and conducts electricity
  • Nucleus, the core of the cell, separated from the cytoplasm by a thin membrane called the nuclear envelope, which contains the cell’s chromosomes and genetic material
  • Mitochondria, self-replicating structures called organelles that generate the energy, in the form of adenosine triphosphate (ATP), the cell needs to function
  • Ribosomes, another type of organelle, which synthesize proteins according to genetic directions the mitochondrial RNA brings to the ribosomes
  • Lysosomes and peroxisomes, also organelles, which contain enzymes to break down cellular wastes into component molecules the cell can recycle

Cell Function

The cell is responsible for all of the functions of metabolism that support the body. Most of the body’s 100 trillion cells have specialized responsibilities. Blood cells transport oxygen, glucose, and other nutrients throughout the body and collect molecules of metabolic waste that cells in the liver and kidneys dismantle, recycle, or eliminate from the body. NERVE cells conduct electrical impulses. Muscle cells contract the heart and move the body. Other cells make hormones, absorb nutrients, fight infection, and so on. Regardless of their specializations, however, the primary activity of all cells is the synthesis of the enzymes and proteins that carry out the biochemical tasks of living.

Cell Division

One of the most important functions of a cell is to replicate itself, as this is the activity that sustains life. Some cells, such as those that line the gastrointestinal tract, replicate every 12 hours. Other cells, such as those in the heart and the liver, divide perhaps once every 12 months or so. Though cells have vast ability to perpetrate themselves in such fashion, there appear to be genemediated limits to the number of times cells may divide.

Cells replicate by dividing themselves, a process called mitosis (somatic cells) or meiosis (gametes). Mitosis is a multistage process during which the cell’s chromosomes pull together and duplicate themselves. When this duplication is complete the cell then pulls apart into two new cells, called daughter cells, with one package of chromosomal content (called a chromatid) going with each daughter cell. In this way each daughter cell receives the full complement of chromosomes. Meiosis has two stages, meiosis 1 and meiosis 2. There is duplication of chromosomal material in meiosis 1 but not in meiosis 2, such that one cell ultimately produces four gametes.

For further discussion of cell structure and function within the context of genetics, please see the overview section “Genetics and Molecular Medicine.”


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