Finding out what cells do

Building themselves

Cells arise from other cells and nowhere else. Once in an organism’s lifetime, at the beginning, two cells fuse to form a new cell. Ever after in that organism’s lifetime, two cells arise from the division of one cell, and all the cells ultimately derive from the first one. This process is how an organism builds itself from one single generic cell, called the zygote, to a complex organism comprising trillions of highly differentiated, highly specialized, and highly efficient cells all working together in a coordinated way. Here’s a look at how a cell goes from one to many.

 

Fusing: the zygote

The organism’s first cell is the zygote, made by the fusion of sex cells: an ovum (egg cell) from the female parent and a sperm cell from the male parent. (See Chapter 14 for more information about sex cells, and see Chapter 15 for more about the zygote.) The zygote has two complete copies of DNA: one from its male parent and one from its female parent. The two copies combine in the zygote’s nucleus. The zygote is said to be diploid — having a complete double-set of DNA.

 

Dividing: mitosis

In the form of cell division called mitosis, one cell divides into two daughter cells, each of them complete but smaller than the original cell. Details of this process are discussed later in this chapter.

The process of mitosis occurs only in diploid cells — cells that have two copies of the DNA. That is, all cells except the mature sex cells, which are haploid — they have only one copy of the organism’s DNA.

 

Differentiating

After mitosis is complete, each daughter cell goes on to its own separate life. One or both may start or continue down a path of differentiation, the name for processes that give cells their particular structures and functions. A cell destined to become a nerve cell starts down one path of differentiation; a cell destined to become a muscle cell starts down another path.

A variation on this mechanism involves a special kind of cell called a stem cell. A stem cell divides by mitosis, and one daughter cell remains a stem cell and goes on dividing again and again, while the other daughter cell goes on to differentiate into a specific type of cell in a particular tissue. Only some tissues have their own special stem cells, such as the skin and the blood.

The details of cellular differentiation are beyond the scope of this book. Its complexity is beyond imagining. This is the only explanation you really need: It’s under genetic control.

 

Building tissues

All tissues are made of cells, and those cells build and maintain it. Cells in a tissue are to one degree or another differentiated or specialized for their anatomical or physiological function in the tissue.

In addition to the cells, many tissues also contain structural proteins (which are made by the cells). Differentiated cells produce different proteins: Some produce only a few different proteins, and some produce many different proteins in response to signals they receive from other cells. The process of protein construction is basically the same in every cell and for every protein.

For the purposes of anatomy and physiology, remember that all cells have certain very important features in common, but they differentiate into a vast array of shapes and sizes, containing vastly diverse structures, and having different functions and life cycles.

 

Transforming energy

Most cells make ATP to fuel their own metabolism. They use glucose in the process of cellular respiration to do so. Flip to Chapter 2 for a description of how cells make ATP.

Some cells, like those lining the small intestine, absorb the glucose only to send it out the other side — allowing other cells access to this valuable resource. Sometimes, the glucose made available by the digestive system is more than the body can use at that moment. Thus, some cells, like those in the liver, function to corral the extra glucose molecules and store them. Later, when glucose levels are low, these cells release some of their stores, making it available to other cells.

 

Making and transporting products

A great many types of cells make special chemicals that are incorporated into tissues and participate in metabolic reactions. Cellular products include thousands of specific proteins and polypeptides, signaling chemicals like neurotransmitters and hormones, small molecules and ions, lipids of many kinds, and structural molecules of many kinds.

Some specialized cells do essentially nothing else but make and export one product for use by other cells; others make products and perform other functions.

Some cells specialize in transporting the products of other cells around the body, or in transporting metabolic waste products out of the body. Some of these transporting cells have other functions as well. Others do nothing but that one job through their entire life cycle. Red blood cells are an extreme example of the one-job model. They lose their nuclei during differentiation and thereafter do nothing but transport gas molecules from one place to another. They don’t divide, don’t produce ATP, and don’t maintain themselves. When their gas-transporting structures wear out, RBCs have nothing to do. They’re removed from circulation and broken down in the liver.

 

Communicating

Some cells transmit signals of various kinds while remaining in one place in the body. Some nervous cells’ sole purpose is to generate and conduct electrical signals. They typically live for years, often until the death of the organism itself. Other cells produce various kinds of signaling molecules, like hormones and neurotransmitters, or receive and react to those signaling molecules.