The animal cell

The term “cell” was first used by the English naturalist Robert Hooke in 1665 to describe the “great many little boxes” he saw when viewing a thin slice of cork through a microscope. He derived the word from the Latin cella, meaning a small room. Hooke was the first scientist to recognize that living matter was built of basic units rather than of continuous material.
Today, with the aid of electron microscopes, we have a more detailed view of living plant and animal cells. Some plant and animal cells have a particular function and are therefore not all exactly the same. The cells of animals exis’t in a wide variety of shapes. They may be round, egg-shaped, square, or rectangular. Some muscle cells are long and thin, and pointed at each end. Some nerve cells, with their long branches, resemble trees.
A system of similar cells forms a tissue for example, nerve cells make up nerve tissue and combinations of tissues in turn form an organ, such as the brain.

An animal cell (below) is characterized by the various structures within it, chief of which are the nucleus (with its nucleolus), mitochondria, and the folded layers of the endoplasmic reticulum.

The structure of cells

Despite this variety of form, all cells have some features in common. A cell consists of a nucleus, which is surrounded by a jellylike substance called the cytoplasm, enclosed in a cell membrane. This cell membrane (also called the plasma membrane) controls the passage of substances into and out of the cell. The membrane, only 0.00001 millimeter thick, is made up of layers of lipid, or fat, molecules sandwiched between layers of protein. The cell membrane is semipermeable that is, it allows certain chemicals to move in and out of the cell and prevents the passage of others.

The cytoplasm contains several kinds of structures, among which are many tiny structures called organelles. Many of the cell’s life activities take place in the cytoplasm. Each organelle plays a vital role in maintaining the life of the cell.

A typical plant cell (above) differs from an animal cell in that it has rigid cell walls, a large permanent vacuole in the center of the cytoplasm; and chlorophyll, located in the chloroplasts.

Organelles

Most of the cytoplasm of a mature cell is filled with elaborate folded membrane systems called the endoplasmic reticulum. The cytoplasm and the soluble proteins of the cell lie on one side of the endoplasmic membranes. On the other side, many cells contain enclosed pockets called cisternae. It seems likely that the endoplasmic reticulum plays an important role in the transport of substances throughout the cell. It also provides a large surface area where essential chemical reactions take place. Many enzymes that are important for the cell’s metabolism are found on the cytoplasmic side, whereas their products are found in the cisternae.
There are two types of endoplasmic reticulum. Smooth endoplasmic reticulum helps manufacture fat molecules. Rough endoplasmic reticulum is involved with the synthesis of proteins. Attached to the walls of the rough endoplasmic membranes are granular structures called ribosomes. But not all of the cell’s ribosomes are attached—some float freely in the cytoplasm. Those that are attached to the endoplasmic reticulum help build proteins that are transmitted to other parts of the body. Those that float freely aid the synthesis of the proteins that remain in the cell.
The rough endoplasmic reticulum also produces packages of enzymes called lysosomes. Under acidic conditions, lysosomes rupture and release their enzymes, which break down the major components of the cell. Lysosomes also release enzymes that destroy damaged or unneeded cells and that digest “foreign” cells, such as bacteria.
The Golgi complex consists of a stack of flat membrane sacs. These sacs process proteins and other substances produced in the cell. Small spheres called vesicles pinch off from the Golgi complex and transport some of these substances across the membrane to other cells or they are used to make the cell’s covering. Other Golgi vesicles remain inside the cell and fuse together to form storage compartments for proteins or other substances.

Mitochondria, another group of organelles found in the cytoplasm, are the powerhouses of the cell. They generate the energy that is needed to keep the cell’s essential processes going. Mitochondria are elongated, fluid-filled structures enclosed by a double membrane. The inner membrane comprises a series of deep folds called cristae. The enzymes needed to extract energy are located on the inner membrane. They oxidize nutrients and release energy in the form of a compound called adenosine triphosphate (ATP), which is used in the synthesis of cell materials in a process called internal respiration, or cellular respiration.

Centrioles, another type of organelle, are important in cell reproduction. They lie near the nucleus and resemble two bundles of rods.

The nucleus

The nucleus controls all the cell’s activities. It is spherical or elliptical in shape and bounded by two membranes that together form the nu-clear membrane. The outer membrane, which seems to be an extension of the rough endoplasmic reticulum, has several small pores through which nuclear material and large molecules pass.
A substance called the nucleoplasm inside the nuclear membrane contains the nucleolus and chromosomes. The nucleolus, a spherical body made up of ribonucleic acid (RNA), helps in the formation of ribosomes, which are the cell’s centers of protein production. RNA is chemically similar to DNA and playsan impor-tant role in protein synthesis. Most cells con-tain one or more nucleoli. The chromosomes, composed chiefly of deoxyribonucleic acid (DNA), are the blueprints for the cell’s struc-ture in the form of a genetic code. In the cell’s resting phase, the DNA is distributed through-out the nucleoplasm in the form of a thread-like material called chromatin. When a cell is dividing, the fine threads of chromatin shorten and thicken to form visible chromosomes.

Liver cells from a salamander are stained (above) so that they can more easily be studied. They would otherwise be impossible to see.