Introduction to Cell

Cell, in biology, the basic unit of all living matter. All forms of life consist of cells or groups of cells. Some, such as bacteria, consist of single cells, while others, such as redwood trees and whales, contain billions of cells. (Viruses, which some scientists consider to be living things, are not made up of cells.)

Most cells are so tiny that they can be seen only through a microscope. A few kinds of cells, however, are large enough to be seen with the unaided eye. The giant amoeba, for example, is a single-celled organism that can stretch to as long as one-fifth of an inch (5 mm) when it is moving actively. Cells in complex multicellular organisms form thousands of different shapes. The shape of a cell and the internal structures of a cell are related to its particular function.

The shape of a cellThe shape of a cell is a result of its particular function.

Robert Hooke, in 1665, was the first to identify cells; he also gave them their name. In the early 19th century, Matthias Schleiden, Theodor Schwann, and others recognized the universal occurrence and importance of cells. Cell biology, one of the most important branches of biological science, includes a number of disciplines, including biochemistry, genetics, and cytology (the study of structure and function of the parts of the cell). Research on the causes of cancer and many other diseases has become a major concern of cell biology, as has research on development of embryos.

Activities of Cells

Cells carry out complex processes. Among these are the synthesis (production) of compounds used by the cell, the selection of the materials that enter the cell, and the excretion from the cell of toxic or useless waste substances. Cells can grow; they can repair and replace their own parts; and they can duplicate themselves.

The different cells that compose complex multicellular organisms are highly specialized, carrying out extremely varied activities. Some cells are specialized for sensitivity to light, heat, sound, or pressure; others for the synthesis of materials such as hair, bone, shell, or bark; and still others for the synthesis of such substances as milk, hormones, and poisons.

In animals, examples of specialized cells are nerve cells, which transmit electrical impulses, and cells that produce antibodies (special proteins that attack invading organisms). In plants, examples are tracheid cells, which conduct water and help support the plant; palisade chlorenchyma cells, which produce food materials; and sieve tube cells, which transport food materials.

All cellular processes require energy, and among the basic properties of cells is the ability to generate energy. The energy is obtained through controlled oxidation (rearrangement of molecules so as to release energy) of foods. The oxidation of foods in cells is known as cellular respiration. For cells of certain living things, including some bacteria, certain protists, and animals, the main food substances oxidized are carbohydrates taken in from the organism's environment. Other living things, including certain bacteria, algae, and plants, make their own carbohydrates by a process known as photosynthesis and in turn oxidize them to obtain energy.

Most cells use free, or molecular, oxygen in the oxidation process. This type of respiration is known as aerobic respiration. Some cells carry on oxidation of food substances without using free oxygen. This type of respiration is known as anaerobic respiration.

After carbohydrates and other foods have been oxidized, the chemical energy thus obtained is locked into the chemical structure of certain specific compounds within the cell, mainly adenosine triphosphate (ATP). When a process requiring energy is to be carried out, the cell utilizes the ATP, releasing chemical energy by breaking down the chemical structure of ATP. The cell must then replenish its supply of ATP through the oxidation of food substances.

Types of Cells and Their Components

Despite the variations in functions and origins, all cells have basic characteristics in common. All cells consist of fluid material called cytoplasm surrounded by an outer covering called the cell membrane. (The contents of a cell are popularly called protoplasm, but this term is little used by scientists.

Eukaryotic cells came into being about 1.5 billion years ago. Most biologists believe that eukaryotic cells originated when certain large prokaryotic cells engulfed smaller prokaryotic cells that then began to function as organelles. This symbiosis (close association between two different organisms) helped both the large and the small cells to survive and was preserved over succeeding generations.

Cell Membrane

Through the cell membrane, food passes into and wastes pass out of the cell. In nearly all plant cells and the cells of certain other organisms, the cell membrane is encased in a relatively thick covering called the cell wall. This wall is usually composed of cellulose, which is inflexible; this is why plant cells are generally more rigid than animal cells. Cell walls help support a plant.

Cytoplasm

In nearly all cells, the cytoplasm contains rounded structures called vacuoles. They are bounded by a membrane similar in function and structure to the cell membrane. Some vacuoles contain food materials in a watery fluid; others contain waste materials and excess water. A mature plant cell often has a large central vacuole; large vacuoles are rare in animal cells.

Organelles are found in the cytoplasm of eukaryotic cells. They differ in various cells, and even the same cell can contain different organelles at different times. Organelles include:

Mitochondria, rod-shaped bodies in which oxygen is trapped and used for the oxidation of carbohydrates. Mitochondria are among the major sources of ATP in the cell.

Ribosomes, spherical bodies in which the synthesis of protein molecules occurs.

Golgi Apparatus, a network of structures, called Golgi bodies, that enclose protein molecules and prepare them for transport to the cell's surface. The Golgi apparatus is usually located near the cell's nucleus.

Plastids, variously shaped bodies common in plants, but rare in animals. The most important plastids are the chloroplasts, which contain chlorophyll (a pigment that is used in photosynthesis and that gives the green color to plant cells).

Centrioles, small, granular structures that lie near the cell nucleus in animals and certain other organisms. There is typically a pair of centrioles in each cell. Centrioles play a role in cell division.

Lysosomes, small granules containing many enzymes. The enzymes stored in the lysosomes are used in breaking down large molecules into smaller ones.

Endoplasmic Reticulum, fine membranes that form a network throughout the cytoplasm. There are two types of endoplasmic reticulum: rough and smooth. The membranes form channels, which are used as avenues of communication between the nucleus and cytoplasm and between the cytoplasm and the exterior of the cell. The rough endoplasmic reticulum contains many of the cell's ribosomes.

The Nucleus

The nucleus, generally more dense than the cytoplasm, is enclosed within a membranethe nuclear membrane. The nucleus controls the process of cell division and governs the functions of the entire cell. The nucleus typically consists of three componentsthe nucleoplasm, chromosomes, and nucleolus. The nucleoplasm is a gel-like substance in which the chromosomes and the nucleolus are suspended.

When the cell is not dividing, the chromosomes are scattered throughout the nucleus as a network of rod-shaped filaments. Chromosomes are composed of proteins and nucleic acids, mainly deoxyribonucleic acid (DNA). Chromosomes are the most important components of the nucleus because they contain the genetic information that determines the characteristics of an organism and directs all the operations carried out by the cell. The information is borne by the chemical structure of the DNA. Segments of the DNA that carry specific units of genetic information are called genes. When a cell divides, an exact copy of its genetic material (DNA) is made and each new cell receives the same information that the parent cell had.

Most cell nuclei also contain at least one nucleolus. Nucleoli are rich in nucleic acid and are usually attached to certain areas of the chromosomes. Nucleoli are involved in the production of ribonucleic acid (RNA).

The Nucleoid

The nucleoid is the region of a prokaryotic cell containing the genetic material. The genetic material is not enclosed in a membrane as in the nucleus of a eukaryotic cell.

Cell Division

All cells develop from previously existing cells through cell division. In a unicellular organism, cell division is equivalent to the reproduction of the organism. In multicellular organisms, cell division is responsible for the growth of the organism and the replacement of old or worn-out cells. At an early stage in the life of a multicellular organism, cell specialization, or differentiation, begins; it is by this process that new tissues and systems are formed.

In cells with nuclei, an important step in cell division is the division of the nucleus. The nucleus (except in sex cells) divides by a process called mitosis. Through mitosis, each new cell receives a copy of the parent cell's genetic material. Mitosis occurs in four successive stages: prophase, metaphase, anaphase, and telophase. There are no sharp dividing lines separating the various stages, and the events of each stage vary somewhat in different organisms. The four stages are preceded by interphase, the stage during which the growth of a cell occurs. Near the end of interphase, each chromosome duplicates itself, forming two identical strands. Each strand is called a chromatid; one is attached to the other at a point called the centromere. Chromosomes are invisible during interphase because their DNA is stretched out. Centrioles, if present, also duplicate during interphaseone pair becoming two pairs. In general, mitosis works as follows:

Prophase

The DNA condenses and the chromosomes become visible. At first, the chromosomes are long and thin and appear to be individual structures. As prophase continues, the chromosomes become short and thick; each chromosome can then be seen to be made up of two chromatids. Filaments collectively called an aster form around each pair of centrioles. Other filaments, called spindle fibers, form between the two pairs of centrioles, which move to opposite poles of the cell. The nucleolus and the nuclear membrane disintegrate.

Metaphase

The chromatid pairs align themselves in the center of the cell and each centromere becomes attached to one spindle fiber from each centriole pair. The centromere divides and the separated chromatids become independent chromosomes.

Anaphase

The new chromosomes move apart from each other to opposite poles. The spindle fibers between the centromeres and centriole pairs shorten.

Telophase

After the chromosomes reach the poles, the spindle fibers disappear and a nuclear membrane forms around each new group of chromosomes.

The cytoplasm then divides by a process called cytokinesis, and the original large cell becomes two smaller cells. Each new cell will take on food and grow, and may in turn divide by the same process.

Gametes (sex cells) are produced by a type of cell division called meiosis. Meiosis consists of two successive cell divisions that resemble mitosis, but the chromosomes are duplicated only once. Gametes therefore have half the number of chromosomes normally found in body cells.

At a very early stage in the life of a multi-cellular organism, the organism contains certain unspecialized cells called stem cells. These cells are capable of dividing indefinitely with little change, but they can also give rise to specialized cells. Stem cells continue to exist in a mature organism and are crucial in the development and balance of an organism's many types of tissue.