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.
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.
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, 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 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.