Difference Between Cells And Tissues
Protoplasm is contained within cells, the structural units of the body. Most cells are too tiny to be seen with the naked eye. In fact, 50 cells of average size laid end-to-end would scarcely spread across the period at the end of this sentence.
Like all animal cells, human cells have a nucleus surrounded by cytoplasm. A thin, slightly elastic membrane forms the outer surface of each cell. Within the cell the filmlike nuclear membrane separates the nucleus from the cytoplasm.
The nucleus is generally round or egg-shaped. It contains one or more dark, round bodies called nucleoli, and a number of extremely fine, slender threads called chromosomes. Chromosomes contain the information that determines how each cell looks and behaves. These attributes are stored in genes, which appear as irregular granules or beads along the surface of each chromosome. A gene consists of a segment of deoxyribonucleic acid—DNA—a twisted double chain of molecules that contains the “recipe” for every physiological process. We know that each chromosome may contain hundreds or even thousands of genes.
With the exception of the reproductive cells, every cell in an individual of a given species has the same kind and number of chromosomes. In human cells, this number is 46. Each mature human reproductive cell has 23 chromosomes. The part that reproductive cells play is discussed in Human Heredity.
The cytoplasm is a watery fluid in which various chemical substances, such as proteins, fats, and salts, are dissolved. The molecules that constitute the cytoplasm are arranged in granules, threads, and other structures, all so small that only the powerful electron microscope can reveal them clearly. These tiny structures within the cell are called the cellular organelles. They are the sites of all important cellular events, such as the controlled release of energy from nutrients and the manufacture of new compounds on which growth depends.
The cytoplasm contains other substances, which range from small, clear fat droplets to fine grains of pigment. In some cases, bacteria and other foreign particles may be seen in the cytoplasm, often undergoing slow digestion.
The human body contains a multitude of different cells. The red blood cells alone number about 25 trillion. And yet every person originates as a single cell—a fertilized egg. From this egg all the body’s cells are then derived by a process called cleavage, or division.
Similar cells are grouped together in a regular fashion to form tissues. Muscle is one type of tissue. It consists of thousands of fine filaments or fibers piled side by side. Each fiber is a muscle cell. While it is true that any tissue is made up primarily of a single cell type, other types are also usually present. Thus, cells of connective tissue, loosely arranged, penetrate the spaces in and around the muscle cells and hold them together. Connective tissue knits various other body tissues into working units.
The human body contains four main kinds of tissues: (1) the protective, or lining, tissues that cover the outer surface of the body and line certain internal organs; (2) the muscular, or contractile, tissues; (3) the connective and supporting tissues; and (4) the conducting, or nervous, tissues.