A cell is the microscopic unit of body organization. The "typical animal cell" is illustrated in figure 1-4. A typical animal cell includes a cell membrane, a nucleus, a nuclear membrane, cytoplasm, ribosomes, endoplasmic reticulum, mitochondria, Golgi apparatus, centrioles, and lysosomes.

Figure 1-4. A "typical" animal cell (as seen in an electron microscope). 1-13. MAJOR COMPONENTS OF A "TYPICAL" ANIMAL CELL

a. Nucleus. The nucleus plays a central role in the cell. Information is stored in the nucleus and distributed to guide the life processes of the cell. This information is in a chemical form called nucleic acids. Two types of structures found in the nucleus are chromosomes and nucleoli. Chromosomes can be seen clearly only during cell divisions. Chromosomes are composed of both nucleic acid and protein. Chromosomes contain genes. Genes are the basic units of heredity which are passed from parents to their children. Genes guide the activities of each individual cell.

b. Cell Membrane. The cell membrane surrounds and separates the cell from its environment. The cell membrane allows certain materials to pass through it as they enter or leave the cell.

c. Cytoplasm. The semifluid found inside the cell, but outside the nucleus, is called the cytoplasm.

d. Mitochondria (Plural). Mitochondria are the "powerhouses" of the cell. The mitochondria provide the energy wherever it is needed for carrying on the cellular functions.

e. Endoplasmic Reticulum. The endoplasmic reticulum is a network of membranes, cavities, and canals. The endoplasmic reticulum helps in the transfer of materials from one part of the cell to the other.

f. Ribosomes. Ribosomes are "protein factories" in the cell. They are composed mainly of nucleic acids which help make proteins according to instructions provided by the genes.

g. Centrioles. Centrioles help in the process of cell division.

h. Lysosomes. Lysosomes are membrane bound spheres which contain enzymes that can digest intracellular structures or bacteria.

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