A "typical" animal cell is illustrated in Figure 2-1.
a. Cell Membrane. As its outer boundary, the animal cell has a special structure called the cell or plasma membrane. All of the substances that enter or leave the cell must in some way pass through this membrane.
b. Protoplasm. The major substance of the cell is known as protoplasm. It is a combination of water and a variety of materials dissolved in the water. Outside the cell nucleus (see below), protoplasm is called cytoplasm. Inside the cell nucleus, protoplasm is called nucleoplasm.
c. Organelles. Within the cytoplasm, certain structures are called organelles. These organelles include structures such as the endoplasmic reticulum, ribosomes, various kinds of vacuoles, the Golgi apparatus, mitochondria, and centrioles.
(1) The endoplasmic reticulum resembles a circulatory system for the individual cell. It is a network composed of unit (single-thickness) membranes.
(2) The ribosomes are granular particles concerned with protein synthesis. They may be found free, clustered, or attached to the endoplasmic reticulum.
(3) The vacuoles are small spaces or cavities within the cytoplasm. These serve functions at the cellular level such as digestion, respiration, excretion, and storage.
(4) The Golgi complex is a portion of the endoplasmic reticulum that aids in the final preparation of certain proteins and mucus-like substances and in the movement of these substances. It is best-developed in secretory cells.
(5) The mitochondria are the "powerhouses" of the cell. They "recharge" ADP molecules to form ATP molecules.
(6) There are ordinarily two centrioles. These organelles play a major role in cell division.
d. Nucleus. Within the cell is the nucleus. This structure has a nuclear membrane separating it from the cytoplasm. Within the nucleus is the chromatin material, made up of the protein deoxyribonucleic acid (DNA). At the time of cell division, this chromatin material is aggregated into individual structures known as chromosomes. Each chromosome has a set of specific genes, which determine all of the physical and chemical characteristics of the body, which represent its structure and function.
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