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Chapter: Anatomy and Physiology for Health Professionals: Levels of Organization : Cells

The cell is the basic unit that performs all the vital physiologic functions in the body.


The cell is the basic unit that performs all the vital physiologic functions in the body. The three parts of the cell are the semipermeable cell membrane, the cytoplasm, and the nucleus. The cell membrane is mostly made up of lipids and proteins, usually in a double layer of phospholipid molecules. The mem-brane lipids are mostly phospholipids, but also con-tain glycolipids, cholesterol, and lipid rafts. There are two distinct types of membrane proteins: integral and peripheral. The peripheral proteins may be anchoring proteins, recognition proteins, enzymes, ­receptor proteins, carrier proteins, or channels. There are alsomembrane ­carbohydrates, which have anchoring, locomotion, binding specificity, lubrication, protec-tion, and recognition functions.

The cytoplasm is a gel-like material suspending the cell’s organelles. The organelles in the cytoplasm each have specific actions that help to carry out the cell’s activities. They are vital to the life of the cell, tis-sue, and organism. Microtubules are hollow tubes that are the largest components of the cytoskeleton. They form and dissemble various components and allow for movement of organelles. Microfilaments of actin and myosin provide cell movement and contraction. The cell nucleus is the control center for cellular operations­. Inside the nucleus, a fluid called ­nucleoplasm sus-pends the nucleolus, nucleosome, and chromatin. The nucleus also contains chromosomes. The DNA con-trols protein synthesis in the nucleus. Gene activation­ in protein synthesis involves temporary removal of histones and utilizes messenger RNA to carry the information required to synthesize proteins. Transla-tion is the forming of a linear amino acid chain, which allows functional polypeptides to be assembled in the cytoplasm. A cell’s nucleus has either direct or indirect control of its cell structure and functionThe cell membrane uses passive and active mech-anisms to allow various substances to enter or leave the cell. Cell membranes may be freely permeable, selectively permeable, or impermeable. Passive cell mechanisms include diffusion, osmosis, and filtration. Primary active transport, via hydrolysis of ATP, allows molecules to move across cell membranes. Active cell mechanisms require energy and specific carrier mol-ecules, and include active transport, endocytosis, and exocytosis. Phagocytosis (cell eating) and pinocyto-sis (cell drinking) are forms of endocytosis. Mitosis occurs in somatic cells, with cell numbers increasing as cell nuclei divide. The stages of mitosis include prophase, metaphase, anaphase, and telophase. Game-togenesis is the process in which germ cells form and later mature into sperm (via spermatogenesis) or ova (via oogenesis). In meiosis, cell division reduces the amount of chromosomes by half, as genetic material from both parents mixes.

A neoplasm is a mass of tissue produced by abnormal cell growth and division. The two types of tumors are benign or malignant. Tumors that are malignant spread into surrounding tissues in a process known as metastasis.

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